This presentation by ACWADAM deals with geological mapping. The occurrence of water within a watershed is governed by the rock types occurring both at the surface and within the subsurface, their sequence and their structure. The physical structure which plays host to the water within the watershed is geological. Every watershed programme must therefore include a fundamental understanding of this framework so that the development and management measures planned through such programmes are meaningful and properly justified technically.

The first step in this direction is systematic and detailed “geological mapping” of the watershed. The mapping of the watershed includes two basic components – surface and sub surface mapping. The geological mapping of a watershed is undertaken with the help of certain available maps which describe the physical configuration of any area (slopes, contours or elevation from mean sea level, village locations, roads, forest cover, some wells, railway lines etc). These are called topographic maps or toposheets and are standard maps prepared by the Survey of India (SoI).

Toposheets must be used as the fundamental maps during watershed mapping. In addition to these maps, satellite images of the watershed can also be obtained, at cost, to support mapping. These images are useful in mapping regional surface features and the regional geology and in delineating structural features like fracture and fault zones, regional folding in rocks, soil moisture variation and other features like land use and cropping patterns which are useful in watershed evaluation and planning.

Surface drainage courses can be mapped using these images but groundwater cannot be directly observed. However these toposheets and satellite imageries are useful in the general geological mapping of the watershed, which in turn can lead to systematic mapping of the water resources in the watershed and especially in developing a correct understanding of the groundwater resources.

This presentation is part of the training modules on planning, development and management of groundwater with special reference to watershed management programmes by ACWADAM. Please write to ACWADAM at acwadam@vsnl.net for sourcing these presentations.