Irrigation Management


It's a certainty that producers want their crops to have enough water. It's also clear that people have very different ideas about how to manage irrigation. Some very important components for proper irrigation management is understanding crop water use, irrigation efficiency, how much water to apply, and irrigation scheduling.

Components of Crop Water Use

Crops use water in two ways: evaporation (E) from the soil surface and transpiration (T) from the crop leaves. The sum of these is called evapotranspiration or ET for short. Over a growing season, 70-80% of all ET is made up of water that cools the plant by moving from the soil through the crop's root system and is transpired from the leaves. The remaining 20-30% of ET is direct evaporation from the soil.

The rate at which crops use water varies depending on plant and soil characteristics, weather, and the stage of growth the plant is in. The figure below shows the effect of weather on water use by a crop with full canopy cover.


Irrigation Measurements and Efficiency

An important step in irrigation management is knowing how much water you apply. The total volume of water pumped is easily determined by using a water meter on the irrigation pipeline. The NRD has ultrasonic flow meters that they are able to hook up an irrigation system that determines a fairly accurate flow rate measurement. Using the simple calculation of flow rate multiplied by pumping time gives you the total estimated volume of water pumped.

Total Volume Pumped = Pumping Rate (acre-in./ hour) x Pumping Time (hours)

No irrigation system is 100% efficient; part of the water applied will not be available for use by the crop. An estimated value of irrigation system efficiency must be used to calculate the gross amount of irrigation water that needs to be pumped to the field in order to apply a given net amount of irrigation water.

The Irrigation System Application Efficiency is a measure of the amount of water that is made available for crop use by an irrigation. Application efficiency is defined as:

Application Efficiency = Net Irrigation Depth / Gross Irrigation Depth

Net Irrigation Depth is the water that infiltrates into the soil and is stored in the root zone.

Irrigation Scheduling

Irrigation scheduling includes deciding when to irrigate and how much water to apply. As a rule of thumb irrigations should be scheduled so that the plant available soil moisture in the crop root zone remains above 50% of the available water-holding capacity.

Irrigation scheduling helps to:

  • assure the plant water needs are met
  • conserve water supplies
  • avoid excess water application
  • reduce nitrate leaching losses
  • save pumping costs

Irrigation Management Information Courtesy of: