Comment on hess-2022-196

Abstract

The naturalized streamflow, i.e., streamflow without water management effects, in the Yellow River Basin (YRB) has been significantly decreased at a rate of -3.71×108 m3 yr-1 during 1982–2018 although annual precipitation experienced insignificantly positive trend. Explicit detection and attribution of naturalized streamflow is critical to manage limited water resources for sustainable development of ecosystem and socio-economical system. The effects from temporally explicit changes of climate variables and underlying surfaces on the streamflow trend were assessed using Variable Infiltration Capacity (VIC) model prescribed with continuously dynamic leaf area index (LAI) and land cover. The results show a sharp increase of LAI trend and land use change as a conversion of cropland into forest-grass in the basin. The decrease in naturalized streamflow can be primarily attributed to the vegetation changes including interannual LAI increase and intra-annual LAI temporal pattern change, which accounts for the streamflow reduction of 1.99×108 m3 yr-1 and 0.45×108 m3 yr-1, respectively. The impacts of LAI change are largest at the sub-region of Longmen-Huayuankou where LAI increasing trend is high and land use change is substantial. Attribution based on simulations with multi-year average LAI changes obviously underestimates the impacts of interannual LAI change and intra-annual LAI temporal change on the natural streamflow trend. Overall, the effect climate variation on streamflow is slight because 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 surface 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 YRB between 1982 and 2018.