A semi-distributed drainage model for monthly drainage water and salinity simulation in a large irrigation district in arid region

The complex and dense irrigation and drainage networks and diverse irrigation scheduling result in uncertainties and complexities in the simulation of drainage water and salinity at different regional scales. We propose a semidistributed model to simulate monthly drainage water and salinity, considering both the surface return flow and groundwater drainage to drainage ditches. The model uses sub-drainage command areas (SDCAs) controlled by main irrigation canals and drainage ditches as simulation units and uses a conceptual model to estimate monthly drainage water and salinity from each SDCA. The proposed model is applied in the Hetao Irrigation District (HID) in North China with 19 SDCAs, and calibrated and validated using monitoring data during 2008-2010 and 2012-2013, respectively. The results reveal better model performance in estimating the drainage water and salinity at SDCA, sub-irrigation district and irrigation district scales. The average annual drainage is 47,021 * 10(4)m(3) in HID, accounting for 10.5 % of the annual irrigation water amount. The surface return flow and groundwater drainage account for 32.7 % and 67.3 % of total drainage, respectively. The ratio of surface return flow to groundwater drainage is higher in the upstream area and lower in the downstream area. Higher surface return flow ratio ( > 5 %) is observed in the western Wulanbuhe, middle and southern Jiefangzha, and northern Wulate sub-irrigation areas, indicating the necessity to improve irrigation water management in these areas. Lower groundwater drainage capability is observed in western and southern HID, while enhanced groundwater drainage capability is observed in northern HID. Average annual salt leaching through the drainage system is 126.5 * 10(4) t, which is only 43 % of the total salt introduced from irrigation. The Wulate sub-irrigation district performs best on salt leaching, while other sub-irrigation district should improve the drainage system and irrigation water use efficiency to control the salt accumulation.