Irrigate when the root zone has used up a set share of its stored water, and apply enough to refill that share, which is the soil’s available water per metre times the root depth times the allowed depletion fraction. For example, 100 mm per metre of soil, 0.5 metres of roots and half of it used gives 25 millimetres, or 25 litres per square metre. Check by digging after watering, and give the water at the crop’s critical stages, such as flowering and grain filling, first if supply is short.
Irrigation scheduling decides when to water and how much, by tracking how fast the crop and the soil surface use water and how much the root zone can hold.
The Root Zone Is A Tank With A Known Size
Roots draw from a volume of soil, and that soil holds only so much water that plants can use. The depth of water to apply is the available water per metre × root depth × the fraction you allow the plant to use up before refilling.
Worked Example
Illustrative inputs (soil and crop determine the real ones):
- Available water: 100 mm per metre of soil.
- Root depth: 0.5 m.
- Allowed depletion: 0.5 (half).
- Depth to apply: 100 × 0.5 × 0.5 = 25 mm, which is 25 L/m².
- Crop use: 5 mm/day. Interval: 25 ÷ 5 = 5 days.
If the crop’s use is 3 mm/day, the interval is 25 ÷ 3 = about 8 days. Longer, deeper watering encourages roots to go down; frequent light watering wets only the surface. Values of 100 mm/m, 0.5 m and 0.5 are placeholders to show the method, not recommendations.
How To Find The Numbers
- Available water: differs with texture; sandy soils hold less than loams and clays (Soil Texture).
- Root depth: dig up a plant and look.
- Crop water use: crop use is approximately a coefficient times the evaporation from open water, and the coefficient rises as the crop covers the ground. An improvised pan is a shallow tray of water, with the level measured each day (top it up to a mark). A pan loses more than a crop uses, so a coefficient below 1 applies; calibrate by comparing the pan with your watering results. With a pan loss of 6 mm/day and a coefficient of 0.5 (a placeholder), the crop uses 3 mm/day. For 10 m² that is 30 L/day.
What You Need
- A shovel or auger for checking.
- A shallow pan and ruler.
- A record sheet of rainfall, pan loss and irrigation.
- A way to measure the water you apply (a container, a timed flow).
Method
Step 1: Fix the root depth and the allowed depletion (half is a cautious start).
Step 2: Check soil moisture by feel at root depth before watering. A handful that crumbles without holding together is dry in a loam; one that holds a ball when squeezed has water.
Step 3: Water when the soil reaches the trigger, not on a calendar.
Step 4: Apply the planned depth, then dig the next day to see how deep it went. If it wet deeper than the roots, you gave too much; if not as deep, too little.
Step 5: In short supply, protect the critical stages. Flowering and early fruit or grain fill are the stages where stress costs most yield. Withhold from vegetative growth before you withhold from these.
Step 6: Time the watering for early morning or evening, when less is lost to evaporation.
Common Mistakes
- Watering by a calendar — it ignores weather.
- Light daily watering — shallow roots and surface loss.
- Watering after the crop has wilted — the damage is done.
- Applying more than the root zone can hold — the excess drains below the roots.
What’s Next
- Learn next: Mulching & Soil Moisture — lowering the demand
- Combine with: Rainwater Collection and Storage — where the water comes from