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

Evaporation losses from a watertight 6.34 ha rain forest catchment under we t maritime tropical conditions in the Luquillo Experimental Forest, Puerto Rico, were determined using complementary hydrological and micrometeorologi cal 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 exceptio nally high. Rainfall interception (Ei), as evaluated from weekly throughfal l 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. Avera ge transpiration rates (E-t) 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 dr in 1996 and 1997, respectively. Both estimates compared reasonably well with the water-budget-based estimates (ET - E-i) of 1.7 mm d(-1) and 2.2 mm d(-1). Inferred rates of wet canopy evaporation were rough ly 4 to 5 times those predicted by the Penman-Monteith equation, with night time 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-intens ity rain is thought to be the most likely explanation of the observed discr epancy between measured and estimated E-i.