| The sustainable use of agricultural water is a priority for social-economic development and ecological security in inland river basins located in arid northwest China.It is of great significance to make a thorough understanding of agro-hydrological processes and its response to water-saving practices and climate changes,for saving water and efficient water use in agriculture.In this dissertation,the middle oasis of the Heihe River basin(HRB),a typical inland river basin,was chosen as the study area.A distributed agro-hydrological model and regional economical optimization model was constructed based on a field scale agro-hydrological model(SWAP-EPIC model).The distributed simulation of agro-hydrological processes,analysis of regional irrigation water balance and evaluation of water use efficiency were conducted on different scales.The optimizing strateges for irrigation water use were further simulated and discussed under different scenarios of water-saving and climate changes.This will provide scientific references for improving irrigation management and formulating appropriate water-saving schemes in the middle HRB.The main research contents and conclusions are as follows:(1)The Yingke Irrigation District(YID),a typical irrigation district in the middle HRB,was taken as a case study for assessing irrigation water use on irrigation district scale,using a distributed agro-hydrological model.Results showed that crop evapotranspiration(ETa)averaged at 589 mm and deep percolation(Dp)was 125 mm in average,which accounted for 21%of total irrigation.Water productivity(WP)spatially varied and the mean field application efficiency(FAE)is 0.8,while the excessive irrigation in the upstream region led to its large Dp and lower FAE and WP.Analysis of water-saving scenario indicated that the improvement of water allocation and irrigation scheduling could reduce 30%of Dp,and save 15%of field irrigation water without negative effects on crop yields.(2)The major irrigation system of the middle HRB(MOIS)was chosen as the study area for the distributed agro-hydrological modelling and irrigation water balance analysis on oasis scale.The spatial distribution of irrigation water depth,ETa Dp and crop yields were presented,and related impact factors were systematically analyzed for MOIS.The regional irrigation water budget indicated that only 53%of total regional irrigation water use(including effective rainfall)was consumed by ETa,while 22%and 25%was respectively wasted though deep percolation and canal conveyance losses.(3)On the basis of the distributed agro-hydrological modelling,efficiency and productivity of irrigation water use was further evaluated both on field and district scales for MOIS.Results provided detailed spatial distribution of beneficial water use fraction(BWUF)and WP in MOIS as well as its impact factors.The BWUF was 0.70 and 0.52 respectively on field and irrigation district scale,with great spatial variations.About half of irrigation districts have regional BWUF below the national mean value(0.5).Crop water productivity(WPC)for major crops was at an upper-middle level across the Hexi Corridor region,with small spatial variation.The field and regional WPs were significantly reduced as compared to WPC,showing a larger spatial variation.Water-saving scenario analysis indicated that about 15%of total regional irrigation amount could be saved efficiently through the timely and uniform water distribution and improved irrigation scheduling.(4)Using the dynamical downscalling weather data in 2021-2050 under RCP4.5 climate scenario,it was simulated and analyzed that the response of irrigation water balance and water use efficiency to climate changes under four water-saving scenarios.Meanwhile the adaptations of four water-saving schemes on future climate changes were evaluated.Results showed that the average changes of mean crop yield,ETa,WP and BWUF were small in the future 30 years under scenarios relative to improved irrigation scheduling(A and C),as compared with that of water-saving scenario at present.While the improved irrigation scheduling led to significant reduction of crop yields and WP in some unfavorable climate conditions.The scenarios relative to drip irrigation(B and D)were more efficiently to save water and to improve crop yieds and water use efficiency in the future.Overall,the improved irrigation scheduling has limitations to a certain extent in the future unfavorable climate.The drip irrigation showed better adaptation on climate changes,and was benefical for ensuring crop yields and water-saving.(5)A process-based regional economic optimization model(PBREOP)was developed with combined use of an ago-hydrological model(SWAP-EPIC)。It is a two-level optimization model based on the decomposition-harmonization method for large systems.The first and second level respectively dealt with the optimal allocation of irrigation water and cropping area in each subsystem,and the optimal strategy for irrigation water allocation among different subsystems.The crop water production functions(CWPF)were an important component of the first-level objective function.They were derived with the SWAP-EPIC model considering various crop-soil distribution and different climate conditions.The model was then applied in the optimization of irrigation water use in YID.Optimized results showed that the total economic benefits of irrigation in YID could be increased by 15%in average through optimizing irrigation water allocation and cropping pattern under the present water supply level.The total irrigation water could be reduced by 23%in average without benefit reduction relative to the present benefit.The application indicated that the proposed PBREOP model was an efficient tool for optimizing irrigation water use on a regional scale. |
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