Soil moisture monitoring to fine-tune irrigation scheduling

Page last updated: Tuesday, 27 August 2019 - 2:52pm

Please note: This content may be out of date and is currently under review.

Determining how water moves through soil and is used by plants following irrigation is difficult. Using soil moisture probes to fine-tune irrigation scheduling is recommended to maximise production and water use efficiency. Tailoring irrigations to the water-holding capacity of the crop root zone, stressing crops to produce specific fruit quality (for wine grapes) and preventing water stress are benefits of using and understanding monitoring equipment.

This page explains how to interpret soil moisture readings to improve irrigation scheduling and water use efficiency.

Irrigation principles

Water use is determined by crop type, growth stage and evaporation.

Evaporation is determined by weather and can be measured with an evaporation pan or calculated from weather station data. Formulae that take account of solar radiation, wind speed, temperature and humidity are used to calculate evaporation representative of a Class A pan or evapotranspiration representative of a standard sward of grass.

Plant water demand changes with growth stage, generally increasing from planting until maturity then decreasing through senescence or harvest.

Water demand as a proportion of evaporation is called a crop factor or crop coefficient. Factors affecting the water demand include crop type, growth rate, canopy area, fruit production or grain fill and senescence.

Crop factors (CFs) are shown for drip irrigated tomatoes in Figure 1.

Graph displaying the proportion of evaporation replacement required based on weeks in the cycle. The first few weeks begin at 0.5CF and the peak is around week 13 at just over 1.4CF.
Figure 1 Recommended crop factors for growing tomatoes on sands of the Swan Coastal Plain based on commercial plantings

Daily water demand = evaporation x crop factor — or — evapotranspiration x crop coefficient.

Further information on how to schedule using evaporation and crop factors can be found in the evaporation-based irrigation scheduling page.


Rohan Prince