From Headwaters to the Ocean – Taniguchi et al. (eds) © 2009Taylor & Francis Group, London, ISBN 978-0-415-47279-1 A continuous rainfall-runoff model as a tool for the critical hydrological scenario assessment in natural channels L. Brocca ∗ , F. Melone &T. Moramarco National Research Council, Research Institute for Geo-Hydrological Protection, Perugia, Italy V.P. Singh Department of Biological and Agricultural Engineering, Texas A & M University, Texas, USA ABSTRACT: Hydrological models are being increasingly considered as reliable tools in providing insights to decision makers on potential scenarios of catchment management when climatic and anthropogenic factors are changing. In this context, an appropriate identification of the physical processes involved in rainfall-runoff transformation, starting from the soil moisture conditions of the basin, is required. This paper, addressing the soil moisture issue, investigates, for a pluriannual period, the scenarios for part of a natural channel network which is fundamental for water resources management in the upper Tiber River basin, Central Italy. The analysis is based on the extended formulation of theTiber River basin semi-distributed hydrological model, including a new component for the soil water balance, which represents a useful tool for runoff prediction in a climatic and land use changing scenario. In particular, the model permits, for each single uniform unit (sub-basin), to assess both the average soil moisture condition and the different runoff components at the basin outlet. The model requires an estimate of a few physically based parameters and hydrological quantities routinely measured, as input data. The model performance is found quite accurate in terms of runoff prediction at gauged sections, belonging to the investigated channel network. The daily discharge assessment during the pluriannual period has allowed for evaluating the contribution of each river branch and hence the suffering temporal ranges of the channel system for low flows. Keywords: rainfall-runoff modeling; water resources; drought; low flows; soil moisture The water resources and catchment management deeply depends on climate changes. As emphasized in Xu et al. (2005), the effects of these changes can be analyzed through a procedure which can be schematized in three steps. The first provides global scenarios by using General Circulation Model (GCM); the second one allows downscaling the CGM out- comes at regional scale so that they can be suitable for hydrological applications; the third step addresses hydrological models to assess the effects at different scales of climate changes. At local scale, the man- ager of water resources is involved in facing the direct impact of climate changes on the supply of fresh- water. In this context, hydrological models represent a reliable tool for providing insights to the decision maker on potential scenarios of catchment manage- ment when climatic and anthropogenic factors are ∗ Corresponding author (l.brocca@irpi.cnr.it) changing. However, the contribution of hydrologi- cal models is sound, provided that they are able to provide an appropriate identification of the physical processes involved in rainfall-runoff transformation, starting from the soil moisture conditions of the basin. Therefore, the assessment of this quantity is a key fea- ture of the hydrological modelling and due to a limited knowledge on the spatial variability of soil mois- ture, many uncertainties affect its estimation. Recently, Brocca et al. (2008a) proposed a simple soil water bal- ance model to assess the surface soil moisture for small catchments. This approach has been incorporated as a module into a semi-distributed rainfall-runoff model providing, in principle, a useful tool for runoff pre- diction in a climatic and land use changing scenario. Indeed, the model requires estimates of a few phys- ically based parameters and hydrological quantities routinely measured as input data. Therefore, the objective of this paper is to analyze, for a pluriannual period, the suffering scenarios for 175 © 2009 Taylor & Francis Group, London, UK