Correspondence: Flawed assumptions compromise water yield assessment

In their recent study, Zhou et al.1 (hereafter Z15) aim at mapping global patterns of the relative contributions of climate and landcover change on the water yield coefficient. However, implicit and not discussed assumptions on ‘typical changes’ of the considered forcing factors2,3 raise questions on the physical integrity of substantial aspects of their analysis4. In fact, we show here that central findings of Z15 are reversed if physically and mathematically more justified assumptions are accounted for. Z15 analyse partial differentials of a rearranged version of Fu’s equations5,6 that predicts the long-term mean ratio of precipitation (P) and runoff (R), which they refer to as the water yield coefficient, as