What the Air Is Telling You: Understanding Vapor Pressure Deficit and Why It Matters in Yuma Agriculture

Vapor Pressure Deficit (VPD) is the difference between the amount of moisture the air can hold when fully saturated and the amount of moisture actually present in the air. In simple terms, VPD measures the atmosphere's demand for water. VPD is determined by both air temperature and relative humidity. As temperatures rise and humidity decreases, VPD increases. Conversely, cooler temperatures and higher humidity result in lower VPD values. Because VPD integrates the effects of both temperature and humidity, it often provides a better indication of crop water demand than either variable alone. Two days with the same air temperature can place very different demands on a crop if humidity levels differ substantially.

Among its many applications, VPD is particularly valuable for irrigation management. Higher VPD values increase transpiration, causing crops to lose water more rapidly through their leaves. As atmospheric demand increases, crops require greater water uptake from the root zone to maintain normal physiological functions. For this reason, VPD can help explain why crop water requirements may change from day to day, even when temperatures appear similar. Monitoring VPD alongside ET estimates and soil moisture measurements can improve irrigation scheduling and help growers maintain adequate soil moisture during periods of elevated atmospheric demand. When VPD remains high for several consecutive days, crops may experience substantial increases in water use. Understanding these conditions can help growers anticipate periods of increased irrigation demand and make more informed water management decisions.

VPD directly influences crop growth, productivity, and physiological performance. Under favorable conditions, plants maintain open stomata that allow carbon dioxide to enter the leaf for photosynthesis while water vapor exits through transpiration. This process supports plant cooling, nutrient transport, and biomass production. However, when VPD becomes excessively high, plants often respond by partially closing their stomata to reduce water loss. Although this protective mechanism conserves water, it also limits carbon dioxide uptake and photosynthesis. Prolonged exposure to high VPD conditions can reduce growth rates, decrease biomass accumulation, and negatively affect yield and crop quality.Conversely, extremely low VPD conditions can suppress transpiration and reduce nutrient movement within the plant. As a result, both excessively low and excessively high VPD conditions can influence crop performance.

VPD can also provide useful insight into environmental conditions that influence pest and disease development. Low VPD conditions are typically associated with higher humidity and longer periods of leaf wetness. These conditions may favor the development of fungal and bacterial diseases in susceptible crops. High VPD conditions, on the other hand, can increase plant stress and influence crop susceptibility to certain insect pests and other environmental stresses. While VPD alone does not predict pest outbreaks, it helps explain environmental conditions that affect crop health and pest dynamics. Integrating VPD information into crop monitoring programs can therefore support both irrigation and IPM decision-making

To better understand VPD conditions in the lower Colorado River region, daily weather data from the Yuma Valley AZMet station were analyzed from January 2020 through May 2026. The analysis revealed a remarkably consistent seasonal pattern (Figure 1). The 2020–2025 monthly mean VPD ranged from a low of 0.91 kPa during January and December to a peak of 3.78 kPa in July. Atmospheric demand increased steadily during spring, rising from 1.33 kPa in March to 2.50 kPa in May. The highest VPD values occurred during the summer months, with June through August averaging between 3.41 and 3.78 kPa.

For practical purposes, these seasonal patterns can be grouped into four production periods (Table 1). Winter months experience relatively low atmospheric demand, while summer months are characterized by extremely high evaporative demand. The difference in atmospheric demand between winter lettuce production and summer crop production is more than three-fold.

March 2026 provides an example of why monitoring current conditions remains important. The monthly average VPD reached 2.39 kPa, approximately 80% above the long-term March average of 1.33 kPa. Growers relying solely on historical expectations would have underestimated crop water demand during that period.

Table 1. Seasonal VPD conditions in Yuma

Perhaps the most remarkable aspect of these findings is not the magnitude of VPD itself, but the ability of Yuma growers to consistently produce high-quality crops under some of the most challenging atmospheric conditions for agriculture in North America. Summer VPD values in Yuma routinely approach 4 kPa, reflecting an environment where the atmosphere exerts an intense demand for water. Despite these conditions, Yuma growers continue to achieve exceptional productivity through efficient irrigation systems, precise water management, and decades of agronomic expertise. Their success demonstrates that agricultural productivity and water conservation can go hand in hand. Through innovation, adoption of advanced irrigation technologies, careful scheduling, and science-based management practices, Yuma growers continue to produce more crop per drop while maintaining the region's position as one of the most productive agricultural areas in the nation.

Data source: Arizona Meteorological Network (AZMet), Yuma Valley station, University of Arizona Cooperative Extension (azmet.arizona.edu).