Water balance analysis based on remote sensing observation of surface water cycle in the Heihe River watershed

Water cycle in a basin is constantly changing. It is an important basis for the allocation of water resources to survey the state of water cycle and analyze the water balance. The significant hydrological change period of the basin from 2000 to 2019 is taken as the research period in this study, and the data, including TRMM and GPM satellite precipitation, remote sensing evapotranspiration, in situ data of meteorological and hydrological gauges, are synthetically used to analyze the water cycle balance of the watershed. The results show that (1) the Qilian mountain area is the main runoff producing area, with an average annual discharge of about 45.11×10⁸ m³ to the middle reaches, and the middle reaches is the main consumption area of the runoff water resources. About 66% (29.92×10⁸ m³) of the upstream incoming water is used in the middle reaches, and about 34% (15.19×10⁸ m³) is used to supplement the downstream area. (2) The Minle-Zhangye basin is the main consumption area of water resources in the middle reaches, with an average annual consumption of about 43.97×10⁸ m³ including water from upper reaches and precipitation at local area (accounting for 75% of the consumption in the middle reaches), and the evapotranspiration consumption of farmland in the middle reaches is about 20.3×10⁸ m³ (accounting for 35% of the evapotranspiration water in the middle reache areas). (3) The increase in precipitation of the upper reach is the main reason for the increase in runoff in the upper reaches of the main stream of the Heihe River. The contribution rate of precipitation to the increase of runoff is 96%, resulting in an average increase of runoff of 0.35×10⁸ m³/a, and potential evapotranspiration has little contribution to the increase of the runoff. (4) According to the current water resource consumption, if the upstream runoff decreases in the future, there will be a risk of prominent contradiction between water used for agricultural development in the middle reaches and water used for ecological protection in the downstream area. Remote sensing observation of the surface water cycle can be used as one of the technologies of the basin water balance analysis to analyze the spatial distribution of basin surface water resources, reveal the temporal change trend and the causes, and support the rational allocation of water resources. The actual evapotranspiration on the land surface is the main source of the analysis uncertainty, and the key to improve the reliability of water balance analysis is to accurately estimate the actual evapotranspiration of different types of landcover. The estimation method of land actual evapotranspiration based on complementary correlation is relatively simple, but the multiplicative empirical coefficient in the Priestley-Taylor formula, which is used to calculate evapotranspiration in wet environment, is greatly affected by terrain, and the use of same value across the region will have a significant impact on the results.