Evapotranspiration

Evapotranspiration is the sum of water evaporated from soil, canopy storage, surface water column and from plant transpiration.

Reference Evapotranspiration

Reference evapotranspiration is an input for MOHID Land and can be provided by user or calculated. In any case, a property named reference evapotranspiration must exist in the Basin data file. This property can have a constant value in time, can be set to be calculated or data can be provided with a Timeseries file, a HDF file or any other valid input data file (see Module FillMatrix).

<beginproperty>
  NAME                               : reference evapotranspiration
  UNITS                              : mm/h
  DESCRIPTION                        : fao evapotranspiration
  DEFAULTVALUE                       : 0.2
  REMAIN_CONSTANT                    : 1
  NO_INTERPOLATION_OR_ACCUMULATION   : 1
<endproperty>

The above example sets a constant reference evapotranspiration of 0.2 mm/h during the entire simulation period.

To make MOHID Land calculate the reference evapotranspiration, the REMAIN_CONSTANT keyword must be set to 0 (false) and no input data files should be provided. MOHID Land will then calculate the reference evapotranspiration using a set of properties that must be set in the Atmosphere data file (see Module Atmosphere). Below is an example of the reference evapotranspiration property set to be calculated:

<beginproperty>
  NAME                               : reference evapotranspiration
  UNITS                              : mm/h
  DESCRIPTION                        : fao evapotranspiration
  DEFAULTVALUE                       : 0.0
  REMAIN_CONSTANT                    : 0
  NO_INTERPOLATION_OR_ACCUMULATION   : 1
<endproperty>

Evaportransipiration is calculated with the 'FAO crop reference evapotranspiration', a standardized Penman-Monteith equation.

All necessary information can be obtained from the FAO website: [1]

ATTENTION: The "UNITS" chosen for the reference evapotranspiration property are important. Internally MOHID Land will convert the values provided with the user units to m/s (meters by second). The units chosen for the property must match that of the input file if one is used. The units are always in LENGTH/TIME format.

The valid units for length are: m, cm, mm
The valid units for time are  : d (for days), h (for hours), s (for seconds)

Crop Evapotranspiration

If Module Vegetation is used, then the reference evapotranspiration will be adjusted using the crop coefficient property, that must be provided in the Vegetation data file. Optionally, the user can provide a "potential leaf area index" property (also in the Vegetation data file). In this case, the crop coefficient (Kc) will be adjusted using the LAI (leaf area index) before the crop evapotranspiration is calculated. This is done using the equation below:

$K_{corr}=K_{c}-[1.0-(\frac{LAI}{LAI_{dense}})^{0.5}]$

Where $LAI_{dense}$ is the potential LAI (potential leaf area index property)

If the potential leaf area index property is not provided, then the crop coefficient value is used without modification.

$K_{corr}=K_{c}$

After the Kcorr is found, the reference evapotranspiration is adjusted using it:

$CropEvapotrans=RefEvapotrans \times K_{corr}$

Basin data file keywords related with the Crop Coefficient (Kc) adjustment

When using the LAI to adjust the crop coefficient value, it's possible to set a minimum Kc and a value for Kc when LAI is ZERO. This is done using these keywords in the Basin data file:

DEFAULT_KC_WHEN_LAI_ZERO
KC_MIN

The DEFAULT_KC_WHEN_LAI_ZERO keyword tells that Kcorr must be set to this value when LAI is ZERO. The KC_MIN keyword tell that if the adjusted value found is lower than it, so the min value will be used instead. It's a good idea provide at least a KC_MIN value (usually 0.3 for soil without vegetation).

Potencial Evaporation & Potencial Transpiration

If the keyword EVAPOTRANSPIRATION_METHOD in Basin data file was set to 2 (separete evaporation and transpiration), then the crop evapotranspiration will be separated into a Potential Evaporation and a Potencial Transpiration.

This is done using LAI (leaf area index), according with the equations below:

$PotentialTranspiration=CropEvapotrans \times (1.0-e^{-0.463 \times LAI})$

$PotentialEvaporation=CropEvapotrans-PotentialTranspiration$

Actual Evapotranspiration

The actual (or real) evapotranspiration is calculated using the crop evapotranspiration (if EVAPOTRANSPIRATION_METHOD = 1) or using the PotentialEvaporation and PotentialTranspiration (if EVAPOTRANSPIRATION_METHOD = 2) and is subjected to the soil water content, stress factors and other limitations (like Head limits).

Basin Output Timeserie

Depending on the value of the EVAPOTRANSPIRATION_METHOD keyword, the 2D output timeserie file for basin (extension srb) will have a set or all of the columns listed next:

EvapoTranspiration_Rate_[mm/hour]   => Actual Evapotranspiration rate (actual transpiration + actual evaporation)
Potential_Crop_EVTP_[mm/hour]       => Crop Evapotranspiration rate  (reference evapotranspiration rate corrected using Kc)
Potential_Evaporation_[mm/h]        => Potential Evaporation rate    (only if EVAPOTRANSPIRATION_METHOD = 2)
Potential_Transpiration_[mm/h]      => Potential Transpiration rate  (only if EVAPOTRANSPIRATION_METHOD = 2)
Actual_Evaporation_[mm/h]           => Actual Evaporation rate       (only if EVAPOTRANSPIRATION_METHOD = 2)
Actual_Transpiration_[mm/h]         => Actual Transpiration rate     (only if EVAPOTRANSPIRATION_METHOD = 2)
Reference_Evapotranspiration_[mm/h] => Reference Evapotranspiration rate