Low flow and high flow responses to converting natural grassland into bluegum ( Eucalyptus globulus) in Nilgiris watersheds of South India

A concern has been raised in many parts of the world over the effect of large scale planting of Eucalyptus on hydrological behaviour of small watersheds. Hydrological response of watersheds due to conversion of natural grasslands into bluegum ( Eucalyptus globulus) plantations on low flows and high flows has been presented in this paper. The concept of using low flow index (LFI) as a tool to study and quantify the effects of bluegum plantation on low flow regime has been demonstrated. Conversion of natural grasslands into bluegum plantations has resulted in decreased low flow volume as well as peak flow, which in turn increased the soil moisture losses. These effects were more pronounced during the second rotation (i.e. first coppiced growth) as compared to the first rotation. Significant reduction in low flow as a result of decline in base flow could be predicted with LFI decreasing by 2.0 and 3.75 times, in the first and second rotation, respectively. Moderation in peak discharge rates was also observed as a result of bluegum plantation. Probability plots of peak discharge tend to suggest that the effect of bluegum plantation on peak flows become insignificant for the floods with higher return periods. These results clearly suggest that caution need to be exercised while planning large scale conversion of natural grasslands into bluegum plantations in the catchments of hydro-electric reservoirs in the Nilgiris which adversely affects water availability especially during lean flow period.