Impact of land use evolution on streamflow and some water qualities of Petchaburi basin

Impact of land use evolution on water quantity was studied basing on various sources of historical data including forest area made available by the Royal Forest Department, water quantity and suspended sediment recorded by Royal Irrigation Department, the precipitation data collected by the Royal Irrigation Department and Department of Meteorological. The aforementioned data during 1973-1991 were analysed using the multiple regression analysis. The results of the study indicated that during the mentioned period, land use changes is the major factor that caused the degradation of forest area in the Lower Petchaburi, Huai Mae Prachan, Upper Petchaburi and Petchaburi watersheds from 24.38, 85.87, 81.52 and 68.49 percent to 8.60, 38.67, 62.82 and 45.39 percent respectively. Most of the area are converted to cultivated land. The water quantity and the characteristic of the water flow are significantly related to the precipitation within Huai Mae Prachan and Petchaburi watersheds, where the maximum value is occurred in October. In addition, they also significantly related to the water released from the reservoir of the Upper Petchaburi watershed, in which its maximum discharge was shown in August and september. The Upper Petchaburi, Huai Mae Prachan and Petchaburi watersheds have shown the potential to provide the runoff of about 36.42, 15.14, and 7.46 percent of the basin precipitation. It was also found that after changing the land use pattern, the runoff potential within Huai Mae Prachan and Petchaburi watersheds was increased, But within the Upper Petchaburi watershed, it depends on the quantity of the water regulated from the reservoir. The amount of suspened sediment is significantly related to the annual water quantity and shows the relationship in forms of exponential function. Forest degradation showed insignificant impact on water quantity and suspended sediment in all watersheds. However, it was found that urban and agricultural areas of Petchaburi watershed, which induced the aquaculture activities are the major factors that caused the degradation of water quality to a level which unsuitable for fishery conservation. This water quality is classified as type 3 and 4 of water quality standard for surface water exclude sea water area. It was recommended that the national policy should pay more attention to seek for the suitable measures to reduce the rate of forest degradation. In addition, it is also important to introduce the integrated agriculture system, the reservoirs to reduce the drought problem within Huai Mae Prachan watershed, and water treatment control from the community area and other activities in urban area.