Recent Studies in Deficit Irrigation
Deficit irrigation is the generic term for applying less water than the full needs of a crop and can take many forms. It can be a planned, sophisticated strategy or an unplanned fact of life when water scarcity arises. Planned deficit irrigation is widely used with grapes to improve quality. Unplanned deficit irrigation occurs widely on forage crops that depend on diversions from mountain streams as the runoff pulse declines in late summer.
Deficit irrigation has been investigated because economists have long known that maximizing crop yield is not the same as maximizing profits. Less water applied could potentially mean dollar savings in labor, water and other inputs, assuming water savings could be monetized. In theory, a farmer could increase profits by optimizing the use of all of these inputs. More recent studies have begun to examine deficit irrigation as a tool to address water scarcity. Below are summaries of recent research relating to deficit irrigation.
“Deficit irrigation and surface residue cover effects on dry bean yield, in-season soil water content and irrigation water use efficiency in western Nebraska high plains.” Dean Yonts, Amir Haghverdi, David L. Reichert, Suat Irmak
A study based in the Nebraska Panhandle in Scottsbluff involved two experiments in 6 consecutive growing seasons (2010-2015) on deficit irrigation of edible dry beans. One of the highest producing edible dry bean regions in the U.S. is the Central High Plains (i.e. Colorado, Wyoming, and Nebraska).
The experiments examined the impacts on dry bean production, irrigation water use efficiency and soil water dynamic after applying full irrigation, deficit irrigation, and rainfed irrigation. Deficit irrigation was measured at 75%, 50%, and 25% of full irrigation. The areas were sprinkler irrigated on a sandy loam soil with a pH of 8, organic matter of 1% and groundwater depth of 14 meters. This part of Nebraska annually receives 12 to 13 inches of precipitation.
The results demonstrated that a 25% reduction of irrigation water only caused a 6% decrease in yields relative to full irrigation
However, applying only 50% of irrigation water resulted in significant declines (30%) in dry bean yields
When pre and early season rainfall was abundant, deficit irrigation treatments before flowering outperformed treatments after flowering
However, under normal and dry conditions, yield decline was more pronounced due to early season water stress compared to later season water stress
Average yield across irrigation treatments was 14% lower for plots covered with crop residue compared to bare soil plots
The dynamic soil water content in the root zone and irrigation water use efficiency were similar in plots covered with residue and bare soil plots
“Benefits and impacts of partial season irrigation on alfalfa production.” Perry Cabot, Joe Brummer, Sumit Gautam, Lyndsay Jones, and Neil Hansen
A study to determine the effects of implementing partial season irrigation on lower elevation alfalfa hayfields on forage yield, nutritional quality, and recovery was conducted on six sites throughout the Western Slope of Colorado. There were three different irrigation plans: (1) full season irrigation, (2) stopping irrigation after the 2nd cutting, and (3) stopping irrigation after the 3rd cutting. These three different irrigation schedules were implemented for two consecutive years. In the third year, all the fields received full irrigation. The plots where irrigation was cut after the 2nd cutting were labeled as “low-risk” and the plots that didn’t received irrigation after the 1st cutting were “high-risk.”
Below is a table of the locations, elevation, annual precipitation and other information where the experiments took place:
At the end of three years, there were several patterns in the results:
Total fiber concentrations were greatest in the control (34.6%) and lowest in the “high-risk” plots (28.2%)
Deficit irrigation’s effects on crude protein were inconsistent
Plots under deficit irrigation had higher digestibility than the plots with normal irrigation
Reduced irrigation may increase forage quality, but will significantly reduce yields.
When irrigation is returned the following year, alfalfa yields may fully recover depending on length and severity of reduced irrigation.
Below are the charts for the yields of each experimentation plot by location. REF is the plot under full irrigation, and “SA2” is when irrigation was terminated after the second cutting, and “SA1” is when irrigation was terminated after the first cutting. Yields are measured in kilograms per hectare (kg/ha). One hectare is about 2.47 acres, and just over 900 kilograms is one ton.
“Deficit Irrigation with Alfalfa: What are the Economics?” Michael J. Ottman and Daniel H. Putnam
A review of recent deficit irrigation studies in the Western United States that examines the profitability of such practices, and includes other topics such as pest control, hay quality, residual effects, and irrigation water use efficiency. These studies are all focuses on alfalfa. The results from these studies do not apply to other crops, only alfalfa.
A few key takeaways from this report:
Many research studies on deficit irrigation have been conducted show that deficit irrigation does not pay. For deficit irrigation to be “profitable” or lose less money than standard irrigation, the cost of water has to higher or the value of the hay lower than what they are currently. Irrigators would have to be compensated for these losses
Alfalfa is usually very responsive to irrigation water and yield usually increases in a linear fashion with irrigation water application up to a certain point. So, on average, it is difficult to see how deficit irrigation will save money
Deficit irrigation may have residual effects that could negatively affect the profitability of this practice. In other words, if yields do not fully recover or insect or weed problems are worse after full irrigation is resumed, then these would be residual effects of deficit irrigation. Stand loss and temporary or permanent yield loss after resumption of full irrigation is more likely with sandy soils or with a long duration of irrigation termination in regions with hot and dry weather conditions
Deficit irrigation may change the quality of the hay produced. Water stress reduces stem growth relative to leaf growth and the leaf to stem ratio may be higher with deficit irrigation
Deficit irrigation will reduce water cost but could also change cost of controlling insects or weeds in a positive or negative direction depending on the situation