Water Resources Inventory Area 1 Watershed Management

Introduction to Climate and Evapotranspiration


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To estimate the amount of water that is available for future allocation in WRIA 1, a water budget needs to be computed for the study area. Essential components of this budget are water that enters WRIA 1 as precipitation and water that leaves it as evapotranspiration (defined as water evaporated from soils and wet plant surfaces and water transpired by vegetation).

Precipitation can be estimated using data from several climatic stations in and near the WRIA 1 study area. These stations have been operated for different durations, they collect different types of data, and they are operated by different agencies (see Locations of Climate and Snow-Accumulation Stations in and near the WRIA 1 Study Area). Generally, however, most climatic stations collect daily precipitation and daily minimum and maximum air temperatures. A few stations measure additional climatic variables including solar radiation and wind speed and direction.

The table on this web page with climate data statistics (Data Statistics for Climate and Snow-Accumulation Stations in and near the WRIA 1 Study Area) refers to the "climate normal." At the end of each decade, the World Meteorological Organization defines the latest 30-year period as the climate normal, which is the arithmetic average of a meteorological variable (such as precipitation or air temperature) over 30 years. The latest climate normal is for 1961 through 1990. In the Pacific Northwest, this particular climate normal includes both dry and wet years and is thus considered to be a good measure of average longer-term climatic conditions in the study area.

Evapotranspiration (Et) is similar to a financial transaction in that water from soil and plant surfaces is given up to the atmosphere in return for the receipt of energy from the atmosphere. Thus, the Et rate depends both on the availability of moisture at plant and soil surfaces and on the supply of energy from the atmosphere, where the latter can be in the form of solar radiation or turbulent sensible heat (warm air). Et can be measured by any of several scientifically rigorous techniques, but more commonly it is estimated using simple empirical Et models that include variables that account for water availability and energy supply. Empirical Et models, if applied correctly, can be useful tools for evaluating water budgets in WRIA 1 (see Estimating Evapotranspiration from Croplands).