Assessing global water mass transfers from continents to oceans over the period 1948–2016

Abstract. Continental water mass change affects ocean mass change (OMC). Assessing the net contribution, however, remains a challenge. We present an integrated version of the WaterGAP global hydrological model that is able to simulate total continental water storage anomalies (TWSA) over the global continental area (except Greenland and Antarctica) consistently by integrating the output from the global glacier model of Marzeion et al. (2012) as an input to WaterGAP. Monthly time series of global mean TWSA obtained with an ensemble of four variants of the integrated model, corresponding to different precipitation input and irrigation water use assumptions, were validated against an ensemble of four TWSA solutions based on GRACE satellite gravimetry over January 2003 to August 2016. The overall fit to GRACE, measured by the Nash–Sutcliffe efficiency (NSE) coefficient, was found to be 0.87. By decomposing the original TWSA signal into its seasonal, linear trend and inter-annual components, we find that the seasonal amplitude and phase are very well reproduced (NSE = 0.88), the linear trend is overestimated by 30–50 % (NSE = 0.65) and inter-annual variability is captured to a certain extent (NSE = 0.57) by the integrated model. During the period 1948–2016, we find that continents lost 34–41 mm of sea level equivalent (SLE) to the oceans, with global glacier mass loss accounting for 81 % of the cumulated mass loss and glacier-free land water storage anomalies (LWSA) accounting for the remaining 19 %. Over 1948–2016, the mass gain on land from impoundment of water in man-made reservoirs, equivalent to 8 mm SLE, was offset by the mass loss from water abstractions, amounting to 15–21 mm SLE and reflecting a cumulated groundwater depletion of 13–19 mm SLE. Climate-driven LWSA are highly sensitive to precipitation input and correlate with El Niño Southern Oscillation multi-year modulations. Significant uncertainty remains in trends of modelled LWSA, which are highly sensitive to simulation of irrigation water use and man-made reservoirs.