Raised beds are designed to achieve:
- A deepened seedbed that is not dense and does not constrain root growth
- A seedbed with more roots and a significant proportion of large pores for good aeration, infiltration and drainage, and
- A short distance and a reasonable height from the bed centres to the base of a furrow for a substantial hydraulic gradient to stimulate lateral drainage
- A controlled traffic style of farming
- Soil in the beds that remains unsaturated and does not subside
The extra runoff from areas of raised beds is likely to be between 5 and 10% of the growing season rainfall. This is roughly equivalent to an extra 15 to 30 millimetres (mm) of water annually, which from an area as small as 100 hectares (ha) amounts to between 15 000 and 30 000 cubic metres per year. If a 20mm rainfall event fell on an already wet field of raised beds the water shed from 100ha by this single event could amount to 4 000 metres (m).
Whilst these amounts of water may seem large in their own right they are only 5 to 10% more than normal. However, the water harvesting efficiency of a system of raised beds is such that the water will run off far quicker than normal and may therefore seem to be excessive, which it is not.
The efficient disposal of this general amount of runoff ought to make a real contribution to reducing groundwater recharge and salinity. Annual amounts of recharge of between 15 and 30mm per year are about the size considered to have caused the shallow water tables and dryland salinity that is so widespread in Western Australia.