This article is the first in a series about conditions in Yuma Valley. Read part 2, A Drier Atmosphere in Yuma Valley: Long-Term RH Trends and Their Implications for Irrigation Management, Crop Performance, and IPM.
The production environment in Yuma Valley is being shaped by gradual warming and highly variable rainfall rather than a strong long-term rainfall trend. Annual precipitation does not exhibit a strong linear trend, and any increase suggested by the trendline is minor relative to year-to-year variability. In contrast, annual average maximum temperature and annual average minimum temperature indicate gradual warming. Together, these changes point to higher crop water demand and tighter irrigation margins in lettuce and leafy green production systems. These trends increase sensitivity to irrigation timing, accelerate canopy water loss during warm periods, and increase the likelihood of crop water stress. At the same time, over-irrigation can increase deep percolation and nutrient losses, complicate salinity management, and create canopy and soil conditions conducive to disease. For crop productivity and management in Yuma Valley, the overall message is that precise irrigation management, responsive crop management, and field-based Integrated Pest Management (IPM) are becoming increasingly important under warming and operationally meaningful rainfall variability.
Arizona’s lettuce and leafy green industry is a cornerstone of specialty crop production and is concentrated in Yuma County, which supplies a major share of U.S. winter leafy vegetables. This industry contributes approximately $2 billion to Arizona’s economy, making the long-term sustainability of Yuma lettuce production a high priority for growers, consumers, and the State of Arizona. Lettuce and leafy greens in Yuma depend on reliable irrigation supplies from the Colorado River, and increasing uncertainty in Colorado River water availability creates an urgent need to strengthen irrigation precision while protecting the environmental conditions that support specialty crop productivity.
Long-term rainfall patterns and what they mean for production
In Yuma Valley, precipitation is not the primary driver of crop water supply; irrigation is. Yet rainfall still matters because it can act like a biological switch, briefly reshaping field conditions in ways that influence weeds and plant disease. A single storm can alter soil surface moisture, stimulate germination, increase canopy humidity, and adjust field access and irrigation timing. For IPM, these short windows often have outsized consequences compared with the annual total
Using annual precipitation totals from the Yuma Valley station for 1987–2025, the long-term trend is slightly upward, but the signal is extremely weak relative to natural variability. The long-term mean annual precipitation is 2.63 inches, while annual precipitation in 2025 was 5.91 inches, substantially above the long-term mean by 3.28 inches (AZMet-Yuma Valley Station). The slope of 0.0111 inches per year corresponds to roughly 0.11 inches per decade, and R² = 0.0074 indicates that the linear trend explains less than 1% of year-to-year variability. The key message is that annual rainfall in Yuma Valley does not exhibit a strong long-term linear trend, and any increase suggested by the trendline is minor relative to variability. Years like 2025 highlight that single-year anomalies can be operationally meaningful even when the multi-decadal trend is weak.
Although rainfall seldom replaces irrigation in Yuma, rainfall variability still has important management implications. Rain events can affect field access and irrigation timing, indirectly shaping canopy structure and pest habitat. In addition, the timing and intensity of rainfall events can influence whether water is retained in the soil profile or lost through runoff. The intensity and distribution of rainfall events during the growing season can strongly influence whether water is retained in the soil profile or lost through runoff and flooding. Additional effects may include increased relative humidity, increased need for plant protection, greater risk of nutrient leaching as the magnitude of a single rainfall event increases, and loss of soil organic matter and erosion. Rainfall pulses can elevate near-canopy humidity, increase leaf wetness, and create short periods that are favorable for weed emergence and disease development. These shortterm effects are often more important to production and management than the annual total itself.
Figure 1. Annual total precipitation at the AZMet Yuma Valley station (1987–2025). Points/line represent annual totals; the horizontal reference indicates the long-term mean (2.63 in). The fitted linear regression trendline indicates a slight increasing tendency over the period.
Warming trends in annual maximum and minimum temperatures
Climate summaries from the Yuma Valley station indicate gradual warming in both annual average maximum temperature and annual average minimum temperature, with Tmax increasing at approximately 0.036°F/year (about 0.36°F/decade) and Tmin increasing at approximately 0.035°F/year (about 0.35°F/decade). These long-term changes matter because even a small increase in minimum or maximum air temperature can substantially affect crop physiological functions, thereby affecting crop growth, development, and yield, even under well-managed farming operations. Transpiration is mainly driven by sunlight, air temperature, and soil and plant water availability during the daytime, and it results in dry matter production and accumulation by the plant. Minimum air temperature is the primary driver of nighttime plant maintenance respiration, and increases in minimum temperature can accelerate respiration, increase dry matter consumption for plant maintenance, and affect crop growth, development, and yield.
For irrigation management, warming in both daytime and nighttime temperatures increases crop water demand and tightens the margin for error in day-to-day irrigation scheduling. Under these conditions, over-irrigation can increase deep percolation and nutrient losses, complicate salinity management, and create canopy and soil conditions conducive to disease, while under-irrigation can induce crop water stress that reduces biomass accumulation and lowers marketable yield and quality. Increased air temperature may also increase soil evaporation and crop evapotranspiration, which may further limit freshwater availability for agricultural production (Mohammed & Irmak, 2022).
For crop performance, warmer conditions increase canopy water loss and intensify the consequences of poor irrigation timing. For IPM, warm conditions combined with inadequate irrigation alignment can increase transient crop water stress, indirectly affect pest dynamics, and tighten scouting and management windows.
Figure 2. Distribution and long-term trend in annual average maximum air temperature (Tmax) at the AZMet Yuma Valley station (Yuma Valley, AZ) for 1987–2025; the dashed line shows the linear trend, and the horizontal line indicates the long-term mean.
Figure 3. Distribution and long-term trend in annual average minimum air temperature (Tmin) at the AZMet Yuma Valley station (Yuma Valley, AZ) for 1987–2025; the dashed line shows the linear trend and the horizontal line indicates the long-term mean.
Implications for irrigation management
Despite extensive irrigation in Yuma, the adoption of soil moisture sensors remains low among lettuce and leafy green growers. Growers and crop advisers are increasingly requesting practical, field-friendly tools that can be used daily to improve irrigation and agronomic decision-making. The specific challenge is inadequate application of water conservation technologies in on-farm irrigation decisions and continued reliance on traditional scheduling approaches, which can contribute to inefficient water use and unnecessary environmental impacts under increasingly variable and demanding conditions.
Under warming and variable rainfall, irrigation management will increasingly depend on improving irrigation precision, reducing unnecessary water loss, and protecting crop productivity. Increased air temperature can increase soil evaporation and crop evapotranspiration, while variable rainfall can complicate irrigation timing and field access. These dynamics require improved and more carefully designed crop, soil, and water management practices developed locally for local production conditions (Irmak & Mohammed, 2023; Mohammed & Irmak, 2022). Existing management practices may not fully account for the impacts of changing climate variables. For growers and advisers, practical responses include using weather-based and soil-based information to improve irrigation timing, avoiding unnecessary irrigations following rainfall events, reducing over-irrigation that can increase nutrient losses and salinity management problems, and improving inseason irrigation adjustments during periods of elevated temperature and atmospheric demand.
Implications for crop performance
Changes in climate variables affect agricultural and agro-ecosystem productivity through both direct and indirect processes. Direct processes include increased air temperature and changes in hydrologic parameters such as precipitation, runoff, and stream flow. Indirect processes include changes in the intensity and frequency of disturbances from pests and diseases. In Yuma Valley, the combination of warming and variable rainfall means that crop performance will increasingly depend on how well management responds to greater sensitivity to irrigation timing and more frequent periods of crop water stress. Warmer conditions can increase canopy water loss, intensify the consequences of poor irrigation timing, and increase the likelihood of stress during periods of high atmospheric demand. Rainfall events remain important because they can alter soil surface moisture, stimulate germination, increase canopy humidity, and adjust field access and irrigation timing, even though rainfall is not the main source of crop water supply. For growers and advisers, this means that maintaining crop performance will increasingly depend on timely irrigation adjustments, close attention to short-term weather changes, and management practices that reduce crop stress during warm periods and after rainfall events. Improving crop, soil, and water management practices will be important for maintaining optimum productivity under changing climate conditions.
Implications for integrated pest management
For IPM, the most important message is that long-term climate trends do not act alone. Day-to-day risk remains primarily governed by crop stage, irrigation practices, canopy microclimate, and episodic weather. Rainfall pulses can rapidly activate weed germination in disturbed soils. Even modest rainfall can elevate leaf wetness and near-canopy humidity, creating short infection windows for moisture-sensitive pathogens. Rain-driven changes in host availability and microclimate can also influence insect survival, dispersal, and population growth. Warm conditions and poor irrigation alignment can increase transient crop water stress, which may indirectly affect pest dynamics and tighten scouting and management windows. In addition, irrigation method and timing, canopy density, and short-term weather events can create localized humid microclimates and wet leaves, even when the broader atmosphere is relatively dry. These conditions can favor foliar disease development and increase the need for careful field monitoring. For growers and advisers, practical implications include intensifying scouting after rainfall events, paying close attention to canopy humidity and leaf wetness periods, and coordinating irrigation management with pest and disease monitoring. The postrain window is often when scouting provides the highest informational value because conditions can shift quickly. Overall, the combined effects of warming and rainfall variability suggest the need for field-based IPM that is closely tied to irrigation practices, canopy conditions, and short-term weather windows.
Conclusion
The long-term climate signal in Yuma Valley is not a simple story of more or less rainfall. It is a story of gradual warming and highly variable rainfall. Annual precipitation does not exhibit a strong long-term linear trend, and any increase suggested by the trendline is minor relative to variability. By contrast, both annual average maximum and annual average minimum temperatures indicate gradual warming. Together, these trends indicate higher crop water demand and tighter irrigation margins in lettuce and leafy green production systems. The practical implication is that existing management practices may not fully account for the impacts of these changing climate variables. Improving irrigation precision, adjusting crop management to protect crop productivity, and strengthening field-based IPM tied to crop stage, canopy conditions, irrigation practices, and short-term weather will be increasingly important for sustaining lettuce and leafy green production in Yuma Valley.
Acknowledgments
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.
References
Arizona Meteorological Network (AZMet). (2026). Arizona climate data – Yuma Valley Station. University of Arizona. Retrieved from https://weather.arizona.edu, accessed April 26th 2026.
Irmak, S., & Mohammed, A. T. (2023). Maize nitrogen uptake and use efficiency, partial factor productivity of nitrogen, and yield response to different nitrogen and water applications under three irrigation methods. Irrigation and Drainage, 72(2), 375–392. https://doi.org/10.1002/ird.2868
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.2022.10779