Effects of Streambed Morphology and Biofilm Growth on the Transient Storage of Solutes

Microbial biofilms are the prime site of nutrient and contaminant removal in streams. It is therefore essential to understand how biofilms affect hydrodynamic exchange, solute transport, and retention in systems where geomorphology and induced hydrodynamics shape their growth and structure. We experimented with large-scale streamside flumes with streambed landscapes constructed from graded bedforms of constant height and wavelength. Each flume had a different bedform height and was covered with a layer of gravel as substratum for benthic microbial biofilms. Biofilms developed different biomass and physical structures in response to the hydrodynamic conditions induced by the streambed morphology. Step injections of conservative tracers were performed at different biofilm growth stages. The experimental breakthrough curves were analyzed with the STIR model, using a residence time approach to characterize the retention effects associated with biofilms. The retained mass of the solute increased with biofilm biomass and the biofilm-associated retention was furthermore related to bedform height. We tentatively relate this behavior to biofilm structural differentiation induced by bed morphology, which highlights the strong linkage between geomorphology, hydrodynamics, and biofilms in natural streams and provide important clues for stream restoration

Complementarity between Combined Heat and Power Systems, Solar PV and Hydropower at a District Level: Sensitivity to Climate Characteristics along an Alpine Transect

Combined heat and power systems (CHP) produce heat and electricity simultaneously. Their resulting high efficiency makes them more attractive from the energy managers’ perspective than other conventional thermal systems. Although heat is a by-product of the electricity generation process, system operators usually operate CHP systems to satisfy heat demand. Electricity generation from CHP is thus driven by the heat demand, which follows the variability of seasonal temperature, and thus is not always correlated with the fluctuation of electricity demand. Consequently, from the perspective of the electricity grid operator, CHP systems can be seen as a non-controllable energy source similar to other renewable energy sources such as solar, wind or hydro. In this study, we investigate how ‘non-controllable’ electricity generation from CHP systems combines with ‘non-controllable’ electricity generation from solar photovoltaic panels (PV) and run-of-the river (RoR) hydropower at a district level. Only these three energy sources are considered within a 100% renewable mix scenario. Energy mixes with different shares of CHP, solar and RoR are evaluated regarding their contribution to total energy supply and their capacity to reduce generation variability. This analysis is carried out over an ensemble of seventeen catchments in North Eastern Italy located along a climate transect ranging from high elevation and snow dominated head-water catchments to rain-fed and wet basins at lower elevations. Results show that at a district scale, integration of CHP systems with solar photovoltaic and RoR hydropower leads to higher demand satisfaction and lower variability of the electricity balance. Results also show that including CHP in the energy mix modifies the optimal relative share between solar and RoR power generation. Results are consistent across the climate transect. For some districts, using the electricity from CHP might also be a better solution than building energy storage for solar PV

Hyporheic Flows in Stratified Beds

Surface-subsurface exchange fluxes are receiving increasing interest because of their importance in the fate of contaminants, nutrients, and other ecologically relevant substances in a variety of aquatic systems. Solutions have previously been developed for pore water flows induced by geometrical irregularities such as bed forms for the cases of homogeneous sediment beds and idealized heterogeneous beds, but these solutions have not accounted for the fact that streambed sediments are subject to sorting processes that often produce well-defined subsurface structures. Sediments at the streambed surface are often coarser than the underlying material because of size-selective sediment transport, producing relatively thin armor layers. Episodic erosional and depositional processes also create thick layers of different composition within the porous medium, forming stratified beds. A series of experiments were conducted to observe conservative solute transport in armored and stratified beds. An analytical solution was developed for advective exchange with stratified beds and provides appropriate scaling of the physical variables that control exchange flows. The results show that armor layers are too thin to significantly alter the advective pumping process but provide significant solute storage at short time scales. Stratified beds with layers of significant thickness favor development of horizontal flow paths within the bed and change the rate of solute transfer across the stream-subsurface interface compared to homogeneous beds

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