Identifying Stacked Conservation Practices that Optimize Water Use in Agriculture: Phase II

Our first two objectives focus on research and are presented here. The third objective centers on education efforts and will be discussed in the education plan section.

Objective 1: Identify combinations of six water conservation practices with the greatest ability to optimize crop production, crop profit, and water use.

Three long-term water optimization trials were established in Utah in Phase I of this project. The first was established in Logan in 2019, the second in Vernal in 2019, and the third in Cedar City in 2021. At each site, a new linear move sprinkler (“lateral”) was installed with funding and support from several federal, state, and local partners. These stakeholders have invested in the infrastructure and Phase I helped establish the trials, while Phase II is critical to take advantage of the investments.

The treatments at each of the three sites include three replications of:

1. four irrigation systems [the conventional mid-elevation spray application (MESA) system vs. low-elevation spray application (LESA), low energy precision application (LEPA), and mobile drip irrigation (MDI)];
2. four irrigation rates (50, 50, 75, and 100% of evapotranspiration). One of the 50% irrigation rates is a partial irrigation strategy focused on applying more water during critical crop growth stages; and
3. up to five crop management treatments depending on the crop:

• Corn has five treatments (conventional, drought-tolerant genetics, no-tillage, cover crops, and a fifth treatment that combines the latter three conservation practices)
• Alfalfa has two treatments (conventional vs. drought tolerant genetics)
• Small grain forage has two treatments (tillage vs. no-tillage)
• Alternative crops (teff, sorghum-sudangrass, and Italian ryegrass) do not have crop treatments but are tested against other traditional crops

The sixteen irrigation treatments (four systems x four rates) are arranged along each lateral (Fig. 2). Each span (150-180 ft long) has a different irrigation system with a random placement of the irrigation rates in four equally sized quarters. The rate differences were achieved by modifying sprinkler nozzles in each quarter. Crop management treatments were arranged perpendicular to the direction of the irrigation system to create replicated combinations of all crop management and irrigation system combinations.

At each site, one block of about six acres is utilized for alfalfa. Three random strips perpendicular to the lateral were planted to a conventional alfalfa variety and another three were planted to a drought-tolerant alfalfa variety (Ladak II). Alfalfa yield has and will be measured in 96 plots at each location (16 irrigation treatments x 2 varieties x 3 replicate strips; Fig. 3) for each alfalfa cutting utilizing research or commercial harvesters. Another 6-acre block is used for a corn-corn-small grain forage crop rotation. This area has 256 small plots (20 x 30 ft) that includes three replications of the 16 irrigation treatments and five crop treatments. Corn is planted parallel to the lateral so sprinklers can irrigate and move between and not across corn rows. Corn silage yield has and will be measured each year in all plots using a two-row Gehl corn chopper with a pull-behind weigh wagon. Small grain forage yield is measured using the same methods as alfalfa. Finally, a 2-acre block is reserved for alternative crops at each site. This allows for three replicated strips each of two alternative crops. The two crops are rotated annually and have included safflower, teff, triticale, and sorghum-sudangrass in Phase I. Alternative crops have and will be harvested with research harvesters.

Forage quality will be estimated on all crops using near infrared reflectance spectroscopy (NIRS) and standardized calibrations. Forage quality parameters differ by crop but all parameters that influence crop selling price will be measured to allow profit calculation.

In order to measure water use efficiency, we will measure irrigation rates using flow meters that were installed on each lateral. We will also continue to monitor hourly soil moisture content at 40 plots at each site. In each plot, water content sensors are installed to three depths (6, 18, and 30 inches). The plots include eight of the irrigation treatments (4 systems x 2 irrigation rates – 50 and 100% evapotranspiration) in all three crops as well two of the crop treatments for alfalfa (conventional vs. drought-tolerant) and two for corn/small grains (conventional vs. the combined no-tillage, drought-tolerant genetics, and cover crops).

Phase I allowed us to install all the irrigation systems and soil moisture sensors, and to establish all the crop treatments. We were able to collect data mostly on corn because it is an annual crop and could be established rapidly (Fig. 4). Alfalfa, a perennial, and the most dominant crop on western farmland, took several years to establish. We trained two graduate students who have published preliminary data on early impacts of irrigation systems and deficit irrigation on corn, establishing alfalfa, and alternative crops.

In Phase II, we will examine how water optimization practices influence established alfalfa. We will also gather additional and needed data for two existing alternative crops (teff, sorghum-sudangrass) and two new alternative crops (triticale and Italian ryegrass) that were highly recommended by local producers and crop advisors. The production, profit, and water use of these crops will be compared to alfalfa and corn. Our database of irrigation system evaluations will also grow, and we will expand examination of a new irrigation system [low-elevation rotator sprinkler (LENA)] in place of MDI at Cedar City. We will replace MDI because it has been extremely problematic in terms of water quality and repairs. Furthermore, now that our no-tillage and cover crop treatments have been in place for one to three seasons, we can begin to more adequately examine how they influence crop production and water use.

In summary, Phase II will include conducting the three water optimization trials at Logan, Vernal, and Cedar City for the 2022-2024 seasons following methods and procedures discussed above and vetted in Phase I. We will have at least 5-13 site-years of data on all the alternative crops, alfalfa, and corn. We will also have 8-13 site-years of comparisons of irrigation systems and crop treatments (no-tillage, drought-tolerant genetics, and cover crops). This additional data will greatly strengthen our results and demonstrate the crop production and water use impacts of water optimization techniques for major crops of the West.

Objective 2: Demonstrate variable-rate irrigation systems that improve water and energy use efficiencies.

Variable-rate irrigation (VRI) utilizes site information and irrigation technology to apply spatially variable irrigation rates that match site-specific growing conditions. VRI aims to improve energy and water use efficiency by avoiding areas of over- or under-irrigation. Two types of VRI are zone control and speed control. Phase I of this project developed and tested a zone control VRI system at two Idaho study locations (Pocatello and Rexburg). Zone control VRI allows differing irrigation rates for zones of any shape by regulating individual sprinklers. The zone control VRI system we developed in Phase I consists of:

• defining irrigation zones,
• characterizing soil water holding properties by zone using sensors,
• real-time, zone specific tracking of soil water depletion, and
• VRI recommendations that maintain soil water within a target depletion range.

We documented a 15% irrigation water savings and similar total yield compared to the grower’s standard practice (Fig. 5).  Other research has documented similar advantages of VRI (Sadler et al., 2002; Hedley and Yule, 2009; West and Kovacs, 2017).

Interest in VRI is increasing in southern Idaho because of recent water rights adjudications that allowed producers with zone control VRI to develop new irrigated land area equal to the unfarmed, un-irrigated rock outcroppings common in this area. However, adoption rates of zone control VRI are expected to proceed slowly. During Phase I, feedback from stakeholders indicated that VRI could be applied more extensively if approaches were simplified and developed for speed control VRI. Speed control VRI is limited to wedge-shaped zones created by changing the travel speed of pivots. Although less flexible, this approach can be implemented on nearly all existing pivots with minimal investment, while still gaining some of the water conservation advantages of zone control.

Long-term zone VRI trials

In Phase II of this project, we will continue to monitor and demonstrate zone VRI practices in 2022- 2024 at the long-term field sites in Pocatello and Rexburg, Idaho. Phase I allowed us to collect data mainly on wheat production. In Phase II, we will be able to assess the impacts of zone VRI on potato production in Pocatello (wheat in 2022-2023 and potato in 2024) and alfalfa production in Rexburg. At both sites, we will continue to test uniform irrigation zones vs. VRI to determine how zone VRI impacts crop yield, quality, profit, and water use. We anticipate that the advantages of VRI will be most pronounced in potato due to its high value but should also offer many advantages in alfalfa due to its high water use.

New speed control VRI on-farm trials

We propose additional on-farm trials that modify our VRI system for a simplified, speed control application that captures advantages similar to those of zone control VRI but are more easily adopted by producers. In Phase I, we used intensive soil sampling throughout the growing season under uniform irrigation to estimate crop water use to define VRI zones. This information was coupled with yield to estimate spatial variation of crop water productivity and to delineate precise zone control VRI zones. We then tested how well simple variables [electrical conductivity (ECa), slope, relative elevation, yield] could predict VRI zones. VRI zones predicted from relative field elevation most closely matched those derived from crop water productivity. In Phase II of this project, we will evaluate yield and water savings with speed control VRI zones delineated using relative field elevation at four new farm fields. This will include two sites in Idaho (Aberdeen and Pocatello) and two in Utah (Monroe and Elberta). At the Pocatello location, we will expand from one field location to include an additional speed control VRI field, with farm operations led by our producer representative, Ryan Christensen. The other three sites will be new collaborations with the producers that provided letters of commitment.

The four new speed-control VRI on-farm trials will be conducted for two growing seasons (2023-2024) with wheat or barley. We will develop and test speed control VRI prescriptions by dividing each field into eight large sectors. Four of the sectors will be four replications of uniform irrigation and the other four will have speed-control VRI. There will be four major steps to establishing sector-VRI prescriptions:

1. Define VRI zones. We will work with cooperating producers to select one of their fields with variation in water need. Producers will share any prior yield maps from their field and we will collect digital elevation maps and soil EC_a using a direct contact system (Veris V3150). These datasets will be paired to provide a starting point for a speed-control VRI prescription.

2. Characterizing average soil water holding properties of soils. In Phase I, we documented that soil water retention properties (e.g., volumetric water content at field capacity, -10 kPa) varied among VRI zones within a field. In Phase II, we will install pairs of volumetric water content sensors (Meter, Teros 12) and soil water potential sensors (Meter, Teros 21) in each VRI zone. Paired sensors will allow us to determine zone specific, volumetric water content at irrigation upper and lower thresholds.

3. Real-time, zone specific tracking of soil water depletion. The paired sensors in each VRI zone will be used to determine soil water depletion by comparing current volumetric water content values to the zone specific upper irrigation thresholds. We will monitor sensors in near-real time with remote access to the Meter Zentra Cloud system.

4. VRI recommendations that maintain soil water within a target depletion range. Real-time, zone specific soil water depletions will be used to calculate VRI rates for every irrigation. The timing and rates of irrigation will be calculated to maintain soil water potential values between -10 kPa and -200 kPa. Soil data will be collected remotely each week through telemetry and prescriptions will be developed and shared with cooperating producers for implementation. This can be easily accomplished working with pivot manufacturer software such as FieldNet® as we have done in the past.

At the end of each growing season, we will work with cooperating producers to obtain a grain yield map of the entire field and the water use as recorded by their pivot software. We will also collect crop quality samples from each of the eight sectors prior to or at harvest by hand sampling. These data will be combined to assess how speed-control VRI impacts crop yield, quality, and profit, along with water use efficiency.