Hydrological behaviour of the Nilgiri sub-watersheds as affected by bluegum plantations, part I. The annual water balance

Abstract

The Nilgiri Plateau in S. India receives an average annual rainfall of 1300 mm distributed over 190 to 220 rainy days. It is estimated that the Nilgiri District alone is responsible for about 50% hydro electric power generation in Tamil Nadu State. In this paper, the effect of converting natural grasslands (grazed) in the catchments of hydel reservoirs to bluegum ( Eucalyptus globulus) plantations on expected water and sediment yield has been discussed. It is estimated that bluegum, during a first rotation of ten years, brings about 16% reduction in the total expected water yield from natural grasslands. This figure is found to be significant both at 1 and 5% levels. The reduction in total flow on an average works out to be 87 mm yr −1 during the first rotation of ten years. During the first rotation, bluegum extracts moisture mostly from the upper soil layers and does not directly interfere with the groundwater table. The reduction in soil moisture status in the upper 50 cm soil depth is found to be significant, though at 1 m sampling depth it is significant only at 5% level. The maximum growth rate is observed from the fourth to the seventh year of plantation. Neither the natural grasslands nor the bluegum add any sediment load to the reservoirs. Caution may, however, have to be exercised while planning large-scale conversion of natural grasslands into bluegum plantations.