Weather Data Requirements and Availability

The FAO-PM method requires that the following data be available for the daily (or monthly) calculations:

• temperature, that is, maximum and minimum daily air temperature (Tmax and Tmin);

• humidity, that is, dewpoint temperature (Tdew), maximum and minimum relative humidity (RHmax and RHmin), or dry- and wet-bulb temperature (7¿ry and Twet);

• radiation, that is, net radiation Rn, shortwave incoming solar radiation (Rs), or bright sunshine hours per day (n).

An optimal situation occurs when all four types of these observations are made in a weather station (see "Daily ET calculations with FAO-PM Method," below). Many weather stations have only limited instrumentation and do not provide data for all types of required meteorological variables. This calls for the use of appropriate estimation methods (see "Estimation of ET0 with Limited Weather Data," below) or for the application of other ET equations (see "Estimation of ET0 with Other Equations," below). ET0 also can be estimated from pan evaporation data (see "Pan-Evaporation Method," below). Unfortunately, it is not enough that observations be made, but also that integrity of data be assessed. Often, data sets are incomplete or do not comply with homogeneity requirements. Although accuracy of measurements is important, errors resulting from inadequacy of the site may be more important than normal errors of observations.

The quality of reference ET depends upon the quality of the weather data utilized. It therefore is advisable that the quality of weather data be assessed before these data are utilized in ET0 calculations. Data integrity screening should be a part of every agricultural weather network to ensure that high-quality and representative data are being collected and provided to users. Procedures presented by Allen [78] concern the calculation of clear sky envelopes for solar radiation, computational validation of net radiation measurements, and expected trends and relationships between air vapor content and air temperature.

Because the ET0 definition and concept apply to a grass crop that is adequately watered and actively transpiring, it is important to follow these conditions in selecting and screening weather data. Any deviations in weather measurements from those expected within an agricultural situation will violate equilibrium boundary-layer profiles and energy exchange conditions representative of a grass reference condition.

ET in an arid, nonirrigated environment is low during periods of low rainfall. Air temperatures T and VPDs in these environments are higher relative to an irrigated environment because of a greater conversion of net radiation into sensible heat. Use of data (particularly humidity and temperature) from nonirrigated weather stations may introduce a bias into ET equations, generally causing overestimation [79]. Differences in mean daily air temperatures as high as 4-5°C on a monthly basis between arid and irrigated stations located less than 10 km apart caused an overestimation of 16% in seasonally computed ET0. Procedures for analysis and correction of weather data from nonreference sites are presented by Allen [78] and Allen et al. [10].

Daily ET0 Calculations with FAO-PM Method

Several parameters characterizing the state of humidity in the atmosphere and the vapor transfer from the canopy into the air, and the energy available at the surface are required in the ET calculations with the FAO-PM. Details on these parameters are given by Allen et al. [60]. Calculations of ET0 that are made using monthly average weather data are very similar to calculations made using daily average weather data on daily calculation time steps and summed over each day of the month [60]. The standardized daily ET0 calculation procedures are described in the following subsections:

Slope of Vapor Pressure Curve (A)

The relationship between air temperature and saturation vapor pressure is characterized by the slope of the vapor pressure curve A (kPa °C-1) as follows:

where T is the mean daily air temperature (°C)

Psychrometric Constant (y)

The saturation vapor pressure at wet-bulb temperature is related to the actual air conditions by

where y is the psychrometric constant (kPa °C-1), P is the atmospheric pressure (kPa), and k is the latent heat of vaporization = 2.45 MJ kg-1.

A simplified equation derived from the ideal-gas law can be assumed for the atmospheric pressure as a function of elevation z (m):

Saturation Vapor Pressure (es)

The saturation vapor pressure is given by

where e0(T) is the saturation vapor pressure function (kPa) and T is the air temperature (°C). For 24-h time periods, es should be computed for the maximum and minimum daily temperature (Tmax and Tmin):

Computations using the mean daily temperature should be avoided because the temperature-saturation vapor pressure relationship is not linear, and this would cause underprediction of es. An estimation of Tmax and Tmin from Tmean is given in Eqs. (5.46) and (5.47).

Actual Vapor Pressure (ea)

The actual vapor pressure ea is the saturation vapor pressure at dewpoint temperature (Tdew). The following approximations [60] are given, in a decreasing order of accuracy, for daily ea computations.

Dewpoint Temperature. When available, it provides the best estimate:

where ea is the actual vapor pressure (kPa) and Tdew is the dewpoint temperature (°C).

PsychrometerMeasurements. Using measurements with dry- and wet-bulb thermometers ea can be approximated by use of of the following equation:

where yasp is 0.00066 for Assmann aspiration at 5 ms-1, 0.0008 for natural ventilation at 1 ms-1, and 0.0012 for indoor ventilation of 0 ms-1(°C-1); Tdry is the dry-bulb temperature (°C); Twet is the wet-bulb temperature (°C); P is the atmospheric pressure (kPa), and e0(Twet) is the saturation vapor pressure at wet-bulb temperature (kPa).

Hygrometer (or Psychrometer) Measurements of Relative Humidity (RH). When two RH measurements are available daily at early morning and at early afternoon, the following equation can be used:

where e0(Tmin) and e0(Tmax) are the saturation vapor pressure (kPa) computed at Tmin and Tmax, respectively, and RHmax and RHmin are, respectively, the maximum and minimum daily RH (%).

When only mean daily relative humidity (RHmean) data are available, average daily vapor pressure ea can be computed as


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