Mediterranean Catchment Research Articles

Disentangling the effect of future land use strategies and climate change on streamflow in a Mediterranean catchment dominated by tree plantations

Climate change (CC) along with Land Use and Land Cover Change (LULCC) have a strong influence in water availability in already fragile Mediterranean ecosystems. In this work the Soil and Water Assessment Tool (SWAT) was implemented for the 2006–2018 period in a rainfed catchment of central Chile (36°) to test the hypothesis that adaptive plantation strategies could mitigate the impacts of climate change and increase streamflow. We also hypothesize that afforestation with exotic tree plantations will reduce water availability in Mediterranean catchments, acting in synergy with climate change. Five LULCC scenarios are analyzed: i) current long-term national Forest Policy (FP), ii) extreme scenario (EX) with large afforestation surfaces, both including the replacement of native shrublands with Pinus radiata; iii) adaptive plantation management scenario (FM), with lower planting density, iv) forced land displacement scenario (FLD), where plantations at the headwaters are moved to lowland areas and replaced with native shrublands, and v) pristine scenario (PR), with only native vegetation. Each LULCC scenario was run with present climate and with projections of different CMIP5 climate models under the RCP 8.5 scenario for the period 2037–2050, and then compared against simulations based on the present land cover and climate. Simulations with the five LULCC scenarios (FP, EX, FM, FLD and PR) with present climate resulted in variations of −2.5, −17.3, 0, 2.3 and 10.9% on mean annual streamflow (Q), while simulations with the current land cover and CC projections produced a 32.1% decrease in mean annual Q. The joint impact of CC and LULCC leads to changes in mean annual Q ranging from −46.2% (EX) to –23.3% (PR). Afforestation with exotic pines will intensify the reduction in water yield, while conservative scenarios focused on native forests protection and restoration could partially mitigate the effect of CC. We make a strong call to rethink current and future land management strategies to cope with lower water availability in a drier future.

Future changes in the Dominant Source Layer of riparian lateral water fluxes in a subhumid Mediterranean catchment

The ‘Dominant Source Layer’ (DSL) is defined as the riparian zone (RZ) depth stratum that contributes the most to water and solute fluxes to streams. The concept can be used to explain timing and amount of matter transferred from RZs to streams in forest headwaters. Here, we investigated the potential impact of future climate changes on the long-term position of the DSL in a subhumid Mediterranean headwater catchment. We used the rainfall-runoff model PERSiST to simulate reference (1981–2000) and future (2081–2100) stream runoff. The latter were simulated using synthetic temperature, precipitation, and inter-event length scenarios in order to simulate possible effects of changes in temperature, rainfall amount, and rainfall event frequency and intensity. Simulated stream runoff was then used to estimate RZ groundwater tables and the proportion of lateral water flux at every depth in the riparian profile; and hence the DSL. Our simulations indicated that future changes in temperature and precipitation will have a similar impact on the long-term DSL position. Nearly all scenarios projected that, together with reductions in stream runoff and water exports, the DSL will move down in the future, by as much as ca. 30 cm. Shallow organic-rich layers in the RZ will only be hydrologically activated during sporadic, large rainfall episodes predicted for the most extreme inter-event length scenarios. Consequently, terrestrial organic matter inputs to streams will decrease, likely reducing catchment organic matter exports and stream dissolved organic carbon concentrations. This study highlights the importance of identifying vertical, hydrologically active layers in the RZ for a better understanding of the potential impact of future climate on lateral water transfer and their relationship with surface water quality and carbon cycling.

Introducing Nature into Cities or Preserving Existing Peri-Urban Ecosystems? Analysis of Preferences in a Rapidly Urbanizing Catchment

Nature-based solutions (NBS) are increasingly being promoted as a means to address societal and environmental challenges, especially flood risk reduction. In the context of rapidly urbanizing catchments, NBS can take part of the development of sustainable cities, either by conserving peri-urban ecosystems from urban sprawl or by developing green infrastructure in the cities. Both can provide a wide range of co-benefits (e.g., climate regulation, air quality regulation), but also generate some negative effects (e.g., mobility issues, unsafety, allergens). We develop and implement a Discrete Choice Experiment survey to analyse people’s perception of co-benefits and negative effects, and associated preferences for the two types of NBS at a catchment scale. The results obtained from 400 households living in a French Mediterranean catchment highlight that people associate numerous co-benefits to NBS, but also negative effects. Our estimations reveal that resident households are ready to contribute large amounts through a tax increase for the development of NBS (from 140 to 180 EUR/year, on average). There is however a strong heterogeneity of preferences at the catchment scale influenced by income, location of the respondent along an urban–rural gradient, and perception of the importance of ecosystem services. These differences may reflect urban environmental inequalities at the catchment scale, which are important to take into account in order to avoid distributive inequalities.

On the potential of variational calibration for a fully distributed hydrological model: application on a Mediterranean catchment

Abstract. Calibration of a conceptual distributed model is challenging due to a number of reasons, which include fundamental (model adequacy and identifiability) and algorithmic (e.g., local search vs. global search) issues. The aim of the presented study is to investigate the potential of the variational approach for calibrating a simple continuous hydrological model (GRD; Génie Rural distributed involved in several flash flood modeling applications. This model is defined on a rectangular 1 km2 resolution grid, with three parameters being associated with each cell. The Gardon d'Anduze watershed (543 km2) is chosen as the study benchmark. For this watershed, the discharge observations at five gauging stations, gridded rainfall and potential-evapotranspiration estimates are continuously available for the 2007–2018 period at an hourly time step. In the variational approach one looks for the optimal solution by minimizing the standard quadratic cost function, which penalizes the misfit between the observed and predicted values, under some additional a priori constraints. The cost function gradient is efficiently computed using the adjoint model. In numerical experiments, the benefits of using the distributed against the uniform calibration are measured in terms of the model predictive performance, in temporal, spatial and spatiotemporal validation, both globally and for particular flood events. Overall, distributed calibration shows encouraging results, providing better model predictions and relevant spatial distribution of some parameters. The numerical stability analysis has been performed to understand the impact of different factors on the calibration quality. This analysis indicates the possible directions for future developments, which may include considering a non-Gaussian likelihood and upgrading the model structure.

What about reservoirs? Questioning anthropogenic and climatic interferences on water availability

AbstractWater resources in semi‐arid regions like the Mediterranean Basin are highly vulnerable because of the high variability of weather systems. Additionally, climate change is altering the timing and pattern of water availability in a region where growing populations are placing extra demands on water supplies. Importantly, how reservoirs and dams have an influence on the amount of water resources available is poorly quantified. Therefore, we examine the impact of reservoirs on water resources together with the impact of climate change in a semi‐arid Mediterranean catchment. We simulated the Susurluk basin (23.779‐km2) using the Soil and Water Assessment Tool (SWAT) model. We generate results for with (RSV) and without reservoirs (WRSV) scenarios. We run simulations for current and future conditions using dynamically downscaled outputs of the MPI‐ESM‐MR general circulation model under two greenhouse gas relative concentration pathways (RCPs) in order to reveal the coupled effect of reservoir and climate impacts. Water resources were then converted to their usages – blue water (water in aquifers and rivers), green water storage (water in the soil) and green water flow (water losses by evaporation and transpiration). The results demonstrate that all water resources except green water flow are projected to decrease under all RCPs compared to the reference period, both long‐term and at seasonal scales. However, while water scarcity is expected in the future, reservoir storage is shown to be adequate to overcome this problem. Nevertheless, reservoirs reduce the availability of water, particularly in soil moisture stores, which increases the potential for drought by reducing streamflow. Furthermore, reservoirs cause water losses through evaporation from their open surfaces. We conclude that pressures to protect society from economic damage by building reservoirs have a strong impact on the fluxes of watersheds. This is additional to the effect of climate change on water resources.

Hydrological modeling of three rivers under Mediterranean climate in Chile, Greece, and Morocco: study of high flow trends by indicator calculation

The study of the influence of climate change on the evolution of floods in areas under the influence of a Mediterranean climate is very important. This work aims to provide new analytical elements to decision-makers in flood management and forecasting in three Mediterranean catchments (Lonquen, Evrotas, and Azzaba), in order to better understand their hydrological behavior and to quantify the flow and its vulnerability to climate change. A semi-distributed conceptual model (HBV-A model) was used to simulate flows with a daily time step. The model results (simulated flows) were used to examine the evolution of high flows using two different approaches (linear regressions and statistical test). The Chilean and the Greece catchments show a decrease in flooding. However, the Moroccan catchment reacts differently, since a positive trend in high flows is observed. The overall results confirm the influence of climate change on the rainfall/runoff relationship.

Comparing the impacts of wildfire and meteorological variability on hydrological and erosion responses in a Mediterranean catchment

Land degradation and water resources pollution caused by catastrophic wildfires is of growing concern in fire-prone regions. Studies on the effects of wildfire on hydrology and erosion have mostly been conducted at plot or hillslope scale, while relatively few studies investigated post-wildfire hydrological responses and erosion at the meso-catchment scale (~ > 10 km2) in the Mediterranean. This study used measured discharge and suspended sediment at the outlet of a burnt catchment in southern Portugal, before and after a wildfire, to investigate post-wildfire changes in hydrological and erosion responses to rainfall. Hydrological and sediment connectivity patterns were derived to investigate changing dynamics induced by the fire within the catchment. The main findings were: (a) although a large part of the catchment experienced moderate to high severity burning, post-wildfire hydro-sedimentary response was considerably limited; (b) meteorological variability determined hydrological responses and erosion more strongly than wildfire effects; and (c) during the post-wildfire vegetation recovery period, only rainfall events with a high return period (~ 2 years) enhanced the hydrological and erosion responses. This can be explained by the spatial scale dependency of these processes and limited fine sediment supply, or relatively low connectivity in the study catchment. While connectivity is only an indicator, this implies that, at the meso-catchment scale, pollution of downstream water bodies by contaminated soil and ash may not occur immediately. Rather, because sediments and associated ashes and contaminants are first being transported to the areas around the stream networks, they only reach the outlet during heavy events which do connect the entire catchment. Thus, dynamic indices of connectivity that take rainfall event characteristics into consideration need to be further tested to assess and manage post-wildfire soil and water contamination risk.

FingerPro: an R Package for Tracking the Provenance of Sediment

Soil loss by erosion processes is one of the largest challenges for food production and reservoir siltation around the world. Information on sediment, nutrients and pollutants is required for designing effective control strategies. The estimation of sediment sources is difficult to get using conventional techniques, but sediment fingerprinting is a potentially valuable tool. This procedure intends to develop methods that enable to identify the apportionment of sediment sources from sediment mixtures. We developed a new tool to quantify the provenance of sediments in an agroforest catchment. For the first time, the procedure for the selection of the best combination of tracers was included in the tool package. An unmixing model algorithm is applied to the sediment samples to estimate the contribution of each possible source. The operations are compiled in an R package named FingerPro, which unmixes sediment samples after selecting the optimum set of tracers. An example from a well-studied Mediterranean catchment is included in the package to test the model. The sediment source apportionments are compared with previous results of soil redistributions where 137Cs derived rates validate the unmixing results, highlighting the potential of sediment fingerprinting for quantifying the main sediment provenance. Fingerprinting techniques will allow us to better comprehend sediment transport to water ecosystems and reservoirs and its detrimental effect on the quality of the water and aquatic habitats. The FingerPro package provides further understanding of the unmixing procedure through the use of graphical and statistical tools, offering a broader and easier application of the technique.

Species-specific macroinvertebrate responses to climate and land use scenarios in a Mediterranean catchment revealed by an integrated modelling approach

Climate and land use changes are altering flow and nutrient regimes in catchments that affect stream habitats and aquatic biodiversity. This study applied an integrated modelling approach to quantify the impacts of climate and land use scenarios on the macroinvertebrate community in Sixth Creek catchment, South Australia. Gradient forest (GF) determined flow as most important driver affecting macroinvertebrate assemblages, and identified most flow-sensitive species based on highest cumulative importance along flow gradients. A catchment model developed using the Soil and Water Assessment Tool (SWAT) simulated flow under the following scenarios: (1) deforestation, (2) reforestation, (3) climate change, and (4) a 10% increased urbanisation as projected by local authorities over the next 30 years. The urbanisation scenario predicted a 2% increase of average flow compared to 17% flow increase predicted by the deforestation scenario. In contrast, diminished monthly flow was suggested by both the climate change scenarios due to declining precipitation and higher temperatures, and the reforestation scenario most likely due to declining surface run-off. In reforestation and climate change scenarios, SWAT results predicted the highest decline in flow during late autumn and late spring, which may shift this unusual permanent flowing stream towards intermittency. The Hybrid Evolutionary Algorithm (HEA) was used to develop flow-driven models for population dynamics of taxa identified by GF as flow sensitive i.e. Hydrobiidae spp., Austrocerca tasmanica, Offadens spp., Cloeon spp., Micronecta spp. and Tasmanocoenis tillyardi based on 14 years of data monitored at a representative stream site of Sixth Creek. The HEA models achieved coefficients of determination r2 between 0.85 and 0.97 for predicted vs. observed abundances. While the abundance of Cloeon spp. diminished in all scenarios, it was most affected by low flow predictions of the climate change scenario. Hydrobiidae spp. appeared most tolerant to altered flow conditions by reaching highest abundances at high flow conditions caused by deforestation and urbanisation. Overall, this study highlighted the non-linear response of species within the commonly used Ephemeroptera, Plecoptera and Trichoptera (EPT) grouping and a need to redirect focus of such studies from community to species level. The integrated modelling approach based on GF, SWAT and HEA proved to be suitable for simulating complex species-specific macroinvertebrate responses to relevant climate and land use scenarios. Learning more about these interrelationships is a prerequisite for sustainable catchment management.

Analysis of post-fire suspended sediment sources by using colour parameters

After a wildfire, total or partial removal of vegetal biomass and changes in physicochemical soil properties lead to an increase in overland flow and sediment yield. Eventual damage must be counteracted urgently by identifying erosion hotspots and by implementing post-fire management programmes and sampling campaigns. In this context, the sediment source fingerprinting technique is widely used to determine the origin of suspended sediments in catchments and to evaluate the effectiveness of sediment management programmes. It traditionally relies on the use of physical, biochemical and geochemical properties as tracers. However, measuring these tracers in the laboratory is often expensive and time-consuming. Colour tracers have been shown to greatly reduce time and cost, especially if a normal office scanner is used for measurements. Here we explored whether colour parameters can be used to investigate suspended sediment origin in burned catchments. To this end, sediment and ash were mixed artificially to verify colour linear additivity and ash influence on colour parameters. Colour parameters were then used for source ascription of suspended sediment samples (n = 9) collected during two years after a fire in a small Mediterranean catchment (Mallorca, Spain). In addition, reflectance-derived colour parameters were compared with those obtained using a normal office scanner. The close correlation between most chromatic indexes (obtained using both methods; p < 0.01) suggested that scanning is a good alternative for measuring soil and sediment colour. A Bayesian tracer mixing model (MixSIAR) determined the relative contribution of each source. The type of mixing model enables appropriate representation of natural and sampling uncertainty in tracer data. During the first events, suspended sediment originated mainly in burned surfaces, whereas the contribution of these decreased throughout the study period. Tracing results obtained using colour parameters were compared with calculations using 137Cs and 210Pbex, as recognized tracers to discriminate between surface and subsurface sediment sources after wildfires. Estimated source ascriptions with both methods (i.e. reflectance-derived colour parameters and radionuclides) coincided in predicting the dominant source in 7 of the 9 samples measured. Colour tracers proved useful in discriminating between burned and unburned sources, making them suitable for suspended sediment source ascription and monitoring as part of post-fire management strategies.

Runoff and soil moisture as driving factors in suspended sediment transport of a small mid-mountain Mediterranean catchment

Soil moisture conditions play a key role in runoff and sediment load dynamics in river catchments of contrasting ecosystems. However, the highly marked seasonality of Mediterranean climate affects hydrological processes and sediment transport strongly, as evapotranspiration determines a succession of wet, transition and dry periods throughout a hydrological year. This study examines results of soil moisture, water and sediment fluxes during five hydrological years in a representative small mid-mountain Mediterranean catchment. It aims to assess the sediment load contribution and its variability at different temporal scales. Precipitation, runoff and suspended sediment load were calculated at annual, seasonal and event scales to assess inter- and intra-annual variability. A database with 45 events was used to identify the main controlling factors over sediment load thought bivariate relationships and an analysis of soil moisture-discharge and discharge-suspended sediment concentration hysteresis. At the annual scale, lithology, land use and soil conservation structures characteristic of the catchment caused low values of sediment yield, although inter-annual rates varied by up to three orders of magnitude (0.08 to 11.86 t km−2 y−1). Seasonal analysis of accumulated sediment load showed that 80% was generated during the autumn and winter seasons. At the event scale, streamflow and soil moisture were the controlling factors for the largest sediment load contributions. The highest frequency of clockwise discharge-suspended sediment concentration hysteresis revealed that most of the sediment was generated from nearby sources, although the largest sediment loads were contributed under wet antecedent conditions, as depicted by the soil moisture-discharge hysteresis assessment.

Water and sediment budgets unveiling contrasting hydro-sedimentary patterns in a mountainous Mediterranean catchment

Mountain regions have a key role in the generation of runoff, and in the production and transfer of sediments to fluvial networks, especially in Mediterranean catchments where these processes are affected by marked changes in climate and land use (i.e. global change). This paper presents the water and the sediment budgets of the Ribera Salada (224 km2), a meso-scale Mediterranean forested catchment located in the Southern Pyrenees. Field monitoring follows an integrated basins scheme (five nested sub-catchments), where hydrological and sediment transport data were collected continuously over a two-year period (2012–2013). Precipitation was obtained using radar images, which allowed the elaboration of rainfall maps used to characterize the spatial distribution of rainfall across multiple scales. Results indicate that the catchment is hydrologically divided in two areas which show contrasting fluvial regimes: the upper part of the catchment is considered wet and has a constant flow regime, supplying the majority of the water, while the lower part is drier, with ephemeral tributaries and water losses into the alluvial aquifer of the main river channel. In contrast to water yield, most of the suspended sediment load (i.e. 80%) is supplied by the driest part of the catchment where sediment availability was greater and where there is a greater connectivity between sediment sources and the channel network. The sediment yield of the whole catchment and the respective sub-catchments sits in the lower bounds of values reported for the Mediterranean region, indicating the generally low intensity of hydrological and geomorphic processes in the area. Once more the sediment budget approach matched to sound hydrological data proves efficient to characterize sediment dynamics in river basins, with special interest in areas such as the Mediterranean mountain catchments, where the effects of global change appear to be more acute.

Mountain ecosystem services affected by land use changes and hydrological control works in Mediterranean catchments

The use of check-dams in mountain environments to regulate fluxes of water and sediments is widely applied across Mediterranean mountains. Besides the use of “grey infrastructures” such as check-dams, other restoration and hydrological control measures rely more on the use of “green infrastructures” or “nature-based solutions” (e.g. reforestation, buffer lines) or a combination of both types of measures. It is widely accepted that both are complementary, and that prioritization should be based on economic, ecological and cultural criteria. This paper brings together all the knowledge generated during more than one decade concerning the impact of land use changes, reforestation, and hydrological control works on several ecosystem services in a representative Mediterranean catchment. The work evaluates different management scenarios aiming to optimize provision of ecosystem services in the area.The study area is a medium-sized catchment (∼300 km2) in Southeast Spain, representative of Mediterranean mountains that experienced agricultural land abandonment, greening up, and restoration works in the second half of the twentieth century. The methods combined: (i) previous research results for the area that were organized in an ecosystem services framework, providing data for three representative scenarios of catchment management; (ii) the use of value content analysis of the existing management plans for the area to understand the view of the managers; and (iii) a multicriteria analysis of the management scenarios to determine the most sustainable scenario to optimize different ecosystem services. The results of the evaluation were later validated with the stakeholders (technicians and managers involved in the management plans) through interviews.Our results show that solutions that respect landscape and ecological dynamics are more sustainable and cheaper in the medium and long term than scenarios based on “grey infrastructures”, although the latter could have more desirable short-term impacts. The value analysis reflects how there are some concepts, such as ecosystem services, that could easily be further incorporated into several management plans. When choosing a management scenario, this needs to be adapted to the local environmental conditions and to the specific objectives of the restoration works. Tailor-made management scenarios taking into account two factors (local conditions and specific management objectives) can optimize resources and achieve medium to long-term sustainability.

The impact of reservoir construction and changes in land use and climate on ecosystem services in a large Mediterranean catchment

The Mediterranean region has been identified as one of the most affected global hot-spots for climate change, which is already manifested by faster increasing temperatures than the global mean and significant decreases in annual precipitation. Besides, over the past decades, important land cover changes have occurred, such as reforestation, agricultural intensification, urban expansion and the construction of many reservoirs. Here we study the impacts of these changes in a typical large Mediterranean catchment and focus on relevant ecosystem services, i.e. primary production, water supply, food production, water regulation, flood and erosion control, and cultural ecosystem services. We applied a hydrological model, coupled with a soil erosion and sediment transport model, for the period 1971–2010 to the entire catchment, and for an extended period (1952–2018), with more pronounced land use changes, to one of its subcatchments that plays a crucial role for regional drinking water provision. Climate change in the evaluated period is characterized by a decrease in precipitation and an increase in temperature. Land use changes included an increase in natural vegetation in the headwaters, agricultural intensification and reservoir construction in the central part, and urban expansion in the downstream part of the catchment. The positive impacts on ecosystem services were quantified by a decrease in hillslope erosion, sediment yield, sediment concentration and flood discharge, which are mainly attributed to climate change and reservoir construction. The negative impacts were quantified by an increase in plant water stress and a decrease in reservoir storage, for which the latter is mainly attributed to reforestation in the headwater catchments. While grey infrastructures like reservoir construction showed important positive impacts on some ecosystem services, such as water regulation and flood control, they also potentially induce negative impacts, such as an increase of water demand and ecological integrity. We argue that a shift is needed to green infrastructures and nature-based solutions, such as reforestation and sustainable land management, which may lead to similar benefits on ecosystem services, but without the negative impacts caused by grey infrastructures. This supports the need for integrated land-use management to sustainably protect ecosystem services, considering the expected climate change impacts on Mediterranean environments and societies.

Geohydrology of a Reference Mediterranean Catchment (Cilento UNESCO Geopark, Southern Italy)

In this paper, we studied the geo-hydrological structure and behavior of a reference catchment, located in the Cilento UNESCO Global Geopark, southern Italy, representative of the hilly, terrigenous and forested headwaters of the Mediterranean eco-region. Based on detailed hydrogeological and hydro-geomorphological surveys and geomorphometric analysis, starting in 2012, a hydro-chemical monitoring activity at the catchment and sub-catchment scale started, and a hydro-chemical dataset was progressively recorded at daily and sub-hourly time steps. Based on this dataset, the authors performed an original procedure to identify different runoff components, derived by applying cascade mass balance filtering. The integration of hydrological and geomorphological approaches allowed us to obtain an interesting conceptualization of the storm flow generation using hydro-chemical signatures related to different runoff components produced during the increasing–decreasing cycle of the flood event magnitude. The hydro-system activated progressively different runoff sources (i.e., groundwater, riparian corridor, hillslope and hollow) and involved various mechanisms (i.e., groundwater ridging, saturation-excess, infiltration-excess and soil pipe exfiltration). The geo-hydrological conceptualization was validated using a hysteresis Q-EC loop analysis performed on selected events that showed how hysteretic indices could be used to characterize the events in respect to their origins, mechanisms and pathways in similar catchments.

Slackwater Sediments Record the Increase in Sub-daily Rain Flood due to Climate Change in a European Mediterranean Catchment

In this work we propose an original method to determine the magnitude of the discharge, the intensity of the precipitation and the duration of short-rain floods in small torrential basins ( 24 h). The need is evident for safety designs of civil infrastructures and flood risk mitigation strategies in the current climate change scenario.

Hydrometeorological Ensemble Forecast of a Highly Localized Convective Event in the Mediterranean

The uncertainties that affect hydrometeorological modelling chains can be addressed through ensemble approaches. In this paper, a convection-permitting ensemble system was assessed based on the downscaling of all members of the ECMWF ensemble prediction system through the coupled atmospheric-hydrological WRF-Hydro modelling system. An exemplary highly localized convective event that occurred in a morphologically complex area of the southern Italian coast was selected as a case study, evaluating the performance of the system for two consecutive lead times up to the hydrological forecast on a very small (11.4 km2) catchment. The proposed approach accurately downscales the signal provided by the global model, improving up to almost 200% the quantitative forecast of the accumulated rainfall peak in the area affected by the event and supplying clear information about the forecast uncertainty. Some members of the ensemble simulations provide accurate results up to the hydrological scale over the catchment, with unit peak discharge forecasts up to 3 m3∙s−1∙km−2. Overall, the study highlights that for highly localized convective events in coastal Mediterranean catchments, ensemble approaches should be preferred to a classic single-based simulation approach, because they improve the forecast skills and provide spatially distributed information about the forecast uncertainty, which can be particularly useful for operational purposes.

Drought assessment in a south Mediterranean transboundary catchment

ABSTRACT Predicting the impacts of climate change on water resources remains a challenging task and requires a good understanding of the dynamics of the forcing terms in the past. In this study, the variability of precipitation and drought patterns is studied over the Mediterranean catchment of the Medjerda in Tunisia based on an observed rainfall dataset collected at 41 raingauges during the period 1973–2012. The standardized precipitation index and the aridity index were used to characterize drought variability. Multivariate and geostatistical techniques were further employed to identify the spatial variability of annual rainfall. The results show that the Medjerda is marked by a significant spatio-temporal variability of drought, with varying extreme wet and dry events. Four regions with distinct rainfall regimes are identified by utilizing the K-means cluster analysis. A principal component analysis identifies the variables that are responsible for the relationships between precipitation and drought variability.

Multi-model assessment of hydrological and environmental impacts on streambed microbes in Mediterranean catchments.

Microbes inhabiting intermittent streambeds are responsible for controlling and developing many biogeochemical processes essential for the ecosystem functions. Although streambed microbiota is adapted to intermittency the intensification of water scarcity and prolonged dry periods may jeopardise their capacity to cope with hydrological changes. This study aims to evaluate whether, and to what extent, the duration of dry periods affects streambed microbial density, diversity, composition (16S rRNA gene diversity) and functions (extracellular enzyme activities and respiration). Our results highlight the fact that hydrology modulates the community composition and, to some extent, the functions carried out under different environmental conditions. The relative abundance of certain taxa inhabiting the driest intermittent communities differs significantly from those found at sites with continuous flow. Microbial functional metrics revealed a progressive increase in recalcitrant carbon degradation activity at sites with an extended dry phase. In contrast, bacterial density and diversity were mainly influenced by the catchment land use, agriculture enhanced density but reduced diversity, and the presence of riparian vegetation supported greater streambed bacterial diversity. From this perspective, a combination of prolonged dryness with reduced riparian vegetation and increased agricultural land cover could compromise the ecosystem functioning by threaten microbially mediated processes linked to the carbon cycle.

Extreme Floods in Small Mediterranean Catchments: Long-Term Response to Climate Variability and Change

Climate change implies changes in the frequency and magnitude of flood events. The influence of climate variability on flooding was evaluated by an analysis of sedimentary (palaeofloods) and documentary archives. A 500-year palaeoflood record at Montlleó River (657 km2 in catchment area), eastern Spain, revealed up to 31 palaeofloods with a range of discharges of 20–950 m3 s−1, and with at least five floods exceeding 740–950 m3 s−1. This information contrasts with the available gauged flood registers (since year 1971) with an annual maximum daily discharge of 129 m3 s−1. Our palaeoflood dataset indicates flood cluster episodes at (1) 1570–1620, (2) 1775–1795, (3) 1850–1890, and (4) 1920–1969. Flood rich periods 1 and 3 corresponded to cooler than usual (about 0.3 °C and 0.2 °C) climate oscillations, whereas 2 and 4 were characterised by higher inter-annual climatic variability (floods and droughts). This high inter-annual rainfall variability increased over the last 150 years, leading to a reduction of annual maximum flow. Flood quantiles (&gt;50 years) calculated from palaeoflood+gauged data showed 30%–40% higher peak discharges than those using only instrumental records, whereas when increasing the catchment area (1500 km2) the discharge estimation variance decreased to ~15%. The results reflect the higher sensitivity of small catchments to changes on flood magnitude and frequency due to climate variability whereas a larger catchment buffers the response due to the limited extent of convective storms. Our findings show that extended flood records provide robust knowledge about hazardous flooding that can assist in the prioritization of low-regret actions for flood-risk adaptation to climate change.