Rainfall, Runoff and Elevation Relationships in the Luquillo Mountains of Puerto Rico*

Long-terrn rainfall and discharge data from the Luquillo Experimental Forest (LEF) were an- alysed to develop relationships between rainfall, stream-runoff and elevation. These relationships were then used with a Geographic Information System (GIS) to determine spatially-averaged, mean annual hydrologic budgets for watersheds and forest types within the study area. A significant relationship exists between 1) elevation and mean annual rainfall; 2) elevation and the average number of days per year without rainfall; 3) annual stream runoff and the weighted mean elevation of a watershed; and 4) annual stream runoff and the elevation of the gaging station. A comparison of rainfall patterns between a high and a low elevation station indicated that annual and seasonal variations in rainfall are similiar along the elevational gradient. However, the upper elevation station had greater annual mean rainfall (4436 mm/yr compared to 3524 mn/yr) while the lower station had a greater variation in daily, monthly, and annual totals. Model estimates indicate that a total of 3864 mm/yr (444 hm 3 ) of rainfall falls on the forest in an average year. The Tabonuco, Colorado, Palm, and Dwarf forest types receive an estimated annual rainfall of 3537, 4191, 4167, and 4849 mm/yr, respectively. Of the average annual rainfall input, 65% (2526 mm/yr) is converted to runoff and the remaind- ing 35% (1338 mm/yr) is lost from the system by evapotranspir ation and other abstractions. In comparsion to other tropical forests, the LEF as a whole has more evapotranspiration than many tropical montane forests but less than many lowland tropical forests. RESUMEN.—Mediante un analisis de regresion, se identificaron relaciones entre lluvia, descarga, y elevacion, utilizando datos obtenidos de estaciones localizadas en la Estacion Experimental de Luquillo (EEL). Estas relaciones fueron integradas a un Sistema de Informacion Geografico para lograr un analisis espacial y balance de agua para las cuencas y tipos de bosque en el area de estudio. Existen relaciones significativas entre: 1) elevacion y lluvia promedio anual: 2) elevacion y el numero promedio de dias al ano sin lluvia; 3) descarga promedio anual y la elevacion promedio; y 4) descarga anual y la elevacion de esa misma estacion. Una comparacion de datos de lluvia obtenidos en partes altas y bajas del bosque demostro un patron de lluvia anual muy similar. La estacion localizada a una mayor elevacion tuvo una mayor cantidad de lluvia promedio anual (4436 mm/yr vs. 3524 mm/yr), mientras que la estacion localizada en la parte baja del bosque presento una mayor variacion en los datos de lluvia darios, mensuales y anuales. Utilizando los modelos desarrollados, podemos estimar un promedio de 3864 mm/yr (444 hm