HYDROLOGICAL PROCESSES Hydrol. Process. 21, 2100–2111 (2007) Published online 1 May 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.6373 Improvement of the hydrological component of an urban soil–vegetation–atmosphere–transfer model A. Lemonsu, 1 * V. Masson 1 and E. Berthier 2 1 M´ et´ eo-France, Centre National de Recherches M´ et´ eorologiques, 42 avenue Coriolis, 31057 Toulouse Cedex, France 2 DREIF-Laboratoire de l’Ouest Parisien, Division Eau-Environnement, 12 rue Teisserenc de Bort, 78190 Trappes, France Abstract: A numerical study was conducted on the Rez´ e suburban catchment (Nantes, France) to evaluate the hydrological component of the town energy balance (TEB) scheme, which simulates in a coupled way the water and energy balances for the urban covers. The catchment is a residential area where hydrological data were continuously collected from 1993 to 1998 by the Laboratoire Central des Ponts et Chauss´ ees (LCPC), notably the runoff in the stormwater drainage network. A 6-year simulation with the TEB and interaction soil–biosphere–atmosphere (ISBA) schemes in off-line mode enabled the comparison of modelled and observed runoff. Some weaknesses of the TEB were uncovered and led to improved parameterization of water exchanges: (1) calibration of the maximum capacity of the rainfall interception reservoir on roads and roofs and (2) inclusion of water infiltration through the roads, according to a simple formulation. The calibration of this water flux gives results that are consistent with direct measurements of water infiltration performed on the Rez´ e site and from the literature. The new parameterization produces better runoff in terms of timing and magnitude, which are comparable to those obtained by the LCPC with other hydrological models. It shows also the impact of the water infiltration through the roads, corresponding to a water transfer from the TEB to ISBA, on the water balance: the water contents of road, roof and soil reservoirs being modified, the evaporation from artificial surfaces decreases, while the evapotranspiration from natural covers increases. Through the evaporative flux, such a modification of the water balance induces large repercussions on the surface energy balance. Copyright  2007 John Wiley & Sons, Ltd. KEY WORDS urban hydrology; SVAT; urban physically based model Received 19 September 2005; Accepted 22 May 2006 INTRODUCTION The water balance is largely modified in the urban environment due to human intervention (Oke, 1987). The same hydrological processes that occur in natural areas are disturbed. The origins of these perturbations are the urban geometry (pattern of roads and roofs), the properties of artificial materials, and the drainage network design. The major characteristics of the urban surface water exchanges are a reduced water storage on built-up covers coupled with a reduced evaporation (Oke, 1982) and a greater surface runoff than from natural surfaces (Boyd et al., 1994). Urban surface runoff that reaches the drainage network is the most important water source for the total runoff observed at the outlet of the network. It should be noted that, in certain residential districts, the outdoor water usage for garden watering can represent an important water source and has a significant impact on the evapotranspiration (Grimmond and Oke, 1999a). In the urban soil, the water exchanges are much more complex due to the large heterogeneities in the ground arrangement. Although some experimental studies allowed a better knowledge of the hydrological behaviour and the water balance specific to the urban areas, the * Correspondence to: A. Lemonsu, M´ et´ eo France, Centre National de Recherches M´ et´ eorologiques, 42 avenue Coriolis, 31057 Toulouse Cedex, France. E-mail: aude.lemonsu@meteo.fr different processes are difficult to quantify: urban surface runoff (Hollis and Ovenden, 1988a; Boyd et al., 1994; Ragab et al., 2003a), water infiltration through the roads (Hollis and Ovenden, 1988b; Ragab et al., 2003b), water storage on artificial surfaces (Boyd et al., 1993), ground- water infiltration into the sewer network (Belhadj et al., 1995). Modelling studies tried to parameterize these complex mechanisms: Boyd et al. (1994) for the runoff, Hollis and Ovenden (1988b) for the depression storage, and Dupasquier et al. (1998) for the ground-water infiltra- tion into the sewer network. The recent urban parame- terizations used in soil–vegetation–atmosphere–transfer (SVAT) models describe the energy exchanges between the urban surfaces and the atmosphere in a realistic way. But the hydrological component is often non-existent or very simple. When the water exchanges are taken into account, both energy and water balances are resolved and interact through the evaporative term, as in the Grimmond et al. (1986) model and in the town energy balance (TEB; Masson, 2000) scheme. The water balance for a catchment, composed of natural and built-up surfaces, can be written thus: P C I D E C R C D C W [kg m 2 s 1 ] ⊲1⊳ where P is rainfall, I is the external pipe-water supply (i.e. irrigation and garden watering), E is evaporation, Copyright  2007 John Wiley & Sons, Ltd.