Application of SCS Model in Estimation of Runoff from Small Watershed in Loess Plateau of China

Chin. Geogra. Sci. 2008 18(3) 235 241 DOI: 10.1007/s11769-008-0235-x www.springerlink.com Application of SCS Model in Estimation of Runoff from Small Watershed in Loess Plateau of China (College of Geography and Planning, Ludong University, Yantai 264025, China) Abstract: Soil Conservation Service (SCS) model, developed by U. S. Soil Conservation Service in 1972, has been widely applied in the estimation of runoff from an small watershed. In this paper, based on the remote sensing geo-information data of land use and soil classification all obtained from Landsat images in 1996 and 1997 and conventional data of hydrology and meteorology, the SCS model was investigated for simulating the surface runoff for single rainstorm in Wangdonggou watershed, a typical small watershed in the Loess Plateau, located in Changwu County of Shaanxi Province of China. Wangdonggou watershed was compartmentalized into 28 sub-units according to natural draining division,and the table of curve number (CN) values fitting for Wangdonggou watershed was also presented. During the flood period from 1996 to 1997, the hydrograph of calculated runoff process using the SCS model and the hydrograph of observed runoff process coincided very well in height as well as shape, and the model was of high precision above 75%. It is indicated that the SCS model is legitimate and can be successfully used to simulate the runoff generation and the runoff process of typical small watershed based on the remote sensing geo-information in the Loess Plateau. Keywords: runoff simulation; small watershed; SCS model; Loess Plateau 1 Introduction A watershed is a physiographic or hydrologic-ecological unit composed of interrelated parts and functions. With the social and economic development, human activities and land use change have dramatically affected the watershed runoff generation and flow paths (Xia et al., 2005). The changes in runoff characteristics induced by intensive human activities are important to well understand the effects of land use/cover change (LUCC) on the hydrological processes of watershed surface (Liu et al., 2004; Zepp et al., 2005). Therefore, there has been a growing need to quantify the impacts of land use changes on hydrology for small watershed management from the standpoint of anticipating and minimizing potential environmental impacts. In the Loess Plateau of China, the accurate information on runoff was scarcely available in most small watersheds because the traditional hydrological models used in practice could not represent the distributed nature of watershed hydrological properties like soil type, slope and land use (Hellweger and Maidment, 1999). Moreover, it was impossible to input spatial characterization data (e.g. land cover type, terrain slope) of the underlying surface concerning parameters relevant to runoff formation directly to most traditional hydrologic models owing to the incompatibilities between the traditional hydrological model structure and characteristics of remote sensing data (Gert, 19; Greene and Cruise, 1996; Qi, 2001). Thus, the hydrological model based on GIS or remote sensing may become an inevitable requirement for the precise simulation of runoff. With the development of GIS and remote sensing techniques, the growing useful information on spatial data is provided. Hence the hydrological watershed models have been more physically based and distributed to enumerate various interactive hydrological processes considering spatial heterogeneity. The purpose of this study is to use the Soil Conservation Service (SCS) model, which fully considers physiographic heterogeneity (e.g. topography, soil, and land use), to simulate the rainfall-runoff relationship of small watershed in the Loess Plateau of China. The model will Received date: 2007-10-08; accepted date: 2008-03-04 Foundation item: Under the auspices of National Natural Science Foundation of China (No. 40101005) Corresponding author: LIU Xianzhao. E-mail: xianzhaoliu@sina.com

236 also serve as an efficient tool for land use management of typical watershed in the Loess Plateau for the 21st century. Finally, the SCS model was validated by comparing simulated runoff with the measured runoff of few selected events in flood season in the period of 1996 1997. 2 Materials and Method 2.1 Study area Wangdonggou watershed (35º12 35º16 N, 107º40 107º 42 E), situated at the west of Changwu County of Shaanxi Province, was selected as the study area, because it is one of the main control small watersheds in the Loess Plateau of China. It has a total area of 0ha with an elevation of 940 1226m a.s.l. The average annual precipitation is 5mm, more than 70% of which occurs during the months from June to September. The maximum and minimum temperature is 32 and 15.3 respectively. The mean relative humidity varies from a minimum of 48% in April to a maximum of % in September. The climate of the area can be classified as a semiarid, warm temperature climate. The groundwater table is about 50 80m below the surface. The soil is mainly loam occupying the area with a slope varying from 0 o to 15 o, even up to 45 o. Major physical properties of the main soils are given in Table 1. This small watershed comprises a mosaic of ecosystem with 12 land use patterns. The average annual erosion module is 677.8t/(km 2 yr). Runoff observation station was built at the outlet of the watershed. In the recent years, with economic unprecedented development, the effect of human activities on runoff generation in Wangdonggou watershed has been rising rapidly. Table 1 Particle composition and hydrologic properties of soils in study area Soil type Particle composition (%) Saturated soil moisture Field capacity Wilting point Bulk density >0.05mm 0.05 0.005mm <0.005mm (g/kg) (g/kg) (g/kg) (g/cm) Dark loess soil 3.5 65.6 30.9 4.3 255.3 100.0 1.21 Loessial soil 6.0 61.9 32.1 4.7 297.1 122.4 1.35 Note: All data are mean values of 0 60cm soil layer 2.2 Data sources In this study, a variety of data including conventional land use map (Li and Su, 1996), soil map (Li and Su, 1996), standard 1.. 5000 topographic map (Shaanxi Province Civil Administration Bureau, 1971) and hydrometeorological data obtained from various sources have been used as data sources. Boundaries of different land uses were digitized in Arc/Info. The original 12 land use types were grouped into five types, i.e., farmland, forestland, grassland, orchard and residential area. Digital elevation model (DEM) derived from 1.. 5000 topographic map of the watershed. Digital soil map of the watershed was traced, scanned and rectified in Erdas Imagine 8.7 through using the 1.. 5000 topographic map. Meteorological data including daily and monthly rainfall for the period of 1958 2000 were collected from local meteorological station. The daily runoff volume and runoff process of some typical rainstorms (1996 1997) were obtained from hydrology station located at the outlet of the watershed. 2.3 SCS model and method SCS model, developed by U.S. Soil Conservation Service in 1972, also known as the Hydrologic Soil Cover Complex Model, has been widely used internationally for water resources management, urban storm water modeling and runoff estimation (Hawkins, 1993; Greene and Cruise, 1995; Mishra et al., 2005; Tsihrintzis and Hamid, 1997; Lewis et al., 2000; Sharma et al., 2001; Chandrmohan and Durbude, 2001; Sharma and Kumar, 2002; He, 2003) because of its versatility. It has also been introduced and applied by some scholars in China because it gives consistently useful results (Pan, 1996; Liu et al., 2005; Jin et al., 2003). Because major input parameters for the SCS model are land use, soil type, terrain slopes, etc., the model is potentially compatible with remote sensing input and can reflect the impact of human activities on runoff yield. The SCS model can be expressed as: 2 ( P 0.2 S), P 0.2S Q= P+ 0.8S (1) 0, P < 0.2S where Q is runoff depth (mm), P is storm rainfall (mm), and S is potential maximum retention or infiltration (mm). For convenience and standardization application of Equation (1), S is expressed in the form of a dimensionless runoff curve number (CN):

Application of SCS Model in Estimation of Runoff from Small Watershed in Loess Plateau of China 237 25400 S = 254 CN (2) where CN represents the runoff potential of the land cover-soil complex characteristics governed by soil antecedent moisture condition (AMC), soil type, and land use and treatment. Three AMCs were defined as dry, moderate and wet, and denoted as AMC Ⅰ, AMC Ⅱ, and AMC Ⅲ, respectively. CN values range from 0 to 100. In order to quantitatively differentiate and reflect the spatial unevenness information on the watershed underlying surface conditions by the CN value in the SCS model, the prime remote sensing data source DEM (Fig. 1) derived from topographic map on the scale 1.. 5000 was used to extract physical characteristics of slope (Fig. 2) and sub-divided basin areas by hydrological GIS tools. From different data sources such as topography, soil and land use, 28 ecological hydrology subunits (EHSUs), which were homogeneous areas with the same land use and pedo-topo-ecological conditions controlling their unique hydrological dynamics, were derived as described in Fig. 3. The land use map (Fig. 4) and the soil map (Fig. 5) were superimposed by using Arc/Info based topographic map. The areas of five land use patterns (Table 2) and soil combinations were obtained in the attribute selection menu by using logical expression. Appropriate different CN values according to standard tables (United States Department of Agriculture, Soil Conservation Service, 1972) were assigned to each EHSU considering AMC. The maximum potential reten- tion was calculated through Equation (2). Then the direct runoff values from each EHSU were estimated using SCS model for rainfall events. The cumulative runoff from the watershed outlet was calculated by Muskingum model (Hassanuzzaman, 1993). The estimated runoff was then validated by comparing it with observed runoff for that period. 3 Results 3.1 Determination of CN value We can know from the above analysis that the calculation of runoff generation in the SCS model mainly relied on CN values, which was a function of AMC, slope, soil type and land use. The CN value reflected the runoff potential. Under the same precipitation condition, low CN values mean that the surface has a high potential to retain water. While high values mean that the rainfall can be stored by the land surface only to a small extent. Therefore areas with high CN values produced a large amount of direct runoff and thereby contributed strongly to the flood peak. In the SCS model, the CN value was influenced by the AMC derived from rainfall measurements of the preceding five days. The CN value for AMC Ⅱ condition can be converted into CN values for AMC Ⅰ and AMC Ⅲ conditions by using available conversion factors as suggested by Delmar et al. (2005). Fig. 1 DEM of Wangdonggou watershed Fig. 2 Surface slope of Wangdonggou watershed

238 Fig. 3 Ecological hydrology subunit (EHSU) of Wangdonggou watershed Fig. 4 Land use map of Wangdonggou watershed Classified Landsat images integrated Geographic Information System with soil map applied the curve number tables (Table 3 and Table 4) provided by Soil Conservation Service to calculate curve numbers of each EHSU. To estimate curve numbers of EHSU, vector coverage of soil map, and land cover and land use classification were overlain each other. The soil layer and classified image were both converted to a 10-m grid cell in order to calculate curve number value. The CN value (Table 5) fitting for different AMCs and different land uses in Wangdonggou watershed was assigned. Curve numbers of the each EHSU (Table 6) were calculated by means of chapter 4 guidelines in National Engineering Handbook (NEH4) (United States Department of Agriculture, Soil Conservation Service, 1972). Land use Fig. 5 Soil map of Wangdonggou watershed Table 2 Land use/cover condition in studied watershed Area (ha) Proportion to total area (%) Main plant species Farmland 353.0 42.5 Triticum aestivum Forestland 246.9 29.7 Robinia pseudoacacia Grassland 56.2 6.8 Erigeron annuus, Stipa glareosa Orchard 117.7 14.2 Crataegus pinnatifida, Malus pumila Residential area 56.2 6.8 3.2 Model verification In order to validate the SCS model output, daily rainfall data of seven selected events in 1996 and 1997 in Wangdonggou watershed were collected and the curve number of the watershed was used for the estimation of runoff and runoff process. The validity and feasibility of the SCS model based on the geographic information was verified by comparing the estimated runoff with measured values. The results show that the hydrographs of the estimated runoff process and the observed runoff process coincided very well in height as well as shape

Application of SCS Model in Estimation of Runoff from Small Watershed in Loess Plateau of China 239 Table 3 CN values of different soil types and land use types under medium soil moisture Land surface infiltration category Land use type A B C D Wasteland 76 94 94 Tuber crops 70 80 Grapery 64 73 Cereal and feedstuff crops 64 76 Exuberantly natural grassland 49 69 Sparsely natural grassland 68 79 Planted grassland 30 58 Exuberant forestland 25 55 77 77 Common forestland 36 60 79 79 Sparse forestland 45 66 Hard surface (cement surface) 100 100 100 100 Notes: A, B, C, and D are four hydrologic soil groups, which are used to determine curve number. Soils in group A have well drained, in group B moderately-drained, in group C poorly drained, and in group D very poorly drained Source: United States Department of Agriculture, Soil Conservation Service, 1972 Table 4 Types of antecedent soil moisture condition for SCS model Antecedent moisture condition Plant growth stage Other Five-day antecedent rainfall (mm) stage AMCⅠ(Dry) < 30 < 15 AMC Ⅱ (Moderate) 30 50 15 30 AMC Ⅲ (Wet) > 50 > 30 (Fig. 6), which indicates that the SCS model used in Wanggonggou watershed on the Loess Plateau has high precision and practicability. The absolute errors between the estimated runoff and observed runoff range from 0.06mm to 0.96mm and the relative errors were from 6.68% to 23.34%, which were within the permissible limit (Table 7). The simulated results of the SCS model were soundly consistent with the actual situation. Thus, the SCS model could be applied to simulating the runoff process of small watershed in the Loess Plateau. Table 5 CN values of different land use types in Wangdonggou watershed Antecedent moisture condition Farmland Forestland Grassland Orchard Residential area AMC Ⅰ (Dry) AMC Ⅱ (Moderate) AMC Ⅲ (Wet) 76 79 93 96 Table 6 CN value of each ecological hydrology subunit (EHSU) in Wangdonggou watershed EHSU number AMCⅠ AMC Ⅱ AMC Ⅲ EHSU number AMCⅠ AMC Ⅱ AMC Ⅲ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 64 81 77 79 67 81 93 95 93 71 14 15 16 17 18 19 20 21 22 23 24 25 26 27 77 80 80 79 71 81 94 Table 7 Comparison between measured and simulated values of runoff yield by SCS model in Wangdonggou watershed Runoff volume (mm) Relative error Absolute error Date Rainfall depth (mm) Observed Estimated (%) (mm) 1996-07-27 22.64 2.700 2.0 6.68 0.18 1996-07-29 32.20 3.001 2.647 11.80 0.35 1996-07-31 18.10 0.2 0.349 20.76 0.06 1996-09-14 23.72 2.279 2.5 13.56 0.31 1996-09-17 37.53 4.935 5.8 19.37 0.96 1997-07-28 19.40 0.1 0.675 22.50 0.20 1997-08-06 31.80 1.101 1.358 23.34 0.26

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