FLOW, RUNOFF AND EROSIONAL LOADING OF STREAMS IN PUERTO RICO

As of 2001, a mean annual runoff of 25.1 inches- and corresponding 69% evapotranspiration– was calculated for the lowermost 31 gages of catchments in Puerto Rico unaffected by the island's abundant karst terrane. Their mean areal discharges of 1.85 cfs/mi² produces an annual runoff of 2174 cfs within an area of 1163 mi², a third of the island's area. Just five rivers account for 55 % of this total.

As expected, long-term mean annual discharge of catchments increased with their area, an R² of 0.70. Isohyetal-area calculations of the mean 30-year rainfall within each drainages corresponded similarly with areal discharge (R²=0.68), demonstrating the disproportionate importance that the mountainous one-third of Puerto Rico has for the discharges eventually reaching more arid lower elevations. The annual rainfall for these basins is 79.9 inches, significantly above the mean 30-year island precipitation (NOAA, 1961-90) of 63 inches. The x-intercept of 50 inches/y is an amount largely corresponding to those non-karst areas without stream networks, and suggests a lower limit below which evapotranspiration exceeds potential runoff.

Puerto Rico is the world's most densely stream-gaged tropical location. In 2001, on this mountainous island of 3435 mi², the US Geological Survey maintained 87 continuous gages of streams (plus canals, and sediment and water quality stations). The lowermost gages of these 30 catchments of 1553 mi² record a mean annual flow near 3100 cfs, but the calculated areas of contribution do not include underground drainage from (or losses to) karst, complicating efforts to estimate gross island runoff.

Excluding about 94 mi² of areas below 50 inches/y, and adding the remaining 2182 mi² (including the karsts) of about 15 inches runoff, Puerto Rico's mean instantaneous discharge may perhaps be as low as 4600 cfs. Solute loads calculated from USGS water quality stations would average 2600 tons/ day and total suspended solids about 450 tons/ day. The latter figure is depressed by storage behind numerous dams on the island's rivers.