This study quantified Crop Water Productivity (CWP) in organic and conventional iceberg lettuce systems at the University of Arizona Yuma Agricultural Center during the 2024–2025 season. Both systems were managed under subsurface drip irrigation using sensor-based and traditional irrigation scheduling strategies. Results showed that conventional lettuce produced about 40–50% more yield per unit of crop evapotranspiration than the organic system; this difference may be related to differences in crop growth and nutrient availability. While short-term CWP was higher under conventional management, organic systems may enhance soil structure, water-holding capacity, and infiltration over time; however, these soil changes were not measured in this single-season study. Sensor-based irrigation scheduling in most cases improved irrigation water-use efficiency across both systems, demonstrating that precision irrigation technologies can help Yuma growers make better use of water resources. Combining precision irrigation with organic farm practices that focus on soil management and improving soil properties offers a promising path toward sustaining productivity and conserving water in Yuma’s desert agriculture.
Introduction
Yuma County is one of Arizona’s most productive agricultural areas and is known nationally as the “Winter Lettuce Capital of the World.” Despite receiving only about three inches of annual rainfall (Arizona Meteorological Network–Yuma Valley Station (AZMET)), Yuma’s access to Colorado River water and its fertile river valley soils support a year-round agricultural economy. Agriculture in Yuma accounts for roughly 78% of total county water demand, emphasizing the importance of efficient water use for long-term sustainability (Water Resources Research Center [WRRC], 2023).
Because lettuce is a shallow-rooted, cool-season crop grown in a hot, arid climate, wise irrigation management is critical for achieving high yield and quality while minimizing water losses and preventing over-irrigation that causes diseases. Variability in soil texture, irrigation uniformity, and climate conditions across the Yuma Valley can greatly affect crop water use and irrigation efficiency. As pressures on Colorado River allocations increase, Yuma growers continue to look for ways to produce more lettuce with less water through improved irrigation application strategies (i.e., laser land leveling technology in agriculture to create precise field slopes (0-0.2%) for uniform water distribution, saving significant water and improving yields) and scheduling.
One of the most effective ways to evaluate how well crops use water is through Crop Water Productivity (CWP), often described as “more crop per drop.” CWP reflects how much yield is produced for each unit of water the crop consumes through crop evapotranspiration (ETc), which includes both water lost from the soil and water transpired by the plant leaves through small openings called stomata. A higher CWP means more lettuce produced per unit of crop water use (ETc) , which is especially valuable in Yuma’s hot, dry, and windy climate, where water losses can be high compared to sub-humid and/or humid climate zones. Measuring and improving CWP helps ensure that every acre-foot of irrigation water supports strong, profitable production.
A range of factors can influence CWP in lettuce production, including climate, soil texture and soil organic matter, crop variety, growth stage, irrigation scheduling, and overall field and agronomic practices management. For example, lettuce requires the most water during the 2 The University of Arizona Cooperative Extension heading stage, making precise irrigation timing essential to avoid water stress or overwatering. Practices that improve soil water retention or reduce unnecessary evaporation, such as good residue management and effective weed control, can also improve CWP in desert systems.
Interest in organic lettuce production has been growing in the Yuma Valley. Organic systems emphasize soil health through compost and cover crops, which can improve soil structure and water-holding capacity and increase nutrients in the active root zone. However, conventional systems often achieve higher yields due to faster plant growth and the availability of soluble nutrients. Understanding how these two production systems compare in their use of limited water resources is important for growers making management and investment decisions.
Studies across arid and semi-arid regions have shown that crop water productivity generally increases when irrigation is applied more efficiently or when limited irrigation strategies are used to reduce non-beneficial water losses (Howell et al., 1998; Djaman & Irmak, 2012; Mohammed & Irmak, 2022). In similar systems, drip irrigation has been shown to improve CWP compared with surface (not laser-levelled as in Yuma) or sprinkler methods, as it provides better control of soil moisture and reduces evaporation losses (Mohammed & Irmak, 2022).
To better understand these differences, a field study was conducted during the Fall 2024–Spring 2025 growing season at the Yuma Agricultural Center to compare Crop Water Productivity between organic and conventional iceberg lettuce systems. Both systems were grown under similar soil and climate conditions using subsurface drip irrigation. We aimed to compare how each system converts irrigation water into crop yield. This work aims to provide practical information that helps Yuma growers improve crop productivity under two cropping systems of organic and conventional, maintain profitability, and support long-term water sustainability in the desert Southwest.
Materials and methods
This study was conducted during the 2024–2025 growing season at the University of Arizona Yuma Agricultural Center, located in the Lower Colorado River Basin. The site represents Yuma’s typical arid desert conditions, characterized by annual rainfall below 3 inches (Arizona Meteorological Network- Yuma Valley Station (AZMET)) and clay loam soils (Field Capacity: 31.9%, Wilting Point: 15.5%) (Mohammed, 2025a, 2025b, 2025d; Mohammed et al., 2025c).
The experiment compared organic and conventional iceberg lettuce systems managed under a subsurface drip irrigation (SDI) method to evaluate their effects on Crop Water Productivity (CWP).
The SDI system was selected because it provides uniform water distribution directly to the root zone, minimizes surface evaporation, and improves irrigation efficiency under Yuma’s high-temperature and low-humidity conditions. Drip tape is in the middle of the bed, and the distance between centers is 42 inches. Hose Diameter 5/8" (16 mm) Emitter Spacing 8" (20 cm) Wall thickness 8 mil (0.20 mm) Emitter flow rate 0.27 gph @ 8 psi. The tapes were injected before planting at 3–4 inches deep (Figure 1).
Then, installing sprinkler irrigation that irrigated 8 beds at a time for 5–7 days for 6–8 hours, depending on the time of the year, to bring soil moisture to field capacity and ensure germination (Figure 2).
Figure 1. Subsurface drip irrigation (SDI) setup used in the 2024–2025 iceberg lettuce trial at the University of Arizona Yuma Agricultural Center (Yuma, AZ).
Figure 2. Sprinkler irrigation used for stand establishment prior to SDI operation in the 2024–2025 iceberg lettuce trial at the University of Arizona Yuma Agricultural Center (Yuma, AZ).
Both fields were planted on October 29, 2024, using presprinkler irrigation to ensure uniform germination. Two irrigation scheduling strategies were applied within each production system:
- Sensor-Based Irrigation (SI): Scheduled by soil moisture sensors installed at multiple depths to trigger irrigation when depletion thresholds were reached.
- Traditional Irrigation (TI): Managed following standard grower practices and fixed irrigation intervals typical of Yuma lettuce production. Each system also included treatments with and without biostimulant applications, resulting in eight treatment combinations (four organic, four conventional), replicated three times in a Randomized Complete Block Design (RCBD).
Organic treatments
- OSB: Organic sensor-based irrigation + biostimulant
- OSI: Organic sensor-based irrigation (no biostimulant)
- OTB: Organic traditional irrigation + biostimulant
- OTI: Organic traditional irrigation (no biostimulant)
Conventional Treatments
- CSB: Conventional sensor-based irrigation + biostimulant
- CSI: Conventional sensor-based irrigation (no biostimulant)
- CTB: Conventional traditional irrigation + biostimulant
- CTI: Conventional traditional irrigation (no biostimulant)
Each treatment was managed independently with separate irrigation lines and fertilizer application systems to prevent cross-contamination between organic and conventional management practices. The study incorporated a total of 24 experimental plots (8 treatments × 3 replications) arranged in a uniform field layout to minimize spatial variability. Each plot received water and nutrient inputs according to its designated treatment specifications.
Fertilization differed by production system.
The conventional system received 200 lbs/acre of nitrogen (N) as pre-plant fertilizer, while the organic system received 2,000 lbs/acre of chicken pellets (4-4-2) and a top-dress of 1,800 lbs/acre of organic fertilizer (9-6-1). Biostimulants were applied twice during the season in both systems through the subsurface drip irrigation system.
The biostimulant used included multiple components, including a concentrated extract of plant-active soil organic matter components, chelating organic acids, sugars, and micronutrients such as 6% Zn and 1% Mn. These formulations are designed to work through both the soil and foliage.
Soil moisture and weather data were continuously recorded using Sentek Drill & Drop sensors and a Soiltech Wireless weather station to measure soil water content, air temperature, humidity, and wind speed. These data supported the calculation of seasonal crop evapotranspiration (ETc), which, together with measured lettuce yield, was used to determine Crop Water Productivity (CWP) for each treatment.
CWP was calculated using the following relationship:
CWP=(Y/ETc)
where CWP is expressed in kg m⁻³, Y is the marketable yield (g m⁻²), and ETc is the seasonal crop evapotranspiration (mm).
Comparisons among irrigation strategies, production systems, and biostimulant treatments were used to identify management practices that enhanced water-use efficiency and overall productivity under Yuma’s desert growing conditions.
Results and discussion
Across all treatments and irrigation scheduling strategies, CWP for iceberg lettuce ranged from 5.6 kg m⁻³ (OTI) to 11.2 kg m⁻³ (CSI) under subsurface drip irrigation (Figure 3). Among the organic treatments, CWP values ranged from 5.6 kg m⁻³ (OTI) to 7.6 kg m⁻³ (OSB). The highest CWP under organic management occurred in the sensor-based irrigation with biostimulant (OSB) treatment, reflecting the benefits of precise irrigation scheduling and improved nutrient uptake compared with traditional irrigation practices.
In the conventional system, CWP values ranged from 9.6 kg m⁻³ (CTI) to 11.2 kg m⁻³ (CSI). The conventional sensor-based irrigation without biostimulant (CSI) treatment achieved the highest overall CWP, while conventional traditional irrigation (CTI) had the lowest. This indicates that accurate irrigation scheduling scheduled by soil moisture sensors had a greater impact on water productivity than biostimulant use alone.
When comparing the highest-performing treatments, conventional lettuce produced about 40–50% more yield per unit of crop water use (ETc) than the organic system. This difference may be related to differences in canopy development, plant growth, and nutrient availability between the two management systems. These results suggest that under Yuma’s desert conditions, where evapotranspiration is high, irrigation management and nutrient management may influence crop water productivity.
However, while short-term CWP values were lower in organic plots, organic systems may provide long-term benefits for water and soil conservation. Soils managed organically tend to build higher levels of organic matter, improving soil structure, water-holding capacity, and infiltration. These improvements reduce surface runoff and evaporation losses while enhancing moisture storage in the root zone. These potential soil-health benefits are supported by the broader literature but were not directly measured in this study The observed differences among treatments also highlight the strong influence of irrigation management on crop water productivity. Sensor-based irrigation in most cases outperformed traditional scheduling across both production systems. By applying water based on soil moisture data, sensor-guided irrigation minimized deep percolation and evaporation losses, maintaining optimal soil moisture during critical lettuce growth stages, particularly heading, when water demand peaks.
Overall, these results demonstrate that combining precision irrigation technologies with appropriate nutrient management strategies can substantially enhance water-use efficiency in desert lettuce production. For the long term, integrating organic soil-building practices with sensorbased irrigation offers a promising path toward maintaining productivity, conserving water, and improving soil health in the Yuma Valley. This approach supports the broader goal of achieving “more crop per drop” while protecting the region’s most critical agricultural resources.
Figure 3. Crop water productivity (CWP; kg m⁻³) of organic and conventional iceberg lettuce under SDI by irrigation scheduling (sensor-based vs. traditional) and biostimulant treatment, Yuma Agricultural Center, 2024–2025.
Conclusion
This study demonstrated that under Yuma’s arid climate and subsurface drip irrigation management, Crop Water Productivity (CWP) for iceberg lettuce varied notably between production systems and irrigation strategies. Conventional systems achieved higher short-term CWP values up to 11.2 kg m⁻³, resulting in greater yield per unit of crop water use (ETc). In contrast, organic systems showed lower short-term CWP values but may provide soil and water conservation benefits over time, although soil changes were not measured in this study. The results confirm that sensor-based irrigation significantly improves water-use efficiency in both organic and conventional lettuce production by maintaining optimal soil moisture and minimizing non-beneficial water losses. This precision irrigation approach ensures that every drop of water contributes directly to crop growth, helping growers meet production goals while conserving valuable Colorado River resources.
Although organic systems may not yet match conventional yields, they may offer long-term advantages for sustainable agriculture in Yuma’s desert environment. Regular applications of compost and organic fertilizers can enhance soil organic matter, improve structure, and increase waterholding capacity. Over time, these improvements can lead to better infiltration, reduced runoff, and greater resilience to drought and salinity stress.
Integrating organic soil-building practices with precision irrigation technologies such as sensor-based scheduling presents a balanced strategy for the region’s future— supporting both productivity and sustainability. For Yuma lettuce growers, this combined approach offers a pathway toward producing “more crop per drop” while maintaining healthy soils and protecting limited water supplies for generations to come.
Acknowledgement
The author gratefully acknowledges the support and collaboration of the University of Arizona Cooperative Extension, the Yuma County Cooperative Extension, and the School of Plant Sciences for their support. Special thanks are extended to the staff and faculty of the University of Arizona, Yuma Agricultural Center, for their technical assistance, field management, and dedication throughout the study. The author also recognizes the contributions of the Western Alliance to Expand Student Opportunities (WAESO)/ National Science Foundation (NSF) intern students for their valuable assistance with data collection, field measurements, and laboratory analysis. Appreciation is extended to Soiltech Wireless for providing weather and soil monitoring equipment and to NatureSafe Fertilizers (Darling Ingredients Inc.) for supplying organic fertilizer materials used in the organic production system.
References
Water Resources Research Center (WRRC). Arizona Water Factsheet: Yuma County (Water in Yuma County). University of Arizona. November 2023.
Howell, T. A., Tolk, J. A., Schneider, A. D., & Evett, S. R. (1998). Evapotranspiration, yield, and water use efficiency of corn hybrids differing in maturity. Agronomy Journal, 90(1), 3–9. https://doi.org/10.2134/agronj1998.00021962009000010002x
Djaman, K., & Irmak, S. (2012). Soil water extraction patterns and crop, irrigation, and evapotranspiration water use efficiency of maize under full and limited irrigation and rainfed settings. Transactions of the ASABE, 55(4), 1223–1238. https://doi.org/10.13031/2013.42262 elibrary.asabe.org
Mohammed, A. T., & Irmak, S. (2022). Maize response to irrigation and nitrogen under center pivot, subsurface drip and furrow irrigation: Water productivity, basal evapotranspiration and yield response factors. Agricultural Water Management, 271, 107795. https://doi.org/10.1016/j.agwat.2022.107795
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Mohammed, A. T., Discua Duarte, S., & Masson, R. (2025c, June). Evaluating biostimulant effects on growth and yield of iceberg lettuce in organic and conventional systems under subsurface drip irrigation in Yuma, AZ (Publication No. AZ2138). University of Arizona Cooperative Extension. UA Cooperative Extension
Mohammed, A. T. (2025d, October). Performance evaluation of nitrate-nitrogen sensing technologies in organic and conventional iceberg lettuce systems under subsurface drip irrigation (Publication No. AZ2169). University of Arizona Cooperative Extension. UA Cooperative Extension