The average annual rainfall in Taiwan is about 2500 mm. Because of the uneven distribution of rainfall in time and space, the steep terrain, short rivers, and rapid flow, water resources cannot easily be stored. In recent years, due to the vigorous development of the social economy, development of lowlands has become saturated, and development has gradually expanded towards slope land; this has brought tremendous changes to the original hydrological cycle system. Therefore, understanding the impacts of changes in land use and rainfall patterns on water resources conservation is an important issue at present. The Shui-Li Creek watershed in central Taiwan was selected for this study. A quantitative assessment of water resources conservation in the study area was conducted using the BASINS/HSPF hydrological model which was developed by the US Environmental Protection Agency. The following research results were documented. (1) The parameters were calibrated and verified by flow data of the years 2009 and 2010, respectively. The results showed that the correlation coefficients (r) were 0.87 and 0.72, the determination coefficients (R2) were 0.75 and 0.52, and the efficiency coefficients (E) were 0.39 and 0.11, respectively. This indicates that the model was reliable. (2) Simulation results of land use patterns in four periods showed that rates of annual water resource conservation were 23.37 to 23.82%. After a cross comparison of land use, it was found that patterns of forests and buildings had the greatest impacts on water resources conservation in watershed areas. (3) Three patterns of land use (forest, agriculture, and buildings), which can easily affect water resource conservation, were selected for six simulation scenarios. Results showed that land use change from forests to buildings had the most obvious impact on water resource conservation. (4) Comparisons of water resource conservation of the annual rainfall in high-flow and dry years showed that rainfall which was stored in the watershed area in a high-flow year was better due to the abundant rainfall. The rate of annual water resource conservation was about 26%.