THE POWER OF OBSERVATION--ESTABLISHING A SCIENTIFIC BASIS FOR STATEWIDE LANDSCAPE AND GROUND WATER RECHARGE MAPPING

With the growing availability of spatial data coverages in digital format, a Pandora’s Box full of landscape characterization alternatives has been flung open. Our challenge in traversing this maze is to relate mapped attributes to real-world observations and accepted scientific concepts.

In North Carolina we have developed statewide landscape maps, built upon county soil surveys and the concepts of nested drainage basins and valley morphology of flats, slopes, and bottom-land landscape units. Along the bottom lands of major Coastal Plain drainages, a digital elevation model (DEM) was used to identify and equilibrate relic channel boundaries and recode outlier soil mapping units. Landscape units are based on an elevation sequence that differentiates soils having similar hydrological properties. These hydrologic properties can be used to develop derivative maps, such as ground water recharge.

To map recharge, we first had to expand our landscape unit classification to differentiate the more clayey Piedmont soils impeding the infiltration of water, then create recharge units by grouping landscape units having similar recharge properties. Next, we developed a relationship between these recharge units and stream base flow, first employing the Rorabaugh-Daniel hydrograph separation model in selected USGS gaged drainages to estimate base flow. Assuming that estimated base flow equaled groundwater discharge, which could be equated with recharge, we calibrated recharge unit recharge rates with the estimated base flow in each drainage, weighted by mean rainfall, using Monte Carlo simulation to minimize the error between aggregate estimated recharge and base flow for all selected drainages. The final recharge maps illustrate the role landscape position and underlying geology play in regulating ground water recharge.