Water Management in Desert Agriculture
Following more than two decades of drought in the western U.S., many watersheds are experiencing critical water shortages. In the Colorado River basin, the two largest reservoirs, Lakes Powell and Mead, are now at less than 20% capacity. In contrast, these reservoirs were nearly full in 2000. With agriculture responsible for 70-80% of the Colorado River water diversions, a great deal of scrutiny is being applied to crop production systems in this region.
Crop production systems in the desert Southwest are experiencing increasing pressure to document and improve irrigation efficiency. There are several ways to define irrigation efficiency. The primary definitions for irrigation efficiency include orientations with water conveyance and delivery systems (engineering efficiency), and also agronomic, economic, and environmental efficiencies.
It comes as no surprise that as an agronomist and soil scientist I tend to focus on the agronomic efficiency of irrigation for the benefit of the crop being grown. The crop plants are the centerpiece of the production process and the soil-plant system itself represents the primary objective of irrigation management.
Agronomic (crop and soil) considerations are centered on our ability to provide irrigation water for the sustainable production of a crop in the field. The three primary demands for crop water management include: 1) providing water for seed germination and stand establishment, 2) providing irrigation water to match crop-consumptive water use and avoid crop water stress, and 3) provide sufficient irrigation water to leach soluble salts from the root zone so that the soils can support crop production in a sustainable manner (Figure 1).
Agronomic efficiency at the field level focuses on the crop water demand (CWD) through crop consumptive use (crop evapotranspiration, ETc, which is the combination of evaporation and transpiration from the crop) and leaching requirements (LR) which are dependent upon the crop and salinity of the irrigation water.
The leaching requirement (LR) can be estimated by use of the following calculation:
This is a good method of LR calculation that has been utilized extensively and successfully in Arizona and the desert Southwest for many years. We can easily determine the salinity of our irrigation waters (ECw) and we can find the critical plant salinity tolerance level from readily available tabulations of salinity tolerance for many crops (Ayers and Westcot, 1989). Additional direct references are from Dr. E.V. Maas' lab at the University of California (Maas, 1984: Maas, 1986; Maas and Grattan, 1999; Maas and Grieve, 1994; and Maas and Hoffman, 1997).
To deal with crop water management at the field level agronomically, the crop and soil factors, there are some fundamentals that we can refer to for assistance.
Dr. Jeremy Weiss, program manager for the University of Arizona AZMET system, has recently developed two valuable tools that provide actual crop evapotranspiration (ETc) estimates from several AZMET sites in the lower Colorado River Valley and for several key vegetable crops, including lettuce (iceberg and romaine), broccoli, cauliflower, cabbage, and spinach.
The first tool provides accumulations of ETc values over the previous week and a range of dates and the planting date selected by the user. In both models, the Kc values presented in FAO-56 are used for appropriate stages with each crop. The reference evapotranspiration (ETo) measurements are taken directly from each AZMET site listed. Thus, by this method ETc estimates are made by use of equation 3.
To access this first crop-water estimate tool please refer to the following link:
The second tool provides accumulations of ETc values over the crop production cycle from planting (or the wet date) and the date of harvest.
This second tool gives us the opportunity to estimate agronomic efficiency of irrigation management. With this tool we can review total crop water use estimates (ETc) and include the leaching requirements (LR) to compare with the irrigation water applied (IWA) as described in Equation 1.
https://viz.datascience.arizona.edu/azmet/azmet-crop-water-use-estimates-post-harvest/
For example, using lettuce (either iceberg or romaine) with a 10 October 2022 planting date through 9 January 2023, we can see from this model the reference evaporation, listed as "water use" (ETo) and the cumulative crop evapotranspiration (ETc, or total crop-water use) of lettuce since the selected planting date.
Since planting on 10 October 2022 through 9 January 2023, a total crop water use (evapotranspiration), or the ETc, has been 11.27 ~ 11.3 inches in the Yuma Valley. We can also determine crop water use (ETc) last week (3-9 January), has been 0.52 inches in the Yuma Valley.
Similar estimates are provided by this model for several AZMET weather stations in the Yuma production area: Roll, Yuma North Gila, Yuma South, and the Yuma Valley. The Yuma Valley station is at the University of Arizona Yuma Agricultural Center and the Yuma South site is near Somerton, Arizona. A map of AZMET sites can be found in the following link:
https://ag.arizona.edu/azmet/mapsites.htm
When we include the leaching requirement for the crop, an overall estimate of field level irrigation efficiency can be made. We can estimate the leaching requirement as follows assuming an electrical conductivity for Colorado River water of 1.1 dS/m and the lettuce crop salinity tolerance of 1.3 dS/m.
Leaching Requirement (LR) = ECw/((5 X ECe)-ECw)
LR = 11.3 X 0.2 = 2.26 ~ 2.3 inches
Thus, total crop water demand in this case = 11.3 + 2.3 = 13.6 or ~ 14 inches total
Contrasting this estimate of total crop-water use with the amount of irrigation water that was applied to the field can provide an estimate of the agronomic efficiency for a given field of lettuce. We can do the same thing using this tool for other primary leafy green vegetable crops.
We are working in times of decreasing water availability and increasing scrutiny in agriculture. It serves us well to measure and understand the relationship between crop demand (consumptive use plus leaching requirements) versus the irrigation water that is applied to a given field.
Khokhar, T. 2017. World Bank.
https://blogs.worldbank.org/opendata/chart-globally-70-freshwater-used-agriculture
2022 Growing Season
With the start of 2022 growing season we are gearing up for field trials on vegetables. Listed below are our research trials for this growing seasons addressing the need of the agriculture community. For more information and/or collaboration contact: bpoudel@arizona.edu
Soil amendments for Fusarium wilt in lettuce
We will be conducting trials using different soil amendments available in market to see their effect on fusarium wilt of lettuce. Last years results available.
https://doi.org/10.1094/PDMR16
N. Singh, M. Porchas, J. Furr and B. Poudel-Ward (2022). Evaluation of different soil amendments to manage Fusarium wilt of lettuce in Yuma, AZ, 2021. PDMR: Volume 16.
. Funded by Specialty crop block grant.
Downy mildew in Lettuce
We will conduct trial on downy mildew of lettuce this year as well.
Sclerotinia drop of Lettuce
We will continue to do field trials on sclerotinia drop of lettuce on both species S. sclerotiorum and S. minor.
Powdery mildew of lettuce
We will continue with field trials on powdery mildew of lettuce.
Downy mildew of Spinach
Downy mildew of spinach field trials won't start until January of next year. But we are requesting industry reps to contact us ahead of time. Protocols for lettuce trials: Please submit by end of October. Material has to be sent in mid December. We start applications on first week of January. For more information contact Bindu Poudel-Ward (bpoudel@arizona.edu).
Big Picture View on AgTech – CES 2023
For those of you interested in the latest thoughts on ag technology, you might want to check out the keynote address given by John Deere1 at CES 2023 earlier this week. The presentation can be found on YouTube at: https://youtu.be/1kjZMHZl538. As you might expect, there is a fair amount of company self-promotion, but it is interesting to hear their view on the current state and future of agricultural technologies. A couple of key takeaways for me were that telematics and computer vision systems will play key roles in the future and that increased precision will continue to be a focus.
You asked. . .We Answered: Spinach Trial
Growers and PCAs asked about the potential injury that can be caused by a mix of RoNeet (Cycloate) with Dual Magnum (S-Metholachlor) to spinach. We did a small evaluation including the treatments suggested by our friends to collect data. A 1.33pt rate for Dual M and 4.0 pt of RoNeet was sprayed for this trial. The lower Dual M recommended rate by the SLN for AZ in 2017 of 0.33 - 0.67 pt per acre was not included in the trial1. Here's the treatment list:
RoNeet 2.0pt + Dual M 1.33pt
RoNeet 4.0pt
Dual M 1.33pt
Plots consisted of two rows 30ft long replicated four times with 10 seed lines, and the test was established in a randomized complete block design with four replications. All treatments were applied preemergence. A CO2 backpack with a 4 flat fan nozzle boom spaced at 20" was used delivering 20 gallons/acre.
Planting was done on Nov 17, 2022, then the next day the treatments were applied and incorporated immediately with sprinkler irrigation. Crop injury was evaluated on December 13, 21 and 27. The combination of RoNeet plus S-Metolachlor at the rates above mentioned presented the highest phytotoxicity symptoms in the form of chlorosis, stunting, and reduced stand. Also, Dual Magnum alone at the rate of 1.33 pt/a presented injury in lower proportion compared to the combination with RoNeet. In this evaluation the 4pt/a rate of RoNeet did not present injury to the spinach as you can see comparing with the untreated plots below. Weed control data was not collected due to inconsistent and low populations. Only phytotoxicity was evaluated.
The most representative images of the plots are below:
Figure 1. Preemergence herbicide injury evaluation on spinach
https://searchagriculture.az.gov/esd/slns/AZ120006.pdf
Area wide Insect Trapping Network (July 27, 2022)
Area wide Insect Trapping Network VegIPM Update, Vol. 13, No. 15, Jul 13, 2022
THE MOST RECENT RESULTS
CEW moth counts increased slightly in the past 2 weeks and remains higher than the previous 2 summers.
Trap counts stay active in some locations (Yuma Dome Valley and Tacna) but lower than last summer
DBM moths were not caught in any trap in the past 2 weeks; average for this time of the year.
Adult movement continuing to increase, particularly in Gila, Yuma and Dome Valleys consistent with melon harvest.
Aphid movement is absent consistent with what we typically see in the summer.
Adult activity has highest in Wellton and Dome Valley, and above average for early July.