Hydrological studies on rainwater harvesting, groundwater recharge and de-siltation of reservoirs to augment recharge in the drought prone Aravali hills ecosystem

Abstract

Low rainfall, sandy soils, harsh climate, installation of large number of bore wells and change in cropping pattern from low water requiring to high water demanding cash crops lead to sharp drop in water table in Tijara block of Alwar district of Rajasthan representing a typical Aravali ecosystem. A corporate house (SRF LTD) established their chemical plant in this block and adopted 35 villages around the plant and funded a natural resources management project with the help of an NGO. In the period from 2006-07 to 2015-16, 206 earthen dams were constructed across gullies emerging from the nearby Aravali hills for rainwater harvesting and groundwater recharge. As the water crises deepened in spite of water harvesting, the long term hydrological data generated was reviewed and formed the basis of this paper. A decreasing trend in the yearly rainfall was observed. The cumulative groundwater recharge by 206 dams localy called Paals was 2832.51ha.m from 2006-07 to 2020-21, while the cumulative extraction by the SRF plant was 2307.15 ha.m. However, the data of 44 observation wells selected in the vicinity of dams revealed a consistent drop varying from 3 to 8 meters in all the wells and fifty percent study wells went dry in this period in spite of intensive rainwater harvesting. Out of 2006 reservoirs, 65 fed by smaller catchments stopped storing any water as the private lands above the dams were leveled and only 141 reservoirs were receiving variable amount of runoff. As a result, the runoff contributing area of 8942.33ha in 2006- 07 was reduced to7480.45ha. This situation raised a question mark on the sustainability of irrigated agriculture with no other source of recharge except rains. From the analysis of emerging issues of ground water depletion, it emerged that there is no scope of constructing more dams as suitable sites have been exhausted. Secondly, the reversion from wheat, cotton and vegetable crops to traditional Pearl Millet- Mustard looked improbable because of their better economic returns. As regards the option of efficient use of available water, more than 85 percent of best lands were already sprinkler irrigated. The deposition of fine silt layer on the surface choked the reservoirs resulting in prolonged storage with more loss of water as evaporation and reduced share of percolation which recharged the aquifer. At the end of the day, it was decided that Paals should be de-silted to augment the ground water recharge. Nine typical reservoirs were de-silted in last two years. The percolation rates varied from 2.77 to 13.71cm/day before and increased from 10.0 to 25.14cm/day after de-siltation with overall mean increasing from 7.08 to 16.29 cm/day across the nine de-silted Paals. The ground water recharge increased from 7.721 to 9.311 ha.m but the overall evaporation loss decreased from 12.23 to 5.36 percent after de-siltation. As a result of these positive results, a regular program of reservoir de-siltation has been initiated.