# HydroGeoSphere/Evapotranspiration

NOTE: Moisture contents referred to below and in section 2.5.3 are actually input in **grok** as saturations, which are less prone to error as they always vary between zero and 1, while moisture content varies between zero and porosity.

## Default evapotranspiration valuesEdit

Unless you modify the default values, all zones (and elements) in the ET domain will be assigned the default properties listed in Table 5.19:

Parameter | Value | Unit |
---|---|---|

Name | Default ET (grass) | - |

Canopy storage parameter | 0.04 | m |

Initial interception storage | 0.04 | m |

Transpiration fitting parameters: |
- | - |

C1 | 0.5 | - |

C2 | 0.0 | - |

C3 | 1.0 | - |

Transpiration limiting saturations: |
- | - |

Wilting point | 0.05 | - |

Field capacity | 0.2 | - |

Oxic limit | 0.6 | - |

Anoxic limit | 0.8 | - |

Evaporation limiting saturations: |
- | - |

Minimum | 0.5 | - |

Maximum | 0.1 | - |

Leaf area index | 1.0 | - |

Root zone depth | 0.2 | m |

Evaporation depth | 0.2 | m |

For each instruction we will again indicate its scope (i.e. `.grok`, `.etprops`). Recall that if an instruction is used in the *prefix*`.grok` file, it will affect the current set of chosen zones, while in a properties (e.g. `.etprops`) file, it will only affect the named material of which it is a part.

## Canopy storage parameterEdit

`Scope: .etprops`

**cint_et**Canopy storage parameter [L], in Equation 2.73.

**• • •**

## Initial interception storageEdit

`Scope: .etprops`

**init_sint_et**initial canopy interception storage value [L], in Equation 3.59.

**• • •**

## Transpiration fitting parametersEdit

`Scope: .etprops`

**C_1**Coefficient [dimensionless] in Equation 2.75.**C_2**Coefficient [dimensionless] in Equation 2.75.**C_3**Coefficient [dimensionless] in Equation 2.77. By default, this coefficient is set to 1.0, which gives a linear ramping function whereas higher values would give higher order ramping functions.

**• • •**

## Transpiration limiting saturationsEdit

`Scope: .etprops`

**thwp_et**Saturation [dimensionless] at wilting point, in Equation 2.77.**thfc_et**Saturation [dimensionless] at field capacity, in Equation 2.77.**tho_et**Saturation [dimensionless] at oxic limit, in Equation 2.77.**than_et**Saturation [dimensionless] at anoxic limit, in Equation 2.77.

**• • •**

## Evaporation limiting saturationsEdit

`Scope: .etprops`

**the2_et**Saturation [dimensionless] below which evaporation is zero, in Equation 2.77.**the1_et**Saturation [dimensionless] above which full evaporation can occur, in Equation 2.77.

**• • •**

The following instruction can be used to modify the default value (1.0 for all time) of the leaf area index *LAI*:

## Lai tables...EndEdit

`Scope: .etprops`

**time(1), lai(1)**First entry.**time(2), lai(2)**Second entry.

- ...etc.
- n.
**time(n), lai(n)***n*^{th}entry.

Causes **grok** to begin reading a group of time vs leaf area index instructions until it encounters an End instruction.

Paired values of time *T* and leaf are index *LAI* should be entered from earliest to latest time. The number of entries in the list are counted automatically to determine the table size.

Observed values of leaf are index [*Scurlock et al.*, 2001] and maximum rooting depth [*Canadell et al.*, 1996] for various terrestrial biomes are shown in Table 5.20:

Vegetative cover | Leaf area index | Maximum rooting depth |
---|---|---|

Desert | 1.31 | 9.5 |

Tundra, circumpolar and alpine | 2.69 | 0.5 |

Wetlands, temperate and tropical | 6.34 | - |

Grasslands, temperate | 2.50 | 2.6 |

Grasslands, tropical | 2.50 | 15.0 |

Crops, temperate and tropical | 4.22 | 2.1 |

Shrubland, heath or Mediterranean-type vegetation | - | 5.2 |

Forest: |
- | - |

boreal deciduous broadleaf | 2.64 | 2.0 |

boreal evergreen needleleaf | 3.50 | 2.0 |

boreal/temperate deciduous needleleaf | 4.63 | 2.0 |

temperate deciduous broadleaf | 5.12 | 2.9 |

temperate evergreen needleleaf | 6.70 | 3.9 |

temperate evergreen broadleaf | 5.82 | - |

tropical deciduous broadleaf | 3.92 | 3.7 |

tropical evergreen broadleaf | 4.90 | 7.3 |

Plantations (managed forests) | 8.72 | - |

Overall mean | 5.23 | - |

**• • •**

## Root depthEdit

`Scope: .etprops`

**root_depth_et**Maximum root depth [L].

A normalized root depth function is mapped onto porous media elements above this maximum depth. Currently, four root depth functions are available; constant, linear, quadratic and cubic, as shown in Figure 5.16. These functions are defined such that the area under the normalized function is 1.0:

**• • •**

By default, the linear form of the depth function is used. The following instructions are available for using the other forms:

- Rdf constant function
- Rdf quadratic decay function
- Rdf cubic decay function

## Evaporation depthEdit

`Scope: .etprops`

**evap_depth_et**Evaporation depth [L].

Evaporation as a function of depth is treated in a similar fashion as the root zone depth described above.

**• • •**

By default, the linear form of the evaporation function is used. The following instructions are available for using the other forms:

- Edf constant function
- Edf quadratic decay function
- Edf cubic decay function

## Echo et at pointEdit

`Scope: .grok`

**x1, y1***xy*-coordinate.

This instruction finds the column of nodes that the given coordinate falls within and then writes the pertinent evapotranspiration information to the `o.eco` file.

**• • •**

For example:

```
ET properties for element column
ZONE: 1
ET MATERIAL: dense deciduous forest
Canopy storage parameter = 0.0
Transpiration fitting parameter C1 = 0.3
Transpiration fitting parameter C2 = 0.2
Transpiration fitting parameter C3 = 3.0E-6
Moisture content at wilting point = 0.32
Moisture content at field capacity = 0.2
Moisture content at oxic limit = 0.76
Moisture content at anoxic limit = 0.9
Moisture content at at energy limiting maximum = 0.32
Moisture content at at energy limiting minimum = 0.2
Initial interception storage = 0.0
Tabular data:
Time LAI
0.00000 2.08000
0.100000E+42 2.08000
Maximum evaporative zone depth = 2.0
Depth EDF
0.00000 2.00000
1.00000 0.00000
Maximum root zone depth = 4.0
Depth RDF
0.00000 2.00000
1.00000 0.00000
ELEMENT ET FLAG DEPTH EDF RDF
12889 T 0.00000 0.281045 0.146305
11528 T 0.303244 0.234783 0.134740
10167 T 0.606488 0.188522 0.123174
8806 T 0.909732 0.142261 0.111609
7445 T 1.21298 0.959994E-01 0.100044
6084 T 1.51622 0.497381E-01 0.884783E-01
4723 T 1.81946 0.765174E-02 0.769130E-01
3362 T 2.1227 0.00000 0.653477E-01
2001 T 2.4259 0.00000 0.537824E-01
640 T 2.7292 0.00000 0.422171E-01
! ------------ ------------
Totals 1.00000 0.942610
```

In this case, the element column falls in ET zone 1, and the default linear root depth and evaporation depth functions have been used and mapped onto it. All of the evaporative potential has been distributed to the element column, as indicated by a total EDF value of 1.0. However, only a portion (0.942610 or 94%) of the total RDF of 1.0 has been mapped onto it, because the maximum root zone depth (4 m) exceeds the total depth of the element column at this point (2.7292 m).