Soil moisture temporal stability at different depths on two alpine hillslopes during wet and dry periods

This paper investigates the temporal stability of near-surface soil moisture at various depths at the hillslope scale. Detailed soil water content data were acquired at 0-6 cm, 0-12 cm and 0-20 cm during three 30-day field campaigns in 2005, 2006 and 2007. Two small alpine hillslopes with relatively homogeneous soil properties and vegetation cover but contrasting morphology were chosen to assess the persistence of spatial organization of soil moisture over time and along the soil profile, to identify the representative sampling locations and to evaluate the temporal stability during wet and dry states. Results show that both study hillslopes exhibited a strong degree of time stability, as revealed by very high autocorrelation values persisting for several days. The ranking stability approach allowed the identification of sampling locations representative of the average hillslope soil water content. These locations, one for each experimental site, proved to act as good indicators of soil moisture at other depths and even on the other hillslope. The spatial structure of soil moisture fields was not affected by the occurrence of piezometric response and was well preserved at all depths during both wet and dry periods, with a slightly higher degree of temporal stability in dry conditions and for deeper layers. The remarkable persistence of soil moisture spatial patterns over time and along the soil profile on the study sites was mainly related to the macro- and micro-topographic properties of the two hillslopes but the soil wetness conditions generally skewed towards the wet state and the negligible variability of climatic forcing due to the small study scale might have contributed significantly.