Soil moisture in the development of hydrological processes and its determination at different spatial scales

The interactions between the earth’s surface and atmosphere are a crucial part of the processes determining the spatio-temporal dynamics of surface and subsurface water. The soil moisture in the surface layer contributes to this interaction through the impact on energy fluxes and by supplying the troposphere with water vapor by evaporation from bare soils and evapotranspiration from vegetated soils. These two processes lead to a depletion of subsurface water balanced by the increase of precipitable water in the troposphere, that has a positive effect on rainfall generation. Then, at the beginning of a rainfall period the soil moisture vertical profile plays an important role in the subdivision between infiltration and hortonian overland flow. These considerations suggest that the spatiotemporal dynamics of soil moisture represents an important topic to be investigated to explain many processes of major significance in hydrological practice (see also Vereecken et al., 2008) such as, for example, the rainfall-runoff transformation, recharge of aquifers and transport of pollutants in the vadose zone. Furthermore, the depth of the wetting front in the stages of infiltration and redistribution of soil water influences the evolution of vegetation and determines the timing of irrigation. An overall analysis of these elements indicates that studies of soil moisture as a function of time at different spatial scales, ranging from the local to the field scale to the watershed one, should be effectively performed. In addition, temporal scales up to the daily one would be acceptable for a wide number of applications. Continuous local measurements of soil moisture in the surface layer, as well as of its vertical profile, have a crucial role in focusing the upscaling and disaggregation methods required to obtain moisture estimates with the resolution necessary for applications at the field or catchment scale. However, these measurements are still complex, costly, and not so common and thus, in spite of the large number of research papers published on this subject in the last 20 years, many problems are not yet adequately solved. The development of new sensors suitable for observations of soil moisture in the uppermost layer, more representative of the soil–atmosphere interface than the measurements commonly performed at a minimum depth of about 5 cm, would lead to an important improvement in understanding the processes of evaporation, infiltration and redistribution of soil water. The development of local modeling to represent the temporal dynamics of soil moisture in the surface layer and as a function of depth is of primary importance when simulations under given hydrometeorological conditions have to be performed. This modeling is generally used as a component of complex hydrologic models and should be extended at field or larger scales. What is required in this context is to set up rather simple and tractable theoretical approximations of the conceptual or semi-analytical type.