Irrigation Scheduling

Good irrigation management is required for efficient and profitable use of water for irrigating crops. A major part of any irrigation management program is the decision-making process for determining irrigation dates and /or how much water should be applied to the crop for each irrigation. The decision-making process is referred to as "irrigation scheduling".

Irrigation scheduling is impacted by all of the components described in the list to the left. They include the regional reference evapotranspiration rate, the crop coefficient, the growth stage, the root zone, and soil triangle. All of these components form the basis of the water budget analysis which is used to anticipate when a crop will just enter water depletion stress.

A major goal in good irrigation management is to prevent yield and/or quality reducing crop water stress by maintaining the soil water content above a certain level. This is done by keeping track of soil water content and knowing how dry the soil can get before this crop water stress level is reached. This stress level is also known as the yield threshold depletion or YTD. The YTD value is dependent upon the crop sensitivity to water stress, soil available water, root density, and root depth. For most situations YTD is usually around 75% of the field capacity. The ultimate choice of how much water to deplete before an irrigation is made by the irrigation manager depends on cultural practices, labor, water deliveries, energy requirements, and other considerations. Irrigation is timed depending on the managed allowable depletion (MAD), which is the percent of soil available water which the irrigator will allow the plants to deplete before irrigating or the depth of water that the irrigator will allow plants to extract fro the root zone between irrigations. In irrigation practices, the MAD is selected to be less than or equal to the YTD. Most MADs in the IMS program are between 75 and 85% of the field capacity. Another term commonly used is soil moisture depletion (SMD). This is the amount of water required at any time to fill the root zone to field capacity.

The water budget balance is the starting point to determine when to irrigate. For irrigation scheduling, soil water content is balanced using a checkbook approach. The amount of water that is lost as crop evapotranspiration (ETc) is analogous to writing checks against an account. The water that enters the soil reservoir (as rain or irrigation) is analogous to depositing funds into that account. By keeping records of these transactions it is possible to know how much water is in the soil reservoir account at anytime.

Knowing when to irrigate is only half of the irrigation scheduling process. The other half is determining how long to run an irrigation system to replace the depleted moisture. To do this you need to know the irrigation system efficiency. The system can be evaluated, but this needs to be done during an actual irrigation. Depending on the design, maintenance and management of an irrigation system, the efficiency can vary substantially. The results of the system evaluation will define how much water is being discharged by the system (usually in inches of water per hour) and how much of this discharged water is actually reaching the root zone of the crop (usually as a percentage). An example of this is a portable sprinkler system that has an average discharge of 0.33 in/hr at 75% efficiency. Therefore, for any given hour of use this system is delivering 0.33 X 0.75 or .25 in/hr of water to the root zone of the crop.

Once the irrigation system efficiency and SMD are know, then is possible to calculate how long to run the irrigation system to replace the depleted soil moisture on any given day. To continue with the above example, this portable sprinkler system is in an apple orchard that has a 3.0” MAD.   The grower would then need to set a run time equal to the MAD divided by the system efficiency or (3.0 inches ÷ 0.25 in/hr) = 12 hours.

There are a number of irrigation systems and crop MAD’s in the District. The irrigation system systems have discharge rates ranging from 0.05 in/hr to over 0.3 in/hr with 75% to nearly 95% efficiencies while the MADs range from 1.5 to over 4 inches. This produces irrigation durations from 10 to over 80 hours per irrigation event.  Further, there is a wide variety of crop grown in the District with crop coefficient's (Kc) ranging from 0.2 to 1.1.  As a result some fields may require one irrigation per season while others require 12 or more.

Effective irrigation scheduling will conserve water and energy while protecting the land from erosion.  Additionally, this information allows the grower to schedule cultural practices in the fields.  An example would be a grower with overhead sprinklers.  If the grower notices a pest problem in the field he can determine if he should irrigate the field before implementing a control program for the pest.  Most pesticides can be removed with 2" or less of applied water so it would be prudent to apply the material after an irrigation event instead of just prior to the irrigation.