METHODS OF IRRIGATION CHAPTER 1

Transcription

1 METHODS OF IRRIGATION CHAPTER 1 Deinition o Irrigation Irrigation may be deined as the science o artiicial application o water to the land, in accordance with the crop requirements throughout the crop period or ull-ledged nourishment o the crops. Following are the actors which govern the importance o irrigation Insuicient rainall Uneven distribution o rainall Improvement o perennial crops Development o agriculture in desert area Course Teacher: Pro. Dr. M. R. Kabir Advantages o irrigation Increase in ood production Optimum beneits Elimination o mixed cropping Improvement o cash crops Source o revenue General prosperity Generation o hydroelectric power Domestic water supply Facilities o communications Inland navigation Aorestation Disadvantages o irrigation Rising o water table: water-logging Problem o water pollution (nitrates seepage into GW) Formation o marshy land Dampness in weather Loss o valuable lands Types o Irrigation Surace Irrigation Sub-surace irrigation Surace irrigation In the surace methods o irrigation, water is applied directly to the soil surace rom a channel located at the upper reach o the ield. Water may be distributed to the crops in border strips, check basins or urrows. Two general requirements o prime importance to obtain high eiciency in surace methods o irrigation are: distribution systems to provide adequate control o water to the ields and proper land preparation to permit uniorm distribution o water over the ield. They are also designed to minimize labor and capital requirements. Eective management practices are dependent on the type o irrigation, and the climate and topography o the region. 1

2 System o Irrigation Surace Irrigation System Lit irrigation Flow irrigation By man or Animal power By mechanical or electrical power Inundation Irrigation Perennial Irrigation Open well Shallow Tube well Deep Tube well Direct Irrigation Storage Irrigation Subsurace irrigation In sub-irrigation, water is applied below the ground surace by maintaining an artiicial water table at some depth, depending upon the soil texture and the depth o the plant roots. Water reaches the plant roots through capillarity action. Water may be introduced through open ditches or underground through pipelines such as drains or mole drains. The depths o open ditches or trenches vary rom 30 cm to 100 cm and they are spaced about 15 m to 30 m apart. The water application system consists o ield supply channels, ditches or trenches suitably spaced to cover the ield adequately and drainage ditches or the disposal o excess water. Sub-main Pipe Fig: Sub-surace Method Perorated Pipe Sub-Surace Irrigation System Natural Sub-irrigation Artiicial Sub-irrigation Methods o Irrigation Free Flooding Border Flooding Check Flooding Basin Flooding Furrow irrigation method Sprinkler irrigation method Drip irrigation method 2

3 Free Flooding or Ordinary Flooding o Ditches are excavated in the ield o Water is applied rom ield ditches without any levee to guide its low. o Movement o water is not restricted, it is sometimes called wild looding o It is suitable or close growing crops, pastures etc. o It is practiced large where irrigation water is abundant and inexpensive. o It involves low initial cost o land preparation, extra labour cost in the application o water. o Application o eiciency is low. o This method may be used on rolling land (topography irregular) where borders, checks, basins and urrows are not easible. M A I N S U P P L Y D I T C H Outlets Main Supply Ditch Subsidiary Ditch Fig: Free looding (plan view) Border Flooding o The arm is divided into a number o strips (width 10 ~ 20 m and length 100 ~ 400 m) separated by low levees or borders. o Water is turned rom the supply ditch into these strips along which a low slowly toward the lower end, wetting the soil as it advances. When the advancing water ront reaches the lower end, the stream turned o. o The surace is essentially level between levees and lengthwise slope is somewhat according to natural slope o the land (0.2 ~ 0.4%) o It is suitable to soils having moderately low to moderately high iniltration rates and to all closely growing crops. o Uniorm distribution and high water application eiciencies are possible. o Large streams can be used eiciently. o It involves high initial cost. o Ridges between borders should be suiciently high o The land should be perpendicular to the low to prevent water rom concentrating on either side o the border Ditch Gate 100 to 400 m Low levees borders 10 to 20 m Fig: Border looding (Plan view) 3

4 Determination o required time to irrigate in border looding A relationship between the discharge through the supply ditch (), the average depth o water lowing over the strip (y), the rate o iniltration o the soil (), the area o the land irrigated (A), and the approximate time required to cover the given area with water (t), is given by the equation: y t 2.3 log A Where, = Discharge through the supply ditch y = Depth o water lowing over the border strip = Rate o iniltration o soil A = Area o land strip to be irrigated t = Time required to cover the given area A Derivation o Equation: t 2.3 y log A Supply Channel Border or Leevee Area da covered with water in time dt Border strip Area A covered with water in time t Considering small area, da o the border strip o area (A) Depth o water, y over this area (A) Assume that in time dt, water advances over this area (da). Now, the volume o water that lows to cover this area = y.da (1) During the same time dt The volume o water that percolates into the soil over the area (A) =.A.dt (2) The total quantity o water supplied to the strip during time (dt) =.dt (3) From equation (1), (2) & (3).dt = y. da +. A. dt y. da dt =. A For getting time required to irrigate the whole land, we have to integrate the above equation and considering y,, and as constants y. da ƒdt = ƒ. A 4

5 Ater integrating the above equation, we get t = y ln + C (constant) (4). A But at, t = 0, A = 0 From equation (4) Finally, t = 0 = 0 = y ln y ln.0 + C y y ln + C = ln(1 ) A y + C = 0 + C = 0 + C C = 0 or, t 2.3 y log A This above equation can be urther written as t. log Now, let 2.3. y A Then, x = log 10 x = A A.10 x.a.10 x = (10 x 1) =.A.10 x x 10 1 A = x.10 x 10 Further, considering the maximum value o 10 We get, A max = x 1 = 1 t y = x Problem: Determine the time required to irrigate a strip o land o 0.04 hectares in area rom a tube-well with a discharge o 0.02 cumec. The iniltration capacity o the soil may be taken as 5 cm/h and the average depth o low on the ield as 10 cm. Also determine the maximum area that can be irrigated rom this tube well. Solution: Here, A = 0.04 hectares = m 2 = 400 m 2 = 0.02 cumec = 0.02 m 3 /s = m 3 /hr = 72 m 3 /hr = 5 cm/hr = 0.05 m/hr y = 10 cm = 0.10 m Now, Time required or irrigating the strip o land, t = 2.3 log = 0.65 hr = 39 min Maximum area that can be irrigated is given by the equation: A max = 72 m 2 = 1440 m 2 = 1440/10 4 hectares = hectares 0.05 y log A 5

6 Check Flooding o Similar to Ordinary looding o Water is controlled by surrounding the check area with low and lat levees o The check is illed with water at a airly high rate and allowed to stand until the water iniltrates o The conined plot area varies rom 0.2 to 0.8 hectares Ditch Openings Levees along the contours Fig: Check looding (Plan view) Adaptability: It is suitable or low as well as high intake soils and or rice or other crops which can withstand temporary looding. Advantages: Eective leaching. Maximum use o seasonal rainall High application eiciencies. Limitations: Soil crusting Unsuitable or crops that cannot accommodate inundation. Basin Flooding o Special type o check looding o Adopted specially or Orchard trees o One or more trees are generally placed in the basin o Surace is looded as in check method by ditch water Subsidiary ditches Connecting Levee Entry o water through a bank hole or by a hose siphon MAIN DITCH Trees Basins Fig: Basin Flooding (plan view) 6

7 Adaptability: It is suitable or low as well as high intake soils. Advantages: Eective leaching Maximum use o seasonal rainall High application eiciencies Limitations: Soil crusting Unsuitable or crops that cannot accommodate inundation. Application eiciency is comparatively high. Furrow irrigation method o Furrow are narrow ield ditches, excavated between rows o plants and carry water through them o Spacing o urrows is determined by proper spacing o the plants. o Furrows vary rom 8 to 30 cm deep and may be as much as 400 meters long o Deep urrows are widely used or row crops. o Small shallow urrow (called corrugations) suitable or irregular topography and close growing crops such as meadows and small grains. o Water diverted into the urrows by using rubber hose tubing. o Hose prevents the necessity o breaking o the ditch bank and provides a uniorm low into the urrow. FURROWS OR FIELD DITCHES ENTRY OF WATER THROUGH A BANK HOLE OR BY A HOSE SYPHON Fig: Plan view (Furrow irrigation method) Adaptability: It is suitable or row crops (like potatoes, groundnut, sugarcane etc.) and or medium to moderately ine textured soil. Advantage: Only about one-ith to one-hal o the land surace is wetted by water. So, it results in less evaporation, less pudding o soil. Disadvantages: Excessively long urrows may result in too much percolation near upper end and too little water near the down-slope end. It involves high initial cost and salt accumulation in the ridges. 7

8 Sprinkler irrigation method o Water is applied to the soil in the orm o a spray through a network o pipes and pumps. o It is kind o an artiicial rain and gives good results o It is a costly process and not widely used in our country. o It can be used or all types o soils and or widely dierent topographies and slopes. o It ulills the normal requirement o uniorm distribution o water. Adaptability: This method can be used or almost all crops (except rice and jute) and on most soils. It is best suited to sandy soils that have a high iniltration rate. It can be applied to any topographic conditions without extensive land preparation. Crops Sprinkling Water Water Supply Fig: Plan view (sprinkler irrigation method) Types o sprinkler systems: Permanent system: In permanent system, pipes are permanently buried in such a way that they do not interere with the arming operations. Semi-permanent system: In semi-permanent system, the main lines are buried in the ground, while the laterals are portable. Portable system: In the portable system, the mains as well as laterals are portable. These portable networks can be moved rom arm to arm Advantages o sprinkler irrigation: o Seepage losses, which occur in earthen channels o surace irrigation methods, are completely eliminated. Moreover, only optimum quantity o water is used in this method o Land leveling is not required and thus avoiding removal o top ertile soil, as happens in other surace irrigation methods. o No cultivation area is lost or making ditches results in increasing about 16 % o the cropped area o The water is to be applied at a rate lesser than the iniltration capacity o the soil, and thus avoiding surace run o. o This method leaches down salts and prevents water-logging or salinity o It is less labor oriented and hence useul where labor is costly and scarce. o Up to 80% o applied water can be stored in the root zone o plants. o Fertilizers can be uniormly applied, because they can mixed with irrigation water Limitations o sprinkler irrigation: o High winds may distort sprinkler pattern, causing non-uniorm spreading o water on the crops. o In areas o high temperature and high wind velocity, considerable evaporation losses o water my take place. o They are not suited to crops requiring requent and larger depths o irrigation, such paddy. o Initial cost o the system is very high, and the system requires a high technical skill o A constant water supply is needed or commercial use o equipment o Only sand and silt ree water can be used, as otherwise pump impellers liting such waters will get damaged. 8

9 Drip irrigation method o It is the latest ield irrigation technique (also called trickle irrigation) o Irrigation water is applied by using small diameter (12 to 32 mm) plastic lateral lines. o The lateral lines contains some devices called emitters at selected spacing to deliver water to the soil surace near the base o the plants. o It is best suited or widely spaced plants, salt problems and or areas with water scarcity. o In this method, water is slowly and directly applied to the root zone o the plants or minimizing the losses by evaporation and percolation o This method is being used or small nourishes, orchards, or gardens. Water supply Fig: Section view (Drip irrigation method) Beneits: Conventional losses such as deep percolation, runo and soil water can be minimized by applying a volume o water approaching the consumptive use o the plants. Small area is wetted thereby reducing weed growth, insects and diseases etc. Soil crusting and intererence with harvesting is minimized. Greater crop yield and better quality can be obtained. For widely spaced like ruit trees, the system may be even more economical than sprinkler method o irrigation. Disadvantages: High initial cost o the deep irrigation equipment sometimes limits its use to orchard and vegetables in water scarcity areas. Clogging o emitter may disrupt the irrigation system. Plastic pipes may damaged by rodents. Wind erosion can harm the pipes. Like the sprinkler method, drip irrigation permits the simultaneous application o ertilizers through the system. When compared to the sprinkler system, the drip method operates on much lower line pressure, thus providing a saving in energy requirements. 9

10 Irrigation Project Surveying Availability o Irrigation Water When it is ound necessary to take up an irrigation project, the availability o required water should be investigated. The ollowing points should be considered Whether any perennial river is available near the command area or not. I an inundation river is available, the maximum discharge o that river is to be ascertained rom the highest lood level mark (as indicated by the villagers residing near the bank o the river). From various investigations (i.e. maximum discharge, rainall etc.) it is necessary to ascertain whether the river will be able to meet the total water requirement or not. Selection o probable site or Barrage or Dam When the source o water is available, the suitable site or the barrage or dam should be ound out considering the ollowing points, The course o the river should be straight at least or a distance o about one kilometer both on the upstream and down-stream side o the site. The width o the river should be minimum and the section o the river should be welldeined. A suitable basin should be available or the storage reservoir. The elevation o the site should be higher than that o the culturable command area. The storage reservoir should not submerge much valuable land. The capacity o the reservoir should ulill the total water requirement. Discharge observation or the river The gauge and discharge observation station should be established at the proposed site to collect the ollowing data: The daily discharge, maximum discharge and minimum discharge o the river throughout the year should be recorded. Silt analysis should be carried out in rainy season (when the river carries much silt) to determine the nature o sedimentation in river or reservoir. Discharge observations should also be carried out or all the rivers crossing the proposed canal. This is required or designing cross-drainage works. Marking o GCA and Cultivable area When it is decided to make up the project, the gross command area should be marked on the topographic map. The culturable areas should be deined on the map to ind the culturable command area that is to be included in the project. Marking o alignment o main canal The alignment should be marked on the topographical map o the concerned area. While marking the alignment the ollowing points should be kept in mind. The alignment o the main canal should be taken in such a way so that unnecessary cutting and banking is avoided. The alignment o the main canal should be such that the branch canals can be taken suitably to cover the whole culturable area. The alignment should cross the rivers, roads, railways lines etc. perpendicularly as much as possible. The alignment should not be taken through the valuable agricultural land. The alignment should not pass through the thickly populated areas, religious places, burial grounds, etc. 10

11 Preliminary location survey The reconnaissance survey should be carried out along the alignment to record necessary data such as obstacles, road crossings, railway crossings, river crossings, etc. This survey involves the ollowing procedures: The approximate distance along the alignment should be measured by pacing and the magnetic bearings o the traverse legs (open traverse) and it should be noted in the ield book. The objects and the nature o the ground on both sides o the alignment should also be noted in the ield book. The alignment may be diverted to avoid religious places, valuable structures, etc. The alignment should be made to cross the rivers perpendicularly. An index should be prepared or the alignment. Final Survey: a) Final location o Barrage or Dam: The inal location o the barrage or dam site involves the ollowing steps: The centre line o the barrage or dam site should be marked with pillars on both banks o the river. The cross-section o the barrage site should be taken very precisely. Cross-sections should be taken at regular interval on the upstream side o the barrage site to ascertain the storage capacity o the reservoir. Boring test should be carried out along the centre line o the barrage site to determine the depth and nature o oundation. b) Route survey: A prismatic compass survey or plane table survey should be conducted along the alignment o the main canal to prepare a route survey map o the area covering a distance o about 30 m on both sides o the alignment. c) Longitudinal leveling: The longitudinal leveling should be done along the alignment o the main canal. Generally, the sta readings are taken at an interval o 20 m along the centre line o the main canal. The magnetic bearings o the lines (traverse legs) should also be noted in the level book. Longitudinal leveling or the branch canals should also be done. d) Cross-sectional leveling: The cross-sectional leveling at regular intervals along the alignment o the main canal should be taken. The cross-sections or the branch canals also should be taken. These cross-sections are required or the computation o volume o earth work. e) Data or cross drainage works: At the places o river crossings, road crossings, railway crossings etc. additional data should be collected or designing cross-drainage works. At the sites o river crossings the gauge and discharge observation stations should be established. ) Soil survey: The soil survey should be conducted along the alignment. It consists o collecting the sample o soil by boring up to the depth until impervious layer is obtained. g) Well observation: Well observation should be carried out along the alignment. This operation consists o measuring the water level o the wells existing on both sides (within 50 m) o the alignment. This is done to know the nature o water table along the course o the canal. 11

12 Preparation o drawings Route survey map (to suitable scale). Longitudinal sections or the main and branch canals with ormation level (to suitable scale). Cross-sections o main and branch canals with ormation level (to suitable scale). Contour map along the alignment. Design o curves with setting out table. Oice works The sections o the canals should be designed. The detailed estimate should be prepared to know the volume o earth work in cutting or banking along the main canal and branch canals. The total land width required should be marked on the route survey map. The design o the barrage or dam, cross-drainage works and other allied structures should be completed. The detailed report should be prepared or the compensation. It includes the names o owners, location, amount o properties, valuation o the land, etc. The total cost o the project should be ascertained by considering all the aspects. Justiication o the selection o inal alignment Ater preliminary survey, the estimates or the tentative alignments (i taken) are prepared. Then by comparing the total costs, working easibility, etc. with the alignments the inal alignment is selected. Final location survey The inal location survey o the approved alignment o the canal should be carried out or the execution o the project works. It includes the ollowing: The center line o the main and branch canals should be marked with concrete pillars at intervals o 30 m or 50 m. The total land width required or the main and branch canals should be marked with pillars at suitable intervals. 12

13 Irrigation Project Report Introduction: The introduction o the project includes the ollowing points: Aim o the project Location o the project Total area to be covered within the project. Total population to be beneited by the project. Future prospect i irrigation is practiced. Stages o uture development. Total cost o the project. Necessity and economic justiication To justiy the necessary and economical development o the area, the ollowing points should be clearly illustrated: Amount o yearly rainall. Nature o distribution o rainall during the crop season. Types o major crops grown in the area. Total water requirement o the crops. Amount o water requirement by irrigation system. Expected increase in yield o crops, i irrigation is practiced. Total revenue expected. Report on land acquisition and compensation A detailed statement should be prepared showing the names o owners, types o properties, quantity, amount o compensation, etc. The procedure adopted or the land acquisition should be clearly mentioned. Details o design and drawing o hydraulic structures The detailed design procedure and drawing o hydraulic structures, canals and other allied structures should be incorporated. Detailed estimate The detailed estimate or all the works o the project should be incorporated. Speciication The speciications o the construction materials and dierent works should be clearly mentioned. Availability o materials and laborers The source o construction materials and places o recruitment o laborers should be mentioned. Communication The existing communication to the selected barrage or dam site should be pointed out. I new communication is required or inaccessible site, the possible route should be pointed out and the expenditure or the new route should be included in the project. Maps Topographical map o the area showing the canal alignment and barrage or dam site. Route survey map. Longitudinal sections. Cross-sections. Contour map o alignment. Detailed drawing o barrage or dam, cross-drainage works, etc. Conclusion and recommendation Ater urnishing all the aspects o the project, the proposal is orwarded to the higher authority with proper recommendation or the necessary approval. 13