This paper describes coupling field experiments with surface and groundwater modeling to investigate rangelands of SE Arizona, USA using erosion-control structures to augment shallow and deep aquifer recharge. We collected field data to describe the physical and hydrological properties before and after gabions (caged riprap) were installed in an ephemeral channel. The modular finite-difference flow model is applied to simulate the amount of increase needed to raise groundwater levels. We used the average increase in infiltration measured in the field and projected on site, assuming all infiltration becomes recharge, to estimate how many gabions would be needed to increase recharge in the larger watershed. A watershed model was then applied and calibrated with discharge and 3D terrain measurements, to simulate flow volumes. Findings were coupled to extrapolate simulations and quantify long-term impacts of riparian restoration. Projected scenarios demonstrate how erosion-control structures could impact all components of the annual water budget. Results support the potential of watershed-wide gabion installation to increase total aquifer recharge, with models portraying increased subsurface connectivity and accentuated lateral flow contributions.

Response functions for drawdown, velocity, storage losses, and capture describe the spatial and temporal reaction of an aquifer to a unit pumping stress. This paper extends the use of response functions to the notion of capture for multilayered aquifer systems. Capture describes the pumping‐induced quantity of water gained by the aquifer from internal or boundary sources. Internal sources include rivers and hydraulically connected aquifers. Boundary sources include constant head and head‐dependent boundaries. Although response functions are defined only for linear systems, a methodology is demonstrated for implementing response functions in cases of nonlinear capture. The value of response functions lies in their utility as hydrologic constraints in management optimization models. An example demonstrates the use of separable programming to determine the nonlinear effects of drawdown below the bottom of a streambed.

Abstract:The San Pedro River originates in Sonora, Mexico, and flows north through Arizona, USA, to its confluence with the Gila River. The 92-km Upper San Pedro River is characterized by interrupted perennial flow, and serves as a vital wildlife corridor through this semiarid to arid region. Over the past century, groundwater pumping in this bi-national basin has depleted baseflows in the river. In 2007, the United States Geological Survey published the most recent groundwater model of the basin. This model served as the basis for predictive simulations, including maps of stream flow capture due to pumping and of stream flow restoration due to managed aquifer recharge. Simulation results show that ramping up near-stream recharge, as needed, to compensate for downward pumping-related stress on the water table, could sustain baseflows in the Upper San Pedro River at or above 2003 levels until the year 2100 with less than 4.7 million cubic meters per year (MCM/yr). Wet-dry mapping of the river over a period of 15 years developed a body of empirical evidence which, when combined with the simulation tools, provided powerful technical support to decision makers struggling to manage aquifer recharge to support baseflows in the river while also accommodating the economic needs of the basin.

Abstract:Groundwater pumping along portions of the binational San Pedro River has depleted aquifer storage that supports baseflow in the San Pedro River. A consortium of 23 agencies, business interests, and non-governmental organizations pooled their collective resources to develop the scientific understanding and technical tools required to optimize the management of this complex, interconnected groundwater-surface water system. A paradigm shift occurred as stakeholders first collaboratively developed, and then later applied, several key hydrologic simulation and monitoring tools. Water resources planning and management transitioned from a traditional water budget-based approach to a more strategic and spatially-explicit optimization process. After groundwater modeling results suggested that strategic near-stream recharge could reasonably sustain baseflows at or above 2003 levels until the year 2100, even in the presence of continued groundwater development, a group of collaborators worked for four years to acquire 2250 hectares of land in key locations along 34 kilometers of the river specifically for this purpose. These actions reflect an evolved common vision that considers the multiple water demands of both humans and the riparian ecosystem associated with the San Pedro River.

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