The adjustment of crop planting structure can change the process of water and material circulation, and thus affect the total amount of water and evapotranspiration in the irrigation district. To guide the allocation of water resources in the region, it is beneficial to ascertain the effects of changing the crop planting structure on water saving and farmland water productivity in the irrigation district. This paper takes Yingke Irrigation District as the background. According to the continuous observation data from 2012 to 2013, Based on the modified Soil and Water Assessment Tool (SWAT) model and taking advantage of monthly scale remote sensing EvapoTranspiration (ET) and crop growth parameters (leaf area index and shoot dry matter), we tested the simulation accuracy of the model, proposed irrigation efficiency calculation methods considering water drainage, and established the scenario analysis method for the spatial distribution of crop planting structure. Finally, we evaluated the changes in water savings in irrigation district projects and resources, the irrigation water productivity and the net income water productivity under different planting structure scenarios. The results indicate that the efficiency of irrigation has increased by 15~20%, while considering drainage, as compared with conventional irrigation efficiency. Additionally, the adjustment of crop planting structure can reduce regional evapotranspiration by 14.9%, reduce the regional irrigation volume by 30%, and increase the net income of each regional water area by 16%.

Spatiotemporal Variability of Potential Evaporation in Heihe River Basin Influenced by Irrigation Congying Han1,2, Baozhong Zhang1,2, Songjun Han1,21 State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038, China.2 National Center of Efficient Irrigation Engineering and Technology Research-Beijing, Beijing 100048, China.Corresponding author: Baozhong Zhang (zhangbaozhong333@163.com) Potential evaporation is a key factor in crop water requirement estimation and agricultural water resource planning. The spatial pattern and temporal changes of potential evaporation calculated by Penman equation (EPen) (1970-2017) in Heihe River Basin (HRB), Northwest China were evaluated by using data from 10 meteorological stations, with a serious consideration of the influences of irrigation development. Results indicated that the spatial pattern of annual EPen in HRB was significantly different, among which the EPen of agricultural sites (average between 1154 mm and 1333 mm) was significantly higher than that of natural sites (average between 794 mm and 899 mm). Besides, the coefficient of spatial variation of the aerodynamic term (Eaero) was 0.4, while that of the radiation term (Erad) was 0.09. The agricultural irrigation water withdrawal increased annually before 2000, but decreased significantly after 2000 which was influenced by the agricultural development and the water policy. Coincidentally, the annual variation of Epen in agricultural sites decreased at -40 mm/decade in 1970-2000 but increased at 60 mm/decade in 2001-2017, while that in natural sites with little influence of irrigation, only decreased at -0.5mm/decade in 1970-2000 but increased at 11 mm/decade in 2001-2017. So it was obvious that irrigation influenced Epen significantly and the change of Epen was mainly caused by the aerodynamic term. The analysis of the main meteorological factors that affect Epen showed that wind speed had the greatest impact on Epen of agricultural sites, followed by relative humidity and average temperature, while the meteorological factors that had the greatest impact on Epen of natural sites were maximum temperature, followed by wind speed and relative humidity.

Irrigation schedule based on actual production conditions and dynamic irrigation schedule based on crop water demand theory exhibit significant differences in irrigation amount and frequency. Under the actual irrigation schedule, the surface temperature in the oasis of HRB decreases by an average of approximately 1-2.5°C, the air temperature declines by an average of about 0.7°C, relative humidity rises by 6%, and the maximum reduction of daily potential evapotranspiration is about 1mm/day. Based on the meteorological conditions from 2009 to 2013, the dynamic irrigation schedule in the Heihe Oasis increased the “cooling effect” and ” wet island effect” by approximately 1°C and 24.5%, respectively, compared to the actual irrigation schedule. The daily potential evapotranspiration is overestimated by about 0.5 mm/day. The dynamic irrigation schedule, due to its higher irrigation water quantity and frequency, exaggerated the climate effects of irrigation.

Evaporation is the key to the basin’s water cycle. Agricultural irrigation has resulted in a significant variation of regional potential evaporation (E _pen ). The spatiotemporal variation of E _pen and the influencing factors in the natural, agricultural, and desert areas in different developmental stages of irrigation in the Heihe River Basin (HRB) from 1970 to 2017 are comparatively analyzed in this study. This work focused on the correction effect of irrigation on the variation of E _pen . The agricultural water consumption in HRB significantly varied around 1998 due to the agricultural development and water policy. Under the influence of irrigation, the annual variation of E _pen in the agricultural, natural, and desert areas was significantly different. From 1970 to 1998, the annual trend slope of E _pen in the natural area only reduced by 1 mm decade ^-1 , while that in the agricultural area significantly decreased by 39 mm decade ^-1 . After the implementation of water-saving irrigation, the E _pen in the natural and agricultural areas increased by 11 and 54 mm decade ^-1 , respectively, from 1998 to 2017. In contrast with the natural and agricultural areas, E _pen in the desert area decreased by 80 mm decade ^-1 from 1970 to 1998 and continuously decreased by 41 mm decade ^-1 from 1998 to 2017. However, the regulatory effect of irrigation on E _pen in the desert area started to manifest due to the expansion of the cultivated land area in the desert area from 2010 to 2017. Irrigation has a significant regulatory effect on the variation of E _pen in HRB. The regulatory effect is mainly reflected on the aerodynamic term (E _aero ). The analytical results of the main meteorological factors affecting Epen in different regions indicated that the main meteorological factors influencing the variation of E _pen in each region are the wind speed 2 m above the surface (U ₂ ) and the water vapor pressure difference (VPD).