The evapotranspiration process is a key ingredient in the overall soil surface hydrological balance and therefore should be carefully evaluated in any geotechnical problem that is markedly affected by weather conditions, such as slope stability. This issue was investigated with reference to the clayey slope underlying the town of Orvieto in Central Italy. The site has been monitored for more than 30 years with piezometers and inclinometers and a clear correspondence between movement characteristics and pore pressure variations, and indeed rainfall events, was observed at different locations and depths. In order to better estimate the rainfall quantity that effectively infiltrates into the slope thus affecting the pore pressure regime, a study was conducted to evaluate the reference evapotranspiration ET0 at the investigated site. In order to assess the reliability of the temperature-based estimate of ET0 through the Penman-Monteith equation suggested by FAO, at a nearby meteorological station values estimated through the FAO procedure were compared to those obtained by a more refined estimate that utilizes measured meteorological data. In addition, the calculation of solar radiation was refined by introducing the effects of slope gradient and orientation as well as those of the shadowing produced by the rock cliff overlooking the clay slope. It was found that the temperature-based FAO estimate of ET0 is significantly larger than that calculated with a wide set of measured parameters. Finally, the repercussion of the different ET0 estimates on the infiltration process was investigated with reference to the water content available in the shallower part of the slope. Calculations, conducted through a simplified approach, indicated that in this specific application the difference in ET0 calculation plays a negligible role. Keywords: evapotranspiration, clayey slope, landslides, hydrological balance.
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