USING MODELS TO ESTIMATE GROUNDWATER RECHARGE

Groundwater recharge is the movement of water from land surface or a surface-water body to the water table. Recharge rates affect groundwater supplies, groundwater discharges to ecosystems, and movement of potential contaminants from land surface to the water table. Therefore, knowledge of the rate, location, and timing of recharge is critical for effective management of water resources. Of the many different methods that are used for estimating groundwater recharge, models are particularly powerful tools. Models can be used to estimate current and past rates of recharge; models can also be used to predict the effects of future land-use and climate change on recharge rates.

Model complexity varies greatly. Some models are simple accounting models. Other models attempt to accurately represent the physics of water movement through each compartment of a hydrologic system; still other models are entirely empirical in nature and have no physical basis. Physically based distributed-parameter models can provide insight on processes that affect recharge; however, these models may require large amounts of data and expertise in model application. Empirical models, on the other hand, are easy to apply and are amenable to upscaling with GIS; empirical models, however, provide little information on factors that influence recharge. Some models provide estimates of recharge directly; for example, a model based on the Richards equation can simulate water movement from the soil surface through the unsaturated zone to the water table. Other models, such as groundwater-flow models, estimate recharge indirectly.

Space and time scales of recharge estimates vary for different models and model applications. Applications of watershed or groundwater-flow models may assume that recharge is uniform over the simulated domain, or recharge may be allowed to vary in space within the domain. In terms of time scales, recharge can be estimated on a daily, monthly, or annual basis; or recharge may be assumed constant over time.

Models for estimating recharge can be divided into the five categories: unsaturated zone water-budget models, watershed models, groundwater-flow models, combined watershed/groundwater-flow models, and empirical models. Models within each category share a common approach, but complexity and features of models vary substantially. An overview of models within each category will be presented, representative models will be described and their underlying assumptions analyzed, and examples of model applications will be discussed.