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Abstract An analytical model is developed for mean annual groundwater evapotranspiration (GWET) at the watershed scale based on a three‐stage precipitation partitioning framework. The ratio of mean annual GWET to precipitation, defined as GWET ratio, is modeled as a function of climate aridity index (CAI), storage capacity index, the shape parameter ‘a’ for the spatial distribution of storage capacity, and the shape parameter ‘b’ for the spatial distribution of available water for GWET. In humid regions, GWET ratio tends to increase with increasing CAI due to the limited energy supply and shallower depth to water table (DWT) for a given storage capacity index. In contrast, in arid regions, the GWET ratio tends to decrease as the CAI increases because of the limited water availability and the presence of a deeper DWT for a given storage capacity index. In arid regions, the GWET ratio decreases as the parameter ‘a’ increases, mainly because of increased ET from a thicker unsaturated zone in environments with a deeper DWT. GWET ratio increases as parameter ‘b’ increases due to more watershed area with larger available water for GWET. The storage capacity index and shape parameters are estimated for 31 study watersheds in Tampa Bay Florida area based on the simulated GWET from an integrated hydrologic model and for 21 watersheds from literature. A possible correlation has been identified between the two shape parameters in the Tampa Bay watersheds. The analytical model for mean annual GWET can be further tested in other watersheds if data are available. 

Abstract A three‐stage precipitation partitioning framework is proposed to study the climate controls on mean annual groundwater evapotranspiration (GWET) for 33 gauged watersheds in west‐central Florida. Daily GWET, total evapotranspiration (ET), groundwater recharge, base flow, and total runoff are simulated by the Integrated Hydrologic Model, which dynamically couples a surface water model (HSPF) and a groundwater flow model (MODFLOW). The roles of GWET on long‐term water balance are quantified by four ratios. The ratios of GWET to total available water, watershed wetting, ET, and recharge decrease exponentially with watershed aridity index (WAI), which is defined as the ratio of potential evapotranspiration to total available water. In the one‐stage precipitation partitioning framework, the contribution of GWET to the ratio between total ET and available water for ET (i.e., they‐axis of Budyko curve) decreases with WAI. In the two‐stage precipitation partitioning framework, the contribution of GWET to the ratio between total ET and watershed wetting (i.e., Horton index) decreases with WAI. The changes in GWET caused by intra‐monthly (IM) climate variability are the highest among the temporal scales of climate variability investigated to understand controls on GWET. The inter‐annual, intra‐annual, and IM climate variabilities lead to increase of GWET; but the sub‐daily climate variability results in decrease of GWET. For the third stage of partitioning, given the same ratio of potential GWET to available water for GWET, higher percentage of forest and wetland and lower percentage of impervious land contribute to higher ratio of GWET to available water for GWET.