F.A.Q. Frequently Asked Questions about Subsurface Drainage
1. Why are farmers installing subsurface drainage?
Subsurface drainage installation has accelerated in South Africa as well as other parts of Africa during the last 5 years. The recent interest in this practice is primarily due to seasonally high water tables. Soil salinity is also a problem in the arid irrigation areas and is related to water table behaviour and soil moisture. Subsurface drainage is a management practice that offers the potential to control and reduce salinity in poorly drained soils.
2. Do my soils have too much clay to install sub-surface drainage?
Subsurface drainage has been practiced successfully on a wide range of soil textures, from sandy to clayey. Coarser soils (silts and sands) can be drained with wider drain spacing, whereas finer soils (loams and clays) require narrower drain spacing. Soils with significant coarse silt or fine sand content may need a special “sand slot” pipe to prevent particle to enter the pipe. Soils in which shrinking/swelling clays or peat predominate, or soils that are sodic, may need special consideration with regard to tile drainage. Soils are classified sodic when the pH is in excess of 8.5 and the amount of sodium in the soil complex is much greater than the combined amount of calcium and magnesium.
3. Are my fields too flat to drain?
Level fields can be drained as long as minimum grades of 0.08 to 0.1 percent are maintained for pipe laterals and mains. A pipe at 0.1 percent grade has 1 meter of fall per 1,000 meter. On level ground, this means that the pipe depth would vary by 1 meter over 1,000 meter. A typical drainage system provides an outlet where tile can drain freely (by gravity) into a surface ditch.
4. How do I determine if a pump station is needed?
Where topography or depth of the outlet ditch does not allow for a gravity outlet, pumped outlets are used, provided a surface waterway exists to discharge the drainage water. A pumped outlet or “lift station” provides the lift required to get the drainage water from the elevation of the pipe to the ground surface or higher and into the receiving waterway. Pumped outlets increase the initial investment and operation/maintenance costs of the drainage system but may be economically feasible in many situations. A pumped outlet station includes a sump, pump, discharge pipe and usually an electric control panel. Important design features include the storage volume of the sump and capacity (flow rate) of the pump.
5. Am I experiencing negative effects from inadequate drainage on my farm, and how will sub surface drainage affect my overall farming operation?
A significant negative effect of inadequate drainage relates to the timeliness of spring and fall field operations. Inadequate drainage can delay spring field operations from days to weeks and interrupt field traffic patterns due to non-uniform drying of the field. Machinery traffic on soils that are too wet will cause increased soil compaction. Delays in planting mean a shorter growing season and fewer accumulated heat units for the crop. Once the crop has been planted, inadequate drainage can cause stunted and shallow root growth, and sometimes complete crop failure due to excess-water stress (lack of oxygen in the root zone). Planting delay, soil compaction and excess-water stress combined can translate into significant negative crop yield impacts. The magnitude of the yield impact for a growing season depends on crop and variety, soils and the season’s rainfall pattern.
6. Can the effects of salt build up in soils be mitigated with sub surface drainage?
Soluble salts may accumulate in the root zone during a period of years with high water tables. Studies have shown that leaching water through the profile and removing the salt via sub surface drainage will reduce the salt concentration in the root zone through time. Depending on seasonal rainfall or ability to irrigate, reducing the salt enough in high-concentration areas for optimum agricultural production may take a few years. This effect may occur more frequently in years with higher rainfall and may not occur at all in dry years. Reclaiming the land with a sequence of more tolerant crops such as barley is important before planting a salt-sensitive crop.
7. Will random or targeted sub surface drainage help control salt levels in saline seeps?
Saline seeps may occur where soil water from high land slowly seeps laterally to lower areas and carries dissolved minerals (salts) with it. If the water comes near, or seeps out of the surface in the low area, it may evaporate and leave the salts behind. Through time, salts can increase to an extent where the soil no longer can support crop growth. Draining these low areas, along with the side slopes, will lower the water table and, depending on the amount of precipitation, eventually will leach the salts. A targeted drainage system of relatively few drainage lines may be all that is needed to address a saline seep situation.
8. What are the economics of sub surface drainage for the crops that I produce?
The economics of drainage systems depend on crop yield response, initial capital investment for the materials and installation of the system, and any annual operation and maintenance costs (such as electricity for pumped outlets). Although crop yield response to drainage can be assessed directly, the impacts of inadequate drainage on soil quality (structure, microbial activity, etc.) are more difficult to measure and assign economic value. Many field crops show a positive response to drainage (on previously poorly drained soils), often with the best response from a combination of surface and sub-surface drainage. The level of yield increase for a given year depends greatly on how poorly drained the soil was prior to drainage, and the timing of seasonal rainfall. Research has shown that during many growth seasons, average yields may increase around 10 to 15 percent, depending on the aforementioned factors. Research on a clay loam soil has shown that wheat yield will be reduced by 42 percent and sugar beet yield will be reduced by 29 percent of potential yield when the water table stays 375mm to 500mm below the surface for extended periods during the growing season. In addition to yield increases associated with adequate drainage, operating expenses on the farm may be cut due to reduced cropping inputs, less fuel consumption, and timely field operations.
9. Will drainage stress my crop in dry years?
Sub surface drainage does not remove “plant available” water from the soil; it merely removes “gravitational” water that would drain naturally if unimpeded by confining layers in the soil. The greatest benefits of sub surface drainage typically are realized in wet years, but because drainage promotes deep root development, crops often will have better access to soil moisture in dry years. In general, where poorly drained soils exist, crop yields will be more uniform from year to year with sub surface drainage. Drainage control structures (also known as controlled drainage or drainage water management) can be installed to provide the potential for limiting the release of drainage water into the ditch and conserve more soil water in the root zone. Similarly, the pump in a lift station can be turned off when drier growing conditions become a concern.
10. Can I install a sub-surface drainage system myself or have a neighbour do it to reduce costs?
Do-it-yourself (DIY) drainage is certainly an option that is being considered by many farmers/ landowners. With good equipment, good design and the necessary commitment of time and resources, DIY drainage may be a sound option and may save on installation costs. However, like any other field operation, an investment in specialized equipment and knowledge is required for DIY drainage. Pipe depth and grade, pipe size and field layout are all extremely important in design and will determine the quality of performance of your system. Above all, making sure the drainage system is designed and installed properly is important so it will perform well for many years to come.
11. When do I need to use a fine/narrow-slot tile?
The need for a fine slotted, or narrower slots, on the drainage pipe depends on the soil texture in the region of the pipe depth in the field. Generally, poorly graded fine sands and coarse silts require the use of fine slotted pipe. In general, clay, silty clay, sandy clay, silty clay loam, silts and loams do not require envelopes due to their natural cohesiveness
12. What is “controlled” drainage or “drainage water management”?
Controlled, or managed, drainage systems incorporate structures that allow the producer/manager to raise the outlet elevation at strategic locations in the drainage system to control the release of drainage water and potentially maintain a shallower water table. Controlled drainage systems offer the potential to conserve soil water in the root zone and reduce drainage flows and the loss of dissolved nutrients (nitrogen and phosphorus) from the field. If the timing of rainfall is favourable, controlled drainage creates the potential to store water for drier periods during the growing season. One or more special control structures, or the pumped outlet itself, may be used to control the drainage system. Control structures utilize stop-logs or baffles to set the desired water table elevation at the location of the structure; a pumped outlet may be turned off to create the same effect. Considering the option of drainage water management in the initial design of the drainage system is important so that the layout of the system accommodates the goal of drainage management to the fullest extent and maximizes the effectiveness of the practice. Typically, fields with average field grade from 0 to 0.5 percent are best suited for the practice, but other factors such as field slope uniformity and access to control structure locations are important, too. A field that is nearly flat may require only one control structure (or a pumped outlet) to implement the practice, whereas a field with more grade may require several control structures. The benefit of drainage water management is that producers have one more tool to manage production risks.
13. Can I irrigate through the sub surface drainage system, or “subirrigation”?
“Subirrigation” is the practice of providing water to the root zone through a drainage water management system. If a source of irrigation water is available and the drainage system is designed appropriately, water can be introduced into control structures, special inlets or the sump of a pumped outlet to raise the water table and make water available to the crop. To make this practice work, a sufficient source of water is needed to supply the water needs of the crop. As with drainage water management, for this practice to be effective, the subirrigation system must be designed before installation of the pipe. A system designed for subirrigation generally will require closer drain spacing than a system designed only for conventional drainage.
14. Are there any water quality issues associated with sub surface drainage?
The water quality impacts of sub surface drainage are positive and negative. In general, when compared with surface drainage only, phosphorus and sediment losses via surface runoff are lower from sub surface drained fields, while losses of nitrate-nitrogen and other dissolved constituents in the root zone are greater. The extent of the increase or decrease of these constituents also depends on farm management practices, and the magnitudes of the losses are highly variable from year to year.
15. What is the relationship between sub surface drainage and downstream flow and flooding?
Sub surface drainage impacts on downstream flow and flooding have been the subject of much debate for more than a century. Some of the important factors that will determine the impact of sub surface drainage on downstream flow and flooding include soil types, rainfall amount and intensity, point of interest (near the field outlet or over a larger watershed), time frame of interest, existing soil moisture conditions, and the extent of surface drainage (including surface intakes) and channel improvements. Despite this complexity, the research on sub surface drainage and stream flow contain some areas of general agreement. For the poorly drained, low-permeability soils where sub surface drainage typically is used, sub surface drainage will lower the water table, which increases soil water storage capacity and infiltration. This reduces the amount of surface runoff and the peak flows coming from the field. Discharge from sub surface drainage occurs during a longer time period than surface runoff.