As with all aspects of the water cycle, the interaction between precipitation and surface runoff varies according to time and geography. Similar storms occurring in the Amazon jungle and in the desert Southwest of the United States will produce different surface-runoff effects. Surface runoff is affected by both meteorological factors and the physical geology and topography of the land. Only about a third of the precipitation that falls over land runs off into streams and rivers and is returned to the oceans. The other two-thirds is evaporated, transpired, or soaks (infiltrates) into groundwater. Surface runoff can also be diverted by humans for their own uses.
As an amateur, I have to wonder about this, for the following reasons:
1. From what I've been reading, isn't the soil heavily saturated with water? Wouldn't that increase runoff?
2. It's February, and it's wet. How much evaporation will there be?
3. From what I've been reading, there is about 30 inches of water in the snowpack. The incoming storm is said to consist of tropical moisture, i.e. warm, and the snow has already been rained on, which makes it more likely to melt.
Only one-third of the rain goes into the reservoir? This seems pretty hopeful to me. I hope you are correct.