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

Abstract

Abstract. Ocean mass and thus sea level is significantly affected by water storage on the continents. However, assessing the net contribution of continental water storage change to ocean mass change remains a challenge. We present an integrated version of the WaterGAP global hydrological model that is able to consistently simulate total water storage anomalies (TWSAs) over the global continental area (except Greenland and Antarctica) 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 TWSAs 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 the Gravity Recovery and Climate Experiment (GRACE) satellite gravimetry from January 2003 to August 2016. With a mean Nash–Sutcliffe efficiency (NSE) of 0.87, simulated TWSAs fit well to observations. By decomposing the original TWSA signal into its seasonal, linear trend and interannual components, we found that seasonal and interannual variability are almost exclusively caused by the glacier-free land water storage anomalies (LWSAs). Seasonal amplitude and phase are very well reproduced (NSE=0.88). The linear trend is overestimated by 30 %–50 % (NSE=0.65), and interannual 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 LWSAs accounting for the remaining 19 %. Over 1948–2016, the mass gain on land from the impoundment of water in artificial 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 LWSAs are highly sensitive to precipitation input and correlate with El Niño Southern Oscillation multi-year modulations. Significant uncertainty remains in the trends of modelled LWSAs, which are highly sensitive to the simulation of irrigation water use and artificial reservoirs.

Global sea-level budget and ocean-mass budget, with focus on advanced data products and uncertainty characterisation

“…Comprehensive insights into the model sensitivity to choices of irrigation water use assumptions and climate input data were acquired, enabling a first uncertainty estimation. The good fit of simulated monthly total water storage anomaly (sum of land water storage and glacier storage) to GRACE-derived estimates, in particular regarding seasonality and de-seasonalised long-term variability, enhanced the confidence in the simulated land water contributions (Cáceres et al, 2020).…”

“…A comparison of the monthly time series of total water and ice storage anomaly (CMC) over the continents (except Greenland and Antarctica) as derived from GRACE and from the non-standard WaterGAP version with glacier integration showed a very good fit, with a modelling efficiency of 0.87 (Cáceres et al, 2020). The GRACE trend during 2003-2016, however, was 26% weaker than the trend from the non-standard WaterGAP version.…”

Reply on RC1

“…For the AIS contribution, the new time series of Antarctica mass change from satellite radar altimetry is the result of an improved processing chain and a better characterisation of uncertainties. With a timeevolving ice and snow density mask and a new method for interpolating surface elevation change in areas located beyond the latitudinal limit of satellite radar altimeters and in between satellite tracks, we (sum of land water storage and glacier storage) to GRACE-derived estimates, in particular regarding seasonality and de-seasonalised long-term variability, enhanced the confidence in the simulated land water contributions (Cáceres et al, 2020).…”

“…The single exception from this agreement applies to the LWS contribution. While SROCC reports a However, a new GRACE-based assessment of continental mass change (Cáceres et al, 2020, updated by Gutknecht et al, 2020cf. Sect.…”

Comment on gmd-2020-367

“…The authors declare that they have no conflict of interest. S3 -S7 S8 -S13 S26-S28 S40-S45 Moore and Clarke,1981 / Rsat / f(gw) Beven and Kirkby,1979 (Schewe et al, 2019;Hock, 2018, Zekollari et al, 2019;Cáceres et al, 2020); c. couple the climate, lands use, hydrology, and human components including their feedbacks and interactions (Nazemi &Wheater, 2015;Pokhrel et al, 2016;Wada et al, 2017;Thiery et al, 2017;; d. land-use dynamics scheme (Sood and Smakhtin, 2015) Measurements and data (Blöschl et al, 2019) Challenges of simulating terrestrial water cycle on the global scale, identified through the present study 16. How can we use innovative technologies to measure surface and subsurface properties, states, and fluxes at a range of spatial and temporal scales?…”