The Sacramento Model is a continuous rainfall-runoff model used to generate daily stream flow from daily rainfall and potential evapotranspiration data. It uses soil moisture accounting to simulate the water balance within the catchment (i.e. functional unit). The conceptual layout of the model is shown in Figure 1. At each model time step the sequence of calculations is:
1. Soil moisture depletion by evapotranspiration and soil moisture redistribution.
2. Soil moisture replenishment by rainfall and percolation, and streamflow generation.
The internal Sacramento Model calculations represent water quantities using units of depth (in millimetres). The model outputs are converted to volumes by multiplying by the catchment area.
There are five stores in the Sacramento Model:
l Upper zone tension water (UZTW);
l Upper zone free water (UZFW);
l Lower zone tension water (LZTW);
l Lower zone primary free water (LZFWP); and
l Lower zone supplementary free water (LZFWS).
The tension water stores represent the volume of water that is held in the soil matrix by surface tension. Water can only be removed from the tension water stores by evapotranspiration. In the case of the free water stores, water can move through the soil vertically and laterally to other stores, and be discharged as interflow (upper zone) or baseflow (lower zone).
The Sacramento Model divides the catchment into impervious and pervious areas. The impervious area is the portion of the catchment that is covered by lakes, rivers, pavement and other impervious surfaces that are directly connected to the stream network. The impervious area produces runoff from any rainfall while the pervious area only produces runoff when rainfall is sufficiently heavy. The Sacramento model also allows the user to specify a variable impervious area, which is a portion of the catchment that can become impervious when the catchment is quite saturated.
Streamflow generated by the Sacramento Model is made up of four main forms:
1. impervious runoff from permanent impervious areas and direct runoff from variable impervious areas,
2. surface runoff, which occurs when Upper Zone Free Water storage is full and the precipitation intensity exceeds the rate of percolation and interflow
3. interflow resulting from the lateral drainage of the Upper Zone Free Water storage
4. baseflow, which is composed of primary and supplemental baseflow
Impervious runoff, direct runoff and surface runoff have no time delay in the Sacramento model, occurring in the same time interval as the rain that generated the runoff component. Interflow generally has a time delay in terms of days, supplemental baseflow a delay of weeks or months and primary baseflow a delay of months or years. The generation of surface runoff, interflow and baseflow depends on the amount of water in each soil moisture store relative to that store’s capacity, and the rate at which water moves into and out of the stores.
https://wiki.ewater.org.au/display/SD41/Sacramento+Model+-+SRG