Impact of water table depth and shallow groundwater use on salt leaching cycle in irrigated areas

Water table management in irrigated areas may increase shallow groundwater use by crops. But the high salt content of groundwater results in faster buildup of salinity in crop root zone,which in turn affects leaching schedule of the irrigation districts. Based on general salt and water balance in crop fields in irrigated areas, a simplified model was proposed in this paper to calculate leaching cycle for crops that use shallow groundwater at different water table depth, considering the salt accumulation process in root zone of crops. Subsequently,leaching cycles were calculated for two study sites with soil salinity measurements. For the case study in a semi-arid irrigation area,under the current irrigation scheduling and the average rainfall condition,the calculated leaching cycle for cotton fields is 100 days for water table depth at 1 m and 140 days for water table depth at 1.5 m with the groundwater salinity maintained at 4.43 g/L;when the water table depth is greater than 2 m,the calculated leaching cycle is longer than the growing period. In another case study in an arid irrigation area with saline groundwater buried at 1.5 m deep,the calculated leaching cycle for plastic mulched and drip irrigated cotton is 78 days,applying slightly saline irrigation water with salinity of 2.81 g/L. These results indicate that managing water table depth under controlled drainage practice may increase crop use of shallow groundwater that contains dissolved salts, the rate of salinity buildup, however, is relatively slow, leaving a time window for making proper leaching schedule. The results from this study provide theoretical reference for salinity management in irrigated agricultural regions that adopt controlled drainage technique.