It’s time to start thinking about your irrigation water management

 by Vasu Sharma, Extension irrigation specialist and Taylor Becker, Extension educator

What is irrigation water management?

Irrigation water management is the timing and regulating of irrigation water applications in a way that satisfies the water requirement of the crop without wasting water, plant nutrients, or energy and without degrading the soil. It means applying water according to your crop needs and how much your soil can hold (consistent with the intake characteristics of the soil) (NRCS Irrigation Guide). The very first step in good irrigation water management is knowledge of your soil type and crop water use.

Why do we need it?

Proper irrigation water management is key to improving crop production and reducing costs. Applying too much or too little water at wrong times is a common problem. Too much water leads to leaching of nutrients, soil erosion, water logging, and waste of water and energy, ultimately leading to reduced yields and higher costs. Too little water at a time when crop needs it the most (critical crop growth stages) can significantly reduce the crop yield. Proper irrigation water management can:

• Increase crop yield and quality
• Reduce leaching and runoff, maintaining or improving quality of groundwater and downstream surface water
• Prevent excessive use of water
• Minimize pumping costs
• Prevent excessive soil erosion
• Reduce labor

How is it done?

When it comes down to it, irrigation scheduling is simply a decision of when to irrigate the crop and how much water to apply. There are different tools that decision makers can use to develop their irrigation strategies. These tools help in estimating or measuring the soil water depletion in the crop root zone. To set up and operate an effective irrigation scheduling program the following steps need to be followed for each field:

1. Determine the crop's active rooting depth and the corresponding available water-holding capacity for each soil type in the field.
2. Select the predominant soil type(s) that should be used for irrigation water management purposes.
3. Define the management allowable soil water depletion (MAD) limits for the selected soil types and the crop(s) to be grown.
4. Establish a soil moisture monitoring system and regularly (at least twice a week) keep track of the soil water deficit or depletion (Two main methods to measure or estimate soil water depletion are listed below).
5. Initiate an irrigation event when the soil water deficit is expected to approach the selected management allowable soil water depletion limit by the time the irrigation cycle is completed.

There are two main methods of soil moisture monitoring and irrigation scheduling that a grower can easily use:

Soil-based method 

Soil based scheduling involves taking a physical measurement of a soil. In this method, plant water needs are measured by directly measuring the water in the soil profile. The soil moisture can be measured by soil moisture sensors or by hand feel. Details about different soil moisture sensors and how to use them for irrigation scheduling can be found at the extension website page: Soil moisture sensors for irrigation scheduling.

Climate-based method

The climate-based method uses weather data to estimate crop water use over time and calculates when to replace it through irrigation. The general approach to climate-based methods is to maintain a running balance of current soil moisture available (or depletion/deficit relative to field capacity) to the plant by keeping track of the evapotranspiration losses and the additions from irrigation and precipitation. More information about the climate based method can be found here: climate based irrigation scheduling. The climate-based method is much easier to use and less expensive than soil-based method, especially with computers and internet sites where current weather data are easily available. One thing that decision makers should note here is that the climate-based method estimates soil water or soil depletion, as opposed to measuring a physical aspect of it, so it is very important to periodically validate the estimations by measuring a physical parameter, such as the soil moisture. Different climate-based irrigation scheduling tools that a grower can use in MN are:

• Irrigation management assistant (IMA) is automated irrigation software that works on the principles of the water balance irrigation scheduling method as described in this article. At this time, the tool is available for Benton, Otter Tail, Becker, Hubbard, Wadena and Todd counties.
• The checkbook spreadsheet method is another climate-based irrigation scheduling tool that can be used in the form of an excel spreadsheet.