River ecosystems at risk

Worldwide water managers are increasingly challenged to allocate sufficient and affordable water supplies to different water use sectors without further degrading river ecosystems and their valuable services to mankind. Since 1950 human population almost tripled, water abstractions increased by a factor of four, and the number of large dam constructions is about eight times higher today. From a hydrological perspective, the alteration of river flows (temporally and spatially) is one of the main consequences of global change and further impairments can be expected given growing population pressure and projected climate change. Implications have been addressed in numerous hydrological studies, but with a clear focus on human water demands. Ecological water requirements have often been neglected or addressed in a very simplistic manner, particularly from the large-scale perspective. With his PhD thesis, Christof Schneider took up the challenge to assess direct (dam operation and water abstraction) and indirect (climate change) impacts of human activities on river flow regimes and evaluate the consequences for river ecosystems by using a modeling approach. The global hydrology model WaterGAP3 (developed at CESR) was applied and further developed within this thesis to carry out several model experiments and assess anthropogenic river flow regime modifications and their effects on river ecosystems. To address the complexity of ecological water requirements the assessment is based on three main ideas: (i) the natural flow paradigm, (ii) the perception that different flows have different ecological functions, and (iii) the flood pulse concept. The thesis shows that WaterGAP3 performs well in representing ecologically relevant flow characteristics on a daily time step, and therefore justifies its application within this research field. For the first time a methodology was established to estimate bankfull flow on a 5 by 5 arc minute grid cell raster globally, which is a key parameter in eFlow assessments as it marks the point where rivers hydraulically connect to adjacent floodplains. Management of dams and water consumption pose a risk to floodplains and riparian wetlands as flood volumes are significantly reduced. The thesis highlights that almost one-third of 93 selected Ramsar sites are seriously affected by modified inundation patterns today, and in the future, inundation patterns are very likely to be further impaired as a result of new major dam initiatives and climate change. Global warming has been identified as a major threat to river flow regimes as rising temperatures, declining snow cover, changing precipitation patterns and increasing climate variability are expected to seriously modify river flow regimes in the future. Flow regimes in all climate zones will be affected, in particular the polar zone (Northern Scandinavia) with higher river flows during the year and higher flood peaks in spring. On the other side, river flows in the Mediterranean are likely to be even more intermittent in the future because of strong reductions in mean summer precipitation as well as a decrease in winter precipitation, leading to an increasing number of zero flow events creating isolated pools along the river and transitions from lotic to lentic waters. As a result, strong impacts on river ecosystem integrity can be expected. Already today, large amounts of water are withdrawn in this region for agricultural irrigation and climate change is likely to exacerbate the current situation of water shortages.

Impact of sward type, cutting date and conditioning temperature on mass and energy flows in the integrated generation of solid fuel and biogas from semi-natural grassland

Energy production from biomass and the conservation of ecologically valuable grassland habitats are two important issues of agriculture today. The combination of a bioenergy production, which minimises environmental impacts and competition with food production for land with a conversion of semi-natural grasslands through new utilization alternatives for the biomass, led to the development of the IFBB process. Its basic principle is the separation of biomass into a liquid fraction (press fluid, PF) for the production of electric and thermal energy after anaerobic digestion to biogas and a solid fraction (press cake, PC) for the production of thermal energy through combustion. This study was undertaken to explore mass and energy flows as well as quality aspects of energy carriers within the IFBB process and determine their dependency on biomass-related and technical parameters. Two experiments were conducted, in which biomass from semi-natural grassland was conserved as silage and subjected to a hydrothermal conditioning and a subsequent mechanical dehydration with a screw press. Methane yield of the PF and the untreated silage was determined in anaerobic digestion experiments in batch fermenters at 37°C with a fermentation time of 13-15 and 27-35 days for the PF and the silage, respectively. Concentrations of dry matter (DM), ash, crude protein (CP), crude fibre (CF), ether extract (EE), neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent ligning (ADL) and elements (K, Mg, Ca, Cl, N, S, P, C, H, N) were determined in the untreated biomass and the PC. Higher heating value (HHV) and ash softening temperature (AST) were calculated based on elemental concentration. Chemical composition of the PF and mass flows of all plant compounds into the PF were calculated. In the first experiment, biomass from five different semi-natural grassland swards (Arrhenaterion I and II, Caricion fuscae, Filipendulion ulmariae, Polygono-Trisetion) was harvested at one late sampling (19 July or 31 August) and ensiled. Each silage was subjected to three different temperature treatments (5°C, 60°C, 80°C) during hydrothermal conditioning. Based on observed methane yields and HHV as energy output parameters as well as literature-based and observed energy input parameters, energy and green house gas (GHG) balances were calculated for IFBB and two reference conversion processes, whole-crop digestion of untreated silage (WCD) and combustion of hay (CH). In the second experiment, biomass from one single semi-natural grassland sward (Arrhenaterion) was harvested at eight consecutive dates (27/04, 02/05, 09/05, 16/05, 24/05, 31/05, 11/06, 21/06) and ensiled. Each silage was subjected to six different treatments (no hydrothermal conditioning and hydrothermal conditioning at 10°C, 30°C, 50°C, 70°C, 90°C). Energy balance was calculated for IFBB and WCD. Multiple regression models were developed to predict mass flows, concentrations of elements in the PC, concentration of organic compounds in the PF and energy conversion efficiency of the IFBB process from temperature of hydrothermal conditioning as well as NDF and DM concentration in the silage. Results showed a relative reduction of ash and all elements detrimental for combustion in the PC compared to the untreated biomass of 20-90%. Reduction was highest for K and Cl and lowest for N. HHV of PC and untreated biomass were in a comparable range (17.8-19.5 MJ kg-1 DM), but AST of PC was higher (1156-1254°C). Methane yields of PF were higher compared to those of WCD when the biomass was harvested late (end of May and later) and in a comparable range when the biomass was harvested early and ranged from 332 to 458 LN kg-1 VS. Regarding energy and GHG balances, IFBB, with a net energy yield of 11.9-14.1 MWh ha-1, a conversion efficiency of 0.43-0.51, and GHG mitigation of 3.6-4.4 t CO2eq ha-1, performed better than WCD, but worse than CH. WCD produces thermal and electric energy with low efficiency, CH produces only thermal energy with a low quality solid fuel with high efficiency, IFBB produces thermal and electric energy with a solid fuel of high quality with medium efficiency. Regression models were able to predict target parameters with high accuracy (R2=0.70-0.99). The influence of increasing temperature of hydrothermal conditioning was an increase of mass flows, a decrease of element concentrations in the PC and a differing effect on energy conversion efficiency. The influence of increasing NDF concentration of the silage was a differing effect on mass flows, a decrease of element concentrations in the PC and an increase of energy conversion efficiency. The influence of increasing DM concentration of the silage was a decrease of mass flows, an increase of element concentrations in the PC and an increase of energy conversion efficiency. Based on the models an optimised IFBB process would be obtained with a medium temperature of hydrothermal conditioning (50°C), high NDF concentrations in the silage and medium DM concentrations of the silage.