Technological advances for skimming fresh water in coastal rainfed areas

Abstract

The inland aquifers are suffering from the maladies of over exploitation of ground water by way of unscrupulous pumping, the coastal aquifers encounter the danger of sea water intrusion and saline water upconing. Seasonally recharged fresh water skimming is the only alternative in coastal zones to stabilize crop production. Along the sea coast of Andhra Pradesh, there are 8.23 lakh ha of sandy soils, of which 1.74 lakh ha have shallow watertable varying from 0.3 to 3.0 m. These soils occur in narrow strip of 10 to 15 km wide and 972 km long eastern coastal strip extended from Ichapuram in Srikakulam district to Tada in Nellore district with mean rainfall of 855mm. In the coastal sands, entire precipitation percolates into the soil because of the high permeability of coastal sands and the infiltrated rain water with lesser density form fresh water lenses which floats on the sub- surface saline ground water. Most of these coastal sandy soils do not have the provision of canal irrigation facility and have only shallow depth of good quality waters. Poor recharging rate and occurrence of clay in deeper layers are the major constraints for installation of tube wells/deep wells. In most of these coastal sands, fresh water floating at a depth of 0.5 to 3.0 m below ground level and to a thickness of 3 to 4 m over saline ground waters/clay layers. The waters cannot be extracted in high quantities by conventional tube wells as they are of watertable aquifers with poor quality waters underneath. In these soils during summer months the watertable fall up to 1.8 to 3.0 m below ground level. The ground water gets recharged during the monsoon season, accumulates and by middle of November the ground water rise up to 0.3 to 0.5 m bgl in most of the areas. As irrigation supplies are not possible either though canal or tube wells in most of the coastal sandy soils, the situation has forced the farmers to search for alternatives and the technique innovated traditionally is to draw water manually that collects in the dug out conical pits locally called ‘doruvus’. Similarly in Tamil Nadu, the coastal belt occupies about 700 km, stretching from Pulicat lake in the north to Cape Comarine in the south and 0.68 m ha is identified as coastal sandy soils. Among different types of soils, salt affected lands occupy nearly 2.04 lakh ha or 25 per cent of coastal area of Tamil Nadu. Under the situation, where ever the fresh water exist, the farmers tap them with the development of ‘Oothu Kuzhi ’ (Spring) like ‘doruvu’ system practiced in Andhra Pradesh. The survey carried-out in the coastal belt of Guntur district, Andhra Pradesh., revealed that the groundwater being harvested through various structures such as traditional doruvu (skimming pot irrigation), traditional doruvu with 1 hp pumping unit, radial skimming well with horizontal collectors using 1- 5 hp pumping units, shallow tube wells with 1 hp pumping unit, two-strainer tube wells with 1/5 hp pumping unit The survey results with features of the structures are presented in Table 1.