Often the most limiting nutrient in irrigated crop production is water! Many irrigation farmers tend to under-irrigate their crops, which limits yield potential. Often the main reasons for under-irrigation are simply not checking soil moisture frequently and starting the irrigation system too late.
Knowing the answers to these four questions can help.
1. How much water does each irrigation system apply?
First, you need to know the application rate of each irrigation system on your farm. The majority of irrigated fields in Western Canada are irrigated with pivot systems.
Table 1 (above) shows the gross water application of a low-pressure quarter section pivot, irrigating 133 acres, with various water outputs in U.S. gallons/minute (U.S. gpm) at different applications times. The gross water application rates are in millimeters (mm). Table 2 (below) shows the net water application, assuming an 80 per cent efficiency rate. Net water application is the amount of water that is actually stored in the soil.
Typically, low-pressure pivot systems with drop tubes and pressure regulated spray nozzles apply water with 80 to 85 per cent efficiency, depending on environmental factors including air temperature and wind speed. High-pressure pivot systems with impact nozzles typically have water application efficiency in the range of 75 per cent. Side roll/wheel move systems have water application efficiency in the range of 70 per cent.
As an example, a low pressure pivot with a 900 U.S. gpm output that makes a full circle in 48 hours has a gross water application rate of 14.6 mm but the net application rate at 80 per cent efficiency is only 11.6 mm (see the tables).
A common mistake made by irrigators is not estimating the net water stored in soil. Further, if a pivot can only apply six mm of water per day, the irrigation system will not be able to keep up with water use during peak periods. Many crops will use eight mm of water per day at peak use!
2. What is the soil’s water holding capacity in each field?
Soil’s water holding capacity should be viewed similar to the varying sizes of fuel tanks on vehicles. Sandy soils have a lower water holding capacity or a small fuel tank; clay loam soil has a relatively larger water holding capacity or large fuel tank. Table 3 (below) provides the approximate amounts of total available soil water for five major soil texture classes and the amount of readily available soil water in the zero to 50 cm and zero to 100 cm depths.
Total available soil water is the total amount of water the soil can hold, which is also called field capacity. As crops are taking up water, the amount of available water is drawn down. As this occurs, remaining soil water is held more strongly within soil pores and plants must gradually work harder to take up the remaining soil water.
Readily available water is the amount of water crops can easily take up, but after this water is used, crops begin to suffer, wilting during the day and yield losses will occur.
Two soil terms irrigators need to know and understand are field capacity and safe depletion point. Field capacity is the total amount of water a soil will hold or total available soil water. Safe depletion point is the amount of water that can be safely used by crops without crop yield loss. When when this point is reached it is time to irrigate soil back to field capacity.
Ideally, for most irrigated crops, you should only use about 30 to 40 per cent of the total available water and then it is time to irrigate soil back to field capacity. Annual crops such as wheat and canola typically will root down to about 90 to 100 cm (36 to 40 inches) and can effectively take up moisture to 100 cm by the time of heading for cereals and flowering for canola. Under ideal conditions at heading of wheat or flowering of canola, crops will take up 70 per cent of water requirements in the zero to 50 cm depth and take up 30 per cent of water requirements in the 50 to 100 cm depth. To prevent yield loss for most crops, only 30 to 40 per cent of total available water should be used on the top 50 cm of soil, then irrigation is required to raise soil moisture back to field capacity.
3. How much water does each crop use at various growth stages throughout the growing season?
Moisture use starts off at relatively low rates in early May, but as vegetative growth rapidly increases, so does daily water use. The graphs show typical water use curves for wheat and canola in southern Alberta.
Peak water use can range from six to nine mm per day, depending on evapotranspiration conditions including maximum air temperature and wind speed. Southern Alberta farmers have access to the Irrigation Management Climatic Information Network (IMCIN) weather stations which provide daily crop water use information at sites across southern Alberta. This is useful in predicting irrigation requirements.
4. What’s the best way to check field soil moisture levels?
As crops are growing, irrigators must regularly monitor soil moisture in fields to ensure moisture levels don’t decline below the safe depletion point. There are various ways to monitor soil moisture. A simple and easy way is the “hand-feel method.” Use a hand auger to auger down to 100 cm, then feel the soil moisture level at 25 cm incremental depths. For full descriptions about how different types of soil will feel at different moisture levels, see the table in the 2013 Alberta Irrigation Management Manual.
A good place to start is to check soil moisture in fields the day after irrigating so you know how your soils feel at field capacity. Then get to know how soils feel at 75 and 50 per cent of field capacity. This takes a bit of time and practice but can actually be an effective, easy and inexpensive way to check soil moisture on a weekly basis.
To pull all this knowledge together:
- At least once per week or more, check soil moisture in each field.
- Know how much water each crop is using on a daily basis during the growing season.
- Estimate how many days you have before readily available water is used up based on soil moisture levels and crop water use.
- Estimate when the next irrigation will be required and how much water to apply.
- Take into consideration the time needed to apply the water and estimate the net water application.
In summary, irrigation should be managed to maintain soil moisture between 60 and 100 per cent of field capacity throughout the growing season in the top 50 cm of soil. For most crops, it is best to ensure the root zone in the top 100 cm soil depth is irrigated to field capacity during vegetative grow stages to ensure enough water is available to the crop during peak water use. Regularly checking soil moisture to ensure soil moisture is maintained between field capacity and safe depletion point will go a long way to achieve optimum production.