Agricultural Engineering Notes | EduRev

Watershed Planning and Management Notes - Agricultural Engg

Agricultural Engineering : Agricultural Engineering Notes | EduRev

The document Agricultural Engineering Notes | EduRev is a part of the Agricultural Engineering Course Watershed Planning and Management Notes - Agricultural Engg.
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In-Situ Rainwater Conservation Techniques

In this Section, in-situ rainwater conservation techniques are described. They can provide lasting solution to the drinking water problem, when adopted on a sustained basis. Now let us briefly discuss the rural drinking water problem in India.


According to the National Drinking Water Mission of India, a village is classified as a problem village if:

  • The source of water i.e., a well or a hand pump is located at a distance of more than 1.4 km from the habitat.

  • The source dries up during the summer months.

  • The source has inadequate supply. The Government of India norm for adequate supply in rural areas is 40 liters/capita/day (lpcd) and 30 liters/cattle/day.

  • The source contains total dissolved solids/arsenic/fluoride/iron in concentrations above their permissible limit.

According to Agarwal (2000), the number of such problem villages identified in 1972 was 150,000 and out of these, 94,000 were provided a source of drinking water by 1980, as per Government of India (GoI) records. The number of remaining problem villages should then have become 56,000. However, a separate inventory showed that the total number of problem villages had now become 231,000 in the same year 1980. Again according to Government of India reports, 192,000 villages were provided a source by 1985, but 140,975 villages remained without a source. Out of these, by 1997 the problems of 110,371 villages were apparently addressed, but still the number of remaining problem villages was 61,747 instead of 30,604 which one can obtain through simple subtraction.

This bewildering and confusing statistical jugglery is a sad reflection on the failure of the methodology used till now in solving the acute water problems, which have also increased manifold over the years. It also means that:

  • The solutions found to problem villages were not sustainable.

  • Some new villages which were earlier having an adequate source have turned ‘problematic’, possibly because of over-exploitation and

  • The increasing practice of tapping deeper aquifers has led to problems of drinking water quality.

One important point to be noted is that in all the Government machinery approaches so far, the methodology adopted was to locate what was already provided for by nature if possible, at the problem village itself, or otherwise provided through expensive pipelines from elsewhere, where the natural source was available at that point in time.

There was hardly any attempt made at creating a new source at the problem village itself through harvesting and conserving the precipitation endowment received annually. The second reason was that the Government efforts failed at the operation and maintenance level. This happened because the people were not involved in the process of solving the problem. The people were neglected and in turn they neglected the upkeep of the sources made available by the Government departments. Rainwater roof catchment systems (RRCS) as existing in many individual homes of the states mentioned were also surveyed and studied. Among the north-eastern states of India viz., Arunachal Pradesh, Nagaland, Mizoram, Manipur, Meghalaya, Assam and Sikkim, RRCS are accepted particularly in those places where the homes are scattered and the piped water supply tem could not reach individual homes. In contrast, the RRCS cannot be readily applied in densely populated areas, where there are many industries/factories or excessive traffic load is causing to precipitate acid rains. In all the states mentioned above, the average annual rainfall is of the order of 1500 - 3000 mm and there is no concentration of industries.

Rain Water Harvesting

The term rainwater harvesting (RWH) refers to direct collection of precipitation falling on the roof or onto the ground without passing through the stage of surface runoff on land. It is sometimes used to describe the entire gamut of water harvesting. We shall use it here only in the specific sense. There are two types of rainwater harvesting viz., roof water harvesting and ‘In situ’ water harvesting. In this lesson, rainwater harvesting methods are described.

Basically, there are two types of rainwater harvesting schemes - those designed for agricultural use and those designed for human use. Rainwater catchment schemes intended for agricultural use require large catchment areas. In this case, use of the ground surface is the obvious choice. However, water for human use should be more convenient and cleaner than water for agriculture use. Roofs are an obvious choice for a catchment surface as their elevation protects them from contamination and damage which are common to ground surface catchments.

The advantages of rainwater roof catchment system are:

  • The quality of rainwater is high, if collected and stored in a hygienic manner. The system is independent, and therefore suitable for scattered settlements.

  • Local materials and craftsmanship can be used in rainwater system construction.

  • No energy costs are needed to run the system.

  • Ease of maintenance by the owner/user.

The disadvantages of rainwater roof catchment system are:

  • The high initial cost.

  • The water available is limited by rainfall and roof area. For long dry seasons, the required storage volume may be too high, which is very expensive.

  • Mineral free water has no taste while people may prefer the taste of mineral rich water.

  • Mineral free water may cause nutrition deficiencies in people who are already on mineral deficient diets.

Roof Water Harvesting

Traditionally, rain water harvesting comprises collection of the precipitation falling onto the roof or terrace of a building and storing it in a waterproof sump at ground level for use year round or in periods of scarcity of supply from other sources such as a pond or a well. Roof water harvesting was practiced, as a matter of necessity, mostly in the low rainfall areas of the country, having annual rainfall less than 500 mm per year. Roof water harvesting and storage systems are common features of all old buildings in North Gujarat, Saurashtra and Western Rajasthan. Roof water harvesting was also practiced in some coastal areas where the groundwater was brackish. Modern construction during the last 50 years, especially in urban areas, has no provision for the collection and storage of roof water. The increase of population and inefficient system of distribution of municipal water supply has led to seasonal scarcity of domestic water supply in practically all the urban agglomerates. The utility of roof water harvesting is now being realized and the movement of roof water harvesting is slowly gathering momentum in urban areas. In prevailing situations of uncertain supply, having a captive source of potable water is a main source of water security for a dweller. The rainwater stored over the ground in a sump or recharged into a dug well or an open well, provides much-needed succour during the summer months. Traditionally, the rainwater collected from roofs was always stored in sumps. In modern days, the roof water is stored in a sump and/or recharged into the local aquifer. The practice of using rainwater for directly recharging the local aquifer is becoming popular in urban areas.

Roof water harvesting has also become necessary now in hilly areas having high rainfall. The traditional perennial sources of water in such areas are springs. However, the yield of these springs has either dwindled over the years due to deforestation or the total amount supplied by them has become inadequate because of increase in population.

In some parts of Andhra Pradesh, Madhya Pradesh, Gujarat and Rajasthan, the level of fluoride in groundwater is above the permissible limit, (i.e., 1.5 mg/1). In parts of West Bengal and Bangladesh, the groundwater also contains arsenic above the permissible limit of 50 μg/l. In these situations also, roof water harvesting is desirable although there may be no shortage of groundwater. Rainwater is practically free of dissolved solids and also does not have substances such as arsenic and fluoride.

There are areas where there is no problem with groundwater quality, or where the water table in the monsoon season does not rise up to ground level. In such areas, it is desirable and cheaper to recharge the collected precipitation into the groundwater reservoir through a percolation pit in the ground or through an existing open well or a tube well. However in areas where the groundwater quality is poor due to excess occurrence of dissolved salts/fluoride/arsenic or due to anthropogenic pollution, surface storing in sumps or other storage structures becomes a necessity. Sumps are also the only option for storing the roof harvested water in the case of a hilly terrain, having slopes or a laterite cover. Generally in such areas, aquifers having adequate storage capacity are generally absent.

If roof water harvesting is practiced on a large scale in an urban area, then it also helps in reducing the severity of floods, which follow a heavy downpour. Similarly if it is used for spot recharging by large number of households, then it helps in restoring the water table and also in improving the quality of water. Another benefit accruing from roof water harvesting in an urban area is that it reduces the demand on the municipal water supply system -that in general is inadequate to meet the needs of each and every household.

19.2 Rainwater Conservation through Storage

The Storage Tank: A satisfactory storage tank is the most important part of the roof water harvesting System. It is difficult to construct and must be a durable device; hence it is the most expensive component of the system. The materials used are masonry, concrete, ferro-cement, plastics, metal sheets etc. The design stage of the project involves sizing the storage tank. There are a number of methods that can be used to determine the tank volume.

Dry Season Demand versus Supply: This approach considers the length of the dry period as a design constraint. The tank is designed so that it accommodates the household demand during the dry season. For this reason, the method is most appropriate where there is a definite wet/dry period during the year. The length of the dry period can be estimated by:

  • Asking farmers and residents about the longest drought they remember.

  • By estimating from official weather analysis data the number of consecutive dry months per year. The dry season demand versus supply method should also consider the maximum drought length in the light of its probability of occurrence.

The dry season demand versus supply gives only a rough estimate of supply and demand. However, it does not take into account variations in annual rainfall patterns. A better method of tank sizing involves the Mass Curve Analysis technique.

Mass Curve Analysis: A more accurate method of sizing a tank involves an analysis of data using the mass curve technique. Successful use of the technique requires approximately 10 years of data.

First, an approximation of the runoff coefficient [i.e., ratio of runoff to rainfall] is required. Some rainwater will be lost during collection. This amount is accounted in the runoff coefficient. This is not a precise value but is estimated on the basis of the type of roof, the condition of gutters and piping, and the evaporation expected from the roof and tank. Approximate runoff coefficient values are given in Table 19.1 as follows:

Table 19.1. Runoff Coefficients  

Type of Roof

Good Gutters

Poor Gutters

Metals

0.9

0.8

Other roofs

0.8

0.7

The Filter: Whenever it is apprehended that water may contain dust or other organic matter from the roof, simple filtration device using crushed charcoal, sand and gravel or coconut fibers or some combinations thereof as media may be installed over the storage tank.

Operation and Maintenance

Rooftop catchment Surfaces collect dust, organic matter and bird droppings, which can clog channe1s, cause sediment buildup on the tank bottom apart from contamination of the stored water. During periods of no rain, these materials are accumulated on the roof and they are washed off with the first rain.

The following steps are necessary for proper maintenance of various components:

Roof: (i) Roof must be periodically cleaned out of dust, branches of trees, leaves, bird droppings etc. (ii) Corrugated metal sheet requires to be painted preferably before each monsoon. (iii) Clay tiles are to be checked from time to time and broken tiles are to be replaced. (iv) Support Structure of the roof is to be checked time to time.

Gutter System: (i) The gutter must be cleaned frequently to prevent overflowing during heavy rains. (ii) The joints of the gutter should be checked periodically and made correct if there is a likelihood of leakage. The joints can be sealed with tar or rubber and the jointing compound should not contaminate water. (iii) The slope of the gutter should also be checked from time to time. (iv) Metal gutters are required to be painted when required. The support of the gutter should also be checked. This can be accomplished by tying wire around the gutter and fastening it to the roof [Fig. 19.1].

Storage Tank: The maintenance requirements of the tank will eventually depend on the effectiveness of the first flush system and the frequency of roof and gutter cleaning. Contamination can be avoided by diverting the first 10 - 20 liters of rain from the tank. Flush traps can be used to prevent the first flush from reaching the tank. In this case, the plastic pipe over the reservoir collects the first flush water from the roof and the removable end allows discharge after each rainstorm.

 Agricultural Engineering Notes | EduRev

Fig. 19.1. A Typical Roof Water Harvesting System. 

Another important factor is the quality of the tank cover and screening in the inlet and outlet. Sunlight reaching the water will promote algae growth. Unprotected openings will also encourage mosquito breeding. So, the following steps are necessary: (i) The inside of all tanks require periodic cleaning. The tank walls should be scraped annually. Vinegar, baking soda or bleaching powder solutions are commonly used as cleaning agents. (ii) Sediments should be removed annually. (iii) Care must be taken not to contaminate the next volume of incoming storage water. (iv) If cracks in the tank wall are observed, they should be re-plastered after each cleaning of the tank surface. (v) Only after cleaning and disinfection, the water should be allowed to enter. (vi) The tank cover should be checked for tightness so that mosquito and other insects cannot find entry into the tank. (vii) The entry pipe and the overflow pipe should be checked for proper screening arrangements to prevent entry of flies etc. (viii) Sheet metal tanks require to be painted periodically to prevent formation of rust.

Filter: Wherever filters are fitted over storage tank, they require frequent inspection, cleaning of the media and periodical flushing to prevent bacterial builds up on the filter medium. However, in most instances the use of a filter is impractical due to the frequent maintenance required.

Instead of disinfecting all the water stored in the storage tank, it is desirable to disinfect only that portion of water, which will be consumed for drinking and cooking. So, covering of the storage vessels inside the house is always required.

Keywords: Rainwater conservation, Roof water harvesting, Water harvesting structures, First flush devices, Gutter system.

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