Hydrological behaviour of first generation coppiced bluegum plantations inthe 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 a nd sub-surface hydrology has been a major concern for researchers and plann ers. This paper critically evaluates the implications of coppiced bluegum p lantations on hydrological behaviour during the 10 years of the second rota tion using the paired watershed technique in a montane temperate humid clim ate. The coppiced bluegum growth (1982-91) on 59% of the catchment area red uced the mean annual total runoff by 25.4% and base flow by 27% over the na tural grassland as compared with 16% and 15%, respectively, during the firs t rotation of 10 years. Regression relationships between observed and compu ted monthly values indicated a relatively higher reduction in the total as well as base flow during the first coppiced growth as compared with the fir st rotation. Reduction in runoff was at a maximum during July to October, w hich uas ascribed to greater availability of rain water and hence its utili zation during this period. The reduction during the dry period (January-Apr il) is crucial for sustaining dry weather flow in the hydro-electric reserv oirs. The increased utilization of water during the second rotation was as a result of the deep root system right from the earliest growth which produ ced 42% more biomass and 40% higher economic returns (at 1982-83 price leve l) than the first rotation. For the year following the harvest of bluegum t rees of the first rotation, the hydrological behaviour was similar to the p re-treatment conditions. The maximum growth of the coppiced shoots height a nd diameter at breast height (DBH) was noticed during second to fifth year as compared with fourth year onward in the first rotation. Moisture extract ion 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 wi th the ground water table was not observed during the second relation even through an extensive network of lateral roots was present. (C) 1998 Elsevie r Science B.V. All rights reserved.