The negative effect of rains on the stability of the slopes is a problem that, added to anthropic factors and population settlements, currently generate not only material but also human losses. Therefore, the evaluation of threat by mass movements has become a first order problem. However, one of the aspects that presents the greatest uncertainty in these evaluations is the effect of soil saturation. This paper presents a methodology for evaluating the effect of rainfall by estimating the probability of soil saturation using the Richards equation and the first order and second moment method-FOSM. The methodology was applied considering two residual soils from the area of north-western Colombia named Aburra valley. For this purpose, a characterization of each material was made, evaluating the variability of shear strength parameters and hydraulic parameters. Subsequently, infiltration models were made using the Richards equation with a historical rain event that occurred between October 27 and November 13, 2010, which exceeded the failure thresholds established for the Aburra Valley and generated several landslides. The advance of the wetting front was evaluated, and the probability of saturation was determined. It was found that, in all the evaluated soils, full saturation reaches depths between 600 and 6000 mm and the probability of saturation is greater in soils from Stock de Altavista that report a lesser values of air entry suction. The mean values of φ b varies between 1.3° and 6.5° for soils from Stock de Altavista. © Published under licence by IOP Publishing Ltd.