Controls on soil moisture variability on two Mediterranean hillslopes during dry and wet periods using wavelet coherence analysis

Although topography and evapotranspiration rates are the main determinants of soil moisture, climatic forcings play a crucial role. In the Mediterranean climate, the marked sensitivity of soil moisture to alternations between wet and dry periods exerts a strong control on hydrological and ecohydrological processes at the hillslope and catchment scales.

We monitored soil moisture in two hillslopes in the Re della Pietra experimental catchment, Appennine mountains, Tuscany, central Italy. The two hillslopes (HS1 and HS2) show different morphological characteristics, such as elevation (≅ 670m asl for HS1 and 940m asl for HS2), slope (≅ 26° for HS1 and 36° for HS2), and tree composition (fruit chestnut grove converted into coppice in HS1 and pure beech forest in HS2). For two years, soil moisture was measured in each hillslope, at three different positions and two different depths (15 and 30 cm) along a longitudinal transect. We used wavelet coherence analysis to evaluate the dominant factors controlling soil moisture variability in the two hillslopes during dry and wet periods.

Preliminary results reveal a clear coupling of soil moisture at 15 cm and 35 cm on both hillslopes during wet periods, indicating a relatively homogeneous soil water content across the two depths. Conversely, a decoupling occurs during dry periods when soil moisture values at 35 cm are greater than those at 15 cm, reflecting significant solar radiation, atmospheric demand, and tree water uptake from shallow soil layers. During dry periods, we observed significant differences in soil moisture between the two depths in HS1 compared to HS2, suggesting that local conditions affect hillslope-scale soil moisture response.

Ongoing analyses investigate the role of rainfall, solar radiation, vapor pressure deficit, and tree transpiration on soil moisture spatio-temporal variability on the two hillslopes.