Detailed analysis of soil-atmosphere interactions in two sample sites in Oltrepò Pavese

Large-scale quantitative assessment of water resources is generally carried out using models that take into account the soil-atmosphere interaction and hydraulic behaviour of an unsaturated soil. Volumetric water content and pore water pressure are the main basic characteristics to be considered when assessing the hydraulic behaviour of the soil in relation to rainfall events. These quantities are very useful because they constitute input data in different types of water balance models. Some models require knowledge of water retention curves that indicate a sort of “biunivocal” link between water content and pore water pressure. Most of these models include a single hysteresis, due to in situ wettingdrying processes to which soils are subjected under natural conditions (Bordoni et al., 2017). Experimental evidence shows that the “biunivocal” relationship between the quantities considered does not allow to adequately reproduce the real behaviour of un saturated soils. In the modelling chain, this mismatch can lead to an inappropriate estimate of water resources, in relation to the amount of rain. This note reports a detailed description of the interaction between soil and atmosphere in a specific area in Northern Italy. Data from continuous monitoring of two sample sites in Oltrepò Pavese, namely Montuè and Costa Cavalieri, representing two different geological contexts, were used (Bordoni et al., 2015). Different time spans have been considered, even those including single rainy events. Field measurements of both water content and pore pressure allow to clearly identify not only the seasonal fluctuations of the hydraulic properties of the soil, but also the hysteresis cycles that characterize the hydraulic behaviour of the unsaturated soil in correspondence of single rain events. In order to achieve this objective, the data recorded over a long period of time were processed and graphs representing the link between pore pressure and water content were obtained. The trends were also compared with the data on precipitation and air temperature. Analysis of the data shows that the dynamics characterising variations in water content and pore pressure at both test sites are closely linked to the alternation of wet and dry periods. However, the response is different in the two sample sites because it depends on the geological context and on the type of shallow soil. It seems that the response is also affected by the presence of preferential flow paths, especially in cracked clayey soil. In general, it can be observed that in winter and spring months, after rain events followed by a prolonged drying period, the response of soil layers up to 0.6 m from ground level is faster than that of the underlying layers. In fact, by increasing depth, the interaction between soil and atmosphere is delayed. Moreover, it is evident that the soil behaviour is not characterized by a single wetting-drying hysteresis, but by numerous cycles that correspond to different isolated rainy events.