Two different soil hydraulic properties, the water retention curve and the unsaturated conductivity function, needs to be determined in order to solve the water balance equation (2.2). Both properties are considered functions of the water content with or without hysteresis effects (hysteresis is described in detail in section Soil water flow). The temperature effect is neglected for the water retention curve but included for the hydraulic conductivity.
To determine these hydraulic properties there is naturally a need to parameterize the model according to measured data. There is plenty of data on soil hydraulic properties for many different soils in the database that can be used as an alternative to own measurements. However, if measurements have been made and the user would like to add them to the model, the level in the soil where the samples were taken very seldom coincides with the heights of the layers in the model. The points of measurement can also be very unevenly distributed in the profile (for example many at the top and few at lower layers). Therefore the measurements are given to the model in a parameter table together with the sampling depth. The model then uses the measured values to interpolate parameter values for each model compartment. This procedure is described in detail in the section “Soil Profile” in “Common Characteristics”. The interpolated values can be viewed in this section in the parameter tables “model boundaries” or “model layers”. Each parameter table in which measured values are added is called “measured horizons” and thus have a corresponding table for interpolated values.
Some parameters can be estimated from others if they are not measured explicitly. This procedure is described at the end of this section.