Introduction
The soil and water conservation measures in vogue, namely, contour bunding, graded bunding etc. have their inherent structural and functional limitations. Contour bunds cause yield reduction to the tune of 25 to 30% due to water stagnation in about 16% of the inter-bund area for prolonged periods. On the other hand, graded bunds drain out the excess rainwater too far, out of the reach of small farmers.
Vertisols and associated soils occur over an area of 72 million hectares (22.12% of total geographical area) of our country. They are mostly confined to the semi-arid region of the Deccan plateau, spread over the states of Madhya Pradesh, Maharashtra, Andhra Pradesh, Karnataka and Tamil Nadu.
Situated in tropics, a large part of this area receives low (<750mm) and erratic rainfall. In spite of low rainfall, short but high intensity (32 to 120 mm h-1) rains are common. This causes sheet erosion (an average of 12.5 t ha-1year-1 on 1-2 %land slopes) in easily erodible black soils due to low intake rates, poor aggregation and poor vegetation cover.
Conservation ditching technique, developed and tested at Central Soil & Water Conservation Research & Training Institute, Research Centre, Bellary has shown great promise to conserve water resource at field level for increasing the productivity of semi arid black soils.
About the Technology
- Conservation ditching is primarily adaptable in deep black soils with low infiltration rates (<1 mm hr-1) receiving low annual rainfall (< 750 mm).
- Conservation ditch is a shallow trapezoidal dugout, laid on contour to serve the dual purpose of soil conservation and as a water storage structure at the individual field level in low rainfall area of deep black soils. This can also be vaguely described as an ‘inverted contour bund’ with flatter upstream sides and steep downstream sides.
- As a terrace structure, a well-designed conservation ditch will store the inter-terrace design runoff within its confines; thus drastically cutting down its potential to initiate/accelerate soil erosion on the downstream of the ditch.
- As a storage structure, due to low infiltration rates in the deep black soils, sufficient volume of the stored water will be available in the ditch for about a week to ten days after the runoff collection in the ditch. The available water at low lifts could be manually lifted and applied to the downstream area to augment the soil profile moisture and stabilize yields of dryland crops.
Design Specifications
Runoff storage capacity
The ditch is designed to store 200 to 300 m3 of runoff from a hectare of inter terrace land in black soil region of Deccan plateau.
Spacing of ditch
The spacing between ditches is same as that of contour or graded bund for similar soil, climatic and topographical conditions. However, for all practical purposes, the spacing is decided as follows for different field slopes in black soils:
| Field slope(%) | Horizontal spacing (m) |
| 0.5 – 1.5 | 50-60 |
| 1.5 – 2.5 | 40-50 |
| 2.5 – 3.0 | 35-40 |
Depth
The depth of the ditch is made in the range of 0.6 to 0.9 m depending upon the storage volume and soil depth.
Bottom width
Bottom width can be made 0.6 to 0.9 m. However, 0.6 m is considered given construction convenience.
Side slopes
The upstream slope is made 5:1 to provide safety against scouring and the downstream slope is made 1.5:1 to conform to the angle of repose.
Cross section of ditch=0.5 x [Top width (T) + bottom width (B)] x Depth (D)
= 0.5 x (4.57+0.6) x 0.6= 1.57 m2
Hence, design capacity = 1.57 m3 per running metre of ditch
Length of ditch (L)
About 30 to 300 m long ditch is constructed in segments of 100 m. In case of series of ditches, the unditched stretches (4 to 5 m) should be staggered for effective erosion control.
Bottom level (bed slope)
The bed slope of the ditch must be maintained at zero level throughout. In valley portions, a small embankment is to be raised suitably to constantly maintain the depth of storage. The layout, dimensions of the typical size of a ditch are shown in Fig 1&2.
Technology Implementation
Ditch layout
The following steps are followed for laying out a conservation ditch:
- Collect the cadastral map from revenue office/village secretary. Do the reconnaissance survey of the field along with farmer. Draw a rough sketch showing the survey no., field boundaries on upstream and downstream, waterways, valleys portions across the slope, soil depth, etc.
- The difference in elevation of the field is to be determined by using Hydro marker or Abney level. One of the spacings mentioned earlier is selected depending upon the field slope.
- For the alignment of the ditch on watershed/sub-watershed scale, professional surveyors may prepare contour maps, which are then super-imposed on cadastral/village map. Slope and drainage pattern is found out from this map for the purpose of making the alignment on the map and to transfer to the ground with permissible deviation.
- Theoretically, conservation ditches are to be laid out along exact contour points. However, permissible deviations need to be applied to negotiate local valley/depression or ridge so that excessive curvature is avoided. In places where straightening/smoothening of the ditch is required, it is suggested that in the valley portions, the ditch sidewall height is suitably raised by filling the valley line. For this purpose, dug out soil could be used. But care should be taken to see that such filled-up portions are raised slightly (15-20 cm) above the adjoining portions of the bed of ditch to prevent surplus water from flowing over such filled-up (made-up) soil.
In black soils, overland flow transforms into rill erosion. Left unchecked, the rill head at the entry to the ditch with a fall of about 0.6 m tends to work upwards into the catchment area simultaneously damaging the ditch shape.
Hence, a vegetative barrier, which mechanically filters out the soil allowing runoff water to flow down into the ditch and checking effectively the rill head advancement, is quite necessary. For creating the barrier, two rows of Agave species with a spacing of 60 cm x 60 cm are planted in a staggered manner.
Construction Procedure
Step 1: As shown Fig 4, the square pits of side ‘a’ equal to the bottom width of the ditch and depth equal to the design depth are dug at 20 m intervals along the aligned line.
The bottom width ‘a’ is kept at 0.6-0.9 m to facilitate easy execution with the cutting of central box portion with permissible deviations on alignment. Pegs are driven into these pits such that their tops correspond to the bottom level of the ditch, which is constant. Since the ditch is on contour, it can be laid out to any length limited to field borders. However, a maximum length of 300 m is appropriate.
It is preferred to construct ditches in multiple lengths of 100 m, by leaving 4 to 5 m lengths unditched in between for free passage of human beings, cattle and farm implements from the field to field. Then, a trench (box portion) joining all the square pits are dug (Fig 5).
Step 2: From the central line, borders marking the edges of the square pits are made, and also the upstream and downstream edges of the ditch are marked using the side slope ratios. The upstream side slope of the ditch is kept flat, i.e. 5:1 (Horizontal: Vertical) to avoid scouring due to the action of incoming runoff. The downstream side is kept at 1.5:1 conforming to the angle of repose of the soil so that caving in of the soil after saturation does not take place.
Step 3: To start with, the full sections of the ditch are dug near the already existing box trench 20 m apart. A template made of GI/PVC pipe/bamboo conforming to the cross section of the ditch is used for maintaining the design shape of the ditch. The trench is covering upstream, and downstream sides are dug concerning aligned line, by periodically checking the shape of the ditch so formed with the template.
Step 4: The soil could, preferably, be spread safely over low-lying areas and depressions to make them nearly level.
Bullock drawn level blade/buck scrapper, commonly available in black soil region, can be used for spreading the spoil.In the same manner along downstream bank horticulture plants such as mango, sapota, ber, anjura, orange, drumstick etc. can be grown as shown in Fig 6.
Step 5: Rooted slips of the Dichantium annulatum can be planted on side surfaces of the ditch, which not only thrives well under the prolonged submergence condition but also produces fodder grass. The survival percentage of grass is more than 90% requiring 10% mortality replacement. Leguminous species used as green manure are also preferred for growing on downstream bank for utilizing the built up moisture regime (Box 1)
Maintenance / Management
In some cases, the ditch may come across the waterway. At the meeting point of waterway and ditch, the waste weir should be constructed on the upstream side of the ditch for protection against scouring. The waste weir should also be constructed on the downstream bank at lowest vulnerable point or depression of the ditch to prevent gully formation. The gaps in Agave rows should be replanted periodically till the vegetative barrier is established. The grass on ditch side slopes should be maintained. Disilting of ditch may be done once in three years.
Cost
| Work | Top width (T) (m) | Bottom width (B) (m) | Depth (D) (m) | Quantity per running metre {0.5 x (T+B) x D}(Cum) | Rate(Rs.) | L (m) | Total cost(Rs.) |
| Earth work | 4.57 | 0.61 | 0.6 | 1.57 | 55 (per cum) | 200 | 17270 |
| Plantation on banks | – | – | – | – | 15 (per RM*) | 200 | 3000 |
*RM = running meter
Recycling of Stored Runoff Water
The ditches are designed to store 90% of 10-year 24-hour maximum run off. The ditch holds entire run off during below normal years and 70 – 90 % of annual runoff during normal rainfall year. It is advantageous to use the stored water at the earliest opportunity for irrigating the downstream land whenever the ditch is full with runoff water. This technique prevents unproductive storage losses.
After every ditch-filling rainfall, the water stays for about a week to ten days (Photo 1). Since the water is available at low lifts (0.6-0.9 m), traditional low-lift and low-cost hand operated lifting devices such as swing basket and Archimedean screw can be used for lifting the water. The downstream area can easily be irrigated by gravity. The water is available for irrigation at different stages of crop growth and also for pre-sowing irrigation. Horticulture or agro-horticulture system is to be preferred in the downstream area as more area (about 40% of catchment area) could be irrigated from ditch as against the limited area (15-20% of catchment area) under annual crops.
Resource Conservation Benefits
Besides harvesting and use of runoff, the nutrient rich silt deposited on the bed of the ditch can be harvested back to the cultivated land @ 4.5 t ha-1yr-1 in the first year, 2.6 t ha-1 yr-1 in next 4 years and 1.8 t ha-1 yr-1 after 4 years. The harvested soil is clay-to-clay loam in texture. The nutrients @ 357 kg ha-1 of N, 38.5 kg ha-1 of P2 O5 and 851 kg ha-1 of K2O are retained along with silt in the ditch.
Production / Output
Conservation ditching is effective in mitigating the drought condition. Continuous charging of downstream land with ditch water can help to realize 5 to 7 times more yield as compared to the situation of withering crops in drought years. Ditch irrigation can produce additional yields of 650-850 kg ha-1 in jowar and 298 kg ha-1 in safflower, which works out to 35-48% increase over unirrigated condition. The water use efficiency in
The water use efficiency in jowar can be around 124 kg ha-1 cm-1. Under normal rainfall condition the maximum grass yield (Dichantium annulatum) of 16 t ha-1 year-1 is obtained by the second year. Two cuttings, the first one in October and the second one in December, can be taken. This serves as an incentive to the farmer for proper maintenance of ditch. Other potential uses are reflected in Box 2.
Economics
On an average, the additional sorghum grain yield expected due to ditching is 628 kg ha-1. The net present value of additional net benefits is Rs. 2843 ha-1 year-1 at 10% discount rate considering a 10-year life span with sorghum as the test crop.
Scope of Application
This technology has the potential for application in black soil region of Karnataka, Andhra Pradesh, Maharashtra and Tamilnadu. The technology is becoming popular in Akola region of Maharashtra where it is being promoted by Dr Panjabrao Deshmukh Krishi Vishvavidyalaya, Akola.
Source-
- Central Soil and Water Conservation Research and Training Institute, Karnataka
