Abstract
Abstract. Global-scale assessments of freshwater fluxes and storages by hydrological models under historic climate conditions are subject to a variety of uncertainties. Using the global hydrological model WaterGAP (Water – Global Assessment and Prognosis) 2.2, we investigated the sensitivity of simulated freshwater fluxes and water storage variations to five major sources of uncertainty: climate forcing, land cover input, model structure/refinements, consideration of human water use and calibration (or no calibration) against observed mean river discharge. In a modeling experiment, five variants of the standard version of WaterGAP 2.2 were generated that differed from the standard version only regarding the investigated source of uncertainty. The basin-specific calibration approach for WaterGAP was found to have the largest effect on grid cell fluxes as well as on global AET (actual evapotranspiration) and discharge into oceans for the period 1971–2000. Regarding grid cell fluxes, climate forcing ranks second before land cover input. Global water storage trends are most sensitive to model refinements (mainly modeling of groundwater depletion) and consideration of human water use. The best fit to observed time series of monthly river discharge or discharge seasonality is obtained with the standard WaterGAP 2.2 model version which is calibrated and driven by daily reanalysis-based WFD/WFDEI (combination of Watch Forcing Data based on ERA40 and Watch Forcing Data based on ERA-Interim) climate data. Discharge computed by a calibrated model version using monthly CRU TS (Climate Research Unit time-series) 3.2 and GPCC (Global Precipitation Climatology Center) v6 climate input reduced the fit to observed discharge for most stations. Taking into account uncertainties of climate and land cover data, global 1971–2000 discharge into oceans and inland sinks ranges between 40 000 and 42 000 km3 yr−1. Global actual evapotranspiration, with 70 000 km3 yr−1, is rather unaffected by climate and land cover uncertainties. Human water use reduced river discharge by 1000 km3 yr−1, such that global renewable water resources are estimated to range between 41 000 and 43 000 km3 yr−1. The climate data sets WFD (available until 2001) and WFDEI (starting in 1979) were found to be inconsistent with respect to shortwave radiation data, resulting in strongly different actual evapotranspiration. Global assessments of freshwater fluxes and storages would therefore benefit from the development of a global data set of consistent daily climate forcing from 1900 to present.
Highlights
The quantification of global-scale freshwater fluxes, in particular river discharge, is essential to assess availability and scarcity of water resources for humans and the environment for both present (Hoekstra et al, 2012; Oki and Kanae, 2006; Prudhomme et al, 2014) and scenario conditions (Döll and Müller Schmied, 2012; Masaki et al, 2014; Schewe et al, 2014)
Using the global hydrological model WaterGAP (Water – Global Assessment and Prognosis) 2.2, we investigated the sensitivity of simulated freshwater fluxes and water storage variations to five major sources of uncertainty: climate forcing, land cover input, model structure/refinements, consideration of human water use and calibration against observed mean river discharge
The best fit to observed time series of monthly river discharge or discharge seasonality is obtained with the standard WaterGAP 2.2 model version which is calibrated and driven by daily reanalysis-based WFD/WFDEI climate data
Summary
The quantification of global-scale freshwater fluxes, in particular river discharge, is essential to assess availability and scarcity of water resources for humans and the environment for both present (Hoekstra et al, 2012; Oki and Kanae, 2006; Prudhomme et al, 2014) and scenario conditions (Döll and Müller Schmied, 2012; Masaki et al, 2014; Schewe et al, 2014). As groundwater plays an important role for humans, e.g., for irrigation purposes, related fluxes such as groundwater recharge (Döll and Fiedler, 2008; Koirala et al, 2014; Portmann et al, 2013) or, as consequence of an overexploitation of groundwater resources, groundwater depletion (Döll et al, 2014b; Wada et al, 2010) are the focus of modeling activities. These examples show attempts to quantify global-scale freshwater fluxes as well as water storages; the methodologies used differ largely. In the last 20 years, a number of GHMs have been developed using different conceptual approaches; e.g., VIC (Variable Infiltration Capacity; Nijssen et al, 2001), WBM (Water Balance Model; Vörösmarty et al, 1998), Mac-PDM (Macro Probability Distribution Model; Gosling and Arnell, 2011), WASMOD-M (Water And Snow balance Modeling system – Macro scale; Widén-Nilsson et al, 2007), H08 (Hanasaki et al, 2008), WaterGAP (Water – Global Assessment and Prognosis; Alcamo et al, 2003; Döll et al, 2003) and PCR-GLOBWB (PCRaster GLOBal Water Balance; Sperna Weiland et al, 2010)
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