4.3: Measurement of Evapotranspiration

Plant water use is an important management input; thus, it is critical to quantify ET. Several methods have been developed to measure ET. A few are summarized here.

Aerodynamic Methods

One method of determining ET is to measure the rate of water vapor leaving the plant canopy. The vapor pressure of the air and air flow velocities can be measured at several levels above a plant canopy. By evaluating these measurements, the instantaneous ET rate can be determined. Summing measurements provides an estimate of ET for a day. This technique requires very accurate equipment because the air moves erratically above the canopy.

Another method relies on the Bowen ratio to estimate ET. The Bowen ratio is the ratio of the amount of energy used to heat the air relative to the amount used to evaporate water. Equipment has been developed to measure the Bowen ratio and to compute ET. A major problem is that advection is ignored in the Bowen ratio method which may be an unacceptable assumption for some locations.

Soil Water Methods

Figure 4.6. Diagram illustrating the components of the soil water balance.

Soil water is the source for ET, and several methods have been used to relate changes in soil water to plant water use. The primary components of the soil water balance are illustrated in Figure 4.6. The soil water balance can be expressed as:

ET = AW_b - AWe + P + d_g + U_f + R_i - R_o - d_p (4.1)

where: ET = amount of ET during the period,

AW_b = amount of available soil water in the root zone at the beginning of a period,

AW_e = amount of available soil water in the root zone at the end of a period,

P = total precipitation during the period,

d_g = gross irrigation during the period,

U_f = groundwater contribution to water use during the period,

R_i = surface water that runs onto the area during the period,

R_o = surface runoff that leaves the area during the period, and

d_p = deep percolation from the root zone during the period.

The ET can be estimated from Equation 4.1 if all other terms are known or can be approximated. If the groundwater table is more than 6 ft below the soil surface, the contribution from groundwater can be ignored. Rain and irrigation from sprinklers are usually measured with rain gages or similar devices. Measuring devices have been developed for surface irrigation applications. Soil water content can be measured using neutron scattering or other techniques described in Chapter 2. Deep percolation is difficult to measure and is often assumed to be insignificant unless large rains occur, or large irrigations are applied. A significant problem with the soil water balance technique is that repetitive measurements must be made throughout the season. One week is usually the shortest period for using the soil water balance method to estimate ET. If deep percolation or runoff is significant, the soil water balance method is further limited because of the lack of measuring capabilities

Lysimetry

Lysimeters are specially designed open-top tanks that are filled with soil, preferably undisturbed soil, and planted to the same crop as the surrounding area. The tanks are buried in the field. Water used for ET by plants grown in the lysimeter must come from the soil water within the tank. ET can be measured by monitoring soil water contents and water applications from irrigation or rain. The soil tank is used to isolate soil water from the surrounding area and to prevent run on, runoff, upward groundwater flow, and drainage. For some applications drainage is allowed and the volume of deep percolation is measured. The soil water within the tank can be measured with traditional methods such as neutron probes. The amount of water in the tank can also be determined by weighing the tank, soil, plants, and soil water. Since soil water is the only item that changes significantly over short time periods, the change in weight equals the amount of water used for ET.

Various types of lysimeters have been utilized to measure ET. The most elaborate and accurate lysimeters are called weighing lysimeters (Figure 4.7). These lysimeters use weighing devices to measure water lost from the soil tank. Large plants with deep root zones usually require large lysimeters. Short plants, with shallow root systems, can be measured by lifting small lysimeters and weighing with a scale. The most sophisticated weighing devices are high precision and can be used to measure small changes of weight. A good description of precision lysimeters is given by Marek, et al. (1988). Such systems have counter balanced weighing systems resulting in a measurement accuracy approaching 0.001 inches of ET. The high accuracy is required for daily measurements.

Figure 4.7. Examples of weighing lysimeters (picture of large lysimeter is courtesy of USDAARS, Bushland, Texas).

Other types of lysimeters do not weigh the soil-plant-water system. Non-weighing lysimeters function the same as the field water balance method except upward flow of groundwater and runoff or run on are prevented by the sides and the bottom of the lysimeter. A drainage system is usually installed in the bottom of the lysimeter to measure deep percolation and to prevent water from ponding at the bottom of the lysimeter. Water table lysimeters, common in humid regions, are a second type. With this design, deep percolation is prevented, and a water table is maintained in the lysimeter. Changes in soil water and the elevation of the water table are measured along with other soil water balance terms. Non-weighing and water table lysimeters are usually only accurate enough for estimating the amount of ET over a period of approximately 1 week. More elaborate methods are needed to measure daily or hourly ET rates. Besides being expensive to install and operate, lysimeters pose several problems. Using lysimeters to measure ET was summarized by Allen, et al. (1991). The best lysimeters are those filled with an undisturbed soil column. These are termed monolithic lysimeters, and if they are large, their filling can be difficult and expensive (Figure 4.7). Regular and careful maintenance of the lysimeter and the surrounding area is required to maintain accuracy. Spatial variability can be significant when measuring ET and several lysimeters may be required. Lysimeters are usually research tools and are too complex and labor intensive for water management.

Plant Monitoring Methods

Plant transpiration can be measured using several techniques. One of these is the autoporometer. With this instrument, a small chamber is clamped onto a growing plant leaf and changes in the humidity and temperature of the air within the chamber are used to compute transpiration during that period. The transpiration rate and other plant responses change very rapidly due to external factors. Therefore, the porometer can only remain on the leaf for a few minutes. Another limitation of the porometer is that only a small part of one leaf is used for measurement. Characterizing the transpiration for an entire crop canopy requires numerous measurements. Further, these measurements only provide instantaneous transpiration rates. Generally, irrigation management requires plant water use for daily and longer time periods. Thus, porometers are primarily used in experiments to investigate plant response to stress and for very short-term water use estimates.

A second method uses infrared thermometers to predict transpiration based upon the difference between the plant temperature and the air temperature. The infrared thermometer has been used successfully to detect plant stress and to predict irrigation timing. If the incoming solar radiation and other energy terms are known, the ET rate can be estimated using the techniques of Hatfield (1983) and Jackson (1982). These techniques are complex and require extensive calculation as well as continuous monitoring of plant temperature. The infrared plant monitoring method can be used to help schedule and manage irrigation but needs further development to estimate ET.