Pick Our Brain - April 2009

How much of the snow and rain that falls ends up running off into rivers and reservoirs?

There is a common misconception that 100 percent of snow and rain makes it into rivers and reservoirs. Several contributing factors, however, affect the percentage of runoff out of each watershed, so that there will never be 100 percent yield. These factors include evaporation, transpiration (plant release of water), sublimation (conversion of snow and ice directly to water vapor), added soil moisture, groundwater recharge, and human use, mainly for agriculture.

The rates of these factors that keep precipitation from reaching rivers and reservoirs vary depending upon the geographic location. Climate plays an important role in these differences. The more arid the area, the more these factors intensify.

Location of the river basin in California has a major impact on percent runoff. In general, the farther north in California the river is located, the greater percentage of precipitation will be converted to runoff. The farther south in California the river is, the lower the percentage of precipitation that will become runoff. Human modification of the river basins through the building of dams and irrigated agriculture also affect the percent of runoff. California rivers range from more than half of the precipitation running off as river water (Russian River) to only two percent running off as river water (Santa Ana River).

California Rivers - Russian (53% Runoff), Eel (52% Runoff), Klamath (44% Runoff), Sacramento (32% Runoff), San Joaquin (10% Runoff), Salinas (10% Runoff), Santa Margarita (4% Runoff), Santa Ana (2% Runoff)