FOLLOWING IN THE FOOTSTEPS OF MARY STOERTZ: USING MODELS TO UPSCALE ESTIMATES OF HUMID-CLIMATE RECHARGE

Mary Stoertz’s graduate research investigated a variety of methods for estimating groundwater recharge in humid climates. While her research included intensive field measurements of a variety of parameters from detailed soil profiles, she also recognized the need to upscale recharge estimates for use on larger scale problems. To address this need, she and Ken Bradbury developed an ingenious modification to the USGS MODFLOW code that calculated recharge from a measured head field. This method has been applied to systems ranging from sand aquifers to wetlands. In the years since, models continue to be developed and applied to estimate recharge. The approaches have ranged from simple soil water balance approaches to full-blown coupled groundwater-surface water models. Given nearly 20 years of work since Mary’s research, some overarching observations can be made.

First, unsaturated zone thickness is an important consideration, both for timing and volume of recharge – a phenomenon well recognized in semi-arid settings, but now recognized in humid settings as well. Second, water balance approaches need to be constrained and evaluated on an upscaled basis in order to judge representativeness of the results. Thirdly, as models become more sophisticated, they can outpace the ability of commonly collected field observations to constrain model parameters. For example, fully coupled groundwater-surface water models often include climatic/atmospheric drivers that depend on energy balance parameters, in addition to the hydrologic parameters used to simulate processes in the soil, unsaturated, and saturated zones. The leverage of a stream baseflow observation is likely not sufficient to elucidate and constrain the values of the all parameters (like recharge) needed for its simulation. This suggests a need for a diverse suite of field observations at multiple scales to overcome issues of parameter correlation and non-uniqueness – an observation very much in line with the breadth of approaches and field data used to estimate recharge in the original graduate work of Mary Stoertz.