Geochemical mass balance in a small watershed in the seasonal tropical region of the Western Ghats: The case of a tributary in the source region of Koyna, India

Abstract

The objective of the present study is to establish the rate of weathering profile development and the rate of denudation in a small watershed in Western Ghats. The Sonpar watershed is drained by the major right bank tributary of the Koyna river in the latter's source region. The basin covers an area about 45.59 sq.km (17.6 sq miles) and lies in the Raigad district of Maharashtra. Besides calculating the geochemical balance the paper provides an insight in the methodology applied, which involved number of steps like the assessment of the average weathering profile thickness, establishment of hydrological balance, calculation of hydrochemical analysis, assessment of silica losses during the course of weathering, calculation of denudation rate by estimating the total solutional and suspended loss. The rate of silica losses provides the rate of weathering profile development. All these calculations were carried out for the year 1989, by monitoring the river discharge and analysing chemically the samples of weathered material from the profile. The whole basin is characterized by steep slopes, some residual relatively flat, narrow stream terraces, a relatively high relative relief and is being dissected by streams and gullies. The estimated age of regolith formation and denudation seems to lie between 20,000 to 30,000 years.