Quantifying water use and groundwater recharge under flood irrigation in an arid oasis of northwestern China

Accurate knowledge of evapotranspiration (ET) and irrigation return flow is critical for the sustainable management of water resources in arid inland river basins. Flood irrigation with large amounts of groundwater and resulting return flow may be not suitable for cropland with long cultivation history. In the midstream of the Heihe River Basin of northwestern China, flood-irrigated cropland with rotation of spring wheat and spring maize from 2005 to 2014 was selected as study area. Deep percolation fluxes below the 1.5 m, 3.0 m and 5.0 m layers were determined based on soil water balance, for which, soil evaporation, transpiration and soil water content (SWC) dynamics during the growing seasons were calibrated and validated by the HYDRUS-1D simulation. Mean annual soil evaporation and transpiration were 64.11 mm and 352.36 mm for spring wheat and 84.03 mm and 447.43 mm for spring maize, respectively. Large amounts of water discharged within the first few days after irrigation from the second to the last irrigation events. Deep percolation below the 1.5 m soil were 841.87 mm and 973.73 mm, occupying 67.10 % and 68.00 % of applied water during the growing seasons of spring wheat and spring maize, respectively. About 108.86 mm and 70.72 mm water percolated from the 0–1.5 m soil were stored in the 1.5–3.0 m layer, respectively. Groundwater recharge (the amount of deep percolation below 5.0 m) amounted to 734.03 mm and 1002.10 mm, occupying 58.86 % and 70.20 % of applied irrigation during the growing seasons of spring wheat and spring maize, respectively. Mean water productivity were 4.24 kg ha−1 mm−1 for spring wheat and 5.86 kg ha−1 mm−1 for spring maize. The impact of decreasing irrigation amount on deep percolation was assessed by assuming 80 % (80 % I), 60 % (60 % I), 50 % (50 % I) and 40 % (40 % I) of current irrigation quota (I) of each event throughout the growing seasons from 2005 to 2014. Even the 40 % I treatment would satisfy water demand of spring wheat and spring maize, and deep percolation would decrease remarkably. The percentage of percolated water from the 0–1.5 m soil to irrigation would decrease on average by 8.31 %, 22.43 %, 33.88 % and 49.78 % under the treatments of 80 % I, 60 % I, 50 % I and 40 % I, respectively. Groundwater recharge would occupy 48.90 %, 34.20 %, 24.45 % and 21.79 % of irrigated water for spring wheat and 62.32 %, 50.21 %, 42.68 % and 40.44 % of irrigated water for spring maize, respectively. The 50 % I treatment would improve water productivity by at least 2.52 kg ha-1 mm-1 for spring wheat and 5.26 kg ha-1 mm-1 for spring maize. The hypothesized irrigation schedule with current frequency and 50 % quota of each event is promising to improve crop water utilization and protect groundwater resources. Optimized irrigation strategy for irrigated agriculture in the midstream will aid the comprehensive management of water resources at a watershed scale in the arid inland river basins.