PERCHED AQUIFER HYDROLOGY: AN UNEXPLORED FRONTIER?

Nearly two-hundred years ago Henry Darcy pursued fundamental knowledge on flow through porous media via experimentation and observation. The casual observer might then conclude that virtually all of the “fundamental knowledge” on groundwater has been collected and polished since Darcy. To the contrary, recent work on ecohydrology indicates that perched aquifers could be vital to certain ecosystem functions, yet perched aquifer hydrology fundamentals seem to be relatively unexplored. Analysis of groundwater and surface water interaction in typically heterogeneous alluvial deposits (Central Valley, CA) suggests that aquifer perching plays an important, perhaps unanticipated, role in stream base flow, vernal pool (seasonal wetland) function, and vegetation establishment, especially in basins that are substantially overdrafted. In such basins, conventional wisdom states that until the regional water table rises and intersects the surface, groundwater management will have no effect on base flow or wetland status. If, however, aquifer perching can play a significant role, the interesting possibility of perched aquifer management is raised. Such management would rest on a scientific foundation that provides general understanding of the potential function of perched aquifers in ecosystems and, for example, the conditions (e.g., perching layer geometry and hydraulic conductivity, recharge rates, layer slope, etc.) necessary to generate a perched aquifer capable of prolonging base flow or establishing vegetation at the key times. This scientific foundation, although within reach of current technologies and methods, appears to be virtually nonexistent because hydrogeologists have largely pursued analysis of aquifers that could be exploited for water supply rather than perched aquifers that are usually irrelevant to groundwater supply development. Now that the role of groundwater in ecohydrology is becoming more apparent, and not just for exploitation of water supplies, a new opportunity has emerged for pursuit of hydrologic fundamentals.