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Runoff generation on a semi-arid agricultural catchment: field and experimental studies
Journal of Hydrology 196 (1997) 99–118
Runoff generation on a semi-arid agricultural catchment: field and experimental studies T.X. Zhu a ,*, Q.G. Cai b, B.Q. Zeng c a
Department of Geography, University of Toronto, Toronto, Ont. M5S 1A1, Canada b Institute of Geography, Chinese Academy of Sciences, Beijing, 100101, China c Shanxi Institute of Soil and Water Conservation, Lishi, Shanxi Province, 033000, China Received 21 September 1995; revised 27 September 1996; accepted 9 October 1996
Abstract Runoff generation on a small semi-arid agricultural catchment with mixed land use in the Loess Plateau of China was monitored over a 19-year period. To investigate the possible spatial variations in runoff generation on the catchment, eight plots were set up on various sections of the hillslope and monitored for 6 years. Only a small proportion of rainfall events generated runoff, contrary to the assumption that rainstorms in this area are characterized by high intensity and short duration, and thereby lead to runoff generation. Runoff occurrence and yields were also found to be highly variable within the catchment. To explore further the results obtained from the field observations, portable and downspraying sprinklers were used in field experiments. Both produce similar rainfall intensities but the raindrops from the portable sprinkler have very low kinetic energy and do not break down aggregates and form crusting, while raindrops produced by the downspraying sprinkler have similar characteristics to natural rainfall. A comparison of the experimental results obtained by those two kinds of sprinklers clearly demonstrated that runoff generation in this area is largely affected by surface crusting. The effects on runoff generation of the crusts formed during previous storms and the present storm were examined experimentally. Finally, the impacts of cultivation and plowing on runoff generation were determined through field investigation and experiments. This study suggests that there is considerable potential to reduce runoff and erosion, and to increase soil moisture and crop yields on the Loess Plateau through changes of currently inappropriate land use and the improvement of land management. q 1997 Elsevier Science B.V.
1. Introduction Much work has been done on runoff generation mechanisms in humid environments * Corresponding author. 0022-1694/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S00 22-1694(96)033 10-0
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(Beston, 1964; Hewlett and Hibbert, 1967; Kirkby, 1978; Anderson and Burt, 1978; Burt, 1979). However, comparatively few studies have been conducted in semi-arid and arid areas. Among the latter, the most notable work has been done by Yair, Bryan and their associates (Yair and Lavee, 1974; 1976; 1985; Bryan et al., 1978). Their studies indicated that the runoff generative processes in semi-arid and arid areas where infiltration-excess overland flow is dominant are highly variable and dependent on rainfall characteristics, properties of surface materials and topographic features. However, their studies were confined to non-agricultural badlands. Since over 40% of the world’s cultivated lands are in semi-arid areas (Zhao and Zhao, 1991), an understanding of runoff generation processes there, and especially of the impacts of land use and of cultivation activities is essential. The Loess Plateau, located in the middle reaches of the Yellow River of China, has an area of 380 000 km 2 and has been cultivated over thousands of years. Currently, three severe problems exist in this region and its neighboring area. First, water deficit in the soil is the crucial factor to hamper agricultural development (Zhao and Zhao, 1991). Second, erosion caused by storm runoff is one of the most severe environmental problems in China. The average and maximum erosion rates are 150 mg ha −1 year −1 and 390 mg ha −1 year −1, respectively (Chen and Luk, 1989). Finally, flood risks in the lower reaches cause severe social problems. The storm runoff and eroded soil eventually find their way into the Yellow River and the alluviation in the lower reaches has raised the bed of river channels above the surrounding flood plain by more than 10 m. As a result, the lives and property of millions of inhabitants are in a hazardous flood zone (Whitney and Chen, 1989). These problems can be attributed to the undesirable movement of water. One of the fundamental solutions to these problems is to change current inappropriate land use and improve land management so as to increase infiltration and thereby reduce runoff and erosion. Clearly, understanding of runoff generation processes is the prerequisite to implement them. The objectives of this study are: (1) to observe runoff generation and its spatial variations through field monitoring; (2) to explore the mechanisms responsible for them using the experiments.
2. Study site The study site, Yangdaogou catchment, has an area of 20.3 ha and is located about 4 km north of Lishi town, Shanxi Province, China (Fig. 1). It is a typical first-order drainage subbasin in the Loess Plateau. The climate is semi-arid warm temperate. Local deposits consist mainly of thick silty loess, which is believed to be wind-borne dust derived from central Asia in Quaternary (Liu, 1964). The Tertiary clayey red earth is widely exposed at the lower slope section near the basin outlet. The major physical and chemical properties of red earth and loess are shown in Table 1. As an unmanaged contrasting experimental basin, the land use had undergone limited changes before the 1980s. However, after then, a check dam with a sedimentation pond was built at the basin outlet and the upper slope has been partially terraced. Like the other first-order subbasins in the Loess Plateau hilly region, the hillslope in the Yangdaogou can be divided into four vertical zones from the divide to gully bottom. Zone
T.X. Zhu et al./Journal of Hydrology 196 (1997) 99–118
Fig. 1. The Yangdaogou catchment, plot locations, measurement sites and topography.
101
Loess Red earth
Materials
13.5 6.4
. 0.05 mm
Particle size (%)
58.1 38.1
0.05–0.005 mm
, 0.005 mm 28.4 55.5
Physical and chemical properties of red earth and loess
Table 1
1.13–1.19 1.27–1.40
Bulk density (g cm −3) 12.73 8.85
CaCO 3 (%)
1.029 0.737
Organic matter (%)
1.7 6.6
Al 2O 3 (%)
4.3 5.52
Fe 2O 3 (%)
0.08 0.10
MnO (%)
2.23 2.28
MgO (%)
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1 is dominated by terraces and cultivated lands with slopes of less than 58. Additionally, small areas of forest lands are distributed on the hilltops. Zone 2 is mainly composed of steep cultivated slopelands, with a slope of about 108 on the upper parts to nearly 258 on the lower part. Zone 3 is marked by a sharp break in slope and is characterized by a substantial increase in gradient to more than 308. This section of slope is too steep to be cultivated and the dominant land use is grazing by sheep and goats. Before the construction of the check dam, Zone 4 was narrow gully bottom consisting of red earth; it is now a largely flat alluvial area although a narrow gully bottom can still be found upstream of the alluvial pond. Zones 1, 2, 3 and 4 account for 4.27%, 46%, 38.56% and 1.17% of the whole basin area, respectively. The soils in the Yangdaogou were developed from Quaternary loess and Tertiary clay and are classified as Typic Ustothorents and Lithic Ustothorents, respectively (Soil Survey Staff, 1975). They occupy about 80 and 20% of the total basin area (Hamilton and Luk, 1993). Cultivation activities take place only at Zone 1 and Zone 2. The crops in the cultivated lands include maize, beans, wheat, sunflower and millet. On the lower slope (Zone 3), shrubs including Caragana korshinski, Abrotanum Lavandulaefolia and Periploca Sepium are present.
3. Field setup and experimental design In order to obtain data on processes and rates of runoff generation under a wide range of rainfall conditions, a field monitoring program was conducted in two periods (1956–1970 and 1987–1990). During the first period (1956–70), a Parshall flume was constructed at the basin outlet to monitor the hydrological processes on the storm event basis. During each storm, the water level readings and sediment samples were taken manually with a varied time interval from 0.5 to 20 min. The time interval decreases with the runoff discharge and its changes over time. Based on the calibration formulae, the water level readings were first transferred into discharge, and the sediment discharge was then calculated. In order to investigate the spatial variation of runoff generation on the hillslope. Eight field plots were set up on various zones on the hillslope in different parts of the basin and monitored from 1963 to 1968. Total runoff discharge for each storm event was recorded. Six of the field plots were selected for detailed analysis in this study (Fig. 1). During the second period (1987–90), although the focus of the monitoring program had shifted to tunnel flow (Zhu and Luk, 1997), the hydrological processes of intermittent stream flow were still monitored. Owing to the formation of the pond, the outflow flume was moved upstream to the end of the alluvial pond. As a result, the catchment area that contributes runoff to the flume was reduced from 20.3 ha to 12.6 ha. Basic sampling procedures were the same as in the first period except for a shorter time interval (1– 3 min). In both periods, rainfall data were recorded in detail. In order to explore the mechanisms responsible for runoff generation and its spatial variation, two kinds of rainfall simulators, portable and downspraying sprinklers, were employed to conduct field experiments. The portable rainfall sprinkler described in detail by Li (1991) ejects 1.5-m fall-height water drops which hit a 0.5-m screen with a net size of 1 mm 2 at a height of 0.5 m to create a spatially uniform rainfall. The simulated rainfall
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Table 2 Description of experimental conditions at the plots Experiment code
Land use
Sprinkler
Slope (8)
YDG-1 ZJG-3 YDG-2 YDG-10 GM TTM HTL HP
Forest land Terrace land Cultivated slopeland Barren slope Forest land Terrace land Cultivated slopeland Barren slope
Portable Portable Portable Portable Downspraying Downspraying Downspraying Downspraying
10 ,5 25 31 ,5 3 12 27
Initial moisture (%) 8 10.2 NA 9.15 NA 14.5 13.5 12.5
Rainfall intensity (mm min −1) 1.39 1.39 1.40 1.40 0.90 0.85 1.20 1.13
NA, no data available.
intensity is constant during any one experiment but can be adjusted from 0.9 mm min −1 to 1.8 mm min −1. The plot size is 0.5 × 0.5 m. Owing to the small drop sizes and low fallheight, water drops have very low kinetic energy and are thereby unable to break down aggregates and form crusting. Details of the downspraying sprinkler can be found in Luk et al. (1986). Briefly, simulated rainfall was sprinkled from Sparaco Full Cone nozzles, pointing downwards from a height of 4.57 m. The kinetic energy of simulated rainfall is about 90% of natural rainfall and rainfall intensity is similar to that of the portable sprinkler. The experimental plots were laid out with a standard dimension of 1 m crossslope and 5 m downslope (measure horizontally). Rainfall simulation experiments were carried out on forest land, terrace land (Zone 1), cultivated slopeland (Zone 2) and barren gully slope (Zone 3) with portable and downspraying sprinklers, respectively. All terrace land and slopeland plots were heavily tilled before the experiments. The experimental conditions are summarized in Table 2. To evaluate the impacts of crusts formed during previous storms and the present storm, a pair of cultivated slopeland plots (Plots A and B) with identical gradients (approximately 118) were selected for downspraying-sprinkler experiments only. Both plots were plowed first and subjected to rainfall simulation experiments for 60 min to develop a crust. Three days later, the crust on Plot A was carefully broken to a depth of 1–2 cm and Plot B was left as it was. Subsequently, both plots were exposed to the simulated rainfall for another 60 min. During all of the experiments, runoff discharges were measured with a time interval of 1–3 min.
4. Results and discussion 4.1. Rainfall and basin runoff Scarcity of matched rainfall–runoff records for small watersheds in semi-arid areas has left the relationship between them still unclear over a wide range. Fairly long-term and detailed records of rainfall and runoff processes in the Yangdaogou basin have enabled this problem to be tackled. Fig. 2 shows annual rainfall, rainfall at rainy season (May to
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Fig. 2. Annual rainfall, rainfall during rainy seasons, runoff-generation rainfall and runoff yield over the monitoring periods, 1956–1970 and 1987–1990.
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Table 3 Correlation matrix between annual rainfall, rainfall in the rainy season, runoff-generation rainfall and runoff yield over the periods of 1956—1970 and 1987–1990 (n = 19) Annual rainfall Annual rainfall Rainfall in rainy season Runoff-generation rainfall Runoff yield
1 0.72 0.52 0.25
Rainfall in rainy season
Runoff-generation rainfall
1 0.65 0.41
Runoff yield
1 0.48
1
September), runoff-generation rainfall and runoff yield in the basin over a 19-year monitoring period. Average annual precipitation is 496.7 mm with extreme values of 243.5 and 756.3 mm. Over 70% of rainfall and almost all runoff events occur within the rainy season. The runoff-generation storms only account for an average of 155.4 mm yearly with a range of 15.3–416.3 mm. The mean annual runoff yield is 29.6 mm with a minimum and maximum of 0.2 and 94.5 mm, respectively. Two very characteristic features can be found from the comparison of these four sets of data. First, the coefficients of variation of annual precipitation, rainfall at the rainy season, runoff-generation rainfall and runoff yield (24%, 29%, 63% and 93%, respectively) show that runoff-generation rainfall and runoff yield have a much higher inter-annual variability. Second, good correlation exists between rainfall variables but not between rainfall variables and runoff yield (Table 3). Of the latter, the weakest is between annual rainfall and runoff yield (R 2 = 0.25). As an example, the highest recorded annual precipitation was 756.3 mm in 1964 but it only yielded runoff of 20.6 mm, which was even below the average of 29.6 mm. From the perspective of individual events, the number of rainfall events sharply decreases with event rain amount and over 80% of events have a rain amount less than 10 mm (Table 4). The percentage of runoff-generation storms and the runoff coefficient show a reverse order. Storms with rainfall amounts of over 40 mm, which in total account for 12% of annual precipitation, generate over 50% annual runoff yields. Overall, only 8% of the total rainfall events generate runoff, which is contrary to the assumption by many workers that rain storms in this area are characterized by high intensity and short duration, and thereby lead to runoff generation. However, runoff-generation and non-runoffTable 4 Rainfall features of different event rain amount categories derived from rainfall data, 1956–1970 and 1987–1990 Event rainfall (mm) Total events Total precipitation (mm) Runoff events Total runoff-generation precipitation (mm) Total runoff depth (mm) Average runoff coefficients (%)
, 10
10–20
20–30
30–40
40–70
. 70
1246 3475.8 34 215.5 20.64 9.6
170 2438 44 656.7 110.41 16.8
63 1306.8 23 533.2 73.71 13.8
31 1025.7 11 356.6 65.5 18.4
12 732.2 12 732.2 162 22.1
5 459.1 5 459.1 129.5 28.2
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Fig. 3. The rainfall intensities to initiate runoff for all 129 runoff-generation storms observed in the monitoring periods, 1956–1970 and 1987–1990.
generation storms share a wide range of event rain amount. The lowest recorded rain amount of a runoff-generation storm was 1.9 mm while the highest non-runoff-generation was 39.8 mm. Hence, rainfall amount cannot be used as a criterion to differentiate nonrunoff-generation storms from runoff-generation storms in this area. Instead, rainfall intensity is a more appropriate measure. Owing to the small catchment area and very quick response of stream flow to rainfall, the rainfall intensity which leads to runoff initiation for each storm can be identified by comparing the hyetograph and the streamflow hydrograph. Fig. 3 shows the rainfall intensity to initiate runoff for all 129 runoffgeneration storms observed during the monitoring periods. It can be seen that the minimum rainfall intensity to runoff generation was around 0.09 mm min −1. In order to understand the between-event variations in rainfall and basin runoff, 15 rainfall variables and six basinflow hydrological variables were selected for correlation analysis using the complete data set of 116 events (1956–70). The aim was to identify those cause-and-effect relationships that would merit further study with experiments. The results are given in Table 5. Three points deserve to be noticed. First, even for the runoffgeneration storms only, the best rainfall indicator (highest r 2) to predict runoff yields is not the total rainfall amount P but the rainfall amount with an intensity of over 0.2 mm min −1 P 0.2. Second, as expected, runoff peak time shows a strong positive relation with rainfall peak time. However, the relationship between peak intensity and peak runoff discharge is not as high as expected. Instead, once again, rainfall amounts with an intensity of over 0.2 mm min −1 P 0.2 are the better indicators to estimate peak discharge Q p. Third, although runoff initiation is almost solely caused by infiltration-excess processes, weak or no correlation exists between times to stream flow initiation T ri and antecedent rainfall P a or rainfall intensity at the beginning of the storm I pb. In order to exclude the effects of
1 0.58 0.06 0.00 0.26 0.50 0.57 0.04 0.11 0.62 0.66 0.56 0.08 0.19 0.02 0.08 0.19 0.00 0.00 0.02 0.06
Qp
1 0.00 0.00 0.05 0.57 0.34 0.03 0.01 0.41 0.55 0.57 0.07 0.40 0.12 0.24 0.39 0.12 0.00 0.05 0.00
Dr
Rc
1 0.05 1 0.02 0.07 0.13 0.35 0.00 0.04 0.61 0.35 0.00 0.22 0.00 0.13 0.00 0.12 0.00 0.00 −0.03 0.00 −0.15 −0.03 −0.10 0.00 −0.03 0.00 −0.10 −0.02 −0.11 −0.02 −0.22 0.01 0.67 0.18
T ri
1 0.65 0.21 0.00 0.27 0.00 0.72 0.07 0.03 0.00 0.00 −0.02 −0.13 −0.06 0.00 −0.10 −0.08 0.32 0.92
T rp
Pa
Dp
1 0.55 0.64 0.14 0.00 0.06
1 0.35 0.04 0.05 0.00
Max. Max. I ri I 10 I 30
1 0.26 0.33 0.19 0.17 0.00 0.13
I ave
1 0.25 0.53 0.58 0.31 0.04 0.02 0.02
P 0.5
1 0.21 0.03 0.09 0.13 0.04 0.00 0.00 0.00
P 0.4
1 0.20 0.52 0.13 0.32 0.52 0.18 0.00 0.04 0.00
P 0.3
1 0.85 0.14 0.36 0.08 0.23 0.48 0.08 0.00 0.07 0.03
P 0.2
1 0.89 0.72 0.10 0.20 0.03 0.12 0.34 0.03 0.00 0.10 0.05
P 0.1
1 0.14 1 0.08 0.00 1 0.00 0.46 0.00 1 0.21 0.86 0.01 0.15 0.29 0.72 0.00 0.07 0.24 0.56 0.01 0.02 0.05 0.08 0.03 0.00 0.17 0.13 0.00 −0.01 0.12 0.00 0.00 −0.19 0.11 0.05 −0.01 −0.06 0.21 0.23 0.00 0.00 0.03 0.00 −0.01 −0.08 0.00 −0.01 −0.01 −0.09 0.00 0.21 0.00 0.24 0.00 0.25 0.00 0.72
P
a
1 0.09 0.00 0.11
I pb
1 0.02 0.09
P il
1 0.34
T pp
Antecedent rainfall is calculated by the equation: Pa = ∑nt= 1 Pt − Rt K t , where: P a is antecedent rainfall; P t and R t are daily precipitation and runoff depth t days prior to present day. Q s, storm flow per unit area (mm); Q p, peak discharge (m 3 s −1); T rp, peak discharge time (min); T ri, runoff initiation time (min); D r, duration of storm runoff (h); R c, runoff coefficients (%); P, storm rainfall (mm); P a, antecedent rainfall (mm); D p, duration of storm (h); P 0.1, P 0.2, P 0.3, P 0.4, P 0.5, precipitation with intensity over 0.1, 0.2, 0.3, 0.4, 0.5 mm min −1, respectively; I ave, average rainfall intensity of the storm; Max. I 10, Max. I 30, intensity for the most intense 10 and 30 min during the storm, respectively (mm min −1); I ri, intensity to runoff generation (mm min −1); I pb, intensity at the beginning of storm (mm min −1); P il, initial losses of rainfall before runoff generation (mm); T pp, time of peak rainfall intensity (min).
Qs Qp T rp T ri Dr Rc P Pa Dp P 0.1 P 0.2 P 0.3 P 0.4 P 0.5 I ave Max. I10 Max. I30 I ri I pb P il T pp
Qs
Table 5 Correlation between rainfall and hydrologic variables
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Fig. 4. Times to runoff initiation under various antecedent rainfall and rainfall intensity conditions.
temporal changes in rainfall intensity on such relationships, storms with relatively unchanged intensity before runoff initiation were selected for analysis. Fig. 4 shows the times to runoff initiation in those storms. It is clear that antecedent rainfall only exerts a limited impact on runoff initiation in the storms beginning with relatively low intensities, while in the storms beginning with rainfall intensity exceeding 0.5 mm min −1, runoff was initiated within 10 min of rainfall onset during almost all events no matter how low the antecedent rainfall. 4.2. Vertical zonation in runoff generation Monitoring the various plots enabled the spatial variation in runoff generation on the hillslope to be determined. It was assumed that runoff was only generated in Zone 4 (gully bottom) if runoff was recorded at the basin outflow flume but not on any of the hillslope plots. Fig. 5 shows the runoff generation source areas. Among the total of 52 runoffgeneration storms over the period from 1963 to 1968, 11 storms generated runoff on Zone 4 only; 11 storms on Zones 3 and 4, four storms on Zones 2, 3 and 4 and 26 storms on all zones. Runoff generation usually required a rainfall intensity in excess of 0.1 mm min −1 on Zone 4, 0.2 mm min −1 on Zone 3 and 0.3–0.4 mm min −1 on Zones 1 and 2, respectively. With the expansion of runoff source areas, hydrological variables of the basin outflow such as runoff coefficient, runoff yield and peak discharge show an overall increase as well although a wide overlap still exists between them (Table 6). Runoff yields also show great variations on the hillslope (Table 7). Table 7 should be
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Fig. 5. Runoff generation source areas on the hillslopes in the runoff-generation storms over the period from 1963 to 1968. Zone 1: hilltop (gentle cultivated land); Zone 2: upper slope (steep cultivated land); Zone 3: lower slope (barren gully slope); Zone 4: gully bottom.
read in conjunction with Fig. 1. The total runoff yield on the whole hillslope (Plot 4), 144.3 mm, was comparable to that of the basin, 148.8 mm, and also to the weighted average of upper and gully slope sections (Plots 3 and 5), 151.5 mm. This may suggest that the overall run-on losses over the slope length were limited because of the relatively steep terrain and the crusted surface. Plots 1, 2 and 3 have identical land use (cultivated lands) and surface material (loess), differing only in slope. The greater runoff yield from Plot 3, compared with that of Plot 2, is consistent with the field experimental result that runoff yield increases with slope gradients (Luk et al., 1992). However, Plot 1 has a gentler slope but produced greater runoff yields than Plots 2 and 3 despite the lower number of runoff events, which will be discussed in detail later. The greater runoff yield from Plot 5 compared with Plots 1, 2 and 3 is a result of the steeper gradient and different land use Table 6 Runoff yield, runoff coefficient and peak discharge of basin outflow with the difference in runoff source areas on the hillslope over the period of 1963–1968. Values in parenthesis are the average Runoff source area
Runoff yield (mm)
Zone 4 Zones 3 and 4 Zones 2, 3 and 4 Zones 1, 2, 3 and 4
0.01–0.5 (0.18) 0.1–4.8 (1.32) 0.3–4.2 (1.52) 0.2–26.9 (4.49)
Runoff coefficients (%) 0.1–8.3 (2.4) 1.3–27.5 (9.2) 1.3–21.4 (12.3) 2.2–43.4 (13.6)
Peak discharge (m 3 s −1) 0.009–0.147 (0.051) 0.027–1.003 (0.374) 0.110–3.087 (0.996) 0.049–6.678 (1.188)
Zones 1 1 and upper 2 1 and 2 1, 2 and 3 3 3
Plot no. 1 2 3 4 5 6
Crop land Crop land Crop land Crop land + barren slope Barren slope Barren slope
Land use
Table 7 Description of field monitoring plots and runoff yields
4 13 17 29 36 29
Mean slope (8) NE NE NE NE NE NW
Slope aspect 40 70 105 185 90 50
Slope length (m) 200–400 600 1855 4167 1655 630
Horizontal area (m 2)
Loess Loess Loess Loess Loess Red earth
Surface materials
129.5 84.7 104.1 144.3 204.7 268.3
Runoff yield (1963–68) (mm)
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although the data cannot be used to evaluate the relative importance of these two factors. Plots 5 and 6 differ in slope gradient, slope aspect and surface material. The steeper gradient of Plot 5 should result in greater runoff but the higher runoff yield from Plot 6 indicates that the difference in surface material is the most important factor. Red earth has a considerably higher clay content and bulk density than loess (Table 1). Additionally, being a locally weathered material, loose red earth debris is only confined to the upper 20– 30 cm below which it becomes almost impermeable. As loess is a wind-borne material, changes of bulk density with depth are much smaller than red earth. All these features of red earth are favorable to runoff generation. Simple regression analysis was conducted between runoff yields on different zones and total rainfall amount or rainfall amount above a certain intensity (Table 8). The results once again clearly show that event rainfall amount is not the best index to predict spatial variation in runoff yields on the hillslope. Instead, the rainfall amount above a certain intensity is the better one. 4.3. Sprinkler experiments The field observation reveals the fundamental characteristics of rainstorms and the spatial variation in runoff generation on hillslopes in this area. The simple regression analyses also suggest some possible cause-and-effect relationships between rainfall and runoff variables. However, owing to the widely varied antecedent conditions and uncontrollable rainfall conditions, it is, more often, difficult to assess the relative significance of individual factors on runoff generation and its spatial variation. Moreover, regression analysis cannot indicate the actual mechanisms responsible for them. For this reason, it is important to get assistance from experiments. Figs 6 and 7 illustrate the infiltration processes on forest land, terrace land, cultivated slopeland and barren gully slope using portable and downspraying simulators, respectively. It can be seen that in both sets of experiments, runoff initiation time decreased in the order of forest land, terrace land, cultivated slopeland and barren gully slope with a range from more than 20 min to less than 1 min. Final infiltration rates were highest on forest lands and the difference between the two types of rainfall simulator experiments was very limited: 1.0–1.1 mm min −1 for the portable-sprinkler method and 0.8–0.9 mm min −1 for the downspraying method. For each rainfall simulator method, terrace land and cultivated slopeland had similar final infiltration rates but the final rates for the downspraying sprinkler (0.2–0.3 mm min −1) was much lower than for the portable-sprinkler (0.6–0.7 mm min −1). In both sets of experiments, final infiltration rates on barren gully slope were low and there was little difference between rainfall simulation methods. Portable and downspraying sprinklers produce similar rainfall intensities but the very low kinetic energy raindrops from the portable sprinkler do not breakdown soil aggregates and no, or very weak, crusts are developed on the surface even with long-duration and large-amount simulated rainfall. This was verified by visual observations. In contrast, the downspraying sprinkler produces raindrops comparable to natural rainstorms and for terrace land and cultivated slopeland, the large difference in final infiltration rates between the two methods is due to the formation of crusts when using the downspraying-sprinkler method. However, on forest lands, a thin layer of litter on the surface and relatively high
Zone 1 Zone 2 Zone 3 (red earth) Zone 3 (loess) Zones 1 and 2 Zones 1, 2 and 3 Subbasin
Plots of subbasin 0.622 0.288 0.381 0.278 0.261 0.473 0.457
Total rain amount (mm) 0.774 0.368 0.355 0.282 0.332 0.539 0.510
Rain amount with I . 0.1 mm min −1 0.642 0.42 0.587 0.485 0.535 0.617 0.704
Rain amount with I . 0.2 mm min −1 0.468 0.488 0.659 0.615 0.61 0.641 0.730
Rain amount with I . 0.3 mm min −1
Correlation analysis between runoff yields from various zones and rainfall variables over the period of 1963–1968
Table 8
0.176 0.466 0.660 0.643 0.477 0.548 0.599
Rain amount with I . 0.4 mm min −1
0.129 0.286 0.531 0.505 0.191 0.347 0.491
Rain amount with I . 0.5 mm min −1
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Fig. 6. Infiltration processes on different land use in the portable-sprinkler experiments with low kinetic energy of simulated rainfall.
Fig. 7. Infiltration processes on different land use in the downspraying-sprinkler experiments with high kinetic energy of simulated rainfall.
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Fig. 8. Effects of the crusts formed during the previous storms and the present storm on infiltration processes. Plot A: cultivated slopeland with crust formed by earlier treatment; Plot B: cultivated slopeland as Plot A but crust was broken before the experiment.
organic matter in the soil largely prohibit the development of crusts, and final infiltration rates are very high using both rainfall simulators. Low final infiltration rates for both methods on barren gully slopes are ascribed to the pre-existing crusts before experiments. The rates of infiltration for the experiments to determine the effects of crusts are shown in Fig. 8. At Plot B, runoff was initiated very quickly and the infiltration rates decreased very sharply with time. In contrast, Plot A had a longer runoff initiation time and gentler decrease in the infiltration rate over time. The infiltration rates became comparable near the end of the experiment on both plots, which suggests that new crusts were gradually formed on Plot A. With differing infiltration processes, the runoff yields from the 60-min experiments are 30.7 and 48.8 mm, for Plots A and B, respectively. 4.4. Effects of cultivation and plowing: evidence from field observation Cultivation and plowing activities occur only on Zone 1 and Zone 2 in Yangdaogou and break up pre-existing crusts. The cultivated land is plowed with animal-driven plows, mainly in May, before seeding. During the crop growth season, occasional hoeing may occur from time to time. However, because the owners of small land parcels have different habits, such activities are more or less random in both space and time. On gully slopes (Zone 3), crusts exist all the time since no cultivation activities occur there. Table 9 lists the runoff yields of the plots on different zones in the first runoff-generation storm in each of the 6 years of the monitoring period (1963–1968). For these storms, the
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Table 9 Runoff depth from the plots or basin in the first major runoff-generation storm of rainy season from 1963–1968 Year
1963 1964 1965 1966 1967 1968
Date
5/23 5/20 7/19 6/29 7/17 5/22
Rainfall (mm) Runoff depth (mm)
62.3 23.7 15.3 11.0 37.5 25.3
Plot 1
Plot 3
Plot 5
1.1 0.2 0 0 / 0
1.0 0.1 0 0 3.8 0.1
14.0 1.5 2.6 2.0 12.2 0.8
Subbasin 5.0 0.4 0.2 0.5 3.9 1.4
runoff yields from the cultivated lands (Plots 1, 3) were much smaller than those from the gully slope (Plot 5), typically a factor of 10 less, compared with a factor of 2 for the average over the 6 years. This early rainy season effect can be attributed to the plowing on the cultivated lands. After several rainfall events, the development of crusts on the cultivated lands increases the runoff yield and the difference in runoff yields between cultivated lands and gully slope decreases. Thereafter, occasional hoeing may exert smallscale and random effects on runoff generation. Another remarkable effect of cultivation is the formation of a plow pan on the cultivated lands in Zone 1 owing to the long history of cultivation. Although cultivated lands are also extensively distributed in Zone 2, no obvious plow pan is found. This is because cultivated lands in Zone 2 have steeper slope and downslope displacement of earth caused by plowing largely depresses the development of plow pans. Field investigation indicates that the plow pans are located at 15–30 cm below the surface and have a thickness of 10– 20 cm. The bulk densities of the surface soil layer, plow pan and subsoil layer are 1.13– 1.19, 1.23–1.35 and 1.19–1.25 g cm −3 (Cai et al., 1990), respectively. Portable sprinkler experiments indicate that the infiltration rate for plow pans is less than 0.1 mm min −1 (Li, 1991). This is even lower than the infiltrability of crusts. During rainstorms, when the infiltration wetting front reaches the plow pan, rainfall may gradually accumulate above it owing to the difference in infiltrabilities between surface layer and plow pan. Finally, the soils above the plow pans become saturated, which needs a rainfall amount of 80 mm or so (Li, 1991). Thus, the effects of plow pans on runoff generation are important only in storms with extremely large amounts of rainfall. Table 10 lists two large storm events occurring in the 6-year plot-monitoring period, with rainfall amounts of 80 mm and 95.5 mm, respectively. Owing to the relatively low intensities in these two storms, the runoff yields from Plot 1 (Zone 1) are not only higher than those from Plot 3 (Zone 2) but also than Plot 5 (Zone 3). This seems to be attributed to the primary effect of the plow pan in Plot 1. Simple regression analyses indicate that the best rainfall indicator to estimate the runoff yield on Zone 1 is rainfall amount with an intensity of over 0.1 mm min −1 (Table 8), which is also consistent with the infiltrability of plow pan. The fewer runoff events on Zone 1 than Zones 2 and 3 are due to the gentler slope and thereby larger rainfall detention capacities. However, owing to the small proportion of the total area, Zone 1 exerts a limited effect on the runoff yields in the whole hillslope and the basin.
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T.X. Zhu et al./Journal of Hydrology 196 (1997) 99–118 Table 10 Effects of plow pans on runoff yields on hillslopes and subbasin Date
7/6/1963 9/6/1964
Antecedent rainfall (mm) 9.5 14.7
Rainfall (mm) 95.5 80
Max. I 10 (mm min −1) 0.33 0.55
Runoff depth (mm) Plot 1
Plot 3
Plot 5
11.7 17.3
0.6 7.0
3.9 10.9
Subbasin 5.1 7.7
5. Conclusions 1. In this area, runoff generation is predominantly caused by infiltration-excess processes although some of the runoff may reach the channels via deep-seated tunnel systems (Zhu and Luk, 1997). Only a small proportion of rainfall events is able to generate runoff. However, runoff-generation and non-runoff-generation storms share a large range of rainfall amount. Thus, event rain amount cannot be used as a criterion to differentiate them. Instead, rainfall intensity is the appropriate indicator. 2. Both runoff source areas and runoff yields show spatial variations over the hillslope. The spatial variation in runoff generation on the hillslope is mainly ascribed to the difference in soil infiltrability. 3. Of all the factors affecting soil infiltrabilities in this area, crusting is the most important. Formation of crusts can reduce the infiltrability of loess soils from 0.6–0.7 mm min −1 to 0.2–0.3 mm min −1 or so. 4. Soil crustability can be significantly affected by land use. On forest lands, no or very weak crusts are developed owing to the existence of a litter layer on the surface and the high organic matter contents in the soil. On cultivated lands, plowing and hoeing can break pre-existing crusts and thereby reduce runoff. Appropriate land management also decreases soil crustabilities. Zeng et al. (1992) found runoff was reduced by 30% after 4-year manure input in comparison with the plots without manure input.
Acknowledgements This study could not have been completed without generous funding from a number of sources such as the Canadian International Development Agency, the Chinese Natural Science Foundation, the Water and Soil Conservation Bureau of Shanxi Province. We also wish to thank S.H. Luk, G. Li, S.J. Ma, Z.J. Jia, Z.G. Zhang, G.P. Wang, H. Hamilton and H. Taylor for their assistance in the field monitoring and experiments. Anonymous reviewers contributed significantly to the clarity and accuracy of the text.
References Anderson, M.G. and Burt, T.P., 1978. The role of topography in controlling throughflow generation. Earth Surf. Processes, 3: 331–344. Beston, R.P., 1964. What is watershed runoff? J. Geophys. Res., 69: 1541–1552.
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Bryan, R.B., Yair, A. and Hodges, W.K., 1978. Factors controlling the initiation of runoff and piping in Dinosaur Provincial Park badlands, Alberta, Canada. Z. Geomorphol., Suppl. Band, 29: 151–168. Burt, T.P., 1979. Diurnal variations in stream discharge and throughflow during a period of low flow. J. Hydrol., 41: 291–301. Cai, Q.G., Chen, H. and Luk, S.H., 1990. The formation processes of plow pan and its effects on sediment and runoff yield and soil fertility on slope land. In: Y.Z. Chen (Editor), Sino-Canadian Symposium of Experimental Researches on the Law of Soil Erosion in the Loess Plateau Region, Western Shanxi. Water Conservancy Press, Beijing, pp. 87–99 (in Chinese). Chen, Y.Z. and Luk, S.H., 1989. Sediment sources and recent changes in the sediment load of the Yellow River, China. In: Proceedings, 5th International Soil Conservation Conference, Bangkok, Thailand. Vol. 1, pp. 313– 323. Dept. of Land Dev., Bangkok. Hamilton, H. and Luk, S.H., 1993. Nitrogen transfers in a rapidly eroding agroecosystem: Loess Plateau, China. J. Environ. Qual., 22: 133–140. Hewlett, J.D. and Hibbert, A.R., 1967. Factors affecting the response of small watersheds to precipitation in humid areas. In: W.E. Sopper and H.W. Lull (Editors), International Symposium on Forest Hydrology. Pergamon Press, New York, pp. 275–290. Kirkby, M.J. (Editor), 1978. Hillslope Hydrology. Wiley, Chichester, 389 pp. Li, G., 1991. Experimental study on rainfall infiltration – one of the fundamental of hillslope amelioration in Loess Plateau. Unpublished Ph.D. Thesis, Institute of Geography, Chinese Academy of Sciences. Liu, D.S., 1964. Loess Deposits in the Middle Reaches of Yellow River. Science Press, Beijing, 234 pp. (in Chinese). Luk, S.H., Abrahams, A.D. and Parsons, A.J., 1986. A simple rainfall simulator and trickle system for hydrogeomorphological experiments. Phys. Geogr., 7: 344–356. Luk, S.H., Cai, Q.G. and Wang, G.P., 1992. Effects of surface and slope gradient on runoff and soil loss in the loess region. In: F.T. Wang (Editor), Soil Erosion Management and Application of Geographical Information System in the Loess Plateau Region, Western Shanxi. Science Press, Beijing, pp. 66–79 (in Chinese). Soil Survey Staff, 1975. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. USDA-SCS Agricultural Handbook, US Government Print Office, Washington, DC, 436 pp. Whitney, J. and Chen, Y.Z., 1989. Introduction and progress report. In: J. Whitney and Y.Z. Chen (Editors), CIDA SEMGIS (CHINA) Project Annual Report. pp. 1–7 (unpublished). Yair, A. and Lavee, H., 1974. Areal contribution to runoff on scree slopes in an extreme arid environment: a simulated rainstorm experiment. Z. Geom., Suppl., 21: 106–121. Yair, A. and Lavee, H., 1976. Runoff generative process and runoff yield from arid talus mentled slopes. Earth Surf. Processes, 1: 235–247. Yair, A. and Lavee, H., 1985. Runoff generation in arid and semi-arid zones. In: M.G. Anderson and T.P. Burt (Editors), Hydrological Forecasting. Wiley, Chichester, pp. 183–220. Zeng, B.Q., Zhang, Z.G. and Li, J.Y., 1992. Experimental study on the effect of soil organic matter on water and soil losses. In: F.T. Wang (Editor), Soil Erosion Management and Application of Geographical Information System in the Loess Plateau Region, Western Shanxi. Science Press, Beijing, pp. 151–156 (in Chinese). Zhao, X.Q. and Zhao, J.B., 1991. Soil Moisture in the Dry Farmlands. Agronomy Press, Beijing, 459 pp. (in Chinese). Zhu, T.X. and Luk, S.H., 1997. Deep-seated, complex tunnel systems – a hydrological study in the Loess Plateau, China. Geomorphology, in press.
Runoff generation on a semi-arid agricultural catchment: field and experimental studies T.X. Zhu a ,*, Q.G. Cai b, B.Q. Zeng c a
Department of Geography, University of Toronto, Toronto, Ont. M5S 1A1, Canada b Institute of Geography, Chinese Academy of Sciences, Beijing, 100101, China c Shanxi Institute of Soil and Water Conservation, Lishi, Shanxi Province, 033000, China Received 21 September 1995; revised 27 September 1996; accepted 9 October 1996
Abstract Runoff generation on a small semi-arid agricultural catchment with mixed land use in the Loess Plateau of China was monitored over a 19-year period. To investigate the possible spatial variations in runoff generation on the catchment, eight plots were set up on various sections of the hillslope and monitored for 6 years. Only a small proportion of rainfall events generated runoff, contrary to the assumption that rainstorms in this area are characterized by high intensity and short duration, and thereby lead to runoff generation. Runoff occurrence and yields were also found to be highly variable within the catchment. To explore further the results obtained from the field observations, portable and downspraying sprinklers were used in field experiments. Both produce similar rainfall intensities but the raindrops from the portable sprinkler have very low kinetic energy and do not break down aggregates and form crusting, while raindrops produced by the downspraying sprinkler have similar characteristics to natural rainfall. A comparison of the experimental results obtained by those two kinds of sprinklers clearly demonstrated that runoff generation in this area is largely affected by surface crusting. The effects on runoff generation of the crusts formed during previous storms and the present storm were examined experimentally. Finally, the impacts of cultivation and plowing on runoff generation were determined through field investigation and experiments. This study suggests that there is considerable potential to reduce runoff and erosion, and to increase soil moisture and crop yields on the Loess Plateau through changes of currently inappropriate land use and the improvement of land management. q 1997 Elsevier Science B.V.
1. Introduction Much work has been done on runoff generation mechanisms in humid environments * Corresponding author. 0022-1694/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S00 22-1694(96)033 10-0
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(Beston, 1964; Hewlett and Hibbert, 1967; Kirkby, 1978; Anderson and Burt, 1978; Burt, 1979). However, comparatively few studies have been conducted in semi-arid and arid areas. Among the latter, the most notable work has been done by Yair, Bryan and their associates (Yair and Lavee, 1974; 1976; 1985; Bryan et al., 1978). Their studies indicated that the runoff generative processes in semi-arid and arid areas where infiltration-excess overland flow is dominant are highly variable and dependent on rainfall characteristics, properties of surface materials and topographic features. However, their studies were confined to non-agricultural badlands. Since over 40% of the world’s cultivated lands are in semi-arid areas (Zhao and Zhao, 1991), an understanding of runoff generation processes there, and especially of the impacts of land use and of cultivation activities is essential. The Loess Plateau, located in the middle reaches of the Yellow River of China, has an area of 380 000 km 2 and has been cultivated over thousands of years. Currently, three severe problems exist in this region and its neighboring area. First, water deficit in the soil is the crucial factor to hamper agricultural development (Zhao and Zhao, 1991). Second, erosion caused by storm runoff is one of the most severe environmental problems in China. The average and maximum erosion rates are 150 mg ha −1 year −1 and 390 mg ha −1 year −1, respectively (Chen and Luk, 1989). Finally, flood risks in the lower reaches cause severe social problems. The storm runoff and eroded soil eventually find their way into the Yellow River and the alluviation in the lower reaches has raised the bed of river channels above the surrounding flood plain by more than 10 m. As a result, the lives and property of millions of inhabitants are in a hazardous flood zone (Whitney and Chen, 1989). These problems can be attributed to the undesirable movement of water. One of the fundamental solutions to these problems is to change current inappropriate land use and improve land management so as to increase infiltration and thereby reduce runoff and erosion. Clearly, understanding of runoff generation processes is the prerequisite to implement them. The objectives of this study are: (1) to observe runoff generation and its spatial variations through field monitoring; (2) to explore the mechanisms responsible for them using the experiments.
2. Study site The study site, Yangdaogou catchment, has an area of 20.3 ha and is located about 4 km north of Lishi town, Shanxi Province, China (Fig. 1). It is a typical first-order drainage subbasin in the Loess Plateau. The climate is semi-arid warm temperate. Local deposits consist mainly of thick silty loess, which is believed to be wind-borne dust derived from central Asia in Quaternary (Liu, 1964). The Tertiary clayey red earth is widely exposed at the lower slope section near the basin outlet. The major physical and chemical properties of red earth and loess are shown in Table 1. As an unmanaged contrasting experimental basin, the land use had undergone limited changes before the 1980s. However, after then, a check dam with a sedimentation pond was built at the basin outlet and the upper slope has been partially terraced. Like the other first-order subbasins in the Loess Plateau hilly region, the hillslope in the Yangdaogou can be divided into four vertical zones from the divide to gully bottom. Zone
T.X. Zhu et al./Journal of Hydrology 196 (1997) 99–118
Fig. 1. The Yangdaogou catchment, plot locations, measurement sites and topography.
101
Loess Red earth
Materials
13.5 6.4
. 0.05 mm
Particle size (%)
58.1 38.1
0.05–0.005 mm
, 0.005 mm 28.4 55.5
Physical and chemical properties of red earth and loess
Table 1
1.13–1.19 1.27–1.40
Bulk density (g cm −3) 12.73 8.85
CaCO 3 (%)
1.029 0.737
Organic matter (%)
1.7 6.6
Al 2O 3 (%)
4.3 5.52
Fe 2O 3 (%)
0.08 0.10
MnO (%)
2.23 2.28
MgO (%)
102 T.X. Zhu et al./Journal of Hydrology 196 (1997) 99–118
T.X. Zhu et al./Journal of Hydrology 196 (1997) 99–118
103
1 is dominated by terraces and cultivated lands with slopes of less than 58. Additionally, small areas of forest lands are distributed on the hilltops. Zone 2 is mainly composed of steep cultivated slopelands, with a slope of about 108 on the upper parts to nearly 258 on the lower part. Zone 3 is marked by a sharp break in slope and is characterized by a substantial increase in gradient to more than 308. This section of slope is too steep to be cultivated and the dominant land use is grazing by sheep and goats. Before the construction of the check dam, Zone 4 was narrow gully bottom consisting of red earth; it is now a largely flat alluvial area although a narrow gully bottom can still be found upstream of the alluvial pond. Zones 1, 2, 3 and 4 account for 4.27%, 46%, 38.56% and 1.17% of the whole basin area, respectively. The soils in the Yangdaogou were developed from Quaternary loess and Tertiary clay and are classified as Typic Ustothorents and Lithic Ustothorents, respectively (Soil Survey Staff, 1975). They occupy about 80 and 20% of the total basin area (Hamilton and Luk, 1993). Cultivation activities take place only at Zone 1 and Zone 2. The crops in the cultivated lands include maize, beans, wheat, sunflower and millet. On the lower slope (Zone 3), shrubs including Caragana korshinski, Abrotanum Lavandulaefolia and Periploca Sepium are present.
3. Field setup and experimental design In order to obtain data on processes and rates of runoff generation under a wide range of rainfall conditions, a field monitoring program was conducted in two periods (1956–1970 and 1987–1990). During the first period (1956–70), a Parshall flume was constructed at the basin outlet to monitor the hydrological processes on the storm event basis. During each storm, the water level readings and sediment samples were taken manually with a varied time interval from 0.5 to 20 min. The time interval decreases with the runoff discharge and its changes over time. Based on the calibration formulae, the water level readings were first transferred into discharge, and the sediment discharge was then calculated. In order to investigate the spatial variation of runoff generation on the hillslope. Eight field plots were set up on various zones on the hillslope in different parts of the basin and monitored from 1963 to 1968. Total runoff discharge for each storm event was recorded. Six of the field plots were selected for detailed analysis in this study (Fig. 1). During the second period (1987–90), although the focus of the monitoring program had shifted to tunnel flow (Zhu and Luk, 1997), the hydrological processes of intermittent stream flow were still monitored. Owing to the formation of the pond, the outflow flume was moved upstream to the end of the alluvial pond. As a result, the catchment area that contributes runoff to the flume was reduced from 20.3 ha to 12.6 ha. Basic sampling procedures were the same as in the first period except for a shorter time interval (1– 3 min). In both periods, rainfall data were recorded in detail. In order to explore the mechanisms responsible for runoff generation and its spatial variation, two kinds of rainfall simulators, portable and downspraying sprinklers, were employed to conduct field experiments. The portable rainfall sprinkler described in detail by Li (1991) ejects 1.5-m fall-height water drops which hit a 0.5-m screen with a net size of 1 mm 2 at a height of 0.5 m to create a spatially uniform rainfall. The simulated rainfall
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Table 2 Description of experimental conditions at the plots Experiment code
Land use
Sprinkler
Slope (8)
YDG-1 ZJG-3 YDG-2 YDG-10 GM TTM HTL HP
Forest land Terrace land Cultivated slopeland Barren slope Forest land Terrace land Cultivated slopeland Barren slope
Portable Portable Portable Portable Downspraying Downspraying Downspraying Downspraying
10 ,5 25 31 ,5 3 12 27
Initial moisture (%) 8 10.2 NA 9.15 NA 14.5 13.5 12.5
Rainfall intensity (mm min −1) 1.39 1.39 1.40 1.40 0.90 0.85 1.20 1.13
NA, no data available.
intensity is constant during any one experiment but can be adjusted from 0.9 mm min −1 to 1.8 mm min −1. The plot size is 0.5 × 0.5 m. Owing to the small drop sizes and low fallheight, water drops have very low kinetic energy and are thereby unable to break down aggregates and form crusting. Details of the downspraying sprinkler can be found in Luk et al. (1986). Briefly, simulated rainfall was sprinkled from Sparaco Full Cone nozzles, pointing downwards from a height of 4.57 m. The kinetic energy of simulated rainfall is about 90% of natural rainfall and rainfall intensity is similar to that of the portable sprinkler. The experimental plots were laid out with a standard dimension of 1 m crossslope and 5 m downslope (measure horizontally). Rainfall simulation experiments were carried out on forest land, terrace land (Zone 1), cultivated slopeland (Zone 2) and barren gully slope (Zone 3) with portable and downspraying sprinklers, respectively. All terrace land and slopeland plots were heavily tilled before the experiments. The experimental conditions are summarized in Table 2. To evaluate the impacts of crusts formed during previous storms and the present storm, a pair of cultivated slopeland plots (Plots A and B) with identical gradients (approximately 118) were selected for downspraying-sprinkler experiments only. Both plots were plowed first and subjected to rainfall simulation experiments for 60 min to develop a crust. Three days later, the crust on Plot A was carefully broken to a depth of 1–2 cm and Plot B was left as it was. Subsequently, both plots were exposed to the simulated rainfall for another 60 min. During all of the experiments, runoff discharges were measured with a time interval of 1–3 min.
4. Results and discussion 4.1. Rainfall and basin runoff Scarcity of matched rainfall–runoff records for small watersheds in semi-arid areas has left the relationship between them still unclear over a wide range. Fairly long-term and detailed records of rainfall and runoff processes in the Yangdaogou basin have enabled this problem to be tackled. Fig. 2 shows annual rainfall, rainfall at rainy season (May to
T.X. Zhu et al./Journal of Hydrology 196 (1997) 99–118
105
Fig. 2. Annual rainfall, rainfall during rainy seasons, runoff-generation rainfall and runoff yield over the monitoring periods, 1956–1970 and 1987–1990.
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Table 3 Correlation matrix between annual rainfall, rainfall in the rainy season, runoff-generation rainfall and runoff yield over the periods of 1956—1970 and 1987–1990 (n = 19) Annual rainfall Annual rainfall Rainfall in rainy season Runoff-generation rainfall Runoff yield
1 0.72 0.52 0.25
Rainfall in rainy season
Runoff-generation rainfall
1 0.65 0.41
Runoff yield
1 0.48
1
September), runoff-generation rainfall and runoff yield in the basin over a 19-year monitoring period. Average annual precipitation is 496.7 mm with extreme values of 243.5 and 756.3 mm. Over 70% of rainfall and almost all runoff events occur within the rainy season. The runoff-generation storms only account for an average of 155.4 mm yearly with a range of 15.3–416.3 mm. The mean annual runoff yield is 29.6 mm with a minimum and maximum of 0.2 and 94.5 mm, respectively. Two very characteristic features can be found from the comparison of these four sets of data. First, the coefficients of variation of annual precipitation, rainfall at the rainy season, runoff-generation rainfall and runoff yield (24%, 29%, 63% and 93%, respectively) show that runoff-generation rainfall and runoff yield have a much higher inter-annual variability. Second, good correlation exists between rainfall variables but not between rainfall variables and runoff yield (Table 3). Of the latter, the weakest is between annual rainfall and runoff yield (R 2 = 0.25). As an example, the highest recorded annual precipitation was 756.3 mm in 1964 but it only yielded runoff of 20.6 mm, which was even below the average of 29.6 mm. From the perspective of individual events, the number of rainfall events sharply decreases with event rain amount and over 80% of events have a rain amount less than 10 mm (Table 4). The percentage of runoff-generation storms and the runoff coefficient show a reverse order. Storms with rainfall amounts of over 40 mm, which in total account for 12% of annual precipitation, generate over 50% annual runoff yields. Overall, only 8% of the total rainfall events generate runoff, which is contrary to the assumption by many workers that rain storms in this area are characterized by high intensity and short duration, and thereby lead to runoff generation. However, runoff-generation and non-runoffTable 4 Rainfall features of different event rain amount categories derived from rainfall data, 1956–1970 and 1987–1990 Event rainfall (mm) Total events Total precipitation (mm) Runoff events Total runoff-generation precipitation (mm) Total runoff depth (mm) Average runoff coefficients (%)
, 10
10–20
20–30
30–40
40–70
. 70
1246 3475.8 34 215.5 20.64 9.6
170 2438 44 656.7 110.41 16.8
63 1306.8 23 533.2 73.71 13.8
31 1025.7 11 356.6 65.5 18.4
12 732.2 12 732.2 162 22.1
5 459.1 5 459.1 129.5 28.2
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107
Fig. 3. The rainfall intensities to initiate runoff for all 129 runoff-generation storms observed in the monitoring periods, 1956–1970 and 1987–1990.
generation storms share a wide range of event rain amount. The lowest recorded rain amount of a runoff-generation storm was 1.9 mm while the highest non-runoff-generation was 39.8 mm. Hence, rainfall amount cannot be used as a criterion to differentiate nonrunoff-generation storms from runoff-generation storms in this area. Instead, rainfall intensity is a more appropriate measure. Owing to the small catchment area and very quick response of stream flow to rainfall, the rainfall intensity which leads to runoff initiation for each storm can be identified by comparing the hyetograph and the streamflow hydrograph. Fig. 3 shows the rainfall intensity to initiate runoff for all 129 runoffgeneration storms observed during the monitoring periods. It can be seen that the minimum rainfall intensity to runoff generation was around 0.09 mm min −1. In order to understand the between-event variations in rainfall and basin runoff, 15 rainfall variables and six basinflow hydrological variables were selected for correlation analysis using the complete data set of 116 events (1956–70). The aim was to identify those cause-and-effect relationships that would merit further study with experiments. The results are given in Table 5. Three points deserve to be noticed. First, even for the runoffgeneration storms only, the best rainfall indicator (highest r 2) to predict runoff yields is not the total rainfall amount P but the rainfall amount with an intensity of over 0.2 mm min −1 P 0.2. Second, as expected, runoff peak time shows a strong positive relation with rainfall peak time. However, the relationship between peak intensity and peak runoff discharge is not as high as expected. Instead, once again, rainfall amounts with an intensity of over 0.2 mm min −1 P 0.2 are the better indicators to estimate peak discharge Q p. Third, although runoff initiation is almost solely caused by infiltration-excess processes, weak or no correlation exists between times to stream flow initiation T ri and antecedent rainfall P a or rainfall intensity at the beginning of the storm I pb. In order to exclude the effects of
1 0.58 0.06 0.00 0.26 0.50 0.57 0.04 0.11 0.62 0.66 0.56 0.08 0.19 0.02 0.08 0.19 0.00 0.00 0.02 0.06
Qp
1 0.00 0.00 0.05 0.57 0.34 0.03 0.01 0.41 0.55 0.57 0.07 0.40 0.12 0.24 0.39 0.12 0.00 0.05 0.00
Dr
Rc
1 0.05 1 0.02 0.07 0.13 0.35 0.00 0.04 0.61 0.35 0.00 0.22 0.00 0.13 0.00 0.12 0.00 0.00 −0.03 0.00 −0.15 −0.03 −0.10 0.00 −0.03 0.00 −0.10 −0.02 −0.11 −0.02 −0.22 0.01 0.67 0.18
T ri
1 0.65 0.21 0.00 0.27 0.00 0.72 0.07 0.03 0.00 0.00 −0.02 −0.13 −0.06 0.00 −0.10 −0.08 0.32 0.92
T rp
Pa
Dp
1 0.55 0.64 0.14 0.00 0.06
1 0.35 0.04 0.05 0.00
Max. Max. I ri I 10 I 30
1 0.26 0.33 0.19 0.17 0.00 0.13
I ave
1 0.25 0.53 0.58 0.31 0.04 0.02 0.02
P 0.5
1 0.21 0.03 0.09 0.13 0.04 0.00 0.00 0.00
P 0.4
1 0.20 0.52 0.13 0.32 0.52 0.18 0.00 0.04 0.00
P 0.3
1 0.85 0.14 0.36 0.08 0.23 0.48 0.08 0.00 0.07 0.03
P 0.2
1 0.89 0.72 0.10 0.20 0.03 0.12 0.34 0.03 0.00 0.10 0.05
P 0.1
1 0.14 1 0.08 0.00 1 0.00 0.46 0.00 1 0.21 0.86 0.01 0.15 0.29 0.72 0.00 0.07 0.24 0.56 0.01 0.02 0.05 0.08 0.03 0.00 0.17 0.13 0.00 −0.01 0.12 0.00 0.00 −0.19 0.11 0.05 −0.01 −0.06 0.21 0.23 0.00 0.00 0.03 0.00 −0.01 −0.08 0.00 −0.01 −0.01 −0.09 0.00 0.21 0.00 0.24 0.00 0.25 0.00 0.72
P
a
1 0.09 0.00 0.11
I pb
1 0.02 0.09
P il
1 0.34
T pp
Antecedent rainfall is calculated by the equation: Pa = ∑nt= 1 Pt − Rt K t , where: P a is antecedent rainfall; P t and R t are daily precipitation and runoff depth t days prior to present day. Q s, storm flow per unit area (mm); Q p, peak discharge (m 3 s −1); T rp, peak discharge time (min); T ri, runoff initiation time (min); D r, duration of storm runoff (h); R c, runoff coefficients (%); P, storm rainfall (mm); P a, antecedent rainfall (mm); D p, duration of storm (h); P 0.1, P 0.2, P 0.3, P 0.4, P 0.5, precipitation with intensity over 0.1, 0.2, 0.3, 0.4, 0.5 mm min −1, respectively; I ave, average rainfall intensity of the storm; Max. I 10, Max. I 30, intensity for the most intense 10 and 30 min during the storm, respectively (mm min −1); I ri, intensity to runoff generation (mm min −1); I pb, intensity at the beginning of storm (mm min −1); P il, initial losses of rainfall before runoff generation (mm); T pp, time of peak rainfall intensity (min).
Qs Qp T rp T ri Dr Rc P Pa Dp P 0.1 P 0.2 P 0.3 P 0.4 P 0.5 I ave Max. I10 Max. I30 I ri I pb P il T pp
Qs
Table 5 Correlation between rainfall and hydrologic variables
108 T.X. Zhu et al./Journal of Hydrology 196 (1997) 99–118
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Fig. 4. Times to runoff initiation under various antecedent rainfall and rainfall intensity conditions.
temporal changes in rainfall intensity on such relationships, storms with relatively unchanged intensity before runoff initiation were selected for analysis. Fig. 4 shows the times to runoff initiation in those storms. It is clear that antecedent rainfall only exerts a limited impact on runoff initiation in the storms beginning with relatively low intensities, while in the storms beginning with rainfall intensity exceeding 0.5 mm min −1, runoff was initiated within 10 min of rainfall onset during almost all events no matter how low the antecedent rainfall. 4.2. Vertical zonation in runoff generation Monitoring the various plots enabled the spatial variation in runoff generation on the hillslope to be determined. It was assumed that runoff was only generated in Zone 4 (gully bottom) if runoff was recorded at the basin outflow flume but not on any of the hillslope plots. Fig. 5 shows the runoff generation source areas. Among the total of 52 runoffgeneration storms over the period from 1963 to 1968, 11 storms generated runoff on Zone 4 only; 11 storms on Zones 3 and 4, four storms on Zones 2, 3 and 4 and 26 storms on all zones. Runoff generation usually required a rainfall intensity in excess of 0.1 mm min −1 on Zone 4, 0.2 mm min −1 on Zone 3 and 0.3–0.4 mm min −1 on Zones 1 and 2, respectively. With the expansion of runoff source areas, hydrological variables of the basin outflow such as runoff coefficient, runoff yield and peak discharge show an overall increase as well although a wide overlap still exists between them (Table 6). Runoff yields also show great variations on the hillslope (Table 7). Table 7 should be
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Fig. 5. Runoff generation source areas on the hillslopes in the runoff-generation storms over the period from 1963 to 1968. Zone 1: hilltop (gentle cultivated land); Zone 2: upper slope (steep cultivated land); Zone 3: lower slope (barren gully slope); Zone 4: gully bottom.
read in conjunction with Fig. 1. The total runoff yield on the whole hillslope (Plot 4), 144.3 mm, was comparable to that of the basin, 148.8 mm, and also to the weighted average of upper and gully slope sections (Plots 3 and 5), 151.5 mm. This may suggest that the overall run-on losses over the slope length were limited because of the relatively steep terrain and the crusted surface. Plots 1, 2 and 3 have identical land use (cultivated lands) and surface material (loess), differing only in slope. The greater runoff yield from Plot 3, compared with that of Plot 2, is consistent with the field experimental result that runoff yield increases with slope gradients (Luk et al., 1992). However, Plot 1 has a gentler slope but produced greater runoff yields than Plots 2 and 3 despite the lower number of runoff events, which will be discussed in detail later. The greater runoff yield from Plot 5 compared with Plots 1, 2 and 3 is a result of the steeper gradient and different land use Table 6 Runoff yield, runoff coefficient and peak discharge of basin outflow with the difference in runoff source areas on the hillslope over the period of 1963–1968. Values in parenthesis are the average Runoff source area
Runoff yield (mm)
Zone 4 Zones 3 and 4 Zones 2, 3 and 4 Zones 1, 2, 3 and 4
0.01–0.5 (0.18) 0.1–4.8 (1.32) 0.3–4.2 (1.52) 0.2–26.9 (4.49)
Runoff coefficients (%) 0.1–8.3 (2.4) 1.3–27.5 (9.2) 1.3–21.4 (12.3) 2.2–43.4 (13.6)
Peak discharge (m 3 s −1) 0.009–0.147 (0.051) 0.027–1.003 (0.374) 0.110–3.087 (0.996) 0.049–6.678 (1.188)
Zones 1 1 and upper 2 1 and 2 1, 2 and 3 3 3
Plot no. 1 2 3 4 5 6
Crop land Crop land Crop land Crop land + barren slope Barren slope Barren slope
Land use
Table 7 Description of field monitoring plots and runoff yields
4 13 17 29 36 29
Mean slope (8) NE NE NE NE NE NW
Slope aspect 40 70 105 185 90 50
Slope length (m) 200–400 600 1855 4167 1655 630
Horizontal area (m 2)
Loess Loess Loess Loess Loess Red earth
Surface materials
129.5 84.7 104.1 144.3 204.7 268.3
Runoff yield (1963–68) (mm)
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although the data cannot be used to evaluate the relative importance of these two factors. Plots 5 and 6 differ in slope gradient, slope aspect and surface material. The steeper gradient of Plot 5 should result in greater runoff but the higher runoff yield from Plot 6 indicates that the difference in surface material is the most important factor. Red earth has a considerably higher clay content and bulk density than loess (Table 1). Additionally, being a locally weathered material, loose red earth debris is only confined to the upper 20– 30 cm below which it becomes almost impermeable. As loess is a wind-borne material, changes of bulk density with depth are much smaller than red earth. All these features of red earth are favorable to runoff generation. Simple regression analysis was conducted between runoff yields on different zones and total rainfall amount or rainfall amount above a certain intensity (Table 8). The results once again clearly show that event rainfall amount is not the best index to predict spatial variation in runoff yields on the hillslope. Instead, the rainfall amount above a certain intensity is the better one. 4.3. Sprinkler experiments The field observation reveals the fundamental characteristics of rainstorms and the spatial variation in runoff generation on hillslopes in this area. The simple regression analyses also suggest some possible cause-and-effect relationships between rainfall and runoff variables. However, owing to the widely varied antecedent conditions and uncontrollable rainfall conditions, it is, more often, difficult to assess the relative significance of individual factors on runoff generation and its spatial variation. Moreover, regression analysis cannot indicate the actual mechanisms responsible for them. For this reason, it is important to get assistance from experiments. Figs 6 and 7 illustrate the infiltration processes on forest land, terrace land, cultivated slopeland and barren gully slope using portable and downspraying simulators, respectively. It can be seen that in both sets of experiments, runoff initiation time decreased in the order of forest land, terrace land, cultivated slopeland and barren gully slope with a range from more than 20 min to less than 1 min. Final infiltration rates were highest on forest lands and the difference between the two types of rainfall simulator experiments was very limited: 1.0–1.1 mm min −1 for the portable-sprinkler method and 0.8–0.9 mm min −1 for the downspraying method. For each rainfall simulator method, terrace land and cultivated slopeland had similar final infiltration rates but the final rates for the downspraying sprinkler (0.2–0.3 mm min −1) was much lower than for the portable-sprinkler (0.6–0.7 mm min −1). In both sets of experiments, final infiltration rates on barren gully slope were low and there was little difference between rainfall simulation methods. Portable and downspraying sprinklers produce similar rainfall intensities but the very low kinetic energy raindrops from the portable sprinkler do not breakdown soil aggregates and no, or very weak, crusts are developed on the surface even with long-duration and large-amount simulated rainfall. This was verified by visual observations. In contrast, the downspraying sprinkler produces raindrops comparable to natural rainstorms and for terrace land and cultivated slopeland, the large difference in final infiltration rates between the two methods is due to the formation of crusts when using the downspraying-sprinkler method. However, on forest lands, a thin layer of litter on the surface and relatively high
Zone 1 Zone 2 Zone 3 (red earth) Zone 3 (loess) Zones 1 and 2 Zones 1, 2 and 3 Subbasin
Plots of subbasin 0.622 0.288 0.381 0.278 0.261 0.473 0.457
Total rain amount (mm) 0.774 0.368 0.355 0.282 0.332 0.539 0.510
Rain amount with I . 0.1 mm min −1 0.642 0.42 0.587 0.485 0.535 0.617 0.704
Rain amount with I . 0.2 mm min −1 0.468 0.488 0.659 0.615 0.61 0.641 0.730
Rain amount with I . 0.3 mm min −1
Correlation analysis between runoff yields from various zones and rainfall variables over the period of 1963–1968
Table 8
0.176 0.466 0.660 0.643 0.477 0.548 0.599
Rain amount with I . 0.4 mm min −1
0.129 0.286 0.531 0.505 0.191 0.347 0.491
Rain amount with I . 0.5 mm min −1
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Fig. 6. Infiltration processes on different land use in the portable-sprinkler experiments with low kinetic energy of simulated rainfall.
Fig. 7. Infiltration processes on different land use in the downspraying-sprinkler experiments with high kinetic energy of simulated rainfall.
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Fig. 8. Effects of the crusts formed during the previous storms and the present storm on infiltration processes. Plot A: cultivated slopeland with crust formed by earlier treatment; Plot B: cultivated slopeland as Plot A but crust was broken before the experiment.
organic matter in the soil largely prohibit the development of crusts, and final infiltration rates are very high using both rainfall simulators. Low final infiltration rates for both methods on barren gully slopes are ascribed to the pre-existing crusts before experiments. The rates of infiltration for the experiments to determine the effects of crusts are shown in Fig. 8. At Plot B, runoff was initiated very quickly and the infiltration rates decreased very sharply with time. In contrast, Plot A had a longer runoff initiation time and gentler decrease in the infiltration rate over time. The infiltration rates became comparable near the end of the experiment on both plots, which suggests that new crusts were gradually formed on Plot A. With differing infiltration processes, the runoff yields from the 60-min experiments are 30.7 and 48.8 mm, for Plots A and B, respectively. 4.4. Effects of cultivation and plowing: evidence from field observation Cultivation and plowing activities occur only on Zone 1 and Zone 2 in Yangdaogou and break up pre-existing crusts. The cultivated land is plowed with animal-driven plows, mainly in May, before seeding. During the crop growth season, occasional hoeing may occur from time to time. However, because the owners of small land parcels have different habits, such activities are more or less random in both space and time. On gully slopes (Zone 3), crusts exist all the time since no cultivation activities occur there. Table 9 lists the runoff yields of the plots on different zones in the first runoff-generation storm in each of the 6 years of the monitoring period (1963–1968). For these storms, the
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Table 9 Runoff depth from the plots or basin in the first major runoff-generation storm of rainy season from 1963–1968 Year
1963 1964 1965 1966 1967 1968
Date
5/23 5/20 7/19 6/29 7/17 5/22
Rainfall (mm) Runoff depth (mm)
62.3 23.7 15.3 11.0 37.5 25.3
Plot 1
Plot 3
Plot 5
1.1 0.2 0 0 / 0
1.0 0.1 0 0 3.8 0.1
14.0 1.5 2.6 2.0 12.2 0.8
Subbasin 5.0 0.4 0.2 0.5 3.9 1.4
runoff yields from the cultivated lands (Plots 1, 3) were much smaller than those from the gully slope (Plot 5), typically a factor of 10 less, compared with a factor of 2 for the average over the 6 years. This early rainy season effect can be attributed to the plowing on the cultivated lands. After several rainfall events, the development of crusts on the cultivated lands increases the runoff yield and the difference in runoff yields between cultivated lands and gully slope decreases. Thereafter, occasional hoeing may exert smallscale and random effects on runoff generation. Another remarkable effect of cultivation is the formation of a plow pan on the cultivated lands in Zone 1 owing to the long history of cultivation. Although cultivated lands are also extensively distributed in Zone 2, no obvious plow pan is found. This is because cultivated lands in Zone 2 have steeper slope and downslope displacement of earth caused by plowing largely depresses the development of plow pans. Field investigation indicates that the plow pans are located at 15–30 cm below the surface and have a thickness of 10– 20 cm. The bulk densities of the surface soil layer, plow pan and subsoil layer are 1.13– 1.19, 1.23–1.35 and 1.19–1.25 g cm −3 (Cai et al., 1990), respectively. Portable sprinkler experiments indicate that the infiltration rate for plow pans is less than 0.1 mm min −1 (Li, 1991). This is even lower than the infiltrability of crusts. During rainstorms, when the infiltration wetting front reaches the plow pan, rainfall may gradually accumulate above it owing to the difference in infiltrabilities between surface layer and plow pan. Finally, the soils above the plow pans become saturated, which needs a rainfall amount of 80 mm or so (Li, 1991). Thus, the effects of plow pans on runoff generation are important only in storms with extremely large amounts of rainfall. Table 10 lists two large storm events occurring in the 6-year plot-monitoring period, with rainfall amounts of 80 mm and 95.5 mm, respectively. Owing to the relatively low intensities in these two storms, the runoff yields from Plot 1 (Zone 1) are not only higher than those from Plot 3 (Zone 2) but also than Plot 5 (Zone 3). This seems to be attributed to the primary effect of the plow pan in Plot 1. Simple regression analyses indicate that the best rainfall indicator to estimate the runoff yield on Zone 1 is rainfall amount with an intensity of over 0.1 mm min −1 (Table 8), which is also consistent with the infiltrability of plow pan. The fewer runoff events on Zone 1 than Zones 2 and 3 are due to the gentler slope and thereby larger rainfall detention capacities. However, owing to the small proportion of the total area, Zone 1 exerts a limited effect on the runoff yields in the whole hillslope and the basin.
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T.X. Zhu et al./Journal of Hydrology 196 (1997) 99–118 Table 10 Effects of plow pans on runoff yields on hillslopes and subbasin Date
7/6/1963 9/6/1964
Antecedent rainfall (mm) 9.5 14.7
Rainfall (mm) 95.5 80
Max. I 10 (mm min −1) 0.33 0.55
Runoff depth (mm) Plot 1
Plot 3
Plot 5
11.7 17.3
0.6 7.0
3.9 10.9
Subbasin 5.1 7.7
5. Conclusions 1. In this area, runoff generation is predominantly caused by infiltration-excess processes although some of the runoff may reach the channels via deep-seated tunnel systems (Zhu and Luk, 1997). Only a small proportion of rainfall events is able to generate runoff. However, runoff-generation and non-runoff-generation storms share a large range of rainfall amount. Thus, event rain amount cannot be used as a criterion to differentiate them. Instead, rainfall intensity is the appropriate indicator. 2. Both runoff source areas and runoff yields show spatial variations over the hillslope. The spatial variation in runoff generation on the hillslope is mainly ascribed to the difference in soil infiltrability. 3. Of all the factors affecting soil infiltrabilities in this area, crusting is the most important. Formation of crusts can reduce the infiltrability of loess soils from 0.6–0.7 mm min −1 to 0.2–0.3 mm min −1 or so. 4. Soil crustability can be significantly affected by land use. On forest lands, no or very weak crusts are developed owing to the existence of a litter layer on the surface and the high organic matter contents in the soil. On cultivated lands, plowing and hoeing can break pre-existing crusts and thereby reduce runoff. Appropriate land management also decreases soil crustabilities. Zeng et al. (1992) found runoff was reduced by 30% after 4-year manure input in comparison with the plots without manure input.
Acknowledgements This study could not have been completed without generous funding from a number of sources such as the Canadian International Development Agency, the Chinese Natural Science Foundation, the Water and Soil Conservation Bureau of Shanxi Province. We also wish to thank S.H. Luk, G. Li, S.J. Ma, Z.J. Jia, Z.G. Zhang, G.P. Wang, H. Hamilton and H. Taylor for their assistance in the field monitoring and experiments. Anonymous reviewers contributed significantly to the clarity and accuracy of the text.
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