Saturated macropore flow is the dominant hydrological process in tropical and subtropical hilly watersheds of northeast India.The process of infiltration into saturated macroporous soils is primarily controlled by size,network,density,connectivity,saturation of surrounding soil matrix, and depthwise distribution of macropores.To understand the effects of local land use,land cover and management practices on soil macroporosity,colour dye infiltration experiments were conducted with ten soil columns (25 × 25 × 50 cm)collected from different watersheds of the region under similar soil and agro-climatic zones.The sampling sites included two undisturbed forested hillslopes,two conventionally cultivated paddy fields,two forest lands abandoned after Jhum cultivation,and two paddy fields,one pineapple plot and one banana plot presently under active cultivation stage of the Jhum cycle.Digital image analyses of the obtained dye patterns showed that the infiltration patterns differed significantly for different sites with varying land use,land cover,and cultivation practices.Undisturbed forest soils showed high degree of soil macroporosity throughout the soil profile,paddy fields revealed sealing of macropores at the topsoil due to hard pan formation,and Jhum cultivated plots showed disconnected subsoil macropores.The important parameters related to soil macropores such as maximum and average size of macropores,number of active macropores,and depthwise distribution of macropores were estimated to characterise the soil macroporosity for the sites.These experimentally derived quantitative data of soil macro- porosity can have wide range of applications in the region such as water quality monitoring and groundwater pollution assessment due to preferential leaching of solutes and pesticides,study of soil structural properties and infiltration behaviour of soils,investigation of flash floods in rivers, and hydrological modelling of the watersheds.

Being the highest specific discharge river system in the world, the Brahmaputra river experiences a number of long-duration flood waves during the monsoon season annually. In order to assess the flood characteristics at the basin and tributary scales, a physically based macro-scale distributed hydrological model (DHM) has been calibrated and validated for 9 wet years. The model performance has been evaluated in terms of prediction of the flood characteristics such as peak discharge, flood duration, arrival time of flood wave, timing of the peak flow and number of flood waves per season. Future changes in the flood wave characteristics of the basin have been evaluated using the validated model with bias-corrected future-projected meteorological scenario from a regional climate model (RCM). Likelihood analysis of the simulated flow time series reveals that significant increase in both peak discharge and flood duration is expected for both the pre-monsoonal and monsoonal seasons in the basin, but the number of flood waves per season would be reduced. Under the projected climate change scenario, it is expected that there will be more catastrophic floods in the basin.