Rain Distribution in a Mountainous Watershed

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Abstract
The orographic and temporal gradients of rainfall in a mountainous watershed in southwestern British Columbia have been analyzed and streamflow has been estimated using a watershed model. The study watershed is the Jamieson Creek watershed located approximately 30 km north of Vancouver in the Coastal Mountains. The purpose of the study was to determine whether rainfall follows a definable pattern in this mountainous watershed. Regression analysis has been performed for the total rainfall depth per event and hourly intensity for the period 1972-1975. Data is taken from the rainfall season of June to mid-November in order to avoid complications of combined rain and snow events. In this analysis, the rainfall data from a gauge at the lower elevation was used as the set of independent variables and the data from the other four gauges in the watershed as dependent variables. The results showed that the rainfall depth per event increased up to the mid-elevation of the watershed, and then decreased at the upper elevations. On the other hand, the hourly rainfall intensity was found to decrease with increase of elevation in the watershed, so that longer duration of rainfall events occurs at the middle and upper watershed. The regression equations, developed from the analysis of the distribution of the hourly intensity, were used for the prediction of rainfall events of the years 1976-1977. The agreement between the predicted and the observed rain was statistically good. Also, the simulation of the watershed streamflow using the predicted rainfall gave good results. Consequently, because the rainfall follows a definable distribution as a function of elevation, it is possible to use data from one station located at the lower elevation in combination with the developed predictor equations to accurately describe the rainfall over the watershed.

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