AROMATIC AND MEDICINAL PLANTS RESEARCH STATION, ODAKKALI
A sub-surface dyke constructed with the support of SIDA for the conservation and utilisation of ground water is a unique feature of this station. It is a living example for ground water conservation technology for mid-lands and for water-shed management. The success story of the sub-surface dyke has demonstrated that it is one of the most feasible methods for the conservation and exploitation of the ground water resources of the state.
SUBSURFACE DIKE FOR GROUND WATER CONSERVATION
Abstract: A subsurface dike was constructed at Aromatic and Medicinal Plants Research Station, Odakkali during 1988 with the objective of conserving ground water. The experience at Odakkali indicated that subsurface dike was an efficient barrier to arrest the subsurface flow of water and conserve ground water. The rate of depletion of drainable water in the catchment area due to the typical undulating topography could be efficiently arrested by the dike. The dike resulted in the maintenance of higher water table in the catchment area for a longer period of time. The water received through summer showers could be efficiently conserved in the catchment area by the dike. Plastic sheet can also be used as a cheaper alternative to masonry structure.
Kerala receives, on an average, more than 3000mm of rainfall per annum which is concentrated to the months of June to October. Due to the typical undulating topography of the state, the excess rain received is mostly lost through runoff. Immediately on cessation of rains subsurface water drains off along the slope resulting in poor retention of water in the catchment area. In spite of the heavy rainfall received in Kerala, the state experiences severe drought during summer months due to inefficient utilisation of rain water. In a state like Kerala, where the pressure on land is very high surface storage of water in large reservoirs with their adverse ecological impact is not an economic proposition. Alternatively, structures like subsurface dike are most ideal for in situ conservation of excess rainfall received in a watershed.
Subsurface dike is a structure that is built in an aquifer with the intention of obstructing the natural flow of ground water, thereby raising the ground water level and increasing the amount of water stored in the aquifer. The ideal location for the dike is a well defined, wide, greatly sloping valley with a narrow outlet having limited thickness of loose soil or porous rock on the top with massive or impervious rock below (Figure 1). Subsurface dike has many advantages. It does not require additional surface reservoir. There is no loss of agricultural land. There is minimum evaporation loss since the storage is subsurface. There is no siltation and loss of reservoir capacity. The cost of construction is low and maintenance is negligible. It is environment-friendly and it can be implemented with locally available materials.
The problems faced at the field level are mainly in locating a suitable site in the field. There is also the problem of reduction in efficiency of the existing abstraction structure in an area, in terms of discharge, lowering water level, etc. There is also a possibility of deterioration of water quality due to concentration of fluoride, nitrate, iron and other pollutants over time.
The first subsurface dam in Kerala was constructed during 1962-‘64 at Ottappalam. Another one was constructed by Central Water Board in 1979 at Anangandadi, but it was a failure. Based on these previous experiences a subsurface dam was constructed at Aromatic and Medicinal Plants Research Station, Odakkali with the main objective of assessing the feasibility of using subsurface dikes for conservation of ground water.
Materials and Methods
The subsurface dike at AMPRS, Odakkali was constructed during 1988, jointly by Kerala Agricultural University and Swedish International Development Agency (SIDA). Central Ground Water Board was also actively involved in the geophysical and hydrological studies of the site. The total cost of the dike was around Rs.1.00 lakh.
The initial site selection survey was carried out using air photo interpretation, map study and field visits. The selection criteria were basically hydrogeological and topographical conditions. Water use and construction feasibility were also considered. The topographical features considered were valley shapes and gradients. Optimally a valley should be well defined and wide with a narrow outlet. The gradient of the valley floor should not be too high since that would reduce the storage volume. The general hydrogeological considerations used were water level fluctuations, storage and flow characteristics of the overburden and the hydraulic conductivity of the rock underlying the porous aquifer to be dammed. The basic justification of constructing a subsurface dam is the depletion of ground water storage by natural ground water flow. A sufficiently high seasonal fluctuation of the ground water level was therefore, an important criterion. Infiltration and flow and storage characteristics of the overburden were estimated in the field. The imperviousness of the rock in the valley bottom was assessed by studying rock outcrops, well sections and the rate of seasonal ground water level declines.
A trench was made across the valley at the narrow outlet down to the bed rock. A masonry wall was constructed from the bed rock level upto one metre below the ground level. The brick masonry wall was 80m in length with a maximum depth of 7.2m (Figure 2). The thickness of the wall was 60cm at the bottom and 25cm at the top. The upstream side of the wall was cement plastered and a low density plastic sheet was laid as an additional lining. In order to study the effect of plastic as an alternative material to check the subsurface flow, a piezometer was installed between the masonry wall and the plastic sheet at one point along the dike. Since structure was 1m below ground level the ground water overflowed above the structure and hence the downstream was not deprived of water. Further, water logging of the upstream was not observed. The dam was also provided with a sluice to allow for drainage, in case of water logging. Nine pairs of peizometers have been installed on the upstream and the down stream sides of the wall at a constant interval of 10 m for monitoring the ground water table fluctuations over time. The refilling on both sides of the dike was of river sand in order to achieve equal pressure conditions and to improve drainage to an open well which was constructed at the deepest point in the valley on the upstream side near the wall for pumping out water.
Weekly data on the ground water level were recorded using water level recorder from the piezometers as well as the observation wells situated near the dike. Rainfall and temperature were also monitored.
Results and Discussion
The ground water table fluctuations on the upstream side and the downstream side of the subsurface dike during all the seasons indicated that higher water table was maintained in the catchment area due to the dike (Figure 3). The water table fluctuation across the catchment area during the non-rainy period (Feb to Apr) reflected rapid depletion of drainable water in the catchment area. This rate of depletion could be considerably reduced by the dike. The water thus conserved could be utilised for irrigation and other purposes.
Taking into account the mean depth of soil as 2 m the catchment area as 1.5ha and the drainable porosity as 10%, the total water storage capacity of the dike would be 3000 m3. Assuming that coconut tree requires irrigation for 5 months at the rate of 40 l/day, 2.8 ha of coconut could be irrigated with this quantity of water. The amount of water conserved would be much higher when the withdrawal of water for irrigation and other purposes and also the recharge through rains were accounted.. In addition, the water table was maintained at a higher level throughout the catchment area for a longer period than that would have been maintained without the dike. The crops cultivated in the catchment area would be benefited by way of constant capillary feeding of the rhizosphere.
It is evident from the figure 4 that in the upstream side full storage capacity was reached immediately after the peak rains and was maintained for about 3 months (Jun-Aug) whereas in the down stream side during the corresponding period this level was not reached and water table fluctuated with rainfall. Following the cessation of rain, there was a steep fall in the water table at the downstream side. Considerable quantity of water was conserved by the dike as indicated by the difference in water table on the upstream and downstream sides through out the year. It is worth noting that the summer showers received during March-April could be effectively conserved in the catchment area as evident from the water table rise in the upstream side of the dike.
The data recorded from the piezometers installed on the upstream side, downstream side and one in between the brick wall and the plastic sheet indicated that plastic sheet effectively arrested the subsurface flow of water and hence it could be used as an alternative cheaper material for subsurface dam. However, further detailed studies are needed to confirm this.
In areas of a well defined watershed with a narrow outlet and undulating topography, which is typical of Kerala, subsurface dike is an efficient system to conserve and utilise the rainfall that is received in a watershed. The water conserved in the dike can be utilised for irrigation and other purposes. It also allows for recycling of irrigation water and nutrients in the catchment area. Plastic sheet can also be used as an alternative cheaper materials to masonry wall as an efficient barrier against the subsurface flow of water.
Nilsson A 1988. Ground Water
Dams for small scale water supply. IT publications.69p
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