FAO 56

The FAO 56 Soil water model is a simple water balance based model that follows the recommendations provided in FAO 56 /3/ for use with the dual crop coefficient method. More details are given in the following sections:

· FAO 56 Soil model properties

· FAO 56 Soil model computations

FAO 56 Soil model properties

The following parameters need to be specified for the FAO 56 Soil water model (see also Table A.3.1 for suggested representative values):

In the Soil moisture content dialogue:

· Initial. This is the starting soil moisture content at the beginning of the crop season [0-1]. The Initial soil moisture content is set for the Top soil and the Root zone. For the Lower zone the initial soil moisture is by default, at Field capacity.

· Field capacity. This is the maximum water content held by the soil against gravity [0-1]. Water cannot be retained in the top soil and the root zone layer when the water is above field capacity, as it will drain away under gravity. Soil moisture below field capacity is available for evapotranspiration until the soil moisture reaches wilting point.

· Wilting point. This is the lowest soil moisture content at which plants can draw water from the top soil and the root zone layers [0-1].

· Porosity. This is the maximum soil moisture content that the soil can contain [0-1].

· Depth of evaporable layer. This is the depth of the top soil layer Ze from which evaporation occurs [m].

Example: The amount of water in a soil column depends on the soil moisture content and the length of the soil column. If the field capacity is 0.15, the wilting point is 0.05, and the length of the root zone is 0.5 meter, then the available amount of water for transpiration is: 500 mm.

Table A.3.1 The representative soil property values and maximum depletion by evaporation for an evaporation layer of 0.1 m
Soil type	Field capacity	Wilting point	Difference	Max. depletion by evaporation [mm]
Sand	0.07-0.17	0.02-0.07	0.05-0.11	6-12
Loamy sand	0.11-0.19	0.03-0.10	0.06-0.12	9-14
Sandy loam	0.18-0.28	0.06-0.16	0.11-0.15	15-20
Loam	0.20-0.30	0.07-0.17	0.13-0.18	16-22
Silt loam	0.22-0.36	0.09-0.21	0.13-0.19	18-25
Silt	0.28-0.36	0.12-0.22	0.16-0.20	22-26
Silt clay loam	0.30-0.37	0.17-0.24	0.13-0.18	22-27
Silt clay	0.30-0.42	0.17-0.29	0.13-0.19	22-28
Clay	0.32-0.40	0.20-0.24	0.12-0.20	22-29

FAO 56 Soil model computations

The model keeps track of the soil moisture content in two soil water compartments:

· The top soil layer from which only soil evaporation is drawn. The length of this layer is specified as the "Depth of evaporable layer" and is denoted Ze.

· The root zone layer. The length of this layer corresponds to the actual root depth (Zroot(t)), which can vary in time depending on the crop development. Transpiration takes place from this layer.

The wetting fraction (equals 1.0 for rain and specified by the user for irrigation) is taken into account when the exchange between the evaporable layer and the root zone is calculated. For rain (wetting fraction 1.0) the sub model assumes that the evaporable layer drains to the root zone as soon as the water content has reached field capacity (above which free drainage can occur). For wetting fractions less than 1, water will start draining at average water contents lower than field capacity. (see Wetting fraction time series)

· Water balance for the top soil layer. The water balance, per unit time, is calculated in the following three steps:

– Infiltration of rain and irrigation water (calculated in two steps).
First rainfall is added, and if storage is still available, irrigation is added:

Where:

: Soil moisture content at saturation [-]

: Soil moisture content to the old time [-]

: Soil moisture in the top soil layer after rain is applied [-]

P: Net rainfall after runoff [mm]

Irr: Irrigation demand [mm]

Ze: Depth of the top soil layer [m]

Wf: Wetting fraction [-]

– Percolation to the root zone: After water from rain and irrigation has been applied, the amount of water above field capacity percolates to the root zone layer assuming that water is instantaneously drained, by gravity, out of the top soil layer.

Where:

: Updated water content after irrigation has been applied [-]

: Soil moisture content at field capacity [-]

– Updating the water content: The water content is now updated taking into account the evaporation from the top soil layer. Notice that the water content in the top soil during periods with high infiltration will be equal to field capacity minus the evaporation.

Where:

: Final soil moisture content in the top soil at the new time [-]

Ev: Evaporation [mm]

· Water balance for the root zone layer.The water balance, per unit time, is calculated in the following two steps:

– Root zone percolation. The percolation out of the root zone layer is assumed to take place, by free drainage, instantaneously as soon as the soil moisture content is above field capacity. It is calculated from the following equation:

Where Zroot is the thickness of the root zone. Zroot is time varying depending on the crop stage. is the intermediate soil moisture content in the root zone layer after it has been updated with the percolation from the top soil layer.

– Updating the water content. The new water content after the time step is then calculated by:

Where Et is the transpiration [mm].