More of Contemporary Practices

A look at some of the common practices adopted to harvest rain...

Check dams
A check dam is generally constructed on small streams and long gullies formed by the erosive activity of water. The ideally a check dam is located in a narrow stream with high banks.

A check dam serves many purposes.

  • It cuts off the runoff velocity and reduces erosive activity
  • The water stored improves soil moisture of the adjoining areas and allows percolation to recharge the aquifers
While constructing a series of check dams on along stream course, the spacing between two check dams should be beyond their water spread. The height of the check dam should be such that even during the highest flood, water does not spill over the banks.
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Contour trenches
Contour trenches are used both on hill slopes as well as on degraded and barren waste lands for soil and moisture conservation and afforestation purposes. The trenches break the slope and reduce the velocity of surface runoff. It can be used in all slopes irrespective of rainfall conditions (i.e., in both high and low rainfall conditions), varying soil types and depths.

Specifications: Trenches can be continuous or interrupted. The interrupted one can be in series or staggered, continuous one is used for moisture conservation in low rainfall areas and require careful layout. Intermittent trenches are adopted in high rainfall areas. The trenches are to be constructed strictly on contours irrespective of the category.

Layout: The size of the trench depends upon the soil's depth. Normally 1,000 sq cm to 2,500 sq cm. in cross section are adopted. The trench may be of 30 cm base and 30 cm top width and square in cross section or it can be trapezoidal with side slopes 1:1. Based on the quantum of rainfall to be retained, it is possible to calculate the size and number of trenches.

Slope of the land
20 %
45 %
50 % (with soil of 30cm depth )
60 % (with soil of 30cm depth)
Horizontal interval 7.5m 9m 7.5m 9m
Vertical interval 1.5m 4m 3.75m 5.85m
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Bunds are small earthen barriers provided in agricultural lands with slopes ranging from 1 to 6 percent. They control the effective length of slope and thereby reduce the gain in velocity of runoff flow to avoid gully formations. Bunds are constructed with the following objectives:
  • To increase the time of concentration of rainwater where it falls and thereby allowing rainwater to percolate into the soil
  • Converting a long slope into several ones as to minimise velocity and thereby reducing the erosion by runoff water
  • To divert runoff either for water harvesting purposes
Types of bunds

a) Graded bunds: Graded bunds are constructed in medium to high rainfall area - having annual rainfall of 600mm and above - and in soils with poor permeability or those having the crust formation tendency.
b) Contour bunds: Contour bunds are constructed in relatively low rainfall areas- having annual rainfall of less than 600 mm ; particularly in the areas having light textured soils. They are essentially meant for storing rainwater received during a period of 24 hours at 10 years recurrence interval. The major considerations are maximum depth of water to be impounded, design depth of flow over waste weir and desired free board
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Contour Stone wall
It is constructed with stones across the hill slopes thereby intercepting the surface runoff. These terraces help in retarding the soil loss and conserving soil moisture. Spacing of such stone walls are not rigid. Spacing ranging from 10 m to 30 m can be adopted depending upon slope of the terrain. For the construction, a shallow trench has to be dug and the stones collected and packed directly on to the foundation and in the super structure to form the terrace. The stones should be properly interlocked. The soil excavated to form the foundation for the terrace is used for forming a small bund on the upstream side of the terrace. Terrace is stabilised by planting suitable vegetation on the bund.
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Gully control
Gully erosion generally starts as small rills and gradually develop into deeper crevices. Ravines are a form of extensive gully erosion. Gully erosion not only damages the land resources but the same time contribute larger amount of sediment load to river system.

Classification of gullies:
For the purpose of gully control measures gullies are classified based on several factors. One method takes into consideration the gully depth and catchment area. The following table give the classification of gullies:
Description Gully depth Catchment area
Small 1m or less 2 ha. Or less
Medium 1 to 5m 2 - 20 ha.
Large Greater than 5m Greater than 20 ha

Gully plugs are earthen embankments usually constructed for blocking the active and erosion prone gullies for their stabilisation.

a) brushwood dams
b) loose rock dams
c) woven wire dams

Use locally available vegetative cutting in their construction. In the woven dam a wire mesh is used to hold the stone in place. All the check dams involving stones are to be adopted in areas where stones are available easily and in plenty. The rock fill dam and the woven wire dam are more lasting than the loose rock dam. There are no standard principles of the design of these structures. These are to be designed and constructed based on the needs and availability of materials in a given situation. The overall height of temporary check dams use for this purpose should not be more than 75 cms; an effective height of about 30 cms is satisfactory.

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Sub-Surface Dams
Groundwater dams are structures that intercept or obstruct the natural flow of groundwater and provide storage for water underground. They have been used in several parts of the world, notably India, Africa and Brazil. Their use is in areas where flows of groundwater vary considerably during the course of the year, from very high flows following rain to negligible flows during the dry season.

The basic principle of the groundwater dam is that instead of storing the water in surface reservoirs, water is stored underground. The main advantages of water storage in groundwater dams is that evaporation losses are much less for water stored underground. Further, risk of contamination of the stored water from the surface is reduced because as parasites cannot breed in underground water. The problem of submergence of land which is normally associated with surface dams is not present with sub-surface dams.

Figure - 1

There are two main types of groundwater dam: the sub-surface dam and the sand storage dam. A sub-surface dam intercepts or obstructs the flow of an aquifer and reduces the variation of the level of the groundwater table upstream of the dam. It is built entirely under the ground (see figure 1).

Figure - 2

The sand storage dam is constructed above ground. Sand and soil particles transported during periods of high flow are allowed to deposit behind the dam, and water is stored in these soil deposits (see figure 2). The sand storage dam is constructed in layers to allow sand to be deposited and finer material be washed downstream (see figure 3).

Figure - 3

A groundwater dam can also be a combination of these two types. When constructing a sub-surface dam in a river bed, one can increase the storage volume by letting the dam wall rise over the surface, thus causing additional accumulation of sediments. Similarly, when a sand-storage dam is constructed it is necessary to excavate a trench in the sand bed in order to reach bedrock, which can be used to create a sub-surface dam too. Groundwater dams are built across streams or valleys. A trench is dug across the valley or stream, reaching to the bedrock or other stable layer like clay. An impervious wall is constructed in the trench, which is then refilled with the excavated material. Various materials may be used for the construction of groundwater dams. Materials should be waterproof, and the dam should be strong enough to withstand the imposed soil and water loads. Dams may vary from 2 to 10 metres high. Materials include compacted clay, concrete, stones and clay, masonry wall or plastic sheets.

The reservoir is recharged during the monsoon period and the stored water can be used during the dry season. Excess water flows over the top of the dam to replenish aquifers downstream. Water may be obtained from the underground reservoir either from a well upstream of the dam or from a pipe, passing through the dam, and leading to a collection point downstream (see figures 1 and 2). Groundwater dams cannot be a universally applicable as these require specific conditions for functioning. The best sites for construction of groundwater dams are where the soil consists of sands and gravel, with rock or a permeable layer at a depth of a few metres. Ideally the dam should be built where rainwater from a large catchment area flows through a narrow passage. The Central Ground Water Board has sited and constructed a number of sub-surface dams in Kerala in the 1980s. Presently, Shri Vivekananda Research and Training Institute (SVRTI), under the guidance of K C B Raju is involved in constructing groundwater dams in Kutch district of Gujarat.

Source: Dr. K.C.B. Raju, Nanda Gautam, 492 10th Cross Sadashiv Nagar, Bangalore - 560 080

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Percolation ponds
A percolation pond, like an irrigation tank, has a structure to impound rainwater flowing through a watershed, and a wasteweir to dispose of the surplus flow in excess of the storage capacity of the lake created. The section of the bund is similar to that of an irrigation tank, except that the cut-off trench is taken to a depth equal to half the height of the bund. The purpose of the cut-off in the case of the percolation tank is just to prevent erosion of the downstream slope of the bund due to piping. The cut-off should be shallow enough to permit the percolating water to pass downstream into the aquifer. The percolation tank bund has a hearting and a casing, and is provided with stone pitching on the upstream face and turfing on the downstream slope. A masonry waste weir is also necessary to pass surplus water. Drains are provided under the bund to lead water percolating into the bund safely downstream. The percolation tanks of Maharashtra have, on an average, a larger storage capacity than the rapats of Rajasthan. The storage capacity of percolation pond is around 30 to 60 million litres
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No water
Water war
Look up!
How to harvest
    In rural areas
 n In urban areas
Jal yodhas
Way ahead

Geographical Information System

Remote sensing coupled
with the use of Geographical Information Systems (GIS) can be used to identify runoff potential zones and location of suitable sites for water harvesting. They can also be used to identify sites in watersheds that have not been gauged, and where, due to very steep slopes, the runoff drains out fast. This system was used to identify 18 suitable sites for rainwater harvesting structures in the watershed of the Song river at Bandal, Uttaranchal.

GIS is a software used for presenting and interpreting the attributes of a particular land area such as topography, soil type and vegetation. The Integrated Land and Water Information System (ILWIS 2.1), a GIS software developed at the computer centre of the International Institute of Aerospace Survey and Earth Science (ITC, 1997), the Netherlands was used for the conventional operations. ERDAS IMAGINE - 8.2 was used for digital image processing and classification.

The methodology consisted of preparing various resource maps such as landuse/land cover by using IRS-1C, LISS data by digital image processing techniques, coupled with ground truth data. Digital elevation model, slope map, aspect map, classified map, soil map, drainage and buffer maps for village and agriculture areas were created in a GIS environment. Input parameters deduced from the basic thematic maps were then integrated with field data to generate runoff potential zoning. This model uses rainfall data, temperature data, soils, landuse and rooting depth of different types of vegetation for calculating the soil moisture deficit, soil moisture surplus, evapo-transportation, surface runoff and other parameters.

Based on an analysis of the data generated, some of the rainwater harvesting structures that could be constructed in this sub-watershed include 14 farm ponds, three groundwater recharge structures and one percolation tank.

Despite several government programmes, Ramanathpuram district of Tamil Nadu faces severe water shortage. Vivekanand Kendra, a voluntary organisation, tries to convince villagers in Mudukulattur block to participate in reviving ooranis (traditional tanks) and harvest rain from the rooftops. The organisation set about motivating villagers through film shows and weekly group discussions that raised the issue of community-based water management. Work began with desilting of ooranis. To increase the storage capacity, the bed of the ponds was widened. The ponds were also deepened, while care was taken to avoid saline water ingress from the sea.
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