Module 1: Fundamentals of Reservoir and Farm Ponds
Module 2: Basic Design Aspect of Reservoir and Far...
Module 3: Seepage and Stability Analysis of Reserv...
Module 4: Construction of Reservoir and Farm Ponds
Module 5: Economic Analysis of Farm Pond and Reser...
Module 6: Miscellaneous Aspects on Reservoir and F...
Lesson 1 Introduction to Rainwater Harvesting
1.1 Definition and Scope of Rainwater Harvesting
The present trend of agriculture is passing through a transition phase. Pressure on the most vital natural resources like land and water is increasing due to rise in population, increasing demand on food grains and urbanization. More and more marginal lands are being utilized today across the globe for crop production consequent upon the rising demand on the food grains by the growing population. Major chunk of this land is located in the arid or semi-arid regions where, the rainfall occurs irregularly and in small quantity. Much of this scarce rainwater is soon lost as surface runoff resulting in frequent agricultural droughts. Consequently, the very existence of human beings and livestock population is threatened and in some pockets the situation is observed to be seriously critical. While irrigation is assumed to be the most obvious response to drought, it has proved to be costly and can only benefit a fortunate few. Therefore, an increasing interest on a low cost alternative to conventional irrigation methods is observed in rainfed areas and this is generally referred to as ‘rainwater harvesting’.
In spite of remarkable achievements in the field of science and technology, nature remains to be a mystery for human beings. Sophisticated technologies have enabled us to derive fresh water from sea water through desalinization and avert drought situations by artificial raining through cloud seeding in some parts of the developed countries. Amidst such developments, the shortage of water even for drinking purpose has stood up as a threat across the world, especially in under developed and developing countries like India.
The never ending exchange of water from the atmosphere to the oceans and back again is known as the hydrologic cycle. This cycle is the source of all forms of precipitation and thus, all water. Precipitation stored in streams, lakes and soil evaporates while the water stored in plants transpires to form clouds that store water in the atmosphere. Making the most efficient use of the scarce and precious resource has become very much imperative. It includes using appliances and plumbing fixtures to conserve rainwater without wasting and taking advantage of alternative water sources such as grey water reuse and rainwater harvesting.
Water harvesting, in broad sense, can be defined as the 'collection of runoff for productive uses'. It also can be defined in various ways such as the process of collecting natural precipitation from catchments for beneficial use or the process of concentrating precipitation through runoff and storing it for beneficial use.
Rainwater harvesting is the process of direct collection of rainwater and the generated surface runoff out of it. The conservation of rainwater refers to storing of the collected rainwater for direct use or for recharging the ground water. Runoff may be harvested from rooftops and land surfaces as well as from intermittent or ephemeral watercourses. Thus rainwater harvesting and conservation aims at optimum utilization of the natural resource i.e. rainwater, which is the first form of water obtained from the hydrologic cycle. Hence, it is known as the primary source of water. On the other hand, the rivers, lakes and underground reservoirs are the secondary sources of water. In present times, in absence of rainwater harvesting and conservation, we depend entirely on such secondary sources of water and in the process it is forgotten that rain is the ultimate source that feeds to these secondary sources. The value of this important primary source of water must not be lost. Rainwater harvesting and conservation mean to understand the value of rain and to make the optimum use of rainwater at the place where it falls.
Water harvesting techniques, which are used to harvest runoff from rooftops or land surfaces, fall under the term rainwater harvesting. All other systems which collect discharges from watercourses are grouped under the term floodwater harvesting.
1.2 History of Rainwater Harvesting
Various forms of water harvesting (WH) have been used traditionally through centuries. Some of them, as practiced across the Middle East in ancient agriculture, were based on techniques such as diversion of ‘wadi’ flow (spate flow from normally dry watercourses) onto agricultural fields. WH systems dating back 4000 years or more have been discovered in the Negev Desert of Israel. These schemes involved the clearing of hillsides from vegetation to increase runoff, which was then directed to fields in the plains.
Floodwater farming was in practice in the desert areas of Arizona and northwest New Mexico for last 1000 years. The Hopi Indians on the Colorado Plateau were carrying out crop production in the fields situated at the mouth of ephemeral streams. These fields, where the streams fan out, are called "Akchin". Micro-catchment techniques used in southern Tunisia for growing trees were discovered in the nineteenth century by some travelers. In "Khadin" system of India, floodwater was impounded at the upstream of earthen bund sand crops were grown at the points of infiltration under residual soil moisture.
The importance of traditional, small scale systems of WH in Sub-Saharan Africa has just begun to gather recognition. Simple stone lines are used, for example, in some West African countries, notably Burkina Faso, and earth bunding systems are found in Eastern Sudan and the Central Rangelands of Somalia for water harvesting.
1.3 Need and Importance of Rainwater Harvesting
The need of rain water harvesting and conservation can be understood by the fact that the wettest place on the earth i.e. Cherrapunjee in Meghalaya state of India, which receives 12063.3 mm of average annual rainfall (1973 – 2002), suffers from acute shortage of drinking water. The reason attributed to inadequate provision of rainwater harvesting leading to quick draining of runoff down the slope along the hilly tracts. The annual rainfall over India is estimated to be 1170 mm, which is much higher than the global average of 800 mm. Moreover, 80 per cent of it occurs in about 70 rainy days during monsoon months (June – September). It makes clear that the sub-continent receives highly intensive and erratic rains in short periods. Practically, it is not possible to arrest all the rains coming in a short duration even through some gigantic structures and thus, it leads to draining out of the runoff at a faster rate leaving little scope for recharging of ground water. Consequently, most part of the country is facing shortage of water even for domestic uses. In regions where crops are entirely rainfed, a reduction of 50% in the seasonal rainfall, for example, may result in a total crop failure. If, however, the available rain can be concentrated on a smaller area, reasonable yields will still be received. Of course in a year of severe drought there may be no runoff to collect, but an efficient water harvesting system will improve plant growth in the majority of years.
Again, the arrival as well as departure of the south-west monsoon in the country is quite uncertain. The timing of onset of monsoon rain, for example in eastern region of India, fluctuates from the last week of May to second week of July leaving the field preparations for kharif crops in a state of quandary. Further, reports reveal that at least two critical dry spells are expected to occur during the rainy season and these two events are coincidental to important field operations and crop growth stages. A dry spell during kharif season if continues for at least 10 days or more is said to be a critical dry spell. When this dry spell occurs during beusan or transplanting stage of rice, the operation is either delayed or deferred in rainfed agriculture. Both the operations are very much essential for a better harvest from rice. In case the operation is delayed, the crop production reduces drastically and when it is deferred, the crop fails. Further, when the critical dry spell coincides with the critical growth stage of the crops, which extends from flowering to grain formation in most of the crops, the crop yield is severely reduced. In order to safeguard the rainfed crops from such drought like situations, rainwater harvesting is imperative.
Apart from the risks of dry spells in kharif season, it is observed that growing of a second crop in rainfed areas following withdrawal of monsoon is a chance factor. It is because of quick depletion of soil moisture from the seeding zone due to cessation of monsoon rain. Studies reveal that successful germination of the seeds of many of the oilseed and pulse crops in winter is very much essential for getting a good yield. Adequate soil moisture in the seeding zone of the crops is required to be maintained at the time of sowing of the crops. Water balance study in the crop root-zone of rainfed areas in eastern India reveals that the soil moisture in the seeding zone remains deficient for germination in more than 60% of the years. A provision of pre-sowing irrigation would be of immense help for this purpose. Thus, lack of a source of irrigation is a major constraint for growing a second crop in rainfed areas. Further, the provision of gravity fed irrigation for these areas is an uphill task in the part of the government. It implies that the rainfed areas will remain mono-cropped along with a chance factor of good harvest in rainy season unless and otherwise the rainfall excess during the late season stage of the kharif crop is harvested. In order to make the second green revolution in the rainfed areas of eastern India successful, a second crop needs to be grown with the provision of supplemental irrigation from the conserved rainwater.
1.3.1 Benefits of Water Harvesting System
A water harvesting system offers the following benefits:
In arid and semi-arid regions, water harvesting is a guarantee of optimum crop production against vagaries of monsoon provided other production factors with respect to soil and crop are favourable. This is especially important when no other source of water is available for irrigation.
Water harvesting system can provide water to take care of the irregularities of rainfall and supplement the soil moisture deficiency for increasing and stabilizing crop production. As the cropping risk is reduced, application of organic or inorganic fertilizers becomes economically viable resulting in increase of the potential yields.
Water harvesting can meet water needs for domestic uses and livestock consumption where public supplies are not available.
The extent of arid areas suffering from desertification increases due to want of water harvesting. The provision of water harvesting in those areas helps increase vegetative cover and consequently environmental degradation is checked. It has been also found effective in recharging the groundwater aquifers.
Generally, water harvesting is a low-external-input technology and not difficult to implement. With few exceptions, it does not require use of pumps or input of energy to convey or apply harvested water.
The implementation of water harvesting may however have a number of detrimental effects as follows:
Increase soil erosion when slopes are cleared to promote runoff
Loss of habitat of flora and fauna due to clearance of slope
Loss of habitat of flora and fauna in depressions (temporary wetlands)
Conflicts among people living upstream and downstream of watershed for the use of harvested water
Conflicts between farmers and herders in dry environment when the harvested water is used for livestock.
1.4 Components of Rainwater Harvesting
Water harvesting is the process of collecting and storing water from an area that has been treated to increase runoff generation from precipitation. Regardless of the purpose and type, all water harvesting systems have the following components (Fig. 1.1).
Catchment Area: Catchment area, watershed and drainage basin are the synonymous terminologies used in rainwater harvesting. It is the geographical area that contributes runoff, resulting from precipitation, which passes through a single point into a water harvesting unit, a large stream, a river, lake or an ocean. Therefore, it is also called as the runoff area. The catchment may be only a few hectares for small ponds or hundreds of square kilometers for large streams, rivers. After all, each catchment area is an independent hydrologic unit and any change made in its land use affects the runoff yield of the catchment.
Storage Facility: Water harvesting systems are not only for storing water to meet the crop water requirement but also for meeting the demand of households and livestock consumption. The storage facility refers to the structure where harvested runoff is held until it is used by crops, animals or people. Water may be stored on the ground for example in ponds and reservoirs, in the soil profile as moisture or recharged into the underground aquifers.
Fig. 1.1. Major components of typical water harvesting system.
(Source: Owesis et al., 2012)
Target: The target groups of a water harvesting system may be the plants, animals or human beings. They are the end users of the system. While in agriculture, the target group is comprised of plants and animals, it is the people and their needs in domestic use. Complex or large scale water harvesting systems usually have additional components for conveying and diverting runoff water to the target and/or storage facility.
Keywords: Rainwater harvesting, Components
Critchley, W. and Siegert, K. (1991). A Manual for the Design and Construction of Water Harvesting Schemes for Plant Production, Food and Agricultural organization.
Owesis, T. Y., Prinz, D. and Hachum, A. Y. (2012). Rainwater harvesting for agriculture in the dry areas. CRC Press publication.
Panigrahi, B., Panda, S.N. and Mull, R. (1999). Dry and wet spell analysis for planning supplemental irrigation. Proceedings of International seminar on Civil and Environmental Engineering – New frontiers and Challenges. Asian Institute of Technology, Bangkok, Thailand, IV: 53-62.
Tiwari, K.N. and Saxena, A. K. (1987). Dry spell and wet spell analysis with reference to the water harvesting- a case study – IV. Indian J. of Power and River Valley Dev., (Nov-Dec.): 295-301.