ADVERTISEMENTS:
Here is an essay on ‘Water Harvesting’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Water Harvesting’ especially written for school and college students.
Essay on Water Harvesting
ADVERTISEMENTS:
Essay Contents:
- Essay on the Introduction to Water Harvesting
- Essay on the History of Water Harvesting
- Essay on the Need for Water Harvesting
- Essay on the Concept and Definitions of Water Harvesting
- Essay on Soil Requirements for Water Harvesting
- Essay on Water Harvesting Design Model for Catchment Cultivated Area Ratio
- Essay on Site and Technique Selection for Water Harvesting
- Essay on Water Harvesting Methods
Essay # 1. Introduction to Water Harvesting:
The climate is continuously changing mainly because of change in ecosystem; as result the temperature change is at alarming rate. Also, the duration of various seasons has been changed.
The length of rainy season and winter season has been reduced (less than 4 months), while the span of summer season has been expanded more than 4 months. The quantum of rainfall and the ground water recharge thereof has also been significantly reduced. In response, at several places the ground water structures like tube well etc., have now been started to failure.
ADVERTISEMENTS:
People are not getting water for their domestic needs; crop cultivation becomes impossible; reservoirs are drying etc. In brief, there becomes massive water scarcity. For fighting against these problems, the government is wasting huge amount of money.
In this situation, the harvesting of natural water (rain water) plays very important role to dilute the problem of water shortage.
Essay # 2.
History of Water Harvesting:
ADVERTISEMENTS:
There have been developed various methods of water harvesting, which are in use from very long back, worldwide. As per report, few of very earliest agriculture in Middle -East was based on diversion of “wadi” flow onto agricultural fields, was a kind of water harvesting and recycling of harvested water for irrigation.
In Negev desert (Israel) the water harvesting systems have been reported to get use about 4000 years back or more, where water harvesting was done by clearing the hillsides from vegetation to increase the runoff, and directing the runoff to the fields on the plains.
Similarly, the floodwater farming has been reported in practice in the desert areas of Arizona and north-west New Mexico from at least 1000 years back. In southern Tunisia, which was discovered in the nineteenth century by travelers Pacey and Cullis (1986) described the micro-catchments technique for tree growing.
In India the “Khadin” system, in which floodwater is impounded behind earthen bunds; and crops are planted in the field based on the residual moisture contents, which was due to infiltration from impounded water.
In sub-Saharan Africa the use of traditional and small-scale water harvesting systems has also been reported by the farmers. In some West African countries the method of water harvesting is reported by constructing the simple structure using stones. In Sudan and in Central Rangelands of Somalia the practice of water harvesting has been reported by constructing the bund system.
Essay # 3.
Need for Water Harvesting:
Rainwater is the main source to feed the demand that is either for ground water recharge or filling the surface water bodies. As compared to the last several years, the amount of rainfall has been abruptly reduced might be due to change in eco-system. The reduction in level of rainfall magnitude has resulted declining water table, apart from due to heavy exploitation of ground water, either in agriculture sector, industries or for domestic usage.
And it has been reached at alarming stage, worldwide. As for as India is concerned, in spite of high average annual rainfall of 1170 mm in comparison to the global average rainfall (800 mm), still there is scarcity of water for fulfilling various demands of the country. It is general observation that during rainfall occurrence most of the rain water gets flew away to the fluvial system, which ultimately meets to the sea or big water body.
A little rainwater is left over the ground surface for ground water recharge, which also results into very little enrichment of water table. In most of the countries, the water table has been reached to such an extent that the ground water structures like tube wells etc., have failed to discharge the water. The water table fluctuation in different states of India is presented in Table 16.1.
In addition, due to erratic rainfall the surface water bodies such as natural pond/depressions, reservoirs etc., are not get filled with rainwater, which also result into not fulfilling the demands of various rural needs. All these facts basically force to search the alternatives to harvest the rainwater, anyhow; otherwise, no any means.
In spite, in continuous declining water table condition there is no reduction in water use rate; but at increased rate. As for as Indian condition is concerned, the estimated water requirement under different domestic activities is presented in Table 16.2.
Essay # 4.
Concept and Definitions of Water Harvesting:
In broad sense, the water harvesting is defined as the “collection of runoff for its productive use”. The rainwater (overland flow/runoff) may be harvested from the roofs and ground surfaces or from the intermittent or ephemeral streams (water courses).
A variety of water harvesting techniques for different applications are available. Productive use of harvested water includes to develop provision for domestic and livestock feed, supply for crops, fodder and tree production and also for creating water body for fish and duck cultivation.
The water harvesting assembles various techniques or practices based on the utilization of runoff from un-cropped areas, and to supplement the harvested water to the cropped areas.
In addition, also to store water for irrigation, domestic or livestock use but priority is always given to use the harvested water for crop production. In general, the job of water harvesting is found very suitable in arid and semiarid areas where occurrence of droughts is very frequent and irrigation is not possible.
Various approaches of water harvesting are outlined as under:
i. Runoff Harvesting:
It refers to the harvesting of runoff resulted from bare or sparsely vegetated areas; and it’s recycling for irrigating the cropped areas. The runoff harvesting includes, (a) sheet-flow harvesting, in which rainwater (sheet flow) in the form of sheet-flow is collected from gently sloping land surfaces to such a point from where that can be suitably used for agriculture purposes; and (b) concentrated runoff harvesting, in which runoff is collected from narrow channels such as footpaths, cattle tracks or transient streams in which runoff has been concentrated.
ii. Floodwater Harvesting:
It is the practice of diversion of floodwater from water-courses to a safe place for storage. The storage place may be the farm ponds or micro reservoirs.
iii. Water Spreading:
In this approach the flood water is diverted to a very vast area. Where spread water gets infiltrated in the soil. The retained moisture content in the soil is utilized for growing the crop.
iv. Rooftop Water Harvesting:
It is the method of direct harvesting of rainfall from the roofs. The harvested water can be used for domestic or livestock usage.
In sheet-flow water harvesting the rainwater is collected from a larger catchment area, and is concentrated to a small size cropping area. In this approach the catchment size is decided based on the cropped area to be irrigated by the harvested water.
As a thumb rule, the catchment area should always be greater than the cropped area. The ratio of catchment area to the cropped area generally varies from 1:1 to 3:1. The sheet-flow water harvesting is very effective when slope of catchment area does not exceed 5%.
In the catchments without crop (bare) the runoff yield becomes more, as compared to the cropped condition. In many situations for increasing the runoff rate, the catchment area is treated by using most suitable means. If runoff yield is very high then there is possibility of getting damage of water harvesting structures. In this situation, the use of diversion ditches becomes essential at upslope of the area to check excessive runoff.
Although, the water harvesting in water scare area is very conducive for crop cultivation, but adoption rate is still very less, which may be due to following reasons:
i. Reluctance to maintain clean and weedless runoff areas.
ii. Involvement of high costs.
iii. High labour requirements for constructing and maintaining the water harvesting structures.
iv. Limited availability of land for constructing water-harvesting structures.
Apart from above reasons, the following are also few important points, which are likely to create adverse effects on water harvesting:
i. High risk of crop damage due to long term water stagnation in the area.
ii. Development of high risk of soil erosion and soil degradation in the catchment area.
iii. There is possibility of intense soil erosion in the area.
Essay # 5.
Soil Requirements for Water Harvesting:
The physical, chemical and biological properties of soil affect the yield response of crop to extra harvested moisture. In general, the soil characteristics favoring the water harvesting are the same as for irrigation.
For better water harvesting response, the soil of catchment area should have following properties:
i. High runoff coefficient.
ii. In cultivated area the soil depth should be deep.
iii. Soil should be in the nature of fertile loam.
Apart from above the followings are also very important parameters, which affect the crop performance under water harvesting combinations.
a. Soil Texture:
The soil texture influences several important soil characteristics including infiltration rate and water holding capacity. Soil texture represents the soil composition in terms of mineral particles.
The followings are the classification of soil texture:
i. Coarse textured soils-sand predominance, i.e. sandy soils.
ii. Medium textured soils-silt predominance, i.e. loamy soils.
iii. Fine textured soils-clay predominance, i.e. clay soils.
The medium textured soils are normally the best for water harvesting purpose, because of the reason that these soils are ideally suited for plant growth in terms of nutrient supply, biological activity and water holding capacity.
b. Soil Structure:
This soil property refers to the group of soil particles in the aggregates; and arrangement of the aggregates. A loamy soil includes good structure and relatively high content of organic matter. Due to this reason, the loamy soil is considered to be a good soil for water harvesting.
Under hot climatic condition, the level of organic matter is often low because of rapid rate of decomposition. In general, the application of organic materials such as crop residues and animal manure in loamy soil is very conducive to improve the soil structure.
c. Soil Depth:
The soil depth is very important for better water harvesting. In the condition of greater soil depth, the quantum of runoff harvesting gets increase because of greater available storage space in the soil. The soils with less than 1 m depths are poorly suitable for water harvesting. The ideal soil depth for water harvesting point of view is 2 m or more.
d. Soil Fertility:
The level of soil fertility also affects the water harvesting. In many areas where water-harvesting system has been introduced, the lack of moisture content and low soil fertility are the major constraints to cultivate the crops. For better response of recycled harvested water the attention should always be given to maintain a good fertility level in the soil.
A soil is said to be sodic when it has high exchangeable sodium percentage; on the other hand when soil contains excess soluble salts then it is called saline soil. These soils should be avoided for water harvesting purposes, because there is significant reduction in moisture availability as well as creating harmful effects on plant growth.
The soil types mainly affect the infiltration rate. Depending on the soil types the variation in infiltration rate is presented in Table 16.3.
A soil with very low infiltration rate is not good for profile water harvesting point of view, because of the possibility of development of surface water logging problem in the area.
On the other hand, a low infiltration rate causes a high runoff rate, which is desirable for the catchment area to supply sufficient water for harvesting in reservoirs like structure which water can be utilized for electricity generation or for directing to the canal system for irrigation purposes. In general, the soils of cropped area should be sufficiently permeable to allow adequate moisture availability in the crop root zone without causing water logging problem.
The water holding capacity and ability of soil to release the water are considered very important parameters for water harvesting point of view. Regarding water harvesting structure, in which runoff water is stored, the soil should be such that which could hold the water for long duration, so that the harvested water could be available for the crop use for longer duration.
The AWC has implications for design; for example in a soil of 2 m depth with 200 mm/m available water, there is no ponding of water to the depth more than 40 cm. This quantity of water when infiltrated, is adequate to replenish the soil profile from permanent wilting point to field capacity and any surplus water if there, gets lost by deep percolation; and thus being for a potential water logging hazard.
Essay # 6.
Water Harvesting Design Model for Catchment Cultivated Area Ratio:
The water harvesting system consists of two components, i.e. one is the catchment, which is the collection unit for rainwater, and other is the cultivated area also called concentration area. There is definite relationship between the collection unit and the cultivated area.
For an appropriate design of water harvesting system the ratio between catchment area and cultivated area must be known, which are determined mainly on the basis of rainfall, runoff and crop water requirement data of the area, where water harvesting structures are to be constructed. The accuracy in estimation depends very much on the accuracy in above motioned data.
But in real practice, there is very wide variation in rainfall, runoff or other required data. The rainfall and runoff are characteristically erratic. It is therefore, sometimes becomes necessary to modify the design procedure based on the experience, and by incorporating few safety measures, such as provision of cut-off drains to avoid damage when rainfall exceeds the design limit.
The calculation procedure of ratio of catchment area to cultivated area (C: CA) is described as under:
Computation of Ratio of Catchment Area to Cultivated Area:
The calculation of ratio between catchment area and cultivated area is based on the concept that the quantity of harvested water must be equal to the extra water required for needful, i.e,
Harvested water = Extra water needed … (16.1)
In which, the amount of water harvested from the contributing catchment depends on host of the factors, such as topographical characteristics, land use, rainfall etc., are the main. Effects of these factors are on the runoff producing behaviour of the catchment. There are several methods for computing the runoff, based on the topographical characteristics, land uses and climatic characteristics, but for water harvesting is concerned, the runoff for a defined time scale is predicted by multiplying the design rainfall with the runoff coefficient.
Also for better accuracy, it is additionally multiplied with an efficiency factor for compensating the deep percolation losses. The following formula can be used for computing the amount of water to be harvested based on the catchment area, rainfall, runoff coefficient and efficiency factor –
Amount of water harvested = Catchment area x Design rainfall x Runoff coefficient x Efficiency factor … (16.2)
The amount of extra-required water is computed by multiplying the size of cultivated area and the net crop water requirement. The net crop water requirement is the total water requirement minus rainfall depth taken place. It is presented as under –
Extra water required = Cultivated area X Crop water requirement – Rainfall depth … (16.3)
Equating equations (16.2) and (16.3)
Catchment area x Rainfall depth x Runoff coefficient x Efficiency factor = Cultivated area x Crop water requirement – Rainfall depth … (16.4)
Rearranging the equation 16.4 –
(Crop water requirement – Rainfall depth)/(Rainfall x Runoff coeffi. x Eff. Factor) = Catchment area/Cultivated area … (16.5)
The equation 16.5 can be used for computing the ratio of catchment area and cultivated area to determine the amount of required water harvesting for the given area. This formula is not valid for tree plantations. For trees the following formula can be used –
MC = RA (WR – DR)/DR – K – Eff. … (16.6)
Where,
MC = area of micro-catchment (m2)
RA = area exploited by root system (m2)
WR = water requirement (annual) (mm)
DR = design rainfall (annual) (mm)
K = runoff coefficient (annual)
Eff. = efficiency factor
In above formula, the area exploited by root system is taken as the area of the tree canopy. For few tree species, the size of catchment and cultivated area followed in different countries is presented in Table 16.4. The table values can be taken into consideration for design purposes of water harvesting structures.
However, there is thumb rule about the size of micro catchment, that for multipurpose trees in arid/semi-arid regions the size of micro-catchment per tree (catchment and cultivated area together) should be between 10 and 100 sqm, depending on the aridity of the area and species grown.
For rangeland and fodder cases, it is generally not necessary to calculate the ratio of catchment area and cultivated area (C: CA) for design of water harvesting system for fodder production or rangeland rehabilitation. However, as a general guideline that the ratio of 2:1 to 3:1 for micro catchments, which are normally used, are found appropriate.
Various parameters involved in above relationship are described as under:
1. Crop Water Requirement:
It depends on the type of crop to be grown and climate of the area. A detail view about crop water requirement can be have from the reference Suresh R (2008).
2. Design Rainfall:
It is the seasonal rain depth, based on which the water harvesting system is designed. The rainfall depth data is essential for determining the amount of runoff likely to be generated from the catchment area, which is expected to get store in the water harvesting structure. The design rainfall depth refers to the rainfall of a specified probability.
The consideration of design rainfall makes the system design, more reliable. If the rainfall is lesser than the design rainfall, then there is possibility of risk of crop failure due to moisture stress, along with not availability of excess water for harvesting purpose. And if the rainfall is more than the design rainfall, then runoff will be in surplus, which can be used for water harvesting.
3. Runoff Coefficient:
It is the fraction of rainfall, which gets change into surface runoff. It depends on several factors such as slope steepness, soil type, vegetative cover, antecedent soil moisture content, rainfall intensity and duration. The runoff coefficient varies between 0 and 1.0.
4. Efficiency Factor:
This factor takes into account the losses of rainwater due to deep percolation and evaporation, and non-uniform distribution of water within the field. In level cultivated lands, its value is at higher side. The micro-catchment systems have higher efficiency because depth of water ponding is less.
Essay # 7.
Site and Technique Selection for Water Harvesting:
There are several factors which decide to a most suitable site and technique for design and construction of water harvesting structures; few of them are outlined as under:
It is very important that before selecting any specific technique for water harvesting, the consideration must be given to the social and cultural aspects prevailing in the area because they are paramount, and affect the success of the implemented technique.
In the arid and semi-arid regions of Africa, it is in report of failure of many projects because of not considering the people’s priorities. There, most of the population have experienced basic subsistence regimes, which resulted over the centuries in setting priorities for survival.
Regarding selection of technique, the consideration of cost and risks is very important. The cost and risks must be compared within various techniques, and based on that the selection should made to that which involve less cost and risk, as well. In addition, the water quality, operational and maintenance costs should also be taken into consideration. The area, where water of better quality, cheaper technology, easier to obtain or there is less risk, should always be given priority.
The water harvesting projects are only being sustainable, when they are socially acceptable and economically viable and also when fulfill the basic technical criteria.
Few important criterion are outlined as under:
1. Ground Slope:
It is one of the very important limiting factors for water harvesting. In general, the water harvesting is not recommended for the areas having slope greater than 5%, because of non-uniform distribution of runoff and also involvement of large quantities of earthwork, which makes the work, un-economical.
2. Soil:
The soil should have all those attributes, which are suitable for irrigation. In other words, the soil should be deep; not be saline or sodic and also possess inherent fertility. A serious limitation for water harvesting is with the soils in sandy texture. The sandy soil contains very high infiltration rate; because of this reason there is more possibility of no yield of runoff if rainfall depth is mild. In general, for yield of sufficient runoff from watershed, the infiltration rate should always be lesser than the rainfall intensity.
3. Cost:
Always, there should be the consideration of cost effectiveness technique for water harvesting. To fulfill it, the site should be that, where minimum earthwork is required; construction materials are easily available in the nearby area; people of the area are very progressive in attitude; laborers are easily available in the area etc. These parameters minimize the total input cost.
Essay # 8.
Water Harvesting Methods:
There are several traditional/improved water harvesting technologies, developed world-wide.
The very common, amongst them are described as under:
This is a traditional water harvesting structure (pit like structure) used in arid and semiarid zones of Sahel (FAO, 1991).
The dimension of pits is as below:
Depth = about 15 cm
Diameter = 40 cm
Spacing = 80 cm
These pits are constructed during dry season by digging the soil, down slope. For making safe storage of runoff, the stones are placed at upslope side around the pits. After constructing such pits in the field, about 1 or 2 handful of dry dung (1-2.5 t/ha) are applied to the pits and covered with the soil, about 2 weeks before occurrence of rainfall. The application of dung enables to increase the infiltration rate of the soil, which results into significant storage of water in the soil profile.
The stored water content in the soil profile is utilized for crop production, later on. The crops are sown in the pits when rain gets start. The Tassa pits also enable the farmers to apply small quantity of manure and compost, which are very helpful for improving the productive potential of soil, in addition to water harvesting. Also, this practice has been found very effective for rehabilitating the degraded lands; and an excellent means of establishing the tree seedlings for agro-forestry, in Sahel.
The main demerit of this construction is the requirement of large number of labours for construction. In this way, in labour scare areas this method is not feasible for water harvesting.
However, the followings are few merits of it:
i. It economizes the amount of use of manures, because of its application in the pits, i.e. at the place of crop planting.
ii. This practice encourages re-introduction of soil fauna, i.e. termites etc., which improves the soil properties.
iii. It results into planting of crops in time, because of preparation of land in advance.
iv. Also, very effective for rehabilitation of badly degraded land.
v. Very conducive to result a good yield from the land.
vi. It also replenishes the groundwater table.
This is also a sheet-flow water harvesting structure. The shape of half-moon of bund guides the runoff to enter into their storage space; and also allows to escape excess runoff from around the ends of half-moon bank. Its storage area varies from 10 to 20 m2. It is constructed by digging the soil and placing the cut soil, around, in the shape of half-moon along contour of the area in offset arrangement. Its width varies from 2 to 6 m. The spacing between contour lines is decided on the basis of required ratio between catchment area to the cropping area.
In Niger, this structure has been constructed in the dimension of 20 cm as depth; 2 m as width at 4 m interval along the contour, with 4 m contour spacing (FAO, 1991). This water harvesting structure can be used for establishment of trees around the pits. For which the size of pit may be 60 cm deep and 60 cm square.
These can also be used for taking the grain crops, forage grasses etc. Since, half-moons are constructed by hand as result their construction requires considerable amount of labours. This method has disadvantage for millet and some trees crops is that, the sediments deposited within the half-moons form fairly impermeable crusts, which can impede the emergence.
This type of water harvesting structure is constructed with the help of stone pieces. First, the position of contour lines is fixed in the field; and then a foundation trench is dug along the marked contour line. The depth of foundation trench varies from 5-10 cm and width 35-40cm. After trenching the foundation, the stone pieces are placed in that and tightly packed. The bund height is kept in the range of 25 cm. During rainfall the sheet flow is obstructed by the stone contour bund.
As result a part of rainwater gets infiltrated into the soil, and is stored in the soil profile. The moisture stored in the soil profile is utilized for crop production. In addition, during this process the suspended materials coming with sheet water also get deposited at u/s of stone bund, which is beneficial to enhance the infiltration rate and water storage thereof. Slowly and slowly the deposition of soil particles on upslope side of stone bund attains the form of terrace. It also becomes very effective to enhance water infiltration in the soil.
This practice of water harvesting is commonly used in Ethiopia for crop cultivation and rangeland rehabilitation. On the lands with the slope 1-3%, the stone bunds are constructed at 25 m spacing; and based on the level of water stored in the soil, the cultivation of sorghum is successfully done. Also, by sorghum crop the runoff gets reduce by 23%. Somewhere, the stone bunds are also constructed as the permeable rock dam across the gullies, to divert the rainwater from the gully area, and spread over the other area for water harvesting.
Apart from above methods, there are several other methods for water harvesting. Table 16.5 summarises few more water harvesting methods.
