The Hydrology-Geomorphology
Interface: Rainfall, Floods, Sedimentation,
Jerusalem Conference, May 1999). IAHS Publ. no. 2 6 1 , 2000.
Land Use (Proceedings of the
75
Runoff, erosion, and the sustainability of ancient
irrigation systems in the Central Negev desert
LESLIE SHAN AN
Institute of Earth Sciences, Hebrew University ofJerusalem,
Jerusalem 91904, Israel
Givat Ram Campus,
Abstract Runoff and erosion processes in desert watersheds were investigated
by studying ancient irrigation systems discovered in the 100 mm rainfall
region of the central Negev. Catchments delivering flood waters to these areas
ranged in size from small plots to large watersheds. Runoff from small
watersheds (less than 50 ha) varied from 4—12 mm year" compared to
0.5-2.5 mm year" for large watersheds (greater than 1000 ha). Even in
extreme drought years, small watersheds produced at least 1.4 mm of runoff,
while large watersheds experienced "dry" years (i.e. without any runoff event)
about once in every three years. Ancient irrigation systems using runoff from
small watersheds were much more efficient "water-harvesting" projects than
those diverting flash flood flows from large watersheds. Rates of erosion from
small watersheds averaged 3.6 mm century" (54 t km" year"'), originating
mainly as sheet erosion on the hillsides. Rates of erosion from large
watersheds, where main wadis are stable broad depressions with
deep loessial soils and a good winter vegetation cover, are about
4.6 mm century" (701 km" year" ). In large watersheds where wadi
incision and headcutting processes are active, rates of erosion can be
expected to range from 7.6-12.6 mm century" (115-180 t km" year" ).
Key words climatic changes; erosion; Negev Desert; runoff; sustainable irrigation systems
1
1
1
1
2
2
1
1
2
1
INTRODUCTION
Numerous studies have reported that the technique of using runoff and flash floods for
irrigation has been practised for more than 3000 years in China, India, Egypt, Iraq and
Israel. Parts of these ancient irrigated areas still produce reasonable yields but
extensive sections are now barren and desolate wasteland. There is no general
agreement on the cause of the decline of these systems. Some investigators have
favoured a theory of increasing aridity and climatic change. Our research lends support
to the conclusion following the dendro-archaeological studies of Lipshitz & Waisel
(1978), that the climate of the Negev has not changed significantly during the past
5000 years. The majority of studies on other ancient systems demonstrate that three
principle factors, acting singly or in combination, led to the deterioration and abandon­
ment of projects:
- The strong central authority that planned and operated the systems was replaced by
one that was incapable of managing them.
- The accumulation of sediment in canals and fields reduced the amount of water
available for irrigation, and the heavy burden of maintaining the projects led to
their abandonment.
- Inadequate drainage, water logging, and the accumulation of salts in the soils
reduced yields below financially viable levels.
76
Leslie
Shanan
This paper focuses on the lessons learned from the ancient irrigation systems in the
Central Negev desert and shows how: (a) the presence or absence of a strong central
authority, and (b) the processes of runoff and erosion affected their sustainability.
THE CENTRAL NEGEV DESERT
The Central Negev, the southern desert of Israel, is dotted with extensive remains of
ancient habitation and agricultural systems (Fig. 1). In this 100-150 mm annual
rainfall region, irrigation based on the utilization of surface runoff from the meagre
winter storms was developed to a high technical degree, reaching its peak during the
Nabatean-Roman-Byzantine domination of the region from about the second century
BC to the seventh century AD. Desert agriculture using hillside runoff was already
F i g . 1 T h e N e g e v , s h o w i n g (I) lowland foothills and (II) central h i g h l a n d s . T h e d a s h e d
and dotted line indicates the b o u n d a r y b e t w e e n the N e g e v and J o r d a n and Sinai. T h e
triple dots indicate the ruins of ancient cities: H a l u z a (Halutsa), R u c h e i b a ( R e h o v o t h ) ,
N i z z a n a (Auja-Hafir), Shivta (Subeita), K u r n u b ( M a m s h i t ) a n d A v d a t ( A b d a ) .
Runoff, erosion,
and the sustainabilily
of ancient
irrigation
systems
in the Central Negev
desert
11
practised during the Israelite Period (eighth and ninth centuries BC) at the time of the
Judean Kings (Evenari et al., 1982). The densest settled areas have been discovered in
the lowlands and foothills and the highlands (Fig. 1).
The lowlands and foothills cover about 150 000 ha. The morphological structure of
this subregion is made up mostly of Eocene limestone hills separating wide rolling
plains, with elevations ranging from 200-450 m a.m.s.l. A number of large wadis
whose sources are in the highlands, cut through the plains (Evenari et al, 1982). The
hillsides are generally covered with shallow, gravelly, saline soils with immature
profiles (Table 1).
The highlands cover about 200 000 ha and are composed of a series of parallel
anticlines of Cenomanian Turanian limestones and cherts. Elevations vary between
450-1000 m a.m.s.l. (Evenari et al, 1982). Adjacent to the main wadis (gullies) lie
relatively narrow flood plains, and near the watershed divides, where the wadis have
not cut down to stable base levels, there are a number of expansive plains (Table 1).
T a b l e 1 S u m m a r y of ecological conditions in the N e g e v H i g h l a n d s (after T a d m o r & Hillel, 1956).
Habitat
% ofthe
highlands
area
Soils
Plant associations
Rocky
slopes
80-90
Shallow, gravelly, saline
Artemisietum
Zygophylletum
Loessial
plains
10-15
D e e p loessial soils; salts
l e a c h e d to 30 c m or m o r e
Anabasidetum
Haloxylonetum
Wadi beds
3
D e e p loessial soils or
gravel and silt fill
Retama roetam association
with m a n y annuals
herbae-albae
dumosi
hausknechtii
articulati
W a t e r available
for plant g r o w t h
(mm)
and
10-60
and
20-50
G r a v e l l y wadis:
6 0 - 1 0 0 , loessial
wadis: 4 0 0 - 6 0 0
"RUNOFF FARM" SYSTEMS
In order to investigate the techniques that enabled ancient desert agriculture to exist
under extremely marginal climatic conditions, a research team (the late Professor
M. Evenari, the late Professor N. H. Tadmor and the author) was established in 1954.
During 1954-1959 we surveyed and studied more than 100 ancient farming systems
and irrigation projects (Evenari et al, 1982). We discovered, amongst other findings,
that all the ancient agricultural projects in this desert were based on utilizing runoff
from small and large watersheds—hence the term "runoff farm" systems. In order to
evaluate the hydrological and agricultural potential of these methods, in 1958-1959 the
research team reconstructed two ancient runoff farms, one at Shivta and the other at
Avdat, where there are extensive remains of ancient systems (Fig. 1). Scientific
research at the reconstructed farms continued for 25 years (Evenari et al, 1982).
The selection of the sites was partly influenced by a controversy concerning manmade heaps of stones that had been placed in mounds and long strips covering
thousands of hectares in the Central Negev desert (Fig. 2). These structures had been
observed in the 1870s, but were not investigated until the 1950s.
Various theories regarding the purpose of the mounds have been reviewed by
Evenari et al. (1982). Palmer in 1871, had concluded that they were associated with vine
cultivation on the hillsides because his Bedouin guide had called them tuleilat el enab,
78
Leslie
Shanan
F i g . 2 A n oblique aerial p h o t o of 2 0 0 0 year old patterns of stone strips, near N i z z a n a .
T h e r e m n a n t s of ancient terraced fields, s u r r o u n d e d b y stone fences, can b e seen in the
b a c k g r o u n d in the b r o a d loessial depression at the foot of the slopes. (Photo b y
N . T a d m o r , 1954).
i.e. grapevine mounds. His theory failed to explain how the grapes grew on such
extremely shallow saline soils, without any irrigation in this 100 mm rainfall region.
Some investigators overcame this difficulty by proposing that the additional moisture
was obtained due to the mounds acting as "air-wells", with dew condensing on the
stones. Observations in gravel mounds rebuilt by us, after the ancient model, neither
collected dew (in sealed containers to prevent evaporation) nor did they improve the
soil moisture conditions below them as compared with the surrounding soil.
Mayerson, in 1959, suggested that the ancient farmers irrigated the tens of
thousands of hectares of hillside vines by carrying water from wells or cisterns,
completely disregarding the amount of water that would be required or the human
effort involved. Previously, Kedar (1957) had proposed an entirely different theory and
argued that the main function of the mounds was not viticulture, but to increase
erosion from the hillsides and so accelerate soil accumulation in the cultivated valleys.
Concurrent with these theories, we proposed that the purpose of the mounds, and
also the strips (Fig. 2, which the previous investigators never recorded or referred to)
was to increase runoff, not erosion from the hillsides, with the aim of collecting the
maximum possible runoff from the slopes. It was in this atmosphere of debate
regarding the runoff and erosion processes in the Negev that the two ancient farms
were reconstructed and experiments superimposed on them to study, inter alia, the
hydrological and climatic conditions of the area.
This paper focuses on case studies related to the techniques used by the ancient
farmers in their endeavours to establish sustainable irrigation systems in this harsh
Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central Negev desert
79
desert region, and refers to the results of our research work over three decades (19541985) that deal with runoff and erosion processes of the region.
The case studies include: (a) diversion systems from large watersheds, (b) runoff farm
systems associated with small watersheds, (c) research data from two reconstructed
ancient runoff farms, and (d) sedimentation measurements in two large watersheds not
directly related to ancient agricultural systems.
The erosion data is presented as "depth per unit of time", namely, mm per century
(mm century" ) together with the conventional unit, tons per square kilometre per year
(t Ion" year" ). The unit 1.0 mm century" , is approximately equal to 14 t Ian" year" ,
assuming an average sediment bulk density value of 1.45.
1
2
1
1
2
1
Diversion systems
Case study 1: The Nahal Lavan system Nahal Lavan (Wadi Abiad) (Figs 3and 4)
is the largest wadi in the vicinity of the ancient town of Shivta (Fig. 1) and drains from
the high plateau of the Matred Plain through an area of barren rocky CenomanianTuronian hills. Torrential floods have cut a deep wadi into the alluvial plain that is
narrow in the upper reaches but in the lower reaches, widens out into extensive flood
plains. Today, the wadi flows in a gravel-bed watercourse typical of the area.
Numerous remnants of ancient walls and terraces are found in the alluvial flood plain
(Fig. 5). At a point where the drainage area of Wadi Abiad is about 53 km , an area
covering 200 ha of terraces was studied in detail (Fig. 4).
Because of the superimposition of many terrace systems one on the other, it is
often difficult to differentiate between projects of different periods. However, the size
and capacity of the spillways, canals and drop structures, provided a key to their
understanding. Three types of spillway, all serving to lower water from one terrace to
the next, were found:
(a) spillways with crest lengths of 30-60 m for handling flows of 10-30 m s" (Fig. 6).
(b) spillways with a crest length of 3-8 m for flows in the range of 1-5 m s" .
(c) small spillways up to 1 m wide, for flows less than 1 m s" .
Using these criteria, three different types of developments were distinguished. The
earliest flood irrigation devices were discovered on the west bank of the wadi where
massive stone spillways with 30-60 m crest lengths are the common structure. These
spillways were connected to low earth embankments, stretching across the plain, of
which only faint traces remain today in the form of low banks (Fig. 3). The spillways
are of such large capacity that they were capable of handling the entire flood. The
topographic location of this system indicated that it was used when Wadi Abiad was a
shallow depression and the earth embankments were built to spread the runoff waters
across the broad flood plain. The wide stone spillways served to control and direct
the water as it passed from higher to lower elevations. This flood plain water-spreading
system was in use before Wadi Abiad had become a deep gravel-bed watercourse.
Water-spreading systems with spillways of 3-8 m crest length and diversion canals
able to handle 1-5 m s" , were found mainly on the northeast bank of the wadi, in the
middle and upper reaches of the survey area (Fig. 3). Some of these canals are more
than 1 Ion long, 5-10 m wide and aligned with a gradient of 0.4-0.5%. Each diversion
2
3
1
3
3
3
1
1
1
00
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ANCIENT AGRICULTURE IN WADI ABIAD
I:
Cotiarime feetra)
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Brircl Strip
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Dislriiufm Hitch
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F i g . 3 A m a p o f W a d i A b i a d (Nahal L a v a n ) s u r v e y e d b y L . S h a n a n a n d N . H . T a d m o r in 1 9 5 5 . N o t e t h e series of diversion canals o n b o t h b a n k s of the w a d i
leading part o f the flood flows to the terraced fields. Stage I s y s t e m s are located o n the s o u t h w e s t side o f the p r e s e n t d a y wadi, a n d stage II a n d III systems o n t h e
northeast side. N o t e h o w a w a d i h a s c u t t h r o u g h Stage II s y s t e m t o w a r d s the lower e n d o f the s u r v e y e d s y s t e m . This tributary w a d i w a s " c a p t u r e d " b y W a d i
A b i a d in relatively recent t i m e s ( p r o b a b l y a b o u t the R o m a n P e r i o d , 5 0 B C - 1 5 0 A D ) .
n"bn
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Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central
Negev desert
81
82
Leslie
Shanan
F i g . 5 A w a d i (gully) b a n k stabilization wall in W a d i A b i a d (Fig. 4 ) . N o t e that the
wall w a s built during at least three different p e r i o d s . T h e terraced fields are 4.5 m
a b o v e the w a d i b e d at this point.
F i g . 6 A m a s s i v e spillway a b o u t 50 m long o n the s o u t h w e s t flood plain of W a d i
A b i a d (Fig. 4), b e l o n g i n g to Stage I d e v e l o p m e n t of a large w a d i diversion s y s t e m
(900 B C - 7 5 0 B C , M i d d l e B r o n z e II Period).
canal irrigates an area of about 2-4 ha. The original stone diversion dams have been
washed away. Most of these terraces are still in excellent condition.
The walls of the diversion canals and the associated terrace walls, were built in
stages (Fig. 3) because the silt and sediment that accumulated in the fields and canals
Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central
Negev desert
83
compelled the irrigators periodically to raise the terrace walls and diversion structures.
Some of the walls reach 5 m in height.
An interesting example of river-capture was found cutting through the system
(Fig. 3). The water-spreading system on the northeastern side of Wadi Abiad was once
a continuous project—despite the fact that today, a 4 m deep tributary wadi joins Wadi
Abiad from the north. The alignment and elevation of the walls on both sides of this
tributary wadi, as well as the sizes and capacities of the structures, clearly indicate that
they once belonged to the same system. In earlier times this tributary was not
connected to Wadi Abiad at all, but continued to flow west to a separate waterspreading system. This tributary wadi was "captured" by Wadi Abiad in relatively
recent times, probably after the late Byzantine period (about 650 AD).
The third use of the area was as "runoff farms" (see below) connected to the
adjoining small watersheds and not to the main wadi. These farms adapted the existing
structures and stone walls of the diversion system to their needs. They no longer expl­
oited the floods in Wadi Abiad but used runoff from small catchments in the adjoining
hills. It is in connection with these runoff farms that the small spillways are found.
Most of the flood plain, particularly the area lying west of the wadi is badly
damaged because it has been used for military manoeuvres since the 1960s.
Case study 2: Wadi Kurnub Wadi Kumub cuts a narrow, steep gorge through a
limestone ridge 2 km south of the ancient town of Kurnub (Fig. 7). At the point where
the gorge opens onto the Tureibeh plain, the ancient settlers constructed a large
channel to divert part of the Wadi Kumub flood waters (generated over a 27 km
drainage basin). The diversion chamiel is a solidly built stone structure 5-9 m wide,
with a gradient of 1:2000 over its 400 m length. The channel led the water to an
extensive (10-12 ha) system of terraced fields that are all still in good condition.
Excess water from each terrace flowed to the next lower one, through drop structures.
The diversion structure in the wadi diverted large quantities of silt into the terraced
areas and, in a manner similar to that described above for the Wadi Abiad systems, the
level of the terraced fields rose continually, forcing the farmers from time to time to
raise the level of the terrace walls and diversion structures. The three types of
development—flood plain, diversion system, and runoff farm—as described for the
Wadi Abiad system, were also discovered in the Wadi Kumub system.
2
Runoff farm systems
The term "runoff farm" (Evenari et al, 1982), denotes a group of adjoining terraced
fields surrounded by a stone-wall fence, forming an integral unit of about 0.5-2.0 ha of
cultivated land. A house, cistern and/or watch-tower are often found within the
boundaries of this fence, and are indicative of a sedentary agricultural population (Fig. 8).
The hillside surrounding the farm served as a catchment from which conduits channelled
runoff water to the terraced fields. Catchment and cultivated areas are thus seen as a
clearly defined unit—an integral part of an overall plan of watershed subdivision.
In the desert, rainfall only wets the hillside soil to a shallow depth, and is soon lost
by evaporation. Ancient runoff fanners had to depend on winter runoff water from the
surrounding slopes to supplement the meagre rainfall. Cultivation practices of the
F i g . 7 M a p of the Kurnub (Mamshit) system, surveyed b y L. Shanan and N . H. T a d m o r
in 1954. T h e r e m a i n s of ancient walls related to the e v o l u t i o n of the w a d i and the
terraces during three diversion stages dating from 9 0 0 B C to 6 0 0 A D w e r e clearly
visible in the field and are s h o w n on the m a p . N o t e the d i v e r s i o n ditch leading runoff
from two small catchments to a farm unit belonging to the Byzantine Period (300-650 AD).
.V.
• • l l
*<
••111
••III
F i g . 8 A n oblique aerial p h o t o of a 2 0 0 0 year old runoff farm near N i z z a n a . T h e
h o m e s t e a d and the stone fence s u r r o u n d i n g the terraced fields are clearly seen. A w a d i
(gully) h a s b r o k e n t h r o u g h the farm, destroying sections of the terraced walls and
fields. ( P h o t o b y N . T a d m o r , 1954.)
Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central
Negev desert
85
farmer in more humid regions aimed to prevent hillside runoff and enhance the infiltr­
ation of rain into the soil. The desert runoff farmer, operates the contrary principle. He
aims to minimize infiltration on the slopes, maximize runoff, and lead the runoff from a
relatively large area on the slope to small cultivated field in the bottomlands.
Agricultural development under the physiographic conditions of the Negev
(shallow, gravelly soils and steep gradients), required not only a mastery of the
techniques of using surface runoff for irrigation, but also an understanding of the skills
of land reclamation. One of the important aspects of ancient desert settlement was
terracing the small narrow wadis (Table 1), and it is in these wadis that runoff farm
units or groups of farm units are found.
Case study 3: Runoff farms using runoff from small watersheds Figure 9 is a
map of a number of farm units in the Avdat region where the fields were irrigated by a
Fig. 9 A detailed m a p (surveyed b y L. S h a n a n and N . T a d m o r in 1955) of a n u m b e r of
runoff farm s y s t e m s in the A v d a t area. T h e contour interval is 10 m . T h e farm units,
collecting conduits and the m o u n d and strip s y s t e m s are easily discernible in the
fields. See text for further explanation.
86
Leslie
Shanan
system of hillside collection channels. The small eastern valley A (Fig. 9), is terraced
in its lower reaches and gets part of its water from the upstream wadi. The terraces,
receive additional runoff from stone built channel systems on the adjoining hills. The
western valley B (Fig. 9), is terraced from its lower end to the head gully. Water is
collected from adjoining slopes that have been partly cleared of stones, to form
patterns of stone mounds, strips, and conduits.
The total area of all these systems is about 100 ha, of which 3 ha were cultivated.
The ratio between the water-collecting and the water-receiving area is about 33:1.
The catchments were subdivided into subcatchments, with conduits transporting
runoff from specific parts of the slopes to specific fields. A single conduit generally
collected water from a relatively small area, 0.1 ha to 1.5 ha in size. The runoff was
thus divided into small controllable streams of water, suited to the dry stone structures
built by the ancient farmers. These small flows could be easily controlled during a
flood period.
Figure 10 is a detailed map of a complete runoff farm unit, which is also shown in
the centre of the flood plain on the southwest bank of Wadi Abiad in Fig. 3. The stone
mounds and strip systems (A, B and C, Fig. 10) increased the runoff from the
catchment. Five runoff collecting channels (a, b, c, d and e, Fig. 10) directed the runoff
to the terraced fields. The ratio of the catchment to the cultivated area is a low 3:1
compared to the normal average of 20:1, because the cultivated terraces were
originally part of a larger project planned to receive runoff water from the early
900 BC-750 BC diversion system shown in Fig 3.
The earliest runoff farm units were found in the Mishor Haruach and Matred Plains,
dating to the Israelite III period (850 BC-600 BC). The farms were generally constructed
near forts and water cisterns located along the caravan routes (Evenari et al, 1982).
Negev (1979) dated several runoff farm units in the Avdat area to the late Nabatean
period (c. 100 AD). He suggests that the Nabateans had applied the technology of
collecting hillside runoff water for filling their cisterns, to irrigating the terraced fields.
Runoff farms were used continuously until about the seventh century AD, when
the rising tide of Islam swept the region. They were abandoned because the Arab
civilization had neither religious, economic, or military motives for maintaining them
(Negev, 1979).
RECONSTRUCTED ANCIENT FARMS
Case study 4: Reconstructed farms at Avdat and Shivta
Detailed descriptions of studies carried out at the two reconstructed ancient runoff
farms have been published elsewhere (Shanan & Schick, 1980; Evenari et al, 1982).
The hydrological studies include 20 runoff plots and 13 watersheds.
Avdat The watersheds include eight catchments ranging in size from 1 ha to 345 ha.
The large watershed is a third order basin in which many of the ancient terraced walls
have collapsed. The other seven watersheds are ancient subdivisions of a 30 ha
watershed that resulted from reconstructing the ancient "water-harvesting" hillside
collecting ditches. The subcatchments vary in size from 1 to 7 ha.
Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central
i
Negev
lour
nl.mr
l.iiv
llnr
null m r r
nl.mr
nlm
nlunr,
desert
87
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F i g . 10 A detailed m a p o f a c o m p l e t e runoff farm unit (see centre o f the southwestern
flood p l a i n area, W a d i A b i a d in F i g . 3 ) . T h e stone m o u n d s a n d strip s y s t e m s (A, B and
C) increased t h e runoff from the hillside a n d t h e collecting c h a n n e l s (a, b , c, d a n d e)
directed t h e flows to the cultivated fields. T h e terraces received flood flows from the
earlier flood diversion s y s t e m (see Fig. 3), p r i o r to the e s t a b l i s h m e n t o f the runoff
farm system. S u r v e y e d b y L. S h a n a n a n d N . T a d m o r in 1954.
Shivta The watersheds include five catchments ranging in size from 1.2 ha to
70 ha. The 1.2 ha catchment leads to a roofed cistern excavated out of soft chalk strata
in a manner that enabled a hard limestone formation to serve as a roof, supported by a
rock pillar. The capacity of the cistern is about 150 m . The other four catchments lead
runoff to the reconstructed runoff farm.
3
RESEARCH UNRELATED TO ANCIENT AGRICULTURAL SYSTEMS
Concurrently with the case studies described above, research that was not directly
related to ancient agricultural systems was carried out in two large watersheds, Nahal
Haroeh and Nahal Boqer, situated about 10 and 15 km respectively, north of Avdat.
Case Study 5: Nahel Haroeh
The watershed comprises typical rocky slopes, deep loessial soils and wadis (Table 1).
Ancient terraces for stabilizing the hillside and the loessial wadis are found in several
88
Leslie
Shanan
subcatchments of the watershed. In Nahal Haroeh, a detention-storage dam was
constructed in 1954-1955 by the author to use flood flows from a 43 km catchment
area (Fig. 11). Nativ (1976) provides details of the watershed, the dam and its
construction, and daily and seasonal rainfall and runoff measurements.
Most of the rainfall in the Negev is localized, often coming in convective cells; the
typical cell diameter is less than 10 km. The proportion of the area receiving rainfall on
a given day may be as low as 20% (Sharon, 1972). Rainfall data were recorded at Sde
Boqer at a point 9 Ion south of the catchment centre and 3 Ion outside the watershed
boundary. Because of the spotty, erratic, and unequal distribution of the storms on the
2
Fig. 11 A vertical aerial p h o t o g r a p h of the N a h a l H a r o e h D a m , constructed near Sde
B o k e r in 1 9 5 4 - 1 9 5 5 . D o w n s t r e a m of the d a m are the terraced fields that receive this
s u p p l e m e n t a l irrigation. N o t e also the ancient terraced fields in s o m e wadis,
particularly the b r o a d terraced s y s t e m southeast of the dam. A tributary w a d i cut b a c k
from N a h a l H a r o e h and completely b y p a s s e d the terraced fields, p r o b a b l y during the
late B y z a n t i n e Period, about 600 A D .
Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central
Negev desert
89
watershed, the recorded rainfall values are only approximate estimates of the probable
average values for the catchment. Annual rainfall for the period 1951-1981 ranged
from 30 mm to 170 mm and averaged 93 mm.
The maximum storage capacity of the dam is 250 000 m and the water is used for
irrigation. Seepage and evaporation losses are high (15 cm day" ) and an estimated
20% of the flood flows are wasted through the spillway that functions about once
every 7-10 years.
3
1
Case Study 6: Nahal Boqer
2
The Nahal Boqer watershed (35 km ) adjoins Nahal Haroeh and is physiographically
similar to Nahal Haroeh. Observations relevant to this paper included measurements of
a wadi "back-cutting" process that has been followed by the author since 1951. We
have observed similar wadi incision processes in Ramat Matred, Mishor Haruach and
Nahal Lavan areas.
RESEARCH RESULTS AND OBSERVATIONS
Runoff processes
Small watersheds and runoff-plots Based on two decades of comprehensive
rainfall and runoff measurements in small watersheds and runoff plots in the Avdat and
Shivta reconstructed farms, the runoff processes have been studied in detail (Shanan &
Schick, 1980; Evenari et al, 1982). The principle factors affecting runoff in these two
regions include: soil cover, seasonal infiltration rates, basin shape, basin size, hillside
slope, differential slope contribution, overland flow lengths, and channel losses.
Our studies showed that both stoim and seasonal runoff are mainly functions of
catchment size and slope gradients (Table 2). Average seasonal yields from the three
catchment sizes studied ranged from 2.4 mm (for third order catchments) to 26 mm
(for small plots, 80 m in size). One of the most important factors contributing to these
differences in yield is the initial loss (i.e. the threshold rainfall needed to initiate
runoff, Table 2). The initial loss in the third order basin (7.0-8.0 mm) can be
accounted for in the following manner:
crust wetting
2.5 mm
overland flow losses
2.5-3.0 mm
wadi channel loss
2.0-2.5 mm
2
T a b l e 2 A v e r a g e annual runoff and initial losses from c a t c h m e n t s in the Central N e g e v .
Catchment
A r e a (ha)
A v e r a g e annual runoff ( m m )
Initial loss ( m m )
Plots: 1% slope
1 0 % slope
2 0 % slope
0.08
0.08
0.08
26
22
11
2.5
2.5
2.5
Sub-catchments
1-7
4-12
5.5
Third-order catchments
345
2.4
7.0-8.0
Leslie
90
Shanan
These conclusions explain why ancient runoff farming systems based on small
watersheds were relatively efficient water harvesting projects. In the ancient systems,
subdividing the first order basins by artificial channels reduced initial losses from
5.5 mm to about 3.0 mm and so increased the frequency of runoff events and the
seasonal water yield.
The results of the runoff plot studies showed that slope gradient is an extremely
important factor affecting runoff. The 10% gradient, for example, produced about 60%
more total annual runoff than the 20% gradient. Gradient is not only an indicator of the
topographic condition of an area, but is an index summarizing the physiographic site
conditions including soil depth, stone cover, rock outcrops and vegetation cover.
Moderate slopes with low infiltration rates were found to be the principal contributors
of runoff in the Avdat area. Unequal areal distribution of rainfall further reduced the
runoff contribution from peaks of hills, because valley side slopes were found to
receive almost twice the rainfall received by the peaks and knolls.
Stone clearing was found to have a significant effect on runoff. Stone clearing
increased average annual yields by 24% and 49% from 10% and 20% slope plots,
respectively. The studies indicate that this increase can be explained by regarding
infiltration as a two-phase process that takes into account the movement of air escaping
upwards through the soil profile and water infiltrating downwards. This explanation
supports the theory that the purpose of the gravel mounds, tuleilat el enab, found in the
region, were a result of stone clearing by ancient farmers to increase the runoff yield
(Evenari et al, 1982).
Avdat large watershed (345 ha) Runoff from this watershed was erratic with
about 50%o of the years producing less than 0.5 mm. However, in about 10% of the
years, runoff exceeded 10 mm. The average annual yield was 2.4 mm (Table 2).
This explains why the catchment-to-cultivated area ratio in the large diversion
projects is at least 35:1 (compared to 20:1 for the small watershed runoff farms),
indicating that the large systems are relatively inefficient "water-harvesters".
These studies contributed towards understanding of the water balance of the Upper
Nahal Zin Basin. By evaluating the differential contribution from first, second, and
third order catchments, the research showed that the amount of water available for
plant growth both on hillsides and on wadi bottoms was minimal. On the hillsides,
only about 30-40% of the annual rainfall penetrates below the soil crust and becomes
available for plant growth because a 10-15 mm rainfall wets only 5-10 cm depth,
depending on local soil cracks and fissures. The threshold conditions required for third
order basins to contribute to the regional water table indicate that: (a) a 2-3 h flow in
the wadi wets an average depth of about 40-60 cm; (b) that after a 3-4 h flow, the
maximum saturation depth in the centre of the wadi does not exceed 1.2 m; and (c)
flood water penetrates to the deeper gravel layers in the wadi beds of third order
streams only during exceptional floods lasting more than 8 h.
2
Nahal Haroeh dam (43 km watershed) Based on 35 years of records at Kibbutz
Sde Boqer (1955-1991), average rainfall was calculated as 95 mm. In 13 out of the 35
years (37%) there was no runoff, i.e. about one out of every three years is likely to be a
drought year. However, the spillway functioned five times during the same 35 year
period, averaging once in seven years (15%). The average annual runoff was 2.4 mm.
Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central
Negev desert
91
The records also indicated that:
(a) a threshold daily rainfall of 8 mm was required to initiate a runoff event in Nahal
Haroeh;
(b) a daily rainfall of about 32 mm produced a large enough flood for the spillway to
operate.
Daily runoff, for rainfall amounts of up to 50 mm day" , can be represented by the
equation:
1
R =
D
K{P -%)
D
where, RD = daily runoff [mm], PD = daily rainfall (<50) [mm], and K is a constant
which varies from 0.17 to 0.22, depending on antecedent rainfall conditions and
season. The quantity " 8 " represents a threshold storm rainfall required to initiate runoff
in the watershed. It is interesting to note that the value of the threshold rainfall (8 mm)
and the average annual runoff (2.4 mm) was found to be the same for both the 345 ha
watershed at Avdat (Table 2) and for the 43 km watershed of Nahal Haroeh.
It is also interesting to note that Ionides (1939) found that the threshold rainfall
required to initiate flood flows in the watersheds of Transjordan (now the Kingdom of
Jordan) was 8 mm.
The Avdat watersheds are tributaries of the Nahal Zin. Runoff records of the Israel
Hydrologie Service (Schnitzis et al., 1997) for the period 1954-1955 to 1994-1995 for
Nahal Zin (Waterfall station) with a drainage basin of 233 1cm , show that 33% of the
years were years of no runoff and the annual average runoff was 2.4 mm year" .
2
2
1
Erosion processes
Of all the forces affecting the landforms in a desert, water is by far the most effective.
The water erosion process has been divided into three phases: sheet, rill, and gully
erosion. During the sheet erosion stage, overland flow depths are no more than several
millimetres, velocities are less than 0.1 m s" and flow is laminar. After traversing less
than 40 m, the flow reaches depressions and rills, its depth and velocity increase
rapidly, flow becomes turbulent and velocities often exceed 1.0 ms" . These
concentrated flows with destructive erosive power have significantly increased silttransport capacities that create the gullies and wadis.
1
1
Erosion from the Avdat small watersheds Bed load and suspended sediment
samples collected from four adjacent subcatchments (numbered 1, 2, 3 and 4) at Avdat
with a total catchment area of 20 ha, produced an average sediment yield of
3 mm century" (45 t km" year" ). This included two fractions:
(a) bed load: measured in silt traps over a 20 year period (1960-1980) averaged
0.003 mm year" ,
(b) suspended load: sampled over a two year period (1970-1972) averaged
0.027 mm year"'.
In these small watersheds, 90% of the total sediment yields appeared as fine
particles transported as suspended material in the runoff. Taking into account that the
average ratio of catchment to cultivated area is about 20:1, the rate of silting in the
ancient terraced fields of the small watershed systems would have only been
0.6 mm year" or 6.0 cm century" .
1
2
1
1
1
1
92
Leslie
Shanan
Erosion from the Shivta cistern watershed Sediment accumulated in the cistern
over the 20 year period (1960-1980) at an average rate of 0.042 mm year" from this
1.2 ha watershed. Average annual rainfall was 93 mm year" and runoff 12 mm year" .
1
1
1
1
-2
1
Total sediment yield was therefore equivalent to 4.2 mm century" (63 t Ion" year" ).
Taking into account that only 10% of the total sediment yield was coarse particle
bed load (see above), these low sediment values explain why the silt-traps built by the
ancient settlers at the entrance to these runoff-collecting cisterns (Evenari et al., 1982)
had capacities of generally less than 100 1. The Shivta cistern, for example, produced
an average of about 80 kg (50 1) bed load sediment per year, a quantity that could be
caught and stored in the silt-trap. Cleaning it once per year, or after each storm, was a
simple maintenance task.
Erosion processes in large watersheds with diversion systems All the diversion
projects studied showed a remarkable similarity in their evolution that is characterized
by three development stages:
- Stage I, Flood plain development Many of the major present-day gravel-bed wadis
were wide shallow depressions in the alluvial plains, prior to 1500 BC. Intensifi­
cation of the agricultural use of these fertile areas necessitated the construction of
the stone walls to prevent the flood flows concentrating in the lowest depressions
and creating gullies. These walls were, over time, extended to spread the water to
extensive sections of the flood plain. The main spillways of these systems were
characterized by 30-60 m wide openings capable of passing the entire flood (Fig. 3).
- Stage II, Diversion system At some period these flood plain spreading projects
were abandoned and the system deteriorated through lack of maintenance. After
abandonment, the floods cut a 1-3 m deep gully through the flood plain. The next
settlers in the area developed the technique of diverting the flood water by
constructing low stone barrages in the wadi and building canals to lead the water
to the flood plain. Each diversion canal served a relatively small area (2-6 ha) and
in most cases the new settlers built on the remnants of earlier walls and structures
they found in the floodplains.
The wadi continued to erode, and silt from eroding banks and from upstream headcutting gullies was deposited in the terraced fields. This sediment raised the level
of the fields until a stage was reached when the height of the terraced walls had to
be increased and a new diversion structure and canal constructed further upstream
to irrigate the fields at their higher elevations. The wadi bed was eventually several
metres below the terraced fields in the flood plain, and channelled between high
stone retaining walls (Fig. 3). Any break in these walls would cause serious
damage to the whole system.
The construction of these structures required an understanding of hydrology and
hydraulics. Thus, the period of these diversion systems must have been one in
which the science of engineering was well developed and the projects were under
the control of a central authority that was able to manage the entire watershed and
to enforce rules for distributing the water during the short flood periods that
occurred two or three times every year.
- Stage III, Runoff farms The area was again abandoned at some point in time. The
systems may have become unmanageable because of the silting problem or
exceptional floods may have destroyed the main structures. The next settlers in the
Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central
Negev desert
93
area no longer relied on using flood water from the main wadis, but used runoff
from small watersheds adjoining their terraced fields to obtain supplementary
irrigation water. These runoff farms usually adapted older walls and structures to
their new requirements and used only small sections of the original diversion
system area that were situated close to hillsides.
2
Sediment in the Nahal Haroeh dam (43 km basin) Sediment yields in the dam
(Fig. 11) were calculated on the basis of a revised survey made in 1976 by the
Department of Agriculture (E. Ador) and on flood flows and discharges recorded by
Kibbutz Sde Boker (Nativ, 1976, and personal communication, R. Yahel of Sde Boker)
for the period 1955-1991.
Average annual runoff was 3.0 mm year" and the total volume of sediment that
accumulated in the dam during a 21 year period (1955-1976) was 23 000 m .
Assuming a trap efficiency for the dam of 55% (25-35% delivered to irrigation and
15-20% wasted over the spillway), average annual sediment yield was 0.046 mm year"
(4.6 mm century" , 70 t km" year" ).
This relatively small sediment yield is due to the physiographic and ecological
conditions prevailing in Wadi Haroeh. It is a stable, wide depression with a lush winter
vegetation cover, and in sections of the wadi the ancient terraces are still in good
condition. The relatively low gradients of the main wadi—about 1%—have also
contributed to the stability of the wadi and few gullies have cut back into the flood plains.
1
3
1
1
2
1
2
Gully headcutting in Nahal Boqer (40 km basin) At the lower end of Wadi Boqer,
the flood waters flow in a wide depression that in some sections was stabilized by
ancient stone walls built to keep the flood spread across a 30-50 m wide zone that was
covered with perennial and annual vegetation (Table 1). Base levels of the area have
not changed appreciably since ancient times, perhaps for 5000-10 000 years. However,
in 1950, at the lower end of the valley, probably because a stone stabilizing wall broke
during an exceptional flood, an ever deepening and widening gully began cutting back
into the flood plain destroying the vegetation and ancient walls as it moved upstream.
In 1951 the gully was about 1-2 m wide and 1 m deep. Thirty years later it had
become a wadi 30 m wide with vertical banks 2.5 m high, and it had cut back some
500 m into the original stable floodplain. These observations indicate that an average
of about 1250 m soil had eroded annually during the 30 year upstream advance
of the headcut. This is equivalent to 0.031 mm year" on the entire watershed or
3.1 mm century" (46 t km" year" ). This is of the same magnitude as the rate of
erosion from small watersheds.
This degradation in the main wadi results in initiation of a correlated incision
process in tributary wadis, and a cycle of upstream headwater erosion can be expected
to further increase the total rate of erosion from the watershed.
3
1
1
2
1
Sedimentation in ancient terraced fields The silt-laden flood water used for
irrigation brought about cumulative changes in the levels of the terraced fields. The silt
deposited in diversion canals and in fields raised their levels about 2 m during a
600 year period, averaging about 3.3 mm year" (33 cm century" ).
The rate of rise in the levels of the fields irrigated from diversion canals can also
be estimated from the annual water use by agricultural crops in the ancient systems
1
1
94
Leslie
Shanan
1
which was about 300-400 mm year" (Evenari et ai, 1982). Given that the average silt
content of the flood waters reaching the fields is about 1% by weight, the amount of
silt deposited in the fields was about 3-4 mm year" , i.e. 30-40 cm century" .
This magnitude of rise would require a concomitant raising of the terraced walls at
the same rate (Fig. 5). These estimated sedimentation rates are considerably higher
than those reported for ancient irrigation systems in Mesopotamia (Iraq) which
averaged about 20 cm century" over a 5000 year period (Jacobson & Adams, 1966).
The situation was less acute in the runoff farm systems, because the rate of sediment­
ation from the small watersheds was only 6 cm century" (see above).
1
1
1
1
DISCUSSION
Rates of erosion
The rates of erosion reported above, are summarized in Table 3. These observations
indicate that erosion from small watersheds averaged an extremely low 3.0-4.2 mm century"
(45-63 t km" year" ). The large Nahal Haroeh watershed, where the main wadi is a
shallow wide loessial depression with a good winter annual and perennial vegetation
cover, produced about 4.6 mm century" (70 t km" year" ) of sediment. Incremental
sediment load resulting from gullies headcutting back into a stable deep loessial wadi
(Nahal Boqer), was equivalent to additional 3.1 mm century" (46 11cm" year" ).
Large watersheds, where gully erosion and headcutting processes are active, can
be expected to produce at least 7.6-9.5 mm century" (115-1521 km" year" ) of
sediment as estimated in Table 4. In catchments where the headstream erosion process
is also taking place in the tributary wadis, rates of erosion may increase by an additional,
say, 3.1 mm century" and reach a total of 12.6 mm century" (180 t km" year" ).
Rates of erosion reported for other regions in the Negev are given in Table 5.
Several factors account for the relatively lower rates of erosion in the Central Negev,
principally:
1
2
1
1
2
1
1
2
1
1
1
2
1
2
1
1
T a b l e 3 E r o s i o n rates in the Central N e g e v : s u m m a r y .
Watershed
Area
Rates of erosion:
( m m century" )
(t km" year" )
45
1
2
A v d a t small watersheds
Shivta cistern
1-7 ha
3.0
1.2 ha
4.2
63
Nahal Haroeh dam
43 k m
2
4.6
70
N a h a l B o q e r headcut
35 k m
2
3.1
46
1
T a b l e 4 E s t i m a t e d rates of erosion in subcatchments of a large w a t e r s h e d with active headcutting.
Source
Estimated rate of erosion:
2
( m m century"')
(t km" year"')
Small w a t e r s h e d s
3.6-4.0
54-60
Stable w a d i s
1.0-2.0
15-30
Active headcuts
3.1-3.5
46-62
Total
7.6-9.5
115-152
Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central
Negev desert
95
T a b l e 5 R a t e s of erosion in three selected regions in the N e g e v .
R e g i o n and reference
Annual
rainfall
(mm)
Watershed
area
(km )
Description
2
Erosion rates:
(mm
century"')
(t km" year" )
2
Eilat, N a h a l Y a e l
(Shick & L e k a c h , 1993)
31
0.5
Natural w a t e r s h e d
11.5
170
Machtesh HaKatan
( G r e e n b a u m & Lekach,
1997)
80
7.3
Natural w a t e r s h e d
7.5
120
220
2.9
4 7 % contour ploughed;
51 % loess-mantled
slopes
16.0
259
Beersheba (Lemanim)
( L a r o n n e , 1989)
1
(a) The number of days per year with a rainfall greater than 5 mm averages about 6, of
which only about 3 exceed 10 mm. Rainfall of 25 mm day" has a probability of
less than once every few years. The maximum intensities of these rainfalls are
much lower than in the rest of Israel (Evenari et al, 1982).
(b) Overland flow distances are short, seldom exceeding 40 m.
(c) The loess soil of the area forms a crust after 2-3 mm of rainfall have wetted the
surface, and it remains relatively stable under the low-velocity laminar flow
conditions prevailing before runoff concentrates in rills and gullies.
(d) The stone cover, particularly in the hamada areas, protects the soil surface and
acts like a mulch.
(e) In the deep loess wadis, annual winter and perennial vegetation stabilizes the
depressions. Erosion rates increase significantly however, after a head-cutting
incision process has been initiated.
The ancient farmer, by subdividing the watersheds into relatively small subcatchments
achieved two important advantages:
(a) He increased the amount of runoff that could be harvested from the hillsides by
reducing the overland flow distances and seepage losses.
(b) He decreased the rates of erosion from the watersheds by minimizing or
eliminating the development of rill, gully, and wadi erosion.
Herzog (1998) discovered an ingenious Israelite III period (850 BC-600 BC)
water supply system at Tel Beersheva that diverted flash floods from a large 25 km
watershed into a tunnel leading to four underground cisterns with a total storage
capacity of 500 m . The system was abandoned after 200 years because of serious
sedimentation problems in the tunnel and the cisterns.
During the Hellenistic period (350 BC-167 BC), one of the cisterns (with a
capacity of about 100 m ) was again put into use to store runoff water, collected this
time not from a large watershed, but from a small watershed.
The Tel Beersheva water cistern complex is the only system discovered in the
Negev that was supplied with runoff water from a large watershed. All other cisterns,
dating from the Israelite period through to the Byzantine period (850 BC-650 AD),
used runoff from small watersheds (Evenari et al, 1982). Apparently after about 600 BC,
the engineers realized that the high silt and sediment loads carried by flash floods in
large watersheds, caused rapid rates of sedimentation in the diversion canals, tunnels,
and cisterns. Unable to meet the heavy burden of maintaining these diversion systems,
they decided to use runoff flows only from small watersheds to fill their cisterns.
1
2
3
3
Leslie
96
Shanan
Role of steep rocky slopes in producing runoff
Yair (1983), based on studies in a small watershed near Sde Boqer, concluded inter
alia, that from the point of view of runoff production, a greater emphasis should be
placed on the role played by steep rocky hillsides and less on the gentle slopes and the
stoneless bare soils. Evenari et al. (1982) had recognized the relative contribution of
the rocky hillslopes, but their runoff plot results showed that significant amounts of
runoff from these areas could only reach the fields if two conditions were met,
separately or in combination:
(a) the overland flow distances of the natural catchments were reduced to 40 m or less
by dividing the hillsides into subcatchments with cross-slope collecting conduits;
(b) the annual runoff yield could be increased 20-60% when the stones were cleared
from the surface and placed in mounds and/or strips.
First, it is important to point out that Yair's Sde Boqer experimental site is not typical
of watersheds where ancient agricultural systems are found and the results may not be
applicable to the ancient runoff-collecting systems for the following reasons:
(a) At Yair's experimental site there are neither runoff farm systems or gravel mound
and/or strip systems. Several terraced wadis and wadi stabilization walls are found
in a few subcatchments of the Sde Boker watershed. However, ancient agricultural
systems had been constructed on no more than 1.3% of the Boqer-Ashalim
catchment area, compared to 2.7%, 5.4%, and 3.1% for the Shivta, Avdat and
Nizzana watersheds, respectively (Table 6). The ratio of the catchment-tocultivated area (Table 6) reflects different densities of development and a diversity
in landforms. In the Avdat watershed, the systems are predominately runoff farms
with mounds, strips, and collecting conduits enhancing runoff production from the
hillsides, with an average catchment-to-cultivated area ratio of 18:1; in the Shivta
and Nizzana watersheds, flood plain diversion systems adjacent to the main wadis
are the principal form of development with catchment-to-cultivated area ratios of
38:1 and 32:1, respectively. However, the catchment-to-cultivated area ratio for
Sde Boqer is 84:1 and reflects the paucity and infrequent occurrence of
stabilization walls in the watershed.
(b) This low level of runoff farm development in the Sde Boqer watershed is one of
the reasons why Yair (1983) did not find any "agricultural installations" in the
13 km loessial plain of Mishor Zin. A second reason for the absence of develop­
ment is that, although Mishor Zin is only 8 km due north of Avdat city, it is
separated from the Avdat area by the deep canyon of Nahal Zin, about 150 m deep
and 2 km wide in parts. Access to Mishor Zin from Avdat necessitates a 15 km
trek circumventing this canyon (Fig. 12).
2
T a b l e 6 A n c i e n t agricultural cultivated areas in selected w a t e r s h e d s in the N e g e v (after K e d a r , 1967).
Watershed
C a t c h m e n t area
(km )
2
Cultivated area
(ha)
% watershed
cultivated
R a t i o of c a t c h m e n t - t o cultivated area
84:1
Sde B o q u e r , A s h a l i m
255
303
Shivta
188
495
1.2
2.7
Avdat
125
678
5.4
18:1
Nizzana-Ruth-Lotz
560
1750
3.2
32:1
38:1
Runoff, erosion,
and the sustainability
of ancient
irrigation
systems
in the Central Negev
desert
97
F i g . 12 A vertical aerial p h o t o of the d e e p w i d e c a n y o n of N a h a l Zin, cutting t h r o u g h
M i s h o r Zin. N o r t h of the canyon, the buildings of the B e n G u r i o n University, Sde
B o k e r C a m p u s , can b e seen o n the brink of M i s h o r Zin. N o t e the ancient runoff farm
s y s t e m s located on that section of M i s h o r Zin situated south of the c a n y o n are only
4 k m distant from the ancient t o w n of Avdat.
The predominant intensive runoff farm developments in the Negev (particularly
the gravel mound and strip systems and the collecting conduits), are found mainly
within 5-6 km distance of the main ancient towns (Avdat, Shivta and Nizzana).
Farmers were not prepared to journey more than 6 km to reach and tend their
irrigated fields or walk tens of km to maintain their water collecting systems.
Furthermore, they were not prepared to live too far from the main centres for
security reasons. (In many developing countries in which the author has worked, it
was observed that villages with 3000-5000 inhabitants cultivate irrigated lands
extending over 300-600 ha but located no more than 3 km from the village, for the
same two reasons).
On the southern side of the Nahal Zin canyon, and only a 4 km trek from Avdat, is
a continuation of the landforms of Mishmar Zin (Fig. 12). On this loessial plain,
98
Leslie
Shanan
ancient runoff farms with collecting conduits on the almost stoneless gentle
sloping area are clearly seen in the photo and in the field. This loess plain was
found by the ancient farmers to be satisfactory from the point of view of its water
harvesting potential and its distance from the town of Avdat.
(c) The physiographic and ecological conditions of the Sde Boqer watershed differ
significantly from those at Shivta and Avdat. First, the Sde Boqer geological
formations are Cretaceous limestones, dolomites and chalks; those at Avdat and
Shivta are younger Eocene limestones, "hamadas" and conglomerate "hamadas".
Second, the plant associations present are also significantly different. In the Sde
Boqer experimental site they are primarily Vartenia phionedes-Originum dayi,
indicative of relatively high soil moisture conditions (Yair, 1983); in Shivta the
Zygophyllum dumosum association dominates while Artemesia herba alba
represents the common association in the Avdat area (Evenari et al, 1982).
(d) The studies by Evenari et al. (1982) were carried out at Avdat and Shivta on an
experimental layout superimposed on reconstructed ancient agricultural runoff
collecting systems, while the experimental site at Sde Boqer was in no way
comiected to any ancient runoff inducement and collecting systems.
(e) The Avdat runoff plots, from which Evenari et al. (1982) drew their conclusions
regarding the effect of slope, cover, and rainfall, were carefully designed and
constructed in four separate blocks with the slope of each plot uniform as well as
equal within blocks. The site was chosen so that the geological, pedological, and
ecological conditions were uniform. The experiment was planned in a random
block design, with four treatment replicates and control plots.
The runoff plots of the Sde Boqer site in contrast, differ widely in their shape, size,
dominant hillslopes, stone cover, and geology. The site comprised three limestone
formations: Dorim, Shivta, and Netser, each with its particular ecological
environment. The majority of plots include two different geological formations
(Yair, 1983). In addition, overland flow lengths vary from 55-76 m and hillside
gradients from 12-29%, Furthermore there are wide variations in the slope
gradients within the plots themselves.
Consequently, in the Sde Boqer experimental site, it is impossible to separate out
from the data, the effects of interrelated variables in an analytical, statistical, or
simulation analysis of the complex nonlinear relationships. This problem is
discussed in further detail below.
(f) The runoff plots at Avdat were uniform in size and shape (20 m long and 4 m
wide), and the slope was uniform in each plot and equal within blocks. The
geological formation is also unifonn on the site. The overland flow length on all
plots was standardized (20 m) and the rainfall micro-distribution was recorded
with a representative number of recorders.
The lack of uniformity and wide variation in attributes between and within the Sde
Boqer plots results in basic differences in the statistical populations under study. For
example, Yair et al. (1980, Table 1) found that 82% of the total erosion and 46% of the
runoff originated on three plots representing only 35% of the area, due to the differential
bioturbation activity of porcupines and isopods that was concentrated on these plots.
In contrast, the experimental design of the runoff plots of Avdat limited the
number of variables under study, and evaluated them in a manner that overcame the
Runoff, erosion,
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problem of equifmality (i.e. similar changes resulting from disparate combinations of
input, throughput, and output acting over different periods of time).
Ancient terraces—sediment traps or stabilization walls?
An important aspect of understanding the runoff and erosion processes in the Negev, is
the question of whether the ancient stone terraces on the hillsides and in the valley
bottoms were constructed to trap the silt washed down from the hillsides, or whether
they were built to stabilize existing soils in the wadis and depressions by preventing
gullies "cutting-back" into these potentially productive areas. This issue has been
referred to widely in the literature during the past four decades. There are several
major objections to the hypotheses that the walls were sediment collecting structures:
(a) Our studies have shown that erosion from small and large watersheds in the Negev
does not exceed 5 mm century" and 10 mm century" , respectively. Assuming a
catchment-to-cultivated area ratio of 30:1, soil accumulates behind the terraces at a
rate of 15-30 mm century" This means that the ancient farmers would have to
wait at least 200 years until they had trapped 30-60 cm of soil behind their terraces
before they could expect to produce any viable agricultural crop.
(b) Our archaeological discoveries in the field support the hypothesis that the soils in
the wadis and depressions pre-date the construction of the runoff collecting
systems:
(i) The three-stage evolution of the large wadi floodplains described previously,
shows that the first walls were built specifically to stabilize the wide depressions
and spread the flood flows across the floodplains (Figs 2, 8 and 11) and so prevent
the development of gullies in the depressions.
(ii) Many of the flood plain development projects were operated during the
Israelite Period (1200 BC-1000 BC), at least a millennium before the RomanByzantine period of development.
(iii) We discovered stone mound systems superimposed on the Roman road just
north of Avdat, showing that the Roman road predated these runoff collecting
systems (Evenari et al, 1982).
(iv) Numerous examples were recorded of wadi incision and "wadi capturing"
(Figs 2, 8 and 11) clearly indicating that many of these head-cutting processes
occurred after the areas had been abandoned (probably after 650 AD) when the
systems became dilapidated through lack of maintenance and gullies broke
through the terraced walls.
(c) Terraces to enable the intensive cultivation of hillsides and bottomlands have been
built continuously throughout the ages, for example, in Middle America and Peru
during the first millennium AD, in the USA during the Navaho period about
800 AD, as well as more ancient examples in China, Nepal and North Africa.
Modern terrace development continues in many regions, particularly those
bordering the Mediterranean. All these projects are constructed only on sites with
soil profiles of at least 50 cm depth, and soils that can be cultivated intensively and
adapted to profitable upland crops, vines, olives and orchards. They are always
built as stabilizing structures and are never constructed on barren areas.
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It must be pointed out that the hillside terraces in the ancient runoff farm systems
in the Negev desert are always constructed in hillside depressions or wadis, and the
runoff from the hillsides collected and led in conduits to the terraced fields. However,
in humid regions (1000-2500 mm annual rainfall)—for example in Nepal, Korea, the
Philippines and other southeast Asian countries—hillside terraces are constructed as
level-bench terraces to collect and hold all the rainfall falling directly on the hillside
slopes. Hence, these are not runoff farm systems. The level-bench terraces are usually
constructed of earth embankments and the fields used for rice, upland crops or
orchards. Similarly, in the 500-100 mm annual rainfall region, for example in the
Jerusalem Hills in Israel and in other countries in the Mediterranean region, the walls
of the bench terraces on the rocky slopes are constructed of stone, the fields are not
always level and are used for growing vines, olives, orchards and grain production.
Stabilization terrace walls in both arid and humid regions enhance the infiltration
of water into the soil profile and so contribute to improving yields and production.
Based on the records of the Nizzana Papyri (Evenari et al, 1982), the yields of barley
in the ancient farm systems in Nizzana (Fig. 1) in the seventh century AD were
recorded as being 8.0-8.7 times the amount originally sown, compared to yields
obtained at Avdat in the 1960s of about 10-11 fold increases. The higher modem
yields were due probably to the use of fertilizers. Based on these production estimates,
terracing in the Negev desert enabled hillside and valley fields to be cultivated
intensively, either to increase the range carrying capacity 10-20 times on steep slopes,
or for grain production, of about 1-3 t ha" barley (grain) on the deeper soils of the
gentle slopes (Evenari et al, 1982).
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Gravel mounds and strips-a unique phenomenon
The tuleilat et enab man-made gravel-stone mounds and strips (Figs 2, 9 and 10), are
confined to specific areas in the Negev. They have not been observed in any other part
of the region, nor reported elsewhere in the world. They are therefore a unique and
remarkable aspect of the Negev desert. Their occurrence in the central Negev
Highlands is always associated with hillside runoff-collecting conduits and together
they form an integral part of the runoff farm systems. Our research has shown that they
result from the ancient settlers clearing stones from the hillside slopes and placing
them in mounds and/or strips. The bared soils on the slopes increased the seasonal
amount of runoff harvested from the hillsides, particularly from the rocky slopes and
the rock outcrops.
Why is their occurrence a unique phenomenon? We concluded that a combination
of many physiographic, environmental and sociological conditions must exist in a
particular area to justify their establishment, operation and maintenance. These
preconditions include:
(a) Availability of land with agricultural potential which only requires the addition of
200-300 mm year" of supplemental water to make it productive and economically
viable.
(b) Location of these potential agricultural areas close to existing towns (within a
6 km radius) so that the farmer does not spend more than about three hours a
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work-day, walking to and from his fields to cultivate, sow, irrigate, harvest, and
keep guard over his crops and his water-collecting systems.
(c) The soil in the fields is at least 1.5 m deep so that the water holding capacity of the
profile is not less than 250 mm, the average depth of water that would be supplied
by a flood irrigation.
(d) The only reliable source of water for this supplementary irrigation is runoff from
the adjoining hillsides.
(e) Runoff from hillsides can be increased significantly if the stones are cleared from
the hillside surfaces; the cleared soils comprise impermeable rock outcrops and/or
have the characteristic of forming an impermeable crust after a few mm of rainfall
have wetted the surface, so that most of the subsequent rain becomes runoff.
(f) The natural overland flow distances are less than 40 m, or alternatively, the
catchments can be subdivided into subcatchments with collecting conduits that
also serve to reduce the overland flow length to less than 40 m.
(g) Rainfall occurs mostly in the winter months when the soil and rainfall temperatures
are 0-5°C. Infiltration rates at these low temperatures are significantly less than the
rainfall intensities of the average storm, thus ensuring runoff occurring even with
light rainfalls of 5 mm depth.
(h) The rates of erosion from the catchments are low, less than 4 mm century"
(60 t km" year" ) so that sedimentation in the ditches, fields and cisterns is not a
critical problem.
(i) The area is under the control of a strong central authority that has the political will
and competence to plan and operate the systems and enforce regulations for water
rights and water distribution procedures. The agricultural sector includes farmers
who are willing to introduce and use new techniques,
(j) An economic and social structure that does not rely only on agriculture for its
livelihood but insures a satisfactory and stable income from several productive
economic sectors. This includes: desert caravan convoys continually moving
through the region along international trade routes; military camps and defence
installations, for local and regional security objectives (like the large Nabatean
army camp for about 1000 permanent soldiers at Avdat); churches and monasteries
serving as local and national religious centres of learning and study found in all the
ancient towns (Fig. 1); and a class of entrepreneurs who are prepared to initiate
new economic ventures in the region (like the horse-breeding enterprise at Kurnub
during the Nabatean period (about 100 BC), or the large Nabatean factory at Avdat
manufacturing exquisite hand-painted delicate pottery of high quality and supply­
ing to the entire region, or the luxurious public bath-house projects at Avdat and
Kurnub serving the caravan convoys along the regional trade routes (Negev, 1979).
The concurrent presence of all these circumstances in the central Negev Highlands
enabled the ancient settlers to introduce runoff inducing water-harvesting systems into
limited areas near the ancient towns of Avdat, Shivta and Nizzana.
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Analytical solutions to the runoff process
The scientist often finds himself having to chose between simple or elaborate
analytical mechanisms for understanding complex relationships and has to select from
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a number of widely differing analytical methodologies—mathematical, statistical and
simulation modelling. Systems that can be solved mathematically are generally very
simple sub-processes and essentially only of academic interest, with limited practical
value. Analytical analysis is nevertheless important because it gives an insight into
fundamental aspects of a problem.
Methods of studying the behaviour of involved, even chaotic interacting systems,
have been developed using advanced statistical methods and/or with the simulation of
continuous and parallel systems. In simulation modelling, the state of a system at any
particular point in time, is expressed quantitatively; changes in the system are
described in mathematical statements or as input data. Ecological, mathematical and
programming aspects are interwoven into a simulation model, and the use of
continuous-system-modelling (CSMP) languages has been developed specifically for
this purpose (de Wit & Goudrian, 1978; Shanan & Schick, 1980).
Two independent methodologies have been used to analyse the complex process of
runoff from the Negev desert watersheds: a multivariate analysis for predicting annual
runoff yield, and a digital simulation model for predicting individual storm and total
seasonal runoff (Shanan & Schick, 1980; Evenari et al, 1982) and are briefly reviewed
below.
Multivariate analysis Annual runoff for watersheds varying in size from microcatchments (less than 0.1 ha) to third order watersheds (up to 300 ha) were correlated
with watershed size, annual rainfall, hillside slope, and stone cover. The results are
presented as a nomogram in Fig. 13 (Evenari et al, 1982, Fig. 91). Yair (1983) used
Fig. 13 to extrapolate an extreme value for predicting the threshold amount of annual
rainfall needed to initiate runoff production from a 1.5 ha catchment with a hillside
slope exceeding 20%. He concluded that the nomogram predicts that a minimum
annual rainfall of 75 mm is required for the catchment to provide "a minimum amount
0
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200
300
F i g . 13 N o m o g r a m of rainfall-runoff relationships at A v d a t , s h o w i n g effect of size,
slope, and surface cover of the c a t c h m e n t s o n a n n u a l runoff. Scale 0 - 3 0 0 , runoff
( m ha" ); scale 0 - 1 5 0 , rainfall ( m m ) (after E v e n a r i et al, 1982).
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Runoff, erosion,
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of water". He then commented that annual rainfall amounts of less than 70 mm are
common in the Negev, and if the nomogram result is correct, then the ancient runoff
farm systems were inefficient users of runoff. Unfortunately, Yair used the nomogram
erroneously. Instead of applying the curve titled "Watersheds 5-70%", he applied the
curve titled "Microcatchments >20%." If he had used the nomogram correctly, he
would have concluded that the threshold annual rainfall for this specific watershed to
produce runoff is 25 mm (not 75 mm). During the 38 years of records at Avdat, the
lowest recorded rainfall (1962-1963) was 28 mm, a one-time occurrence. Even in
1962-1963, all the watersheds produced significant amounts of runoff, averaging
1.4 mm (giving an average coefficient of runoff of 5.0% for the extreme drought year).
Simulation modelling The simulation model (Shanan & Schick, 1980) developed
in CSMP language to simulate the runoff process, used storm rainfall as input, and
runoff measurements from plots and catchments as output. The structure of the model
was verified in three stages, with an optimization algorithm minimizing the sum of the
squares of the residuals between observed and simulated events. In the first stage, the
model was confined to runoff plot data so that time and areal variations in rainfall and
the effects of overland flow would be minimized. Functions to account for the effects
of slope, stone cover, infiltration, and evaporation were evaluated. In the second stage,
the model was expanded to simulate the processes on seven sub-basins that were
considered as a combination of plots, modified by areal rainfall distribution patterns
and overland flow conditions. Finally, in the third stage the model was adapted to a
third order basin, which was considered an assemblage of sub-basins, modified for
main channel losses.
The basic premise of the model states that runoff is initiated after a crust is formed.
Infiltration rates of the soils are considered to be greater than rainfall intensities until
the crust is puddled and saturated. This threshold requirement, called the "maximum
saturation deficit", is regarded as including depression and interception storage. When
the crust is partly saturated, the amount of water required to bring it to saturation is
called the "saturation deficit". Runoff is initiated when two conditions are fulfilled:
(a) the crust is saturated (saturation deficit is zero), and (b) the rate of rainfall exceeds
the infiltration rate of the crust.
The sub-basin model was modified to include the effect of areal rainfall
distribution, differential contribution of areas, seasonal infiltration rates as functions of
soil and rainfall temperatures, raindrop impact, stone cover, hillside slope, overland
flow and channel losses. Maximum saturation deficit values are about 2.5 mm and
infiltration rates of the saturated crust about 1.0 mm h" . A normal distribution in the
areal variation of infiltration rates was found to give the best fit for the simulated
results of the plots. The simulation of storm and annual runoff was performed in two
stages: (a) parameter development stage: trial and error runs of 63 storms during the
1964-1967 period; and (b) validation stage: simulation runs of 42 storms during the
1967-1969 period using the "best fit" values developed from the first period.
Although the problem of equifmality was successfully resolved in the plot stages
of the model, it was not solved for the sub-basins and the third order basin because of
the differential effects of hillside slope, slope distribution, overland flow distances, and
channel losses. Nevertheless, the model gives satisfactory results and the inter1
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60 50
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or less
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value
F i g . 14 N o m o g r a m (after Evenari et al., 1982), s h o w i n g the distribution of infiltration
rates o n a runoff plot as influenced b y season, stone cover, slope, and moisture content
of the crust. M a x i m u m saturation deficit ( M D E F ) is the a m o u n t of w a t e r required to
saturate the crust to initiate runoff. T h e n o m o g r a m s h o w s that M D E F is 3.4 m m , and
4.3 m m for the spring, winter, and a u t u m n seasons respectively. T w o e x a m p l e s are
given in the figure: for a 6.0 m m rainfall in the spring season (
) o n a plot with a
1 0 % slope and with stone cover, the m e a n infiltration rate of the plot w o u l d b e a b o u t
4.0 m m h" varying from a m i n i m u m of 2 m m h " ' to a m a x i m u m of 6 m m If for
different points of the plot; and for a 6.0 m m rainfall in the winter s e a s o n (
) for
the s a m e 1 0 % slope and stone cover, the m e a n infiltration rate w o u l d b e 3.2 m m h "
varying from a m i n i m u m of 1 m m h" to a m a x i m u m of 5.0 m m h"'.
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relationship between the factors affecting infiltration rates on plots (cumulative
rainfall, maximum saturation deficit, season, stone cover, hillside slope angle, and a
normal distribution of infiltration rates) is presented in nomogram form (Fig. 14).
The model includes: (a) functions for evaluating the sensitivity of the results so as
to enable the research scientist to decide which parameters mainly control the complex
process and so guide him to further experiments and studies, and (b) functions for
formulating the model stochastically. Consequently, the model has applicability both
as a research tool and an engineering planning technique.
THE SUSTAIN ABILITY OF THE ANCIENT IRRIGATION SYSTEMS
Our studies highlighted the factors that enabled several civilizations (Israeli, Nabatean,
Roman and Byzantine) to establish irrigation projects in the Central Negev desert
during a 1500 year period from about 850 BC to 650 AD. Their operation depended on
the ability of the settlers to understand the complex environmental conditions of the
Runoff, erosion,
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105
desert (particularly the climate, pedology and hydrology), to master innovative
technologies that enabled them to exploit hillside runoff and flash flood flows in the
wadis, and to establish irrigation layout and design criteria for constructing dry stone
structures to serve as stabilizing terrace walls, diversion canals, distribution systems,
and erosion-control measures.
The research also highlighted the environmental and socio-economic constraints
following in the wake of this development and limiting the sustainability of many of
the projects. These included:
(a) Environmental constraints: although erosion, transportation and deposition of
sediment are processes that have occurred throughout geological time, man's
interference with the balance of nature results in changes in the relative levels of
the flood plains and the wadi bottoms at accelerated rates of 10-30 cm century" .
This necessitated the raising of the height of the terraced walls and diversion
structures. The increasing burden of maintaining the projects (cleaning the canals
and raising the heights of the terraced walls) eventually exceeded the capabilities
of the farmers, and the larger systems were abandoned.
(b) Socio-economic constraints: the planning, operation and maintenance of the
projects required a central authority to manage entire watersheds and possess the
power to enforce the laws for distributing the water during the short flash flood
periods. The projects were abandoned when the central authority was no longer
interested or capable of carrying out these duties.
Furthermore, the economic and financial viability of irrigated agriculture based on
small family-sized plots of less than 1 ha in size, was dependent on the overall
economic viability of the community in the region. Agriculture was economically
sustainable provided that it was integrated into a regional economy that comprised
several economic sectors including: large-scale military installations and/or army
camps protecting the security of the region; regional and international trade routes
passing through the area that served as a ready cash market for local goods and
services (such as fresh foods and bath-house facilities); the existence of local entre­
preneurs of industries for exporting goods to other regions (horse-breeding, exquisite
pottery, etc.); and the presence of large-scale regional religious institutions of learning.
Understanding the reasons for the failure of irrigation projects in the past is a
prerequisite to proposing feasible ways of improving the sustainability of irrigation
projects today. During the last four decades, numerous attempts have been made by
governments and international agencies to improve the present short-comings in
irrigation projects, particularly in developing countries (Shanan, 1998). Unfortunately,
the results of improvement schemes have fallen far below the planners' expectations.
The lessons learned from the ancient irrigation projects in the Central Negev desert can
be of great value to planners who are searching for ways to make present-day irrigation
projects sustainable over long periods of time.
1
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Greenbaum, N. & Lekach, Y. (1997) The potential water resource and efficiency of detention storage reservoirs in the
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