FAQ - Drainage & Salinity

Q: How does capillarity affect soil water holding capacity?
A: One important characteristic of soil is its ability to hold water against the force of gravity and supply a portion of that water to plants. Much of this capacity is related to the number and size of pores and channels distributed throughout a soil. Some water can be held so tightly on polar surfaces in the soil that many atmospheres of pressure are required to force this water out. Plant roots must out-compete the forces that hold water in soil to survive, especially as more and more water is removed from the soil. However, much of this water would not even be in the soil in the first place without capillarity. All rainfall would drain rapidly from the soil and not be available for long-term use by many organisms nor would it be available for plant root uptake days or weeks after rainfall events. Most of the water available for plants in soil is that water categorized as capillary water.

Q: What role does soil play in water movement?
A: When water reaches the land surface as precipitation, it can seep downward through pores between soil particles. Soil is made up of tightly packed particles of many shapes and sizes. A high porosity soil has the ability to hold large amounts of water due to the presence of many pore spaces. If the pores are well connected and allow water to flow easily, the soil is permeable. The size and shape of clay particles along with the arrangement of the pores between these particles result in clay soils being relatively impermeable and resistant to infiltration. Sands and gravels allow more rapid infiltration due to their high permeability. The initial water content of the soil is also important. In general, water infiltrates drier soils more quickly than wet soils. The intensity of a storm, or the length of time during which precipitation occurs, can also influence infiltration. If rain or snowmelt reaches the soil surface faster than it can seep through the pores, then the water pools at the surface, and may run downhill to the nearest stream channel. This limitation on the soil's capacity to allow infiltration is one of the reasons why short, high intensity storms produce more flooding than do lighter rains over a longer period of time.

Q: Can I substitute poor-quality, salty water for low-salinity irrigation water as an alternative to limited or deficit irrigation?
A: The answer to this question is: 'it depends' – on what crop is being irrigated and its salt tolerance, how much has the crop been irrigated and with what quality of water before the 'salty' event, what are the assurances, amount, quality and timing of the next irrigation event, what is the growth stage of the crop being irrigated, what type of soil is the crop being grown on, what are the predicted or forecast weather conditions in the near future. In short – a relatively mature, salt-tolerant, perennial crop, grown on a deep, well-drained soil, with assurances of a good supply of comparatively salt-free water in the near future, could be irrigated with a relatively 'salty' water supply, if there was sufficient water and management to assure good leaching during the irrigation event. However, under most irrigated conditions, the answer to this question would likely be NO!

Q: Is there any reason you couldn't get a sufficient leaching fraction with a drip irrigation system?
A: There is absolutely no reason why you couldn't get a sufficient leaching fraction with a drip irrigation system. In fact, if your irrigation system management does not intentionally impose an irrigation regime which included consideration for a leaching fraction - or did not have the assurance that off-season rainfall would cause the necessary leaching - your system would fail within a matter of a few years. The failure would be the result of salinization of the soil within the root zone. The sustainability of any and all irrigation systems is dependent on adequate leaching. In short - without leaching the system will eventually fail due to salinization.

Q: Which irrigation system is likely to result in less salinization of soil?
A: Salinization occurs because salt is allowed to accumulate in the soil in the absence of adequate leaching, assuming the soil was not saline before irrigation began and assuming salinization did not occur as a result of a rising water table close to the soil surface. In most circumstances where salinization occurs, the net direction of water flow in the soil is upward, toward the soil surface. Salinization problems might be less with drip versus other irrigation systems because drip systems typically experience lower evaporation losses, which contribute to salt accumulation. When one looks at water loss in a cropped or planted system, the operative term is evapotranspiration, which is a combination of evaporation and transpiration. It is a well-known fact that evaporation constitutes 80-90% of ET shortly after sprinkler or surface irrigation and as the soil dries, the evaporation component rapidly decreases and the transpiration component rapidly increases. Eventually, transpiration accounts for 80-90% of the water removal from the system and evaporation accounts for every little water removal.

In subsurface drip systems (SDI) or buried systems, there is very little wetting of the soil surface and thus evaporation is minimal. You will still accumulate salt, but the amount of salt accumulation is less than in traditional systems because less water is evaporated. Generally, the net overall result is that less water is used in the drip system and thus less salt accumulates. If the drip system is on or above ground, there presumably is less water dispersed, and only immediately around the plant root zone - thus, again - less water distributed, less water evaporated, less water used.

Q: What are the 'field' conditions that make drainage necessary?
A: Drainage is often necessary to profitably grow most agricultural crops on soils with high water tables or where salinity is a hazard. Most soils grow best where the water table is more than 6 feet below the soil surface. Fields where water stands within six feet of the surface should be drained. (Western Fertilizer Handbook, 8th Edition, page 64).

Q: What role does drainage play in salt and salinity management?
A: Presence of excessive amounts of salt in the soil or soil water solution is a consequence of either: 1) poor or inadequate drainage, 2) inadequate leaching, or 3) a shallow or upward rising water table, all of which result in net upward water movement out of the soil, thereby leaving salts behind. Consequently, adequate drainage is essential to salt and salinity management. Drainage will help facilitate leaching of salts downward in the soil, while also lowering the water table. Reclamation of saline soils, those which have a conductivity of the saturation extract (ECe) greater than 4.0 dS/m and an exchangeable sodium percentage (ESP) less than 15%, can be accomplished by leaching with high quality irrigation water. Chemical amendments are usually not required. Successful reclamation requires adequate drainage and using the appropriate amount of water. (Western Fertilizer Handbook, 8th Edition, page 221).

Q: What role does drainage play in water conservation?
A: Drainage can play a role in water conservation in a positive way, while inadequate drainage or lack of drainage can play a negative role in water conservation in a number of ways. Poorly drained soils typically exhibit a water movement pattern whereby water from precipitation and/or irrigation infiltrates a shallow depth of soil. Subsequently, the net movement of water is upward to the atmosphere through either plant water use or evaporation. In poorly drained soils or those with shallow water tables, the predominant process by which water leaves the soil is evaporation – which is water lost for plant water use or back to the local hydrologic cycle. Another consequence of poor drainage and sustained water logging of soil is that plants grown on such soils often exhibit signs or symptoms of oxygen stress, which is often mistaken for drought. That being the case, unknowing irrigators are likely to attempt to resolve the apparent drought by additional irrigation – which leads to inefficient irrigation water use. A third consequence of poor drainage and sustained water logging of soil is that salt levels build up in the soil, causing stress to crops. Crops often manifest salt stress in a manner similar to drought stress – by wilting, accompanied by dark green tissue color. Unknowing irrigators are likely to attempt to reduce the ‘apparent’ drought stress through additional irrigation.

Q: How does salinity affect soil?
A: Salinity actually helps stabilize soil structure by strengthening the electrical bond between individual soil particles. Aggregate stability is increased and clay particles tend to form relatively stable aggregates in saline conditions. Note: salinity should not be confused with sodic conditions and consequences of sodic conditions should not be mistaken as a consequence of salinity.

Q: How does drainage relate to water conservation?
A: The efficiency of water use by agricultural crops is often quantified by a value termed 'water use efficiency' (WUE). Crop WUE can be calculated a variety of different ways, the most common one being the ratio of harvestable crop per unit of water use. For example: the water use efficiency of wheat production might be something like 7 bushels/acre/acre-inch of water use. Thus, a wheat crop which transpires the equivalent of 7 acre inches of water would yield 49 bushels of wheat/acre (7 bushels/acre/acre-inch x 7 acre inches=49 bushels/acre). In a poorly drained soil, some of the measured water use may actually be 'water loss' due to evaporation. Additionally, the ability of a crop to physiologically perform is impaired by saturated soil conditions and poor aeration status in the root zone. Under those conditions, WUE is likely to be less than under ideal conditions, i.e., good drainage. Thus, either more water is required to produce the crop or less of the available and crop-used water actually contributes to crop production. Basically, a crop growing on a poorly drained soil will have a low WUE, which means more water is needed to produce the crop.

Q: What causes salinity? What factors contribute to salinity?
A: In the simplest terms, salinity is the result of an accumulation of salts in soil. The source of salts in the soil is either water in or entering the soil, fertilizers, or weathering of soil and salts geologically accumulated in the soil. Irrespective of the source of the salts, the factor which contributes the most to salinity is pattern of water movement in the soil. If there are circumstances which have contributed salt, i.e., inflowing water, fertilizers, geologically sourced salts, lack of water movement or drainage through and below the root zone or depth of measurement in the soil is the single-most significant factor contributing to salinity.

Q: Are there amendments that can be added to soil or water to 'neutralize' salinity?
A: There are no amendments which can be added to soil or water to 'neutralize' salinity. Amendments (gypsum) may be added to salt-affected soils to alter soil structure, enhance water flow through the soil, and facilitate leaching of salts from the soil. However, the effectiveness of these amendments is dependent on good drainage.

Q: How does salinity affect plants?
A: Salinity reduces the 'availability' of water to plants. In essence, salinity creates a condition like drought. Plants uptake water from the soil by regulating their internal salt concentration to be higher than the salt concentration of the surrounding soil solution. The amount of energy expended by plants to take up water is directly proportional to the salinity of the soil solution – the higher the soil solution salinity, the greater the amount of energy required by the plant to take up water. This expenditure of energy is usually reflected in reduced plant growth. There is another way that the salts that are causing salinity can affect plants – by being toxic to the plants. Some plants are particularly sensitive to elevated concentrations of certain elements of salts.

Q: What regulatory or environmental restrictions apply to drainage of ag land?
A: Regulations and environmental restrictions applicable to drainage of agricultural land are generally specifically defined by state natural resource management agencies. Discharge of ‘drainage’ water may require a permit as part of the National Pollution Discharge Elimination System (NPDES). Any lands which qualify as 'wetlands', under provisions defined by the U.S. Army Corps of Engineers and the Natural Resources Conservation Service (NRCS), fall under very strict provisions which preclude drainage of such lands without appropriate offset or mitigation.

Q: What options or alternatives do I have if 'drainage' is not an option for land that is poorly drained?
A: Eventually, almost all poorly drained or non-draining soils in arid environments will become saline. Options for management are limited if it is not possible to install drainage. Efforts should be made to reduce the amount of water applied to the land – whether from irrigation or run-on – by construction of surface diversions or by reducing irrigation amounts of water. Other management options might include continuous cropping, planting of shallow-rooted grasses and salt-tolerant plant species.

Q: Water that escapes to the atmosphere (evaporation, transpiration) is lost from the watershed while water lost into the ground (drainage, deep percolation) is still in the watershed and may be available for later use. Should I be focusing on irrigation efficiency improvements that reduce losses to the atmosphere rather than losses to drainage or deep percolation?
A: Practically speaking, an irrigator should focus on irrigation improvements that are most cost effective for the farm operation.

Q: What is subsurface drainage?
A: Subsurface drainage can be simply described as buried pipelines that intercept water below the ground’s surface and direct it to a surface outlet. Subsurface drainage is often referred to as "tile" drainage because up to the 1970's clay or concrete tiles were used to construct the subsurface pipeline. The joints between the tiles allowed water to flow into the pipeline. Since the 1970's, most tile drainage uses corrugated plastic pipe that is perforated. Many homes with basements have plastic tile lines installed around the footings to control basement water seepage. In agriculture, tile drainage is used to control water table elevations in production fields. Many agricultural fields have areas where the water table rises to close to the surface before, during or after the growing season. Tile lines are typically buried 3 to 4 feet below the surface of the field to allow for normal tillage operation and promote full root development of the crop. The tile lines in the field are typically called laterals and a series of laterals empty into sub-mains that in turn convey the water to a main. The main discharges the water into a surface drain. A tiled field may have one or more outlet locations depending on topography and surface drainage in the area. In some places, the elevation of the outlet is higher than the elevation of the main and a pump station must be used to lift the water from the main to discharge into the surface drain. More information can be found at University of Minnesota Extension.

Q: Why do farmers install tile drainage?
A: There are two main reasons why farmers install tile drainage. 1) To control high water table situations that affect planting, cultivation, or harvest conditions and 2) to remove excess salt that accumulates in the soil. In the semi-arid and arid portions of the country, salt accumulation inhibits crop production and is often linked to rising water table conditions. Salt accumulation frequently affects the crop productivity, so a farmer may decide to tile a field to improve their yield.

Q: What does it cost to install tile drainage on a farm field?
A: Tile laterals, sub-mains and mains can be installed in a random pattern or in a parallel pattern. The random pattern is often used to tile only portions of a field where the parallel pattern is used to tile the entire field. A tile contractor will often bid a random pattern based on the length of tile line installed. If the whole field is to be tiled, a contractor will bid the project on a per acre basis. For a whole field tile system that is installed commercially, the costs can be $500 to over $900 per acre. Field costs are determined by the length of each lateral, spacing between laterals, length and diameter of submains and mains, topography, obstructions, and outlet conditions. Many farmers install their own tile and thus significantly reduce the cost of installation. More information can be found at University of Minnesota Extension.

Q: Are there water quality issues associated with tile drainage?
A: Where rainfall triggers tile flow events, the water that discharges from tile drains has some positive and negative quality aspects. Generally, phosphorus and sediment losses from tile drained fields are less than surface drained fields, while loss of nitrogen in the form of nitrate and other dissolved minerals may increase. Tile drains installed to control salinity will discharge water containing higher total dissolved salts for several years with the concentration decreasing each year after installation. Eventually, the discharge will reach an equilibrium value. Where tile drains are installed to control irrigation induced high water tables or salt accumulations, the discharge water will eventually have a similar water quality as the irrigation water, but may contain more nitrate. Over the last ten years, researchers have developed several methods that will reduce nitrate in water flowing from tile drains. These methods include controlled drainage, buried biofilters, reducing the depth and spacing of tile laterals, and improved design of surface intakes. Some of the organizations working on water quality impacts from tile drainage can be found at:

Agriculture Drainage Management Systems Task Force
Agriculture Drainage Management Coalition

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