Continuous:Computation of Soil Moisture

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9.2 - Computation of Soil Moisture

Potential evaporation (PET) is computed hourly from hydro-meteorological inputs using the Penman-Monteith (Montieth, 1965, 1981) method for vegetated soils. PET is first subtracted from any water ponded on the cell surface. If there is no surface water or the amount of water ponded on the surface does not satisfy the PET demand, then actual ET (AET) taken from the soil layer is dependent on the soil moisture. For values of water content higher than the wilting point (θwp), the soil moisture below which plants cannot pull water from the soil matrix, the AET (m/s) is computed as (Dingman, 1994):

Calculation of AET from PET

AET is computed separately for each layer. The PET is allocated to soil layers based on the relative size of the layers. The number of layers in the soil column depends on the method used to calculate infiltration, how the soil is discretized, and the depth of the soil layer, as defined in the SOIL_MOIST_DEPTH. How the soil moisture is calculated when using INF_RICHARDS is described in Section 7. When INF_REDIST is chosen as the infiltration model for long term simulations the soil moisture is calculated at descibed in this document.

Two Layer Soil Moisture Model

The relationship between soil moisture and the ratio of AET to PET looks like the figure below. In GSSHA the exponent on the equation is 1.0, so the relationship is linear. As seen in the figure, non-linear relationships can be made by varying the exponent on the equation. This is not an option in GSSHA at this time.

Aet pet.jpg

When using the two layer model, either one or two layers be specified. The total depth of the soil layer is defined with the SOIL_MOIST_DEPTH file. The SOIL_MOIST_DEPTH can be broken into two layers by specifying a surface layer depth using the TOP_LAYER_DEPTH card. The TOP_LAYER_DEPTH must be equal to or less than the SOIL_MOIST_DEPTH. The default value for TOP_LAYER_DEPTH is the SOIL_MOIST_DEPTH, so that unless a TOP_LAYER_DEPTH is specified, only one soil layer will be present in the GSSHA model. For this method, when no infiltration is occuring soil moisture calculations occur at the HMET update interval, one hour. When infiltration is occuring, the soil moisture is calculated every infiltration time step, on the order of one minute. Downer (2008), link above, gives the details of the method.

If INF_LAYERED_SOIL model of infiltratio is used, the method used to calculate soil moisture is the same as with the INF_RICHARDS method Downer (2008), except neither SOIL_MOIST_DEPTH nor TOP_LAYER_DEPTH are specified. The depths of the soil layers in the invidual cells are taken from the SOIL_LAYER_INPUT_FILE.

GSSHA User's Manual

9 Continuous
9.1     Computation of Evaporation and Evapo-transpiration
9.2     Computation of Soil Moisture
9.3     Hydrometeorological Data
9.4     Snowfall Accumulation and Melting
9.5     Sequence of Events During Long-Term Simulations