Analisi della USLE in contesto agricolo collinare su base LiDAR.

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Runoff events classification based on streamflow-water table hysteresis

A framework for rainfall-runoff events classification helps reduce information into a manageable number of classes, and it allows watersheds comparisons. Hydrological signatures serve as proxies for tracking the catchment behaviour and represent a powerful tool for characterising the catchment response to a storm event. Despite that, they have rarely been used for rainfall-runoff event typology identification. In this study, we propose a general framework for the classification of rainfall-runoff events based on the analysis of the hysteretic relation between streamflow and depth to the water table, and its relation with the event characteristics. Particularly, this study aims to: i) analyse the temporal variability of hysteretic patterns between streamflow and depth to water table in a small headwater catchment, ii) relate a set of hydrological and meteorological characteristics to the hysteretic index at event scale, and iii) identify clusters of events with similar characteristics. The study area is a small forested catchment located in the Italian Pre-Alps, where hydro-meteorological data have been recorded since August 2012. A set of 112 rainfall-runoff events, occurred between 2012 and 2016, was investigated. A simple hysteresis index was applied to each event. The hysteresis index was used to characterize the direction (clockwise or anti-clockwise), the size and the shape of the hysteretic loops. Results show that the hysteresis analysis was particularly useful for the identification of three main clusters of rainfall-runoff events. A first cluster was characterised by a clockwise loop, i.e., there was a faster streamflow response compared to the depth to the water table. The events in this cluster were short, with dry antecedent conditions, small streamflow peaks, event runoff depths and runoff coefficients. The second cluster of events was characterised by an anti-clockwise loop, i.e., there was a faster response of the depth to the water table compared to the streamflow. The events in this cluster were long, with wet antecedent conditions, large streamflow peaks, event runoff depths and runoff coefficients. A third cluster had characteristics similar to the first cluster, i.e. clockwise hysteretic loop and similar storm characteristics, but on average displayed a narrower hysteretic loop. The statistics showed a significant difference (p<0.05) among the clusters. This analysis allowed us to successfully identify three clusters of rainfall-runoff events with specific characteristics and distinct hydrological behaviour. Concluding, the analysis of the hysteresis between streamflow and depth to the water table can be considered a useful tool for classifying rainfall-runoff events

Using tracers and hydrological hysteresis analysis to assess process consistency in a catchment conceptual model application

Assessment of process consistency in hydrological modelling is crucial to get reliable model responses under conditions beyond the range of prior data availability. This is even more important in the case of conceptual catchment models because the assessment of process consistency may drive the selection of the degree of parsimony, which is warranted in a certain model implementation. This study aims to analyse process consistency description for a simple conceptual rainfall-runoff model, by using water isotopic data and by the analysis of hysteretic relations. The continuous hydrological model conceptualizes the catchment dividing it into hillslope and riparian zone. A third conceptual tank represents the groundwater storage. The precipitation is used as input to the hillslope and the riparian areas, that are linked dynamically through a simple linear equation. The model was applied to a headwater forested catchment located in the Italian pre-Alps (598-721 m a.s.l.), where rainfall, discharge, soil moisture and shallow groundwater level were monitored continuously. Moreover, samples for isotopic analyses were collected monthly and during selected rainfall-runoff events from rain and stream water, soil water and shallow groundwater. We applied an index for quantifying hysteresis between streamflow (independent variable) and groundwater level (dependent variable) at the rainfall-runoff event timescale. The index provides information on the direction, the extent and the shape of the loops. A set of 114 rainfall-runoff events were available from 2012 to 2016, to apply the model and compute the hysteresis index. The comparison of observed and modelled hysteretic relations was used to calibrate the hydrological model. This model consistency analysis allowed us to investigate the goodness of the model in capturing the complex hydrological dynamics, keeping the number of parameters to be conditioned at the minimum. In particular, hysteresis analysis allowed to identify model parametrizations, which permitted an adequate mimic of the system-internal processes. Preliminary results show that the combined tracer analysis and examination of the hysteretic patterns provided indications on the degree of internal consistency of the model representation, making the model application more robust when extended beyond the range of data availability for model conditioning

A standardization method to disentangle environmental information from axial trends of xylem anatomical traits.

Anatomical traits such as xylem conduit diameter and vessel connectivity are fundamental characteristics of the hydraulic architecture of vascular plants. Stem xylem conduits are narrow at the stem apex, and this confers resistance to embolisms that might otherwise be induced by large, negative water potentials at the top of tall trees. Below the apex, conduits progressively widen and this characteristic minimizes effects of path length on total hydraulic resistance. While interconnections among xylem vessels have been noted for decades, their role(s) are not fully clarified. For example, we do not know if they allow water to bypass embolized vessels, or increase the risk of spread of embolisms, or how their arrangement varies within a tree. Here we demonstrate the benefit of removing the independent effect of stem length on assessment of effects of external (e.g., climatic) factors on such xylem traits. We measured the hydraulic diameter (Dh) and vessel conductivity index (VCI) along the stem of 21 shrubs/trees of similar height (1.19 < H < 5.45 m) belonging to seven Acacia species, across a wide aridity gradient in Australia. All trees showed similar scaling exponents of Dh (b = 0.33) and VCI (b = 0.53) vs axial distance from the apex (L), thus conforming with general patterns in woody plants. After de-trending for L, neither Dh (P = 0.21) nor VCI (P = 0.109) differed across the aridity gradient. We found that across a wide gradient of aridity, climate had no effect on xylem anatomy of Acacia spp, which was instead dictated by axial distances from stem apices. We argue that the use of standardization procedures to filter out intrinsic patterns of vascular traits is an essential step in assessing climate-driven modifications of xylem architecture