Impacts of water conservancy projects on agricultural water use efficiency and crop productivity in the Nianchu River Basin of China

Abstract: Water conservancy engineering has been one of the most important human activities to impact the hydrological process in the watershed. Among them, the agricultural reservoirs for irrigation can change the various hydrological elements in the watershed, even the water resources utilization and agricultural production. The Manla Reservoir (known as "The first dam in Tibet") has also posed a great challenge to water resources management and agricultural development in the Qinghai-Tibet Plateau. Taking the Nianchu River near the Manla Reservoir as an example, this study aims to determine the optimal parameters using the Soil and Water Assessment Tool (SWAT). The runoff was predicted after the construction of the reservoir using the data before construction. The Modified Normalized Difference Water Index (MNDWI) was combined with the inflow, outflow, and water storage of the reservoir after the SWAT simulation. A Vegetation Interface Processes (VIP) model was used to calculate the cultivated land Gross Primary Productivity (GPP) in the reservoir dispatch scenario using the farmland water balance. The ridge regression was selected to quantify the effects of human activities on the GPP, such as the reservoir irrigation and major climatic factors (Carbon dioxide concentration, Air temperature, Precipitation, and Solar radiation). The results show that: 1) The runoff decreased 3.78×109 m3 after the construction of the Manla Reservoir, according to the simulation of the SWAT model. Among them, 2.5×109 m3 of water was transported through the Manla reservoir for irrigation per year. The utilization rate of runoff increased from 10.3% to 25.1% in agriculture in the basin. Due to the impoundment of the reservoir, the runoff of the Manla Reservoir was reduced by 0.65×109 m3/a. 2) The average annual GPP of cultivated land was between 143 and 853 gC/(m2·a) in the VIP model. The average annual GPP of cultivated land increased by 5.3 gC/(m2·a2). The GPP of more than 90% of the cultivated land areas showed an upward trend during the operation of the reservoir (2000-2019), of which more than 67% was a significant upward trend. There was a changing trend in the spatiotemporal scale for the carbon dioxide concentration, air temperature, and radiation during the operation of the reservoir, indicating an excellent agreement with the GPP. But, there was negatively correlated between the precipitation and GPP. 3) The operation of the Manla Reservoir greatly contributed to the deployment of water resources. The GPP increase was attributed to the irrigation and CO2 concentration, according to the ridge regression model. The average relative contributions were 43.9% and 25.1%, respectively, which were much higher than those of all climatic factors. The relative contributions of air temperature, radiation, and precipitation were only 14.7%, 6.9%, and -5.1%, respectively. Consequently, there was some impact of the construction and operation of the Manla Reservoir on the eco-hydrological elements (runoff, and evaporation) in the basin. The relative contributions of Manla reservoirs were quantitatively analyzed to improve the irrigation level and crop production. The finding can provide a strong reference for the decision-making on water resources allocation and agricultural irrigation instruction.