COMMONLY ASKED QUESTIONS & ANSWERS
1. How does subsurface drainage promote better plant growth on poorly drained soils?
Subsurface or “tile” drainage removes excess water from the soil-water that prevents air and oxygen from getting to plant roots. Without artificial drainage, plants have difficulty establishing a healthy root system on poorly drained soils. Subsurface drainage provides the mechanism for these soils to drain to field capacity in a reasonably short period of time so that plant growth is not significantly impaired. In addition, drainage often permits spring field operations (e.g., tillage, planting) to take place in a more efficient and timely way. Depending on seasonal rainfall, this can have the effect of adding days, to a week or more, to the length of the growing season, providing another source of potential crop yield improvement.
2. Does subsurface drainage remove plant-available water from the soil?
No, drainage does not increase or decrease plant- available water in the soil profile. Drainage removes “drainable” water from the soil the same way a potted plant with a hole in the bottom of the pot drains after watering. Upward flow can occur in soil with tile drains, however, from the water table to the root zone, providing an important source of moisture for crop growth. On lighter (coarser) soils, placing drains too deep can limit this source of moisture.
3. Is groundwater or rainfall the source of subsurface drainage water?
In most situations, flow from drainage systems is shallow groundwater that is replenished by rainfall-the less rainfall there is, the less drainage flow there can be. In some cases, however, drainage systems are designed to intercept lateral flow.
4. Does subsurface drainage cause more water to leave the field compared to undrained conditions?
While not true for all cases and locations, in general, subsurface drainage may cause 10 to 15 percent more water to leave the field than agricultural land with surface drainage only. This number is based on drainage simulation models because variations this small are difficult to measure in the field due to high seasonal variability.
5. How does drainage influence surface runoff and flooding?
Rainfall provides water for surface runoff and infiltrating the soil. The route that water takes as it flows through the landscape plays a very important role in the amount and rate of total runoff, and this is affected greatly by land use. When natural vegetation is disturbed or converted into field crops and pasture, peak runoff rates at the field edge can increase dramatically. Often these conversions are accompanied by some surface drainage practices. In general, subsurface drainage tends to decrease surface runoff (sometimes one- to three-fold) and decrease peak surface runoff rates when compared to surface-drained or undrained land. The decrease occurs because water flows more slowly through the soil to reach the drainage system (and eventually the outlet) than it would as surface runoff. The later arrival of drainage flow may cause the overall peak outflow (surface + drainage) to decrease. Moreover, when the amount of runoff is reduced, the speed of its flow may also decrease. While these processes are well understood and documented at field and farm scales-flooding is a watershed-scale phenomenon. As we look at larger and larger landscapes, the increasing complexity of watershed hydrology makes it more difficult to make statements about drainage that hold true for all watersheds, at all scales and at all times. It can be said, however, that the potential for subsurface drainage to reduce peak flow rates at the field scale does not support the notion that subsurface drainage exacerbates flooding at larger scales. It should also be noted that most researchers agree that large-scale, basin-wide floods, are largely attributable to catastrophic precipitation, not the presence of subsurface or surface drainage systems.
6. What is meant by the “sponge effect” of subsurface drainage?
The combined effect of subsurface drainage and water removal by a healthy, deep-rooted crop provides for an increased storage capacity for water infiltration into the soil, compared to an undrained, high-water table soil. Depending on the timing, depth, and intensity of rainfall, more water has a chance to infiltrate a drained soil compared to a poorly drained soil. This increased storage capacity is often referred to as the “sponge effect.” The extent to which the effect is realized depends on soil type, crop, time of year, and both rainfall and soil moisture characteristics prior to and during precipitation. In poorly drained, high water table soils, subsurface drainage lowers the water table and increases the empty (air-filled) pore space available for infiltrating water. The deeper, healthier plant root structure that is promoted by drainage further enhances this effect by removing still more water from the soil profile, which creates more empty pore space. The combined effect of subsurface drainage and crop growth is most apparent from middle to late growing season.