Surface irrigation is the oldest and most widely used method for applying water to agricultural fields globally, relying entirely on natural forces for distribution. This traditional practice involves delivering water to the field’s edge, where it moves across the soil surface by gravity to wet the land and infiltrate the root zone. The technique is characterized by its simplicity, using the land itself for conveyance and distribution, often without the need for complex machinery. This method of water delivery has been used for over 6,000 years, making it one of the earliest engineering innovations in agriculture.
The Role of Gravity and Land Preparation
The operation of surface irrigation is governed by the principles of gravity, which dictates the flow of water across the soil surface. This reliance means that the topography of the field must be precisely managed to control the water’s movement and infiltration. Land grading or leveling is a necessary preparation step, ensuring a uniform slope or a perfectly flat surface, depending on the specific technique used. Without this careful preparation, water will naturally pool in low spots and bypass high spots, leading to inconsistent wetting and crop growth.
The effectiveness of the system is described by two key time-based concepts: advance time and recession time. The advance time is the duration required for the incoming stream of water to flow from the field’s inlet to its farthest end. Recession time refers to the period it takes for the standing water to drain or infiltrate completely after the water supply is shut off. Ideally, the difference between these two times should be minimal and uniform across the field. If the advance time is too slow, the area near the inlet will be exposed to water for a much longer “opportunity time,” resulting in excessive deep percolation losses and poor uniformity across the field length.
Primary Techniques of Surface Irrigation
The overarching method of surface irrigation is broken down into three primary techniques, each suited to different crops, soil types, and field slopes. Furrow irrigation is designed for row crops like corn, cotton, or vegetables, where water is applied in small, parallel channels created between the rows of plants. The water flows down these furrows, allowing plants to absorb moisture laterally through the channel walls and downward. This approach avoids wetting the entire soil surface, which helps reduce evaporation losses and minimizes soil crusting around the plants.
Basin irrigation, in contrast, involves dividing the field into small, level compartments or plots surrounded by low earthen embankments called dikes. Water is rapidly introduced to these basins until the entire area is covered, allowing it to pond and soak uniformly into the soil. This technique is particularly effective for crops that can tolerate temporary deep inundation, such as rice, or for orchards where a small basin is created around each tree. Precision land leveling is highly valued in basin irrigation to maximize the uniformity of water depth and infiltration across the compartment.
The third technique, border strip irrigation, uses long, sloping strips of land separated by parallel earthen banks or borders. Water is applied to the top of the strip as a sheet and flows slowly down the slope, guided by the borders. Unlike basin irrigation, border strips typically have a free-draining end, allowing excess water to run off or be collected. This method is often applied to closely spaced crops that are tolerant of sheet flow, such as alfalfa, small grains, or pasture, and is suitable for fields with a slight, uniform gradient.
Advantages and Operational Limitations
Surface irrigation remains widely practiced due to its inherent advantages, mainly centered on cost and simplicity. The initial capital investment for a surface system is typically low, as it requires minimal specialized equipment or piping compared to pressurized systems. It functions without external energy sources like pumps if the water source elevation is higher than the field, significantly reducing operational energy costs. The simplicity of the technique allows management to rely on traditional or local knowledge, making it highly accessible to small-scale farmers.
Despite its benefits, the method presents significant operational limitations, primarily concerning water use efficiency. Because water flows over the soil surface, a considerable volume can be lost to deep percolation near the inlet or as surface runoff at the field’s end. Achieving a high degree of water application uniformity demands substantial labor for continuous monitoring and management of the water flow rate. Significant land leveling or grading can be a substantial initial expense and labor investment, especially on uneven terrain. This method is also less suited for soils with very high infiltration rates, such as sandy soils, where water can soak in too quickly near the inlet before reaching the rest of the field.