**Preface to "Rainfall Infiltration Modeling"**

Rainfall infiltration plays a fundamental role within the hydrologic cycle. The spatiotemporal evolution of the infiltration rate under natural conditions cannot currently be deduced by direct measurements at any scale of interest in applied hydrology and, therefore, the use of infiltration modeling is of crucial importance in allowing it to be described through measurable quantities. In spite of continuous developments in infiltration modeling, the estimate of infiltration at different spatial scales, i.e., from the local to watershed scales, is a complex problem because of the natural spatial variability of both soil hydraulic characteristics and rainfall. For many years, research activity has been limited to the development of local or point infiltration models for vertically homogeneous soils. However, in addition to deepening this modeling, other interesting open problems should be addressed, including the modeling of point infiltration into vertically non-uniform soils, infiltration over horizontal heterogeneous areas, and infiltration into soil surface with significant slopes. The main objective of this Special Issue concerning the rainfall infiltration modeling is to put together updated and original contributions on this topic as a basic element to emphasize the critical points that require substantial research development. This Special Issue consists of 10 papers, including a review paper focused on rainfall infiltration modeling, which retraces some important milestones that led to the definition of basic mathematical models both at the local and field scale. In the same paper, some open problems involving the vertical and horizontal inhomogeneity of the soils are explored, while rainfall infiltration modeling over surfaces with significant slopes is considered. In the remaining papers, a wide range of topics and research questions is discussed. More specifically, a new conceptual model for slope infiltration and the effect of wastewater in the infiltration process has been developed, along with different recharge models and original sensitivity analyses in the field of groundwater.

> **Renato Morbidelli, Carla Saltalippi, Alessia Flammini** *Special Issue Editors*
