Attributing trend in naturalized streamflow to temporally explicit vegetation change and climate variation in the Yellow River basin of China

The naturalized streamflow, i.e., streamflow without water management effects, in the Yellow River basin (YRB) has been significantly decreased at a rate of -3.71 x 10(8) m(3) yr(-1) during 1982-2018, although annual precipitation experienced an insignificantly positive trend. Explicit detection and attribution of naturalized streamflow are critical to manage limited water resources for the sustainable development of ecosystems and socio-economic systems. The effects from temporally explicit changes of climate variables and underlying surfaces on the streamflow trend were assessed using the variable infiltration capacity (VIC) model prescribed with continuously dynamic leaf area index (LAI) and land cover. The results show a sharp increase of the LAI trend and land use change as a conversion of cropland into forest grass in the basin. The decrease in naturalized streamflow can primarily be attributed to the vegetation changes including an interannual LAI increase and intraannual LAI temporal pattern change, which account for the streamflow reduction of 1.99 x 10(8) and 0.45 x 10(8) m(3) yr(-1) , respectively. The impacts of the LAI change are largest at the subregion of Longmen-Huayuankou where the LAI increasing trend is high and land use change is substantial. Attribution based on simulations with multiyear average LAI changes obviously underestimates the impacts of the interannual LAI change and intra-annual LAI temporal change on the natural streamflow trend. Overall, the effect of climate variation on streamflow is slight because the positive effect from precipitation and wind speed changes was offset by the negative effect from increasing temperature. Although climate variation is decisive for streamflow change, this study suggests that change in underlying surfaces has imposed a substantial trend on naturalized streamflow. This study improves the understanding of the spatiotemporal patterns and the underlying mechanisms of natural streamflow reduction across the YRB between 1982 and 2018.