Options for the UZ model
In the Bypass flow option, 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, qFC) and zero when the soil is very dry (e.g. moisture content at the wilting point, qWP)
Simple bypass flow is described in the Reference section under Simplified Macropore Flow (bypass flow) (V1 p. 564).
Bypass Constants
The available bypass parameters include:
Maximum bypass fraction - This is the maximum fraction of net rainfall that will infiltrate via bypass flow. Valid values are between 0 and 1.0.
Water content for reduced bypass flow - This is the threshold water content below which the bypass fraction is reduced. If the water content 10cm or 50cm below the ground surface is less than this water content, then the soil is dry and the bypass flow will be reduced.
Limit on water content for bypass flow - This is the minimum water content for bypass flow. If the water content 10cm or 50cm below the ground surface is less than this limit, then the soil is very dry and the bypass flow will be zero.
The actual relationship between the bypass constants and the calculation of the bypass flow is described in Simplified Macropore Flow (bypass flow) (V1 p. 564).