Author : Delorino, Rolando A.

Summary : The rate of sedimentation of the Macagtas reservoir was observed for one year. Stream velocity, cross-sectional area of stream flow and the sediment concentration of the water were monitored during rainfall greater than 18 mm. The relationship between water discharge and sediment inflow for the two streams draining the watershed was analyzed using the simple linear regression model. A direct relationship between these variables was shown; a correlation coefficient value of 0.99 was obtained for both streams. For the entire period of the study, the computed sediment discharge in the reservior was about 1,702 tons. This value is higher than previously computed rates which were based on the estimated annual soil erosion loss rates using the Modified Universal Soil Loss Equation Model. Soil erosion loss rates were estimated based on three land-use maps representing the periods before project construction and during operation phases. Land-use trends indicated various stages of watershed transformation. The estimated soil erosion loss during the period of study was 2,475 t/yr. With a reservior sediment inflow of 1,702 t/yr, the sediment delivery ratio (SDR) was about 69 percent. Sedimentation rates in the past and future were computed based on the above SDR value and an assumed reservior trap efficiency of 93 percent. It is predicted that if the present rate will remain constant, the dead storage volume capacity will be filled up with sediments after 25 years (since 1976) of continuous water impoundment. The sediment inflows for the projected 30-year period under the scenarios with and without watershed management conditions are 35,348 tons (27,191 m3) and 47,672 tons (36,666) m3), respectively. The damage cost of sedimentation was determined by comparing the benefits that can be derived from the with and without watershed management options. However, benefits and costs which were difficult to quantify were not included in the analysis.