Pick Our Brain - June 2009

People talk about ‘wasted’ water ‘lost’ to evaporation from irrigated crops, lawns, etc. Where does this lost water go? Isn’t much of it carried eastward during the warm seasons, eventually condensing into additional rain over the western slopes of the Sierras or the Rocky Mountains? --Robert Meagher, Sacramento

Water that is returned to the atmosphere through evapotranspiration (the combined processes of evaporation and transpiration) is a major component of the hydrologic cycle. The global average residence time for water vapor in the atmosphere is eight to nine days. In high humidity areas of the world, this process makes for efficient local recycling of water. For example, it is estimated that the water transported from the Atlantic Ocean to the Amazon Basin is recycled (precipitation, evapotranspiration and reprecipitation) four times as you move from the eastern to western portion of the Amazon Basin. In areas where humidity is commonly lower, like much of California, the evapotranspired water is carried much further before conditions allow for condensation and precipitation. Where this occurs is dictated by a complex mix of wind speed and direction, humidity and temperature.

Some of the water vapor coming from irrigated crops and lawns in California does condense over the mountains and return as rain. However, climatologists and meteorologists think this is a relatively small fraction of summertime precipitation in arid and semi-arid regions of the Western United States. Summertime precipitation in the West derives mainly from the Gulf of California, Gulf of Mexico and the Pacific Ocean. Most of the evapotranspiration from California is transported eastward, where the moisture interacts with water vapor from additional evapotranspiration from other parts of the continental United States and moist air from the Gulf of Mexico. These processes can generate precipitation or the water vapor can be transported in the atmosphere to the Atlantic Ocean.