Simulation and analysis of irrigation water consumption in multi-source water irrigation districts in Southern China based on modified SWAT model

Abstract: The statistics of irrigation water consumption in irrigation districts are of great significance to implement the most stringent water resources management system. On account of the impossibility of complete measurement, it is quite difficult to accurately calculate the irrigation water consumption in multi-source water irrigation districts in the south of China. Therefore, a precise and effective way is needed to estimate irrigation water consumption in multi-source water irrigation districts. In this study, the Soil and Water Assessment Tool (SWAT) was modified according to the characteristics of hydrologic cycle and irrigation operation in the multi-source water irrigation district in the south of China for accurately estimating irrigation water consumption. The water balance modules of paddy field were modified in SWAT model, in addition, a canal seepage loss calculation was added to SWAT model. Specifically, a multi-source water auto-irrigation module was added as one of the components of SWAT model to estimate the irrigation water consumptions from different types of water sources. Furthermore, the modified SWAT model with a digital elevation model (DEM), a soil map, a land cover map and multi-year meteorological data, was applied to build a distributed hydrological model of Tongjiqiao Reservoir Irrigation District (TID) in Zhejiang Province. Moreover, the observed monthly runoff was used to calibrate (1995-2007) and validate (2008-2015) the simulated runoff via SWAT Calibration and Uncertainty Programs (SWATCUP), and the observed irrigation water consumptions of 4 main irrigation canals in 2017 were used to calibrate the simulated irrigation water consumptions. The results showed that the simulated monthly runoff matched well with the observed values in calibration and validation periods, the absolute relative errors (RE) were less than 12%, the coefficients of determination (R2) were greater than or equal to 0.88, and the Nash-Sutcliffe efficiency coefficients (NS) were greater than or equal to 0.85 in both periods; in addition, the maximum of the absolute relative errors between simulated irrigation water consumptions and the observed values of 4 main irrigation canals was less than 20%, indicated that the modified SWAT model has a good performance in the multi-source water irrigation districts. Additionally, the irrigation water consumptions in different hydrological years in TID, multi-year averages of simulated irrigation water consumptions and water supply proportions of different types of water source were simulated and calculated based on the modified SWAT model, in addition, the effect of water saving reform on irrigation water consumption was also analyzed. And the results indicated that the irrigation water consumption is small in wet year and large in dry year. Moreover, in addition to the key water sources (namely the Tongjiqiao Reservoir and the Puyang River), 41.40% of the irrigation water consumption came from the rivers inside sub-basins, ponds and small-sized reservoirs, indicating that the amounts of water monitored at the head of canals fetching water from the key water sources did not represent the irrigation water consumption in irrigation districts. Beyond that, with the development of water saving reform in irrigation district, the irrigation water use efficiency increased so that the irrigation water consumption decreased. Consequently, the modified SWAT model can be used to simulate and analyze the irrigation water consumption in multi-source water irrigation districts accurately and reasonably, and the simulation of irrigation water consumption in multi-source water irrigation districts based on the modified SWAT model is an effective and rational method for calculation and analysis of irrigation water consumption in irrigation districts in the south of China, which satisfied the requirements of the total amount of water statistics and the most stringent water resources management system.