This thesis aims to enhance the understanding of the hydrological functioning of headwater catchments by focusing on i) how rainfall patterns controls spatial and temporal variability of soil moisture, and ii) how the soil moisture variability provide a control to the catchment response.
A first analysis of the spatial variability of soil moisture was carried out for data at 0-30 and 0-60 cm depth collected on a plot in Grugliasco (Po Plain, Northern Italy), characterized by two land uses (meadow and vineyard). Results showed that the differences in spatial mean and variability of soil moisture for the meadow and the vineyard are likely due to the different vegetation cover. Evaluation of the main physical controls on the spatial mean and the variability of soil moisture was carried out by using a simple bucket model. The model was calibrated by using spatial mean soil moisture and it had a relatively good prediction capability. The model was also shown to be able to capture the main differences between the two sites in terms of spatial variability of soil moisture.
The spatial and temporal variability of soil moisture was also analyzed in relation to throughfall spatial patterns in plot on a forested hillslope in the Italian pre-Alps. Throughfall was measured using two types of throughfall collectors: buckets and rain gauges. The collectors differed in size, number and spatial arrangement. Results showed that buckets and rain gauges measured similar throughfall amounts during rainfall events. However, findings indicate that different collectors can lead to differences in the quantified spatial variability of throughfall and presence of local clusters and outliers.
Near-surface soil moisture was measured upslope of each bucket, at 0-7 and 0-12 cm depth before and after rainfall events. Throughfall and soil moisture spatial patterns were not significantly or only weakly correlated, likely due to the lateral and vertical redistribution of water in the soil profile during the 2-36 hour period between the end of the rainfall event and the start of the soil moisture measurements. The temporal stability of soil moisture was larger than the temporal stability of throughfall and they were also not significantly correlated. The patterns of temporal stability were also not related to canopy characteristics (i.e., canopy openness and leaf area index). The application of the simple bucket model revealed that a large spatial variability in saturated hydraulic conductivity that is correlated with the spatial variability in leaf area index and root fraction weaken the correlation between throughfall and soil moisture patterns. The analysis of field data combined with the model application suggests that in this specific forested hillslope the spatial organization of soil moisture is dominated by a combination of soil properties and vegetation characteristics, rather than by the throughfall spatial patterns.
Saturation at the soil-bedrock interface or the rise of shallow groundwater into more permeable soil layers results in subsurface stormflow and can lead to hillslope-stream connectivity. Networks of spatially-distributed piezometers in five small headwater catchments in the Italian Dolomites and the Swiss pre-Alps were used to quantify and compare the spatial and temporal variability of subsurface connectivity and its relation to streamflow dynamics. Results showed that the time that piezometers were connected to the stream was significantly correlated to the topographic wetness index, for two Swiss pre-alpine catchments, or to the distance to the nearest stream, for the dolomitic catchment with the largest riparian zone. During rainfall events, mainly anti-clockwise hysteretic relations between streamflow and the area that was connected to the stream were observed. Threshold-like relations between maximum connectivity and total stormflow and between maximum connectivity and the sum of total rainfall plus antecedent rainfall were more evident for the dolomitic catchments, where the riparian zone is characterized by a groundwater table near the soil surface. These preliminary results suggest that the delayed increase in subsurface connectivity relative to streamflow is likely not affected by the presence of a riparian zone. However, further analyses are needed to determine if morphology of the catchments affect the observed relations between subsurface connectivity and total stormflow.
Finally, this thesis attempted to develop an index for the quantification of hysteretic loops between hydrological variables at the runoff event timescale. The index provides information on the direction, the shape and the extent of the loop. The index was tested with synthetic data and field data from experimental catchments in Northern Italy. Hysteretic relations between streamflow and soil moisture, depth to water table, isotopic composition and electrical conductivity of stream water were correctly identified and quantified by the index. The sensitivity of the index to the temporal resolution of the measurements and measurement errors was also tested. The index can successfully quantify hysteresis, except for very noisy data or when the temporal resolution of the measurements is not well suited to study hysteresis between the variables. Overall, this metric can be used to test if models reproduce temporal variability in hysteresis or to compare hydrological responses in different catchments or at different spatial scales
