Evaporation from a tropical rain forest, Luquillo Experimental Forest, eastern Puerto Rico

Evaporation losses from a watertight 6.34 ha rain forest catchment under wet maritime tropical conditions in the Luquillo Experimental Forest, Puerto Rico, were determined using complementary hydrological and micrometeorological techniques during 1996 and 1997. At 6.6 mm d -1 for 1996 and 6.0 mm d -1 for 1997, the average evapotranspiration (ET) of the forest is exceptionally high. Rainfall interception (Ei), as evaluated from weekly throughfall measurements and an average stemflow fraction of 2.3%, accounted for much (62-74%) of the ET at 4.9 mm d -1 in 1996 and 3.7 mm d -1 in 1997. Average transpiration rates (Et) according to a combination of the temperature fluctuation method and the Penman-Monteith equation were modest at 2.2 mm d -1 and 2.4 mm d -1 in 1996 and 1997, respectively. Both estimates compared reasonably well with the water-budget-based estimates (ET - Ei) of 1.7 mm d -1 and 2.2 mm d -1. Inferred rates of wet canopy evaporation were roughly 4 to 5 times those predicted by the Penman- Monteith equation, with nighttime rates very similar to daytime rates, suggesting radiant energy is not the dominant controlling factor. A combination of advected energy from the nearby Atlantic Ocean, low aerodynamic resistance, plus frequent low-intensity rain is thought to be the most likely explanation of the observed discrepancy between measured and estimated Ei.