In the mountainous landscapes of the western United States, water resources are dominated by snowpack. As temperatures rise in spring and summer, the melting snow produces an increase in river flow levels. Reservoirs are used during this increase to retain surplus water, which is released to supplement growing season water supply once the peak flows decrease to below water demands. Once there is no longer surplus natural flow of water, the water accounting changes – referred to as the day of allocation (DOA), and water previously retained within the reservoir is used to supplement the lower flow levels. The amount of water stored in the reservoir on the day of this accounting shift determines the water allocated to water right holders for the remainder of the water year. Predicting the day that allocated water will be determined is of special interest to both regulators and those that retain water rights per the Prior Appropriation Law. A method to forecast this day is developed using daily snow water equivalent data for the Boise, Payette, and Upper Snake Rivers in a multiple linear regression model. The melt rates of snowpack are typically comparable to using the maximum accumulation of that snowpack as predictor variables for day of allocation. Therefore, water users can be confident in predictions based on snowpack to determine what crops can be grown. The primary controls on these variances are water demand and volume of water accumulated.
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