Ritchie: Inputs and outputs

The soil evaporation Ritchie model (Ritchie, 1972) requires, as input and parameters:

1. a layer by layer soil representation. For each layer it is required the knowledge of:

• a. the layer thickness input (SLTi, m) whose value ranges from 0.01 to 3 m (default value=0.05);
• b. soil water content input (θi, m3 m-3) whose value ranges from 0 to 0.8 (default value=0.25);
• c. soil water residual content (θr, m3 m-3) whose value ranges from 0 to 0.2 (default value=0). θr here represents soil water content below which no more water can be evaporated from the bare soil;
• d. Clay mass layer by layer (cl, % );
• e. Sand mass layer by layer (sa, %);
• f. Flat biomass at soil surface (Flres, kg ha-1).

2. a value of potential evaporation input (Epot, mm d-1), whose value ranges from 0 to 20 (default value=10);

3. a variable that accumulates layer by layer evaporation values. This variable needs to be updated by the strategy itself;

4. a variable that accumulates the number of days when water availability and soil transmissivity are limiting for evaporation. These variables need to be updated by the strategy itself;

5. the daily rainfall;

6. a parameter that sets an option to exclude mulch effect by computation.

The soil evaporation Ritchie model (Ritchie, 1972) estimates (output):

1. soil water evaporation for each layer (Ei, mm) whose value ranges from 0 to 15 (default value=5). This is not an output from the original method, but it is an original attempt to partition evaporation among layers; the most common condition in agricultural soil is the homogeneity of surface layer texture, due to tillage operation, so this partitioning is coincident to the original method;

2. total water losses due to evaporation (Es, mm), whose value ranges from 0 to 15 (default value=5);

3. maximum soil evaporation effective depth (Devap, mm) whose value ranges from 0 to 130 (default value=0).

Methods for parameter estimation:

The θr parameter is estimated in WEPP by the Baumer equation (personal communication). Otherwise it is possible to set θr at a water content value equal to WP/3 (Stöckle et al., 1994). Several PTFs set SWCr to 0 or to some fixed values. In many cases also with measured data of the retention function, θr is only a fitting parameter without physical significance.

Here it is used the default value from the data-type; option for estimating it as WP/3 or following the WEPP model will be implemented.

Precondition needed to correctly apply The soil evaporation Ritchie model (Ritchie, 1972), other than all the values of input and parameters are within their domain:

1. Generic for soil

a. none

2. Specific for this model

a. Soil water content > θr

Postcondition

1. Total soil water evaporation <= potential soil evaporation.

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