Fate of water pumped from underground and contributions to sea-level rise

Abstract

The contributions from terrestrial water sources to sea-level rise, other than ice caps and glaciers, are highly uncertain and heavily debated 1-5 . Recent assessments indicate that groundwater depletion (GWD) may become the most important positive terrestrial contribution 6-10 over the next 50 years, probably equal in magnitude to the current contributions from glaciers and ice caps 6 . However, the existing estimates assume that nearly 100% of groundwater extracted eventually ends up in the oceans. Owing to limited knowledge of the pathways and mechanisms governing the ultimate fate of pumped groundwater, the relative fraction of global GWD that contributes to sea-level rise remains unknown. Here, using a coupled climate-hydrological model 11,12 simulation, we show that only 80% of GWD ends up in the ocean. An increase in runo to the ocean accounts for roughly two-thirds, whereas the remainder results from the enhanced net flux of precipitation minus evaporation over the ocean, due to increased atmospheric vapour transport from the land to the ocean. The contribution of GWD to global sea-level rise amounted to 0.02 (±0.004) mm yr −1 in 1900 and increased to 0.27 (±0.04) mm yr −1 in 2000. This indicates that existing studies have substantially overestimated the contribution of GWD to global sea-level rise by a cumulative amount of at least 10 mm during the twentieth century and early twenty-first century. With other terrestrial water contributions included, we estimate the net terrestrial water contribution during the period 1993-2010 to be +0.12 (±0.04) mm yr −1 , suggesting that the net terrestrial water contribution reported in the IPCC Fifth Assessment Report report is probably overestimated by a factor of three.Sea-level rise (SLR) is a direct effect of climate change, through the thermal expansion of ocean waters and the contribution of melt water from ice sheets, ice caps and glaciers. In an initial assessment 13 , the net contribution of sub-polar terrestrial water storage change to global sea-level variation was highly uncertain and heavily debated [14][15][16][17][18]

Reassessment of 20th century global mean sea level rise

“…Specifically, we used spatial variations from the hydrological model of ref. 38, which expresses groundwater depletion in a yearly resolution on a 0.5°× 0.5°grid, and the scaled depletion rates everywhere by a factor of 0.8 so that total groundwater depletion matches the results of updated hydrological models (37). Because the crustal motion component is already approximated by the VLM correction, we only correct for spatial variations in the geoid response resulting from loading by changes in TWS (i.e., deflections about a zero mean).…”

“…Changes in TWS (either caused by groundwater depletion or water impoundment behind dams) are accompanied by regional deflections of the solid earth (crustal motion) and sea surfaces (geoid), which can be calculated using Green's functions for vertical displacement and gravitational potential (21,22 (37). Specifically, we used spatial variations from the hydrological model of ref.…”

Human–water interface in hydrological modelling: current status and future directions

“…A notable example is from a study by Wada et al (2016a) where the contribution of groundwater depletion to sea-level change was assessed by including groundwater withdrawal and consumption in the Community Earth System Model (CESM). Pokhrel et al (2015) incorporated a water table dynamics scheme and a pumping scheme into the LSM called the Minimal Advanced Treatment of Surface Interaction and Runoff (MAT-SIRO; Takata et al, 2003) to explicitly quantify the natural and human-induced groundwater storage change.…”

“…Given rising levels of human footprint, and the heavy dependence of the world economy and livelihoods on water, human impacts on land and water systems are pervasive (World Water Assessment Programme, 2016). Agriculture and urbanization affect the delivery and quality of water to river and groundwater systems ; many river flows are regulated and threaten ecological flows (Poff et al, 2010); water use, in particular for irrigation, can be a dominant factor in the hydrological cycle, including effects on land-atmosphere feedbacks and precipitation (Wada et al, 2016a) that can have substantial non-local impacts (Dirmeyer et al, 2009;Tuinenburg et al, 2012;Wei et al, 2013;Lo and Famiglietti, 2013). In an era now designated as the Anthropocene (Steffen et al, 2011;Montanari et al, 2013;Savenije et al, 2014), global hydrology must therefore be treated as a coupled human-natural system.…”

Synthesizing long-term sea level rise projections – the MAGICC sea level model v2.0

“…The linear response function approach for the Antarctic SID component presented in Levermann et al (2014) was adapted to satisfy MAGICC model specifications. In addition, we have implemented the option to include land water SLR contribution estimates based on Wada et al (2012Wada et al ( , 2016, with an extension until 2300.…”