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Artificial recharge ponds have been used increasingly in recent years to store water in underlying aquifers and modify baseline groundwater gradients or alter natural hydrologic fluxes and state variables in an aquifer system. The number of constructed ponds, their geographic spacing, and the volume of water diverted to each pond can have a significant impact on baseline system hydrologic fluxes and state variables such as groundwater head, with the latter sometimes rising to cause waterlogging in cultivated areas. This study seeks to quantify the impact of recharge ponds on groundwater state variables (head, saturated thickness) and associated fluxes within an irrigated stream-aquifer system. We use a numerical modeling approach to assess the impact of a set of 40 recharge ponds in a 246 km2 region of the South Platte River Basin, Colorado on localized groundwater head, regional groundwater flow patterns, and groundwater interactions with the South Platte River. We then use this information to determine the overall influence of recharge ponds on the hydrologic system. A linked agroecosystem–groundwater (DayCent-MODFLOW) modeling system is used to simulate irrigation, crop evapotranspiration, deep percolation to the water table, groundwater pumping, seepage from irrigation canals, seepage from recharge ponds, groundwater flow, and groundwater–surface water interactions. The DayCent model simulates the plant–soil-water dynamics in the root zone and soil profile, while MODFLOW simulates the water balance in the aquifer system. After calibration and testing, the model is used in scenario analysis to quantify the hydrologic impact of recharge ponds. Results indicate that recharge ponds can raise groundwater levels by approximately 2.5 m in localized areas, but only 15 cm when averaged over the entire study region. Ponds also increase the rate of total groundwater discharge to the South Platte River by approximately 3%, due to an increase in groundwater hydraulic gradient, which generally offsets stream depletion caused by groundwater pumping. These results can assist with groundwater resource management in the study region, and generally provide valuable information for the interplay between pumping wells and recharge ponds, and their composite effect on groundwater–surface water interactions. In addition, the developed linked DayCent-MODFLOW modeling system presented herein can be used in any region for which recharge rates should be calculated on a per-field basis. [ABSTRACT FROM AUTHOR]

APEX (A gricultural P olicy/ E nvironmental e X tender) is an oft-used agroecosystem model but has limited use in groundwater-driven watersheds due to a simplistic representation of groundwater processes. This paper presents the linkage of APEX and the groundwater flow model MODFLOW into a single modeling code. The mapping of recharge, groundwater head, and groundwater-surface water interactions are handled internally via subroutines. The APEX-MODFLOW model is applied to three watersheds in the United States for testing code accuracy and hydrologic state variables and fluxes: the Animas River Watershed, Colorado and New Mexico (3543 km2); the Price River Watershed, Utah (4886 km2); and the Middle Bosque River Watershed, Texas (470 km2). Whereas the hydrology of the Animas River and Price River watersheds is driven by snowmelt and spring runoff, the hydrology of the Middle Bosque River Watershed is driven by summer thunderstorms. The model can be used for scenario analysis in groundwater-driven watersheds. • New integrated hydrologic model links APEX model to MODFLOW model. • Model passes recharge, water table elevation, and gw-sw flux rates internally. • APEX-MODFLOW applied to three watersheds in Utah, Colorado, and Texas, USA. • Model results show importance of including physically based groundwater flow. [ABSTRACT FROM AUTHOR]