A subsurface dyke is a barrier impermeable to water that is placed underground to control the groundwater flow in an aquifer, and to raise the water table.
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Although the total amount of water on Earth is generally assumed to have remained virtually constant, the rapid growth in population, together with the extension of irrigated agriculture and industrial development, are putting stress on the quality and quantity aspects of natural system. The sub-surface dyke[1][2][3] constructed at Krishi Vigyan Kendra, Kannur at Panniyur near Taliparamba under Kerala Agricultural University with the support of ICAR as part of demonstrating rain water harvesting technologies has become a living example for an effective method for ground water conservation. 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. The dyke is now the largest rainwater harvesting system in the region.
Subsurface dyke 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.
A trench was made across the valley portion of the undulating topography of the farm down to the bed rock. Three layers of plastic sheets were spread on the wall of the trench from ground level to the bed rock level. The plastic sheet on the wall measured 40m in length with a maximum depth of 8 m. It obstructs the flow of sub surface water raising water table in the catchment area.
The dyke was constructed in the 25 hectare farm of Krishi Vigyan Kendra Kannur in 2007 to solve the problem of water scarcity. The experience of KVK indicated that subsurface dyke 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 dyke. The dyke resulted in the maintenance of higher water table in the catchment area for a longer period of time. There is an existing dug out pond with top length of 12 m, width 8m a depth 5m in the upstream side of the dyke. The pond is the only source of water in the KVK farm. The pond, which usually gets dried up in the summer is now maintaining a high water table throughout the year as the dyke ensures an incessant supply of water to the pond. Weekly data on the ground water level were recorded using water level recorder. The data showed that significantly higher level of water table was maintained in the catchment area (upstream area) compared to down stream area. Taking into account the mean depth of soil as 2 m the catchment area as 1.5ha (the actual area is 15 ha) and the drainable porosity as 10%, the total water storage capacity of the dyke would be 3000 m3. The crops cultivated in the catchment area would be benefited by way of constant capillary feeding of the rhizosphere. 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 dyke are most ideal for in situ conservation of rainfall received in a watershed.
The ideal location for the dyke 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 . Subsurface dyke 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 maintenance is negligible. It is environment-friendly.
The other rainwater harvesting technologies demonstrated at KVK include stone pitched contour bunds, dry rubble check dams, protection of seasonal spring by afforestation, moisture conservation pits, sprinkler and drip irrigation and roof water harvesting system.