Hydrological Sciences - journal - des Sciences Hydrologiques, 28, 3, 9/1983
Rainwater infiltration into a bare loamy sand
K. D, SHARMA*, H, P, SINGH & 0. P. PAREEK
Central Arid Zone Research
Jodhpur 342003,
India
Institute,
ABSTRACT
In order to design micro-catchment water
harvesting systems in the Indian desert, rainwater
infiltration experiments were conducted on a
representative loamy sand soil for a period of six years.
Plots with three slopes - 0.5, 5 and 10%, and five slope
lengths - 5.12, 7.0, 8.5, 10.75 and 14.5 m were used.
With dry antecedent soil conditions, infiltration is
governed by rainfall depth, whereas with wet antecedent
soil conditions, raindrop impact (intensity) which forms
a crust over the soil surface, is the deciding factor.
Infiltration decreases significantly with increasing
slope due to reduction in the time available for rainfall
to infiltrate, but slope length has no significant effect.
A graphical multiple curvilinear model to predict rainfall
infiltration using rainfall and basin characteristics is
developed and the goodness of fit is tested.
Infiltration
de l'eau
de pluie
dans un sable
argileux
nu
RESUME
En vue de mettre au point les projets de
systèmes de très petits bassins pour la mise en réserve
d'eau dans les régions désertiques de l'Inde, des
expérimentations sur l'infiltration de l'eau de pluie sur
un sol représentatif sable argileux ont été effectuées
pendant une période de six ans. Des parcelles avec trois
pentes différentes - 0.5, 5 et 10%, et cinq longueurs de
pente - 5.12, 7.0, 8.5, 10.75 et 14.5 m ont été utilisées.
Lorsque le sol est sec juste avant la pluie l'infiltration
est conditionnée par la hauteur de précipitations, alors
que si le sol est humide c'est l'impact des gouttes de
pluies (intensité), qui forme une croûte de battance à la
surface du sol, qui est le facteur déterminant.
L'infiltration décroît de façon significative lorsque la
pente croît part suite de la réduction du temps dont la
pluie dispose pour s'infiltrer, mais la longueur de la
pente n'a pas d'effet significatif. On a mis au point un
modèle graphique à courbes multiples pour prévoir
l'infiltration de la pluie en utilisant les
caractéristiques de l'averse et du bassin et on a testé
la valeur de l'ajustement.
*Present
793003,
address:
India.
ICAR Research
Complex for
NEH Region,
Shillong
417
418
K.D. Sharmaefa/.
INTRODUCTION
Runoff yield under micro-catchment water har-vesting systems in hot
arid regions is a function of the infiltration capacity of the basin,
which is turn depends upon the rainfall depth and intensity, and
basin characteristics (degree and length of slope and antecedent soil
moisture conditions). Relying on actual field observations,
information on these aspects for selecting the optimum basin
characteristics taking into account the rainfall features of maximum
probability of occurrence, is rather meagre (Dunin, 1976; Evenari et
al.,
1971; National Academy of Sciences, 1978; Sagi, 1969). An
attempt was therefore made to establish relations between rainfall,
basin characteristics and infiltration which will help to formulate
a basis for the selection of optimum basin characteristics for the
adoption of micro-catchment water harvesting systems under the
prevailing agro-climatic conditions in the Indian desert.
EXPERIMENTAL SETUP
Field experiments were conducted on a representative loamy sand soil
(sand - 84.5%, silt - 5.6%, clay 9.9%, and bulk density 1.5 g cm - 3 )
for a period of six years (1975-1980) at the Central Arid Zone
Research Institute, Jodhpur. Precipitation in this region is
characterized by storms with relative high intensity and extreme
variability both in time and space (Bell, 1979). During the study
period total rainfall varied between 245 and 784 mm with a mean of
506 mm as against the 80 years average of 366 mm. 35.5% of storms
having intensities >40 mm h~* resulted in 48.9% of the total rainfall;
64.5% of storms having intensities of <40 mm h~x resulted in 51.1% of
the rainfall; 6.5% of storms at intensities 80-120 mm h - 1 resulted in
7.2 % of the total rainfall. Three slopes (0.5, 1 and 10%) and five
slope lengths (5.12, 7.0, 8.5, 10.75 and 14.5 m) were considered,
thus forming fifteen combinations (Sharma et al.,
1982). Basin width
was kept constant to 2 m. Rainfall depth and intensity were recorded
with a recording raingauge installed at the site. Runoff from the
individual basins was measured with a precalibrated V-notch weir
combined with a stage level recorder. The amount of infiltrating
rainfall was obtained by the difference between the rainfall
received and the runoff from the individual basins assuming
evaporation to be negligible.
METHOD OF ANALYSIS
This study is concerned with a sample of 63 events recorded during
the study period. To determine the effects of rainfall depth and
intensity on the accumulated infiltration, partial correlations were
calculated (Snedecor & Cochran, 1968). Simple analysis of variance
was performed (Snedecor & Cochran, 1968) to describe influences of
slope length as a response to head of ponded water. The significance
of interaction variance was tested against error mean square and that
of slope and slope length variance against interaction mean square.
Finally, the rainfall and basin characteristics were combined
Rainwater infiltration into bare loamy sand
419
together to develop a graphical multiple curvilinear model (Linsley
et al., 1958) to predict the accumulated rainwater infiltration.
The observed (XQ) and predicted (Xe) values of the accumulated
infiltration were used to calculate the sums of square of the
residuals (SSres) from
SST
= (Xn - Xp)'
and the standard error of estimate (SEest) which is obtained from
SE
est
= ss
res/ N
where N is the number of observations in the sample. Goodness of
fit of the graphical multiple curvilinear model was tested from the
chi-square test which is given by
X
(XQ - X e ) /X e
RESULTS AND DISCUSSION
Observed values of the accumulated rainwater infiltration into the
bare loamy sand soil as a function of the rainfall depth with three
classes of the rainfall intensity (0-40, 40-80 and 80-120 mm h -1 )
are plotted in Fig.l. No significant difference was observed in the
accumulated rainwater infiltration among these rainfall intensity
classes. This was attributed to the crust formation due to the
beating action of raindrop impact at all intensities which inhibits
infiltration. However, to differentiate between the effects of
raindrop impact (intensity) and the rainfall depth (soil water
accumulation) values of the partial correlation coefficients are set
out in Table 1. It is revealed that under dry antecedent conditions
40
Sf
1200
40
80
1200
40
SO
R A I N F A L L
D
E
P
T
H (mm)
Fig. 1 Accumulated rainwater infiltration vs. rainfall depth for three intensity classes.
420
K.D. Sharmaef a/.
Table 1 Partial correlation coefficients between the accumulated rainwater
infiltration and rainfall characteristics
Antecedent moisture condition (USDA, 1972)
r123
r
(I) Accumulated rainwater infiltration (mm)
(II) Rainfall depth (mm)
(III) Rainfall intensity (mm h"1 )
0.971
0.882
0.788
-0.366
-0.490
-0.905
(AMCI) the rainwater infiltration is governed by the depth of
rainfall (r^.3 > r 1 3 2 ) .
This implies that rainwater infiltration
in sandy soils is governed by the depth of rainfall at low levels of
initial soil water content, a fact that is established in many of the
experiments. However, under saturated or nearly saturated antecedent
conditions rainwater infiltration is governed by raindrop impact
(r13.2 > r12.3>- This is because at high levels of initial soil
water content, a surface crust is formed due to raindrop impact
(Vehera & Jones, 1974). Therefore, the infiltration regime of the
sandy soils with wet antecedent conditions is determined by the
intensity as a result of the crust formed by raindrop impact and not
by changes in the soil water content due to the increased rainfall
depth. Changes in the soil moisture regime have an effect on the
infiltration of soils, which is the product of the hydraulic gradient
and the hydraulic conductivity. These factors depend upon the
changes in the depth of the wetting front (rainfall) and the soil
moisture content. This well-known process is not the dominant effect
on infiltration when a crust is formed by raindrop impact. The
crust has a much lower conductivity than the soil had before crust
formation, so much of the energy loss occurs in the crust, which thus
becomes the governing factor in the infiltration process. Reduction
in the accumulated rainwater infiltration due to crust formation as
a result of raindrop impact has also been reported by Frevert et
al.
(1955), Gupta & Yadav (1978), Mclntyre (1958) and Morin & Benyamini
(1978).
The nature of the individual storms also has an effect on the
accumulated rainwater infiltration into bare sandy soil (Fig.2).
A storm with peak intensity at its onset gives less infiltration of
rainwater than a delayed storm, which generates the peak intensity
towards the end of the event. This is associated with the time at
which crusting occurs; for storms of the latter type, crusting occurs
later in the storm than for storms of the former type, so that
infiltration is consequently greater.
Table 2 presents the accumulated rainwater infiltration as a
function of the basin steepness and slope length. The variability
in the accumulated infiltration with these basin characteristics is
perhaps due to micro-topographical variations, uneven initial soil
moisture distribution due to varying drying rates and spatial
variation of soil characteristics. However, a more consistent
pattern of the infiltration values with slope and slope length is
obtained if mean values are considered. Thus by increasing the basin
steepness from 0.5 to 5% the accumulated infiltration was reduced
from 17.1 to 13.5 mm which is significant at 0.01 level of
Rainwater infiltration into bare loamy sand
SLOPE 5.0%
SLOPE 0.5%
421
SLOPE 10%
NATURE OF
STORMS
ADVANCE
///
IHTEBtEDIATE
o
DELAYED
A
T
0
to
R
Fig. 2
80
A
I
N
120 0
F
A
«0
L
L
80
120 0
D
E
P
40
T
H
(
m
m
)
Accumulated rainwater infiltration vs. rainfall depth for three storm types.
probability. No significant difference was observed between the
infiltration values on 5 and 10% slopes. This was due to more
reduction in the infiltration opportunity time in the former than
the latter (a reduction of 58.8% between 0.5 and 5% slope as
against 23.3% between 5 and 10% slope for the same slope length).
Similarly, by increasing the slope length from 5.12 to 14.5 m the
accumulated rainwater infiltration increased from 13.9 to 15.5 m,
owing to increased conveyance losses with travel distance in sandy
soils. However, the increase in infiltration with travel distance
is not significant within the observed range of values.
Table 2
Accumulated rainwater infiltration (mm) and basin characteristics
Basin slope (%)
Slope length
(m)
5.12
7.0
8.5
10.75
14.5
Mean
0.5
5.0
10.0
Mean
15.6
16.0
19.5
17.0
17.6
13.1
13.1
13.7
13.9
13.8
12.9
12.7
12.9
16.3
15.0
13.9
13.9
15.4
15.7
15.5
17.1
13.5
14.0
(a) Standard error (SE) for degree of slope ±0.55
Least significant difference (LSD) at 5% 1.79
LSD at 1%
2.62
(b) SE for slope length
±0.71
LSD at 5%
not significant
(c) SE for body of table
±1.87
LSD at 5%
not significant
422 K.D. Sharmaef al.
A multiple curvilinear model (Fig.3) to predict the accumulated
rainwater infiltration is developed by combining the rainfall and
basin characteristics. If the predicted values using this model are
compared with the observed values of the accumulated rainwater
infiltration, a correlation coefficient of r = 0.897 is obtained
which is significant at the 0.01 level of probability (Fig.4).
With a coefficient of determination of 80.5% the model can be used
for predictive purposes. The chi-square value of 43.87 (p < 0.10)
indicates that the model fitting is good and hence it can be used
for suitably designing the micro-catchment water harvesting systems
in different agro-climatic regions of the Indian desert.
CONCLUSION
Rainwater infiltration into sandy soils is greatly influenced by
rainfall characteristics. At low initial soil water content
rainwater infiltration is governed by rainfall depth (soil water
accumulation) whereas at high initial soil water content the
raindrop impact (intensity) forming the surface crusts, is the
deciding factor. Also rainwater infiltration significantly
decreases with increasing basin slope and reducing the slope
length.
120
80
ACCUMULATED
U-
iO
0
INFILTRATION (tnln)
40
ACCUMULATED
1120
1201
80
INFILTRATION
120
(Trim}
M
Fig. 3 Rainwater infiltration model for a representative loamy sand soil of the
Indian desert.
Rainwater infiltration into bare loamy sand
0
20
40
PREDICTED
Fig. 4
60
VALUES
80
423
100
(mm I
Comparison of observed and predicted values from Fig. 3.
ACKNOWLEDGEMENT
We are grateful to Dr K.A.Shankarnarayan for his
help in revising this manuscript.
REFERENCES
Bell, F.C. (1979) Precipitation. In: Arid Land Ecosystems
(ed. by
D.W.Goodall & R.A.Perry), Cambridge University Press, UK.
Dunin, F.X. (1976) Infiltration: its simulation for field
conditions. In: Facets of Hydrology (ed. by J.C.Rodda), 199-227.
John Wiley, New York.
Ivenari, M., Shanon, L. & Tadmor, N. (1971) The Negev.
The
Challenge of a Desert, 228-232. Harvard University Press, USA.
Frevert, R.K., Schwab, G.O., Edminster, T.W. & Barnes, K.K. (1955)
Soil
and Water
Conservation
Engineering,
44-46.
John Wiley, New
York.
Gupta, J.P. & Yadav, R.C. (1978) Soil crust formation and seedling
emergence in relation to rainfall intensity and mode of sowing.
J. Ind.
Soc.
Soil
Sci.
26 (1), 20-24.
Linsley, R.K., Kohler, M.A. & Paulhus, J.L.H. (1958) Hydrology
for
Engineers,
311-321. McGraw-Hill, New York.
Mclntyre, D.S. (1958) Soil splash and the formation of surface
crusts by raindrop impact. Soil Sci. 65, 261-266.
Morin, J. & Benyamini, Y. (1977) Rainfall infiltration into bare
soils. Wat. Resour. Res. 13 (5), 813-817.
National Academy of Sciences (1978) More Water for Arid Lands, 23-27.
National Academy of Sciences. Washington, DC.
Sagi, K. (1969) Infiltration in terms of soil moisture, rain
intensity and depth of rainfall. In: Water in the
Unsaturated
Zone (Proc. Wageningen, Netherlands, Symp), 390-392. IAHS Publ.
424 K.D. Sharma et al.
no. 82.
Sharma, K.D., Pareek, O.P. & Singh, H.P. (1982) The effect of runoff
concentration on growth and yield of Jujube. Agric.
Wat.
Management
5 (1), 73-84.
Snedecor, G.W. & Cochran, W.G. (1968) Statistical
Methods,
238-240.
Oxford & IBH Publishing Co., New Delhi.
USDA (1972) Hydrology,
section 10, 1-24. SCS National Engineering
Handbook no. 4, Washington, DC.
Vehera, G. & Jones, R.L. (1974) Bonding mechanisms for soil crusts.
In: Soil Crusts
(ed. by J.W.Cary & D.D.Evans), 17-28. University
of Arizona, USA.
Received
17 November
1981;
accepted
9 March
1983.