Minor Irrigation & Command Area Development 3(2+1)

LESSON 15. CHAK LAYOUT AND ALIGNMENT OF FIELD CHANNELS

15.1 Layout of Chaks:

Following factors are considered for finalizing chak boundaries and total layout of chaks.

i) Topography (Ridges, vallies, local depressions, high patches, etc.)

ii) Maximum permissible area which can be irrigated in peak rotation by a prescribed discharge 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 so as to limit the boundary of the Chak to natural drain / village boundary / road / railway lines.

15.3 Maximum Permissible Chak Size:

The chak size plays an important role in the efficient scheduling of irrigation. It is the area 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 of Maharashtra 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 14 days. Now the flow period of 6 days in a rotation period of 14 days is adopted for chak design. With the discharge of 30 l/s and the flow period of 6 days in a rotation, the only 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 for crops as per the approved project-cropping pattern. This is generally the NIR at root zone (expressed in mm) considering effective rainfall and the special needs of different crops. Since the water is released at outlet head, the net irrigation requirement at outlet head has to be determined. This can be computed by applying efficiency factors, from field to turnout and from turnout to outlet. The field application efficiency (root zone to the turnout) depends on preparation of land, irrigation methods and the flow rate. It normally varies from 70% to 80%. With the developed lands and good irrigation method, 75% application efficiency can be achieved and may be adopted. The conveyance 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 channel which determine running time of each unit lengths of field channel, etc. However, average conveyance efficiency of field channel from the turnout to the out let may be taken 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 net irrigation requirement for every fortnight of year is calculated for the project crop pattern. Based on this, the master statement of crop water requirement is prepared. The maximum value of the irrigation water requirement is then adopted for the determination 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 of I.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 of field channel as 85%, the overall efficiency between outlet and root zone works out as 75% 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 complaints from tail Enders about non-receipt of desired discharge. Therefore, the maximum length of field channel in one stretch from outlet generally should not be more than 1000 m (1 Km).

15.5 Number of Farmers:

In canal irrigation, the water that is supplied at the Government outlet is to be shared equitably by all the beneficiary farmers. This involves understanding and cooperation and therefore the number of beneficiaries has to be reasonably small. Generally 10 to 15 beneficiaries can be organized properly for water sharing. The number of beneficiaries in a chak will depend upon the sizes of the holdings. Where the average size of holding is smaller, the chak size may be adjusted such that the maximum number 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 lines should be considered while fixing chak boundaries. As far as possible chak boundaries may be restricted upto road/railway line in order to avoid crossings. Considering above-mentioned factors the boundaries of the chaks be fixed and layout of chaks in the whole command of a given canal be finalized. The cost economics of OFD works should 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 the concerned Organizations are marked on the contour plan. Following aspects may be 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 be taken d/s of the road.

If these locations are not suitable for the layout obtained earlier, changes are made by relocating 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 or distributory, so that the outlet can draw its full discharge of 30 l/s even when the minor is flowing half full. From the sill level, the bed level of the field channel near the outlet is obtained.

The alignment of the field channel is then marked along high ground so that it can command maximum area. As far as possible, it should be taken along the boundary of a holding so that the field is not artificially divided by the channel. The field channels will be aligned either along the ridge so that irrigation can be done on both sides or along the contour with irrigation only on one side, depending upon topography. The field channel should be marked down to each individual holding. It should be ensured that the turnout can be placed in the holding it serves and near the boundary. While marking the alignment of the field channel, it should be seen that the channel can command the fields included in the design. For this purpose preferably the water level in 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 case F.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 above the field channel as uncommanded, then it is obligatory to provide a catch water drain along-side 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 outlets and the alignments of the field channels should then be marked on site and verified by the 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 should also be consulted and their suggestions, if found suitable, may be incorporated. After these are approved, the location of the outlets can be finally informed to the Construction Organization so that the construction can proceed. If required, a joint inspection should be carried out by Executive Engineer of Construction Organization and the Divisional Officer of a Land Development Division. The approval of the Divisional Officer will be accorded on the plan (tracing on cloth/paper), on which locations of outlets and the alignments of field channels are shown, in ink.

Table 15.1 FORTNIGHTLY (14 days) NET IRRIGATION REQUIREMENT BY MODIFIED PENMAN METHOD

(For 100 ha of I.C.A.) of Kukadi irrigation project

S. No		Area in ha.	Net irrigation Requirement in mm/m3
			19	20	21	22	23	24	25	26	1	2	3	4	5
			10/9-23/9	24/9-7/10	8/10-21/10	22/10-4/11	5/11-18/11	19/11-2/12	3/12-16/12	17/12-31/12	1/1-14/1	15/1-28/1	29/1-11/2	12/2-25/2	26/2-11/3
1	Wheat (1/11-28/2)	16	-	-	-	80	19	34	59	74	68	75	74	51	7
						12800	3040	5440	9440	11100	10880	12000	11840	8160	1120
2	Local Jawar (R) (1/10-28/1)	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) (1/10-28/1)	12	-	9	28	50	61	63	66	69	65	45	-	-	-
				1080	3360	6000	7320	7560	7920	8280	7800	5400
4	Jawar (Rattoon) (16/10-23/1)	2	-	-	-	12	33	54	66	70	52	27	-	-	-
						1800	4950	8100	9900	10500	7800	4050
5	Peas/Gram (16/10-23/1)	5	-	-	9	22	33	54	68	76	65	44	-	-	-
					450	1100	1650	2700	3400	3800	3250	2200
6	Onion (16/10-12/1.)	2	-	-	83	21	28	44	60	69	61	65	69	5	-
					1600	420	560	880	1200	1380	1220	1300	1380	100
7	Potato (16/10-27/1)	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	8260	1120

 

Figures in numeration indicate NIR in mm and those in the denominator indicate Qty in m³ Peak Demand is in the 26^th Fortnight (17-12 to 31-12)  i.e.(46610 m³/100 ha of I.C.A)

            The crop pattern of this project does not contain any crops growing in Hot Weather season viz 1-3 to 30-6