Influence of land use on soil moisture spatial–temporal variability and monitoring

In this study, the influence of land use on soil moisture dynamics is investigated for monitoring purposes. To this end, 23 measurement campaigns were carried out during a period of 8 months at 5 sites located in central Italy, within a catchment of ∼6 km2. The sites are characterized by different land uses: grassland, woodland (holm oak and hornbeam) olive grove and cropland. Soil moisture was measured with a portable Time Domain Reflectometer for a layer depth of 15 cm under the soil surface. The optimization of the monitoring scheme was addressed through a statistical and temporal stability analysis. Notwithstanding the significant differences in the land use, the temporal patterns of the field-mean soil moisture of the different sites were very similar while the spatial variability, expressed through the coefficient of variation, was found slightly higher (average value equal to 0.27) than that obtained from previous sampling campaigns on the same area but on sites characterized by a homogeneous soil use. The maximum number of required samples, to estimate the areal mean soil moisture within an accuracy of 2% vol/vol, was found ranging between 7 and 11 at the field scale and equal to 20 at the catchment scale. The temporal stability analysis allowed to identify the grassland site as the most representative of the catchment-mean soil moisture behavior (coefficient of determination, R2 = 0.96). Therefore, even though the heterogeneity of the land use increases the spatial variability (as expected), soil moisture exhibits a significant temporal stability and its large scale monitoring from few observations is still feasible.