Reading Your Water Balance Chart: Rainfall vs ET0 Explained
Every OCM Agrisystems site with a weather station generates a water balance chart in the dashboard. It is one of the more information dense charts on the platform, and on first look it can read as a tangle of lines rather than something you can act on. Once you know what is behind those lines, it becomes one of the most useful tools for timing irrigation.
This guide walks through what the water balance chart is actually showing, how rainfall and reference evapotranspiration are calculated, and how to read the chart against your own planting dates.
What the Water Balance Chart Shows
The water balance chart tracks a running total from your planting date: rainfall coming in against reference evapotranspiration going out. When rainfall exceeds evapotranspiration, the balance rises and the soil is building a surplus. When evapotranspiration exceeds rainfall, the balance falls and the crop is drawing down stored soil water.
Water Balance = Cumulative Rainfall − Cumulative Reference Evapotranspiration (from planting date)
What Is Reference Evapotranspiration (ET0)?
Reference evapotranspiration, or ET0, is an estimate of how much water a well watered reference surface would lose to evaporation and transpiration under current weather conditions. OCM dashboards calculate ET0 using the FAO-56 Penman-Monteith method, the internationally recognised standard for this calculation. It draws on temperature, humidity, wind speed and solar radiation from your site's weather station to produce a daily figure.
ET0 is not the same as what your specific crop is using, since actual crop water use depends on growth stage and canopy cover as well. It is a consistent reference point that lets you compare evaporative demand across days, weeks and seasons on the same basis.
Why Rainfall Alone Does Not Tell the Full Story
A week of decent rainfall can look reassuring on its own, but if that same week was hot and windy, evaporative demand may have been high enough to offset most or all of it. The water balance chart accounts for this by netting rainfall against ET0 rather than showing rainfall in isolation. This is what makes it more useful than a rain gauge total for judging actual soil water status.
Reading the Chart Through the Season
A few patterns are worth watching for as the line moves across the season:
A rising line means rainfall is outpacing evapotranspiration and the site is building a water surplus
A falling line means evapotranspiration is outpacing rainfall and stored soil water is being drawn down
A flattening line suggests rainfall and evapotranspiration are roughly in balance for that period
A steep, sustained fall during flowering or pod fill is worth particular attention, since these stages are usually the most sensitive to water stress
Using the Chart for Irrigation Decisions
The water balance chart is most useful alongside growing degree day tracking rather than on its own. GDD accumulation tells you what growth stage the crop is likely at, and the water balance chart tells you whether soil water is trending toward surplus or deficit through that stage. Read together, they give a clearer picture of when irrigation will have the most impact than either chart alone.
Frequently Asked Questions:
What is FAO-56 and why does it matter?
FAO-56 is the Food and Agriculture Organization's reference method for calculating evapotranspiration, based on the Penman-Monteith equation. It is the internationally accepted standard because it accounts for temperature, humidity, wind and solar radiation rather than relying on temperature alone.
How often is the water balance chart updated?
The chart updates as new weather data comes in from your site's station, so the running balance reflects current conditions rather than a periodic estimate.
Can I compare water balance across different planting dates or sites?
Yes. Because the balance is calculated from each planting date, you can compare how different paddocks or seasons are tracking on the same basis, which is useful for spotting where conditions are diverging.