Mean Areal Potential Evapotranspiration

As a whole, the main factors affecting ET in a region or basin are climate (solar heating, etc.), availability of soil moisture, and the type of plant life.  Evaporation and transpiration are a major portion of the annual water budget.  Evaporation is a physical process that is affected by the vapor pressure gradients, wind, and heat.  Transpiration is a process of plants using water.  The two terms are very often combined into a single term – evapotranspiration or ET.  As a whole, the main factors affecting ET are climate (solar heating, etc..), soil moisture supply, and plant life in a region or basin.

Evaporation will occur as long as there is a moisture supply.  Evaporation occurs very nearly year-round throughout much of the U.S. except for Alaska. In the lower 48 states, evaporation ranges from under 35 inches in the Northeast and Pacific Northwest to over 130 inches in the desert southwest. Evaporation occurs from land with all types of vegetation cover and under all kinds of uses. Water can easily evaporate from soils; however, this only affects the top few inches, though capillary action may allow moisture to be supplied to the surface from deeper layers.

Transpiration is the result of plants using water.  Plants will use water as long as it is available.  Water or moisture can meet considerable resistance as it travel through a plant, this results in a rather inefficient process.  Plants pass moisture to the atmosphere through small openings that are protected by guard cells.  These guard cells open and close in response to moisture, which is a function of relative humidity. 

Evapotranspiration, ET, can easily account for more than 60% of the annual water balance in an area.  ET is not constant throughout the year, instead, varying with season in most areas.  In a hypothetical water year (Oct. 1 - Sept. 30), we think of soil moisture (and groundwater) recharge occurring from about October through January or February.  From the February through about April/May we think of the total moisture balance as roughly break-even, as far as soil moisture supply is concerned.  As the growing season begins, we begin to see large volumes of water be consumed in the ET process and we begin to use some of the stored water – thus we are “in the red”.  This is a very simplistic breakdown and will, of course, vary throughout the United States, depending on climate.  This is illustrated in Figure 1, below.

Figure 1

NWSRFS provides an approach for computing ET using observable hydrometeorological data - either (1) the National Weather Service Class A pan data or (2) air temperature, humidity, wind, and radiation data. Over the large areas modeled by RFCs, most of these data are difficult if not impossible to obtain with sufficient spatial representativeness or in a timely enough manner. Therefore, NWSRFS provides another way to estimate ET using a series of curves known as ET demand curves. These curves are typically broken down on a monthly basis, with values being determined or estimated for the middle of the month and the remaining values interpolated. These curves are developed and 'fixed' in the OFS during the calibration/set-up phase. In lieu of using hard-to-get Class A pan observations or wind, and radiation data, most if not all RFCs use ET demand curves. This approach has proved satisfactory in the large majority of hydrometeorological circumstances. Therefore, an OFS run requires no real preprocessing of data for ET computations when ET demand curves are used. Given the importance of ET in basin water balance, however, one can see the importance of having ET demand curves which represent basin processes as accurately as possible.

In the OFS, the ET demand curve is defined as part of the parametric input to the SAC-SMA operation. As a continuous soil moisture accounting model, the Sacramento model calculates the amount of water lost from the soil due to ET. Based on interpolation between the twelve monthly values used to define the ET demand curve, the ET-demand for a single day of the year can be estimated. The actual ET computed by the Sacramento model is then a function of the ET demand for that day and the soil moisture deficit being tracked by the model. If there is no deficit, i.e. the soil is saturated, then the computed ET is equal to the demand. If the soil moisture deficit is high, then only a fraction of the ET demand will be met.