Evaporation-based irrigation scheduling

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.

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'.

Calculating irrigation requirement

A crop’s water requirement is calculated in millimetres so it is convenient to replace it in millimetres. To schedule by evaporation, you need to know the application rate of your irrigation system. For information on calculating application rates of irrigation systems, see the following pages on this web site 'Measuring the delivery of drip irrigation systems' and 'Evaluating sprinkler systems'.

For this example, the application rate of an overhead system is 10mm per hour and the crop is at mid to late growth stage in Table 1. Evaporation for the previous 24 hours was 7.2mm and the soil has readily available water storage of 3mm/100mm depth.

Below is a worked example of how to use evaporation to schedule irrigation:

Step 1

Calculate daily water requirements by multiplying evaporation by the crop factor for the growth stage.

Daily water requirements = Evaporation x crop factor

                         7.2 x 1.1 = 7.9 mm

Step 2

Calculate run time in minutes by dividing the water required by the irrigation system application rate (mm per hour) and multiply by 60.

 (Water required/application rate) x 60 = Run time

                                       (7.9 /10) x 60 = 47 minutes

How the required depth of irrigation is applied is determined by the soil’s water-holding capacity, water quality and the crop growth stage. Irrigation should meet the water-holding capacity for the rooting depth.

Excess irrigation leads to water passing below the root zone lowering irrigation efficiency and  applied nutrients move down the profile beyond the reach of plant roots. For information on calculating the effective water holding capacity of your soil, see the following page on this website: 'Calculating readily available water'.

Step 3

Number of irrigations

Calculate soil water storage by multiplying effective root zone depth by the soil’s readily available water-holding capacity. In this case the readily available water is given as 3mm for 100mm depth of soil.

            RAW storage = Root zone depth x Readily available water (RAW)

                                                200mm root depth x 3mm/100mm depth

                                                200 x 3/100 = 6mm RAW

For this example, daily water required is 7.9mm and the readily available water storage is 6mm. Irrigation will need to be split so the water applied does not exceed soil storage.

Confirming the irrigation effectiveness can be done using soil moisture equipment. Decision support systems such as the Vegetable Irrigation Scheduling System (VISS) can calculate Steps 1 and 2 for individual plantings or irrigation shifts and be viewed online. Some systems will email the requirements for the crops entered.

Alternatively, a simple table can be made as in Table 2. Look up the crop factor — shown along the top row — and daily evaporation — shown down the left-hand column — and the water requirement is shown where the two intersect.

Reported crop factors are higher for sandy soils than for heavier soils because site inefficiencies such as low soil water-holding capacity of sands and irrigation efficiency have been taken into account. This simply reinforces the need to fine-tune application to match the specific farm situation. Beware of overseas information that is not geared to sandy soils.