Growing up on our family farm in Arizona, I remember dad (Pat Murphree) planning “night irrigations.” I also remember him always talking about how important the “night irrigator” was to our farming operation.

Night irrigators’ own experiences working the night shift bring back even more memories. They would tell us that, before soil monitors and other modern technology, one of the best ways at the time to tell when the water had reached the end of the rows at night was simple: after setting up a new “irrigation set,” you’d drive to the far end of the field, lie down in one of the rows with their head facing the ditch, and wait. The moment the water touched the top of your head, you knew it was time to get up, check the rest of the rows, and start the next set.

Only problem, if one is a heavy sleeper, they might end up drenched.

My brother, Brent, was one of the night irrigators. Too wound up to sleep, he wouldn’t sleep at the end of the row; instead, he’d listen to music or even take a swim in the ditch.

“I’d watch for shooting stars, look for snakes and even aliens – the aliens I never saw,” says Brent.

Cool Hours Irrigation, also called irrigation during cool hours or cool-season timing, schedules irrigation during the coolest parts of the day or night. If in the morning, primarily pre-dawn to shortly after sunrise, typically 2 to 8 am). After sunset, farmers will simply tell you it’s “night irrigation.”  The effort aims to minimize evaporation losses and maximize water reaching plant roots and soil.

In Arizona and other western states, Cool Hours Irrigation is a fundamental best practice in arid and semi-arid agriculture and landscaping, driven by high evapotranspiration (ET) rates, and most certainly during the summer months. Keep in mind, when plants need water, irrigation takes place around the clock as well.

Why It Matters in Arizona

Arizona’s low desert regions feature extreme summer heat (often >110°F/43°C), low humidity, high winds, intense solar radiation, and high ET. During Arizona’s mild winters in the valley, this problem is avoided.

Key benefits in Arizona:

  • Water conservation: Critical due to Colorado River allocation cuts, drought, and strict regulations. Efficient timing helps stretch supplies.
  • Better plant uptake: Plants absorb water more efficiently in cooler temperatures (stomata function better below ~90°F/32°C).
  • Lower costs: Less water pumped = lower energy bills; compliance with municipal time-of-day restrictions for homeowners (common in Phoenix metro, Tucson, Prescott, etc.).
  • Microclimate cooling: Irrigation provides a measurable cooling effect on land surface temperatures in central Arizona croplands and urban areas.

Even in urban settings, many cities enforce or strongly recommend watering windows (e.g., before 10 am or after 7–8 pm, or 8 pm–8 am) to encourage this practice. Drip irrigation systems are often exempt from stricter spray restrictions because they lose less to evaporation and are water right near the root system and even subsurface (below ground).

On the farm, I remember the drives around the farm and the microclimate cooling effect. A recently irrigated field of alfalfa was much cooler than driving by a fallowed field with no crop or water on it. With the pick-up windows rolled down, you could feel the difference from field to field.

What Arizona Farmers and Irrigators Consider

Farmers (especially in Yuma for lettuce/vegetables, central Arizona for cotton/alfalfa, or date farms) and landscapers follow these principles, often guided by University of Arizona Cooperative Extension, AZMET weather data, and other tools.

Core factors farmers always evaluate in farming practices:

  1. Timing/Window 
    • Best: Early morning (3–6 am or 2–5 am) when temperatures are lowest (~80–90°F) during the summer, winds calm, and evaporative demand is minimal.
    • Acceptable alternative: After sunset (e.g., 8 to10 pm).
    • Avoid: When they can, Midday/peak heat and strong wind periods.
    • Technology: Use real-time weather data or smart controllers to shift schedules dynamically. Modern-day soil probes are key to knowing and understanding soil moisture levels and then maximize irrigation timing. Pivot irrigation, for example, waters near to the ground to avoid excessive ET. Basin irrigation technology helps lower ET as well.
  2. Water Volume, Frequency & Depth 
    • Deep, infrequent watering promotes deep roots and drought tolerance (vs. shallow/frequent, which weakens plants).
    • Use cycle-and-soak: Multiple short cycles with soak time in between (prevents runoff on slopes or compacted/sandy soils common in Arizona).
    • Target root zone depth: Typically 6 to 12+ inches, depending on crop/plant.
    • Base amounts on ETc (crop evapotranspiration) = ETo (reference ET from AZMET stations) × crop coefficient (Kc). Tools like AZSCHED help calculate this for specific crops.
  3. Site & Crop-Specific Factors (Hydrozoning) 
    • Soil type (sandy loams drain fast ? need more frequent or adjusted timing).
    • Plant/crop water needs and rooting depth (group similar plants on same valve/zone).
    • Slope, exposure (south/west-facing needs more), microclimates, and shade.
    • Crop stage (e.g., young plants or shallow-rooted vegetables like lettuce need more consistent moisture; established dates or trees need deeper applications). Because our vegetable crops in winter enjoy the cooler temperatures, less evaporation takes place and why winter vegetables grown in Arizona are so important to our food system.
    • Seasonal adjustments: Higher needs in summer; reduce in winter/monsoon season when rains supplement.
  4. Technology & Monitoring 
    • Smart controllers, soil moisture sensors, or weather-based systems that automatically adjust for ET, rain, or temperature and can prioritize cool hours.
    • Regular system audits for uniformity, leaks, clogs (especially drip emitters), and pressure regulation.
    • Soil probes or sensors to verify moisture at root depth rather than guessing.
    • All of these irrigation technologies allow farmers flexibility and timing to maximize their water applications, and means managed irrigation wins.
  5. Other Practical Considerations 
    • Regulations & restrictions: Comply with local watering windows and drought stages.
    • Disease/pests: Early morning preferred for most crops; monitor for issues like powdery mildew in humid monsoon periods.
    • Maintenance: Clean filters, flush lines, check for damage—especially important in dusty Arizona conditions.
    • Economics & sustainability: Balance yield/quality with water savings; many programs incentivize efficient systems.
    • Labor/operations: Automated timers allow scheduling without manual labor in extreme heat.

Cool Hours Irrigation is just one of many tools’ farmers use to maximize their water use. On our farm, irrigation timing was a regular consideration and today’s irrigation technology means more efficiency.