How Wet/Nitrate?

Assess how soil water and nitrate have changed over the current fallow season.

Soil water & nitrate

HowWet/Nitrate? uses records from a nearby weather station to estimate how much plant available water (PAW) has accumulated in the soil and the amount of organic nitrogen that has been converted to an available nitrate during a fallow. Howwet? tracks soil moisture, evaporation, runoff and drainage on a daily time step. Accumulation of available nitrogen in the soil is calculated based on surface soil moisture, temperature and soil organic carbon.

HowWet/Nitrate? estimates how much:

  • rain has been stored as plant-available soil water during the most recent fallow period
  • nitrogen has been mineralised as nitrate-N in soil, and
  • and provides a comparison with previous seasons.

This information aids in the decision of what crop to plant and how much nitrogen fertiliser to apply.

Grain growers in regions where stored soil water and nitrate at planting are important in crop management decisions. This is of particular importance to northern Australian grain growers with clay soils where stored soil water at planting can constitute a large part of a crop’s water supply.

Questions this tool answers

  • How much longer should I fallow? If the soil is near full, maybe the fallow can be shortened.
  • Given my soil type and local rainfall to date, what is the relative soil moisture and nitrate N accumulation over the fallow period compared to most years? Relative changes are more reliable than absolute values.
  • Based on estimates of soil water and nitrate-N accumulation over the fallow, what adjustments are needed to the nitrogen supply?


  • A selected soil type and weather station.
  • An estimate of soil cover and starting soil moisture
  • The stand-alone version of HowOften? uses rainfall data input by the user


  • A graph showing plant available soil waterfor the current year and all other years and a table summarising the recent fallow water balance.
  • A graph showing nitrate accumulation for the current year and all other years.


HowWet/Nitrate? uses a standard water-balance algorithms from Howleaky? and a simplified nitrate mineralisation based on the original version of Howwet? Further calibration is needed before accepting with confidence in absolute value estimates.

Soil descriptions are based on generic soil types with standard organic carbon and C:N ratios and as such should be regarded as indicative only and best used as a measure of relative water accumulation and nitrate mineralisation.


HowWet/Nitrate? was first developed by Agricultural Production Systems Research Unit in 1994 and updated on the iOS Smart Phone software by RPS Australia East and DHM Environmental Software Engineering Pty Ltd for Managing CliMate Variability Research and Development Program in 2012.

The Agricultural Production Systems Research Unit and SILO maintain access to the stand alone program and to long term rainfall data for HowOften?


Freebairn, DM, Hamilton AH, Cox PG, Holzworth D. 1994. HOW WET? Estimating the storage of water in your soil using rainfall records. A computer program. Agricultural Production Systems Research Unit, QDNR -QDPI-CSIRO, Toowoomba, Queensland.