MAPPING GROUNDWATER WITHDRAWAL FOOTPRINTS AT THE SUB-BASIN SCALE

A simple method is proposed to display the magnitude of groundwater withdrawals from wells on a gridded map. The map is solely an illustration of the withdrawal magnitudes, spatially centered on wells; it does not depict the capture zones or the contributing areas of recharge flowing to the wells. Pumping-well symbols on maps are commonly sized proportional to withdrawals to display those rates. The maximum size is adjusted so that small wells are clearly visible while closely spaced wells do not overlap too much. In contrast to this graphic-design-guided scaling, the proposed method uses a depth-rate index, in units of length per time, to display withdrawal rates by volumetrically consistent areas, or “footprints”. The product of the depth-rate index (inches per year, for example) and each individual well’s footprint area equals that well’s withdrawal rate. To account for the overlap of individual footprints of closely spaced wells, composite footprints are computed by areal distribution of withdrawal rates on a grid (a raster). Each grid cell is assigned a maximum withdrawal rate corresponding to the product of the depth-rate index and the area of the cell. If total withdrawals for wells within a cell exceeds this maximum, the excess is recursively distributed to adjacent cells, which leads, in the limit, to distributed withdrawal rates equal to the cell’s maximum value, zero, or between zero and the maximum. The resulting map shows circular footprints around isolated wells, but merged footprint shapes where wells are closely spaced. This method also accounts for the non-circular shape of footprints that abut grid boundaries, or domain boundaries within the grid. Example applications use depth-rate indexes corresponding to the rate of water-table decline (scaled by specific yield), stream base-flow statistics (normalized by drainage-basin area), and spatially variable recharge. The groundwater withdrawal footprint map can be used as a standalone illustration of withdrawal magnitudes at the sub-basin scale, or as a display of the magnitude of pumping rates input for groundwater-flow or water-budget models. The proposed algorithm can be incorporated in interactive map applications.