Abstract
Many studies on drought consider precipitation and potential evapotranspiration (PET) impacts. However, catchment water retention is a factor affecting the interception of precipitation and slowing down runoff which also plays a critical role in determining the risks of hydrological drought. The Budyko framework links retention to the partitioning of precipitation into runoff or evapotranspiration. Applied worldwide, we demonstrate that retention changes are the dominant contribution to measured runoff changes in 21 of 33 major catchments. Similarly, assessing climate simulations for the historical period suggests that models substantially underestimate observed runoff changes due to unrepresented water management processes. Climate models show that water retention (without direct water management) generally decreases by the end of the 21st century, except in dry central Asia and northwestern China. Such decreases raise runoff, mainly driven by precipitation intensity increases (RCP4.5 scenario) and additionally by CO2-induced stomata closure (RCP8.5). This mitigates runoff deficits (generally from raised PET under warming) by increasing global mean runoff from −2.77 mm yr−1 to +3.81 mm yr−1 (RCP4.5), and −6.98 mm yr−1 to +5.11 mm yr−1 (RCP8.5).
Highlights
There is compelling evidence that climate change has profound impacts on regional and global hydrological cycles (Huntington 2006, Piao et al 2007)
We propose here a new application of the Budyko framework to diagnose how changing environmental factors affect runoff in both measurements and Earth System Model (ESM) estimates over recent decades, and facilitating comparison
Future projections there are noted deficiencies of ESMs depiction of retention, compared to measurements, due to their non-representation of water management processes, we can still explore future water retention changes within the Coupled Model Intercomparison Project Phase 5 (CMIP5) model ensemble. These changes in n reflect rising atmospheric CO2 concentrations, changing imposed climate, varying vegetation distribution and its water use, along with land use change. We again do this via analysis of the ensemble mean of the CMIP5 models (CMIP5-EM), but here to analyze future runoff changes
Summary
There is compelling evidence that climate change has profound impacts on regional and global hydrological cycles (Huntington 2006, Piao et al 2007). Reconstructions of global river runoff show that one-third of the 200 largest rivers have significantly changed flows since the 1950s More of these rivers experienced a runoff decrease rather than an increase, in an approximate ratio of 3:1 (Dai et al 2009). A majority of Earth System Model (ESM) projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5) show an increase in global runoff for the future. This is true for the high latitudes and humid tropics, while models predict that rivers in most dry tropical regions may experience reduced runoff (Cisneros et al 2014, Prudhomme et al 2014). The strong predictive capability of models is needed to help with decision-making to ensure sustainable future water resource management (Oki and Kanae 2006, Hall et al 2014)
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