Probability Analysis of Carryover Storage to Evaluate Drought Response Capacity of Agricultural Reservoirs in Paddy Irrigation.

Abstract. Agricultural reservoirs are principal water resources for paddy field irrigation in South Korea. In early season of paddy irrigation, a large amount of water is required for land preparation for puddling and rice transplanting, and the water requirement is supplied from agricultural reservoirs. Therefore, carryover storage at the beginning of irrigation season is an important criterion for successful irrigation water supply to overcome agricultural drought, because it is crucial for rice transplanting cultivation in early rice growing stage. The aim of this study is to analyze probability of carryover storage of agricultural reservoirs to evaluate drought response ability at the first day of irrigation season. The first day of irrigation and non-irrigation season was assumed as 1st April and 1st October respectively. To analyze the probability of reservoir storage on 1st April, reservoir storage rates on 1st October, the first day of non-irrigation season, were assumed to fifty, sixty, seventy and eighty percent of the full storage as the initial conditions. Tank model was chosen as rainfall-runoff model to predict amount of runoff from a watershed, because carryover storage is changed from runoff during non-irrigated season. As input data for tank model operation, precipitation data during non-irrigated season were collected for 30 years from 1985 to 2014, and ratios of area of forest, paddy, and upland to watershed were calculated with GIS tools. With initial condition and runoff amount, frequency analysis was conducted in order to analyze probability of filling 90 percentage of reservoir storage at the first day of irrigation season. The result of this study is expected to be used as reference for on and off-season reservoir storage management considering the target storage of the next irrigation season.