The soil drainage class and some characteristic features associated with each class are depicted in the following figure (from Soil Survey). One characteristic feature in the figure is the depth of rooting that typically occurs in each drainage class, providing there are no other restrictions (i.e., compacted layer) to root penetration. Deeper rooting depths are associated with well drained soils, because the depth of the water table below the surface is not restricting root growth and oxygen exchange. Although not all plant species respond the same, for most common agricultural crops, a deeper and healthy root environment translates into higher biomass productivity. Studies in New York have shown 2 to 3 fold yield increases in corn and forage production on well drained soils as compared to those grown on somewhat poorly to poorly drained soils. Another major benefit of well drained soils that affects productivity is that the soil is more conducive to timely tillage operations without creating structural damage. This is particularly important in the Northeast, where the seasonal moisture distribution causes soils to be wet in the spring and fall, and the growing season is limited.

 image source: NRCCA Soil and Water Management Study Guide

The soil drainage classification is based on a variety of physical, chemical and biological indicators and interactions that occurred over a long time period. From a hydrology perspective, the depth of the water table below the surface, the extent of the rise and fall (fluctuation) of the water table, and the time of duration that the water table is near the surface are important indicators. The water table may reach the surface of a well drained soil, but if this happens very infrequently, it doesn't cause it to be a poorly drained soil.

A very poorly drained soil classification arises out of the criteria that the water table not only reach or even exceed the surface (flood), but also that it remain close to the surface for the entire year duration, even during the summer months when evapotranspiration rates are high.  A poorly drained soil on the top of a hill is usually caused by the presence of some impermeable underlying layer. Nevertheless, poorly drained soils are more likely to be found at the base of hills and in low-lying areas of a watershed where water collects.

Long term anaerobic soil conditions reduce manganese and iron which results in grey/green/blue staining of soil peds as shown in the poorly drained classification. Where the water table rises and falls, the soil goes through brief periods of anaerobic and aerobic conditions respectively, which cause iron to oxidize and produce the red/brown/yellow/orange staining (mottles) on soil ped interfaces as shown in the photograph below.

1. Surface Drainage
2. Subsurface Drainage
3. Random Layout
4. Pattern Layout

Surface Drainage

Surface drainage is the shaping, grading, or management of the land surface to provide gradual removal or diversion of water off of the land surface. Surface drainage is accomplished by smoothing out small depressions (land smoothing) or regrading an undulating land surface to a uniform slope, and directing  water to a natural or improved, constructed channel. Ridge tillage is a form of surface drainage, providing excess water that accumulates between the ridges can flow away. Soil aeration or coring is also a form of surface drainage if it facilitates infiltration of water into an unsaturated subsoil. Surface drainage refers to the orderly removal of water, both within a field or to the removal of excess water off site.

Advantages of surface drainage are to minimize the duration of ponded water that inundates crops, and to minimize the prolonged saturation of soil which restricts gas (oxygen and carbon dioxide) exchange with the soil and plant root system or which prevents cultural operations. Surface drainage is most advantageous on flat lands where slow infiltration, low  permeability, or restricting soil layers prevent the ready infiltration of high intensity rainfall.

A disadvantage of surface drainage is that it has a minimal affect on reducing the saturated subsoil occuring as a result of high water table conditions, especially where the source of the water is emerging from lower horizons. Other disadvantages are that if the  water is not removed in an orderly manner, soil erosion may occur, and nutrient and other contaminants may be carried off in the runoff. Phosphorus and many herbicides are normally bound near the soil surface, and these may be transported in the surface drainage water.

Subsurface Drainage

Subsurface drainage is the removal of excess drainable porosity water in the subsoil, with the aim of lowering or controlling the water table depth below the crop root zone. Subsurface drainage is usually implemented with the use of buried corrugated (and perforated) plastic or clay (tile) conduits, but it can be done also by creating an unlined pore (mole drain), constructing blind (or French) drains, excavating deep open drains, or by the use of tubewells (shallow groundwater wells).

A subsurface drain must be installed below the water table (so water can flow from higher to lower energy state) or it will not work. Once the water table drops to the same elevation as the drain, the drain will no longer flow. The primary advantage of subsurface drainage in humid regions is the water table can be lowered so soils classified as poorly drained can be improved to respond more like well drained soils, with the benefits of improved productivity and trafficability. In arid regions, the advantage is mainly to minimize the buildup of excess salinity in the crop root zone.

A disadvantage of subsurface drainage is that its often more costly to implement per unit area compared to surface drainage, especially for fine textured soils. Also, if water ponds on the surface because of surface sealing or a shallow compact layer (plowpan, fragipan), subsurface drainge is not effective in removing this excess water. The environmental disadvantages related to drainage implementation are poorly drained, wetland type habitats may be modified, and the drainage discharge water may carry unacceptable contaminants. Since subsurface drainage lowers the water table and facilitates aerobic soil conditions, nitrification is enhanced and high nitrate concentrations may occur in the drain discharge water.

Random Layout

A random layout refers to the irregular pattern in which surface and subsurface drainage systems are implemented into the landscape. A random layout mimics and takes advantage of natural drainage patterns, and thus surface ditches and/or subsurface drains are randomly arranged in depressional topography to improve the wettest areas of the landscape. Random systems are well suited to undulating landscapes, where the higher areas of the topography are already adequately drained. Random systems are less costly to install per unit land area improved, and generally facilitate more efficient cultural operations by reducing turns (around wet areas), allowing for larger areas to be managed as a single unit. A disadvantage of a random layout is that it may not adequately and uniformly drain the area.

Pattern Layout

A pattern layout is a well organized regular spacing of surface and/or subsurface drains across an area. The pattern can be parallel or herringbone shaped (at angles to a slope, channel, or field boundary). Pattern layouts are well suited to long, uniformly sloping fields where the land slope is generally less than 5 to 8%. In land slopes of 3% or less, the pattern can be oriented in any direction to take advantage of optimal field shapes. A pattern layout provides more uniform drainage improvements, but will be more costly per unit land area improved.

Two types of pattern layout drainage systems: a simple parallel system and a herringbone pattern.

1. Location of Bedrock
2. Topography
3. Organic Soils
4. Type of Crop
5. Outlet

Location of Bedrock

Bedrock that is massive and unfractured creates an impermeable boundary which can restrict soil drainage. Soils that are shallow over this type of bedrock will saturate quickly, producing interflow and surface runoff. When this type of bedrock is within less than three feet of the surface, subsurface drains are ineffective and difficult to install.

Subsurface drains can be beneficial if this type of bedrock is deeper than three feet, and does not interfere with installation. Surface drainage can be used to facilitate the removal of ponding or shallow perched interflow water.

Topography

Slope or changes in topography tend to facilitate the overall drainability of soils as excess water has an opportunity to flow to lower elevations. Soils on convex type slopes are often better drained than those within concave slopes (particularly those at the toe of the slope) because rain and excess water tends to be dispersed rather than concentrated in the landscape.

The topography has a significant influence on the type of drainage systems and methods used because the installation of a drainage system by necessity also must have a place to dispose of or discharge the water that is collected. Since excess water collects at low points in the topography, the drainability of soils in this position is also made worse because outlets may be difficult to find for the water. Dikes and the use of pumping systems may be the only alternative remaining to enhance soil drainage in low lying areas. Thus, topography determines the layout of a drainage system (i.e., interceptor, random, pattern), and the outlet situation (i.e., natural, constructed open ditch, or pump).

Organic Soils

The internal drainability of deep organic soils is rarely a problem because of their prevalence for large pores and high drainable porosity. Some organic soils are shallow over an impermeable clay or marl, which may restrict the internal drainage. However, the larger difficulty with draining most organic soils in the Northeast is usually a result of their landscape position.

Organic soils have mostly developed in low lying parts of the landscape where the natural drainage of excess water away from these areas is minimal.  Since organic soils are most often used and developed for growing high value agricultural crops, more intensive drainage systems are installed to reduce water stress risk. Drains are often installed at closer spacing, and the design is adjusted to remove larger volumes of water in short periods of time.

Type of Crop

The type of crop and its sensitivity to poor drainage conditions establishes the criteria of whether drainage improvements are needed, and to what extent. The rooting depth of a crop is one important factor, as shallow rooted crops may be able to withstand water tables closer to the surface.

For a soil of the similar drainability, one crop may do well whereas another can't compete. This is often observed when a mixed forage seeding of alfalfa, birdsfoot trefoil, and timothy are grown.

Outlet

A soil may have a high drainable porosity, but its drainability depends on whether or not this gravitational water has a place to flow. Impermeable layers or a water table within the soil determine whether the drainable water will flow vertically downward, or laterally through the soil. The impermeable layer and water table also need to have some slope, or the water may not move laterally either.  

Without an outlet of some sort, soils saturate, water accumulates, and eventually landscapes flood. In order for a subsurface drain to lower the water table in the soil, the outlet of the pipe has to discharge to a water surface elevation that is below the elevation of the water table in the soil. If this cannot be achieved by gravity utilizing changes in topography, then pumps are required.

Photo courtesy of NRCS

http://photogallery.nrcs.usda.gov

The major positive environmental effects and concerns with drainage implementation are:

• Increased productivity and value of land – wet soils can be used for more intensive land usesImproved trafficability, timeliness, and cost efficiency
• Reclaim soils laden with salts (in arid regions) or to divert or collect water from other potential contaminant areas (barnyards, septics)
• Vector control and public health – stagnant waters and wet areas are eliminated that are conducive to disease-causing organisms
• Reduced erosion – saturated soils cannot adsorb additional water so these may induce more surface runoff and erosion, and saturated sloping soils are less stable

The major negative environmental effects and concerns with drainage implementation are:

• Land use conversion – wet soils and wetlands converted to agricultural land or other intensive (industrial, urban) land uses
• Habitat conversion – bio-diverse areas converted to mono-culture ecological systems (corn, soybean fields or houses and lawns)
• Water quantity manipulations – drainage water discharges may alter receiving stream hydrographs (i.e., extensive uncontrolled surface drainage may increase downstream peak flows and induce flooding)
• Water quality alterations – water discharged from drainage systems may contain undesirable concentrations of sediment and/or other contaminants that may enter receiving waters, as compared with runoff and leaching which occurs naturally

Hydric soils are defined as soils formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part.  The hydric soil indicators may vary by region and for organic or sandy soils as opposed to loam and clay soils, but they generally must include observations of pronounced organic matter accumulation since plant decomposition is slower under saturated conditions.  Mottled soils near the surface and color hue are key indicators also. The very poorly drained and poorly drained soil drainage classifications usually fit the hydric soil delineation.  The wetland protection rule requires permits be secured prior to draining and altering a delineated wetland.

The factors that influence the actual evapotranspiration of crops, and the overall (seasonal) water requirements and irrigation management decisions are:

• Type of plant
• Rooting depth and distribution
• Adaptation ability to water stress
• Growth and development stage
• Plant health

The soil water budget and balance is similar to balancing one's checking account. In fact one of the simple methods for irrigation scheduling is called the "checkbook" method. Rain is a deposit to the soil water budget, and the actual evapotranspiration is a withdrawal. The field capacity water content sets the upper limit (balance) of what the soil can absorb, or for the checking account analogy it is the minimum balance one may need to sustain. The daily rainfall (deposits) relative to the actual daily evapotranspiration (withdrawals) determines whether the soil water budget is wetting or drying. If the rainfall exceeds the actual evapotranspiration, the soil water increases.

If the soil water is already at its field capacity water content when this occurs, then the excess is drainable (deep percolation or runoff) water. When the soil water content is less than field capacity, the soil absorbs the rain, and the soil water content increases. However, if the rain amount is more than the evapotranspiration and the amount needed to raise the soil to its field capacity, the extra must drain. When there is no rain, the evapotranspiration gradually depletes the soil water.  This can go on for awhile, until the readily available water is gone, and then the plant starts to stress and stops evapotranspiring. Either rain or irrigation must occur at this time to replenish the soil water, preferably enough to raise the water content back to the field capacity.

In the Northeast, the annual or growing season irrigation system water requirements range from about 4 to 12 inches (or acre-inches volume) to offset the difference between the rainfall and actual evapotranspiration amounts.

The four methods of irrigation are:

• Surface
• Sprinkler
• Drip/trickle
• Subsurface

Surface irrigation consists of a broad class of irrigation methods in which water is distributed over the soil surface by gravity flow. The irrigation water is introduced into level or graded furrows or basins, using siphons, gated pipe, or turnout structures, and is allowed to advance across the field. Surface irrigation is best suited to flat land slopes, and medium to fine textured soil types which promote the lateral spread of water down the furrow row or across the basin.

Surface irrigation

Sprinkler irrigation

Sprinkler irrigation is a method of irrigation in which water is sprayed, or sprinkled through the air in rain like drops. The spray and sprinkling devices can be permanently set in place (solid set), temporarily set and then moved after a given amount of water has been applied (portable set or intermittent mechanical move), or they can be mounted on booms and pipelines that continuously travel across the land surface (wheel roll, linear move, center pivot).

Drip/trickle irrigation systems are methods of microirrigation wherein water is applied through emitters to the soil surface as drops or small streams. The discharge rate of the emitters is low so this irrigation method can be used on all soil types.

Drip/trickle irrigation

Subsurface irrigation

Subsurface irrigation consists of methods whereby irrigation water is applied below the soil surface. The specific type of irrigation method varies depending on the depth of the water table. When the water table is well below the surface, drip or trickle irrigation emission devices can be buried below the soil surface (usually within the plant root zone).

The sources of water for irrigation can include surface water sources, groundwater sources, municipal water supplies, grey-water sources, and other agricultural and industrial process wastewaters.

Surface water sources include 'flowing' water supplies (i.e., creeks, streams, canals) and 'standing' or stored water supplies (i.e., ponds, reservoirs, lakes).

Groundwater supplies may come from springs and wells, and although the quality is usually good, the available quantity that can be pumped at any time may again limit the irrigation method.

Grey-water is domestic wastewater, other than that containing human excreta, such as sink drainage, washing machine discharge or bath water.

The quality of agricultural or industrial process wastewaters often limits their use to surface or sprinkler irrigation methods, and in their suitability for fruit and vegetable crop irrigation.

Irrigation scheduling is a soil water budget accounting process to determine when irrigation is needed and the amount of irrigation water to apply. The important components of irrigation scheduling include:

• Rainfall amount
• Potential evapotranspiration amount
• Field capacity soil water content
• Wilting point soil water content
• Allowable crop depletion factor
• Crop rooting depth
• Time of planting

• Soils are characterized by drainage class, which affects agricultural practices on that soil.
• Through drainage systems, humans improve the draining ability of a wet soil, and make better use of the land.  However, there are many aspects to consider before a drainage system is installed.
• Irrigation transports to otherwise dry fields.  The method selected will depend on field characteristics, such as soil structure and topography.