Where does all the rainfall go?

CAT’s view of the landscape

When rain falls on land, a series of processes are modelled by the CAT. Water can be utilised by plants, evaporate from surfaces, runoff to streams or become recharge by percolation to groundwater.

The pathway of water across the landscape depends on many factors, like slope, soil type and landuse. CAT allows the user to consider how this variability impacts the water balance.

The Plant / Soil / Air Interface

For each cell in a standard CAT modelling run, the water balance is calculated. Rainfall is apportioned between surface evaporation (from soil and canopy), plant transpiration, surface runoff, subsurface lateral flow and recharge to groundwater, depending on the values from all input layers at that point in the landscape.

Streamflow - Runoff mechanisms

The processes, shown in red in the diagram opposite, must all be treated differently by the CAT, and the effect of each will vary in magnitude across the landscape.

Surface runoff runs into streams at high velocity after a storm, possibly carrying a sediment load and surface nutrients.

Lateral flow water enters the soil, an impermeable layer causes it to flow downslope, eventually discharging into a stream or re-emerging in the saturated area close to stream (common in duplex soil landscapes).

Base flow is water that has flowed through soil to the watertable, then moved to stream from the watertable. It is usually a slow moving contributor to streamflow with a time delay after significant rain events.