Modern agriculture relies on irrigation to supply crops with the necessary moisture for growth, especially in arid or semi-arid regions. Managing the resulting water balance is an equally important consideration for long-term farming sustainability. When too much water is applied, or natural precipitation is high, the excess moisture must be systematically removed from the soil profile. This regulated removal process, known as agricultural drainage, maintains optimal soil conditions for plant health.
Defining the Irrigation Drain
An irrigation drain, frequently termed a field drain or agricultural drain, is a planned conduit system designed to collect and safely remove surplus water from the soil profile. These systems regulate the water table, which is the upper level of the ground saturated with water, keeping it below the root zone of cultivated plants. Unlike irrigation canals, which deliver water to the fields, drains are engineered to manage water output and prevent soil saturation. The infrastructure can range from simple open ditches to complex networks of underground piping. The primary goal is to manage the soil’s moisture content, ensuring roots have access to both water and air for healthy growth.
The Function of Water Drainage
The most direct function of agricultural drainage is the prevention of waterlogging, a condition where the soil pores become completely saturated with water. When the water table rises into the root zone, it displaces the air, effectively suffocating the plant roots and halting their ability to absorb nutrients and water. Prolonged waterlogging quickly leads to anaerobic conditions, causing root decay and ultimately resulting in significant crop yield losses.
Drainage also plays a fundamental role in controlling soil salinity, particularly in irrigated areas where salts accumulate over time. Irrigation water naturally contains dissolved salts, and when this water evaporates from the soil surface, the salts are left behind, concentrating to potentially toxic levels. This process is exacerbated when a shallow water table allows water to rise near the surface through capillary action, carrying even more salts with it.
By lowering the water table, drains facilitate the leaching process, where fresh irrigation water is used to dissolve the accumulated salts and flush them down and out of the root zone. This allows the salts to exit the field through the drainage system, maintaining a healthy, low-salinity environment for crop growth. Furthermore, the removal of excess water restores soil aeration, permitting oxygen to re-enter the soil structure, which is necessary for beneficial microbial activity and healthy root respiration.
Different Types of Drain Systems
Agricultural drainage systems are broadly categorized based on their placement relative to the soil surface. Surface drains consist of shallow, open ditches, or graded land designed to quickly remove excess water that accumulates on the field surface from heavy rainfall or irrigation runoff. These open channels are relatively simple and inexpensive to construct and are highly effective for managing surface runoff, especially on fields with poor infiltration rates.
However, open drains occupy productive land area, interfere with mechanized farming operations, and require regular maintenance to prevent the buildup of weeds and debris.
Subsurface Drains
Subsurface drains, often referred to as tile drains, are buried networks of perforated pipes or conduits installed below the soil surface at a specified depth. These closed systems work by intercepting groundwater and lowering the water table to a predetermined, safe level, thereby regulating the moisture content in the root zone. Modern subsurface drains typically use corrugated plastic tubing, which is much easier to install than the clay tiles used historically.
While the initial cost is higher and installation is more complex, subsurface systems allow for the full utilization of the field surface and require less frequent maintenance than open ditches. Subsurface drainage is generally the preferred remedial measure when salinization is a major concern, as it directly supports the leaching process. The choice between surface and subsurface drainage often depends on the soil type, topography, and the specific water management goals of the farm.
Operation and Management of Drains
The operation of a subsurface drain system begins when the water table rises above the level of the buried pipes. Water enters the drain through small perforations or seams and then flows by gravity toward a larger collector line or main drain. This collected water, often termed tailwater or effluent, is then transported out of the field and ultimately to a designated disposal point.
The disposal of drainage water presents a significant management challenge because the effluent often contains elevated concentrations of salts, agrochemicals, and nutrients like nitrates and phosphates. In arid irrigated areas, subsurface drainage water has a higher salinity than the original supply water, reflecting the characteristics of the soil through which it has percolated. Due to potential environmental impacts, such as the risk of polluting downstream water bodies or affecting sensitive ecosystems, careful management is required.
In many areas, this water is routed to large drainage basins, rivers, or, in some cases, specialized evaporation ponds where the water dissipates and the salts are left to accumulate. Evaporation ponds, however, can pose risks to wildlife, as seen with issues related to selenium in the San Joaquin Valley. Modern management practices may include treating this water or reusing it for salt-tolerant crops when the quality permits.
Proper management also involves routine maintenance to ensure the system remains functional and efficient. Open surface drains require regular cleaning to remove sediment buildup, weeds, and debris that can impede the flow of water. Subsurface systems may need periodic flushing, known as ‘jetting,’ to clear blockages within the pipes caused by fine soil particles or chemical precipitates like iron ochre.