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 values

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:

Table 5.19: Default Properties for Evapotranspiration

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 parameter

Scope: .etprops

1. cint_et Canopy storage parameter [L], $c_{int}$ in Equation 2.73.

• • •

Initial interception storage

Scope: .etprops

1. init_sint_et initial canopy interception storage value [L], $S_{int}^{0}$ in Equation 3.59.

• • •

Transpiration fitting parameters

Scope: .etprops

1. C_1 Coefficient $C_{1}$ [dimensionless] in Equation 2.75.
2. C_2 Coefficient $C_{2}$ [dimensionless] in Equation 2.75.
3. C_3 Coefficient $C_{3}$ [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 saturations

Scope: .etprops

1. thwp_et Saturation [dimensionless] at wilting point, ${\theta }_{wp}$ in Equation 2.77.
2. thfc_et Saturation [dimensionless] at field capacity, ${\theta }_{fc}$ in Equation 2.77.
3. tho_et Saturation [dimensionless] at oxic limit, ${\theta }_{o}$ in Equation 2.77.
4. than_et Saturation [dimensionless] at anoxic limit, ${\theta }_{an}$ in Equation 2.77.

• • •

Evaporation limiting saturations

Scope: .etprops

1. the2_et Saturation [dimensionless] below which evaporation is zero, ${\theta }_{e2}$ in Equation 2.77.
2. the1_et Saturation [dimensionless] above which full evaporation can occur, ${\theta }_{e1}$ 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...End

Scope: .etprops

1. time(1), lai(1) First entry.
2. 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:

Table 5.20: Observed Values of Leaf Area Index [Scurlock et al., 2001] and Maximum Rooting Depth [Canadell et al., 1996] for Various Terrestrial Biomes

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 depth

Scope: .etprops

1. 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:

Figure 5.16: Normalized Root Depth Functions

• • •

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 depth

Scope: .etprops

1. 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 point

Scope: .grok

1. 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).