Watershed functions

FALLOW includes a simple annual water balance at patch level, with an allocation of incoming rain over evapotranspiration, overland flow and infiltration, that depends on a soil physical quality that changes in a positive or negative direction depending on current land cover type and its assumed supply of food for soil biota. In interaction with soil physical quality, water infiltration is also determined by slope at plot level (see figure below). Surplus from this first filtering step determines the overland flow. Under saturated soil conditions, infiltrated water will flow out as subsurface quick flow and together with the overland flow produce storm flow. Water that reaches the groundwater storage is released as base flow. Overland flow multiplied with a sediment concentration that depends on land cover determines gross erosion. FALLOW also assigns a potential filter function to each plot ¾ depending on contact cover by litter (Noveras, 2002) ¾ and derives a net erosion loss that leads to the sediment load of rivers. The most critical phase of land use/cover change is found within the pioneer phase, due to relatively low filter efficiency (Sinukaban, et. al, 2000). Filter effects only can be exerted along the pathway of overland flow, giving a specific relevance to ‘riparian filter zones’.

Infiltration fraction of a plot depending on slope and soil physical quality; slope is classified according to USLE