 |  | Small Scale Irrigation Systems (Peace Corps) | ||
 |  | Section 11. Drainage | ||
|  | (introduction...) | ||
| | Surface drainage | ||
| | Subsurface drainage | ||
| | Soil salinity | ||
| | Surface runoff | ||
| | Example problem | ||
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Drainage is the removal of excess water from the land to prevent crop damage and salt accumulation, allow earlier planting of crops, increase the root zone, aerate the soil, favor growth of soil bacteria, and to reclaim arable low-lying or swamp areas. Practically every valley where irrigation has been carried on for a considerable length of time has lands needing drainage. To be fit for crop production, several classes of land in irrigated sections require artificial drainage. Man-made swamps, the product of irrigation, often constitute the most of the area needing drainage.
All plans that are developed for introducing water to land, either as a supplemental supply or for new irrigation, should provide for removing excess water from the land.
Drainage problems are usually made apparent by:
1. Standing water or salt deposits on the soil's surface.
2. Scalding of crops by summer water ponding.
3. Propagation of mosquitoes in irrigated fields.
4. Soil compaction and resulting poor water penetration.
5. Difficulty in carrying on farm operations because of poor tractor footing.
6. Salt accumulating in the soil.
7. Poor root growth due to a high water table.
8. Plant root diseases.
9. Development of uneconomical plant communities.
In general, there are two main types of drainage situations; surface and subsurface.
Surface drainage is affected by topography and vegetation. Excess irrigation water must be removed to alleviate ponding and water logging of the lower parts of the field. Rain water must find its way into a drainage channel without causing erosion and without inhibiting aeration.
On some drainage projects, open ditches are used to convey water to distant outlets. High construction and maintenance costs, the inconvenience of moving machinery, and the value of land removed from production, make open-ditch drainage expensive--and often inconvenient. Still open-ditch construction has a place in many systems. The open ditch, as its name implies, is merely a waterway cut into the soil to receive drainage from adjacent land.
Excess rainfall can be controlled and disposed of by terraces and diversions leading to grassed waterways. On flat land, where there is no erosion hazard, shallow surface drains may be used.
Tail ditches generally are shallow open drains large enough to carry away irrigation waste water and storm water runoff. Storm runoff generally governs capacity. The grade of these ditches should be governed by the soil's resistance to erosion. Banks of the ditches must be protected from surfacewater erosion by inlet structures or by establishing vegetation on flattened slopes.
A combination of field ditches and land leveling is most practical. It would take an unreasonably large network of field ditches to do a good job of moving water from most fields without land leveling. Deep channels to carry the final collection into an accepted area are often constructed on field boundaries.
The first step in solving a drainage problem is to determine the source, direction of movement, and amount of excess water. The permeability or hydraulic conductivity of the soil (the rate of movement of water through the soil) is an important factor. Excess groundwater must be removed by either deep open ditches or tile to provide an effective root zone depth.
Subsurface drainage requires a thorough study of subsurface conditions. Test pits, borings, and permeability tests permit one to evaluate a soil's internal drainage capacity. Borings are commonly used to determine depth and fluctuations of the water table, depth of and thickness of the substrata, and to ascertain the character of the substrata.
The two general methods for removing excess water are by interceptor drains or relief drains. The appropriate method depends primarily on flow characteristics of the water, topographic features of the area, and subsoil conditions.
It is always a good idea to intercept excess water before it reaches the point where damage occurs. For this reason, an interceptor drain should be placed to remove water before it reaches the point of damage. In this case, the tile should be placed as deep as possible to intercept the maximum amount of water flowing downslope.
Relief drainage systems are installed in either a systematic or random pattern within an affected area. These laterals drain water to a main line which in turn discharges it into a trunk drain. Lateral-tile lines are placed parallel to the direction of groundwater movement and often in a gridiron or herringbone pattern.
Tile drains are impractical in many countries because of availability and price of the clay tiles or plastic drain pipe usually used. The design of underground tile drains requires expert engineering talent. The design of such systems is beyond the scope of this manual.
When soils become too saline for efficient crop production, crops must be removed or the land abandoned. The excess soluble salts in saline soils impair plant growth and soil productivity. One of the first effects of soil salinity is shown by a plant's inability to absorb enough water because osmotic pressure in the soil solution is too great.
Soils in arid-regions contain relatively large amounts of soluble salts. In more humid regions, salts are leached out by rain water. Small rains of the arid regions do not penetrate the soil deep enough to percolate the salts away. Lack of percolation, along with excessive evaporation, causes soluble salts, which are injurious to plant life, to accumulate on the soil's surface. The basic cause of salinity usually is inadequate application of water, poor drainage, or using water a high concentration of soluble salts.
High salt concentrations in the soil may result from a high water table. During periods between irrigations, a high water table favors upward capillary flow of water to the surface where the water evaporates. Soluble salts carried by the upward moving water cannot evaporate, hence, they are deposited on or near the surface.
The most effective way to remove salt from soil is with water passing through the root zone of the soil. To prevent salt accumulations, and consequent decrease in crop yields, irrigators must remove as much salt as is brought in. In some areas, a limited supply of irrigation water is spread over too many acres, with the result that the soil is not wetted below a few feet.
In other areas, the groundwater table is so shallow that it prevents the leaching of salts from the root zone. Upward flow of water from the shallow water tables results in a continuing accumulation of salts in the surface soil. If it were possible to maintain moisture distribution in irrigated soils so the water flow would be continuously downward, there would be relatively little trouble from salinity, even when moderately saline irrigation water was used.
Adequate drainage is extremely important for either reclaiming saline lands or maintaining lands free from salinity. It is usually essential that water volumes in excess of crop requirements be applied to saline and alkaline lands and be made to percolate through the soil to leach out excess salts. Salts dissolve in water that passes through the soil.
In all cases, water must pass beyond the root zone to remove excess salts from the root zone. Leaching therefore, is impossible without natural or artificial drainage.
Enough water should be applied to assure that all the surface is covered, even if ridges and knolls must be leveled first. When the subsoil is impervious, subsoiling must be done. Waste water should not be allowed to run off but should percolate down through the soil. For this reason, a series of dikes and checks should be built to accomplish adequate ponding. Each should have as large an area as slope and water supply permit. Remember that excessive leaching also removes desirable plant nutrients from the soil, especially nitrates. Overuse of irrigation water may also add to drainage problems.
Permanent reclamation of saline and alkali lands requires several essential steps:
1. lowering the water table,
2. satisfactory water infiltration,
3. leaching excess salts from the soil,
4. intelligent future management of the soil.
Some alkali and saline soils that are low in available phosphorous give better crop yields if phosphate fertilizers are used. Liberally applying barnyard manure, plowing under cover crops, and avoiding plowing and other farm operations when the soil is too wet or too dry all help. Keeping drains open and in good repair, applying only enough water to assure adequate penetration into heavy soils, and preventing excessive evaporation are all essential steps in maintaining permanent relief from waterlogging and a continued soil productivity.
To protect roads, irrigation systems, buildings, and fields, you should determine maximum rate of runoff for all drainage systems. Most structures can be flooded for a short time, but peak rainfall intensities and runoff data should be determined so that the system (bridges, culverts, etc.) can be designed to handle the runoff. It may be most economical to design the structures on a 10- to 25-year recurrence expectancy; that is, the expected runoff would be exceeded only once every 10 to 25 years.
In calculating runoff on small watersheds, this formula has wide usage:
Q = CIA
Q = Expected flood peak, cubic meters per second
C = Runoff coefficient
I = Rainfall intensity, mm per hour
A = Drainage area in hectares
For a drainage area in a diversified farming area, the value of C is often used as 0.50. Some of the figure for C under various conditions are in Table 11-1.
An irrigation field is on a flood plain near a stream. Sloping land, 10 percent slope, above the field is used for dryland farming and extends about 1000 meters above the field. The irrigated field is 500 meters long, so it would intercept water running off the steeper slope. Rainfall records indicate maximum rainfall intensities of 100 mm/hr during a recent 10 year period.
How much surface runoff would a diversion channel have to carry to prevent surface runoff water from reaching the irrigated field?
The area to be intercepted is:
A = 1000 X 500 = 600,000 m²
A = 500,000 / 10,000 = 60 Ha
From Table 11-1 the runoff coefficient is estimated to be 0.002. The quantity of runoff then becomes:
Q =.002 X 100 X 50 = 10 m³/sec.
Table 11-1. Runoff coefficients
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0-5% Slope |
10-30% Slope |
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Cultivated land |
0.0018 |
0.002 |
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Pasture land |
0.0010 |
0.0012 |
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Timber land |
0.0005 |
0.0006 |
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