ET to you - 1996

The use and misuse of the term ET (evapotranspiration) has become pronounced with the current water shortage and a brief review of the term seems timely.  In short, ET is the amount of energy arriving at a site that drives water loss.  Water loss is a physical process that is driven primarily by energy from the sun.  The components of this physical process include: 1) day length, 2) solar radiation, 3) temperature, 4) wind speed and direction, and 5) relative humidity. 

As day length, solar radiation (clear or overcast skies affect the amount of radiation hitting the earth), temperature and wind increase, evaporation increases.  As humidity increases, evaporation decreases.  A person feels these factors: a hot, windy, dry day is one when you drink a lot of water.  If it is hot and humid, sweat does not evaporate from the body.  A complex summation of theses five factors determines the total amount of water that can be evaporated from a body of water, such as a lake or ocean.  If the pond is dry, there is no evaporation.  The energy may still be arriving to potentially cause water loss, but unless water is present it can not be evaporated.  We speak then of potential evaporation.

Evaporation pans have been used for over a hundred years to measure this potential evaporative loss.  The National Weather Service has climatological stations which incorporate a Class A pan;  a pan of given dimension that is related to water loss from a larger body of water.  The larger the pan, the closer evaporative loss from it approximates that from a lake.  Because the sides heat up and the pan is more affected by wind, it has a higher rate of water loss than a lake.  A correction factor is used to relate a pan's loss to larger bodies. 

For the Class A pan with a diameter of 3 feet, the daily or weekly loss (drop in depth of the water in the pan) is multiplied by 0.85 to more closely approximate loss from a larger body.  The values should be viewed as rainfall is measured.  Instead of a rain gauge measuring inches of captured water, the pan is measuring inches of water loss.  Nearly any dimension pan can be used, a galvanized wash tub, a plastic play pool, but it is important to determine the correction factor needed to relate that vessels loss to a larger body of water. 

So what do evaporation and pans have to do with ET and CIMIS and plant water loss?  The same physical processes that cause evaporative loss are also those that drive plant water loss or transpiration.  Imagine those little stomata on the underside of leaves as a sea of stomata.  Don't think of the single leaf, but of a series of leaves stacked one on top of another so that the combined surface area of all those little holes, whereby water is transpired, approximates the surface area of a lake.  If the plant is well-watered and transpiration is not impaired, the plant will lose water in a similar fashion to that of a lake.   This is without regard to soil texture, clay or sand, since the soil is simply a reservoir for water storage.

When CIMIS calculates ET from all it's measuring devices, it is calculating the amount of energy arriving at a site that would cause evaporation if water were present or the crop were well-watered.  It is calculating a reference ET or ETo ( ET zero).  However, if there is no crop present, there will be no water loss.  But if a small plant is present, it will absorb some of the energy and transpire some of the water from the ground.  As more and more plants appear, or plants become larger and larger, they intercept more and more energy and hence transpire more.  The amount they transpire in relation to ETo is a direct relationship to the amount of leaf area present.  This is where the crop coefficient, kc, comes in.  The estimate of how much the crop will lose relative to the standard, ETo, is called ETc (ET crop).  Multiplying the ETo by kc gives the amount of water used by the plant.  Knowing the amount of water stored in the soil and subtracting the daily ETc from the stored amount will give the time of the next irrigation.

The purpose of this article is not to explain irrigation scheduling with ET or the water budget method, but to make the connection between evaporation and ET.  For more information on scheduling with ET consult your farm advisor, mobile lab or UC leaflet # 21454 - Irrigation Scheduling: A Guide for Efficient On-Farm Water Management. 

Suffice it to say that it is possible to devise your own pan station in your own orchard and assess the water requirements of your crop.  Using CIMIS is simply another way of arriving at an approximation of plant water needs.  CIMIS also gets around the problems of reading the pan, of filling it and ensuring that birds and coyotes don't alter the readings.  An instrument similar to a pan, an atmometer (atmosphere meter), is available for about $150 and reduces some of the inconvenience of using a pan. 

Whatever method you use, pan, atmometer or CIMIS, it is important that it is used in conjunction with soil and plant evaluations.  Use of ET is just another tool to be used for improving the management of your orchard.