Analysis of the contribution of groundwater to evapotranspiration in an arid irrigation district with shallow water table

Abstract

Groundwater from the shallow aquifers can supply substantial water for evapotranspiration of crops and other vegetation (the groundwater supported evapotranspiration). Regional scale evapotranspiration and the groundwater contribution to evapotranspiration were estimated by the soil water balance equation in Jiefangzha sub-district (JFZSD), a sub-district with shallow aquifers in Inner Mongolia, China. Estimates used an 8-year (2006–2013) hydrological dataset including soil moisture, the depth to water table, irrigation amounts, rainfall data, and drainage water flow. Soil moisture and the groundwater depth varied substantially with irrigation, the fluctuations in rainfall, and the stage of crop growth. The soil moisture with very shallow groundwater depth was greater than it was with groundwater at greater depth. The 8-year mean evapotranspiration was estimated to be 664mm, the mean section evapotranspiration were ranged from 538mm to 899mm during the crop growth period in JFZSD. The mean groundwater supported evapotranspiration (ETg) was estimated to be 228mm, with variation from 145mm to 412mm during the crop growth period in JFZSD. Analysis of the positive correlation between evapotranspiration and the sum of irrigation and rainfall, and the analysis of the negative correlation between ETg/ET and the sum of irrigation and rainfall, reflect the need of groundwater to meet the evapotranspiration demand. Approximately 20% to 40% of the evapotranspiration is from the shallow aquifers in JFZSD. With the policy of decreasing the amount of Yellow River water diverted to the HID in the future, the more groundwater would be used. However, the local groundwater in return depends on irrigation. Besides, the fraction of drainage to irrigation is already low, suggesting that future decreases in water deliveries will lead to soil salinization and loss of productivity in the sub district. These results could be useful for the government to understand the significant role of the groundwater and make reasonable water use policy in the semiarid agricultural regions.