Deriving Both Areas and Boundaries of Groundwater Catchments from Radar-Rainfall and Discharge Records in Puerto Rico

Discharges from three USGS gaging stations (2003-2006) were correlated with areal rainfall obtained from a NEXRAD Doppler radar site 70 km to the east, and used to outline subsurface flow boundaries in the Northern Karst of Puerto Rico. USGS gage #50028000 records the flow (annual mean 1.4 cms) of the Río Tanamá draining an upland catchment (47 km²) developed on volcaniclastics; the gage is located at the boundary of a mature holokarst. The river traverses nine caves as it passes through the karst, and its flow is then recorded at USGS gage #50028400 (mean 2.8 cms), coinciding with its exit from the karst. The difference between the station discharges represents the total influx of groundwater flow in that karst catchment.

When a discharge event was recorded at a gage, radar-rainfall (RR) of the preceding 24 hours was integrated with previous RR amounts associated with other discharge events- using GIS software- to produce an evolving hyetograph image of the local region (about 450 km²). The hypothesis was that a summed sequence of causal precipitation events would highlight and identify those areas contributing to discharge at a gage point.

This procedure was used to establish boundaries between the Tanamá karst catchment and the neighboring karst of the Río Camuy to the west, recorded at gage #50014800 (mean 3.1 cms). RR hyetographs were produced from 11 discharge events that occurred on the Tanamá, but which were absent or minimal on the Camuy, and for 7 discharge events that occurred on the Camuy but not on the Tanamá. The resulting hyetographs corresponded with several known limits of the catchment boundaries, showed a clear separation between the two groundwater catchments (core areas of about 30 and 50 km² respectively), and provided hydrogeological detail about the local influence of non-permeable layers, evapotranspiration, and rain-runoff lag times.