Impact of agricultural water-saving practices on regional evapotranspiration: The role of groundwater in sustainable agriculture in arid and semi-arid areas

Evapotranspiration (ET) is an important component of the water budget process and is characterized by complex spatiotemporal changes, especially in irrigated agricultural areas. The impact of various hydrological processes and human activities on ET is still a meaty theme to study and investigate. A typical agricultural irrigation district with shallow groundwater and arid climate conditions was selected as the case study area in this work. The impact of the supplied irrigation water, shallow groundwater, crop planting pattern, and weather conditions on regional ET was determined after the regional ET was estimated by a Surface Energy Balance Algorithm of Land (SEBAL) model with Moderate Resolution Imaging Spectroratiometer (MODIS) data. The results show that the regional ET in Hetao kept declining in the past 15 years. The positive correlation between the water input (water diversion and precipitation) and ET indicated that reduced water diversion controls the declining ET, also causing the drop of groundwater level. Due to capillary forces and root uptake, the shallow groundwater tended to move upward to support the crop water consumption because the soil suffered from a water deficit. Furthermore, we quantified the contribution of shallow groundwater to regional ET and found that the water supplied from shallow groundwater increased from 5% to 15% during the period of water-saving irrigation. However, the long-term decrease of irrigation water supply and groundwater level caused a soil water deficit over the crop growth period, and the variation of crop planting pattern reduced ET as well. Therefore, groundwater plays an important role in sustainable agricultural development in arid and semiarid areas and the contribution of shallow groundwater to regional water consumption cannot be neglected.