Simulation of Actual Evapotranspiration and Evaluation of Three Complementary Relationships in Three Parallel River Basins

Based on observed precipitation and runoff data, the monthly actual evapotranspiration (ETa) was calculated using the hydrological budget balance method in three parallel river basins. The performance of three complementary relationship methods, namely, the nonlinear advection-aridity (non-AA) method, generalized complementary relationship method (B2015), and sigmoid generalized complementary function (H2018), for simulating ETa was evaluated. The evaluation results showed that the three methods could accurately simulate monthly ETa series. The Nash–Sutcliffe efficiency coefficients between the monthly ETa simulated by the non-AA, B2015, and H2018 methods and the water balance-derived ETa were 0.74, 0.78, and 0.79, respectively. The correlation coefficients were 0.84, 0.89, and 0.90, and the root mean square errors were 10.76 mm mon−1, 10.01 mm mon−1, and 9.78 mm mon−1, respectively. ETa increased spatially from the upstream region to the downstream region at the catchment scale. Annual ETa simulated by the non-AA, B2015, and H2018 models showed significant increasing trends during the years 1956–2018 in the basins, with the increasing magnitudes of 1.53 mm/a, 1.66 mm/a, and 1.47 mm/a, respectively. Research on the influence of meteorological factors and ETa showed a positive correlation between ETa and precipitation, temperature, wind, and hours of sunshine, with average correlation coefficients of 0.40, 0.64, 0.63, and 0.72, respectively. The value between ETa and relative humidity was –0.38. ETa in the basins was highly sensitive to temperature, wind speed, and hours of sunshine, with average sensitivity coefficients of 0.26, 0.21, and 0.27, respectively. Moreover, it was moderately sensitive to relative humidity, with a sensitivity of –0.18.