Applied Geography 23 (2003) 23–45
www.elsevier.com/locate/apgeog
Runoff and soil erosion on cultivated rainfed
terraces in the Middle Hills of Nepal
R.A.M. Gardner a, A.J. Gerrard b,∗
a
b
Royal Geographical Society with the Institute of British Geographers, 1 Kensington Gore,
London, UK
School of Geography and Environmental Sciences, The University of Birmingham, Edgbaston,
Birmingham B15 2TT, UK
Received 16 August 2001; received in revised form 1 October 2002; accepted 1 October 2002
Abstract
The paper presents runoff and soil erosion measurements from plots on outward-sloping
rainfed agricultural terraces in the Likhu Khola drainage basin, Middle Hills, Nepal, for the
pre-monsoon and monsoon periods of 1992 and 1993. Runoff coefficients ranged from 5% to
over 50%, depending on the nature of the rainfall event and the characteristics of the terrace.
Total rainfall amount provided the highest level of explanation for the variation in runoff. Soil
losses ranged from 2.7 to 8.2 t ha–1 for 1993 and up to 12.9 t ha–1 for 1992. The higher losses
were associated with red, finer-grained soils. The majority of these rates are lower than the
rates of soil loss that have been commonly perceived for the Middle Hills of the Himalaya.
However, they are broadly similar to rates obtained from the few other studies that have
examined runoff and erosion under traditional rainfed cultivation. The results suggest that a
re-evaluation of the degree of land degradation in such areas may be necessary. Relationships
between soil loss and rainfall characteristics were highly variable but were improved considerably when vegetation cover was included. This indicates that the maintenance of some form
of ground cover is advisable if runoff and erosion are to be minimized.
 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Agricultural terraces; Himalaya; Nepal; Runoff; Soil erosion
∗
Corresponding author. Tel.: +44-(0)121-414-5543; fax: +44-(0)121-414-5528.
E-mail address: [email protected] (A.J. Gerrard).
0143-6228/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0143-6228(02)00069-3
24
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
Introduction
Soil and water are widely recognized as very important resources in the mountain
kingdom of Nepal (HMG Nepal, 1988). Over the past 20 years, significant concerns
have been raised over the degradation of the soil resource in the Middle Hills of
Nepal as a result of the expansion of agricultural land and the increase in cropping
intensity. These concerns, most notably expressed by Eckholm (1976) and the World
Bank (1979), led to the formulation of the Theory of Himalayan Degradation. This
‘Theory’ has been reviewed critically by Ives and Messerli (1989). Its central tenet
was that deforestation and land use change were causing accelerated runoff and soil
erosion on the steep Himalayan hillslopes. Implicit if not explicit in the ‘Theory’
was the view that, if accelerated runoff and soil loss were occurring, the local cultivators were either oblivious to what was happening or, if aware, were somehow
unconcerned. This would clearly be an unsustainable situation.
By the late 1980s these ideas were coming under increasing scrutiny; a number
of detailed studies were suggesting that deforestation in the Middle Hills was not a
recent phenomenon, and that it was currently occurring at rates of less than 0.1%
per annum (Land Resource Mapping Project, 1986; Mahat, Griffin, & Shepherd,
1986). The contribution of human activity to landsliding was also being questioned
(Ramsay, 1985, 1986). Also some studies, such as that of Fleming (1983), were
indicating that soil erosion on cultivated slopes was not as high as was thought.
Above all, it was recognized that reliable and accurate data with which to test the
various components of the ‘Theory’, both at the hillslope and catchment scales, were
largely unavailable (Ives & Messerli, 1989). The data that did exist were frequently
based on perceptions and observations over relatively short time periods (Gilmour,
1991), usually with a western, developed perspective, and were often contradictory
and fraught with uncertainty (Thompson & Warburton, 1985). In terms of soil erosion, the paucity of data relating to rates, timing and processes was highlighted.
Thus, most studies concluded that there was a clear need to evaluate carefully the
ways by which human activity may be causing accelerated soil erosion through
changing land use, land cover and land management practices.
This paper presents surface runoff and soil loss rates for nine rainfed agricultural
terraces in the Likhu Khola basin in the Middle Hills of Nepal, measured during the
1992 and 1993 monsoon seasons. Terraces are the basic unit of cultivation and soil
management, and the scale of this study reflects this. The findings are also compared
to those obtained from the relatively small number of previous studies on similar
agricultural terraces in the Middle Hills area of the Himalaya.
Study area
The study area comprises five well-defined subcatchments (of 1.14–4.23 km2 in
size) and the valley slopes between them in the drainage basin of the Likhu Khola
(Fig. 1). The Likhu Khola (river) is a tributary of the Trisuli, which flows from the
High Himalaya, in the Langtang region, and ultimately into the Ganges. The drainage
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
Fig. 1.
25
Location of the study area, Middle Hills Nepal.
basin trends from east to west and is located entirely within the Middle Hills region
immediately to the north of the Kathmandu Valley (27°50’N, 85°20’E). Altitude
ranges between 600 and 1850 m, and the lithology comprises gneisses of the
Kathmandu Complex.
The gneisses are extensively weathered to depths exceeding 7 m, giving rise to a
regolith dominated by silts and fine/medium sands (Gardner, 1994). Soils and regolith
are best preserved on the flatter and younger geomorphological terraces (Gerrard &
Gardner, 2000), where soil B horizons may attain thicknesses of up to 2.5 m. Because
of extensive cultivation and a limited amount of erosion, A horizons are rarely preserved. Most of the soils on these freely draining slopes have been mapped, by the
Soil Survey Division of His Majesty’s Government of Nepal, as cambisols and luvi-
Monthly distribution of rainfall events monitored in 1993.
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
Fig. 2.
26
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
27
sols (Maskey, 1995). Surface soil textures are predominantly loams, silty loams and
silty clay loams. On the steeper and more incised areas, which have suffered heavy
past erosion, the soils are dominated by arenosols, regosols and leptisols. These tend
to have coarser textures of sandy loam and sands.
Records from the nearest meteorological station, located at Kakani on the southern
watershed of the Likhu Khola basin, show that about 90% of the annual rainfall in
the area occurs between May and September, during the pre-monsoon and monsoon
seasons, with highest monthly totals in July and August. In terms of total pre-monsoon and monsoon rainfall in the Likhu Khola basin, both 1992 (2141 mm) and
1993 (2211 mm) were below average for the 1981–91 period (mean 2558 mm). The
all-Nepal monsoon record is highly correlated with the Southern Oscillation Index
and 1992 was an exceptionally dry year coinciding with the elongated El Niño of
1992–3 (Shrestha, Wake, Dibb, & Mayewski, 2000).
Rainfall event data from the Likhu Khola, in 1992 and 1993, indicate that highmagnitude events occurred throughout the monsoon and pre-monsoon period, but
with a higher frequency in late July and early August. An analysis of measurements
for 1993 is shown in Fig. 2. The frequency distributions of rainfall magnitudes were
highly skewed, with most of the rainfall occurring in bursts of less than 5 mm.
Maximum continuous rainfall (defined as no breaks in monitored rainfall exceeding
30 minutes) during the two monitored seasons was 102.4 mm. This magnitude falling
within one day has a return period of approximately 1.7 years, using Kakani
maximum 24-hour rainfall data for 1972–86 as the baseline. However, rainfall intensity patterns within storms were complex and highly variable. Short-duration intensities were often high, reaching a recorded maximum of 144 mm hr–1 for maximum
1-minute intensities (Table 1) and 129.60 mm hr–1 for maximum 5-min intensities.
This meant that both rainfall magnitude and rainfall intensity had to be assessed
separately and included as potential explanatory variables in the analysis of water
runoff and soil erosion. The rainfall characteristics of both monitored years were
similar, except that there were slightly more higher-magnitude and higher-intensity
events in 1992.
Table 1
Summary of characteristics of individual rainfall events for May–September 1993 at the Field House site
Total rain
(mm)
Mean
Median
Mode
Standard
deviation
Range
Min.
Max.
EI15
Mean
intensity
(mm hr⫺1)
Duration
(min)
Max. 1-min.
intensity
(mm hr⫺1)
Max 5-min.
intensity
(mm hr⫺1)
7.47
2.20
0.20
12.01
7443.08
221.60
0.00
21067.0
5.54
3.00
1.20
6.52
78.79
41.50
13.00
97.75
27.23
12.00
12.00
26.07
17.08
8.40
12.00
22.56
61.20
0.20
61.40
149359
0.00
149359
40.80
0.40
40.80
746.00
3.00
749.00
132.00
12.00
144.00
129.60
0.00
129.60
28
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
There are two main cultivation regimes in the Middle Hills of Nepal: khet (paddy)
and bari (rainfed). The paddy land, with fields of standing water, generally traps
eroded soil and sediment, other than at times of ploughing during intense rain. This
study concerns the more vulnerable bari lands, which are rainfed cultivated bench
terraces where crops are dependent entirely on the monsoon rains for moisture. The
typical cropping cycle is as follows. Maize is sown after sufficient soil moisture has
accumulated from the pre-monsoon rains, usually in May or early June. Prior to
sowing, the soil has generally been tilled twice and organic manure may have been
incorporated. In general, maize is planted in early May and harvested in early to
mid-August. Millet seedlings are then almost immediately planted out, following
hand turning of the soil and the incorporation of organic residues (maize stalks and
weeds). Millet is harvested in November or early December, well after the end of
the monsoon. In some other areas, especially where rainfall is lower or the growing
season shorter, the maize and millet may be intercropped, or a single crop of maize
grown. The cropping cycles give rise to complex patterns of ground cover during
the pre-monsoon and monsoon periods, which significantly influence patterns of runoff and soil loss.
In the study catchments the bari terraces slope outwards at between 1 and ⬎10°
(Fig. 3). The most frequent individual slope angle was approximately 5°. As over
65% of terrace slope angles were less than 7°, slope was not thought to be a major
factor influencing water runoff and soil erosion. Terrace widths commonly ranged
from 2 to 10 m and riser heights were commonly up to 2 m, but heights of over 4
m also occurred (Fig. 4). Riser angles are usually above 60°. It is only when the
terrace risers have slumped that the slope angle becomes less than 60°. In general,
outward-sloping terraces are far more common than inward-sloping types in the
Middle Hills. Terraces also tend to slope sideways, either away from a central axis
or as a whole, which facilitates drainage of excess water into adjacent steep tributaries and natural ravines. Drainage is also commonly aided, in the study area, by
ditches at the bottom of terrace risers. These ditches lead water and associated sediment into the neighbouring ravines. Farmers indicated that the ditches were needed
in areas of plentiful rain in order to prevent excess water from cascading over several
terraces and causing damaging rills and gullies. Thus, although it has not been inves-
Fig. 3.
Frequency distribution of bari terrace widths and downslope angles.
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
Fig. 4.
29
Frequency distribution of bari terrace riser heights and riser angles.
tigated in detail, it would seem from discussions with farmers that the terrace slopes
and drainage provision are organized so as to balance the threat of gullying from
overland flow with that of terrace slumping from excessive infiltration. These management techniques also ensure that sufficient soil moisture is maintained for crop
growth, suggesting that sustainable use is a major consideration.
Methodology
There have been few attempts to examine, in any systematic manner, the variability of water runoff and soil erosion on different terraces in one area. Also, relationships with independent variables, such as rainfall amount and intensity, and soil and
vegetation characteristics, have only been investigated in a very rudimentary fashion.
The methodology adopted in this study was designed to examine many of these
factors. Following a pilot study of terrace shape and form and surface soil textures,
nine terraces were selected for detailed study (Fig. 5). These were chosen to be
representative of the common terrace morphologies, and to encompass the range of
surface soil textures, including the finer-textured red soils. Thus, the study was
designed to be illustrative of the processes occurring and not universal. The implications of this are discussed below when the significance of the results are examined.
The chosen terraces also included those on the lower, more productive lands and
the higher, more marginal and less productive/less fertile areas. All terraces were
subject to the same cropping cycles and broadly similar land management regimes.
It is important to note that very steep terraces (⬎10°) were not monitored, partly
owing to the physical difficulty of so doing with a reasonable degree of accuracy,
and partly because they are not common in the study area.
The characteristics of the monitored terraces are summarized in Table 2. The particle-size characteristics listed are representative of upper and lower terraces respectively. Bari 1 terraces were wide and possessed a sandy loam texture; in contrast,
Bari 2 terraces were narrower, of similar slope but with a finer-textured, silty clay
30
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
Fig. 5.
Location of the monitored bari terraces in the Likhu Khola catchment.
loam soil and substantially lower infiltration rates. Both Bari 2 and Bari 11 terraces
were on the heavier-textured, red soils, but they differed in infiltration rates, with
those on Bari 11 being higher. Bari 10 and Bari 3 terraces were both on sandy
textured soils with moderately high to high infiltration rates, but Bari 3 was on an
unditched sequence of terraces and therefore had a much longer slope length than
any other plot. It also had lower slope angles on its terraces.
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
31
Table 2
Size, slope angle and soil particle size characteristics of the bari terraces monitored
Terrace
Area (m2)
Plot length
(m)
Slope (0)
% sand
% silt
% clay
Bari 1
u: 535.52
1: 488.70
u: 158.01
1: 83.08
76.12
u: 90.86
1: 133.37
u: 163.73
1: 149.70
u: 9.50
1: 10.30
u: 4.60
1: 3.60
16.80
u: 3.00
1: 6.20
u: 6.40
1: 3.30
u: 3–8
1: 3–8
u: 3–10
1: 3–10
1–2
u: 3–5
1: 3–5
u: 3–9
1: 3–9
74.09
18.53
7.38
65.36
20.44
14.20
80.00
85.14
14.92
12.32
5.08
2.56
56.18
26.91
16.91
Bari 2
Bari 3
Bari 10
Bari 11
u, upper; 1, lower.
Soil erosion can be estimated or measured directly in a number of ways. Direct
measurement, using erosion plots and natural rainfall, was chosen in preference to
formula-driven estimates based upon the Universal Soil Loss Equation or its derivatives (e.g. RUSLE, see Renard, Laflen, Foster, & McCool, 1994). Using instruments
such as the soil erosion bridge (Shakesby, 1993) was not feasible, and the use of
rainfall simulation was considered not practicable. It was important to be able to
estimate water runoff and soil loss associated with individual storms at different
times during the pre-monsoon and monsoon periods. Thus, recognizing that ‘studies
that rely on natural rainfall are essential for obtaining data on long-term average soil
erosion or infiltration amounts, and on variability and extremes resulting from climatic differences’ (Meyer, 1994: 84), erosion plots were used.
The field approach was designed to interfere as little as possible with the cultivation systems, and to minimize the possible errors introduced by selecting small
erosion plots and by extensively bounding plots on land with locally complex topographies. Erosion plots have been criticized because they simplify topography and
ignore storage of runoff and sediment (Boardman, 1996). This is why, in this study,
the units of measurement were either whole terraces or substantial terrace segments.
Use was made of the agricultural ditches at the foot of the terrace risers for channelling runoff and eroded soil to collection drums. This also meant that artificial
bounding of the plots was kept to a minimum. Where artificial metal sheet boundaries
were emplaced, these were located wherever possible along natural drainage divides
on the terraces. Plots were thus arranged to accommodate natural drainage routes.
The riser ditches were left unlined and continued their normal function of trapping
some of the eroded soil from the terrace above. At the time of soil preparation for
the next crop, the ditch is re-excavated and the soil added to the adjacent terrace.
Thus the monitored terraces include the ‘natural’ storage components, both on the
terraces and in the ditches. Land management regimes on the terraces were largely
unaffected as the erosion ‘plots’ were, in general, sufficiently large to accommodate
ploughing. The drawback of this approach was that the erosion plots were not of
32
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
uniform size, but they did represent the farmers’ unit of cultivation and soil management.
With one exception, paired adjacent terraces (upper, UT; and lower, LT) were
selected to provide a check on reproducibility. Each set of terraces was served by
a tipping bucket rain gauge and associated data logger. Such instrumentation was
necessary because of the high spatial variability of rainfall amounts and intensities
in any one storm. Whenever possible, individual storm events were monitored on
each terrace, and on no occasion was the monitored period longer than 24 hours.
Runoff and eroded soil were collected in sets of two or three 200-litre drums; within
each set the drums were separated by 1/10 divisors. Drums, divisors and rain gauges
were calibrated each season. After storms the runoff volume was measured and a
well-mixed sample of suspended sediment collected, filtered in the field, dried and
weighed.
Plots were mapped in the field at a scale of 1:100. Soils on the plots were sampled
and particle size, wetted soil aggregate stability, and total organic carbon measured.
Moisture contents and bulk densities were measured monthly on most plots between
May and September. Infiltration capacity was assessed using a double ring infiltrometer, and soil suction was measured daily on selected plots using soil tensiometers.
Standard methods were used for all analyses (see Goudie, 1992). Vegetation surveys
(canopy and ground cover) were undertaken monthly between May and September
on all plots. Any major changes on the plots were also noted at intervening intervals
and a record was kept of farmers’ management practices.
A sample of rainfall events was monitored at each site. This sample included the
complete range of rainfall magnitudes and intensities as well as a good seasonal
distribution of events in relation to cropping cycles and changing antecedent soil
moisture conditions (an example is shown in Fig. 6). Between 70 and 78 events
Fig. 6.
Nature of rainfall events monitored on Bari 1 in 1993; bars represent total rainfall amounts.
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
33
were monitored on each of the five main terraces that were instrumented for 1992
and 1993; and 34 to 41 events on the other four terraces, which were instrumented
for 1993 only (Table 3). The low (in monsoon terms) mean intensity of many of
the sampled events can be misleading, as most of the larger storms did include
periods of high-intensity rain. This was overcome in the analysis by using a number
of indices of rainfall intensity, including mean intensities and maximum 1-, 5-, 10and 15-minute intensities, and an EI15 index. The last was a modified form of the
EI30 index (Wischmeier & Smith, 1958) that incorporated the kinetic energy equation of Hudson (1965). This was believed to be more applicable to tropical rains
of high intensity. The maximum 15-minute rainfall intensity was used because it has
been found to be more appropriate where the land is moderately-to-sparsely vegetated, such as that most at risk in the Middle Hills (Stocking & Ewell, 1973). The
small number of high-intensity events each month had important implications for
field sampling and data capture. In 1993, approximately half the 90 events where
rainfall exceeded 5 mm were included in the samples on most plots; in 1992 between
25 and 40% were sampled. For all sites, either the highest or the second-highestmagnitude rainfall event of the season, as well as a broadly comparable range of
events, were captured each year.
Results
Runoff
Empirical relationships between total rainfall and runoff allow comparisons to be
made between terraces. The relationships can also be used to produce estimates of
total runoff and runoff coefficients over the pre-monsoon and monsoon periods.
Rainfall/runoff relationships for all terraces in both 1992 and 1993 are shown in Fig.
7. The relationships are clearly non-linear and show that runoff is highly variable
between the terraces. Runoff coefficients (water running off the plot as a percentage
of the total volume of water entering by rainfall) for individual rainfall events varied
Table 3
Numbers of rainfall events monitored on each terrace in each year
Sites
Bari
Bari
Bari
Bari
Bari
Bari
Bari
Bari
Bari
1 upper
1 lower
2 upper
2 lower
3
10 upper
10 lower
10 upper
11 upper
1992
1993
Total
28
28
32
32
18
N/A
N/A
N/A
N/A
49
49
46
46
52
34
34
41
41
77
77
78
78
70
34
34
41
41
34
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
Fig. 7.
Rainfall/runoff relationships and statistical correlations for 1992 and 1993.
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
35
considerably from less than 5% to over 50%, depending on the nature of the event
and the terrace. Much of the inter-event variability on each terrace could be
accounted for by a combination of total rainfall and a measure of rainfall intensity.
Table 4 lists the statistical levels of explanation for a sample of terraces for 1993.
Total rainfall, in general, offered the highest level of explanation (53–80%) for runoff
variance of all the simple independent variables apart from for terrace 11. The EI15
composite index also offered a high level of explanation of between 54% and 82%.
Total rainfall performed better than EI15 as an explanatory variable on plots with
lower infiltration rates, and vice versa. The inclusion of rainfall duration as a second
independent variable with total rainfall further improved the explanation by up to
10%, but only on some plots (Table 4).
Other variables contributed much less, if at all, to the explanation of runoff variance. Antecedent 24-hour rainfall and soil suction both added a little further explanation, as did the percentage of ground cover vegetation. However their overall contribution was small. For example, on Bari 1 lower terrace, the inclusion of both
vegetation cover and antecedent 24-hour rainfall raised the R2 value from 73.0% to
82.3% (each contributed approximately 5%).
The relationships depicted in Fig. 7 indicate that the ranking of terraces in terms
of runoff generation was consistent from 1992 to 1993 for those terraces monitored
in both years. Bari 2 generated substantially more runoff than Bari 1, which generated
greater runoff than Bari 3. Generally the variation between sites was greater than
the variation between the terraces at any one site, although intra-site differences
clearly existed. The differences reflected variations in soil texture, bulk density, infiltration properties, and possibly downslope terrace length. Bari 2 appeared to owe
its high runoff to a combination of a fine-textured soil (clay loam) with a very low
infiltration capacity (0.42–0.78 cm hr–1) and poor soil structure. High runoff may
have been a response to the narrowness of the terrace that offered less surface storage
potential. Of the ditched terraces, Bari 10 experienced the lowest runoff, despite
relatively poor crop growth. This possibly reflected the sandier soil texture and high
infiltration rate in these previously heavily eroded soils. Bari 1 was intermediate in
terms of runoff, surface soil texture and infiltration capacity. At present, it is difficult
to explain the low runoff coefficients on the finer-textured soils of Bari 11.
Table 4
Strength of the statistical correlation, as determined by the coefficient of determination (r2%), between
runoff rates (R) and precipitation amount (P), duration (D) and intensity (EI15), for 1993
Terrace
R:P
R:P/D
R:EI15
1 UT
1 LT
2 UT
2 LT
11 UT
11 LT
76.6
73.0
80.0
68.2
52.7
64.3
80.8
75.3
83.8
73.1
68.3
73.7
63.2
54.0
65.9
60.8
74.1
72.4
UT, upper terrace; LT, lower terrace.
36
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
Bari 3 consistently experienced the lowest runoff, both in 1992 when the farmer
failed to weed the terrace beneath the maize crop, and in 1993 when the maize crop
was weeded as normal. This plot differed from the others in that the terraces possessed a sandy, well-draining soil with no riser ditches. The soil texture encouraged
infiltration and the plot configuration, of low-angle terraces without ditches, gave
more potential for on-plot depression storage and a longer residence time as the
water moves across the three terraces monitored (total length of 16.8 m). Variability
in response was also great for individual rainfall events, as well as seasonal loss and
runoff. This is why meaningful comparisons are only possible through the use of
empirically derived relationships between the key variables at each location.
Regression relationships between total rainfall and runoff, using a complete set
of pre-monsoon and monsoon rainfall events, standardized for each site, have enabled
annual mean runoff coefficients to be calculated. Estimated total runoff as a percentage of total rainfall was 5–10% for Bari 1, 10 and 11, approximately 3% for Bari
3, and 18–26% for Bari 2 (Fig. 8). For comparison, the monsoon runoff coefficients
for good secondary forest cover and highly degraded forest cover were found to be
1% and 33% respectively (Gardner & Gerrard, 2002).
Fig. 8.
1993.
Estimated overall runoff as a percentage of total rainfall for the plots between May and September
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
37
Soil loss
The relationships between amounts of soil eroded and both runoff and EI15 were
non-linear and showed a high degree of scatter on all the terraces. The variance in
soil loss explained by runoff varies tremendously (Table 5). For most terraces, the
EI15 index explained slightly more of the variance in erosion losses than total runoff.
Coefficients of determination for some terraces were poor. This is not unusual in
erosion research. It may suggest that soil particles are being detached and transported
in several stages and in different ways. Thus, their final removal from the terrace may
occur in events of lesser magnitude than those that effected the initial detachment of
the particles. Moreover, small rills develop across the terraces during the rainy-season
crop cycles and episodic erosion into the banks of these may supply eroded material
in bursts. Moreover, the storms are very complex in their magnitude and intensity
profiles, and both the total runoff and EI15 parameters are crude approaches to summarizing the storm profile. Other soil and vegetation parameters will also affect
erosion losses. If vegetation cover is added to the analysis, coefficients of determination are generally increased for both soil loss and runoff and soil loss and EI15.
It must also be remembered that considerable soil loss can occur during a single
rainfall event. Thus, maximum erosion losses recorded in a single event varied from
39.4 g m–2 on Bari 10, to 316.0 g m–2 (equivalent to 3.16 t ha–1) on Bari 2.
As with variations in runoff, some of the variations in soil loss between the terraces
may be due to variation in soil properties. A number of methods are available for
estimating soil erodibility. Some of the simpler approaches using soil physical and
chemical properties have been used here. Erodibility is often highly correlated with
the silt and fine sand fractions (positively), the organic contents (negatively) and
water-stable aggregate contents (negatively). These values for the terrace soils have
been listed in Table 6. The combination of moderately high silt and fine sand fractions, low to moderate clay-size fractions, low organic carbon contents and, for most
terraces, low soil aggregate stabilities, rendered the surface soils moderately to highly
erodible. The high potential erodibility was readily seen in the field by the frequent
Table 5
Strength of the statistical correlation, as expressed by the coefficient of determination (r2%), between soil
loss (S) and runoff (R), rainfall intensity (EI15) and vegetation cover (V), for 1993
Terrace
S:R
S:EI15
S:R/V
S:EI15/V
1 UT
1 LT
2 UT
2 LT
10 UT
10 LT
11 UT
47.9
58.4
39.3
52.5
18.8
14.9
34.1
57.0
66.9
52.3
59.3
23.3
22.3
22.0
60.3
69.4
66.4
73.3
45.3
34.6
47.5
64.0
69.5
59.4
71.2
48.6
32.1
36.5
UT, upper terrace; LT, lower terrace.
38
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
Table 6
Some significant soil characteristics of the monitored rainfed bari terraces
Plot
Bari
Bari
Bari
Bari
Bari
1
2
3
10
11
% sand
⬎63 µm
% fines
⬍63 µm
% organic
Aug/Sept
carbon range infiltration
(cm hr⫺1)
Aug. bulk
density
% water-stable
aggregates
74.09
65.36
80.00
85.14
56.18
25.91
34.64
20.00
14.86
43.82
0.45–1.21
0.68–0.81
0.43–0.47
0.95–1.07
0.27–1.09
1.30
1.41
1.37
1.24
1.53
4.66
3.73
2.40
24.80
6.67
1.80
0.78
4.38
35.40
2.82
occurrence of rainsplash pinnacles on all terraces monitored. The rubified soils of
Bari 2 and Bari 11, with their higher clay contents, possessed a lower erodibility
status, especially where the soil surface was subject to direct raindrop impact because
of the crusting and compacting processes. However, these soils also possess relatively
high fines contents and relatively low aggregate stabilities, which will ensure that
they are still moderately erodible.
Variability in terrace response was also affected by vegetation cover change over
the cropping cycles, and by the differing effects of storms of slightly varying character and timing occurring on different plots. This made meaningful comparisons
between terraces difficult. To overcome some of the difficulties, comparisons have
been made on a seasonal basis. The most sensitive early season, before crops have
produced a blanketing effect, in which it might be expected that the differences will
be greatest between the terraces, has been analysed for 1993. The terraces fell into
two clusters in terms of seasonal response. The first group comprised Bari 3 and
Bari 10, in which erosional response was relatively insensitive to both runoff and
the EI15 index. The sandy texture and relatively high infiltration rates of the soils
on these terraces were probably responsible for the low rate of change of soil erosion
with increasing runoff and rainfall intensity, as well as for the low levels of erosion
relative to the other terraces during the larger rainfall events. As noted previously,
aggregate stability of the soil on Bari 10 terraces was especially high. There were
some differences in erosion losses between Bari 3 and Bari 10 terraces, which probably reflected the longer downslope distance and hence terrace storage on Bari 3,
the unditched terrace.
The second group of terraces comprised Bari 1, 2 and 11. These were much more
responsive to the higher-magnitude events, and the rate of change of soil loss with
increasing runoff and EI15 was greater. However, soil erosion at lower-intensity rainfalls was less than on the sandier soils, probably because of the greater cohesive
forces and hence less particle detachment by splash on the finer-textured soils. Bari
2 showed the greatest rate of change and the highest overall soil losses.
Differences in runoff and soil loss between sets of terraces were consistent over
the two years monitored, and appeared to relate largely to soil properties. The finertextured soils, with higher bulk density and lower infiltration capacities, were most
susceptible to runoff generation and soil losses, and experienced estimated total mon-
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
39
soon season (May–September) soil losses of between 8 and 12 t ha–1. In contrast,
the sandier soils, characteristic of the more marginal land of lower fertility, showed
lower runoff potential and less susceptibility to erosion, especially when arranged
in several terraces without riser ditches. Total monsoon season estimated losses were
low and ranged from 2.7 to 4.4 t ha–1. These figures can be compared to losses of
0.01 t ha–1 for good forest cover and 20 t ha–1 for highly degraded forest and bare
ground (Gardner & Gerrard, 2001).
Estimates of total monsoon season losses
As it was logistically impossible to monitor every event on each plot, estimates
of total monsoon soil loss might be made using the regression relationships as predictive tools, with a complete set of monsoon and pre-monsoon rainfall events. However, the highly skewed distribution of rainfall magnitudes and intensities, combined
with non-linear runoff and soil loss relationships, and continuous changes in vegetation cover through the cropping cycles, meant that the multiple regression relationships (even with 50% sampling of events of all magnitudes) were not sufficiently
well defined, for the highest-magnitude storms, to rely on this technique of estimating
soil losses. A longer-term dataset would be required to define, more fully, the mathematical relationships to enable them to be used with a high degree of confidence.
Thus, a less sensitive but more robust magnitude and intensity categorization
approach was employed to estimate total soil losses from the terraces. Using the
total 530 monitored events for 1992 and 1993, a best estimate figure for soil loss
was derived for combined classes of rainfall magnitude (⬎40 mm, 40–20 mm, 20–
10 mm, 10–2 mm rain), mean intensity (⬍0.15 mm min–1, ⱖ0.15 mm min⫺1) and
seasonal vegetation cover (early maize cycle; maize harvest/early millet cycle; and
all other times when weed cover ⱖ50%) for each terrace.
The estimated losses ranged from approximately 2.7 to 8.2 t ha–1 for the 1993
pre-monsoon and monsoon period (May to September), and up to 12.9 t ha–1 in
1992 (Table 7). These results are comparable to average annual soil losses of 3.65
Table 7
Total erosion losses (t ha⫺1) in pre-monsoon and monsoon seasons for 1992 and 1993 (derived by categorization procedure)
Plot
Bari
Bari
Bari
Bari
Bari
Bari
Bari
Bari
Bari
1 UT
1 LT
2 UT
2 LT
3
10 UT
10 LT
11 UT
11 LT
1992
1993
3.4
5.1
12.9
12.4
0.7
N/A
N/A
N/A
N/A
2.7
4.8
8.2
5.7
4.4
4.0
4.2
5.1
4.2
UT, upper terrace; LT, lower terrace.
40
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
t ha–1 from landsliding on bari terraces for the period 1991–93 in the same area
(Gerrard & Gardner, 2002). There were a greater number of high-magnitude rainfall
events in 1992 (20 events ⬎30 mm). The highest losses were on Bari 2, especially
in 1992, and the lowest on Bari 3, also in 1992, reflecting the previously discussed
differences in soil particle detachment and runoff generation on these very different
terraces. The other terraces were estimated to have lost between a relatively modest
2.7 t ha–1 (Bari 1 UT in 1993) and 5.1 t ha–1 (Bari 11 UT in 1993). Estimates using
the multiple regression relationships with EI15 and vegetation cover were slightly
lower.
Discussion
Few studies have measured runoff and soil losses on agricultural terraces in the
Middle Hills region of the Himalaya. Comparisons with earlier work can only be
based around estimates of gross annual soil losses as few of the published studies
have examined the temporal patterns of loss. Comparisons are also hampered by
differences in methodologies, plot size and configuration, soil characteristics, cropping cycle, and land management practices. These problems make it impossible to
account effectively for the observed differences in previous studies. Study scale and
methodology can have substantial effects on the erosion estimates. When compared
with other areas in the Middle Hills, there are good reasons to believe that some
locations will experience much higher soil losses, even during relatively weak monsoons, than those in the Likhu Khola. This is because: (a) some lithologies, such as
phyllites, develop finer-textured soils; (b) there can be terrace slopes of a much
higher angle; (c) some areas receive substantially more monsoon rainfall; and (d)
not all bari land is treated with chemical fertilizer, which may have increased the
rate and amount of ground cover development on the Likhu Khola terraces.
Previous studies have been undertaken on erosion plots in six main areas of the
Middle Hills: Fakot (Uttar Pradesh, India; Anon, 1984); Nagarkot (Kathmandu Valley, Nepal: Maskey & Joshi, 1991); Banpale (near Pokhara, Nepal: Mulder, 1978;
Impat, 1981); Naini Tal (northwest India: Pandey, Pathak, & Singh, 1984); Tamagi
(near Pokhara: Impat, 1981); Kulekhani (Nepal: Upadhaya, Sthapit, & Shrestha,
1991); Bhandar and Bimti (edge of high mountains, eastern Nepal: Ries, 1994); and
at Pakhribas (eastern Nepal: Sherchan, Gurung, & Chand, 1991). However, not all
of these plots were cultivated using traditional techniques. Some of these studies
have been summarized and evaluated by Ramsay (1985) and Bruijnzeel and Bremmer
(1989). The majority of the studies used bounded plots of between 14 and 80 m2.
Taking all plots together, there is a poor correlation between runoff and soil loss.
The Kulekhani plots have high runoff but low soil losses, and the plots at Chyandanda, Nagarkot, have almost 50 times the soil loss under the same levels of runoff.
Soil losses under traditional tillage practices ranged from 0.6 to 14 t ha–1 on outwardsloping terraces in the Nargakot area, and up to 35 t ha–1 on inward-sloping terraces
developed in red soils in the Pakhribas area. In both areas the terraces were cultivated
traditionally with maize and millet. With the exception of the Pakhribas site, the
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
41
losses under traditional cultivation are low, and lie broadly within a similar range
to those reported in this study. It would seem from the present and Pakhribas studies
that the red soils are subject to both higher runoff and higher soil losses, although
the recorded losses at Pakhribas greatly exceed any reported here. The Pakhribas
results are more consistent with some high losses reported on steeply outwardsloping, unditched, red soil terraces in the Jikhu Khola, where losses up to
20 t ha–1 have been reported (Nakarmi, Schrier, Merz, & Mathema, 2000). Other
cultivation practices, such as vegetables and alley crops, can result in much greater
soil losses. It is interesting to note that the erosion rate on bari land in the Phewa
Basin, Pokhara, calculated using the Universal Soil Loss Equation, was 17.4 t ha–1
(Impat, 1979). In areas where limited conservation measures have been attempted,
it seems clear that mulching, minimum tillage, strip-cropping and improved maize
planting regimes can all reduce soil losses.
The data for runoff from the present and past studies indicate that a good proportion of the rainfall (probably 50–70%) infiltrates into the soil. Until data on interception are available, this cannot be quantified further. The estimates of total soil
losses are significantly less than the hypothetical figures quoted in the past (e.g.
Carson, 1985), suggesting that there may be less need for concern about soil erosion
on agricultural terraces in the Himalayas than hitherto believed. While there has been
no detailed estimate of weathering rate or tolerable soil losses for the region, the
presence of relatively deep soils (⬎1.5 m) and regolith (up to 6 m), at least in the
lower and middle elevations, suggests that annual losses of 10–12 t ha–1 should be
tolerable in the medium term in areas such as the Likhu Khola basin. Thus, reported
losses in ‘average years’ in the Likhu Khola would be within a tolerable level.
Indeed, it has been suggested that losses of up to 25 t ha–1 yr–1 may be tolerable in
young mountain environments (Morgan, 1986); this would encompass expected surface soil losses in most years in the Likhu Khola basin.
There are several reasons that could account for the comparatively low soil losses
even in weak to average monsoons. In the present study, the storage capacity of the
terrace ditches should not be underestimated. A typical ditch 20 cm wide, 10 cm
deep and 50 m long can store 1 m3 of soil, which is approximately equivalent to
the loss of 2.6 t ha–1 over a terrace of 500 m2. By the end of the maize season, the
ditches are typically clogged with soil and are dug out, the soil being placed on the
adjacent terrace. This results in a sequential annual cascade of soil to the terrace
below. The net loser is the farmer cultivating the top of the terrace set. Secondly,
the role of weeds (or other form of ground cover) in reducing soil particle detachment
is critical in minimizing soil losses, and conversely in concentrating losses into two
peak times. Thirdly, there is the farmers’ continual vigilance to prevent major rills
and gullies forming across the terraces and to control and re-terrace small slumps.
Under traditional management regimes there are soil additions each year too,
which should be included in the consideration of net losses. These come from the
annual cutting back of the vegetated terrace risers by about 2 cm. Not all farmers
practise this, but it is common in the low-to-middle-elevation areas. On a terrace 50
m long with a 2 m riser, this will add approximately a further 2 m3 of soil to the
terrace below. Some, but not all, of this material will have been trapped as eroded
42
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
soil passing down the vegetated riser but, nevertheless, the long-term net effect will
be to move the whole terrace set back and up into the hillside. Thus, in a situation
where the terraces are ditched and where annual additions come from the riser, the
net soil losses from the terrace surface, if any, will be very small.
Great care must be taken in the wider applicability of these results. The variability,
in biophysical conditions and land management regimes, within the Middle Hills
should mean that erosion losses are highly variable. The findings in the Likhu Khola
catchment suggest that losses are greater on the finer-textured soils, especially the
red soils. They may also be higher in areas where weed and crop growth is poor or
restricted, and probably where the terraces slope outwards at high angles. They will
probably be lower on the sandier regoliths.
However, if the terraces with sandier soils are high in the catchments, lower erosion loss may be counteracted by the effect of thinner soils. In terms of sustainability
there will be a reduced margin of safety, especially in the medium term. Problems
may also be compounded because these soils have often suffered substantial previous
erosion, have a low fertility status, and the annual cutting back of the riser cannot
take place because the risers are supported by walls or rock benches. These are the
bari lands that are often marginal in terms of productivity and which support the
poorer households. They are also the terraces that suffer most from small-scale
landsliding (Gerrard & Gardner, 1999).
Recorded losses will also vary according to the methodology adopted in the erosion studies. In particular, where part of one terrace surface alone is monitored, losses
may seem very high because the ‘natural’ storage of the traditional cultivation system
is not included in the monitoring. This may be either in the form of vegetated risers,
riser ditches, or terraces below, over which runoff would cascade in the absence of
a ditch. It is essential that the parcel of land monitored reflects the aims of the
monitoring exercise and, in the case of the Middle Hills, the farmers’ unit of land
management. In addition, losses will be seriously underestimated if the small number
of high-magnitude storms in late April and May, after ploughing but before development of a weed cover, are not measured.
Conclusion
A monitoring programme in 1992 and 1993 on nine outward-sloping, rainfed cultivated terraces in the Likhu Khola drainage basin has provided a detailed dataset on
runoff and soil losses in the sensitive Middle Hills of Nepal. The results show the
spatial and temporal variability in such losses, as well as their overall magnitudes,
on cultivated terraces under land management regimes that are traditional in all ways
except in the use of chemical fertilizer. The philosophy behind the field methodology
was to interfere as little as possible with the cultivation systems and the natural
storage potential on the land, and with this in mind whole or substantial parts of
individual terraces were monitored, including the small agricultural ditches at the
backs of the terraces, where present. Terraces were paired in most cases as a check
on reproducibility, and terraces typical in terms of soil texture, morphology, artificial
drainage, and farming system were chosen.
R.A.M. Gardner, A.J. Gerrard / Applied Geography 23 (2003) 23–45
43
The estimated total losses were much lower than commonly perceived for the
‘environmentally threatened’ Middle Hills of the Himalaya. This in part probably
reflects the relatively weak character of the monsoons monitored, and the fact that
‘typical’ rather than extreme (steep and fine-textured) examples of terraces in the
Likhu Khola basin were chosen for study. Moreover, the riser ditches in the Likhu
catchment act as important traps and stores for eroded soil, which is merely redistributed by the farmer onto the adjacent terrace. However, most of the runoff from bari
land is lost rapidly from the hillslope system. Over 80% of the bari terraces in the
study area were drained by riser ditches directly into side tributaries or ravines. Most
of the terraces are designed to aid drainage by sloping both outwards and sideways.
It must also be stressed that a significant proportion of the water that drains into the
side tributaries is then taken off, lower down the slope, to irrigate the khet land.
Thus, not all the water and sediment from the bari terraces reaches the main river
systems. This has significance if overall denudation rates are calculated. Sediment
delivery rates need to be carefully estimated.
The relatively low losses may also reflect the high degree of biophysical and land
management diversity in the region as a whole. However, many of the small number
of previous erosion monitoring studies have reported similar levels of loss under
traditional cultivation practices in other areas of the Middle Hills, although their
differing methodologies make effective comparisons difficult. Perhaps it is also time
to reconsider the common perceptions in an environment where farmers, for the most
part, exercise great care and judgement in managing their limited land resources.
The results of this study appear to suggest that, in the Likhu Khola drainage basin
at least, traditional cultivation of sloping rainfed terraces is sustainable over short
to medium time scales. It is a system of cultivation that has evolved over a considerable number of years. However, there are undoubted pressures for change and some
cultivators are now growing more cash crops, such as vegetables. Soil losses in such
situations may be greater.
Acknowledgements
The project was funded by the UK Overseas Development Administration and
was coordinated jointly by the Soil Science Division (Nepal Agricultural Research
Institute), Royal Geographical Society and Institute of Hydrology (Natural Environment Research Council).
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