Flood irrigation is the oldest method of applying water to crops, relying on gravity to flood the land or channels between rows. This method is generally considered the least efficient when compared to modern pressurized systems. The performance of flood irrigation is highly variable, depending significantly on local soil conditions, field preparation, and the management practices employed by the irrigator. Efficiency rests on how well the water application is controlled and how much is lost before reaching the plant roots.
Quantifying Water Application Efficiency
The efficiency of any irrigation method is measured by its Application Efficiency, which is the percentage of water delivered to the field that is stored in the crop’s root zone for use. Traditional flood irrigation systems are typically rated with an application efficiency ranging from 40% to 70%. This means that 30% to 60% of the applied water is lost to mechanisms other than crop uptake. By comparison, high-efficiency systems like drip irrigation can achieve application efficiencies exceeding 90%.
A related metric is Distribution Uniformity, which describes how evenly the water is spread across the entire field. Poor uniformity is a major inefficiency in flood irrigation, causing some areas to be over-watered while others are under-watered. This uneven application often forces the irrigator to apply excess water to ensure the driest parts of the field receive enough moisture.
Factors Driving Water Waste
The first loss is Deep Percolation, which occurs when water infiltrates below the crop’s root zone. This water becomes inaccessible to the plant and is often lost to groundwater, sometimes leading to rising water tables and waterlogging. Deep percolation is particularly pronounced in sandy soils, where the high infiltration rate allows water to move rapidly past the root zone.
Another major mechanism is Runoff, where water flows off the end of the field into drainage systems. This loss is directly related to the field’s slope, the volume of water applied, and the rate of flow. If the inflow rate is too high for the soil to absorb, or if the field is not perfectly level, the excess water flows away without infiltrating. Poorly managed flow rates can also cause soil erosion, which further degrades the field’s ability to absorb water uniformly.
The third loss factor is Evaporation, which removes water directly from the soil surface into the atmosphere. While flood irrigation generally experiences less evaporation than some spray systems because the water is on the surface for a shorter time and the droplets are not atomized, surface water is still exposed. Water sitting in open furrows or basins, especially during hot, windy conditions, contributes to the overall water wastage before it can soak into the soil.
Modern Improvements in Surface Irrigation Management
The efficiency of surface irrigation can be significantly enhanced through modern management and technology. One of the most impactful improvements is Laser Leveling, which uses GPS-guided machinery to create a field with a precise, uniform slope or a perfectly flat basin. This precision grading ensures water spreads evenly and rapidly across the field, drastically reducing localized deep percolation and end-of-field runoff.
Another practice is Surge Flow, which involves applying water in intermittent pulses rather than a continuous stream. The initial wetting and subsequent brief drying periods cause fine soil particles to settle and “seal” the surface of the furrow. This temporary reduction in the infiltration rate allows the water to travel further down the field more uniformly, improving distribution and reducing the deep percolation losses at the head of the field.
To directly address the runoff problem, growers implement Tailwater Recovery Systems. These systems capture the water that runs off the end of the field in a pond or sump and then pump it back to the head of the field for reuse in the next irrigation cycle. By recycling the runoff water, the overall Application Efficiency can be raised substantially, sometimes bringing the system’s performance into the 80% range or higher.