Introduction
The best irrigation system is only as efficient as the way it is operated, maintained and monitored. To be an efficient irrigator, you must ensure that your irrigation system is delivering water evenly, that your irrigation scheduling is matched to crop water demand and that the impact of irrigations on soil moisture is monitored.
This page describes good practice in evaporation-based irrigation scheduling.
Plant water use and irrigation requirements
Plant water use is determined by environmental and plant factors. Environmental factors are related to climate and weather while plant factors relate to the type of crop, its stage of growth and vigour.
Crop irrigation requirement is also influenced by cultural and site factors, including the method of irrigation delivery, uniformity, scheduling, planting density, variety and soil type.
Environmental factors include climate and weather. Temperature, wind speed, solar radiation and relative humidity determine the amount of water required for productive plant growth. They can be measured directly using an evaporation pan or calculated as evaporation (Epan) or evapotranspiration (ET) from weather stations. Epan and ET data can be sourced from DAFWA, the Bureau of Meteorology or SMS weather services such as the one run by vegetablesWA.
Using long-term averages is not recommended for scheduling irrigation for vegetables growing in sandy soils. Long-term averages do not provide sufficient accuracy of daily measurements. They should only be used for crops that can tolerate some stress or are growing in soils with water storage capacity greater than the crop’s demand over a day or two. For information on estimating a soil’s water-holding capacity, see the 'Calculating readily available water' page on this website.
Crop water requirements
As plants grow larger, produce more leaf area, start producing fruit or approach maturity, the proportion of Epan or ET that needs to be replaced by irrigation changes. Differences in water requirements and the proportion of Epan to be replaced are called crop factors (CF). When using ET, they are called crop coefficients (Kc).
Crop factors and crop coefficients are split into periods of growth that relate to key stages in the plant’s development. Descriptions may detail the development stage or may reference a generic stage or phase of growth when water requirements change. An example of crop factors for an annual crop is shown in Table 1.
| Crop stage | Crop factor | Root depth (mm) |
|---|---|---|
| Transplanting and establishment | 0.7 | 75 |
| Rapid growth | 1.0 | 100 |
| Mid to late growth | 1.1 | 200 |
| Late growth to harvest | 1.3 | 250 |
Calculating daily water use
Daily water use is calculated from weather data and crop stage.
Daily water use (mm) = Evaporation (mm) x crop factor
or
Daily water use (mm) = Evapotranspiration (mm) x crop coefficient
Crop factors and crop coefficients are only a guide to crop water demand and are not definitive. When not developed specifically for a farm’s cultural and site factors, some form of crop or soil moisture monitoring is always recommended to ensure plant water demands are met for the specific situation to which they are applied.
Most crop factors and coefficients are developed for maximum potential yield. Where a specific size or quality of product is required or an area is subject to high disease pressure, generic crop factors may over- or underestimate plant water requirements. Soil moisture monitoring can also assist in these situations.
Information on how soil moisture monitoring can assist with fine-tuning irrigation can be found in 'Soil moisture monitoring to fine-tune irrigation scheduling'.