Why use this calculator
In dryland farming, stored soil water is money in the bank. It decides whether a paddock is worth sowing, how much nitrogen to put out and how much yield is realistic, and GRDC research has long shown that stored summer and autumn rain can add a tonne or more to wheat yield.
But not all rain counts: some runs off, some evaporates from bare soil, and on a light or already full profile some drains below the roots. This calculator separates those losses for your soil texture and ground cover, caps storage at the room left in the profile, and translates the result into days of crop water, which is the number that matters for sowing and topdressing decisions.
How to use it
- Enter the rainfall from the event in millimetres from your gauge.
- Choose the soil texture of the root zone from a soil test or hand texture.
- Choose the ground cover, since stubble cuts runoff and evaporation.
- Enter the root zone depth in centimetres that your crop can actually reach.
- Enter how full the profile was before the rain, from a soil probe or sensor.
- Enter daily crop water use in millimetres for the time of year, lower in winter and higher in a hot spring finish.
- Read the stored rain first, then the days of water in the profile.
What each result means
- Rain stored in the root zone
- Millimetres of this rain that end up held in the root zone where the crop can use them.
- More rain to reach 60% full
- Millimetres of gauge rain still needed to lift the root zone to 60 per cent of its capacity, allowing for the runoff and evaporation losses of your soil and cover.
- More rain to fill the profile
- Millimetres of gauge rain still needed to fill the root zone completely, after runoff and evaporation. Any more than this drains below the roots.
- Lost to runoff and evaporation
- Rain that never entered the soil, running off or evaporating from the surface, which more cover would reduce.
- Drained below the root zone
- Rain that entered the soil but did not fit in the profile, moving deeper than roots reach.
- Plant available water after rain
- Total water the crop can extract from the root zone after the event, including what was there before.
- Profile fullness after rain
- Water in the root zone as a share of its capacity. Above about 60 per cent is a strong position at sowing.
- Days of crop water from this rain
- How many days the stored rain alone would supply the crop at the daily use you entered.
- Days of crop water in the profile
- How many days the whole profile could supply the crop without more rain, at the same daily use.
- Root zone water holding capacity
- The most plant available water the root zone can hold when full, set by soil texture and root depth.
The formula
Root zone capacity
(mm) = plant available water per cm for the soil x root depth (cm): sand 0.6, sandy loam 1.1, loam 1.6, clay loam 1.8, clay 1.7 mm per cm
Water in profile before rain
(mm) = capacity x starting fullness %
Rain entering soil
(mm) = rainfall x soil infiltration share (sand 0.95 to clay 0.70) x cover factor (bare 0.8, partial 0.9, full 1.0)
Stored = the lesser of rain entering
Space left; drained = rain entering minus stored; lost = rainfall minus rain entering
Days of water = mm stored / daily use (mm/day)
How this number is used in practice
Australian growers increasingly make sowing and nitrogen decisions on stored soil water. A push probe after rain shows wetting depth, and soil moisture probes or the plant available water capacity of a characterised soil give a millimetre figure. The rule of thumb used in many GRDC water use efficiency studies is that wheat needs about 110 mm of water just to get going before it yields grain, and then turns each further millimetre into roughly 15 to 22 kg of grain per hectare, so each 10 mm stored is worth something on the scale of 150 to 200 kg of grain in a finishing season.
Ground cover changes the result as much as soil does. Stubble retention and no-till farming have spread across Australian cropping because they cut runoff and evaporation from summer storms and lift the share of rain that is stored. Summer weed control matters just as much: weeds growing on fallow can use the stored water as fast as it arrives.
On sands the profile fills quickly and extra rain drains away, while on heavy clays intense storms run off before they soak in, which the soil comparison table under the results shows side by side.
Worked examples
25 mm on a loam, partial stubble, 60 cm roots, profile 20% full, crop using 4 mm a day
19.1 mm stored, 5.9 mm lost, profile 40% full with 38.3 mm available, about 9.6 days of crop water in the profile.
60 mm summer storm on a sand, bare, 80 cm roots, profile 30% full, 3 mm a day
45.6 mm of rain enters but only 33.6 mm fits, 12.0 mm drains below the roots and the profile is 100% full.
40 mm on a cracking clay, full stubble, 100 cm roots, profile 10% full, 2 mm a day
28.0 mm stored, 12.0 mm lost, profile 26% full with 45.0 mm available, 22.5 days of water in the profile.
When people use it
- Deciding whether to sow a paddock after an autumn break
- Judging whether to topdress nitrogen in a dry spring
- Showing the value of stubble retention and summer weed control
- Comparing water storage on different soils across the farm
- Estimating how long a crop can last between rain events
Tips
- Use a push probe after rain to check the actual wetting depth against this estimate.
- Control summer weeds quickly; they can use stored water as fast as storms add it.
- Keep stubble cover above 70 per cent where you can to cut runoff and evaporation.
- Check for hostile subsoils such as sodicity, salinity or boron, which cap useful root depth.
- Log soil water at sowing each year to build a record of paddock performance against stored water.
What it assumes, and where it stops being accurate
- Soil water holding and infiltration shares are typical values for each texture; characterised local soils can differ by a third or more.
- The whole event is treated as one soaking; very intense storms run off more and light showers evaporate more than shown.
- Rain entering the soil is assumed to fill the root zone before any drains below it.
- Crop water use is a steady daily figure and does not change with fullness, heat or growth stage.
- Plant available water is the fraction between the drained upper limit and the crop lower limit, not total soil water.
Terms used on this page
- PAWC
- Plant available water capacity, the most water a root zone can hold that crops can extract.
- Infiltration
- Rain soaking into the soil surface rather than running off.
- Fallow efficiency
- The share of rain falling during a fallow that is still stored in the soil at sowing.
- Drained upper limit
- Soil water content after a full soaking once free drainage has stopped.
Questions people ask
- What is plant available water capacity?
- Plant available water capacity, often written PAWC, is the water a soil holds between the drained upper limit, where drainage stops after a soaking, and the crop lower limit, where roots can extract no more. It depends on texture, structure and root depth, and is the ceiling on how much rain the root zone can store.
- Why does sand drain so much rain?
- Sand lets water in quickly but holds only about 0.6 mm per centimetre of depth, so the root zone fills after a moderate fall. Anything beyond that moves below the roots. That is why deep sands are sensitive to rainfall timing and why smaller, more frequent falls suit them better than one big storm.
- How does stubble help store rain?
- Stubble slows water moving across the surface, giving it time to soak in, and shades the soil so less evaporates after rain. Research across Australian cropping regions has found retained stubble and no-till lift fallow efficiency noticeably, which is why most grain growers now keep cover over summer where disease and weed risks allow.
- What daily water use should I enter?
- Winter crops use about 1 to 2 mm a day through winter, 3 to 5 mm in spring and 6 mm or more during hot, windy finishes. Summer crops and irrigated pasture in mid summer can use 7 to 10 mm a day. Evaporation figures from the Bureau of Meteorology give a useful local upper bound.
- How accurate is this estimate?
- It is a planning estimate. Real storage depends on rain intensity, slope, soil cracks, water repellence and the actual depth roots reach. For decisions worth serious money, such as nitrogen topdressing or sowing a marginal paddock, check the estimate with a soil probe or moisture sensor and a characterised soil from your local agronomist.
Use this with
Rainfall & Soil Moisture Calculator answers one part of the question. These pick up where it stops: Irrigation Water Calculator, Growing Degree Days Calculator, Crop Yield Estimator and Stocking Rate Calculator.
Where these figures come from
Figures and rules on this page follow Australian law and ATO guidance.
- Grains Research and Development Corporation. Spray application, farm budgeting, crop water use and phenology research for Australian grain growers
- Bureau of Meteorology. Daily temperature, rainfall and evaporation records for Australian stations
- Primary Industries and Regional Development, Western Australia. Dryland cropping costs, farm water planning and soil water holding data
- Agriculture Victoria. Extension notes on feeding livestock, farm water supplies and soil moisture
Rainfall & Soil Moisture Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.