Large-Scale Controls of the Surface Water Balance Over Land: Insights From a Systematic Review and Meta-Analysis

The long-term surface water balance over land is described by the partitioning of precipitation (P) into runoff and evapotranspiration (ET), and is commonly characterized by the ratio ET/P. The ratio between potential evapotranspiration (PET) and P is explicitly considered to be the primary control of ET/P within the Budyko framework, whereas all other controls are often integrated into a single parameter, . Although the joint effect of these additional controlling factors of ET/P can be significant, a detailed understanding of them is yet to be achieved. This study therefore introduces a new global data set for the long-term mean partitioning of P into ET and runoff in 2,733 catchments, which is based on in situ observations and assembled from a systematic examination of peer-reviewed studies. A total of 26 controls of ET/P that are proposed in the literature are assessed using the new data set. Results reveal that: (i) factors controlling ET/P vary between regions with different climate types; (ii) controls other than PET/P explain at least 35% of the ET/P variance in all regions, and up to approximate to 90% in arid climates; (iii) among these, climate factors and catchment slope dominate over other landscape characteristics; and (iv) despite the high attention that vegetation-related indices receive as controls of ET/P, they are found to play a minor and often nonsignificant role. Overall, this study provides a comprehensive picture on factors controlling the partitioning of P, with valuable insights for model development, watershed management, and the assessment of water resources around the globe. Plain Language Summary Precipitation over long time periods (several years or longer) is partitioned into water leaving a river catchment as runoff, and water that is evaporated or transpired by plants. Different factors have been suggested to directly and indirectly influence how much of the precipitation turns into runoff, and how much is evaporated or transpired. To further assess the relevance of these factors, we gathered observational information about the water balance from numerous previous studies, obtaining a dataset with unprecedented global coverage. Results reveal a similar importance of long-term average evaporative demand (the amount of water that would evaporate under conditions of sufficient water supply) relative to precipitation, and the net effect of all other influencing factors. Among these additional factors we find that the average slope of a catchment and climate-related variables, such as the fraction of precipitation falling as snow and the relative timing of rainfall and evaporative demand during the year, influence the partitioning of precipitation more than other landscape characteristics. Surprisingly, vegetation-related factors are found to play a minor role despite the high attention they have previously received. Overall this study provides valuable insights on processes controlling freshwater resources globally.