Abstract
Abstract. Groundwater is the world's largest accessible source of fresh water. It plays a vital role in satisfying basic needs for drinking water, agriculture and industrial activities. During times of drought groundwater sustains baseflow to rivers and wetlands, thereby supporting ecosystems. Most global-scale hydrological models (GHMs) do not include a groundwater flow component, mainly due to lack of geohydrological data at the global scale. For the simulation of lateral flow and groundwater head dynamics, a realistic physical representation of the groundwater system is needed, especially for GHMs that run at finer resolutions. In this study we present a global-scale groundwater model (run at 6' resolution) using MODFLOW to construct an equilibrium water table at its natural state as the result of long-term climatic forcing. The used aquifer schematization and properties are based on available global data sets of lithology and transmissivities combined with the estimated thickness of an upper, unconfined aquifer. This model is forced with outputs from the land-surface PCRaster Global Water Balance (PCR-GLOBWB) model, specifically net recharge and surface water levels. A sensitivity analysis, in which the model was run with various parameter settings, showed that variation in saturated conductivity has the largest impact on the groundwater levels simulated. Validation with observed groundwater heads showed that groundwater heads are reasonably well simulated for many regions of the world, especially for sediment basins (R2 = 0.95). The simulated regional-scale groundwater patterns and flow paths demonstrate the relevance of lateral groundwater flow in GHMs. Inter-basin groundwater flows can be a significant part of a basin's water budget and help to sustain river baseflows, especially during droughts. Also, water availability of larger aquifer systems can be positively affected by additional recharge from inter-basin groundwater flows.
Highlights
Groundwater is a crucial part of the global water cycle
In order to understand variations in recharge and human water use affect groundwater head dynamics, lateral groundwater flow and groundwater surface water interactions should be included in global-scale hydrological models (GHMs), especially as these GHMs progressively move towards finer resolutions (Wood et al, 2012; Krakauer et al, 2014)
Higher coefficient of variation (CV) are found for the Sahara and Australian desert, where recharge is low, transmissivities are high, and groundwater levels become disconnected from the surface
Summary
Groundwater is a crucial part of the global water cycle It is the world’s largest accessible source of fresh water and plays a vital role in satisfying basic needs of human society. It is a primary source for drinking water and supplies water for agriculture and industrial activities (Wada et al, 2014). By supplementing the water budget, incoming interbasin groundwater helps to sustain baseflows during droughts thereby increasing surface water availability for human water needs (de Graaf et al, 2014). The current generation of GHMs typically does not include a lateral groundwa-
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