Growth responses of crops and natural vegetation to irrigation and water table changes in an agro-ecosystem of Hetao, upper Yellow River basin: Scenario analysis on maize, sunflower, watermelon and tamarisk

Due to water scarcity, water saving practices (WSPs) are being implemented in the irrigation districts of the upper Yellow River basin, causing a series of consequences, e.g. the reduction of field irrigation amount and groundwater depth (GWD) increase. The impacts of irrigation water reduction and GWD increase in canal systems with heterogeneous land covers is complex and remains unclear, thus the need to be elucidated to efficiently promote WSPs. Based on the two-year field observations in 2012 and 2013 and former studies, the HYDRUS-dualKc model, which has been well calibrated and validated in a typical canal system in Hetao, was used to predict the changes in transpiration, evaporation and salt accumulation considering scenarios with various GWDs and irrigation strategies. Results showed that under present irrigation scheduling, plant transpiration of maize, watermelon, tamarisk and sunflower had an initial slight increase, then reached a peak value, and finally decreased in the two years with GWD increase, while soil evaporation and salt accumulation both declined continuously. Based on integration results from the three crops and the natural vegetation simulation scenarios, an optimum GWD of between 1.7-2.3 m for the crop fields and 1.4-2.0 m for the natural land was recommended. With optimum GWD, 15%-30% of water diversion from the Yellow River during the growing season can be saved each year. When considering the irrigation strategies, salt accumulation increased and crop transpiration declined with irrigation reduction, yet the impact on soil evaporation was limited. It also showed that only sunflower had the potential to reduce irrigation amount (about 20%). Future water saving should pay more attention to the water delivery process and the optimization of irrigation during the non-growing season. An additional irrigation to watermelon during its development stage and a pre-season irrigation to tamarisk can largely improve their growth, and the recommended irrigation depths were 100 mm and 250 mm, respectively. These results can be reference criterion for the further implementation of WSPs.