THE ARNO RAINFALL-RUNOFF MODEL

This paper describes in detail a semi-distributed conceptual rainfall- runoff model known as the ARNO model, which is now in widespread use b oth in land-surface-atmosphere process research and as an operational flood forecasting tool on several catchments in different parts of the world. The model, which derives its name from its first application t o the Arno River, incorporates the concepts of a spatial probability d istribution of soil moisture capacity and of dynamically varying satur ated contributing areas. The ARNO model is characterized by two main c omponents: the first and most important component represents the soil moisture balance, and the second describes the transfer of runoff to t he outlet of the basin. The relevance of the soil component emerges fr om the highly nonlinear mechanism with which the soil moisture content and its distribution controls the dynamically varying size of the sat urated areas mainly responsible for a direct conversion of rainfall in to runoff. The second component describes the way in which runoff is r outed and transferred along the hillslopes to the drainage channels an d along the channel network to the outlet of the basin. Additional com ponents, such as the evapotranspiration, snowmelt and groundwater modu les, are also described. A discussion on the advantages of the model, calibration requirements and techniques is also presented, together wi th the physical interpretation of model parameters.Finally, after desc ribing the original calibration of the ARNO model on the Arno basin, a nd a comparison with several conceptual models, recent applications of the ARNO model, as part of a real-time flood forecasting system, as a tool for investigating land use changes and as an interesting approac h to the evaluation of land-surface-atmosphere interactions at general circulation model (GCM) scale, are illustrated.