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Assessment of Land Use and Climate Change Impacts on Stream Flow
Abstract:
The impact of land use and climate change on stream flow of the Binahaan watershed in Pagbilao, Quezon was determined using appropriate and hydrologic model.
In this study, the choice of hydrologic model was limited to Brook5 due to its simple model structure and data requirements, and its ability to simulate runoff and other hydrologic variables in a tropical watershed like the Philippines.
Primary data gathering included actual measurements of stream height and stream velocity undertaken for four months using available conventional instrumentations and measurement techniques such as staff gauge and floatation method. Secondary data, on the other hand, were existing maps from various sources and a 30-year daily rainfall and mean temperature data from the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) which were used as inputs in the Geographic Information System (GIS) and hydrologic model. Statistical analysis was employed for both the simulated and observed stream flow.
The results of this study included the following: (1) impact of observed rainfall and mean temperature and four different land use conditions on the monthly runoff; (2) monthly runoff under increased temperature and normal precipitation; (3) rate of monthly runoff and daily stream flow under different land use, rainfall, and mean temperature conditions; and (4) impact of future land use change scenario on monthly runoff.
Simulation results revealed that in general, a decrease in forest land or its conversion to other uses such as the one that occurred in Binahaan watershed from 1975 to 2001 resulted to an increased runoff. However, the results may not always be true especially during El Niño phenomenon as exhibited by the lowest rate of runoff for the year 1984. The simulation of the impacts of an increased temperature and observed rainfall on runoff showed an exactly opposite relationship because of decreasing values of runoff under an increasing temperature. This study somehow validated earlier findings that an increased temperature will likewise increase the evaporative demand.
The results of model simulation by interchangeably applying the temperature/ rainfall under different land use conditions showed that in general, the decrease in forest cover and subsequent increase in agricultural and open areas will also cause increase rate of runoff.
Results of correlation and regression analysis showed that the observed stream flow values were not significantly different from the model simulated stream flow for a period of four months.