Drainage Problems and Methods

T. W. Edminster and Ronald C. Reeve.

Good management of the land means good management of the water. Excess water in the soil interferes with crop growth and the timely performance of tillage, seeding, cultivation, and harvesting. Poor drainage also abets the accumulation of salts in soils of and regions.

The source of the excess water determines the severity of drainage problems and the ways to solve them.

The problems are: Periodic flooding of lands by overflow from streams or by tidal action in coastal areas; overflow of low-lying flat lands from hillside runoff or return-flow seepage on sloping land; accumulation of too much water in soils when subsoil drainage is restricted; accumulation of excess water in depressions or low-lying areas, such as old ponds and lakebeds; buildup of a high water table as a result of applying excess irrigation water; buildup of a high water table from seepage losses from irrigation canals; and the development of a high water table because of the movement of artesian water.

Lands are drained primarily to insure agricultural productivity and increase efficiency of farming operations.

There are other beneficial effects. Poorly drained areas require large expenditures annually to build and drain highway subgrades and to prepare construction sites. Mosquitoes and some disease problems may be related to poor drainage.

About 103 million acres of land, encompassing nearly 2 million farms, were in organized district and county drainage enterprises in 1950. These public projects were in 40 States and covered areas from less than 100 acres to more than 1 million acres. The average enterprise covered about 7 thousand acres.

The annual cost of constructing, operating, and maintaining public drainage projects was about 32 million dollars in 1949. It was estimated in 1956 that some 15 million acres were still too wet for cultivation and that crop losses were frequent on an additional 10 million poorly drained acres.

The Soil Conservation Service in 1955 estimated that more than 20 million acres would require drainage by means of group drainage facilities.

An estimated 15.5 million acres of land have been drained through private projects. Soil Conservation Service in 1955 estimated that 67 million acres still required drainage improvement through individual activity.

The development of a drainage system will embrace three basic points: The drainage requirements, the water transmission properties of soils, and physiographic features.

THE DRAINAGE REQUIREMENTS involve the adequacy of drainage whether there is too much water on or in the soil and the amount of water to be drained.

In humid regions, where rainfall supplies most of the moisture for crops, the drainage requirements are related to the oxygen status and the soil-water relationships that will influence crop growth. The optimum moisture content of the soil for farming is particularly important.

The drainage requirement usually is referred to as a drainage coefficient and is expressed in terms of the time required to remove a given depth of water for example, a drainage coefficient of three-eighths means that the drainage system would permit the removal of three-eighths inch of water from the soil surface in 24 hours. The drainage requirement may be adjusted to the susceptibility of the crop to damage by exposure to excess water or to the cost of delaying farm work.

The salinity factor alters greatly the drainage requirements in and regions. Irrigation water contains soluble salts, which are concentrated in the soil by evaporation and transpiration. To deep the salts in the soil solution from becoming so concentrated that they hurt crop growth, excess water must pass through the root zone and flush away, or leach out, soluble salts.

Information concerning the consumptive use the amount of water lost by evaporation and transpiration helps one to estimate the amount of water that must pass through and beyond the root zone to provide the required amount of leaching. That amount is determined on the basis of the salt content of the irrigation water and the salt concentration that can be permitted in the drainage water.

The leaching requirement is an estimate of the fraction of the surface-applied water that must be leached through the root zone to control soil salinity at any specified level. The total amount of water to be drained will be greater than the leaching requirement by an amount equal to the losses in conveying and applying the water to the land, plus the water from other sources, such as management waste and artesian waters.

The amount of water that must be removed from the land may be expressed as a rate of flow per unit area of land. One must also establish a minimum allowable water-table depth to prevent damage to crops, either from excess water in the root zone or from the concentration of salts in the soil by upward flow.

The minimum depth is governed by the crops to be grown, the soil conditions, and the salt content of the drainage water. The main requirement in any case is that the depth to the water table must be such that the upward movement of salts from ground water into the root zone can be conveniently controlled.

If an adequate water-table depth cannot be maintained, irrigation and management practices can sometimes be altered to allow crop production. Improvement of irrigation efficiencies and more uniform application of water are examples of ways by which drainage conditions can be improved and a net movement of salts maintained.

Irrigation efficiency, losses in conveyance and distribution, regulatory losses, rainfall, the amount of salt in the irrigation water, and salt tolerance of crops all enter into the problem of determining drainage requirements.

The ability of soils to transmit water has primary importance in the drainage of farm lands. The rate at which water enters a soil after a rain or after an irrigation and the rate that water tables can be lowered and excess waters drained away are directly related to the rate at which water can move through the soil. Of the factors that govern the flow of water in soils, the transmission properties are the hardest to evaluate, primarily because soils vary so much.

THE PHYSIOGRAPHIC FEATURES bear strongly on drainage design. The topography, stratigraphy (or the vertical arrangement of strata or layers of different types of soil), and location of outlets and sources of water must be considered in the design.

Topography is important when excess water originates as an application on the land surface, either as rainfall or from irrigation. The location of drains depends largely on topography.

Stratigraphy must be considered in subsurface drainage. The occurrence of permeable layers, such as sands or gravel, within the soil profile at a convenient depth to be tapped by open or tile drains may permit a wider spacing of drains and thereby reduce the total number of drains. On sloping hillsides or at the bottom of a slope, interception drains may drain relatively large acreages, but on relatively level lands a large number of ditches or tile drains may be required to control the water table.

The effectiveness of drainage structures is related directly to their location with respect to subsoil layers. Water that enters the soil in one area from rainfall or deep percolation from irrigation may come to the surface in a nearby area or may appear as a seep on a hillside, according to the path of flow through the subsoil. The drains should be located to remove water from the more permeable soil layers through which such flow occurs.

Highly permeable layers may discharge excess water from an area or may conduct excess water from one area to another. Impermeable soil layers may intercept conducting layers and block the free movement of ground water away from the farmland.

Proper orientation and placement of the drains with respect to stratigraphy may be the most important single factor in the design of a good system.

In general, the most effective method of drainage for the control of ground water tables is one that takes advantage of the most permeable materials in the profile for intercepting, collecting, and discharging excess waters from the land.

Crops may be affected in a number of ways by high water tables or excess soil moisture. Some affect root development aeration and temperature of the soil, nutrient uptake, and plant disease. Salinity problems may also develop and affect crop growth in and soils where water tables are just under the surface.

The roots of most cultivated crops will not penetrate saturated soil areas. When a rising water table inundates a root, there is usually an early change in the appearance of the crop, reflecting changes in the ability of the root to function properly.

POOR SOIL AERATION is a primary factor in this adverse response of roots and crops. Gas diffusion drops rapidly if the larger pore spaces are filled with water. The oxygen level declines and the carbon dioxide level rises as organic matter decomposes when the soil is saturated. Oxygen, which helps convert insoluble plant nutrients into a soluble form, also is a critical agent in the decomposition of organic materials. It is essential for seed germination and the development of root hairs. When the oxygen supply is cut off to roots of most cultivated plants, the root suffocates and dies, the intake of water and plant food is lowered, and the plant wilts and dies.

Plants that normally grow on well-drained and aerated soils usually are most sensitive to the lack of oxygen. Even plants, such as cranberries, which can remain covered during a long dormant period, however, will suffer from poor aeration in summer when the plant uses more water and nutrients. Plants that can withstand long periods of little oxygen have special tissues in their stems and roots that can conduct the oxygen to the roots.

Because carbon dioxide rarely occurs in the soil in amounts sufficient to harm the roots, the reduction in the oxygen level is the most critical result of soil saturation.

The gaseous balance changes more rapidly under higher temperatures because of greater biotic activity one reason why flooding in summer often is more damaging than flooding in winter. A second reason is that the reduced plant growth in winter places fewer demands on the injured and unhealthy roots that may occur under winter flooding.