Amazonia's Hydrologic Cycle and the Role of Atmospheric Recycling in Assessing Deforestation Effects

Abstract

Atmospheric and hydrologic phases of the water cycle in Amazonia are modeled coherently by subdividing the basin (which extends from about 47.5° to 77.5° west longitude) into six adjacent regions each 5° wide. Due to long-distance travel of the air masses, the recycling of vapor evaporated from upwind land surfaces is important. For example, it is found that 88% of the water precipitating on the westernmost region falls at least a second time from the air. The modeling of the longitudinal profiles of precipitable water in the atmosphere as well as exchangeable moisture in the soil is supplemented by the modeling of the complementary longitudinal profiles of annual mean surface and air temperature. In a climatonomical experiment, the forest cover in central Amazonia (i.e., in the two regions between about 57.5° and 67.5°W) is assumed to be reduced to one-half of the original value, thereby increasing the evaporative flushing rate of soil moisture in the two partly deforested regions. Computations show that due to recycling the precipitation increases at all longitudes between 57.5° and 77.5°W; however, the primary augmentation of evaporation outweighs the precipitation increase. Consequently, total runoff from the entire basin decreases from the original 1075 mm year^-1 to 1018 mm year^-1, but the atmospheric throughflow (i.e., precipitable water moving westward across the continental divide) increases by 3%, which could benefit the “dry side” of the Andes cordillera. Soil moisture in the Amazonas basin is found to decrease at all longitudes eastward of the midpoint of the partly deforested area.