When irrigating, the objective is to apply water when the plant needs it to maximise plant growth but not overfill the soil which wastes water. To estimate the irrigation rate the Overseer model estimates soil moisture. Soil moisture depends on the amount of rainfall, evaporation, transpiration, water drainage and eventual water runoff.
Within the model, for each day the soil moisture is calculated by adding the day’s rain and irrigation to the previous day’s soil moisture, then subtracting modelled evapotranspiration (AET), surface runoff and drainage from the root zone. The daily runoff, drainage, irrigation, transpiration loss and evaporation loss are totaled into monthly values. An average of the daily soil moisture values is created for each month.
If the soil moistures of two blocks are different then the irrigation rate modelled will also be different. Because crops and pasture will have different vegetative covers and rooting depths, the soil moisture of these blocks will be different, even if the blocks have the same soil types and so the Overseer model will allocate different application rates.
While this may not always be what happens on farm, the Overseer model “assumes good management practice” which in the case of irrigation management means:
- Managing the amount and timing of irrigation inputs to meet plant demands and minimise risk of leaching and runoff.
- Design, check and operate irrigation systems to minimise the amount of water
It is also important to remember the climate used in Overseer is a 30-year average and so it is important that irrigation is consistent and aligns with the climate used in the model. This means that although modelled irrigation volumes may not match actual water applied, the combination of modelled rainfall + irrigation applications and the drainage which is resulting should reflect the situation on site to a reasonable degree. Irrigation volumes should not be adjusted to match individual years applications.