Reversibility of forest conversion impacts on water budgets in tropical karst terrain

A conceptual model of the control of tropical land use and vegetative cover on bedrock recharge is developed for highly permeable geologic substrates. A case study of water budgets is then developed from field data and simple modeling for upland sites with three different vegetative covers (cropland, intensively grazed pasture and forest regrowth) in Leyte, Philippines. Water budget model results show that annual precipitation is divided primarily between evapotranspiration and overland flow for the pasture, but apportioned more to evapotranspiration and inputs to bedrock storage for the crop and forest sites. Modeled evapotranspiration from the forest (1906 mm) was not sufficiently greater than that for either the crop (1661 mm) or pasture (1476 mm) sites to offset the greater overland flow from those sites. The differences in overland flow are related to depth profiles of soil bulk density, which decreased between crop and forest and increased between crop and pasture, and drainable porosity, which increased between crop and forest and decreased between crop and pasture. Dry season streamflow is assumed to be primarily base flow and dependent on wet season bedrock recharge, which was dramatically lower for the pasture (106 mm) than for the crop (1134 mm) or forest covers (1320 mm), for 2946 mm of rainfall. The results support the premise that for landscapes with adequate storage in bedrock fractures, forest regrowth can increase recharge to perched aquifers, and hence dry season baseflow, relative to cropping and that dramatic reductions in overland flow and increases in dry season baseflow may be achieved by reforestation of compacted pastures.