Rainfall and streamflow analysis of a large tropical rainforest watershed

An examination of ten years (1974-1983) of hydrometeorological data at the Sungai Jelai watershed in Pahang, Malaysia, showed that mean monthly rainfall was 169.41 mm with the maximum (507.29 mm) occurring in the North-east monsoon season. Mean annual rainfall was 2,057.91 mm. Streamflow followed the rainfall trend closely. Mean monthly streamflow was recorded at 61.98 mm; maximum flow occurred in November and minimum flow in August. The mean annual streamflow was 742.86 mm. Mean monthly forest evaporation was found to be 84.33 mm while the annual forest evaporation was 1,012.01 mm. Monthly temperatures in the watershed fluctuated between 16.0 degrees Celsius and 19.3 degrees Celsius with the mean at 17.7 degrees Celsius. It was observed that about 36% of rainfall, on the average, left the watershed as streamflow. A higher percentage of 54% was found for the month of January. On the other hand, about 49% of rainfall was lost through the process of evapotranspiration. In some months, the combined effects of evapotranspiration and streamflow far exceeded that of rainfall. This situation could have been due to storage water. Prediction equations were developed using streamflow as the dependent variables. Two prediction equations were finally selected. The first one had streamflow related to the rates of rainfall and forest evaporation and antecedent rainfall. This equation showed highly significant t-values and had an R2 value of 0.74. The second equation is a simple one with only two independent variables. Streamflow was found to be related to rainfall and the reciprocal of forest evaporation. The t-values were highly significant and R2 value was 0.70. Using the prediction equations, tables of rainfall and forest evaporation were constructed to facilitate the use of such equations.