Water Resources Research Act Program

Principal Investigators: Shahram Khosrowpanah, Yuming Wen

Abstract: Surface runoff and sediment losses from soil erosion are major contributors to reduction in surface water quality and subsequent degradation of the coral reefs in Southern Guam. A study of the Ugum watershed on Guam indicates that soil erosion from vegetated savanna grassland in the watershed is approximately 70 tons ha-1 yr-1 but can be as high as 547 tons ha-1 yr-1 in unvegetated sloping sites known as "badlands" (NRCS, 1996). in contrast, agricultural lands in the Ugum watershed were estimated to have an average soil erosion loss of 45 tons ha-1 yr-1 (NRCS, 1995). Additional problems associated with soil erosion include loss of soil productivity at the eroded site, reduced water storage capacity in streams and lakes, and loss of wildlife habitat. The negative impact of sediment loading on the aquatic environment of Guam has been recorded by several researchers (Rogers, 1990; Richmond, 1993). These researchers observed that coral reef decline, due to sediment deposition, is directly linked with reduction in the quantity and quality of solar radiation. Corals growing in areas subjected to high sediment loads in stream runoff are especially vulnerable. Undesirable effects associated with the degradation of coral reefs include declining fish populations and a negative impact on tourism. Effective land use planning and proper erosion control requires: 1) a basic understanding of runoff and erosion rates at the plot, on hill slopes, and at small catchment scale and how these vary across the landscape, 2) a means to identify areas that have the potential for high soil erosion, and 3) a means of implementing proper soil reduction practices that are both effective and economic. For the past several years, various agencies such as the Natural Resources Conservation Service (NRCS) and the Guam Department of Agriculture have been implementing erosion control practices by carrying out extensive tree planting programs. However, it is important to investigate: 1) the impact of tree growing in reducing soil erosion, 2) the identification of other kind of land coverage (e.g., Vetiver grass) that be more effective in reducing soil erosion, and 3) the cost of applying of these erosion reducing practices. The objective of this project is to use a GIS based erosion model, recently developed by WERI researchers, to investigate the effectiveness of various vegetative cover schemes in reducing soil erosion and attempt to determine the relative costs of applying these erosion prevention schemes. The benefit of this project is that that NRCS and the Guam Department of Agriculture will be able to better choose between soil erosion prevention alternatives in the future.