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Estuaries are an important component of the global carbon budget as sites of removal and transformation for carbon between land and coastal ocean. Drought conditions can lengthen river and estuarine water residence time, which can extend the retention and alter the cycling of organic carbon and nutrients. To better understand the functioning of an estuary under the current threat of climate change related droughts, we use the Elbe Estuary as an example, examining a period since 1997, when annual mean DIC in the mid to lower Elbe Estuary increased significantly, and with focus on the drought conditions since 2014. The recent (1997-2020) significant DIC increase by 6 to 15 µmol L-1 yr-1 we found is due to increase in upper estuary POC content of 8-14 µmol L-1 yr-1 in late spring and summer (May-August). The significant increase in POC was associated with dominating autotrophy (with negative AOU and pH > 9), and an overall improvement in water quality shown in significant (> 50%) decrease in BOD7 since 1997. We found that microbial respiration of organic matter from upstream regions accounted for most of the DIC produced in the mid-estuary, therefore, the increased POC is efficiently remineralized to DIC by the mid-estuary region.The Elbe River and estuary was subject to significantly lower river discharge between 2014 and 2020 (468 ± 234 m3 s-1), nearly 40% of the long-term average (1960-2020, 690 ± 441 m3 s-1). In addition, May was the only month with a significant negative trend in mean monthly river discharge from 1997, and down to 264 ± 19 m3 s-1 by 2020, a discharge usually observed during summer and early autumn. During the recent drought period (2014-2020), the internal gain in the carbon load as DIC in the mid to lower estuary was significantly higher, by up to 3 times, compared to the non-drought period (1997-2013). This suggests that the drought in the Elbe watershed caused a significant reduction in the average river discharge in May, likely increasing the residence time in the estuary, subsequently permitting a longer period for remineralisation of POC and greatest production of DIC in the mid-lower Elbe Estuary.

Estuaries are an important contributor to the global carbon budget, facilitating carbon removal, transfer and transformation between land and coastal ocean. Estuaries are also susceptible to global climate change and anthropogenic perturbations. We find that a long-term significant increase in dissolved inorganic carbon (DIC) of 6–21 µmol kg-1 yr-1 (1997–2020) in a temperate estuary in Germany (Elbe Estuary), was driven by an increase in upper estuary particulate organic carbon (POC) content of 8–14 µmol kg-1 yr-1. The temporal POC increase was due to an overall improvement in water quality observed in the form of high rates of primary production and a significant drop in biological oxygen demand. The magnitude of mid-estuary DIC gain was equivalent to the increased POC production in the upper estuary, suggesting that POC is efficiently remineralized and retained as DIC in the mid-estuary, with the estuary acting as an efficient natural filter for POC. In the context of the significant DIC increase, the impact of a recent extensive drought period (2014–2020) significantly lowered the annual mean river discharge (468 ± 234 m3 s-1) compared to the long-term mean (690 ± 441 m3 s-1, 1960–2020). During the drought period, the late spring internal DIC load in the estuary doubled. This suggests that the drought induced a longer dry season, starting in May (earlier than normal), increased the residence time in the estuary and allowed for a longer remineralization period for POC. Annually, 77–94 % of the total DIC export was laterally transported to coastal water, reaching 89 ± 4.8 Gmol C yr-1, and thus only a maximum of 23 %, at 10 Gmol C yr-1, was released via carbon dioxide (CO2) evasion, between 1997 and 2020. Export of DIC to coastal waters decreased significantly during the drought (2014–2020: 38 ± 5.4 Gmol C yr-1), on average by 24 % compared to the non-drought period. In addition, we have identified that seasonal changes in DIC processing in an estuary require consideration in order to understand both the long-term and future changes in air-water CO2 flux, DIC export to coastal waters, as well as the impacts of prolonged droughts on the land-ocean carbonate system.

Climate projections show high temperature extremes, meteorological droughts and heavy precipitation events are set to increase across Europe (Barros et al., 2014), where the decadel average has already increaed, with temperature in 2002-2011 already 1.3°C±0.1°C above the 1850-1899 mean (Barros et al., 2014). The observed seasonal precipitation pattern presents drier summers and wetter winters across Europe, also mirrored in river flow changes. Across small river catchments in Europe from 1962 to 2004, winter period showed positive trends whereas summers were characterized by negative trends in river flow (Stahl et al., 2010). Such changes can alter the residence time of an estuary. Estuaries are biogeochemical hotspots, and critical zones for carbon cycling, and changes in the hydrological balance, still largely not well characterized, may influence processes within the water column. The present study will assess the potential impacts of droughts on the carbonate system in the Elbe estuary. One of the largest in central Europe, the Elbe River catchment spreads over approximately 150,000 km2 in four countries. Between 2014 and 2018, regions of Northern Germany have been under drought conditions during certain months (UFZ, 2018), reducing discharge in the Elbe River. From 2014, annual Elbe river discharge has been relatively low, where 2018 exhibited the lowest annual mean river discharge of 441 m3 s-1 since 1992. Model projections show the annual river discharge for the Elbe river is likely to remain low at 410 m2 s-1 in 2046-2055 compared to >550 m2 s-1 in 1960-1990 (Krysanova et al., 2005).Analysis of the long-term FGG Elbe (Flussgebietsgemeinschaft Elbe) records of dissolved inorganic carbon (DIC) in the mid to lower Elbe estuary show that over spring and summer months DIC values have increased with time (1997-2018). In this period, DIC increased from the freshwater to the mesohaline region, followed by a decrease to the polyhaline zone. This is opposing to previous DIC patterns in the early 1980s, where DIC decreased towards the mid-estuary after which increased to the outer estuary. An increase in DIC in the mid-estuarine region coincided with increased turbidity and extended residence time, and during the productive months with higher organic matter from upstream regions. This could suggest that more time for heterotrophic activity and availability of labile organic matter, acts to enrich DIC within the water column in the turbid regions, thus changing carbon cycling within the estuary. Further analysis will focus on the changes in river discharge and inorganic carbon during the past two decades, thus inclusive of low discharge and drought conditions.

Water reservoirs around the world suffer from accelerated sediment loads and, consequently, contamination. Notably, in water-scarce regions such as Jordan, this poses a threat to an important water source, and identifying the sediment sources is an important task. Thus, a sediment fingerprinting study in the Wadi Al-Arab catchment of northern Jordan was implemented with special attention directed to the development of suitable correction factors necessary to improve the comparability of source and sink sediments. The selection of seven conservative elements for the sediment fingerprinting was made, with specific attention directed to the chemical environment of the reservoir. Thirty-six samples from six different surface and subsurface sources and 38 sink samples from the Wadi Al-Arab reservoir were collected. In total, 27 organic and inorganic elements as well as radionuclides were analysed. Two vertical physicochemical water profiles provided information on the pH and Eh conditions and common element concentrations. The stepwise multiple regression analysis model (SMRAM) was developed to explore parameters that influence the element concentrations and their interrelations, and to calculate an element-specific correction factor. The standard selection procedure was expanded by the comparison of water and sink sediment element concentrations, a literature review concerning the pH and Eh conditions and, in selected cases, a correlation analysis. The combination of Al, Cr, Mn, Fe, 232Th, 228Th and 137Cs provided the best source discrimination, and based on Monte Carlo simulations, the mixing model revealed the existence of three major sediment source areas. These are as follows: (i) olive orchards on slopes, which delivered 59 ± 8 % of the sediments in the sink; (ii) cultivated fields on plateau and saddle positions contributed 11 ± 9 %; and (iii) slopes with natural vegetation used for grazing contributed 29 ± 15 % of the deposited sediment. With a mean residual error of 1.04 %, the sum of the source concentrations differs only slightly from sink concentrations and proves that the model is reliable. The SMRAM model revealed that the different inorganic (total inorganic carbon, TIC) and organic (total organic carbon, TOC) carbon contents and the clay/sand content influence the element concentrations of the sediment samples. Due to the carbonatic environment, it was mainly necessary to correct for TIC. Applying an expanded literature review regarding the chemical environment under investigation, in addition to the standard mass conservation and Kruskal-Wallis test, prevented possible non-conservative elements from entering the discriminant analysis.

Roman cisterns served as rainwater storage devices for centuries and are densely distributed in parts of northern Jordan. A major earthquake hit the region ca. A.D. 750 and in a short time many settlements were abandoned. As a consequence, most cisterns were not maintained, and they filled with sediments that today provide a postabandonment depositional record. In two field surveys, we mapped the locations of more than 100 cisterns in the Wadi Al-Arab basin and selected two for detailed stratigraphic analysis that included 14C and optically stimulated luminescence dating. Catchment basin area for each cistern was determined by differential GPS. Both cisterns filled with sediments after the great earthquake and consequent abandonment of the region. Calculated sediment volumes are translated to long-term average sediment export rates of 2.6–6.6 t ha−1a−1, which are comparable to erosion and sediment yield rates from other studies within the Mediterranean region. Our pilot study suggests that this approach can be applied elsewhere to calculate long-term sediment export rates on hill slopes containing relict cisterns.

Over the last few decades many quantitative erosion studies have revealed that olive orchard expansion and increased mechanization in southern European countries have led to increased soil erosion under olive trees. Consequently, these studies have suggested different methods of mitigation. In light of the 2014 European trading zone expansion to countries east and south of the Mediterranean, a further intensification of olive plantations is postulated to meet market demands. To attain first medium-term estimates of erosion in Northern Jordan and its driving factors, a new method measuring olive mounds was implemented. Seven fields with clearly erosive structures were chosen throughout the Wadi Al-Arab catchment in Northern Jordan. Topographic measurements were used to reconstruct the historical and recent surface level and calculate the volume eroded since the planting of the trees. A total of 81 bulk density measurements and 14 tree cores allowed the estimation of the soil loss in tons per hectare. The combination of modified land use map and slope information helped to identify similar olive fields with high erosive potential. Results show that the method provides medium-term quantitative estimates for averaged soil loss consistent with some existing results from similar research areas in the Mediterranean. They clearly indicate the significant potential for erosion in olive orchards with around 95 ± 8 t ha − 1 yr − 1 . Tillage practice and water erosion were identified as critical erosion processes, both depending on tillage characteristics, tillage timing, and soil parent material. The investigated fields represent about 19% of the catchment's surface area and are likely to contribute to the measured yearly sediment yield that fills up the Wadi Al-Arab reservoir with sediments.

Riverine dissolved organic carbon (DOC) supports the production of estuaries and coastal ecosystems, constituting one of the most actively recycled pools of the global carbon cycle. A substantial proportion of DOC entering oceans is highly aged, but its origins remain unclear. Significant fluxes of old DOC have never been observed in temperate headwaters where terrestrial imports take place. Here, we studied the radiocarbon age of DOC in three streams draining forested headwater catchments of the river Mulde (Ore Mountains, Germany). In a 4 week summer precipitation event DOC aged at between 160 and 270 years was delivered into the watershed. In one stream, the DOC was modern but depleted in radiocarbon compared to other hydrological conditions. The yield was substantial and corresponded to 20–52 % of the annual DOC yields in wet and dry years, respectively. The analysis of long-term data suggested that the DOC export in extreme precipitation events added to the annual yield and was not compensated for by lower exports in remaining periods. We conclude that climate change, along with additional processes associated with human activities, channels old soil carbon into more rapidly cycled carbon pools of the hydrosphere.

High availability of phosphorus in agricultural soils leads to increased phosphorus (P) losses from land to water and contributes to the eutrophication of surface water. The most important variable for the transport of soluble P via surface runoff is the labile P content in the top soil. Until now, a detailed understanding and quantitative estimates of the seasonal and spatial dynamics of labile P in the top soil layer at the catchment scale have been lacking. The objective of this paper is to analyze the spatial and temporal variability of labile P and to quantify concentrations in the top soil by coupling the process-based one dimensional P soil model ANIMO with a hydrological model (WaSim-ETH). For testing the spatially distributed ANIMO model, a sampling scheme for soil P compounds was carried out. The scheme comprised of 80 sampling points in a 150 m grid in the 1.44 km2 study catchment Schafertal, Germany. For a period of 2 years water soluble P (WSP), total phosphorus (TP), oxalate extractable P/Fe + Al, pH value and Corg were analyzed. Close correlations with a high level of significance were observed between P compounds (from 0.733 to 0.737) and between P compounds and Corg (from 0.508 and 0.527). It was found that, in addition to crop rotation and management practice, spatial variability of WSP was controlled by topography affected microclimate and soil water balance. The combined modeling approach was successfully calibrated for WSP in the top soil with a R2 of 0.94. Although the accuracy of simulation results for the validation period was lower, the model reasonably reproduced the spatial and seasonal variation of WSP. The simulations identified areas of high WSP concentrations and revealed a spring depression with low WSP concentrations in the top soil. The new modeling approach can be used to better understand WSP availability and P yield at the catchment scale.

Quantification of soil water flow is a prerequisite to accurate prediction of solute transfer within the unsaturated zone. The monitoring of these fluxes is challenging because the results are required to answer both scientific and practical questions regarding protection of groundwater, sustainable management of agricultural, forestry, mining or set-aside industrial areas, reducing leachate loss from landfills or explaining the fate of environmentally harmful substances. Both indirect and direct methods exist for estimating water-flux rates and have been used with varying success. In Europe, the use of direct lysimetry methods for measuring water and solute fluxes in soils has increased in recent years. This technique ensures reliable drainage data, but requires relatively large investment and maintenance expenses. Other research groups, especially in the USA, have developed alternative techniques. In this paper we compare the functioning of a passive-wick sampler, especially the deep-drainage meter type (DDM), with two different types of drainage lysimeters (weighing and non-weighing) under field conditions in Germany for the measurement period from May 2004 until April 2009. The study showed that under sandy soil conditions no significant differences occurred between the measurements from DDM and both drainage lysimeter types. Only in periods with increased precipitation was there a tendency of drainage over-estimation by the DDM in comparison with the lysimeters tested. For longer periods, no significant differences in the amount of drainage or the pattern of drainage formation were found between weighing and non-weighing gravitation lysimeters. The practical use of DDMs is restricted because the groundwater level must be >2 m from the soil surface. Suggestions are made for the technical improvement of the DDM as well as the testing of the device with more cohesive soils.

Several pressure sensors were deployed in a small lake to determine its storage change. It could be shown that a deep enough deployment and an averaging over a time interval of 1 hour and 5 measuring points allowed for a measurement of 1 or 2 kg/m 2 (i.e. 2 mm of water column) of changes in the storage on the scale of the lake size. This accuracy for the lake storage could not be achieved by other methods, especially if conditions were difficult, e.g. snowfall, or in cases when precipitation was small. Finally, the pressure measurement – originally intended to roughly determine the water level – turned out to be a direct measurement of water mass in the lake, which was the proper magnitude for exchanges between atmosphere and lake. Hence the measurement of lake storage could become an interesting approach even for meteorological measurements, such as precipitation and evaporation on a water surface.

Recent studies about soil erosion show that erosion rates in winter can reach or exceed erosion induced by summer events. A factor of particular importance is the incidence of frozen soil, which modifies the surface runoff generation and also the erodibility of the soil. In this paper, investigations are conducted to characterize the snowmelt erosion events in the 1·44 km 2 ‘Schafertal’ research catchment that is located c. 150 km southwest of Berlin. Two runoff events of different initial conditions are compared and analysed. Although the net erosion rate in the catchment is relatively low, the described event with soil frost is considerably different from the event without soil frost. For example, the net erosion, maximum and median suspended sediment concentrations are significantly higher for the event with frozen soil. The results presented suggest that the sediment source areas differ for both situations. On one hand the channel or sediment flushing is identified as the source, while on the other hand hill slope processes and intra-storm variations are recognized for the soil frost situation. A model modification is presented to improve the estimation of spatial differentiation of surface runoff within the continuous hydrological model WaSim, which is linked to the erosion model AGNPS. Based on measurements of topsoil temperature at locations with diverse exposition and land use, an algorithm is developed. The average agreement of air temperature to the calculated topsoil temperature is r 2 = 0·75. The spatial information about soil frost is utilized to modify the infiltration characteristics of the soil. Hence, the spatial aspects of runoff generation in winter conditions and the related transport of sediment is considered in a more realistic way. The spatial results of the modelling are plausible but the estimation of erosion needs further improvement. Copyright © 2005 John Wiley & Sons, Ltd.

Owing to the marginal success in abating pollution sources in water systems, integrated catchment management plans are being increasingly implemented in hopes of improving water quality. Transferring data and concepts between subbasins in the catchment is often used to fill knowledge and data gaps. Conducted here is a comparative study of three catchment areas of similar size, two neighboring stream basins in Uganda and one in central Russia, to investigate important prerequisites for transferability of data between basins. Compared were ions (Ca, Cl, Fe, K, Mg, Mn, Na), nutrients (NH4-N, NO 3 -N, dissolved and particulate P, SO 4 ), discharge, suspended sediment, pH, and alkalinity with attention given also to the climatic, soil, and land-use characteristics of each catchment. Significant statistical differences were found in the parameters, even of the adjacent lying catchment areas in Uganda. The substance regime in the Russian stream is dominated by the snowmelt events whereas that in Uganda by the rainy season pattern. The phosphorus dynamics were markedly different between the two adjacent catchments in Uganda. Although the climates and the chemical composition of the soils of the two basins are quite similar, slight shifts in land-use distribution can alter the processes dominating in the transport of phosphorus. This is indicative of the sensitivity that parameters of individual basin characteristics (even small neighboring basins) can have on the outcome of the substance regimes and fluxes. These sensitivities highlight the precision required in defining the processes to make transferability of data and models across subbasins in a catchment management system as successful as possible. Keeping such systems flexible and modular (object-oriented approach) can alleviate these difficulties.

Temporal variability and spatial heterogeneity of surface runoff generation triggers the dynamics of source areas of sediment and sediment-associated nutrient transport. Reliable modelling of hydrological special situations i.e. snowmelt is of high importance for the quality of erosion and sediment yield modelling. Data from the research catchment Schafertal demonstrate the individuality of snowmelt events in terms of runoff coefficient and delivery ratio. This 1.44 km2 low mountain catchment is characterised by a high portion of arable land with a winter grain/winter rape crop rotation. The integrated winter erosion and nutrient load model (IWAN) considers these dynamic aspects by coupling a hydrological model with a sediment load model. Cell size of this raster-based approach is 10 × 10 m2. Additionally, snowmelt rill erosion is simulated with a newly developed physically based model that is firstly applied on a catchment scale. A sensitivity analysis of this model system component demonstrates the plausibility of the model approach and the overall robustness of the model system IWAN. The results of the long-term hydrological modelling from 1991 to 2003 are reliable and form the basis for the simulation of six snowmelt events which were observed in the Schafertal catchment. The estimated total runoff volumes for these events match the observations well. The modelled overland runoff coefficients vary from 0.001 to 0.72. The mean values of cell erosion, which were modelled with one set of parameters for all six events range from 0.0006 to 0.96 t ha− 1. The total modelled erosion for the events with unfrozen soil and low amount of surface runoff is of a factor 50 below those with partly frozen soil. In addition to these distinctions, the major differences are caused by flow accumulation in shallow depressions in variable parts of the catchment. However, the validation of these results on the single event scale is restricted due to limited spatial data. Total simulated sediment yield at the catchment outlet was as high as 13.84 t which underestimates the observed values, with the exception of one event. Oversimplification of the modelled channel processes may be a reason. The temporal variability and spatial heterogeneity of the surface roughness parameter, which was identified to be sensitive, also causes uncertainty in the parameter estimation. Despite these findings, the model system IWAN was applied successfully on the catchment scale and the simulated results are reliable.

Abstract Current research suggests that strategies to control sediment and phosphorus loss from non-point sources should focus on different runoff components and their spatial and temporal variations within the river basin. This is a prerequisite for determining effective management measures for reducing diffuse source pollution. Therefore, non-point source models, especially in humid climatic regions, should consider variable hydrologically active source areas. These models should be able to consider runoff generation by saturated overland flow, as well as Hortonian overland flow. A combination of the hydrological model WaSiM-ETH and the erosion and P-transport model AGNPS was chosen for this study. The models were run in the WaSiM runoff generation mode (Green & Ampt/TOPMODEL or Richards equation approach) and the SCS curve number mode to assess the effect of these different runoff calculation procedures on the dissolved phosphorus yield. A small and a medium-sized river basin, of the area of 1...

Rainfall erosivity is a basic mean of characterizing and understanding erosion processes. The empirically based EI 30 index, which is frequently used as part of the Universal Soil Loss Equation (USLE), has a number of limitations and requires adaptation for different climatic regions. In contrast, the A index, that was developed to calculate rainfall erosivity from rainfall simulation, is a physically based weight-function formula. This study demonstrates the functionality of this index using calculations for a number of rainfall events from two Russian regions as well as from Sardinia/Italy. Descriptive statistics and correlation analysis show, that values of the A index can replace with high accuracy the calculation of the EI 30 with a relation of 1:1 without data adaptation for the three test regions, which vary in climate and rainfall development. Although the values of erosivity for both indices are of high similarity, differences occur for single rainfall events with high internal variability. The results indicate a good estimation of rainfall erosivity for single rainfall events with the physically based A index. As there is a lack of data and an adaptation has to be carried out for the EI 30 index to fit the climatic conditions, it is feasible to substitute the EI 30 values with a calculation of the A index.