Surface-cover controls on reference-to-actual evaporation in silty pyroclastic soils of Campania region, Italy: evidence from an 12-year lysimeter experiment

Abstract

Study region pyroclastic soils investigated in this study are widespread on natural slopes, not only in Italy (particularly in the Campania region), but also in numerous volcanic regions worldwide. Their intrinsic and state-dependent properties make them highly sensitive to soil–atmosphere interactions, which strongly influence both rainfall infiltration and water losses through evaporation. Study focus estimating potential and actual evaporation under different land-cover conditions is essential for several hydrological applications. This study derives empirical functions for the FAO single-crop approach, namely the crop coefficient (k c ) and the water-content reduction function (k r (θ)), for a non-plastic silty volcanic soil. The analysis is based on a 12-year lysimeter experiment performed under three surface conditions: bare soil, grass-covered soil, and bare soil covered by a wooden ember cover (WEC). A procedure based on cumulative evaporation fluxes was adopted to reduce short-term measurement scatter and identify the water-content thresholds separating energy-limited and water-limited regimes. New hydrological insights for the region results highlight substantial differences in potential evaporation among the investigated surface conditions and distinct forms of k r (θ), including a bilinear decay for the WEC associated with its capillary-barrier effect. Vegetation maintains higher evaporative efficiency under dry conditions, whereas the WEC markedly reduces actual evaporation despite its high radiative absorption. The calibrated parameters were used to develop predictive abaci for estimating monthly evaporation from atmospheric variables alone, providing practical tools for hydrological modelling, agricultural applications, and weather-induced landslide hazard assessment.