Web GIS-based simulation of water fluxes in the Miyun catchment area

Abstract

Change of land use and agricultural management, combined with severe droughts during the last two decades, are the main reasons for a strong decrease of water quality and quantity in the Miyun reservoir (China). Due to semi-arid monsoon climate, episodic water fluxes have to be considered adequately in an integrated water resources management in the total catchment area. To get insight into runoff generation processes, sediment and nutrient source areas and transfer dynamics, a monitoring approach was established in the Miyun catchment area spanning from plot measurements with weighable gravitation lysimeters and in situ measurements to a subcatchment scale of approx. 10 km 2 . One of these subcatchments (Sheyuchuan) was selected to adapt the methodologies to the regional runoff pattern using the web GIS-based modeling tool STOFFBILANZ. Calibration of the daily runoff modeling is based on the lysimeter observations from Shixia station and runoff observations in Sheyuchuan subcatchment for the reference year 2011. Despite the fact that the climatic water balance in the region is negative, the precipitation event on 24 July 2011 with a rainfall of 107 mm caused a significant leachate at the lysimeter. The daily modeling for the lysimeter core by the FAO-ETc-adj approach showed a similar deep percolation and a similar distribution of evapotranspiration for the year considered. In the next step, the calibrated model was applied to the subcatchment Sheyuchuan to compare the results of direct runoff and groundwater runoff (deep percolation) with the runoff observations at the weir. Deep percolation was extremely small (1.7 mm yr -1 ), and direct runoff was 26.9 mm yr -1 in 2011. In comparison to the monitoring results at the weir, a disagreement in the quantity of runoff was found between the observation on the one hand and the simulation on the other hand, which can be explained by water storage in reservoirs for irrigation purposes. Additionally, the water fluxes in the total catchment area were simulated at meso-scale, obtaining a good correlation with observed values.Keywords: Bottom-up, Direct Runoff, Lysimeter, Meso-Scale, Miyun, Model, Percolation a grid with variable cell size (125 × 125 m up to 500 × 500 m; approx. 366 000 grid cells) by using a QUADTREE procedure (Fig. 1). Cropland as well as residential areas got the highest resolution in order to optimize the indication of critical source areas.The major part of the Miyun catchment area is mountainous with steep inclinations with elevation ranging from 60 to 2200 m a.s.l. Annual average precipitation in the sub-humid continental monsoon climate is lower than 500 mm yr -1 and occurs mainly in summer, whereas winter and spring are extremely dry (Chen et al. 2010). Precipitation increases from east to west, annual average temperature increases from south-east to north-west, ranging from 6.3 to 10.9 °C. Land cover is dominated by rocky mountain forest (54.8%) with a smooth transition to shrubbery, heath and dry grassland (31.5%). Cropland (10.7%) can be foun...

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