2. ET measurement

#### 2.1 Energy balance

Many ET estimation methods use or are based on the energy balance. The energy balance concept describes the processes of radiation in the atmospheric boundary layer. Solar radiation is the sole energy input for radiation processes. The incoming shortwave and longwave radiation is either reflected or absorbed by the surface of the earth. The net radiation (Rn) is the amount of radiation absorbed by the earth's surface and is measured by subtracting the reflected radiation from the total incoming radiation. The absorbed radiation contributes to soil heat flux, sensible heat flux, and latent heat flux. Soil heat flux (G) is the amount of radiation gained or lost by the soil surface though conduction. Sensible heat flux (H) is the energy that increases the temperature of the atmosphere causing advection, and latent heat flux (LE) is the energy available for the evaporation of water. Due to the law of thermodynamics, the net radiation must be distributed among the other three fluxes. This yields the basic energy balance equation:

$$R\_n = LE + H + G \tag{5}$$

Pullman clay loam soil profile with subsurface drip irrigation at 23 cm depth or mid elevation sprinkler irrigation. The soil container rests on a large agronomic scale equipped with a counterbalance and load cell system. Initial design and installation details of the lysimeter are provided by [15, 16]. The lysimeters were later equipped with drainage effluent tanks suspended from the lysimeter by load cells for separate measurement of drainage mass without changing total lysimeter mass. Load cell output is measured and recorded by a precision data logger. Load cell voltage outputs are converted to mass using calibration equations, and 5 minute means are used to develop a base dataset for subsequent processing [17]. Lysimeter mass in kg is converted to a mass-equivalent relative lysimeter storage value (mm of water) by

Large weighing lysimeters at the USDA-ARS Conservation and Production Research Laboratory (CPRL), Bushland,TX. The view from the surface (a) shows the outline of the lysimeter container during the growing season with a crop planted. A photo taken during installation (b) illustrates the size of the container and the

be expressed in terms of water flux, defined as mm of water lost or gained per unit time. The lysimeter data logger mass resolution is better than 0.001 mm when converted to equivalent depth of water. Lysimeter accuracy is, however, determined by the RMSE of calibration, which has ranged from 0.05 mm to 0.01 mm [14, 17]. Lysimeter quality assurance and quality control (QA/QC) and data

Calculating ET in units of equivalent depth of water requires that the change in lysimeter mass be divided by the effective evaporating and transpiring area of the lysimeter [13]. The Bushland lysimeter inside surface area is 8.95 m<sup>2</sup> [14]; however, the area of contribution from captured precipitation or irrigation, as well as ET, is beyond the lysimeter container, resulting in an effective area larger than the physical area of the lysimeter. The reported the outside lysimeter surface area was

). Equivalent mass values allow for changes in lysimeter mass to

) and the density of

) is applied to ET

dividing it by the relevant surface area of the lysimeter (9 m<sup>2</sup>

9.35 m2 [10]. In this case, a correction factor of 1.05 (9.35/8.95 m<sup>2</sup>

processing techniques are provided by [18].

measurements from the lysimeter.

9

water (1000 kg m<sup>3</sup>

below ground access housing.

Field-Scale Estimation of Evapotranspiration DOI: http://dx.doi.org/10.5772/intechopen.80945

Figure 2.

Net radiation can be measured by a variety of instruments where the incoming solar radiation will be measured in addition to the reflected radiation. The soil heat flux can be measured by soil heat flux plates which measure the amount of energy gained or lost by the soil. H and LE require advanced instruments and methods for measurement. Since LE is the energy used for evaporating water, it can be converted to ET by dividing by the latent heat of vaporization. Measuring H and LE is more challenging and requires more sophisticated instrumentation.

#### 2.2 Lysimeter

A lysimeter is considered the most accurate ETa measurement instrument. A lysimeter consists of a mass of soil in an enclosed container which can be accurately weighed to determine the amount of water lost or gained per unit time. Lysimeters can be very complex and expensive to install and operate but are a direct measurement of soil water storage. Thus, lysimeters are considered the most accurate for ET measurement [13, 14]. Lysimeters are point measurements and only have the measurement area of the container. However, if the surrounding field is properly managed to match the lysimeter, the ET data can represent field conditions. This intensive management is typically only possible at research locations. One example of large continuously weighing lysimeters are those located at the USDA-ARS Conservation and Production Research Laboratory (CPRL) in Bushland, TX. This location houses four lysimeters within a 20 ha (50 acre) field, divided into four quadrants.

Each large weighing lysimeter measures 3 by 3 m on the surface by 2.3 m deep over a fine sand drainage base (see Figure 2). It contains an undisturbed monolithic Field-Scale Estimation of Evapotranspiration DOI: http://dx.doi.org/10.5772/intechopen.80945

#### Figure 2.

neutrons are counted by the probe counter. The neutron count is related to the soil moisture by a calibration. The amount of lower energy neutrons that is reflected back to the sensor provides an accurate indication to the soil moisture

correlated to the soil moisture content with lower moisture contents having larger

Many ET estimation methods use or are based on the energy balance. The energy balance concept describes the processes of radiation in the atmospheric boundary layer. Solar radiation is the sole energy input for radiation processes. The incoming shortwave and longwave radiation is either reflected or absorbed by the surface of the earth. The net radiation (Rn) is the amount of radiation absorbed by the earth's surface and is measured by subtracting the reflected radiation from the total incoming radiation. The absorbed radiation contributes to soil heat flux, sensible heat flux, and latent heat flux. Soil heat flux (G) is the amount of radiation gained or lost by the soil surface though conduction. Sensible heat flux (H) is the energy that increases the temperature of the atmosphere causing advection, and latent heat flux (LE) is the energy available for the evaporation of water. Due to the law of thermodynamics, the net radiation must be distributed among the other three

Net radiation can be measured by a variety of instruments where the incoming solar radiation will be measured in addition to the reflected radiation. The soil heat flux can be measured by soil heat flux plates which measure the amount of energy gained or lost by the soil. H and LE require advanced instruments and methods for

converted to ET by dividing by the latent heat of vaporization. Measuring H and LE

A lysimeter is considered the most accurate ETa measurement instrument. A lysimeter consists of a mass of soil in an enclosed container which can be accurately weighed to determine the amount of water lost or gained per unit time. Lysimeters can be very complex and expensive to install and operate but are a direct measurement of soil water storage. Thus, lysimeters are considered the most accurate for ET measurement [13, 14]. Lysimeters are point measurements and only have the measurement area of the container. However, if the surrounding field is properly managed to match the lysimeter, the ET data can represent field conditions. This intensive management is typically only possible at research locations. One example of large continuously weighing lysimeters are those located at the USDA-ARS Conservation and Production Research Laboratory (CPRL) in Bushland, TX. This loca-

measurement. Since LE is the energy used for evaporating water, it can be

tion houses four lysimeters within a 20 ha (50 acre) field, divided into four

Each large weighing lysimeter measures 3 by 3 m on the surface by 2.3 m deep over a fine sand drainage base (see Figure 2). It contains an undisturbed monolithic

is more challenging and requires more sophisticated instrumentation.

Rn ¼ LE þ H þ G (5)

status [12]. In addition, the sphere of influence of the neutron meter is

Advanced Evapotranspiration Methods and Applications

fluxes. This yields the basic energy balance equation:

contributing values.

2. ET measurement

2.1 Energy balance

2.2 Lysimeter

quadrants.

8

Large weighing lysimeters at the USDA-ARS Conservation and Production Research Laboratory (CPRL), Bushland,TX. The view from the surface (a) shows the outline of the lysimeter container during the growing season with a crop planted. A photo taken during installation (b) illustrates the size of the container and the below ground access housing.

Pullman clay loam soil profile with subsurface drip irrigation at 23 cm depth or mid elevation sprinkler irrigation. The soil container rests on a large agronomic scale equipped with a counterbalance and load cell system. Initial design and installation details of the lysimeter are provided by [15, 16]. The lysimeters were later equipped with drainage effluent tanks suspended from the lysimeter by load cells for separate measurement of drainage mass without changing total lysimeter mass. Load cell output is measured and recorded by a precision data logger. Load cell voltage outputs are converted to mass using calibration equations, and 5 minute means are used to develop a base dataset for subsequent processing [17]. Lysimeter mass in kg is converted to a mass-equivalent relative lysimeter storage value (mm of water) by dividing it by the relevant surface area of the lysimeter (9 m<sup>2</sup> ) and the density of water (1000 kg m<sup>3</sup> ). Equivalent mass values allow for changes in lysimeter mass to be expressed in terms of water flux, defined as mm of water lost or gained per unit time. The lysimeter data logger mass resolution is better than 0.001 mm when converted to equivalent depth of water. Lysimeter accuracy is, however, determined by the RMSE of calibration, which has ranged from 0.05 mm to 0.01 mm [14, 17]. Lysimeter quality assurance and quality control (QA/QC) and data processing techniques are provided by [18].

Calculating ET in units of equivalent depth of water requires that the change in lysimeter mass be divided by the effective evaporating and transpiring area of the lysimeter [13]. The Bushland lysimeter inside surface area is 8.95 m<sup>2</sup> [14]; however, the area of contribution from captured precipitation or irrigation, as well as ET, is beyond the lysimeter container, resulting in an effective area larger than the physical area of the lysimeter. The reported the outside lysimeter surface area was 9.35 m2 [10]. In this case, a correction factor of 1.05 (9.35/8.95 m<sup>2</sup> ) is applied to ET measurements from the lysimeter.

The lysimeter is designed to be the representative of the surrounding field so that measured lysimeter ET closely mimics field ET. Experienced support scientists and technicians are responsible for maintaining lysimeter representativeness as compared to surrounding fields. Careful attention is given to agronomic operations including planting, harvesting, tillage, fertilization, irrigation, and pesticide application such that there should be no distinguishable differences, particularly in height, between the crop grown on the lysimeter and that grown in the surrounding field. To confirm this, multiple neutron probe access sites were located both throughout the field and in the lysimeter to monitor the soil profile water content. Weekly soil water content (SWC) readings from the neutron probes throughout the field are compared to SWC readings from the lysimeter to determine similitude representativeness. In addition to SWC readings, plant mapping and stand counts were periodically taken to ensure the crop growth on the lysimeter approximates the surrounding field. The lysimeter box contains a 50 mm freeboard lip that extends above the soil surface to limit runoff or run-on to the lysimeter. Similarly, furrow dikes are used to limit runoff and run-on for the surrounding field.

lysimeters at the SPER, and the open space between the lysimeters, limits their absolute accuracy. However, this facility can provide good comparisons between treatments and soil types. This illustrates that even though the quantitative measurements from some lysimeters may be lacking, the qualitative data can still be quite valuable. More information regarding lysimeter research at the CPRL can be

Even smaller lysimeters can also have value. Temporary "micro lysimeters" have been used to measure soil evaporation on a daily time step (see Figure 4). Lysimeters such as these are useful in research involving partitioning evaporation and transpiration separately. Like the small lysimeters at the CPRL, the micro lysimeters are not perfectly accurate but can still provide meaningful data for certain purposes. Many other lysimeter designs have been used and can be permanent or temporary. Large weighing lysimeters are the most accurate, but other, simpler, and more costeffective options are available. As with most instruments, the accuracy and usefulness of the data will depend on the purpose and management of the lysimeter.

Bowen ratio is a method of partitioning fluxes between latent and sensible heat based on flux-profile relationships for energy and mass exchange [21]. This method assumes flux directions are vertical and no horizontal flux movement occurs. Measurements of air temperature and relative humidity are taken at two different heights in the same location. The relative humidity is used to calculate the vapor pressure. The Bowen ratio is the ratio of sensible heat flux to latent heat flux and can

β ¼ γ

An example of a "micro lysimeter" used to determine soil evaporation. The inner soil container can be removed

ΔT Δe

(6)

found in [20].

Field-Scale Estimation of Evapotranspiration DOI: http://dx.doi.org/10.5772/intechopen.80945

2.3 Bowen ratio

be calculated as

Figure 4.

11

from the outer housing and manually weighed.

The lysimeters at the CPRL are a great example of large weighing lysimeters. However, there are many types and sizes of lysimeters. Some are constantly weighing, such as those at the CPRL, while others are weighed periodically. In addition, lysimeters can vary in size. The large weighing lysimeters at the CPRL are considered highly accurate due to their large size, where the effects of the enclosed space on the plants are minimal. Smaller lysimeters will contain more error, especially if the soil volume is small enough where root growth is impeded. With lysimeters, the accuracy is dependent on the lysimeter design, representativeness, maintenance, and operation. Smaller lysimeters can have value, even if they are not highly accurate. An example of the usefulness of smaller lysimeters is the Soil-Plant-Environment Research (SPER) facility, also at the CPRL (see Figure 3). This facility is equipped with 48 lysimeters, each measuring 1 m by 0.75 m by 2.3 m deep [19]. The 48 lysimeters are comprised of 12 replications each of Ulysses silt loam soil from the Garden City, KS area; Pullman clay loam soil from Bushland, TX; Amarillo sandy loam soil from the Big Spring, TX area; and Vingo fine sand soil from the Dalhart, TX area. These represent the four main soil types of the Southern Great Plains of the United States. The SPER contains an automatically controlled rainout shelter that covers the lysimeters during precipitation events, which allows water additions to be precisely controlled through surface drip irrigation. The size of the

#### Figure 3.

The Soil-Plant-Environment Research (SPER) facility at the Conservation and Production Research Laboratory (CPRL) in Bushland,TX. This facility contains 48 smaller lysimeters consisting of 12 replications of the 4 main soil types throughout the Southern Great Plains region of the United States. An automatic rainout shelter (seen in the background) covers the lysimeters during precipitation events so that water can be precisely controlled through surface drip irrigation.
