The overland and downslope flow of rainwater into channelled flow when the rock or soil is saturated. In rainfall run-off modelling, hydrological models are categorized as lumped—which treat the catchment area as a single unit—or distributed—which represent the catchment as a system of interrelated subsystems, both horizontally and vertically. For deterministic rainfall models, see Ramos et al. (1995) Water Resources Res. 31, 6. For fuzzy conception rainfall models, see Ozelkan and Duckstein (2001) J. Hydrology 253, 1. For a very clear guide through a parametric rainfall-run-off simulation model see Dawdy, Lichty, and Bergmann (1972) US Geol. Surv., Prof. Paper 506–B. For the use of artificial neural networks to relate meteorological variables to run-off, see Antar et al. (2006) Hydrol. Procs 20, 5.