Shallow groundwater plays an important role in enhancing irrigation water productivity in an arid area: The perspective from a regional agricultural hydrology simulation

Agricultural water productivity (WP) is an important indicator to evaluate the implementation of agricultural water saving in arid regions. However, the role of groundwater capillary rise to crop water use and WP is unclear at the regional scale, as the soil texture, irrigation amount, planting pattern and groundwater depth is various for different fields. Based on the calibrated Agricultural Water Productivity Management for Shallow Groundwater (AWPM-SG) model, a five-year regional WP and water budgets assessment was performed. The results showed that the groundwater contribution to crop evapotranspiration (ET) would be up to 65% with a groundwater depth of 1.0-1.5 m, but the agricultural productivity would be relatively low resulting from a waterlogged root zone. Additionally, deep groundwater could result in a reduced WP due to less capillary rise, while WP would be 2.02 and 1.98 kg/m3 with groundwater depth of 2.5-3.0 m and 3.0-4.5 m under irrigation amount of 100-300 mm. Furthermore, limited irrigation can enhance the contribution of groundwater to WP and irrigation water productivity (IWP), which is significant with groundwater depth increasing. While the average IWP were 5.83, 3.62, 2.54 and 1.77 kg/m3, respectively for irrigation amount of 100-300, 300-500, 500-700 and 700-900 mm and the average IWP decreased from 4.79 m to 3.18 kg/m3 with groundwater depth increasing 0.5-1.0 m to 3.0-4.5 m. However, irrigation effective utilization (Cieu) was affected by groundwater depth weakly with irrigation water increasing. Furthermore, the optimal groundwater depth of 2.5-3.0 m was obtained by the impact of groundwater on irrigation water productivity (IWP) and Cieu. Thus at the regional scale, the spatial distribution of groundwater levels needs to be considered for making irrigation decisions.