An analytical model for studying the effect of rainfall on salt leaching under controlled drainage in subhumid irrigation areas

Properly raise water table with controlled drainage may increase shallow groundwater which canbe used by crops,mitigating the water shortage in subhumid irrigated areas. However,salinity in groundwa-ter over there is generally higher,and increasing use of shallow groundwater may speed up salt accumula-tion in the root zone and shorten the salt leaching cycle;this is not desired in these areas because theleaching water can hardly be ensured during the required period. Considering in the subhumid irrigation ar-eas there are certain amount of rainfall that may produce leaching effect for salinity control,an analyticalmodel was developed for calculating salt leaching cycle under controlled drainage. The model was testedsubsequently through a case study in a subhumid irrigation area in which soil was reclaimed from saliniza-tion. With the daily water balance predicted by the field hydrology model-DRAINMOD,we calculated thesalt accumulation in the root zone over a long term period(1951-2005) under controlled drainage and dif-ferent supplemental irrigation depths. The results show that when groundwater depth is controlled at 1.2 m,rainfall has some effect on salt leaching even without irrigation;but soil salinity grows with time in theroot zone. When the supplementary irrigation depth is greater than 150 mm,rainfall can effectively controlthe root zone salinity to reach a favorable salt balance in the case study area. Findings from this researchindicate that in subhumid irrigation areas, considering the leaching effect of rainfall, proper water tablemanagement with controlled drainage can effectively control soil salinity for crop production and improve wa-ter use efficiency.