Hydrological behaviour of first generation coppiced bluegum plantations in the Nilgiri sub-watersheds

The bluegum (Eucalyptus globulus) has been widely grown in different parts of the world for multiple purposes. However, the possible adverse effect of converting natural forest watersheds into bluegum plantations on surface and sub-surface hydrology has been a major concern for researchers and planners. This paper critically evaluates the implications of coppiced bluegum plantations on hydrological behaviour during the 10 years of the second rotation using the paired watershed technique in a montane temperate humid climate. The coppiced bluegum growth (1982-91) on 59% of the catchment area reduced the mean annual total runoff by 25.4% and base flow by 27% over the natural grassland as compared with 16% and 15%, respectively, during the first rotation of 10 years. Regression relationships between observed and computed monthly values indicated a relatively higher reduction in the total as well as base flow during the first coppiced growth as compared with the first rotation. Reduction in runoff was at a maximum during July to October, which was ascribed to greater availability of rain water and hence its utilization during this period. The reduction during the dry period (January-April) is crucial for sustaining dry weather flow in the hydro-electric reservoirs. The increased utilization of water during the second rotation was as a result of the deep root system right from the earliest growth which produced 42% more biomass and 40% higher economic returns (at 1982-83 price level) than the first rotation. For the year following the harvest of bluegum trees of the first rotation, the hydrological behaviour was similar to the pre-treatment conditions. The maximum growth of the coppiced shoots height and diameter at breast height (DBH) was noticed during second to fifth year as compared with fourth year onward in the first rotation. Moisture extraction from deeper soil layers by coppice growth was significant (alpha = 0.01) which was not true for the first rotation. Direct contact of tap roots with the ground water table was not observed during the second rotation even through an extensive network of lateral roots was present.