Flood irrigation, also known as surface irrigation, is a method that relies entirely on the force of gravity to distribute water across a field. Water is introduced at the high end of the plot and flows over the soil surface to irrigate the crops. Because this system is non-pressurized, the effectiveness of the water application is directly controlled by the physical shape of the land. Terrain is therefore the single most limiting factor for achieving high efficiency and uniform water coverage with this ancient, yet widely used, technique.
The Ideal Landscape Requirement
The fundamental requirement for efficient surface irrigation is a highly uniform land surface that allows water to advance across the field predictably. Achieving this uniformity involves managing the slope in two directions: the primary flow direction and the cross-slope. The goal is to ensure a controlled flow velocity that minimizes erosion while still providing adequate drainage.
For graded irrigation methods, such as furrow or border-strip systems, the preferred longitudinal slope typically falls within a narrow range of 0.15% to 0.3% on deep alluvial soils. This slight grade is sufficient to move the water slowly but steadily without causing excessive scour. Slopes should generally not exceed 0.5% in most soil types to maintain water control and prevent significant runoff.
A slope that is too steep will cause the water to rush down the field, leading to severe soil erosion and insufficient infiltration time for the water to soak into the soil. Conversely, perfectly flat land, as is required for level basin irrigation, must be dead-level to prevent ponding and ensure even water application. Any unintended depression in a flat field will accumulate water, causing waterlogging, while a minor rise will remain dry and under-irrigated.
Practical Terrain Modification
Since naturally uniform terrain is uncommon, extensive engineering is required to prepare a field for efficient flood irrigation. This process involves land grading, which is the reshaping of the land surface to a planned, uniform grade for water application and drainage. This is a prerequisite for successful surface irrigation, especially for large-scale operations.
Modern land modification relies heavily on technology like laser-controlled leveling equipment or Global Positioning System (GPS) guidance systems. These tools are used to conduct detailed topographic surveys to map the existing elevations and calculate the necessary cuts (soil removal) and fills (soil addition) to achieve the design plane. The final goal is to create a constant, uniform slope with minimal variation across the entire plot.
Careful planning is needed to avoid excessive soil movement, as deep cuts can expose less fertile subsoil, negatively impacting crop yield. Engineers calculate a balance where the volume of cut soil slightly exceeds the volume of fill to account for soil compaction during the process. In cases where the natural slope is too steep for a single graded field, techniques like bench leveling, or terracing, are employed to create a series of flatter steps.
Impact of Undulating Terrain on Water Distribution
When a field’s terrain is undulating or poorly modified, the water distribution becomes highly uneven, leading to significant inefficiencies and agricultural problems. Water always follows the path of least resistance, concentrating in the lower areas and bypassing the higher spots. This uneven travel leads to a drastic reduction in water use efficiency.
High spots in the field receive insufficient water, resulting in under-irrigation and reduced crop growth in those specific areas. Conversely, the low spots accumulate excess water, leading to prolonged water ponding and the risk of waterlogging. Waterlogging starves plant roots of the oxygen they need, which can cause root death and crop loss, especially in crops that are sensitive to standing water.
The non-uniformity of the slope also causes the water velocity to fluctuate wildly as it moves across the field. Where the slope steepens unexpectedly, the increased flow rate causes scour, detachment, and transport of the topsoil, resulting in erosion. This soil movement not only degrades the field but also further exacerbates the unevenness of the terrain, creating a cycle of increasingly poor irrigation performance.