Chapter 1: Water management
NOTE: This is an excerpt adapted from the Small Grains Field Guide.
For high yields, small grains need 14 to 17 inches of water depending on weather conditions and length of growing season. The water used for optimum growth is a combination of stored soil moisture, rain and irrigation. Small grains require about six inches of water as a threshold for grain yield. Each additional inch of water will provide four to five bushels per acre. In deep, well-drained soils, the roots of small grains will extract water to a depth of 3 to 3.5 feet.
Irrigation water management is most needed when farming on sandy soils to provide sufficient amounts of soil water to minimize moisture stress. Small grains are most sensitive to water stress in the boot to flowering stage of growth. Root-zone water deficits should be reduced to low levels during this period. Small grains are susceptible to fungal infections. Most small grains are irrigated with center pivots, so it is better to apply at least an inch of water per irrigation rather than more frequent small applications. Wheat and barley are particularly susceptible to Fusarium Head Blight (FHB) just prior to flowering through early grain filling. Irrigation during this period should be avoided if possible, so root zone water should be brought to a high level prior to flowering.
During the peak water use period, small grains can use up to 0.30 inches per day depending on air temperature and cloud cover (Tables 1.1, 1.2 and 1.3). Daily crop water use -- often called evapotranspiration or ET -- depends on plant development and local weather conditions. Small grain water use will generally peak between heading and early dough stage. Daily ET estimates in the following table are based on long-term average solar radiation and cloud cover. Daily ET estimates in northwestern Minnesota may be 5 percent to 10 percent greater than estimates found in Table 1.1 for central Minnesota because there is a greater chance for a clear and cloud-free sky.
Table 1.1 Average water use for wheat in inches/day in Central Minnesota
For detailed instructions on how to apply the daily crop water use estimates from these tables within an irrigation scheduling program, review the irrigation scheduling in the Checkbook Method bulletin from the University of Minnesota Extension or order a copy of the North Dakota State University Extension Service Bulletin # AE792.
Table 1.2. Average water use for wheat in inches/day for North Dakota
Table 1.3. Average water use for barley in inches/day for North Dakota
Drainage is a necessary soil water management practice on many farmlands to remove water in excess of field capacity, improve field operations, and stabilize year-to-year yield variability. Both surface and subsurface (tile) drainage are important in water management. The combination of the two often provides the best response to excess moisture. No long-term yield response to drainage has been measured for small grains in Minnesota or North Dakota. Previous research has shown that on a clay loam soil, wheat may reach only 58 percent of its potential yield when the water table is within 15"-20" of the soil surface for extended periods of time. Check with your local Extension office and the local Soil & Water Conservation District (SWCD) office for potential yield response information related to improved drainage.
The first step in developing a drainage plan for a farm is to evaluate the feasibility of drainage by consulting the Minnesota Drainage Guide, state drainage laws, local drainage experience and expertise, and by evaluating soil survey information, wetland restrictions, downstream impacts, and economic factors. Next, further evaluate the site by identifying outlet locations, conducting topography surveys, and making field evaluations (surface and subsurface). Visiting your local NRCS, SWCD, and/or Watershed office is an important first step in getting the process going, and for help in interpreting current wetland legalities and local restrictions.
A drainage outlet must provide for free discharge into a ditch or waterway where the flow can be carried away from the field. A drainage design for any field or farm must begin at the outlet. Tile outlets are typically located 3 to 5 feet below the elevation of the field. The bottom of an outlet pipe must be located above the water level in the receiving ditch or waterway, except during times of very high flows. Drainage outlets must be kept clean of weeds, trash, rodents, and be protected from erosion around the outlet, and damage from machinery or cattle. Where topography 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 tile to the ground surface and into the receiving waterway. Pumped outlets add to the initial monetary outlay and operation/maintenance costs of the drainage system, but have proven to be economically feasible in many situations. A pumped outlet station includes sump, pump, and discharge pipe. Important design considerations include size and shape of sump and capacity of the pump.
Drainage Pipe Size and Grade
Drainage mains and laterals should be selected to provide for the desired amount of water removal, commonly referred to as the drainage coefficient. This will typically range from 3/8 to 1/2 inches of water removal per day. If some surface water is to be drained by open surface inlets, the drainage coefficient for that area should be increased to 3/4 to 1 inch per day. Refinement of these guidelines should be done in consultation with local experts. Table 1.4 shows the maximum land area that different tile sizes can accommodate, at selected grades, for a 1/2-inch drainage coefficient. For other sizes, grades, and drainage coefficients, consult a drainage engineer, contractor, or the Minnesota Drainage Guide. Tile drains must not be installed at less than the minimum recommended grades, shown in Table 1.4, to prevent soil from settling within the tile.
Drainage Tile Spacing and Depth
Although many combinations of tile spacing and depth can produce the desired water removal rate, spacing and depth should be based on soil type, soil permeability and stratification, desired drainage coefficient, and degree of surface drainage. Table 1.6 shows some very general spacing options that might be considered during the early planning phase for a new or improved system. These values should be refined for specific soils with information from the Minnesota Drainage Guide and local experience.
Surface inlets offer timely removal of ponded water within a field. But, these inlets can provide a direct pathway for surface waters that may carry sediment and other pollutants to downstream rivers, which otherwise may have been trapped in the field. The general public, researchers, and others are concerned about the potential impacts these inlets may have on the downstream water flow and quality. Some farmers are converting their open inlet to a "blind" or "rock" inlet. University researchers and others are investigating the flow and water quality impacts of alternative inlet designs, such as raised pipe, blind inlets with rock and sand filters, grass buffer strips and reduced tillage.
Farmers have the option of hiring a contractor to install their drainage system or doing the job themselves with a towed implement. While the latter is certainly an option, it is one that must be carefully considered. One major consideration of your tile installer is experience and familiarity with design procedures and standards of tile drainage systems. Depth, grade, pipe size, and field layout are all extremely important in design and will determine the quality of performance of your system. The lifespan of corrugated plastic tile can be quite long–decades, if not generations. Once the tile is in the ground, it's there to stay, so make sure the installation is done correctly to avoid performance and longevity problems.
Table 1.4 Potential acres drained by selected tile sizes and grades of corrugated plastic tile with drainage coefficient = 1/2 inch/day
Table 1.5 Minimum grades for tile drain lines (when not subjected to fine sand or silt)
|Drain size, Inches||% Grade (ft/100ft)|
Table 1.6 General parallel tile lateral spacing and depths for different soils
|Type of Soil||Drainage Coefficient||Depth|
|Clay loam||Very low||70||50||35||3.0-3.5|
|Silty clay loam||Low||95||65||45||3.3-3.8|
|Silt loam||Moderately low||130||90||60||3.5-4.0|
|Sandy loam||Moderately high||300||210||150||4.0-4.5|
Authors: Jerry Wright, Thomas Scherer, and Gary Sands