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HYDROLOGICAL PROCESSES, VOL. 9, 227-235 (1995)
TOPOGRAPHIC ZONATION OF INFILTRATION IN THE HILLY LOESS REGION, NORTH CHINA
GANG LI* Institute of Geography, Chinese Academy of Sciences, Beijing, China
SHIU HUNG LUK Department of Geography, University of Toronto, Erindale Campus, Mississauga, Canada
AND QIANG GUO CAI
Institute of Geography, Chinese Academy of Sciences, Beijing, China
ABSTRACT Field infiltration tests using portable rainfall infiltrometers were conducted in the Wangjiagou experimental basin in the hilly loess region of north China. Based on data collected at 27 sites, a topographic zonation of infiltration character- istics was observed. The average steady infiltration rate and the average ponding time decreased from the hilltop to the hillslope and further decreased to the gully wall. Such a zonation is closely related to the variations of topography, soil and land use conditions in the study area. A general infiltration model is proposed. Collected field data are used to establish the applicability of the proposed model in the study area.
KEY WORDS infiltration; infiltrometers; infiltration equation; loess region; surface sealing; topographic zonation
INTRODUCTION
Infiltration, as one of the most significant processes which control the fate of water over a catchment, is an essential input in modelling soil erosion processes and an important component in the design of soil and water conservation measures. In the Loess Plateau region, North China, enhancing soil infiltrability has been proposed as a strategy to control soil erosion (Huang, 1983). Similar strategies have been followed in other parts of the arid and semi-arid world (Hudson, 1988).
In the Loess Plateau region, previous determinations of the infiltration rates of surface soils have been obtained using the ring infiltrometer (Jiang and Huang, 1984). However, because of lateral leakage and the absence of raindrop impact to induce soil surface sealing, the estimated infiltration rates are considered to be higher than the actual rates. It has been recognized that the use of a rainfall simulator for this purpose can produce more reliable estimates of infiltration rate.
In this study, which was conducted in a small catchment in the hilly loess region, the objectives are two- fold: (1) to determine the infiltration characteristics of the surface soil under a range of field conditions; and (2) to test the applicability of a general infiltration model to the Loess Plateau region.
*Present address: Department of Geography, State University of New York at Buffalo, Buffalo, USA.
CCC 0885-6087/95/020227-09 0 1995 by John Wiley & Sons, Ltd.
Received 6 May 1992 Accepted 4 May 1994
228 G. LI E T A .
P
'C
-
= P
initial wet- up stage
- actual infiltration rate ....... -.. infiltration capacity ----- rain intensity
declining stage 1 finalsteady j stage
t P t C
Figure 1. Schematic diagram to show proposed infiltration model
THEORY
The process of infiltration into surface soils under high intensity rainfall has been described by a series of theoretical equations (Green and Ampt, 1911; Philip, 1969; Parlange, 1972; Mein and Larson, 1973; Broadridge and White, 1988) as well as empirical equations (Kostiakov, 1932; Horton, 1940; Holtan, 1961; Sharma et al., 1980). In this study, infiltration is divided into three stages: (a) the initial wet-up stage, when the infiltration capacity is greater than the rainfall rate and the infiltration rate is equal to the rainfall intensity; (b) the infiltration declining stage, when the surface soil becomes saturated, ponding begins to take place on the soil surface and the infiltration rate decreases with the increase in soil moisture content; and (c) the final steady stage, when infiltration reaches its final infiltration rate (Figure 1). Thus
For tp < t < t,, i = i, + mt-" (2)
(3) . . For t 2 t,, I = I ,
where i is the infiltration rate (mmjmin), p is the rainfall rate (mm/min), t is the time from the beginning of rainfall (min), i, is a characteristic constant describing the steady infiltration rate (mm/min), m and n are parameters associated with the initial soil surface conditions and rainfall characteristics, respectively, tp is the ponding time (min), t , is the steady time (min), which occurs when the observed infiltration rate is within f 5% of the steady infiltration rate, or
t, = t, + (20m/i,)'/" (4)
and
TOPOGRAPHIC ZONATION OF INFILTRATION 229
In general terms, the proposed model (Figure 1) shares some of the characteristics of a number of previously proposed equations. For instance, if parameter n is specified as a constant of -0.5, it becomes Philip's equation (Philip, 1969), with m equal to 0.5s. If n is taken to be a constant of -1, it becomes Green and Ampt's equation (Green and Ampt, 191 l), with m integrating the term KsS,,Md. In this study, field data collected in the study basin are used to test the validity of the above model.
STUDY SITES AND METHODS
To investigate the infiltration rate on the loess hillslopes, field tests were conducted in the Wangjiagou experimental basin, which is typical of the hilly loess region of north China. The basin is located at 110" 8'E and 37" 32/N, covering 9.1 km2 in area. In this part of the hilly loess region, the mean annual rainfall is about 500mm, mostly occurring during June and September (Jia et ul., 1987). Within the basin itself, there are significant topographic variations. Typically, three topographic zones can be recognized. Zone I, located on the hilltop, is mainly underlain by Malan Loess of Upper Pleistocene age and is usually planted with field crops such as beans, millet and potatoes. Zone I1 is characterized by cultivation on either slopeland or terraced land. This part of the slope is underlain by Lishi Loess of Middle Pleistocene age. Zone I11 is developed on gully walls that are covered by various densities by shrubs such as Curugunu korshinskii and both Lishi Loess of the Lower Pleistocene and Red Earth of the Tertiary are present. However, in this paper, only slopes underlain by the Red Earth were investigated.
In this study, the infiltration characteristics of the surface soil in the study area were determined using a portable rainfall simulator which was built at the University of Toronto based on the design published by Imeson (1977). In the field, water was delivered from 64 1 mm orifices at a height of about 1.5 m. At 0.5 m above ground, the waterdrops were scattered by a suspended sheet of 1 mm mesh to produce a uniform distribution of drops over a target area of 50 x 50cm. The final raindrops produced were of a moderate size, but as the fall height and kinetic energy level are limited, no attempt was made to determine the actual drop size distribution and the kinetic energy delivery.
On the soil surface, a 45cm square steel frame 30cm high was driven lOcm into the soil. Thus the actual mini-plot covers an area which is considerably smaller than the wetted area. A simple Gerlach-type trough was installed on the downslope end of the frame. Before each infiltration test, a calibration run of 20-30 minutes was conducted. This ensures that the rainfall intensity during each experiment falls within the range 1.3-1.5mm/minute. During each experiment, the runoff rate at the trough was determined at three-minute intervals until equilibrium rates were reached. The equilibrium runoff rate is defined for this study by the observation of five successive readings which fall within & 5 % of the average reading. Under the given field conditions, this usually occurs after 65-80 minutes of simulated rainfall. For each experimental site, samples collected at 0-5 cm depth were used to determine the antecedent soil moisture.
w
Figure 2. Location of field sites along cross-gully transacts in Wangjiagou experiment basin
230 G. LI ET AL.
The experimental results reported in this paper are based on field work conducted during 1989 and 1990. In the Wangjiagou basin, sites were mainly selected along transects runnings across the major gully systems in the area (Figure 2). Other sites were added to provide data for a range of slope conditions and parent material. In total, 27 sets of data were considered to be valid for detailed analysis.
RESULTS
In this study, the data collected are manipulated as follows. First, the infiltration rate (i) is calculated by
i = dI/dt x AIlAt (6)
AI = A P - AR (7)
and
for each sampling interval of three minutes. In Equations (6) and (7), i is the mean infiltration rate at three minute intervals, I is the cumulative infiltration over a three minute period, t is the time, P is the rainfall
Table I. Rainfall and infiltration parameters obtained from the field experiments
Zone Site Slope Initial Rainfall ic tP tc m -n gradient moisture intensity (min) (min) (min) (degrees) content (%) (mm/min)
I I I I
Mean
I1 I1 I1 I1 11 I1 I1 I1 I1 I1 I1 I1 I1 I1 I1 I1 I1 I1
Mean 111 I11 I11 111 111 Mean
XSLl ZJGl YDG8 YDGl
YDG2 ZJG6A YDG7 XSL13 HTL12 XSLlO XSL15 XSL14 XSL7 ZJG32 ZJG33 DLGl XSL8 XSL2 XSL9 ML4 XSL3 HTL22
YDG4 xsL42 c c z 3 YDG6 XSL6
3 3 3
10 4.75
25 20 32 7
I5 15 25 15 22 3 3 9 3 3 3 3
10 12
12.50 35 25 27 25 20 26.40
8.9 14.4 9.4 8.0
1018
8.5 9.9
10.2 4.0 8.2
10.4 5.4 4.3 6.6
11.8 11.8 6.3 3.9 8.3 7.9
17.2 5-4 9.1 8.29
4.9 6.6 8.6 4.5 2.1
5.34
1.60 1.40 1 +40 1.40
1.45 1.28 1.34 1.37 1.36 1.40 1.40 1.33 1.52 1.38 1.37 1.72 1.40 1.39 1.30 1.33 2.00 1.40 1.31
1.42 1.86 1.40 1.50 1 .oo 1.40 1.43
0.67 0.70 0.52 0.99 0.72
0.83 0.72 0.70 0.70 0.64 0.59 0.47 0.43 0.39 0.67 0.70 0.84 0.65 0.60 0.49 0.49 0.98 0.91 0.66 0.73 0.70 0.49 0.46 0.46 0.57
3.1 5.3 2.2
23.5
8.52
8.6 5.8 4.4 8.9 1.1 5.1 4.3 6.3 2.0 3.6 1.1 9.6 9.9
11.5 5.7 1 .o 5.1 1.1 5.28
3.3 4.2 2.9 8.0 3.1 4.3 1
40.5 32.5 37.0 82.3 48.06
37.3 78.4 32.5 34.2 33.1 63.6 79.5
233.9 91% 85.2
140.2 41.6 62.8 99.8 96.3
305.0 34.8 15.0 86.95
118.4 36.0 98.0 81.5 61.5 79.08
3,97 10.88 2.30
85.36 25.63
15.12 4.18 5.85
77.40 0.80 6.80 5.1 1 8.18 2.05 2.39 1.06
30.89 3536 24.87 7.39 1.50 2.61 0.43
12.90
3.57 5.28 3.15 9.14 3.80 4.99
1.29 1.65 1.24 1.69 1.47
1-63 1.09 1.47 2.18 0.92 1.31 1.23 1-09 1.03 0.96 0.69 1.77 1.69 1.46 1.25 0.72 1.12 0.83 1.25 0.96 1.40 1.06 1.36 1.24 1.20
ic = Final infiltration capacity (mm/min); m, n = parameters in Equation (2); rp, tc = ponding time and steady time, respectively (min)
Tabl
e 11
. Com
para
tive
perf
orm
ance
of
the
Gre
en a
nd A
mpt
and
Phi
lip’s
equ
atio
ns v
ersu
s th
e pr
opos
ed in
filtra
tion
mod
el
Con
fiden
ce
leve
l Si
te
Phili
p’s
equa
tion
in w
hich
rn =
0.5
s TI
= -0
.5
(r2)
Prop
osed
in
filtra
tion
mod
el
(r2)
XSL
1 Z
JGl
YD
G8
YD
Gl
YD
G2
WG
6A
YD
G7
XSL
13
HT
L 12
X
SL 10
X
SL15
X
SL14
X
SL7
WG
32
WG
33
DL
Gl
XSL
8 X
SL2
XSL
9 ML4
XSL
3 H
TL22
Y
DG
4 x
su
2
ccz3
Y
DG
6 X
SL6
0.83
0.
70
0.50
0.
75
0.7 1
0.
81
0.65
0.
85
0.62
0.
92
0.86
0.
86
0.76
0.
72
0.55
0.
8 1
0.91
0.
86
0.76
0.
76
0-66
0.
51
0.78
0.
74
0.61
0.
80
0.83
erro
r of
es
timat
e n= -1
(m
m/m
in)
rn =
Ks Sav M
d
0.05
0.
0 1
0.71
0.
07
0.01
0,
47
0.14
0.
01
0.41
0.
07
0.01
0.
48
0.04
0.
01
0.58
0.
08
0.01
0.
79
0.08
0.
01
0.47
0.
03
0.0 1
0.
43
0.07
0.
01
0.60
0.
05
0.01
0.
72
0.05
0.
01
0.78
0-
08
0.01
0.
84
0.05
0.0
1
0.75
0.
09
0.01
0.
72
0.09
0.
0 1
0.29
0.
08
0.01
0.
77
0.04
0.
01
0.71
0.
06
0.01
0.
74
0-05
0.0
1
0.74
0-
06
0.0 1
0.
33
0.05
0.
01
0.65
0.
05
0.01
0.
49
0.11
0.
01
0.78
0.
08
0.01
0.
60
0.07
0.
0 1
0.6
1 0.
05
0.01
0.
74
0.05
0.
01
0.75
Stan
dard
er
ror
of
estim
ate
(mm
lmin
)
Stan
dard
er
ror
of
estim
ate
(mm
/min
)
Con
fiden
ce
leve
l
0-07
0.
09
0.15
0.
09
0.05
0.
08
0.10
0-
07
0.07
0.
06
0.07
0.
08
0.05
0.
09
0.1 1
0.
09
0.07
0.
08
0.06
0.
10
0.06
0.05
0.
1 1
0.10
0.
07
0.06
0-
06
0.0 1
>
0.01
>
0.01
>
0.01
>
0.01
0.
01
> 0.
01
> 0.
01
0.0 1
0.
01
0.0 1
0.
01
0.01
0-
0 1
> 0.
01
0.01
0.
0 1
0.01
0.
01
> 0.
01
0.0 1
>
0.01
0.
01
> 0,
01
0.01
0.
01
0.01
0.12
0.
07
0.0 1
0.
23
0.26
0.
43
0.09
0.
09
0.07
0.
30
0.3 1
0.
44
0.15
0.
38
0.38
0.
70
0-33
0.
38
0.40
0.
44
0.34
0.
03
0.43
0.
17
0.25
0.
40
0.17
0.12
0.
12
0.20
0.
1 1
0.07
0.
14
0.14
0.
08
0.1
1 0.
10
0.12
0.
17
0.09
0.
13
0.10
0.
1 1
0.1 1
0.
12
0.09
0.
09
0-08
0.
07
0.17
0.
15
0.10
0.
09
0.1 1
232
Y e .g 0.4- C
g F - .-
0.2-
G. LI ET AL.
I
0
o 0
0
0
Figure 3. Selected infiltration data and fitted curves using the proposed infiltration model
and R is the runoff. In the study area, evaporation occurs at a rate of < 3 mm/h, which is very small com- pared with a rainfall intensity of 90 mm/h. Also, the vegetation cover is sparse. Hence both evaporation and interception by vegetation are ignored in the water budget calculations. Depression storage is also ignored due to the smooth ground surface at the plots.
Then, for each data set, the method of least squares was used to obtain the parameters rn and n pertaining to Equation (2). Also, i, was calculated by averaging the last five readings recorded 65-80 minutes after the beginning of each experiment. The estimated infiltration parameters are summarized in Table I and data from selected sites are presented in Figure 3. It can be shown in every instance that the regression coeffi- cients are significant at the 0.01 level and the standard error of the estimate for infiltration rate is typically less than 0.09 mm/min (Table 11).
For the parameter m, which is related to soil and slope conditions, there is a tendency for it to decrease from the upper to the lower slope (Table I). On the other hand, the parameter n, which relates to rainfall intensity, shows very small variations among the sites in different zones.
For the purpose of evaluating the variation in soil infiltration characteristics with slope zone, the param- eter of final infiltration capacity and ponding time (Table I) are considered in greater detail. It can be seen from this table that for zones 1, 2 and 3, the infiltration rate at the steady stage and the average ponding time are 0.72, 0.65 and 0.57 mm/min and 9.0, 5.1 and 4.6 minutes, respectively. This overall spatial pattern of variation is clearly shown in the data obtained from the Yangdaogou, a small subbasin within the
0.8 I
Figure 4. Effect of slope gradient and surface sealing on infiltration rate
TOPOGRAPHIC ZONATION OF INFILTRATION 233
Table 111. Effect of tillage on infiltration (final infiltration capacity, mm/min) ~~ ~~ ~~
Cropped slopeland Terraced field
Heavily tilled 0.68-0.83 0.65-0'70
Slightly tilled 0'39-0.42 0.4 7 - 0.4 9 Moderately tilled 0.47-0'70 0.60
Wangjiagou experimental basin (Figure 3). Here, the highest steady infiltration rate (0.99 mm/min) with the longest ponding time (24 minutes) was recorded on the hilltop, which has a gentle slope of 5" and which supports stands of planted Robinia pseudoacacia. A moderate value of 0.83 mm/min with a moderate ponding time (about 10 minutes) occurred on the hillslope with a gradient of 25", which has been culti- vated. The lowest infiltration of 0*73mm/min with the shortest ponding time (around five minutes) was registered on the steep gully wall with a gradient of 35" and covered by Caragana koshinskii shrubs.
The data presented above show that a significant topographic zonation of infiltration exists in the study basin. However, it is important to recognize that such variations in infiltration characteristics are related to a range of environmental conditions, including changes in gradient, soil and land use. Unfortunately, the limited data available do not allow a detailed investigation of the relative significance and the isolated effect of each of the controlling factors. Hence, in the following discussion, a comparison of results is only made whenever the effect of the controlling factors can be isolated. Even then, due to the limited sample size, statistical tests for significant differences were not conducted and the differences should be treated as general tendencies only.
For the effect of slope gradient, results obtained from moderately tilled slopeland show a decrease in final infiltration rate with an increase in slope angle (Figure 4). This is generally consistent with the relations obtained by Luk et al. (1993) in a series of rainfall simulation experiments conducted in a nearby site and is also compatible with the results published by Jiang and Huang (1984).
It is expected that the soil type has an important influence on infiltration. For instance, samples from surface soils (0- 15 cm) show that the Red Earth has a bulk density of 1.28 g/cm3 and a median particle size of 16pm, which is generally more compact and finer in size than the surface loess soil that has a bulk density of 1.17g/cm3 and a median size of 24pm. Thus we can infer that infiltration is lower on the gully slope which is underlain by Red Earth and higher on the loess dominated upper slope (zone 1). However, in the field situation, the soil type also varies with slope steepness and therefore the precise effect of soil type on infiltration cannot be isolated.
In addition, the effect of tillage should be considered. In the Loess Plateau, tillage mainly consists of cattle-drawn ploughing (usually 15-25 cm deep) during the autumn or spring and hoeing (normally several centimetres deep) conducted during the growing season. Thus a range of tillage intensities is involved. Here, 'slightly tilled' refers to an unploughed condition since the last harvest; a 'moderately tilled' condition
Table IV. Mean final infiltration capacity for a range of land use in the Wangjiagou experimental basin
Land use Mean infiltration Sample size rate (mmlmin)
Woodland 0.98 3 Grass 1 and 0.96 3 Shrubland 0.56 6 Cropped slopeland 0.61 9 Terraced fields 0.60 9
234 G. LI ET AL.
occurs if ploughing was conducted in the spring, but no hoeing has been conducted in the last seven days before the experiment; and 'heavily tilled' refers to land which was ploughed and hoed during the last seven days before the experiment. According to these definitions, the recorded steady infiltration rate on the slightly tilled slopeland ranged from 0.39 to 0.42 mm/min, which is 46% lower than on heavily tilled slope- land; on the moderately filled slopeland it varies from 0.47 to 0.70mm/min, which is 24% lower than on heavily tilled land. For terraced fields, these differences are smaller (Table 111). Tillage effect may be ascribed to two mechanics. Firstly, tillage loosens earth and augments soil voids (Hill, 1990) which increases the path of the movement of water. Secondly, tillage destroys surface crust which otherwise will heavily block water infiltration (McIntyre, 1958; Romkens et al., 1990).
Finally, the effect of land use is considered. Woodland and grassland show considerably higher steady infiltration than shrubland, cropped slopeland and terrace fields (Table IV). Obviously, in this instance, the lower steady infiltration on shrubland covered with Caragana korshinskii is related to its steeper gradient and the presence of Red Earth. Also, the low steady infiltration for cropped slopeland is due to inclusion of steep gradient sites.
DISCUSSION
One of the major objectives of this study is to identify the zonation pattern of infiltration. It has been shown that steady infiltration decreases from zone 1 and zone 3. Furthermore, the spatial pattern of variation in steady infiltration is related to many factors, including gradient, soil and land use. However, it should be borne in mind that such spatial patterns are determined under conditions where surface soil sealing and crusting were not induced during the experiments due to the low kinetic energy of the rainfall. We view the elimination of sealing effects in this set of experiments to be crucial because it defines the baseline conditions against which the effect of surface sealing and crusting can be gauged.
Some general indication of the effect of surface sealing can be obtained by comparing the data collected in this study with that derived from field experiments conducted at nearby sites where surface sealing was induced by the application of simulated rainfall at an intensity of 1.2 mm/min (which is lower than the rain- fall intensity used in these experiments, but which approximates up to 90% of the kinetic energy of natural rainfall at the same intensity) (Luk et al., 1993). For this comparison, three slightly tilled loess slopeland sites were selected. They are considered to be similar in surface conditions to the field conditions pertaining to the earlier experiments. It can be seen that the observed infiltration rate (at 26-30 minutes after simu- lated rainfall began) for the sites with antecedent surface crusting are consistently lower by a wide margin (Figure 4). This clearly shows the overwhelming importance of surface sealing and crusting in affecting the infiltration of the surface soils in the Loess Plateau. Given that no surface sealing was observed in the pre- sent experiments, whereas the earlier experiments approximate the kinetic energy of natural rainfall at 1.2mm/min, the average decrease of about 0.3mm/min in the infiltration of these loess soils can be considered to be the effect of surface sealing under the specified rainfall intensity of these experiments.
Furthermore, results from Luk et al. (1993) show that the development of surface sealing varies with slope gradient. In fact, infiltration is significantly reduced on gentle slopes (< 10") due to the preferential development of surface crusting and sealing (Figure 4). This result, when compared with data from this study, suggests that for gentle slopes the inverse relation between slope gradient and infiltration under non-crusted conditions may be reversed under crusted conditions. It appears that new knowledge about the effect of surface sealing and crusting under a range of slope gradient may hold the key to rationalizing the wide range of infiltration-slope gradient relations published elsewhere.
Fitting of the collected field data to the proposed infiltration model shows a small standard error of estimate and a significance level which is < 0.01 in every case studied. However, if either Green and Ampt's equation (with n = - 1) or Philip's equation (with n = -0.5 and m = 0.5s) is used (Table II), the fit is far inferior, with a much higher standard error of estimate, and the significance level is > 0.01 in many instances. Obviously, this result is expected because Equation (2) allows for variable values of m and n whereas the other two equations have fixed m and/or n values. However, such a result does suggest that infiltration equations with built-in constant m and n values are not valid in the Loess Plateau region, wherc
TOPOGRAPHIC ZONATION OF INFILTRATION 23 5
significant gradient, soil and land use conditions change within a small area. Thus there appears to be some merit in pursuing the evaluation of the proposed infiltration model for general application in the study region. Unfortunately, due to the limited number of data sets available, the appropriate range of m and n values for different topography, soil and land use conditions cannot be precisely defined. Further work is clearly required in this area.
CONCLUSIONS
1. In the Wangjiagou experimental basin, which is representative of the hilly loess region, north China, a topographic zonation of infiltration characteristics was observed. The average steady infiltration rate and average ponding time were found to decrease from the hilltop to the hillslope and further to the gully wall. Such a zonation is closely related to the variations of gradient, soil and land use conditions in the study area. A general infiltration model was proposed. Collected field data from 27 sites were used to establish the applicability of the proposed model in the study area.
2.
ACKNOWLEDGEMENTS
Funding for this research was provided by the Institute of Geography, Chinese Academy of Sciences, and the Canadian International Development Agency. For the completion of this work, we acknowledge the assistance of Wu Su’an, Li Gaoshe and Ma Shaojia, Institute of Geography, Chinese Academy of Sciences; and Niu Siping and Zeng Boqing, Shanxi Institute of Soil and Water Conservation.
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