Understanding runoff processes using a watershed model—a case study in the Western Ghats in South India M.R.Y. Putty a , R. Prasad b, * a Department of Civil Engineering, The National Institute of Engineering, Mysore-570 008, India b Department of Civil Engineering, Indian Institute of Science, Bangalore-560 012, India Received 6 January 1997; accepted 16 December 1999 Abstract The wet tropical Western Ghat Mountain ranges in South India present an interesting combination of meteorological and physical characteristics. The results of a watershed model analysis carried out to understand the catchment response and the relative importance of different runoff processes in the region are reported in this paper. A lumped parameter model simulating saturated source area runoff, lateral flow through pipes and the saturated zone groundwater flow, has been developed assuming that source area runoff is the only quickflow component. The model has been calibrated on seven catchments using sufficiently long records of daily data. A wide range of tests has been used to show that the model performs reliably. The influence of catchment characteristics on the relative importance of the flow components and the catchment response has been studied. The model simulations have been interpreted to infer that the pipeflow contributions augment the contributions of source area runoff to stream quickflow. Suggestions for further research in the area are given, based on the inferences drawn.  2000 Elsevier Science B.V. All rights reserved. Keywords: Wet mountainous catchments; Variable source area theory; Lumped parameter model; Pipe quickflow; Catchment characteristics; Dynamic contributing volumes; Rainfall influence 1. Introduction A watershed model is a mathematical representa- tion of the catchment processes capable of simulating streamflow and other outputs of the catchment system, corresponding to any given values of the inputs, mainly precipitation. Hence, the model is normally utilised either for generating streamflow or to deter- mine how runoff is affected by factors such as affor- estation (e.g. Aston and Dunin, 1980; Eeles and Blackie, 1993), urbanisation (Smith and Bedient, 1981) or rainfall augmentation (Lumb and Linsley, 1971). As shown by several researchers, however, it is possible to utilise the model as an investigative tool also for learning about catchment response and infer- ring about the runoff processes in the catchment. For example, Betson (1964) inferred the existence of partial source areas of runoff by his regression model. Freeze (1972) made deductions about soil parameters using a subsurface flow model. Smith and Hebbert (1983) use an unsaturated vertical flow model to infer the influence of soil depth and aniso- tropy on source areas and runoff. Ward (1984), comparing streamflow predicted by his catchment model with observed flows, suggests physical processes to which the differences between the two are linked. McCord et al. (1991) employ their model Journal of Hydrology 228 (2000) 215–227 0022-1694/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-1694(00)00141-4 www.elsevier.com/locate/jhydrol * Corresponding author. E-mail address: rama@civil.iisc.ernet.in (R. Prasad).