Abstract
Abstract. Groundwater is a non-negligible component of the global hydrological cycle, and its interaction with overlying unsaturated zones can influence water and energy fluxes between the land surface and the atmosphere. Despite its importance, groundwater is not yet represented in most climate models. In this paper, the simple groundwater scheme implemented in the Total Runoff Integrating Pathways (TRIP) river routing model is applied in off-line mode at global scale using a 0.5° model resolution. The simulated river discharges are evaluated against a large dataset of about 3500 gauging stations compiled from the Global Data Runoff Center (GRDC) and other sources, while the terrestrial water storage (TWS) variations derived from the Gravity Recovery and Climate Experiment (GRACE) satellite mission help to evaluate the simulated TWS. The forcing fields (surface runoff and deep drainage) come from an independent simulation of the Interactions between Soil-Biosphere-Atmosphere (ISBA) land surface model covering the period from 1950 to 2008. Results show that groundwater improves the efficiency scores for about 70% of the gauging stations and deteriorates them for 15%. The simulated TWS are also in better agreement with the GRACE estimates. These results are mainly explained by the lag introduced by the low-frequency variations of groundwater, which tend to shift and smooth the simulated river discharges and TWS. A sensitivity study on the global precipitation forcing used in ISBA to produce the forcing fields is also proposed. It shows that the groundwater scheme is not influenced by the uncertainties in precipitation data.
Highlights
Land surface processes considerably influence the global climate system (Dirmeyer, 2001; Dirmeyer et al, 2000; Douville, 2003, 2004; Koster et al, 2000, 2002). They can affect the water and energy exchanges between land surface and atmosphere, the ocean temperature and salinity at the outlet of the largest rivers (Durand et al, 2011), and the climate, at least at regional scales (Alkama et al, 2007; Douville et al, 2000; Gedney et al, 2000; Lawrence and Slater, 2008; Molod et al, 2004). These land surface processes are parameterized in the continental hydrological systems (CHSs), which are composed of land surface models (LSMs) generally coupled with river routing models (RRMs)
This simulation was forced by the global meteorological forcing from Princeton University (Sheffield et al, 2006), where the precipitation is hybridized with the Global Precipitation Climatology Center (GPCC) datasets since the GPCC climatology certainly appears to be the best dataset for global hydrological applications (Decharme and Douville, 2006)
A methodology based on Vergnes et al (2012) has been used to construct a global groundwater model to investigate the effects of groundwater processes on river discharges and terrestrial water storage (TWS) variations at global scale
Summary
Land surface processes considerably influence the global climate system (Dirmeyer, 2001; Dirmeyer et al, 2000; Douville, 2003, 2004; Koster et al, 2000, 2002). Despite its long response time, groundwater is an important component of the continental part of the global hydrological cycle It represents about 30 % of the continental fresh water reservoir, and its interaction with the soil surface is likely to influence the soil moisture in unsaturated zones and the water and energy exchanges with the lower atmosphere (Anyah et al, 2008; Fan et al, 2007; Shiklomanov and Rodda, 2003). It helps to sustain river base flows during the dry season in temperate zones, whereas it receives seepage from rivers in arid regions (Brunke and Gonser, 1997)
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