Furrow irrigation is a traditional surface method where water flows by gravity down small channels, known as furrows, created between crop rows. The amount of water absorbed is not a fixed quantity; rather, it represents the portion of applied water successfully stored in the crop root zone. This absorption efficiency is highly variable, depending on the physical environment and the management of the system. It is constantly changing based on factors like soil type, field slope, and the irrigator’s actions.
Baseline Efficiency and Water Loss Mechanisms
The typical water absorption efficiency for conventional furrow irrigation ranges widely, often falling between 50% and 80%. Poorly managed systems can see efficiencies drop as low as 45%, while well-designed systems can achieve the higher end. This means that for every 100 gallons of water delivered, only 50 to 80 gallons are absorbed by the soil within the root zone.
The water that is not absorbed is lost through three primary mechanisms. The largest loss is deep percolation, where water infiltrates past the root zone into the groundwater, making it unavailable to the crop. Runoff, or “tailwater,” is the excess water that flows off the end of the field without fully infiltrating. A third, usually smaller, loss is evaporation directly from the wetted surface of the furrow.
Deep percolation loss can account for over 40% of the applied water, especially at the upstream end. This occurs because the water must run for a long time to reach the far end, causing the head of the furrow to receive a disproportionately long soaking period. Runoff is also significant when tailwater is not captured and reused. Controlling these three loss pathways increases the percentage of water absorbed.
Key Variables Controlling Water Absorption
The rate and uniformity of water absorption are determined by the fixed physical characteristics of the field. Soil texture is a dominant factor, as it dictates the infiltration rate, or how quickly water enters the soil profile. Sandy soils have large pore spaces and high infiltration rates; water is absorbed rapidly but moves straight down, increasing deep percolation losses. Conversely, clay soils have small pores, leading to a slower infiltration rate and more lateral water movement.
The slope of the furrow also plays a significant role by governing the speed at which water flows down the channel. Steeper slopes cause the water to advance quickly, reducing contact time and limiting the total volume absorbed, often resulting in higher runoff. Flatter slopes slow the water’s advance, allowing for a longer infiltration opportunity time at the head of the furrow. This improves absorption but can exacerbate deep percolation at the upstream end.
Furrow length is a primary determinant of absorption uniformity across the field. If the furrow is too long, the time difference between water reaching the head and the tail becomes excessive, leading to the head receiving far more water. The initial inflow rate, or stream size, must be large enough to push the water quickly across the field to ensure uniform coverage. An inflow rate that is too small results in slow advance and high deep percolation losses near the supply ditch.
Management Practices to Optimize Water Retention
Farmers can significantly increase the percentage of water absorbed by implementing targeted management practices. One effective technique is surge flow irrigation, which involves applying water in a series of on-off cycles rather than a continuous stream. The initial surge wets the furrow, and the subsequent off-period allows the soil surface to partially seal or consolidate. This reduces the infiltration rate for the next surge, allowing the water to advance much faster down the furrow. This greatly improves distribution uniformity and reduces deep percolation at the head of the field.
Reducing the length of the furrow runs is a direct way to mitigate uniformity problems caused by long advance times. Shorter runs decrease the time needed for water to reach the end of the field, minimizing the difference in infiltration opportunity time between the upstream and downstream ends. Another element is the use of tailwater recovery systems, which capture runoff from the end of the field in a pond or reservoir. This captured water is then pumped back and reused for subsequent irrigations, effectively turning a water loss into an absorbed volume.
Controlling soil surface conditions through compaction and maintenance is also important for managing the infiltration rate. For soils with very high infiltration, gentle compaction of the furrow bottom can help reduce the rate at which water enters the soil, ensuring more water reaches the end of the row. Conversely, using residue management or reducing tillage can help maintain a higher intake rate in soils that tend to seal or crust. These adjustments allow the irrigator to fine-tune the system to achieve a higher percentage of beneficial water absorption.