Gravity irrigation, also known as surface irrigation, is one of the oldest and most widely used methods for watering agricultural fields. This technique relies entirely on the natural force of gravity and the land’s slope to move and distribute water across the soil surface. By harnessing the basic physics of water flowing downhill, it provides a cost-effective and energy-efficient means of delivering water to crops, requiring minimal or no mechanical pumping.
The Core Principle of Gravity Flow
The physics underlying gravity irrigation centers on converting the water’s potential energy into kinetic energy and pressure. This conversion is achieved through a difference in elevation, often referred to as “head.” Water is delivered from a source, such as an elevated reservoir or a canal intake, positioned higher than the field being irrigated. As the water travels downward, gravity pulls it across the field’s surface, and careful land grading and slope manipulation are prerequisites for effective distribution and uniform infiltration.
Key Methods of Surface Irrigation
The application of gravity-fed water is managed through several distinct surface irrigation methods, each suited to different crops and land conditions. Furrow irrigation directs water into small, shallow channels that run parallel to the crop rows. Crops are planted on the ridges between these furrows, and the water infiltrates laterally and downward to the root zone. The speed of the water flow is controlled by the slope and the rate of inflow, which must be managed to prevent excessive runoff.
Basin irrigation is best suited for flat or nearly level land, often utilized for crops like rice or orchards. The field is divided into plots enclosed by small earthen walls called bunds or levees. Water is released into the basin and allowed to pond across the entire surface until it is absorbed into the soil. This method requires precise land leveling to ensure the entire area receives a uniform depth of water.
Border strip irrigation divides the field into long, gently sloping strips separated by parallel earthen dikes. Water is introduced at the high end of the strip as a sheet flow and progresses down the length, guided by the dikes. The length and slope are designed to ensure the water reaches the far end with minimal runoff while achieving sufficient infiltration time. This method is effective for close-growing crops like alfalfa and various grains.
Infrastructure and Water Control
Implementing a functional gravity irrigation system requires a network of structures to manage and regulate the water’s path. Water is first conveyed from the source through main supply canals or primary ditches, which carry large volumes of water to the farm area. From these main canals, smaller field ditches branch off to bring the water directly to the individual plots or application points.
Precise control of water flow is achieved through structures such as control gates, also known as turnouts. These gates are installed at the junctions of the main and field ditches to divert the desired amount of water into a specific area. Check dams, or checks, are small barriers placed within the ditches to raise the water level, allowing it to flow out over the ditch banks and into the field. For certain application methods, temporary structures such as siphon tubes or spiles may be used to draw water over the ditch bank and into the field.