Modeling Steady-State Groundwater and Surface Water Interactions

An existing steady state analytic element model has been expanded to model conjunctive groundwater and surface water flow. The model solution provides groundwater flow fields and baseflows throughout the watershed. In addition to matching modeled and observed heads, the model is calibrated by comparing measured and modeled baseflows at a single surface water gaging station. This additional means for calibrating the model reduces the uncertainty in the areal recharge rate and the hydraulic conductivity. By constraining stream infiltration to available baseflow, more realistic boundary conditions for groundwater flow are obtained and the quality of the groundwater solution is improved. Using the model to investigate the hydrologic impacts of changing recharge and changing boundary conditions for groundwater flow in Michigan's Pere Marquette River Basin, the concept of expanding and contracting groundwatersheds is explored. Under the influence of recharge variations, groundwatersheds expand and contract as local groundwater flow cells expand and contract. On a local scale, as recharge changes, the contributing area for a surface water body adjusts. For some surface water features, these adjustments cause the baseflow to be more stable. For other local sites, baseflow is more sensitive to changing recharge. On a larger regional scale, the local expansion and contraction of groundwatersheds is averaged out and the groundwatershed remains stable through a range of recharge variations.

The model that has been developed here is based on the Dupuit-Forchheimer assumption. However, Toth's theory of regional groundwater flow suggests patterns of regional groundwater flow that include significant vertical components. In Toth's solutions, the constraints of available recharge were not considered, leading to misconceptions concerning the importance of vertical flow in regional aquifers and the distribution of recharge and discharge. Local and intermediate flow cells develop in low permeability aquifers when the ratio of recharge to hydrualic conductivity is unusually large or in moderately permeable aquifers when the terrain is relatively flat or sharply dissected. When recharge is distributed uniformly along the surface of a surficial aquifer, the resulting flow is predominantly horizontal and therefore can be adequately modeled with two-dimensional Dupuit-Forchheimer models.