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. In global hydrological models, groundwater (GW) istypically represented by a bucket-like linear groundwater reservoir.Reservoir models, however, (1) can only simulate GW discharge to surfacewater (SW) bodies but not recharge from SW to GW, (2) provide no informationon the location of the GW table, and (3) assume that there is no GW flowamong grid cells. This may lead, for example, to an underestimation ofgroundwater resources in semiarid areas where GW is often replenished by SWor to an underestimation of evapotranspiration where the GW table is closeto the land surface. To overcome these limitations, it is necessary to replacethe reservoir model in global hydrological models with a hydraulic headgradient-based GW flow model. We present G 3 M, a new global gradient-based GW model witha spatial resolution of 5 ′ (arcminutes), which is to be integrated into the 0.5 ∘ WaterGAP Global Hydrology Model (WGHM). The newly developed model frameworkenables in-memory coupling to WGHM while keeping overall runtime relativelylow, which allows sensitivity analyses, calibration, and data assimilation.This paper presents the G 3 M concept and model designdecisions that are specific to the large grid size required for a global-scale model. Model results under steady-state naturalized conditions, i.e.,neglecting GW abstractions, are shown. Simulated hydraulic heads show betteragreement to observations around the world compared to the model output of de Graaf etal. (2015). Locations of simulated SW recharge to GW are found, as isexpected, in dry and mountainous regions but areal extent of SW recharge maybe underestimated. Globally, GW discharge to rivers is by far the dominantflow component such that lateral GW flows only become a large fraction oftotal diffuse and focused recharge in the case of losing rivers, somemountainous areas, and some areas with very low GW recharge. A strongsensitivity of simulated hydraulic heads to the spatial resolution of themodel and the related choice of the water table elevation of surface waterbodies was found. We suggest to investigate how global-scale groundwatermodeling at 5 ′ spatial resolution can benefit from more highly resolvedland surface elevation data.

geoscientific model development

Challenges in developing a global gradient-based groundwater model (G&amp;lt;sup&amp;gt;3&amp;lt;/sup&amp;gt;M v1.0) for the integration into a global hydrological model