Computational model of pond re-storage times in karst valley area of Chongqing based on DEM and SCS models

Two typical karst valley areas in Zhongliang and Yunfeng town of Jiu Longpo district, Chongqing are selected as the study area which is located in the middle and east sides of Zhongliangshan Mountains. The study area, with an area of 532.3 hm2 and an elevation ranging from 300 to 700 m above mean sea level, lies between the latitudes of 106°23′49″ to 106°25′06″E and the longitudes of 29°26′20″ to 29°29′47″N. The area falls in humid subtropical monsoon climate region, with an average annual precipitation of about 1,027.3 mm and an average annual temperature of 17.3 ℃. Landform types of the area are karst valley and a typical anticline low mountain, where the soil types include yellow limestone soil, dark purple clay and ore sub soil; and land use patterns are dominated by forest land and upland. Hydrologically, as no main stream and tributary run through the area, pond facilities which impound rainwater have become main sources for irrigation in this area. In fact the use of ponding water has been of significance in small scale water supply and to solve the problem of water shortage in southwest China karst region. The number of pond rainwater re-storage is an important parameter in determining the quantity of water supply. Through the analysis of current problem of irrigation systems and the computation of the number of repeated pond water impoundment, it attempts to provide a scientific guidance for future planning of ponding water system in southwest China karst region. In order to predict the repeated impoundment number, SCS model was used to simulate rainfall runoff and crop water requirement, the latter of which was computed by CROPWAT8.0 in this study. Then a water balance analysis of the pond irrigation system was performed and consequently repeated impoundment number of the ponding system was sorted out. The results showed that, (1) Calculated runoff coefficient of the study area by the SCS model was from 0.16 to 0.27, which suggested that the runoff simulated by the model was reasonable; (2) In karst valley area, it is much better to use a catchment as an analytical unit than an administrative area to calculate water supply of the pond facilities; (3) In the low flow year, rainwater can be most efficiently used by the ponds in the karst valleys with a minimum rate of surplus water as 50.98%; (4) The study area of pond re-storage times were 2.51, 2.60, 2.12 in correspondence to the rainfall frequency of 50%, 75%, 95%, respectively.