Unsaturated Zone

There are three methods in MIKE SHE to calculate Unsaturated Flow:

· the Two-Layer Water Balance (V1 p. 563).

The available options in this dialog and the items in the data tree depend on which UZ calculation method chosen in the Simulation Specification (V1 p. 182) dialog.

Initial Conditions

The initial conditions section defines the initial moisture content for the soil profile.

· Equilibrium pressure profile (to field capacity) This is the default option. In this case, the initial soil moisture distribution follows the water content-pressure relationship from the soil database with a minimum water content equal to the calculated field capacity. In other words, the profile will be saturated at the water table and decrease above the water table until field capacity is reached. All cells above this will have a water content equal to field capacity. This is a reasonable initial condition for most temperate climates, where the water content is regularly recharged by rainfall. Generally, this will lead to an initial drainage from the UZ as the water content equilibrates with the rainfall rate.

· Equilibrium pressure profile (to residual moisture content) This option is useful in arid and semi-arid conditions where the natural soil moisture below the root zone is low. When the rate of rainfall is low, then the natural soil moisture distribution will approximate the soil-moisture pressure curve. This option is useful in dry conditions because drainage to dry soil moisture with depth may require a long simulation to reach equilibrium.

· Specified matrix potential and Specified water content These two options for specifying the water content or matrix potential with depth are typically only used in special conditions where the models are very detailed and accurate soil moisture or pressures have been measured. For example, in column experiments. Specifying these values requires you to define either a uniform value, or values and specified depths.

Green and Ampt Infiltration

The Green and Ampt infiltration is an analytical solution to the increased infiltration experienced in dry soils due to capillarity. It is available for the 2-Layer WB and the Gravity Flow UZ solution methods. The Richards equation method already includes capillarity so the Green and Ampt method is not applicable.

Enable column classification

The column classification should be avoided today because the models have become more complex, MIKE SHE has become more efficient and computers have become faster.

If the either Richards Equation or the Gravity Flow Module are chosen for calculating the unsaturated zone flow then the top-level Unsaturated Zone dialogue has an option to turn open a section for the column classification. However, if the Two Layer Method is chosen then the Column Classification section is unavailable.

Calculating unsaturated flow in all grid squares for large-scale applications can be time consuming. To reduce the computational burden MIKE SHE allows you to compute the UZ flow in a reduced subset of grid squares. The subset classification is done automatically by the pre-processing program according to soil and, vegetation distribution, climatic zones, and depth to the groundwater table.

Column classification can decrease the computational burden considerably. However, the conditions when it can be used are limited. Column classification is either not recommended or not allowed when

· the water table is very dynamic and spatially variable because the classification is not dynamic,

· if the 2 layer UZ method is used because the method is fast and the benefit would be limited,

· if irrigation is used in the model because irrigation zones are not a classification parameter, and

· if flooding and flood codes are used, since the depth of ponded water is not a classification parameter

If the classification method is used, then there are three options for the classification:

· Automatic classification

The automatic classification requires a distribution of groundwater elevations (see Groundwater Depths used for UZ Classification). This can be either the initial depth to the groundwater based on the initial heads, or you can supply a .dfs2 map of the groundwater elevations. In both cases, you must supply a table of intervals upon which the classification will be based. The number of computational columns depends on how narrow the intervals are specified. If, for example, two depths are specified, say 1 m and 2 m, then the classification with respect to the depth to groundwater will be based on three intervals: Groundwater between 0 m and 1 m, between 1 m and 2 m, and deeper than 2 m.

If the Linear Reservoir method is used for the groundwater, then the Interflow reservoirs are also used in the classification. However, since feedback to the UZ only occurs in the lowest Interflow reservoir of each subcatchment, the Interflow reservoirs are added to the Automatic Classification in two zones - those that receive feedback and those that don’t.

· Specified classification

Alternatively a data file specifying Integer Grid Codes, where UZ computations are carried out can be specified, with grid codes range from 2 up to the number of UZ columns (see Specified classification). The location of the computational column is specified by a negative code and the simulation results are then transferred to all grids with the an equivalent positive code.

· Calculated in all Grid points (default)

For smaller scale studies, or studies where the classification system becomes intractable, you can specify that computations are to be carried out in all soil columns.

· Partial Automatic

Finally a combination of the Automatic classification and the Specified classification is available. If this option is chosen an Integer Grid Code file must be provide (see Partial automatic classification) with the following grid codes: In grid points where automatic classification should be used the grid code 1 must be given. In grid points where computation should be performed for all cells the grid code 2 must be given.

Macropore Flow

Flow through macropores in unsaturated soil is important for many soil types.

Simple bypass flow - A simple empirical function is used to describe simple bypass flow in macropores. The infiltration water is divided into one part that flows through the soil matrix and another part, which is routed directly to the groundwater table, as bypass flow.

The bypass flow is calculated as a fraction of the net rainfall for each UZ time step. The actual bypass fraction is a function of a user-specified maximum fraction and the actual water content of the unsaturated zone, assuming that macropore flow occurs primarily in wet conditions.

Typically, macropore flow is highest in wet conditions when water is flowing freely in the soil (e.g. moisture content above the field capacity, q_FC) and zero when the soil is very dry (e.g. moisture content at the wilting point, q_WP)

Simple bypass flow is described in the Reference section under Simplified Macropore Flow (bypass flow) (V1 p. 564).

Full Macropore Flow - Macropores are defined as a secondary, additional continuous pore domain in the unsaturated zone, besides the matrix pore domain representing the microporous bulk soil. Macropore flow is initiated when the capillary head in the micropore domain is higher than a threshold matrix pressure head, corresponding to the minimum pore size that is considered as belonging to the macropore domain. Water flow in the macropores is assumed to be laminar and not influenced by capillarity, thus corresponding to gravitational flow.

Full Macropore flow is described in the Reference section under Full Macropore Flow (V1 p. 565).

Max macropore infiltration per time step - This is a stability criteria that prevents too much water from entering the macropores in one time step.

Max macropore matrix exchange per time step - This is a stability criteria that prevents too much water from exchanging between the macropores and the bulk matrix in one node in one time step.

Max macropore-matrix column exchange per time step - This is a stability criteria that prevents too much water from exchanging between the macropores and the bulk matrix in the entire column in one time step.