A Drier Atmosphere in Yuma Valley: Long-Term RH Trends and Their Implications for Irrigation Management, Crop Performance, and IPM
This article is the second is a series about conditions in Yuma Valley. Read part one, Long-Term Trends in Annual Maximum Temperature, Annual Minimum Temperature, and Annual Precipitation.
Relative humidity (RH) is a microclimate variable that strongly shapes crop water relations, canopy wetness duration, and the comfort zone for many pests and pathogens. In leafy greens, RH influences transpiration rate, stomatal behavior, and leaf-surface wetness, which are tightly linked to crop stress and disease favorability. Annual summaries from the Yuma Valley station show that maximum, minimum, and average RH have declined over time, even though year-to-year variability remains evident. This pattern indicates a gradual increase in atmospheric drying power. For production and management in Yuma Valley, declining RH implies higher atmospheric demand for water, greater crop sensitivity to irrigation timing, faster canopy water loss during warm and dry periods, and changing pest and disease risk conditions. The overall message is that declining RH increases the importance of precise irrigation management, crop protection, and field-based IPM under a drier atmosphere.
Arizona’s lettuce and leafy green industry is a cornerstone of specialty crop production and is concentrated in Yuma County, which supplies a large share of U.S. winter leafy vegetables. Yuma County’s agribusiness sector has been described as a multi-billion-dollar industry and a foundation of the county's economy. Lettuce and leafy greens in Yuma depend on reliable irrigation supplies from the Colorado River, and increasing uncertainty in Colorado River water availability, combined with rising salinity pressure under high evaporative demand, creates an urgent need to strengthen irrigation precision while protecting the environmental conditions that support specialty crop productivity.
Long-term relative humidity trends
Relative humidity is a microclimate variable that strongly shapes crop water relations, canopy wetness duration, and the comfort zone for many pests and pathogens. In leafy greens, RH influences transpiration rate, stomatal behavior, and leaf-surface wetness, which are tightly linked to crop stress and disease favorability. Annual summaries from the Yuma Valley station (AZMet-Yuma Valley Station) show that maximum, minimum, and average RH have declined over time, even though year-to-year variability remains evident (Figures 1, 2, and 3). Annual maximum RH declined at about 0.99% per decade, annual minimum RH declined at about 1.2% per decade, and annual average RH declined at about 1.3% per decade. In recent years, they have more frequently sat at or below the historical baseline. This pattern indicates a gradual increase in atmospheric drying power. The relatively low coefficients of determination indicate that substantial year-to-year variability remains, and that factors beyond the linear trend contribute to variation in relative humidity over time.
Implications for irrigation managemen
For irrigation management, a gradual decline in RH implies higher atmospheric demand for water, which can increase crop transpiration and intensify the need for precise irrigation management. Increased atmospheric drying may increase crop evapotranspiration and may further limit freshwater availability for agricultural production (Mohammed & Irmak, 2022). These dynamics require improved and more carefully designed crop, soil, and water management practices, developed locally for local production conditions. Existing management practices may not fully account for the impacts of changing climate variables. Under a drier atmosphere, improving irrigation precision will be increasingly important for protecting crop productivity and conserving limited water resources.
Figure 1. Distribution and long-term trend in annual maximum relative humidity (RHmax) at the AZMet Yuma Valley station (Yuma Valley, AZ) for 1987–2025; the dashed line shows the linear trend, and the yellow dotted line indicates the long-term mean.
Figure 2. Distribution and long-term trend in annual minimum relative humidity (RHmin) at the AZMet Yuma Valley station (Yuma Valley, AZ) for 1987–2025; the dashed line shows the linear trend, and the yellow dotted line indicates the long-term mean.
Figure 3. Distribution and long-term trend in annual average relative humidity (RHavg) at the AZMet Yuma Valley station (Yuma Valley, AZ) for 1987–2025; the dashed line shows the linear trend, and the yellow dotted line indicates the long-term mean.
Implications for crop performance
For crop performance, lower RH can translate into faster canopy water loss during warm, dry periods, greater sensitivity to irrigation timing, potential for physiological stress when high temperature coincides with low RH, particularly during rapid canopy expansion, and more rapid drying of leaf surfaces after irrigation events (Irmak &Mohammed, 2023). Changes in climate variables impact agricultural and agro-ecosystem productivity and land surface–atmosphere relationships through various direct and indirect processes. Direct processes include increased air temperature and changes in hydrologic parameters. Indirect processes include changes in the intensity and frequency of disturbances of pests and diseases. In Yuma Valley, declining RH means that crop performance will increasingly depend on how well management responds to faster canopy water loss and greater irrigation sensitivity under a drier atmosphere.
Implications for integrated pest management
For IPM, declining ambient RH suggests a drier background atmosphere, which may reduce baseline favorability for some foliar diseases. One example is lettuce downy mildew, a disease that requires damp, cool conditions and moisture on leaves for infection and symptom development. However, irrigation method and timing, canopy density, and short-term weather events can still create localized humid microclimates and wet leaves. Lower RH combined with warm conditions can also increase transient crop water stress if irrigation is not aligned with crop demand, indirectly affecting pest dynamics and tightening scouting and management windows. Overall, the long-term RH decline is best viewed as a shift toward greater atmospheric drying power, while day-to-day IPM risk remains primarily governed by crop stage, irrigation practices, canopy microclimate, and episodic weather rather than the trend alone.
Conclusion
A drier atmosphere in Yuma Valley is becoming an important production and management issue. Annual maximum, minimum, and average RH have declined over time, indicating a gradual increase in atmospheric drying power. For irrigation management, this means higher atmospheric demand for water and a greater need for precision. For crop performance, this means faster canopy water loss, greater irrigation sensitivity, and greater potential for physiological stress when warm conditions coincide with low RH. For IPM, this means that disease favorability may shift, but irrigation practices, canopy conditions, and short-term weather events will continue to govern day-to-day risk. The practical implication is that improving irrigation precision, adjusting crop management, and strengthening field-based IPM will be increasingly important under a drier atmosphere 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.2022.107795