Measurement and Estimation of  ET_c

Turf evapotranspiration can be measured or estimated. Measuring methods include use of  lysimeter, water balance and eddy correlation method. Estimation methods include energy balance, mass transfer, combination of energy and heat, mass transfer and the use of crop coefficients. Each method is described next according to Jensen et al., 1990 .

Measuring Methods

Lysimeters

Lysimeters are tanks filled with soil in which crops are grown under natural conditions to measure the amount of water lost by evaporation and transpiration. This method provides a direct measurement of the crop evapotranspiration (ET_c) and is frequently used to study climatic effects on ET_c and to evaluate estimating procedures. By the nature of its construction, a lysimeter prevents the natural vertical flow and distribution of water. Ideally, lysimeters must meet several requirements for the data to be representative of field conditions (Van Bavel, 1961; Miranda et al., 2006). Lysimeters can be grouped into three categories: (1) non-weighing, constant water-table type; (2) non-weighing, percolating-type; and (3) weighing types. Also, large and mini-lysimeters can be used for different applications.

Water Balance

This method has been used on large areas such as valleys in which the inflow and outflow are determined from streamflow and precipitation measurements, and where the basin is confined to eliminate other significant sources of inflow or outflow. This type of method generally provides only gross estimates of the average water evaporated and transpired from cropped and non-cropped areas.

Eddy Correlation

This method overcomes the need to determine each component in the water balance by using the energy balance approach. It also avoids soil surface heterogeneity issues by placing the sensors above the crop canopy and the evapotranspiration can be measured from various type of vegetation.

Estimating Methods

Energy Balance

This method can be used for hourly or shorter values, especially during daylight hours. The Bowen ratio approach is the most commonly used method. The Bowen ratio is the ratio of energy fluxes from one medium to another by sensible and latent heating, respectively. The instrumentation requirements and technical procedures involved generally limit the energy balance method to research studies over relatively short periods of time, but the results can be very reliable if the measurements are accurate because they are obtained under natural environmental conditions.

Mass Transfer

Direct determination of ET can be accomplished for a specific field and crop by using eddy diffusion equations and meteorological data measured on site.

Crop Coefficients

This method estimates ET_c by multiplying reference evapotranspiration (ET_o) to a crop coefficient (K_c), like this:

ET_c = (ET_o)(K_c)

 The ET_o can be calculated from weather data collected from a well watered reference crop surface. Many methods have been developed to estimate the ET_o. The most current method was developed by the American Society of Civil Engineers, Environmental and Water Resources Institute (ASCE_EWRI; Allen et al., 2005) and it is called the 'ASCE-EWRI Standardized Reference ET Equation'. This equation is based on the FAO-56 Penman-Monteith equation for a hypothetical crop is given as:

ETsz = {[0.408 Δ(Rn – G)] + [γ C_n /(T+273) U2 (es – ea)]}/ Δ + γ(1 + C_d U2)            (Eq. 1)

Where ETsz is the standardized reference evapotranspiration for a short reference crop (grass - ETos) or a tall reference crop (alfalfa - ETrs) in units based on the time step of mm d^-1 for a 24-h day or mm h^-1 for an hourly time step, C_n is the numerator constant for the reference crop type and time step and C_d is the denominator constant for the reference crop type and time step (see Table 1 for values of C_n and C_d). However, as early as 1952, turfgrass crop-water requirements studies began in Florida by McCloud and Dunavin (1954). Later on, McCloud (1955) developed an empirical formula based on mean air temperature to estimate ET_o, which resulted relevant for Gainesville climatic conditions. The application of this formula in other locations in Florida resulted in underestimated ET_o values, specifically where mean temperature was above 70F.  It is important to note that this equation is only relevant for Gainesville, FL and its application under another climatic conditions might bring some errors when turf water consumption is estimated.

The estimation of ET_c using crop coefficients requires crop coefficients developed using crop evapotranspiration measurements (ET_c) and ET_o estimates for a specific crop and location.

Table 1: Values for C_n  and C_d in Eq. 1 (after Allen et al., 2005).

Calculation time step	Short reference crop ETos	Tall reference crop ETrs	Units for ETos, ETrs	Units for Rn and G
	Cn                    Cd	Cn                  Cd
Daily Hourly, daytime Hourly, nighttime	900                 0.34 37                   0.24 37                   0.96	1600           0.38 66                0.25 66                1.70	mm d-1 mm d-1 mm d-1	MJ m-2 d-1 MJ m-2 d-1 MJ m-2 d-1

• ET_c Rates of Warm-Season Grasses Commonly Used in Florida

 

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