The amount of green water is determined by soil, terrain and climate, and by management (Table 1):
Table 1 (after Ringersma 2003)
Tilling practices Early soil prep.
Atmospheric demand (PET)
*) As far as it determines the amount of green water
The partitioning of rainfall between infiltration and run-off is determined, in part, by soil surface attributes; some soils have a stable, porous surface structure, others crust and seal, especially when rain fall on a bare surface;
The rate of vertical and horizontal percolation of water is determined by the soil's hydraulic conductivity;
Available wate capacity, the amount of green ater that may be held in the soil, depends on the volume of soil accessible to roots, its texture, structure, organic matter content and the kind of clay minerals: the proportion of available water may vary by a factor ten;
Deep percolation to the groundwater may be hindered by compact layers;
Lateral movement of infiltrated water to rivers is determined by the composition and architecture of unlithified materials down to the bedrock, and the shape of the soil-bedrock interface as well as the characteristics of deeper aquifiers.
Therefore, knowledge of about soil qualities and their spatial distribution is needed to understand the hydrological regime of both farmers' fields and river basins.