The Areal Rainfall Depth, necessary for the hydrological modelling of the basin, is derived from a punctual value attenuated by an Areal Reduction Factor (ARF). ARFs are a function of the rainfall duration and area of interest, and are subject to local aspects, so their extrapolation to remote sites should be studied in each case. In this paper, the areal decay of rainfall depth is studied in the Great San Miguel
de Tucumán by the record of a dense network of 18 meteorological stations. For this analysis, we select the storms that go beyond the maximum rainfall estimated for 5 -years return period, recurrence used in the design of urban sewage. Then, the CoDA methodology (García et al, 2000) is applied to the maximum intensity time intervals of these storms, according to pre-established durations. The incidence of
the spatial and time scale of the storms in the ARFs is analysed, since extensive and prolonged phenomena present less decay when moving away from the nucleus of the storm than intense local episodes.

Finally, the calibrated model generates an analytical expression of the ARF as a function of the duration and area. This result represents a fundamental tool for estimating the spatial distribution of design rainfall in the region.