The SPAW hydrology simulation model embodies the hydrology of two agricultural systems, a farm “field” and a “pond”. The several interlinked principle hydrologic processes represented within these systems are shown schematically on the following diagrams. The farm field hydrology is that of a vertical water budget encompassing the atmosphere well above the plant canopy downward to a soil depth containing the plant roots and any important soil hydrologic features. The pond hydrology is that for any of a wide variety of impoundments to be found in a landscape or farm such as wetland ponds, constructed lagoons, small ponds or larger reservoirs. A more detailed description of the various field and pond hydrologic processes can be found in the Users Manual.