STREAMFLOW RELATIONS IN THE PUERTO RICAN CORDILLERA

Puerto Rico is likely the world’s most densely stream-gaged tropical region. In 2000, on this mountainous island of 3435 mi², the US Geological Survey maintained 85 continuous stream gages, plus sediment and water quality stations. Instantaneous or periodic discharge is recorded for at least 1725 mi² (a large additional indeterminate area is contributed from the island’s abundant karst terrane). The remainder is either below the lowermost gage of a catchment, enters the sea directly, or is low-lying arid land with no channel network.

Currently, 72 stream gages have periods of record of ten years or more, dominantly in northeastern and west-central basins. The lowermost gages within catchments [30] record a mean annual instantaneous discharge of 3124 cfs, for areas given as 1553 mi². This figure is compromised by basinal transfers and irrigation canals, but errs chiefly in ignoring underground drainage from karst. Further complications of the karst are drainage directly to the coast, and unknown discharges to offshore springs. Excluding gages affected by karst catchments, the remaining 1163 mi² has a total mean annual runoff of 2195 cfs, with mean annual areal discharges of 1.89 cfs/mi². Just 5 rivers account for 55% of this total.

As expected at these 32 non-karst gages, total discharge increased with area, with an R² of 0.71. These catchments were then assigned values corresponding to the mean 30-year rainfall within their drainages. Areal discharge also corresponded well with the mean rainfall (R² 0.76), but was a minor improvement over the simple area-flow relation. The x-intercept of 35 inches rainfall per year is an amount largely corresponding to the non-karst areas without stream networks, and suggests a lower limit below which evapo- transpiration exceeds potential runoff. Mean annual rainfall for the entire gaged area is 61.6 inches (island range: 25-170 inches), with annual runoff of 27.6 inches; evapotranspiration is about 55%.

Excluding those areas below 35 inches/y , and adding 900 mi² of karst, the mean total instantaneous discharge from the island is perhaps 5200-5600 cfs. Rainfall increases also correlate with increasing percentage rates of runoff: the mountainous one-third of Puerto Rico is disproportionately important for the discharges eventually found at the more arid lower elevations.