Temporal and spatial variations in reference evapotranspiration (ET₀) from 1980 to 2012 were investigated for 33 meteorological stations in the three-river source region (TRSR). Penman–Monteith reference evapotranspiration in the TRSR significantly decreased at a rate of 0.91 mm a⁻² from 1980 to 2012, and the decrease was actually an offset of the positive effect of increasing air temperature on ET₀ and the negative effect of decreasing net radiation, wind speed, and actual vapor pressure. A differential equation method based on the Penman–Monteith formula was used to attribute the change in ET₀. As for the decrease of reference evapotranspiration from 1980 to 2012, the contributions of net radiation, wind speed, actual vapor pressure, and air temperature were −0.57, −0.49, −0.26, +0.41 mm a⁻², respectively. Thus, ‘net radiation decrease’ was the major reason for the decrease in ET₀ for the TRSR. In addition, it appears that the change in ET₀ was more sensitive to meteorological variables in high-elevation regions than in low-elevation regions.

Pan evaporation is an important indicator of atmospheric evaporative demand, and its long‐term variation is of much concern in studies of climate change. Based on data from 33 meteorological stations from 1962 to 2012, this work considered the temporal and spatial trends of pan evaporation and the meteorological variables that affect them in the Three‐River Source Region (TRSR) of southwestern China. Pan evaporation in the TRSR has decreased significantly since 1988 with an obvious abrupt change from 1993 to 2003. Furthermore, a 27 year period of oscillation has existed over the past 51 years. Pan evaporation reflects the combined effects of four meteorological variables: net radiation (R_n), wind speed (u₂), actual vapor pressure (e_a), and air temperature (T_a). Based on this research, a number of conclusions were drawn. (1) The pace of climate change increased after 1980 and pan evaporation decreased at a rate of −13.3 mm/a² from 1980 to 2012, which is much faster than the rate of −1.2 mm/a² from 1962 to 1979. (2) For the decrease of pan evaporation from 1980 to 2012, the quantifying contributions of R_n, u₂, e_a, and T_a were −8.7, −6.4, −1.8, and +3.6 mm/a², respectively. Thus, it was established for the TRSR that “global dimming” was the main reason, and “wind stilling” was a close second to global dimming for the decrease in pan evaporation. (3) Different regions of the TRSR are affected differently by the effects of the meteorological variables. Low‐elevation regions in the TRSR are more susceptible to the effects of net radiation and wind speed, whereas high‐elevation regions are affected more by actual vapor pressure and air temperature.

As the debates surrounding the negative influences of flood control using dams or reservoirs on the eco-environment become fierce, non-structural flood control measures like land use change gain more attention. This study researched the effect of integrated and single land use changes on three floods at small, medium and large scales, respectively, in Yongding River basin. A SWAT (Soil and Water Assessment Tool) model was used to simulate the effect of integrated and single land use changes on floods of different scales. The single land uses were set as S1, S2, S3 to represent the agricultural, grass and construction land changes. The results showed that: (1) the integrated land use changes reduced the small flood, the medium flood and the large flood by 14%, 13% and 5%; (2) the land use management functioned most effectively on medium-scale floods and least effectively on large-scale floods; (3) S1 decreased the medium floods optimally by 24% with a 7-day maximum runoff volume as the indicator and by 29% with a 1-day maximum flood discharge; (4) S2 reduced the medium floods optimally by 21% with runoff depth volume as the indicator; (5) S3 increased the medium floods optimally by 15% with a 1-day maximum flood discharge as the indicator.