Improving Irrigation Water Use Efficiency
Focus Resource Concern for January 2012
PHOENIX, Jan 3, 2012 – Wise use of irrigation water is one of our highest conservation priorities in Arizona. Irrigation water management is used to apply the amount of water needed, when it is needed by the crop. To improve irrigation efficiency on your farm, there are four things you need to know.
Know Your Crop Water Needs
First, it is important to understand the water needs of the crops being grown. The amount of water each crop uses from the soil profile each day depends on the daily temperatures and amount of wind during the growing season, the growth stage of the crop, the rooting depth of the crop, and the amount of residue on the surface of the soil. For design purposes, daily consumptive use information has been developed for the various crops grown in different parts of Arizona. But to really know how much water your crop is using, you must monitor the soil moisture in your fields.
Know Your Soils
Next, it is important to know a few characteristics about the soils being irrigated. Each type of soil has a different water holding capacity and intake rate. The water holding capacity is primarily determined by the soil textures within the rooting depth of the crops. Water holding capacity is also affected the salinity of the soil, and the amount of organic matter in the soil. The soil intake rate is a measure of how fast water can infiltrate into the soil. The intake rate is determined primarily by the texture of the soil, but it can also be affected by soil chemistry and organic matter levels.
Know Your Water
The third part of the soil-plant-water relationship, is of course the water. Knowing the actual flow of water being delivered to the field is critical to improving irrigation efficiency. As mentioned before, it is also critical to know the soil moisture in your fields in order to know when to irrigate, and how much water will be needed to refill the soil moisture to field capacity. The salinity level of the irrigation water can also affect your irrigation efficiency. If you know the volume of water that needs to be delivered, and the flow rate of the water being delivered, it is pretty easy to determine how long to irrigate.
There is a simple formula you can use to determine how long to irrigate:
T is the time to irrigate in minutes
D is the acre inches that needs to be applied (soil need ac. in./planned efficiency)
A is the number of acres that will be irrigated at a time (set size)
Q is the volume or head of water, in cubic feet per second (cfs) (Note: 450 gpm = 1 cfs)
Know Your Field
So the fourth piece of information needed is the configuration of the field being irrigated. It is important to know the slope of the field, which is used to help determine the depth and proper rate water of applied per furrow or border. It is also important to know the length of the field and the row spacing or border spacing for the crop to calculate the acres that are irrigated in border or each set of furrows.
Inefficient use of irrigation water occurs because excess runoff from the field, or deep percolation below the root zone. Some losses occur in the conveyance system that transports water from the source to the field.
By far, most water is wasted simply because too much water is applied to the field. Reducing and uniforming field slopes, improving the head of water available, changing set sizes, or changing in the kind of irrigation system may also need to be considered to maximize irrigation efficiency.
Poor irrigation efficiency can be caused by several things:
• Not stopping the flow of water to the field when the amount of water needed has been delivered
• Not knowing the exact quantity of water (head) flowing from the water source to the field
• Not knowing the soil moisture level at the time of irrigation
• Not knowing the water holding capacity of the soils being irrigated
• Not applying water based on the intake characteristics of the soil
• Not knowing the daily water use requirements of the crops being grown
• Not using the proper set sizes for the available head of water
• Non-uniform slopes which cause poor water distribution across the field
• Excessive slopes which creates high runoff losses
So you’ll need some information in order to make sound decisions. Contact your local NRCS Field Offices to help you develop a complete conservation plan that includes recommendations for improving your irrigation efficiency.