When it rains on the ground surface, a certain amount of the water may run off, either as surface flow or as subsurface flow or both. The irrigation module of MIKE HYDRO is considering a vertical water balance model for the soil column from the ground surface to the groundwater table. The horizontal extent of this soil column represents the field in which a certain crop sequence is grown.
The total runoff from an irrigation field can be divided into two components, a surface water runoff and a groundwater runoff. The latter is the amount of water, which escapes the root zone and percolates to the groundwater storage. The percolation to groundwater is calculated whether a surface runoff model is included or not, see further details in the Soil model section (wherever the link between irrigation node and catchment groundwater is described).
The task of the runoff model is to calculate the surface flow part of the total runoff. Surface runoff may occur either when it rains, when the crop is over-irrigated, or when a combination of both happens.