HowWet? Putting Farm Rainfall Records to Work   

Did you know that...

• on average, 60% of rainfall is lost as evaporation before plants get a chance to use it

• the amount of water stored in soil during a fallow can range from 10% to 40% of total rainfall, depending on how the rain falls!

• 10% of all rainfall on average is lost as runoff or drainage, and this loss can aggravate erosion and/or salinity!

HowWet? is a computer program which uses farm rainfall records to estimate:

• How much Rain has been stored as Plant Available Water

• How much Nitrogen has been mineralised in soil

• How much Erosion was caused by runoff water during the fallow period

The WINDOWS based HowWet? is a user-friendly educational tool as well as a decision support aid. Outcomes associated with different management scenarios and seasons can be examined by changing soil, management and starting conditions.

Inputs are simple - just select a range of soil types and climate localities from the menus then enter farm rainfall data. Tables of weekly temperatures and evaporation rates are held for each location. Default plant-available water capacities can be selected or overwritten to customise soil responses. The hydrology component of HowWet? is derived from the PERFECT and APSIM models.

Rainfall is entered in a simple spreadsheet calendar. District rainfall data can be entered from meteorological files, but generally the intention of HowWet? is to use farm rainfall data.

Output is given as tables of total rainfall, runoff, evaporation and mineralised nitrogen and a series of graphs depicting:

• Plant available water in the soil profile (calculated from daily evaporation, rainfall and runoff)

• Daily fluctuation of soil water in the surface layer (with animation)

• Accumulation of water in the lower or "safe" layer of the soil profile

• A histogram of daily rainfall and runoff

• A calendar of daily rainfalls

• A graph combining a histogram of actual monthly rainfall and measured rain

• Daily N accumulation in relation to daily temperatures and soil moisture in the surface layer

Denitrification under very wet conditions is not considered.