Modeling the effects of throughfall reduction on soil water content in a Brazilian Oxisol under a moist tropical forest

Access to water reserves in deep soil during drought periods determines whether ornot the tropical moist forests of Amazonia will be buffered from the deleterious effectsof water deficits. Changing climatic conditions are predicted to increase periods of droughtin Amazonian forests and may lead to increased tree mortality, changes in forestcomposition, or greater susceptibility to fire. A throughfall reduction experiment has beenestablished in the Tapajo ́s National Forest of east-central Amazonia (Brazil) to test thepotential effects of severe water stress during prolonged droughts. Using time domainreflectometry observations of water contents from this experiment, we have developed adynamic, one-dimensional, vertical flow model to enhance our understanding ofhydrologic processes within these tall-stature forests on well-drained, upland, deepOxisols and to simulate changes in the distribution of soil water. Simulations using960 days of data accurately captured mild soil water depletion near the surface afterthe first treatment year and decreasing soil moisture at depth during the second treatmentyear. The model is sensitive to the water retention and unsaturated flow equationparameters, specifically the van Genuchten parametersqs,qr, andn, but less sensitive toKsanda. The low root-mean-square error between observed and predicted volumetric soilwater content suggests that this vertical flow model captures the most importanthydrologic processes in the upper landscape position of this study site. The modelindicates that present rates of evapotranspiration within the exclusion plot have beensustained at the expense of soil water storage.