Possible climate change impacts on water resources availability in a large semi-arid catchment in Northeast Brazil.

The semiarid region of Northeast Brazil is characterized by water scarcity, vulnerability of natural resources, and pronounced climatic variability. An integrated model has been developed to simulate this complex situation with an emphasis on a large-scale representation of hydrological processes and on the sensitivity to climate change. Regional climate change scenarios were obtained by empirical downscaling with large-scale climate information from different GCMs which differ strongly in their projections for future precipitation. The results show that due to these differences, it is still impossible to give quantitative values of the water availability in a forecast sense, i.e. to assign probabilities to the simulated results. However, it becomes clear that efficient and ecologically sound water management is a key question for further development. The results show that, independent of the climate change, agriculture is more vulnerable to drought impacts in the case of rainfed compared to irrigated farming. However, the capacity of irrigation and water infrastructure to enhance resilience with respect to climatic fluctuations is significantly constrained in the case of a negative precipitation trend

Model-based analysis of nutrient retention and management for a lowland river

International audienceIn the context of the European Water Framework Directive options for improving the water quality of the lowland river Havel (Germany) were assessed. The lower section of this river is actually a polytrophic river-lake system suffering from high external nutrient loading and exhibiting significant in-river turnover. In order to gain a better understanding of present conditions and to allow integrated scenarios of nutrient management to be evaluated the catchment models SWIM and ArcEGMO-Urban were coupled with a simple, newly developed nutrient TRAnsport Model (TraM). Using the TraM model, the retention of nitrogen and phosphorus in a 55 km reach of the Lower Havel River was quantified and its temporal variation was analyzed. It was examined that about 30% of the external nitrogen input to the Lower Havel is retained within the surveyed river section. A comparison of simulation results generated with and without consideration of phosphorus retention/release revealed that summer TP concentrations are currently increased by 100?200% due to internal loading. Net phosphorus release rates of about 20 mg P m?2 d-1 in late summer were estimated for the Havel lakes. Scenario simulations with lowered external nutrient inputs revealed that persistent phosphorus limitation of primary production cannot be established within the next decade. It was shown that a further reduction in nitrogen concentrations requires emissions to be reduced in all inflows. Though the TraM model needs further extension it proved to be appropriate for conducting integrated catchment and river modeling

Use of soil moisture dynamics and patterns at different spatio-temporal scales for the investigation of subsurface flow processes

Spatial patterns as well as temporal dynamics of soil moisture have a major influence on runoff generation. The investigation of these dynamics and patterns can thus yield valuable information on hydrological processes, especially in data scarce or previously ungauged catchments. The combination of spatially scarce but temporally high resolution soil moisture profiles with episodic and thus temporally scarce moisture profiles at additional locations provides information on spatial as well as temporal patterns of soil moisture at the hillslope transect scale. This approach is better suited to difficult terrain (dense forest, steep slopes) than geophysical techniques and at the same time less cost-intensive than a high resolution grid of continuously measuring sensors. Rainfall simulation experiments with dye tracers while continuously monitoring soil moisture response allows for visualization of flow processes in the unsaturated zone at these locations. Data was analyzed at different spacio-temporal scales using various graphical methods, such as space-time colour maps (for the event and plot scale) and binary indicator maps (for the long-term and hillslope scale). Annual dynamics of soil moisture and decimeter-scale variability were also investigated. The proposed approach proved to be successful in the investigation of flow processes in the unsaturated zone and showed the importance of preferential flow in the Malalcahuello Catchment, a data-scarce catchment in the Andes of Southern Chile. Fast response times of stream flow indicate that preferential flow observed at the plot scale might also be of importance at the hillslope or catchment scale. Flow patterns were highly variable in space but persistent in time. The most likely explanation for preferential flow in this catchment is a combination of hydrophobicity, small scale heterogeneity in rainfall due to redistribution in the canopy and strong gradients in unsaturated conductivities leading to self-reinforcing flow paths

Use of soil moisture dynamics and patterns for the investigation of runoff generation processes with emphasis on preferential flow

International audienceSpatial patterns as well as temporal dynamics of soil moisture have a major influence on runoff generation. The investigation of these dynamics and patterns can thus yield valuable information on hydrological processes, especially in data scarce or previously ungauged catchments. The combination of spatially scarce but temporally high resolution soil moisture profiles with episodic and thus temporally scarce moisture profiles at additional locations provides information on spatial as well as temporal patterns of soil moisture at the hillslope transect scale. This approach is better suited to difficult terrain (dense forest, steep slopes) than geophysical techniques and at the same time less cost-intensive than a high resolution grid of continuously measuring sensors. Rainfall simulation experiments with dye tracers while continuously monitoring soil moisture response allows for visualization of flow processes in the unsaturated zone at these locations. Data was analyzed at different spacio-temporal scales using various graphical methods, such as space-time colour maps (for the event and plot scale) and indicator maps (for the long-term and hillslope scale). Annual dynamics of soil moisture and decimeter-scale variability were also investigated. The proposed approach proved to be successful in the investigation of flow processes in the unsaturated zone and showed the importance of preferential flow in the Malalcahuello Catchment, a data-scarce catchment in the Andes of Southern Chile. Fast response times of stream flow indicate that preferential flow observed at the plot scale might also be of importance at the hillslope or catchment scale. Flow patterns were highly variable in space but persistent in time. The most likely explanation for preferential flow in this catchment is a combination of hydrophobicity, small scale heterogeneity in rainfall due to redistribution in the canopy and strong gradients in unsaturated conductivities leading to self-reinforcing flow paths

Analysis and simulation of nutrient retention and management for a lowland river-lake system

International audienceIn the context of the European Water Framework Directive, we studied the possible impact of reduced emissions on phosphorus and nitrogen concentrations in a lowland river-lake system (Havel River, Germany). As a prerequisite, we quantified the retention of nutrients in the river from mass balances and deduced its seasonal variation. We detected that about 30% of the total nitrogen input is retained within the surveyed river section. In contrast, phosphorus release from sediments was shown to cause a considerable increase in present P concentrations. Average net phosphorus release rates of about 20 mg P m?2 d?1 in late summer were estimated for the Havel Lakes. Based on the observed patterns of N retention and P release we parametrized a newly developed water quality simulation program (TRAM), which allows alternative model approaches of different complexity to be implemented and tested. To account for the future trend of internal P loading, the phosphorus excess in lake sediments was estimated from core samples and included in the model as a state variable. For analyzing scenarios of reduced nutrient emissisions, the water quality simulation program was linked to mesoscale hydrological catchment models for the first time. From scenario simulations we conclude that internal P loading is likely to counteract efforts of emission control for decades. Even by significant reductions in external P loads, a persistent phosphorus limitation of primary production can hardly be established in the analyzed time frame of 13 years. Though in the short run a continued reduction in nitrogen loads appears to be the more promising approach of eutrophication management, we recommend enhanced efforts to diminish both N and P emissions

Modeling of water balance response to an extreme future scenario in the Ötztal catchment, Austria

The aim of the study was to investigate the impact of climate change on the water balance of the Ötztaler Ache catchment in Tyrol, Austria. For this purpose the conceptual hydrological model HBV-D REG was applied. First, the model was calibrated and validated using current observed climate and discharge data. Second, the calibrated model was applied with reanalysis data. Third, downscaled climate scenarios from 2010 to 2099 served as input data to the HBV-D REG. Thereby two extreme land cover scenarios were considered: for water balance modeling a constant glacier coverage from today and additionally for runoff simulations a complete loss of glaciered area. The downscaled climate data were generated with the expanded downscaling method. Scenario simulations indicated an increase in annual areal temperature by 3.4 °C and a slight decrease in annual areal precipitation by 89 mm in the next one hundred years. According to the hydrological modeling, these climate changes caused an increase in evapotranspiration and a decrease in snow coverage. Furthermore model simulations showed an increase in winter and spring runoff, whereas summer runoff was highly sensitive to glacier coverage and decreased with complete loss of glacier coverage

Increasing compound warm spells and droughts in the Mediterranean Basin

The co-occurrence of warm spells and droughts can lead to detrimental socio-economic and ecological impacts, largely surpassing the impacts of either warm spells or droughts alone. We quantify changes in the number of compound warm spells and droughts from 1979 to 2018 in the Mediterranean Basin using the ERA5 data set. We analyse two types of compound events: 1) warm season compound events, which are extreme in absolute terms in the warm season from May to October and 2) year-round deseasonalised compound events, which are extreme in relative terms respective to the time of the year. The number of compound events increases significantly and especially warm spells are increasing strongly – with an annual growth rates of 3.9 (3.5) % for warm season (deseasonalised) compound events and 4.6 (4.4) % for warm spells –, whereas for droughts the change is more ambiguous depending on the applied definition. Therefore, the rise in the number of compound events is primarily driven by temperature changes and not the lack of precipitation. The months July and August show the highest increases in warm season compound events, whereas the highest increases of deseasonalised compound events occur in spring and early summer. This increase in deseasonalised compound events can potentially have a significant impact on the functioning of Mediterranean ecosystems as this is the peak phase of ecosystem productivity and a vital phenophase

Assessing flood risk for a rural detention area

International audienceFlood detention areas serve the primary purpose of controlled water storage during large flood events in order to decrease the flood risk downstream along the river. These areas are often used for agricultural production. While various damage estimation methods exist for urban areas, there are only a few, most often simpler approaches for loss estimation in rural areas. The loss assessment can provide an estimate of the financial provisions required for the farmers' compensation (e.g., in the context of cost-benefit analyses of detention measures). Flood risk is a combination of potential damage and probability of flooding. Losses in agricultural areas exhibit a strong seasonal pattern, and the flooding probability also has a seasonal variation. In the present study, flood risk is assessed for a planned detention area alongside the Elbe River in Germany based on two loss and probability estimation approaches of different time frames, namely a monthly and an annual approach. The results show that the overall potential damage in the proposed detention area amounts to approximately 40 000 ? a?1, with approximately equal losses for each of the main land uses, agriculture and road infrastructure. A sensitivity analysis showed that the probability of flooding (i.e., the frequency of operation of the detention area) has the largest impact on the overall flood risk