23-Studies on hydrological impact of conversion of natural grasslands into bluegum plantations (Eucalyptus globlus) in the Nilgiri Hills.

Abstract

The Nilgiris disrict is drained by the rivers Moyar and Bhavani and their innumerable tributaries which are harnessed for Irrigation and generation of electricity. The natural vegetation comprised of vast stretches of grasslands interspersed with pockets of 'Sholas'. Out of many useful alien trees introduced in the District, bluegum has been extensively used for afforestation by the Tamil Nadu Governments over the past thre2 decades due to its high rate of biomass production, coppicing capacity, shorter rotation and high economic returns. The Central Soil and Water Conservation Research and Training Institute, Research Centre, Udhagamandalam initiated catchment studies during 1968 to quantify the effect of conversion of natural grasslands into bluegum plantations on water yields in the hills through paired watershed technique.The study area located at an average elevation of 2200 metres above sea level experiences an average annual rainfall of 1490 mm, 810• of which is contributed by south west and north east monsoons. Runoff was measured over 2:1 trangular weirs with stage level recorders. After the initial calibration of the watersheds under natural cover of grassland, 'Shola' and swamps, one of the watersheds was planted with eucalyptus in July,1972. At the end of the first rotation of 10 years, the trees were felled during 1982 and during this period the average annual reduction in total runoff due to conversion of grasslands into bluegum was to the tune of 16 . During the first half of the second rotation, the reduction in total flow has further increased to about 20'A. The seasonal distribution of total flow especially during the dry period which is mostly baseflow is. drastically reduced to 50/.. The data on consumptive use by Eucalyptus as a function of age and annual rainfall, five yearly trends in annual and seasonal runoff ratios for the period 1968 to 1986, the implications of reduced inflows into the hydro-electric reservoirs affecting power production,the policy decisions to be taken by the Government to sustain the hydel power based industries and wood based industries are discussed in this parer togeth2r with environmental impact.