Simulation of the Effects of Urbanization on Soil Loss and Runoff from a Small Tropical Watershed Using the ANSWERS Model

Ramsarran, Capildeo

Runoff and loss of soil are important factors of concern in urban development. Two different years, 1971 and 1984 were selected to assess for any significant increase in soil loss, runoff and sediment yield due to a change in urbanization in a small tropical watershed (204.8 ha) on the island of St. John, Virgin Islands. The ANSWERS model, which is a deterministic, distributed, event based model was used to simulate the outputs forth different storms. Data on rainfall, soils, and land management and cropping practices were inputs in the model. The watershed was subdivided into 127 elements for which elemental data were provided. All data were either estimated, taken from existing sources, or deduced from aerial photos, maps, etc. During the simulations all parameters were held constant for both years except the percentage cover. A sensitivity analysis was performed for several soil parameters. The results showed that for just a 9.2 percent change in urbanization from 1971 to 1984, the average soil loss and total sediment-yield increased by 4,598 kg/ha and 941,755 kg respectively with a rainfall depth of 76.9 mm. For small storms of 20.6 mm there were basically no differences in the average soil loss and runoff volume. In 1984, the average soil loss and total sediment yield exceeded those in 1971 by 1,007 kg/ha and 206,256 kg for a medium size storm of 38.1 mm. In general, there were small differences in the runoff volume between the two years. The average soil loss and runoff volume for rainfall amount 38.1 mm in 1971 compared favorably with those of plot studies done in Jamaica and those of small watersheds in the United States. From the sensitivity analyses, the antecedent soil moisture was found to be the most sensitive soil parameter.