Furrow irrigation is a surface irrigation technique used globally for centuries, making it one of humanity’s oldest methods for delivering water to crops. This system relies entirely on gravity to move water across a field, unlike pressurized methods such as sprinklers or drip irrigation. Farmers create small, parallel channels, known as furrows, between rows of crops, allowing water to flow down the slope. The simplicity and cost-effectiveness of this method ensure its continued use worldwide, particularly for row crops that benefit from water application directly at the root zone.
Physical Components and Water Flow
The mechanics of furrow irrigation begin with the primary water delivery components, typically a head ditch (a shallow earthen channel) or a gated pipe (a portable pipe with adjustable openings), located at the highest point of the field. Water is diverted from the main source into this primary delivery system, which then distributes the flow across the top of the field.
From the head ditch or gated pipe, the water is introduced into individual furrows using small siphons, gates, or openings. This initial stage is known as the advance phase, where the water front rapidly moves down the length of the furrow, pulled by gravity. As the water flows, it begins to infiltrate into the soil, moving both downward toward the deeper root zone and laterally into the raised beds where the crops are planted.
The shape and size of the furrow influence this infiltration pattern. Wider, shallower furrows are often used in fine-textured soils like clay to maximize contact time. The goal is for the water to spread sideways sufficiently to wet the entire root zone of the adjacent crop rows. The flow time is carefully managed to ensure the lower end of the furrow receives adequate moisture before the flow is terminated.
Factors Influencing Furrow Design
The physical design of a furrow system is determined by static variables that must be engineered before the first irrigation event.
Land Slope
The slope of the land is a primary factor, as it dictates the speed of water flow and the potential for soil erosion. Most furrow systems are graded, meaning they have a slight, uniform slope, typically between 0.05% and 0.5%, to encourage water movement while preventing erosive velocities.
Furrow Length
Furrow length is another design variable, which is closely linked to the soil’s infiltration rate. In highly permeable, sandy soils, furrows must be kept shorter to prevent excessive water from soaking into the ground near the inlet. Conversely, on fine-textured soils with slower infiltration, longer furrows can be used. An optimal length is necessary because if a furrow is too long, the downstream end may be under-watered.
Initial Flow Rate
The initial flow rate, or stream size, put into each furrow must be carefully balanced to achieve a rapid advance without causing erosion. A larger stream size is initially needed to push the water quickly down the furrow, minimizing the time water spends at the inlet and promoting uniformity. The maximum non-erosive stream size is often estimated by dividing a constant, like 12.5, by the field’s percentage slope, ensuring the flow does not exceed the soil’s ability to resist detachment.
Controlling Water Uniformity and Runoff
Operational management during the irrigation cycle focuses on achieving uniform water distribution and minimizing water waste.
Timing and Uniformity
The advance phase, the time it takes for water to travel from the inlet to the end of the furrow, is a measurement. The time water is allowed to flow, known as the set time, is managed to ensure the water has enough “opportunity time” to infiltrate evenly along the entire furrow length.
The cutoff time is the precise moment when the water application is stopped. Stopping the flow too late leads to excessive deep percolation, where water sinks below the plant root zone and is wasted. A common guideline for maximizing efficiency is the “quarter time rule,” which suggests the advance time should be approximately one-quarter of the total water application time.
Runoff Management
Runoff management is necessary because a certain amount of water reaching the end of the furrow, known as tailwater, is often unavoidable for achieving uniformity. To prevent this tailwater from being lost, systems may incorporate a tailwater recovery system that collects the runoff in a ditch or sump. This collected water can then be pumped back to the head of the field for reuse in a subsequent irrigation set, significantly improving the overall water application efficiency.