GEOTOP: A Hydrological Balance Model: Technical Description and Programs Guide,Version 0.75

The study of the hydrologic cycle is physically focused on the analysis of the interactions between the soil surface (and specifically the soil water content, linked with precipitation) and the low atmosphere, which occur mainly through the mediation of the soil itself, the vegetation and the turbulent and radiative energy transfers which take place on the Earth's surface. In recent years, the hydrologic research has evolved towards a comprehensive theory describing the mass, energy and motion-quantity exchanges between surface and atmosphere at several scales. The practical aims of this efforts are: (1) to improve the mid- and long-term hydrologic forecasts; (2) to increase our capability of describing the impacts deriving from changes in the soil use and in the climate on the hydrologic cycle and on the Earth's ecosystems. The paper illustrate the implementation of GEOTOP, a distributed model of the hydrologic cycle which is meant to give a scientific contribution in this direction. GEOTOP is a terrain-based model, i.e. it is based on the employment of DEMs (digital elevation models); it is a distributed model, since all the simulated variables are returned for each pixel in the basin; it is a model of the hydrological cycle, in the sense that it simulates all the elements of the hydrological cycle, and not only the mass balance but also the energy balance: in fact the two balance equations are coupled by the evapotranspiration terms and by the soil temperature, which controls the soil hydraulic conductivity and the snow cover accumulation. As to the soil-atmosphere interaction, GEOTOP follows the treatment initially developed by Deardorf, and then implemented, with numerous changes, in land surface models either at global scale like BATS, NCAR-LSM, NOAH-LSM - or at larger scales like the VIC or at basin sclae like the DHSVM. The model adopts original solutions for the discharge calculation. In particular, it differs from the TOPMODEL, because it does not assume any stationeries conditions in the subsurface flows.