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The semi-arid region of the Mediterranean basin has climatic conditions characterised by a very erratic pattern of rainfall with respect to time, duration, intensity and amount. Subsequently, the hydrology of semi-arid regions is different from that of humid and sub-humid regions. As a result, one would expect that models developed for humid regions are not necessarily applicable to the semi-arid Mediterranean region. Mediterranean countries could increase their water resources by constructing small hill reservoirs. Gaining more understanding of the dominant hydrological processes operating in such an environment will help in the design as well as in sustainable water use of these reservoirs. Field measurements and modelling studies can achieve these goals. After being validated, models can be used as a management tool. The HYDROMED model has been developed for semi-arid regions in general and for Mediterranean conditions in particular. The model has two sub-models, the Rainfall-Runoff and The Reservoir Storage Capacity and Probability of Failure. In Part 1, the hydrological processes contributing to runoff are described and their mathematical formulations given while the results of model application on three Mediterranean catchments are given in Part 2. In Part 3 the model deals with the storage capacity of a reservoir required to maintain a yield with a given probability of failure. The HYDROMED model is menu driven, easy and friendly to use. The model uses the Genetic Algorithm for optimisation and has default values that can be selected in the absence of measured parameter values. There are different options to calculate the flows, options for the time steps ranging from less than one hour to one month and there is the facility to import text data files and import data directly from data loggers and to output results into data files and graphs. An example of calibration for the El-Gouazine catchment in Tunisia is given. The model successfully showed its ability to simulate Rainfall-Runoff events.