Effects of Tropical Forest Conversion on Hydrologic Response: Process Implications for Potable Water and Sediment Budgets in Karstic Uplands

Abstract:

Conversion forest to agricultural land has been attended by marked hydrological and ecological responses and has arguably made a considerable contribution to global change. These changes have been less well studied in the tropics than in temperate climates, yet the impact of such changes on potable water security is more immediate for rural landholders in the tropics than for industrialized societies with extensive built infrastructure to buffer hydrologic variability. A case study is presented for Leyte, Philippines. The most immediate concern of upland farmers was the decreasing availability of water, both for household and agricultural use. They attributed the changes in hydrology to deforestation, but were unclear about the hydrologic processes involved. The primary goal of development organizations conducting projects in the region was soil conservation, through implementation of erosion control practices. The most common soil conservation/agroforestry approach promoted in the region was alley farming between contour hedgerows. This study was conducted to identify the mechanisms governing hydrologic change and soil loss in a succession of land uses. Forest removal and conversion to swidden agriculture was found to have less dramatic impacts on surface runoff than did soil disturbance associated with tilled agriculture and grazing. Soil erosion was more strongly associated with soil disturbance than with extent of surface runoff. The interplay between the shift in hydrologic processes and tillage frequency for the agricultural land uses resulted in the proposed conservation practice exacerbating the erosion rates within the catchments. Although on site erosion for the pasture catchment was minimal, this treatment was found to have the lowest infiltration capacity and likely contributed greatly to the loss of perennial springs in the uplands and erosion of lowland rice paddies. These results suggest that land uses and agricultural practices that maintain soil infiltration capacity will have the greatest success in maintaining local potable water sources and reducing soil erosion.