Spray irrigation, including systems like center pivots and lateral move machines, applies water to crops through pressurized nozzles, mimicking rainfall. Efficiency is measured by the ratio of water successfully absorbed by the crop root zone compared to the total volume released by the system. Understanding this ratio is important for managing water resources and ensuring crop productivity.
How Irrigation Efficiency is Measured
The performance of a spray irrigation system is measured using specific technical metrics. The primary metric is Application Efficiency, which is the percentage of the water delivered to the field that is actually stored in the crop’s root zone for plant use. This value is directly affected by environmental factors like wind and evaporation, as well as by the system’s management and design.
A separate metric is Distribution Uniformity (DU), which measures how evenly water is spread across the irrigated area. High DU ensures all parts of the field receive a similar amount of water, preventing both over-watering and under-watering. Although distinct, high application efficiency requires good distribution uniformity. For overhead systems, DU is often calculated using the “low quarter” method, comparing the average depth in the driest 25% of the field to the average depth across the whole field.
Typical Efficiency Ranges for Spray Irrigation
The efficiency of spray irrigation varies significantly, depending on the system’s technology and the local operating conditions. Older or less sophisticated systems, such as high-pressure sprinklers and traveling gun systems, typically operate with application efficiencies ranging from 65% to 75%. These systems use higher pressures to throw water farther, which increases water loss to the atmosphere.
Modern center pivot and linear move systems generally achieve higher efficiencies, often ranging from 75% to 90%. The largest gains come from advanced technologies like Low-Energy Precision Application (LEPA) and Low-Elevation Spray Application (LESA) systems. These specialized systems can attain application efficiencies between 85% and 95%, or even up to 98% under optimal management. The wide range results from specific design choices and the level of management employed.
Environmental and Operational Factors Causing Water Loss
Spray irrigation systems face inherent loss mechanisms that reduce water available to the crop root zone. The primary cause is Evaporation, which occurs as water droplets travel through the air from the nozzle to the ground. This loss is greater in hot, dry climates, sometimes exceeding 10% in high-temperature, low-humidity conditions.
Another significant loss is Wind Drift, which is the physical displacement of water droplets away from the intended target area by air movement. This loss increases substantially as wind speeds rise and as the size of the sprayed droplets decreases. The system’s Operating Pressure also plays a role, as high pressure creates smaller droplets that are more susceptible to both wind drift and evaporation loss.
Runoff occurs when water is applied faster than the soil can absorb it, causing pooling and flow away. While more common in surface irrigation, runoff can still be a problem for spray systems, particularly those with high application rates like LEPA. Losses can range from 2% to 20% if runoff is not properly managed.
Modern Techniques for Optimizing Spray System Performance
Technological advancements and improved management practices optimize spray system performance. Low-Energy Precision Application (LEPA) and Low-Elevation Spray Application (LESA) systems drop nozzles closer to the ground or directly into the crop canopy. This reduction in travel distance significantly minimizes losses from wind drift and evaporation, contributing to high efficiency ratings.
System performance is improved through better irrigation scheduling, which determines precisely when and how much water to apply. Modern scheduling relies on real-time data from soil moisture sensors and weather monitoring to prevent over-application, runoff, or deep percolation. Applying smaller, more frequent amounts of water based on crop needs helps manage evaporative losses and ensures effective absorption.
The use of Variable Frequency Drives (VFDs) on pumps helps dynamically manage the system’s operating pressure. VFDs maintain the minimum pressure needed for optimal droplet size and uniformity. This conserves energy and limits the creation of excessively small, drift-prone droplets.