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4/19/2016 MISD: LESSON 15. CHAK LAYOUT AND ALIGNMENT OF FIELD CHANNELS
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LESSON 15. CHAK LAYOUT AND ALIGNMENT OF FIELD CHANNELS
Introduction
The chak or outlet command is a basic unit for irrigation management in thecommand of Minor, distributory or any other parent channel. Therefore,establishing layouts of the chaks in the command of a given channel is the firststep of the planning process. For this purpose, the contour map of the commandof minor or distributory to the scale of 1:2000 (1:4000 if command is very big)with contour interval of 0.2 or 0.3 m should be available.
15.1 Layout of Chaks:
Following factors are considered for finalizing chak boundaries and total layout ofchaks.
i) Topography (Ridges, vallies, local depressions, high patches, etc.)
ii) Maximum permissible area which can be irrigated in peak rotation by a prescribeddischarge in a given flow period
iii) Maximum length of field channel
iv) The number of farmers to be served
v) Other factors like village boundary, road/railway lines, etc.
15.2 Topography:
Mark vallies, local depressions, gullies, natural drains, high patches, ponds, gaothans,roads, railway lines, village boundaries etc. in different colours on the contour plan soas to limit the boundary of the Chak to natural drain / village boundary / road / railwaylines.
15.3 Maximum Permissible Chak Size:
The chak size plays an important role in the efficient scheduling of irrigation. It is thearea served by an outlet. It depends mainly on discharge of the outlet, flow period,crop pattern and peak irrigation water requirements. For example, the Government ofMaharashtra has standardized the capacity of the field channels at 30 l/s. Previously,the flow period in most of the projects used to be 12 days in a rotation period of 14days. Now the flow period of 6 days in a rotation period of 14 days is adopted for chakdesign. With the discharge of 30 l/s and the flow period of 6 days in a rotation, theonly variable factor in determining the chak size is the peak net irrigation requirement.Net irrigation requirement (NIR) based on modified Penman Method is worked out forcrops as per the approved projectcropping pattern. This is generally the NIR at rootzone (expressed in mm) considering effective rainfall and the special needs of differentcrops. Since the water is released at outlet head, the net irrigation requirement at outlethead has to be determined. This can be computed by applying efficiency factors, fromfield to turnout and from turnout to outlet. The field application efficiency (root zoneto the turnout) depends on preparation of land, irrigation methods and the flow rate. Itnormally varies from 70% to 80%. With the developed lands and good irrigationmethod, 75% application efficiency can be achieved and may be adopted. Theconveyance losses in field channel depend on length of field channel, soil strata,channel condition, type of lining, distribution of fields along the length of the channelwhich determine running time of each unit lengths of field channel, etc. However,
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LESSON 13.APPROACH TOPLANNING,CONSTRUCTIONAND ...LESSON 14.Survey AndInvestigationsFor Ofd WorksLesson 14 QuizLESSON 15.CHAK LAYOUTANDALIGNMENT OFFIELD CHAN...Lesson 15 QuizLESSON 16.Chak WaterDelivery System– Channel De...Lesson 16 QuizLESSON 17.Chak WaterDelivery System– Control St...Lesson 17 QuizLESSON 18.Chak WaterDelivery System– Control St...Lesson 18 QuizLESSON 19.Flow MeasuringDevices ForField ChannelsLesson 19 QuizLESSON 20.Materials forliningwatercourses
4/19/2016 MISD: LESSON 15. CHAK LAYOUT AND ALIGNMENT OF FIELD CHANNELS
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average conveyance efficiency of field channel from the turnout to the out let may betaken as 85%. The efficiency from root zone to outlet is therefore 75% x 85% =63.75%. The efficiency figures can be modified if specific data is available. The netirrigation requirement for every fortnight of year is calculated for the project croppattern. Based on this, the master statement of crop water requirement is prepared. Themaximum value of the irrigation water requirement is then adopted for thedetermination of chak size.
One such sample calculation is presented in Table 15.1. given in the end of this lecture.
As per Table 15.1, Fortnightly Peak Net Irrigation Requirement for the given
crop pattern is in the 26th fortnight (17/12 to 31/12) and is 46610 m³ per 100 ha ofI.C.A.
NIR = 46610 m³ for 100 ha of ICA
i.e. 466.10 m³/ha of ICA
Assuming field application efficiency of 75% and average conveyance efficiency offield channel as 85%, the overall efficiency between outlet and root zone works out as75% x 85% = 63.75% i.e. 0.6375.
NIR at outlet head = (466.10/0.6375) = 731 m³
Water delivered at outlet
At the rate of 30 lps (108 m³/hr) = 108 m³ x 6 days (flow period) x 24 hrs
= 15552 m³
Chak Size (ha of I.C.A.) = (15552/731) = 21.27 ha
If intensity of irrigation (ICA/CCA) for project under consideration is Say 80% then
Chak Size (ha of C.C.A.) = (21.27/0.8) = 26.58 ha.
Maximum chak size for the project is say 25 ha.
15.4 Length of Field Channel:
Excessive length of field channel results into more conveyance losses and complaintsfrom tail Enders about nonreceipt of desired discharge. Therefore, the maximumlength of field channel in one stretch from outlet generally should not be more than1000 m (1 Km).
15.5 Number of Farmers:
In canal irrigation, the water that is supplied at the Government outlet is to be sharedequitably by all the beneficiary farmers. This involves understanding and cooperationand therefore the number of beneficiaries has to be reasonably small. Generally 10 to15 beneficiaries can be organized properly for water sharing. The number ofbeneficiaries in a chak will depend upon the sizes of the holdings. Where the averagesize of holding is smaller, the chak size may be adjusted such that the maximumnumber of beneficiaries should not exceed 30.
15.6 Other Considerations:
As far as possible the chak should lie in one village only. Similarly roads, railway linesshould be considered while fixing chak boundaries. As far as possible chak boundariesmay be restricted upto road/railway line in order to avoid crossings. Consideringabovementioned factors the boundaries of the chaks be fixed and layout of chaks inthe whole command of a given canal be finalized. The cost economics of OFD worksshould also have bearing while finalizing the chak area and layout.
15.7 Alignment of Water Courses and Field Channels:
After the layout of the chaks is prepared, the locations of the outlets, jointly decided by
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the concerned Organizations are marked on the contour plan. Following aspects maybe considered while finalizing the location of the outlet.
i) The outlet to be located upstream of drop/fall on parent channel
ii) The FC should command the area within short distance (Say within 15m)
iii) As far as possible it should not cross borrow pit
iv) To avoid the road crossing on F.C., if possible, the location of the outlet should betaken d/s of the road.
If these locations are not suitable for the layout obtained earlier, changes are made byrelocating the outlets and increasing the number where necessary.
The sill level (of the outlet) should be taken at or near the bed of the minor ordistributory, so that the outlet can draw its full discharge of 30 l/s even when the minoris flowing half full. From the sill level, the bed level of the field channel near the outletis obtained.
The alignment of the field channel is then marked along high ground so that it cancommand maximum area. As far as possible, it should be taken along the boundary ofa holding so that the field is not artificially divided by the channel. The field channelswill be aligned either along the ridge so that irrigation can be done on both sides oralong the contour with irrigation only on one side, depending upon topography. Thefield channel should be marked down to each individual holding. It should be ensuredthat the turnout can be placed in the holding it serves and near the boundary. Whilemarking the alignment of the field channel, it should be seen that the channel cancommand the fields included in the design. For this purpose preferably the water levelin the field channel be kept as 15 cm higher than the ground, it is designed to serve.
A field channel must necessarily tail into a drain (natural or otherwise). In any caseF.C. should not be used as a tail of minor or distributory.
In case, the field channel is being aligned on a sloping ground leaving some area abovethe field channel as uncommanded, then it is obligatory to provide a catch water drainalongside the field channel on the upstream side. It should terminate in a natural gully.
The marking of the alignment should be done by the SDO. The locations of the outletsand the alignments of the field channels should then be marked on site and verified bythe Agricultural Supervisor/Junior Engineer and also by the SDO (and so certified)before the plans are submitted by SDO to DO for approval. The beneficiaries shouldalso be consulted and their suggestions, if found suitable, may be incorporated. Afterthese are approved, the location of the outlets can be finally informed to theConstruction Organization so that the construction can proceed. If required, a jointinspection should be carried out by Executive Engineer of Construction Organizationand the Divisional Officer of a Land Development Division. The approval of theDivisional Officer will be accorded on the plan (tracing on cloth/paper), on whichlocations of outlets and the alignments of field channels are shown, in ink.
Table 15.1 FORTNIGHTLY (14 days) NET IRRIGATION REQUIREMENT BYMODIFIED PENMAN METHOD
(For 100 ha of I.C.A.) of Kukadi irrigation project
S.No
Areainha.
Net irrigation Requirement in mm/m3
19 20 21 22 23 24 25 26 1 2 3
10/923/9
24/97/10
8/1021/10
22/104/11
5/1118/11
19/112/12
3/1216/12
17/1231/12
1/114/1
15/128/1
29/111/2
1Wheat
(1/1128/2)16
80 19 34 59 74 68 75 74
12800 3040 5440 9440 11100 10880 12000 11840
4/19/2016 MISD: LESSON 15. CHAK LAYOUT AND ALIGNMENT OF FIELD CHANNELS
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2
LocalJawar (R)
(1/1028/1)
15
7 20 48 67 67 63 66 62 48 24
1050 3000 7200 10050 10050 9450 9900 9300 7200 3600
3
Hy. Jawar(R)
(1/1028/1)
12
9 28 50 61 63 66 69 65 45
1080 3360 6000 7320 7560 7920 8280 7800 5400
4
Jawar(Rattoon)
(16/1023/1)
2
12 33 54 66 70 52 27
1800 4950 8100 9900 10500 7800 4050
5
Peas/Gram
(16/1023/1)
5
9 22 33 54 68 76 65 44
450 1100 1650 2700 3400 3800 3250 2200
6
Onion
(16/1012/1.)
2
83 21 28 44 60 69 61 65 69
1600 420 560 880 1200 1380 1220 1300 1380
7
Potato
(16/1027/1)
3
82 19 24 40 60 75 65 70 4
2460 570 720 1200 1800 2250 1950 2100 120
1050 4080 15070 32740 28290 35330 43560 46610 40100 30650 13340
Figures in numeration indicate NIR in mm and those in the denominator indicate Qty in m3
Peak Demand is in the 26th Fortnight (1712 to 3112) i.e.(46610 m3/100 ha of I.C.A)
The crop pattern of this project does not contain any crops growing in Hot Weatherseason viz 13 to 306