Water retention curve

In the model there are two options for how to express the water retention function as determined by the switch Hydraulic Functions.

In the first function by Brooks & Corey (1964), the pressure head or actual water tension, ψ, is given by:

                                                              (2.19)

where ψ_a is the air-entry tension and λ is the pore size distribution index. The effective saturation, S_e, is defined as:

                                                               (2.20)

where θ_s is the porosity, θ_r is the residual water content and θ is the actual water content, see Figure 2.2.

 

Figure 2.2. Variables in the Brooks and Corey expression.

See viewing functions Measured Unsaturated Conductivity, Pressure Head, single layers and Modelled Water Retention, profile.

As an alternative expression to the Brooks & Corey expressions, the water retention function by van Genuchten (1980) has been introduced:

                                                     (2.21)

where α, g_n and g_m are empirical parameters.

In order to get a good fit in the whole water content range, eqs.(2.19) and (2.21) are fitted only to data corresponding to tensions below a threshold value, ψ_x (Figure 2.3). The relation between water content and tension above this threshold is assumed log-linear:

                      (2.22)

where θ_x is the threshold water content at the threshold tension, ψ_x, θ_wilt is the water content at wilting point, defined as a tension of 15 000 cm water, i.e. ψ_wilt.

In the range close to saturation, i.e. from θ_s to θ_m a linear expression is used for the relationship between water content, θ, and water tension, ψ.

                      (2.23)

where ψ_mat is the tension that corresponds to a water content of θ_s - θ_m. The three different parts of the water retention curve is illustrated for a sandy soil below.

 

Figure 2.3. An example of how three different expressions in the water retention curve are used in different ranges. The pF value corresponds to the logarithm of tension expressed in cm

It is possible to scale the water retention curve so that the curve is shifted either to the right or the left (see switch Scaling retention). This is accomplished by modifying the porosity, θ_s, and the residual water content, θ_r:

                                                        (2.24)

and

                                                             (2.25)

where s_sscale and s_rscale are scaling parameters (see viewing functions Scaling of water retention, porosity and Scaling of water retention, residual water content).