Evapotranspiration (AETI), Land Cover Classification (LCC), Net Primary Productivity (NPP), Precipitation (PCP), Phenology (PHE), Quality layers (QUAL), Reference Evapotranspiration (RET), Soil moisture (RSM), Total Biomass Production (TBP) and Water Productivity (WP)

Reference Evapotranspiration (RET) data component

Reference evapotranspiration (RET) is defined as the evapotranspiration from a hypothetical reference crop. It simulates the behaviour of a well-watered grass surface and can be used to estimate potential ET for different crops by applying predefined crop coefficients. This information can be used in the design of irrigation schemes. Together with estimates of the evaporation, transpiration and interception, crop coefficients may be derived as the ratio between ETIa and RET. This information may be combined with land cover maps, to infer crop coefficients during the growing season for different type of crops. RET is not influenced by land cover and can be calculated using standard weather measurements and solar radiation.

WaPOR data component

The reference evapotranspiration data component (RET) is produced at a spatial resolution of 20 km. The data are produced on a daily basis and pixel values represent the daily Reference ET₂₄ in mm/day. The highest reference evapotranspiration values can be found in the Sahara desert and on the southern part of the Arabian Peninsula where RET values can reach 16 mm/day. Here, temperature and incoming solar radiation are high as it is close to the equator and the relative humidity is low. This creates potential for high evapotranspiration, though little water is actually available to fill this potential. The equatorial rain forests have a lower reference evapotranspiration compared to the desert area as the relative humidity is high, suppressing the potential for evapotranspiration. In this area, the RET ranges from 1 to 5 mm/day.

Table X: Overview of RET data component

Methodology

Reference evapotranspiration is calculated in a similar way as evaporation and transpiration, applying the Penman-Monteith equation. The main differences are that for calculating RET some of the variables are predefined (i.e. crop height, bulk surface resistance and albedo) and E and T are not calculated separately. The theoretical background behind the Penman-Monteith equation is given in the evapotranspiration methodology description.

where:
λ = latent heat of evaporation [J kg^-1]
E = evaporation [kg m^-2 s^-1]
T = transpiration [kg m^-2 s^-1]
R_n = net radiation [W m^-2]
G = soil heat flux [W m^-2]
&rho:_a = air density [kg m^-3]
c_p = specific heat of dry air [J kg^-1 K^-1]
e_a = actual vapour pressure of the air [Pa]
e_s = saturated vapour pressure [Pa]
Δ = slope of the saturation vapour pressure vs. temperature curve [Pa K^-1]
γ = psychrometric constant [Pa K^-1]
r_a = aerodynamic resistance [s m^-1]
The soil heat flux G is considered to be net 0 for the whole day.

The aerodynamic equation for the reference crop is parametrized taking into account the crop height of 0.12m,

where
μ_obs = the wind speed [m^-1] at observation height of 10m
r_s = bulk surface resistance [s m^-1]

The resistance to vapour flow from the transpiring reference crop is set to 70 sm^-1. Δ, γ and ρ_a are a function of air temperature and elevation.

The net radiation R_n, is solved using the radiation balance:

where:
α₀ = the surface albedo [-] (a fixed albedo of 0.23 is used for the reference crop)
R_s = incoming solar radiation [W m^-2]
L* = net long wave radiation [W m^-2]
I = the energy needed for interception [W m^-2], which is set at 0 for calculating RET.
For more information on the parameterization, see FAO report 56, page 21 (Allen, 1998).

Processing approach

The input data components for the production of reference evapotranspiration (RET) are daily meteorological and solar radiation data. The FAO-56 procedure for the calculation of reference evapotranspiration has been split into two steps; one for obtaining the meteorological data, the other for calculating the reference evapotranspiration. The evapotranspiration of a well-watered field of grass is calculated under the current atmospheric conditions with the albedo for grass set to 0.23.

Challenges

Since RET is produced at a resolution of 20 km, it has limited information on spatial variation compared to the other data components at level 2 (100m) and level 3 (30m).

Functions and flowcharts

Meteorological data (NRT processing, using GEOS-5)

Meteorological data (final processing, using AgERA5)

Reference evapotranspiration