Overhead sprinkler systems distribute water by spraying it into the air, allowing it to fall onto the crop or landscape below. This delivery method exposes water droplets to the surrounding atmosphere, creating a risk for significant water loss before the water reaches the ground. Understanding the magnitude and causes of this inefficiency is a primary concern for resource management and the financial viability of irrigation operations. This loss involves a complex interaction between the irrigation equipment and the surrounding environmental conditions.
Typical Ranges of Water Loss in Overhead Systems
The total amount of water lost from an overhead sprinkler system is highly variable, depending on specific operating and environmental conditions. Under ideal conditions, the combined loss may be as low as 1 to 3 percent of the total applied water. However, under less favorable circumstances, total losses can escalate dramatically, sometimes reaching up to 20 percent or more during peak demand periods.
This overall inefficiency is comprised of two distinct forms of loss: true evaporation and wind drift. True evaporative loss occurs when the water droplet changes phase into water vapor while traveling from the nozzle to the ground. Studies suggest that under typical field conditions, this specific type of loss for droplets in flight is often less than 5 percent of the applied water.
Wind drift is a physical loss where water droplets are carried away by air currents and deposited outside of the intended irrigation area. This phenomenon is often the larger contributor to total air loss, sometimes accounting for 50 to 70 percent of the water that does not reach the target. The wide range of total water loss percentages is primarily due to the variation in climate conditions and the size of the water droplets produced by the sprinkler system.
Key Environmental and System Variables that Drive Evaporation
The rate at which water is lost is governed by atmospheric forces and the physical characteristics of the irrigation equipment.
Environmental Factors
Environmental factors like air temperature, relative humidity, and wind speed directly influence the atmosphere’s capacity to absorb moisture. Hotter air and lower humidity create a greater “thirst” in the atmosphere, leading to rapid evaporation.
A more precise measure of this atmospheric thirst is the Vapor Pressure Deficit (VPD). VPD is the difference between the amount of moisture the air can hold when saturated and the amount it currently holds. High VPD significantly accelerates the evaporation rate because the air is much drier than its potential maximum, making VPD a strong predictor of evaporative loss. Wind speed also increases both evaporative loss and drift by constantly replacing the layer of moisture-saturated air surrounding the droplet with drier air.
System Factors
System-based factors determine how exposed the water is to these environmental forces. Droplet size is an important factor, as smaller droplets have a much greater surface-area-to-volume ratio than larger ones. Smaller droplets heat up and evaporate much faster, and they are also more susceptible to wind drift.
The nozzle pressure of the sprinkler system is directly linked to the size of the droplets produced. Higher pressure generally atomizes the water into a finer mist, increasing the percentage of smaller, more vulnerable droplets. Furthermore, the height of the sprinkler head affects the total time the water is exposed to the atmosphere. A higher spray trajectory allows for a longer opportunity for evaporation and wind drift to occur before the water hits the ground.
Operational Strategies to Minimize Water Loss
Optimizing the timing of irrigation is one of the most straightforward ways to reduce atmospheric water loss.
Timing Optimization
Irrigating during periods of lower climate demand, such as late evening or early morning, minimizes the exposure of water droplets to the harshest atmospheric conditions. During these times, air temperatures are lower, relative humidity is higher, and wind speeds are typically reduced. This strategy can cut evaporative loss by as much as half compared to midday operation.
Equipment Adjustments
Adjustments to the sprinkler equipment can significantly increase the size of the water droplets, reducing their susceptibility to evaporation and drift. Operators can achieve this by using low-pressure nozzles or pressure-regulating heads to avoid the high pressures that create a fine mist. Selecting nozzles designed to produce a larger droplet size will ensure more water reaches the soil.
Lowering the physical height of the sprinkler head reduces the travel distance and time a droplet spends in the air, limiting its exposure to wind and high VPD. For center pivot systems, installing drop tubes with low-pressure spray heads brings the water closer to the crop canopy or even below it. Regular system maintenance, including checking for clogs and ensuring proper head alignment, also prevents fine streams or misdirected spray that can exacerbate evaporative loss.
Advanced System Design
For new installations or major upgrades, advanced irrigation designs offer the greatest potential for water savings. Systems like Low Energy Precision Application (LEPA) are engineered to apply water directly to the soil surface or just above it, drastically minimizing air losses. Converting to modern low-pressure sprinkler packages on drop tubes can also provide a substantial gain in efficiency compared to older, high-pressure overhead impact sprinklers.