 |
|
|
|
|
|
|
|
Evapotranspiration (ET) is the loss of water to the atmosphere by the combined processes of evaporation (from soil and plant surfaces) and transpiration (from plant tissues). It is an indicator of how much water your crops, lawn, garden, and trees need for healthy growth and productivity.
Accurate estimates of ET are needed in many circumstances. In agricultural irrigation, for example, estimates of ET are necessary for system design, irrigation scheduling, water transfers, planning, and other water issues.
For ET to take place, the following conditions have to be met. First, water has to be present at the surface. Second, there must be some form of energy to convert the liquid water into a water vapor. Third, there must be a mechanism to transport the water vapor away from the evaporating surface.
Precipitation and irrigation are the two primary sources of water that plants use. Plant leaves and soil surfaces temporarily retain some part of the water applied to the field. This part is readily available for evaporation. The remaining part infiltrates into the soil. Plants extract the infiltrated water through their roots and transport it up to their leaves for photosynthesis, a process by which plants produce glucose (sugar). In addition to water, plants also need carbon dioxide (CO2) and light for photosynthesis. The light comes from the sun and CO2 comes from the atmosphere. In order to take in CO2 from the atmosphere, plants open their stomates, the microscopic pores on plant leaf surfaces. It is during this process that they loose their water to the atmosphere.
As mentioned earlier, the conversion of liquid water into water vapor requires large amounts of energy (about 540 Calories per gram of water at a temperature of 100 °C). This energy is provided by the sun in the form of solar energy. The solar energy is absorbed by water molecules and converted to latent heat energy, the energy that is tied up in vapor molecules. The water vapor thus produced escapes to the atmosphere because of a vapor pressure gradient between the surface and atmosphere. Once in the atmosphere, it is taken further away from the surface by wind (or other mechanisms), creating more gradient between the evaporating surface and the air above it. This process continues as long as the three conditions mentioned above are present.
|
|
|
Estimating ET
|
|
|
Many factors affect ET including: weather parameters such as solar radiation, air temperature, relative humidity, and wind speed; soil factors such as soil texture, structure, density, and chemistry; and plant factors such as plant type, root depth and foliar density, height, and stage of growth. Although ET can be measured using such devices as lysimeters, estimating ET using analytical and empirical equations is a common practice because measurement methods are expensive and time consuming. Most ET equations were developed by correlating measured ET to measured weather parameters that directly or indirectly affect ET. Since there are so many factors affecting ET, it is extremely difficult to formulate an equation that can produce estimates of ET under different sets of conditions. Therefore, the idea of reference crop evapotranspiration was developed by researchers. Reference ET is the ET rate of a reference crop expressed in inches or millimeters.
Reference crops are either grass or alfalfa surfaces whose biophysical characteristics have been studied extensively. ET from a standardized grass surface is commonly denoted as ETo whereas ET from a standardized alfalfa surface is denoted as ETr. The American Society of Civil Engineers (ASCE) recommends the use of ETos and ETrs, respectively, where "s" stands for standardized surface conditions. The logic behind the reference evapotranspiration idea is to set up weather stations on standardized reference surfaces for which most of the biophysical properties used in ET equations are known. Using these known parameters and measured weather parameters, ET from such surfaces is estimated. Then, a crop factor, commonly known as
crop coefficient
(Kc), is used to calculate the actual evapotranspiration (ETc) for a specific crop in the same microclimate as the weather station site.
CIMIS is using a well-watered actively growing closely clipped grass that is completely shading the soil as a reference crop at most of it's over 120 weather stations. Therefore, reference evapotranspiration is mostly referred to as ETo on the CIMIS web site, although there are a few notable exceptions with ETr.
|
|
|
The Equations
|
|
|
There are many theoretical and empirical equations around the world to estimate ETo. The choice of any one method depends on the accuracy of the equation under a given condition and the availability of the required data. For reference surfaces with known biophysical properties, the main factors affecting ETo include solar radiation, relative humidity/vapor pressure, air temperature, and wind speed. Therefore, ETo can be estimated quite accurately using a "model" (a series of complex mathematical equations).
The two models that are used in CIMIS are the
Penman-Monteith
and a version of
Penman's equation modified
by Pruitt/Doorenbos (Proceedings of the International Round Table Conference on "Evapotranspiration", Budapest, Hungary. 1977). The Modified Penman employs a wind function developed at UC Davis. The version used in CIMIS uses hourly weather data to calculate ETo instead of daily weather data. Hourly averages of weather data are used in the "model" to calculate an hourly ETo value.
The 24 hourly ETo values for the day (midnight to midnight) are summed to result in daily ETo. Air temperature, wind speed, and relative humidity are measured directly at each weather station. Vapor pressure is calculated from relative humidity and air temperature. Hourly net radiation is estimated using a method developed by the University of California. This method uses solar radiation, vapor pressure, air temperature, and a calculated monthly cloud coefficient (CK).
|
|
|
|
|
|
|
|
