Photo credit: Rainwater Harvesting
Rooftop rainwater harvesting system rehabilitated at a Govt. school Vijaypura, India
Water harvesting, Conservation and Utilization Techniques in hot Arid Ecosystem of India
by Pratap Narain
Out of 31.7 Million ha hot arid Ecosystem in India, 82 per cent is spread in western Rajasthan and adjoining Gujarat states including about 7.5 Million ha Thar Desert extending across the border into north-eastern Pakistan. Striking features of arid ecosystem are hot climate, less than 350 mm erratic annual rainfall (cv. 60-80 %), high evaporation, negative water balance and low biomass production setting especially on sandy soil with low water holding and poor fertility. Wind erosion is the main cause of land degradation affecting nearly 45% of arid Rajasthan.
Desertification is manifested in drifting of sand/ sand dunes, paralyzing road and rail traffic and depositing sand on fertile cropland and in water reservoirs. Recurring and prolonged droughts once in 2-3 years are the root cause of desertification, crop failures and exacerbate scarcity of water, food and fodder requiring their imports for drought relief. Seasonal migration in search of employment and greener pastures, a traditional way of life of pastoralist and nomads, is declining due to social conflicts. Thar Desert is the most densely populated desert in the world (population density of 127persons per km ² in 2011) with a very high animal population (animal: human ratio is 1: 4-5 against 1: 0.5 in rest of the country). It is also intensively studied region as 82 per cent area of arid zone has been surveyed by CAZRI by conventional and remote sensing.
Agriculture is the main stake of livelihood in the region. Livestock based farming and pastoralism is dependable way of survival in view of uncertainties in cropping particularly in drought years. In hyper arid region, animals are supported by grass lands dominating with Dichanthium, Cenchrus and Lasiurus grass covers. In slightly better rainfall regions and on desert margins, mixed cropping with pearl millet, arid legumes, cluster beans and green gram and agro-forestry with wide variety of multipurpose trees like Prosopis cineraria, Ziziphus mauritiana and Acacia senegal, Techomella undulata, Hardwickia binnata, C. mopane, Faidherbia albida and Ailanthus excelsa is practiced in livestock mixed farming system.
Scanty rainfall is the only source of available water in the hot arid ecosystem. Unique water harvesting, storage and conservation techniques have been evolved and practiced by the desert dwellers. Erstwhile rulers have also constructed magnificent community water structures for public usage. Traditionally, bawari, jhalra (step wells), khadin, nadi (ponds) and tanka (underground cisterns) and roof water harvesting have been utilized for rainwater harvesting for drinking, crops and ground water recharging. In Rajasthan 43 per cent of the rural drinking water supply is sourced from nadi, 35 per cent from tanka, 15 per cent from wells and tube wells and 8 per cent from other sources.
Some of these techniques have been improvised in design for efficient harvesting, storage and utilization of precious rainwater by CAZRI and popularized in the region. Ground water is limited, deep and brackish with high concentration of salts of chloride, fluorides and nitrates, which is being over-exploited for drinking and irrigation despite poor quality. The ground water development in the state of Rajasthan is reported to be 138 per cent, which is a very serious concern. Depleted freshwater aquifers have led to an acute shortage of drinking water. Artificial recharge structures comprising of ponds linked to infiltration wells (in hard rock areas), percolation tanks (in alluvial formations) and sub-surface barriers across ephemeral streams (in sandy beds) have been designed and constructed due to which availability of drinking water has improved considerably. The potential of water conservation and harvesting against drought in Rajasthan has been estimated.
The major outcomes of the study are:
For oral presentation at the Desert Land Conference on 16th-17th June, 2015 in Ghent, Belgium.
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