Quantitative attribution of changes in streamflow due to the non-linear relationship between climate change (CC) and land cover/land use change (LCLUC) in a hydrological system of the Yangtze River Source Region (YZSR) in the Qinghai-Tibetan Plateau (QTP) was measured. A combination of the SWAT model and a statistical technique known as One Factor At a Time (OFAT) and Innovative Trend Analysis (ITA) was carried out to achieve the study objectives. The hydro-climatic data from 1961-2016 and land cover/land use data of 1985, 1990, 1995, 2000, 2005, 2010, and 2015 were used. The results revealed that changes in annual streamflow during 1985-1990 were contributed by CC (+11.4 mm) and LCLU (+9.4 mm), accounting for 54.8 % and 45.2 % of the total combined impact of +20.7 mm. A more dominant effect of CC than LCLU also occurred for the periods between 1995 and 2000, 2000 and 2005, 2005 and 2010, 2010 and 2015, where CC and LCLU contributed to increasing the annual streamflow of 5.9 mm and 0.3 mm, respectively. However, a different pattern was observed in 1990-1995 where changes in streamflow were mainly attributed to LCLU rather than CC. It was observed that major LCLUC were found in 1990-1995 where low grassland decreased (-13,353 km2), bare land increased (+9,048 km2), medium grassland increased (+2,485 km2), water increased (+1,391 km2), high grassland increased (+2,329 km2), and wetland decreased (-1,927 km2). The ITA results showed that there is a rise in temperature, precipitation, and streamflow monotonically in the second half (1990-2016) as compared to the first half (1961-1989). In addition, it was found that temperature and precipitation were positively correlated (P<0.05) in high flow months (July and August), whereas negatively correlated in low flow months (November-March). The results of SWAT model simulation showed that CC (i.e. warmer climate) is the primary source of variations in streamflows of the Yangtze River Source Region.

The Food and Agriculture Organizations’ (FAO) PenmanMonteith reference evapotranspiration (ET0) index is a key parameter in hydrological and meteorological studies. Temporal and spatial variations in ET0 from 1981–2017 were investigated in the topographical rapid change zone in the Hengduan Mountains, China. The results showed a change point around the year 2000 in the area-averaged annual ET0 series. ET0 decreased and increased significantly by 3.103 mm/yrmm/year (p < 0.05) from 1981–2000 and by 3.591 mm/yrmm/year (p < 0.05) from 2001–2017, respectively. The contribution analysis shows that reduction in wind speed (Ws) was the primary driving force for the decrease in ET0 during 1981–2000 in spring, autumn, and winter, and annually, while net solar radiation (Rn) was the dominant force in summer. Reduction in relative humidity (RH) was responsible for the increase in ET0 in all seasons and for the annual scale in 2001–2017. The sensitivity analysis shows that ET0 was most sensitive to Rn, followed by RH, and air temperature (Ta) was the least sensitive of the variables. The trends of ET0 were also found to increase with elevation; we denote this as the elevation-dependence of ET0 changes. The elevation-dependence was also noted for the trends of Ws, RH, and Rn, with higher elevations showing larger changes in these parameters. In addition, the sensitivities of Rn, RH, and Ta decreased with elevation, while that of Ws increased with elevation. A comprehensive investigation into the trends of these climatic variables and their sensitivities revealed complex trends of ET0 along the elevation gradient, with typical increases with elevation over the annual scale despite the large differences in seasons. A more detailed exploration of the mechanisms causing this pattern is required.