Use these simple calculations to estimate how much water your crop needs and how much is available. Confirm moisture stress by checking below.
You can work out how many days it will take before your crop is short of water by dividing available soil moisture by crop water use. The following three step method is adapted from Lafitte 1994.
Step 1. Estimating crop demand
How much water does the non-stressed crop use each day?
Water use per day by the crop (mm) = Evaporative demand (from Table 1 below) x Crop coefficient (from Table 2 below)
Evaporative demand in mm/day (evapotranspiration) can be taken from Table 1 below or weather data, or from your local pan evaporimeter.
Table 1. Evapotranspiration values for different environments (mm/day).
| mm evaporation per day in the: | Average daily temperature (°C) | ||
|---|---|---|---|
| 10-16 | 17-23 | 24-30 | |
| Humid tropics | 3-4 | 4-5 | 5-6 |
| Subhumid tropics | 3-5 | 5-6 | 7-8 |
| Semiarid tropics | 4-5 | 6-7 | 8-9 |
| Arid tropics | 4-5 | 7-8 | 9-10 |
From FAO Irrigation and Drainage Paper 33 (Use the larger value of each pair if the area around the crop has no transpiring vegetation).
Table 2. Evapotranspiration Crop Coefficients for crops reaching 80-90% full ground cover by heading. Reduce the coefficients for crops with less ground cover*.
| Growth stage | Zadoks code | Crop coefficient | Crop ground cover |
|---|---|---|---|
Early vegetative growth |
Z1.0 - Z1.3 | 0.3 | 10-30% |
| Tillering | Z1.3 - Z3.0 | 0.6-0.8 | 30-80% |
| Stem elongation to flowering | Z3.0 - Z6.8 | 1.05 | 70-100% |
| Grain filling | Z6.8 - Z8.7 | 0.40.4 | 50-20% |
Step 2. Estimating available soil moisture
The crop begins to suffer stress and lose yield when around 50% of the available soil water has been used. Calculate the available soil moisture as follows:
Water available (mm) = Present soil moisture (mm) - Soil moisture (mm) when at 50% of field capacity.
Use the table to estimate soil moisture by feel. Estimate the current rooting depth of the crop.
Step 3. Estimating time until moisture stress
Now calculate the number of days that the crop can go without irrigation or significant rainfall before stress begins:
Number of days before stress = Available soil water (mm) (from Step 2) / Daily crop water use (mm) (from Step 1)
Example:
This calculation may seem fairly difficult so try an example to give yourself confidence.
- Crop stage: Heading, good ground cover
- Soil: Clay-loam soil (i.e., soil texture class)
- Environment: Semiarid tropics, mean temperature 20° C.
Step 1: Estimating crop demand
| Evapotranspiration = 7 mm water per day | (Table 1 above) |
| Crop coefficient = 1.0 | (Table 2, column 3 above, crop is at heading) |
| Water demand = 7 mm * 1.0 = 7 mm water/day | (i.e., Step 1 calculation). |
Step 2. Estimating available soil moisture
The soil is a clay loam that forms a ball when you squeeze it, but the ball breaks easily. According to the table to estimate soil moisture by feel your soil contains 110 mm water per m soil depth. Assuming the crop roots reach down to 1 m, then 110 mm water is available for crop use.
The table to estimate soil moisture by feel also shows that at 50% moisture, when stress begins, your soil holds 80 mm of available water, so the crop can use 110 - 80 = 30 mm before stress begins.
Step 3. Estimating time until moisture stress
At 7 mm per day water loss, you do not need to irrigate for about 4 days (i.e. 30/7 days).