chapter 5.pptx: drainage and irrigation engineering
- 1. CHAPTERFIVE : AGRICULTURAL DRAINAGE Definition Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.
- 2. Agricultural land drainage • Agricultural land drainage is the removal of excess surface and subsurface water from the land to enhance crop growth, including the removal of soluble salts from the soil.
- 3. Increased aeration of the soil; Stabilized soil structure; Higher and more diversified crop production; Better workability of the land; Earlier planting dates; Reduction of peak discharges by an increased temporary storage of water in the soil. Positive effects Positive and negative effects of drainage
- 4. Negative effects decomposition of organic matter; soil subsidence; reduced irrigation efficiency; increased risk of drought. excessive leaching of valuable nutrients from the soil; downstream environmental damage by salty or otherwise polluted drainage water; the presence of ditches, canals, and structures impending accessibility and interfering with other infrastructural elements of the land.
- 5. • Drains can be either surface or sub surface or combination based on condition of water to be removed and or a 5.1 Surface drainage system Surface drainage - is the orderly removal of excess water from the surface of land through improved natural channels or constructed ditches and through shaping of the land surface. Types of drainage system
- 7. Surface drainage systems, when properly planned, eliminate ponding, prevent prolonged saturation and accelerate flow to an outlet without siltation or erosion of soil. Surface drainage system is comparatively simple to plan, design and construct and is usually rather inexpensive. • All possible excess water from all sources should be removed before it percolated to the groundwater table and create or intensify a more expensive subsurface drainage problem.
- 8. The various conditions which cause surface drainage problems are: I. Uneven land surface with pockets or ridges which prevent or retard natural runoff. II. Low-capacity disposal channels within the area which remove water so slowly that the high water level in the channels causes ponding on the land for damaging period. III. Outlet conditions which hold the water surface above ground level such as tide water elevation.
- 9. Soils also need surface drains under any of the following situations: I. A hard pan or tight layer exists in the upper zone, II. The subsoil within a depth of 100 cm remains dry even after an extended rainy period, and III. In tropical and subtropical area which receives high intense rainfall and where the soil is heavy and slow permeable
- 10. The basic surface drainage systems are: the random, the parallel, and the cross slope or diversion system. The system to be used will depend upon the requirement of the site.
- 12. The random system When the topography is irregular, but so flat or gently sloping as to have wet depressions scattered over the area, a random system is used. The field ditches should be so located that they will transect as much depression as feasible along a course through the lowest part of the field towards an available outlet. Land grading, smoothing or bedding will usually be necessary on the less permeable soils to assume complete surface water removal.
- 13. The parallel system Where topography is flat and regular, and a random system is impractical or inadequate, field ditches should be established in parallel but not necessarily at equidistance. Orientation of field ditches will depend upon directions of land slope; location of diversions, cross slope ditches and mains and laterals of the disposal system. Usually, field ditches should run parallel to each other across a field to discharge in to field laterals bordering, the field.
- 14. The cross-slope system The cross-slope system is used to drain sloping land, and to prevent accumulation of water from higher land. The system consists of one or more diversions, and field ditches built across the slope. To choose between diversions or field ditches depends on the steepness of the slope, the permeability of the soil, and the possibility of water flowing from higher land onto the field being drained. Field ditches are best on slopes under 2%. Diversions apply to steeper land.
- 15. 5.2 Sub-surface drainage system Subsurface drainage is the removal of excess water and dissolved salts from soils via groundwater flow to the drains, so that the water table and root zone salinity are controlled. Types of Subsurface Drainage System Subsurface drainage aims at controlling the water table control that can be achieved by: open drains, or subsurface drains - pipe drains or mole drains tube well drainage
- 16. Sub-Surface Drainage Using Ditches
- 17. Tube well drainage and mole drainage are applied only in very specific conditions. Moreover, mole drainage is mainly aimed at a rapid removal of excess surface water rather than at controlling the water table. The usual choice is, therefore, between open drains and pipe drains. The difference between them is the way they are constructed: an open drains is an excavated ditch with an exposed water table; whereas, a pipe drains is a buried pipe.
- 18. Sub-Surface Drains Using Buried Drains
- 19. A drainage system has three components: A field drainage system – the network that gathers the excess water from the land by means of field drains, possibly supplemented by measures to promote the flow of water to these drains. A main drainage system - which is a water conveyance system that receives water from the field drainage systems, surface runoff and groundwater flow, and transports it to the outlet. The main drainage system consists of some collector drains and a main drainage canal.
- 20. A Collector drains can be either open drain or pipe drains. The main drainage is the principal drain of an area. It receives water from collector drains, diversion drains, interceptor drains (drains intercepting surface flow or groundwater flow from outside the area), and conveys this water to an outlet for disposal outside the area. An outlet - which is the point where the drainage water is led out of the area.
- 21. The combined systems of surface and subsurface drainage may be appropriate in the following situations: A soil profile with a layer of low permeability below the root zone, but good permeability at drain depth; and Areas with occasional high intensity rainfall that causes water ponding on the land surface, even if a subsurface drainage system is present.
- 22. Arrangements of Sub-Surface Drains
- 23. Singular versus composite drainage systems A singular drainage system is a drainage system in which the field drains are buried pipes and all field drains discharge into open collector drains. A composite drainage system is a drainage system in which all field drains and all collector drains are buried pipes.
- 24. The choice between the singular and composite system depends on: Field size and land loss- Surface water ditch collector (singular system) provides an outlet for excess surface water, Blockage - the outflow of a pipe drain into a ditch collector (singular system) is easy to inspect and malfunctioning easy to localize.
- 25. Maintenance - ditch collectors require much more maintenance than pipe collectors - once or twice a year compared to once per five or ten years. Outlets - the many pipe outlets in the singular system represent weak spots as they are easily damaged. Hydraulic gradient - a pipe collector requires about 5-10 times as much gradient as a ditch collector. Costs - installation costs for composite systems are considerably higher than for a singular system.
- 26. For subsurface drainage, a distinction can also be made between different types of systems. A random system connects scattered wet spots, often as a composite system. If the drainage has to be uniform over the whole area, the drains are installed in a regular pattern. This pattern can be either a parallel grid system, in which the field drains join the collector drain at right angles, or a herringbone system, in which they join at sharp angles. Both regular patterns may occur as singular or composite system. Random versus parallel drainage systems
- 27. Design of drainage • In irrigated areas, water enters the groundwater from: • Deep percolation, • Leaching requirement, • Seepage loss • Conveyance losses from watercourses and canals and • Rainfall for some parts of the world. Drainage Coefficient in Irrigated Areas
- 28. Example • In the design of an irrigation system, the following properties exist: Soil field capacity is 28% by weight, permanent wilting point is 17% by weight; Bulk density = 1.36 g/cm3 ; root zone depth is 1 m; peak ET is 5 mm/day; irrigation efficiency is 60%, water conveyance efficiency is 80%, 50 % of water lost in canals contribute to seepage; rainfall for January is 69 mm and evapotranspiration is 100 mm; salinity of irrigation water is 0.80 mmhos/cm while the acceptable is 4 mmhos/cm. Compute the drainage coefficient.
- 29. Solution: • Readily available moisture (RAM) = ½ (FC - PWP) = 1/2(28 - 17) = 5.5%. In depth, • RAM = 0.055 x 1.36 x 1000 mm= 74.8 mm = Net irrigation • Shortest irrigation interval = RAM/peak ET = 74.8/5 = 15 days • With irrigation efficiency of 60 %, Gross irrigation requirement = 74.8/0.6 = 124.7 mm. This is per irrigation. • (a) Water losses = Gross - Net irrigation = 124.7 - 74.8 = 49.9 mm • Assuming 70% is deep percolation while 30% is wasted on the soil surface (Standard assumption), deep percolation = 0.7 x 49.9 = 34.91 mm
- 30. Solution Contd. (b)Seepage Conveyance efficiency, Ec = Water delivered to farm Water released at dam = 0.8 Water delivered to farm = Gross irrigation =124.7 mm i.e. Water released = 124.7/0.8 = 155.9 mm Excess water or water lost in canal = 155.9 - 124.7 = 31.2 mm Since half of the water is seepage (given), the rest will be evaporation during conveyance Seepage = 1/2 x 31.2 mm = 15.6 mm
- 31. Solution Contd. • (c) Leaching Reqd. = Ecirrig (ET - Rain ) = 0.8 (100-69) • Eca ccept 4 • = 7.75 mm • This is for one month; for 15 days, we have 3.88 mm • • (d) Rainfall = 69 mm; for 15 days, this is 34.5 mm • • Note: In surface irrigation systems, deep percolation is much higher than leaching requirement so only the former is used in computation. • It is assumed that excess water going down the soil as a result of deep percolation can be used for leaching. In sprinkler system, leaching requirement may be greater than deep percolation and can be used instead.
- 32. Solution Concluded Neglecting Leaching Requirement, Total water input into drains is equal to: 34.91 + 15.6 + 34.5 = 85.01 mm This is per 15 days, since irrigation interval is 15 days Drainage coefficient = 85.01/15 = 5.66 = 6 mm/day.