Rainfall infiltration to depth in slopes formed by clayey soils has been generally considered minor, to the extent of being assumed not influential on deep instabilities. As a consequence, rarely monitoring campaigns include piezometric monitoring at large depths. Furthermore, the modelling of seepage at depth is generally not satisfactory because it has to account for the lithostratigraphy of the whole slope and the hydro-geological boundary conditions, seldom surveyed with the appropriate accuracy. The present paper addresses this gap of knowledge in the field of slope-atmosphere interaction, being concerned with the effects of climate on the equilibrium at large depths in slopes formed by clayey soils. In particular, the clayey soils of reference are very widespread in the southern Apennines (Italy), often part of tectonized turbidites, that are in general sequences of fissured clays and fractured rock strata. The paper discusses the results of a research on a case history, the Pisciolo hill-slope (Melfi, Italy), a pilot site largely representative of the slopes in the Daunia and Lucanian Apennines. In the slopes of this region the combination of the clay poor strength properties with the high pore-water pressures prompts the propagation of deep failures, resulting in slow to extremely slow landsliding in slopes of even small inclination. Therefore, the research is aimed to investigate the sources of the large piezometric heads that predispose the slope to instability and the external causes that trigger the slope accelerations. The geological model of the Pisciolo slope and the results of the mechanical and hydraulic characterization of the fissured clays are first presented in the paper. Thereafter, numerical analyses of the seasonal rainfall infiltration and evapo-transpiration are reported with the aim to predict transient flow conditions and to verify the agreement with field monitoring data.