Flood irrigation is a surface irrigation technique where water is delivered to a field and flows across the land purely by gravity. This method is one of the oldest forms of agriculture, dating back thousands of years in ancient civilizations such as Mesopotamia and Egypt. It operates on the principle of letting water spread and saturate the soil as it moves across the surface. This system relies on low technology and minimal infrastructure, making it an accessible option for farmers across the globe.
The Mechanics of Flood Irrigation
A flood irrigation system begins with the delivery of a large stream of water to the field from a source like a canal, ditch, or pipe. Gravity moves the water from the supply channel, which is typically elevated, onto the agricultural land. The water then advances across the field surface, soaking into the soil as it progresses from the highest point to the lowest.
For the water to spread evenly and fully saturate the root zone, the field must be precisely leveled or graded. Without this land preparation, water will flow toward low spots, causing waterlogging in some areas and leaving high spots unirrigated. The goal is to achieve uniform infiltration by controlling the flow rate until the water source is shut off.
Variations in Application Methods
Flood irrigation covers several distinct methods, differentiated by how the field is physically configured to guide the water flow. These field layouts manage the water’s movement and containment across the plot. The three most common controlled surface irrigation methods are basin, border strip, and furrow irrigation.
Basin Irrigation
Basin irrigation involves dividing the field into small, level plots surrounded by raised earthen banks or dikes. Water is introduced rapidly into these basins and is contained until it has fully infiltrated the soil. This method is suitable for crops like rice that can tolerate standing water for extended periods.
Border Strip Irrigation
Border strip irrigation uses parallel earthen ridges to divide the land into long, narrow strips that have a slight, uniform slope. Water is applied from the higher end of the strip and flows down the slope as a thin sheet, guided by the borders. This technique is used for close-growing crops such as grains and forage crops.
Furrow Irrigation
Furrow irrigation utilizes small, shallow channels or grooves, known as furrows, created between the rows of crops. Water is directed down these furrows, and it seeps laterally into the soil to wet the root zone of the plants. This method is common for most row crops, including corn and vegetables, because it avoids wetting the plant stems and leaves directly.
Water Use and Efficiency
Flood irrigation is characterized by a low application efficiency, often ranging between 40% and 60%, which means a significant volume of applied water does not benefit the crop. The primary reason for this low efficiency is the substantial volume of water lost during the application process.
One major loss pathway is deep percolation, where water infiltrates below the plant root zone and becomes unavailable to the crop. Another substantial loss is surface runoff, which occurs when excess water flows off the end of the field and is not captured for reuse. This runoff can lead to the loss of topsoil and nutrients from the field. Additionally, a portion of the water is lost to evaporation from the large, exposed water surface before it has a chance to infiltrate the soil.
Despite its inherent inefficiency in water use, this method remains in widespread use due to several practical advantages. The system requires minimal infrastructure and has a very low initial setup cost compared to pressurized systems like drip or sprinkler irrigation. Furthermore, because it relies on gravity to move the water, the method requires little to no energy input for operation, which translates to low energy costs for the farmer. The simplicity of the system also makes it less complex to maintain and manage.