| 1. |
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| 2. |
- Breuer, L., et al.
(författare)
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Assessing the impact of land use change on hydrology by ensemble modeling (LUCHEM). I : Model intercomparison with current land use
- 2009
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Ingår i: Advances in Water Resources. - : Elsevier BV. - 0309-1708 .- 1872-9657. ; 32:2, s. 129-146
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Tidskriftsartikel (refereegranskat)abstract
- This paper introduces the project on 'Assessing the impact of land use change on hydrology by ensemble modeling (LUCHEM)' that aims at investigating the envelope of predictions on changes in hydrological fluxes due to land use change. As part of a series of four papers, this paper outlines the motivation and setup of LUCHEM, and presents a model intercomparison for the present-day simulation results. Such an intercomparison provides a valuable basis to investigate the effects of different model structures on model predictions and paves the ground for the analysis of the performance of multi-model ensembles and the reliability of the scenario predictions in companion papers. in this study, we applied a set of 10 lumped, semi-lumped and fully distributed hydrological models that have been previously used in land use change studies to the low mountainous Dill catchment. Germany. Substantial differences in model performance were observed with Nash-Sutcliffe efficiencies ranging from 0.53 to 0.92. Differences in model performance were attributed to (1) model input data, (2) model calibration and (3) the physical basis of the models. The models were applied with two sets of input data: an original and a homogenized data set. This homogenization of precipitation, temperature and leaf area index was performed to reduce the variation between the models. Homogenization improved the comparability of model simulations and resulted in a reduced average bias, although some variation in model data input remained. The effect of the physical differences between models on the long-term water balance was mainly attributed to differences in how models represent evapotranspiration. Semi-lumped and lumped conceptual models slightly outperformed the fully distributed and physically based models. This was attributed to the automatic model calibration typically used for this type of models. Overall, however, we conclude that there was no superior model if several measures of model performance are considered and that all models are suitable to participate in further multi-model ensemble set-ups and land use change scenario investigations.
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| 3. |
- Buffam, Ishi, et al.
(författare)
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Spatial heterogeneity of the spring flood acid pulse in a boreal stream network.
- 2008
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Ingår i: Science of the Total Environment. - : Elsevier BV. - 0048-9697 .- 1879-1026. ; 407:1, s. 708-22
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Tidskriftsartikel (refereegranskat)abstract
- Spatial and temporal patterns in streamwater acidity are ecologically important, but difficult to measure in parallel. Here we present the spatial distribution of streamwater chemistry relevant to acidity from 60 stream sites distributed throughout a 67 km(2) boreal catchment, sampled during a period of winter baseflow (high pH) and during a spring flood episode (low pH). Sites were grouped based on pH level and pH change from winter baseflow to spring flood. The site attributes of each pH group were then assessed in terms of both stream chemistry and subcatchment landscape characteristics. Winter baseflow pH was high throughout most of the stream network (median pH 6.4), but during the spring flood episode stream sites experienced declines in pH ranging from 0-1.6 pH units, resulting in pH ranging from 4.3-6.3. Spring flood pH was highest in larger, lower altitude catchments underlain by fine sorted sediments, and lowest in small, higher altitude catchments with a mixture of peat wetlands and forested till. Wetland-dominated headwater catchments had low but stable pH, while the spring flood pH drop was largest in a group of catchments of intermediate size which contained well-developed coniferous forest and a moderate proportion of peat wetlands. There was a trend with distance downstream of higher pH, acid neutralizing capacity (ANC) and base cation concentrations together with lower dissolved organic carbon (DOC, strongly negatively correlated with pH). This apparent scale-dependence of stream chemistry could be explained by a number of environmental factors which vary predictably with altitude, catchment area and distance downstream-most notably, a shift in surficial sediment type from unsorted till and peat wetlands to fine sorted sediments at lower altitudes in this catchment. As a result of the combination of spatial heterogeneity in landscape characteristics and scale-related processes, boreal catchments like this one can be expected to experience high spatial variability both in terms of chemistry at any given point in time, and in the change experienced during high discharge episodes. Although chemistry patterns showed associations with landscape characteristics, considerable additional variability remained, suggesting that the modeling of dynamic stream chemistry from map parameters will continue to present a challenge. (C) 2008 Elsevier B.V. All rights reserved.
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| 4. |
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| 5. |
- Grabs, Thomas, 1980-, et al.
(författare)
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Modeling spatial patterns of saturated areas: A comparison of the topographic wetness index and a dynamic distributed model
- 2009
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 373:1-2, s. 15-23
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Tidskriftsartikel (refereegranskat)abstract
- Topography is often one of the major controls on the spatial pattern of saturated areas, which in turn is akey to understanding much of the variability in soils, hydrological processes, and stream water quality.The topographic wetness index (TWI) has become a widely used tool to describe wetness conditions atthe catchment scale. With this index, however, it is assumed that groundwater gradients always equalsurface gradients. To overcome this limitation, we suggest deriving wetness indices based on simulationsof distributed catchment models. We compared these new indices with the TWI and evaluated the differ-ent indices by their capacity to predict spatial patterns of saturated areas. Results showed that the model-derived wetness indices predicted the spatial distribution of wetlands significantly better than the TWI.These results encourage the use of a dynamic distributed hydrological model to derive wetness indexmaps for hydrological landscape analysis in catchments with topographically driven groundwater tables.
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| 6. |
- Grabs, Thomas, et al.
(författare)
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Modelling spatial patterns of saturated areas: a comparison of the topographic wetness index and a distributed model
- 2007
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Ingår i: Geophysical Research Abstracts. - : European Geoscience Union. ; , s. vol 9-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The spatial distribution of saturated areas within a catchment is a key factor to understanding and predicting hydrological response and stream water quality at the catchment scale. The topographic wetness index (TWI, ln(a/tan(beta))) is a widely used measure for assessing the spatial distribution of wetness conditions and only requires distributed elevation data as input. The predicted pattern is constant in time because the index is a static representation of the landscape. In this study we examined the predictions of saturated areas using this static topographic wetness index and compared the spatial predictions with temporally aggregated simulations of a distributed hydrological model. The model was calibrated against discharge measured at the outlet and at two internal points of a small forested catchment in northern Sweden. After calibration the model was applied to a larger 68 km2 catchment which included the subcatchment used for calibration. The dynamic groundwater level simulations of this model were temporally aggregated into dynamic indices. These indices were compared to the static topographic wetness index (TWI). We used the ability to spatially predict the occurrence of wetlands as a validation of the static and dynamic indices. First results indicate that the dynamic approach is superior to the static TWI.
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| 7. |
- Huisman, J. A., et al.
(författare)
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Assessing the impact of land use change on hydrology by ensemble modeling (LUCHEM) III : Scenario analysis
- 2009
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Ingår i: Advances in Water Resources. - : Elsevier BV. - 0309-1708 .- 1872-9657. ; 32:2, s. 159-170
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Tidskriftsartikel (refereegranskat)abstract
- An ensemble of 10 hydrological models was applied to the same set of land use change scenarios. There was general agreement about the direction of changes in the mean annual discharge and 90% discharge percentile predicted by the ensemble members, although a considerable range in the magnitude of predictions for the scenarios and catchments under consideration was obvious. Differences in the magnitude of the increase were attributed to the different mean annual actual evapotranspiration rates for each land use type. The ensemble of model runs was further analyzed with deterministic and probabilistic ensemble methods. The deterministic ensemble method based on a trimmed mean resulted in a single somewhat more reliable scenario prediction. The probabilistic reliability ensemble averaging (REA) method allowed a quantification of the model structure uncertainty in the scenario predictions. It was concluded that the use of a model ensemble has greatly increased our confidence in the reliability of the model predictions.
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| 8. |
- Jonsson, Christina E., 1965-
(författare)
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Holocene climate and atmospheric circulation changes in northern Fennoscandia : Interpretations from lacustrine oxygen isotope records
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates how variations in the oxygen isotopic composition of lake waters in northern Fennoscandia are recorded in lake sediment archives, especially diatoms, and how these variations can be used to infer past changes in climate and atmospheric circulation. Results from analyses of the oxygen isotopic composition of lake water samples (δ18Olakew) collected between 2001 and 2006 show that δ18O of northern Fennoscandian lakes is mainly controlled by the isotopic composition of the precipitation (δ18Op). Changes in local δ18Op depend on variations in ambient air temperature and changes in atmospheric circulation that lead to changes in moisture source, vapour transport efficiency, or winter to summer precipitation distribution. This study demonstrates that the amount of isotopic variation in lake water δ18O is determined by a combination of the original δ18Olakew, the amount and timing of the snowmelt, the amount of seasonally specific precipitation and groundwater, any evaporation effects, and lake water residence time. The fact that the same isotope shifts have been detected in various δ18Olakew proxies, derived from hydrologically different lakes, suggests that these records reflect regional atmospheric circulation changes. The results indicate that diatom biogenic silica isotope (δ18Odiatom) records can provide important information about changes in atmospheric circulation that can help explain temperature and precipitation changes during the Holocene. The reconstructed long-term Holocene decreasing δ18Op trend was likely forced by a shift from strong zonal westerly airflow (relatively high δ18Op) in the early Holocene to a more meridional flow pattern (relatively low δ18Op). The large δ18Olakew depletion recorded in the δ18O records around ca. 500 cal yr BP (AD 1450) may be due to a shift to more intense meridional airflow over northern Fennoscandia resulting in an increasing proportion of winter precipitation from the north or southeast. This climate shift probably marks the onset of the so-called Little Ice Age in this region.
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| 9. |
- Jonsson, Christina E., et al.
(författare)
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Stable oxygen and hydrogen isotopes in sub-Arctic lake wateras from northern Sweden
- 2009
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 376:1-2, s. 143-151
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Tidskriftsartikel (refereegranskat)abstract
- Lakes in sub-Arctic regions have the potential of retaining many different aspects of water isotope composition in their sediments which can be used for palaeoclimate reconstruction. It is therefore important to understand the modern isotope hydrology of these lakes. Here we discuss the significance of variations in water isotope composition of a series of lakes located in north-west Swedish Lapland. Climate in this region is forced by changes in the North Atlantic which renders it an interesting area for climate reconstructions. We compare δ18Olake and δ2Hlake collected between 2001 and 2006 and show that the lakes in this sub-Arctic region are currently mainly recharged by shallow groundwater and precipitation which undergoes little subsequent evaporation, and that the d18O and δ2H composition of input to the majority of the lakes varies on a seasonal basis between winter precipitation (and spring thaw) and summer precipitation. Seasonal variations in the isotopic composition of the lake waters are larger in lakes with short residence times (<6 months), which react faster to seasonal changes in the precipitation, compared to lakes with longer residence times (>6 months), which retain an isotopic signal closer to that of annual mean precipitation. Lake waters also show a range of isotope values between sites due to catchment elevation and timing of snow melt. The lake water data collected in this study was supported by isotope data from lake waters, streams and ground waters from1995 to 2000 reported in other studies.
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| 10. |
- Köhler, S. J., et al.
(författare)
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Dynamics of stream water TOC concentrations in a boreal headwater catchment : Controlling factors and implications for climate scenarios
- 2009
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 373:1-2, s. 44-56
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Tidskriftsartikel (refereegranskat)abstract
- Two different but complementary modelling approaches for reproducing the observed dynamics of total organic carbon (TOC) in a boreal stream are presented. One is based on a regression analysis, while the other is based on riparian soil conditions using a convolution of flow and concentration. Both approaches are relatively simple to establish and help to identify gaps in the process understanding of the TOC transport from soils to catchments runoff. The largest part of the temporal variation of stream TOC concentrations (4-46 mg L-1) in a forested headwater stream in the boreal zone in northern Sweden may be described using a four-parameter regression equation that has runoff and transformed air temperature as sole input variables. Runoff is assumed to be a proxy for soil wetness conditions and changing flow pathways which in turn caused most of the stream TOC variation. Temperature explained a significant part of the observed inter-annual variability. Long-term riparian hydrochemistry in soil solutions within 4 m of the stream also captures a surprisingly large part of the observed variation of stream TOC and highlights the importance of riparian soils. The riparian zone was used to reproduce stream TOC with the help of a convolution model based on flow and average riparian chemistry as input variables. There is a significant effect of wetting of the riparian soil that translates into a memory effect for subsequent episodes and thus contributes to controlling stream TOC concentrations. Situations with high flow introduce a large amount of variability into stream water TOC that may be related to memory effects, rapid groundwater fluctuations and other processes not identified so far. Two different climate scenarios for the region based on the IPCC scenarios were applied to the regression equation to test what effect the expected increase in precipitation and temperature and resulting changes in runoff would have on stream TOC concentrations assuming that the soil conditions remain unchanged. Both scenarios resulted in a mean increase of stream TOC concentrations of between 1.5 and 2.5 mg L-1 during the snow free season, which amounts to approximately 15% more TOC export compared to present conditions. Wetter and warmer conditions in the late autumn led to a difference of monthly average TOC of up to 5 mg L-1, suggesting that stream TOC may be particularly susceptible to climate variability during this season.
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| 11. |
- Laudon, Hjalmar, et al.
(författare)
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The Krycklan Catchment Study, Sweden: A field based experimental platform for linking small-scale process understanding to landscape patterns
- 2007
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Krycklan Catchment Study (KCS) (http://ccrew.sek.slu.se/krycklan), is a multiscale experimental catchment located in the boreal region of northern Sweden. The catchment is extensively instrumented for hydrological and biogeochemical research, including 15 permanent gauging stations, ranging from 3 ha to 6700 ha in size, intensively sampled and continuously monitored to quantify temporal and spatial variations in water chemistry and discharge. An additional 90 locations are sampled occasionally for water chemistry at different runoff stages. The multi-investigator KCS has been developed to provide a direct insight into the governing hydrological and biogeochemical processes at a range of catchment scales and consists at present of over 30 separate projects. Its location within an established Experimental Forest provides a comprehensive instrumental infrastructure, long-term climate monitoring facilities and a small research catchment where process-based hillslope, hydrological and biogeochemical research has been conducted for three decades. Recently two new major investments are being implemented. The first includes a Riparian Observatory with over 200 soil lysimeters in the riparian zone. The second investment is the use of laserscanning (LIDAR) which makes KCS one of the first large-scale research catchments where high-resolution elevation and ground cover data are available for hydrological and water quality modeling.
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| 12. |
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| 13. |
- Lindgren, Georg, et al.
(författare)
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Nitrogen source apportionment modeling and the effect of land-use class related runoff contributions
- 2007
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Ingår i: Nordic Hydrology. - : IWA Publishing. ; 38:4-5, s. 317–331-
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Tidskriftsartikel (refereegranskat)abstract
- Models simulating nutrient transport at the catchment scale are frequently used for sourceapportionment and thereby for finding cost-efficient management strategies for water quality improvements. Onetypical modelling approach at the catchment scale is the use of leaching coefficients (mass per unit flow ofwater) to compute the nutrient input based on land-use information. In this study two different such modelapproaches, the lumped Fyrismodel and the distributed HBV-N-D model, were compared based on simulationsfor the River Fyris catchment in central Sweden. A major difference between the models were differentassumptions of specific runoff variations between different land-use classes. These differences had aconsiderable effect on the computed source apportionment. The higher specific runoff from agricultural areas inthe HBV-N-D model compared to the Fyrismodel resulted in a larger contribution of agricultural areas to the totalnitrogen export. These results demonstrate the importance of the assumptions of the spatial variation of specificrunoff on source apportionment HBV-N-D model estimations.
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| 14. |
- Lyon, Steve, et al.
(författare)
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Incorporating landscape characteristics in a distance metric for interpolating between observations of stream water chemistry
- 2008
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Ingår i: Hydrology and Earth System Sciences. ; 12, s. 1229-1239
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Tidskriftsartikel (refereegranskat)abstract
- Spatial patterns of water chemistry along stream networks can be quantified using synoptic or ‘snapshot’ sampling. The basic idea is to sample stream water at many points over a relatively short period of time. Even for intense sampling campaigns, the number of sample points is limited and interpolation methods, like kriging, are commonly used to produce continuous maps of water chemistry based on the point observations from the synoptic sampling. Interpolated concentrations are influenced heavily by how distance between points along the stream network is defined. In this study, we investigate different ways to define distance and test these based on data from a snapshot sampling campaign in a 37-km2 watershed in the Catskill Mountains region (New York State). Three distance definitions (or metrics) were compared: Euclidean or straight-line distance, in-stream distance, and in-stream distance adjusted according characteristics of the local contributing area, i.e., an adjusted in-stream distance. Using the adjusted distance metric resulted in a lower cross-validation error of the interpolated concentrations, i.e., a better agreement of kriging results with measurements, than the other distance definitions. The adjusted distance metric can also be used in an exploratory manner to test which landscape characteristics are most influential for the spatial patterns of stream water chemistry and, thus, to target future investigations to gain process-based understanding of in-stream chemistry dynamics.
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| 15. |
- Lyon, S.W., et al.
(författare)
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Estimation of permafrost thawing rates in a sub-arctic catchment using recession flow analysis
- 2009
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Ingår i: Hydrology and Earth System Sciences. - : Copernicus GmbH. - 1027-5606 .- 1607-7938. ; 13, s. 595-604
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Tidskriftsartikel (refereegranskat)abstract
- Permafrost thawing is likely to change the flow pathways taken by water as it moves through arctic and sub-arctic landscapes. The location and distribution of these pathways directly influence the carbon and other biogeochemical cycling in northern latitude catchments. While permafrost thawing due to climate change has been observed in the arctic and sub-arctic, direct observations of permafrost depth are difficult to perform at scales larger than a local scale. Using recession flow analysis, it may be possible to detect and estimate the rate of permafrost thawing based on a long-term streamflow record. We demonstrate the application of this approach to the sub-arctic Abiskojokken catchment in northern Sweden. Based on recession flow analysis, we estimate that permafrost in this catchment may be thawing at an average rate of about 0.9 cm/yr during the past 90 years. This estimated thawing rate is consistent with direct observations of permafrost thawing rates, ranging from 0.7 to 1.3 cm/yr over the past 30 years in the region.
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| 16. |
- Seibert, Jan, et al.
(författare)
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A new triangular multiple flow-direction algorithm for computing upslope areas from gridded digital elevation models
- 2007
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Ingår i: Water Resources Research. ; 43, s. W04501-
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Tidskriftsartikel (refereegranskat)abstract
- Gridded digital elevation data, often referred to as DEMs, are one of the most widelyavailable forms of environmental data. Topographic analysis of DEMs can take manyforms, but in hydrologic and geomorphologic applications it is typically used as asurrogate for the spatial variation of hydrological conditions (topographic indices) andflow routing. Here we report on a new flow routing algorithm and compare it to threecommon classes of algorithms currently in widespread use. The advantage of the newalgorithm is that unrealistic dispersion on planar or concave hillslopes is avoided, whereasmultiple flow directions are allowed on convex hillslopes. We suggest that this newtriangular multiple flow direction algorithm (MD1) is more appropriate for a range offlow routing and topographic index applications.
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| 17. |
- Seibert, Jan, et al.
(författare)
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Gauging the ungauged basin: What is the value of limited streamflow measurements? (solicited)
- 2007
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The long-standing issue of hydrological predictions for ungauged basins has received increased attention thanks to the PUB imitative. Given all the problems on making predictions in totally ungauged catchments one might argue that the best thing to do in an ungauged basin is to actually take a few runoff measurements. In this study we explored how implementing such a procedure might support predictions in an ungauged basin. We used the well-studied Maimai watershed as a hypothetical ungauged basin where we pretend to start with no runoff data and add different sub-sets of the available data to constrain a simple catchment model. These sub-sets were single runoff events or a limited number of point values; in other words these data represent what could be measured with limited efforts in an ungauged basin. Besides these runoff data we used different types of soft data to constrain the model. We recently presented ‘soft data’ as a general framework to facilitate communication between experimentalist and modeler for new ways to test models and quantify uncertainty, parameter identifiabilty and parameter uncertainty. The model simulations were then validated using the available runoff data from different years. We found that surprisingly little runoff data was necessary to derive model parameterizations which provided good results for the validation periods, especially when these runoff data were combined with soft data. We argue that the improved dialog between experimentalist and modeler may be a necessary next step within the PUB movement for moving from calibration-reliant models to ones grounded in understanding and applicability to ungauged basins.
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| 18. |
- Seibert, Jan
(författare)
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Hydrological landscape analysis based on digital elevation data
- 2007
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Ingår i: International Perspectives on Spatial Modeling in Catchment Research, Manchester University, 25-27 June 2007.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Topography is a major factor controlling both hydrological and soil processes at the landscape scale. While this is well-known qualitatively, quantifying relationships between topography and spatial variations of hydrologically relevant variables and other landscape characteristics such as the variation of soil properties still remains a challenging research topic. Several topographic indices, which can be computed from digital elevation models (DEMs), have been suggested to quantitatively describe terrain characteristics. In the following a review of several recent studies is given.While there are many different topographic indices, it is also important to recognize that many of these indices can be computed in different ways. This is especially true for the topographic wetness index (TWI), also called TOPMODEL index (Beven and Kirkby, 1979). The different methods to calculate both upslope area and slope result in significant differences of the computed TWI maps and the correlations between TWI and observed hydrological and other variables (Sørensen et al., 2006). The upslope area can be computed in various ways which differ in the way accumulated area in one cell is portioned between its neighbouring cells. Seibert and McGlynn (2007) recently suggested a new method which combines the advantages of the multiple-directions approach (Quinn et al., 1991) and the D∞ approach (Tarboton, 1997). A new slope term, the downslope index tanαd was introduced by Hjerdt et al. (2004). The idea behind this index was that the local slope tanβ might not always be a good representation of the groundwater table hydraulic gradient because downslope topography more than one cell distant is not considered. In contrast to tanβ, which only considers the cell of interest and its neighbours, tanαd is defined as the slope to the closest point that is d meters below the cell of interest. Both slope estimates give similar results for small values of d, but the results differ for larger values of d (Hjerdt et al. 2004).The topographic influence on soil properties is apparent in the soil catena concept. Seibert et al. (2007) used measured soil properties from the Swedish National Forest Soil Inventory, which is a long-term inventory of permanent sample plots from the Swedish National Forest Inventory. It includes a description of soil types and soil horizons as well as sampling of organic and mineral soil horizons for subsequent chemical analyses. The study focused on Podzols and Histosols, which provided 4 000 sample plots distributed over almost all of Sweden. Plot locations were determined accurately by GPS, which allowed the overlaying of plot data and the DEM. Topographic indices such as the topographic wetness index, TWI (ln(a/tanß)), were computed from gridded digital elevation data for all sample plots. Several significant correlations between topographic indices and soil properties could be found. The thickness of the organic layer increased with TWI and the thickness of the leached E-horizon increased with upslope area. Soil pH in the organic layer increased with TWI, while the C-N ratio decreased. Soil pH in the organic layer was also found to be higher for south facing slopes than for north facing slopes. The ratio between the divalent base cation (Ca and Mg) and the monovalent base cation (K and Na) concentrations in the O-horizon increased with TWI. These correlations confirmed the importance of topography on soil properties, although there was considerable scatter, which could be attributed to heterogeneity in the large data set.Similar correlations were found in data from two study areas where soil samples were taken at plots randomly distributed over 25 km2 (Zinko et al., 2005; 2006). These studies also demonstrated a positive correlation between topographic wetness index and plant species biodiversity (evaluated by the number of species found in 200 m2 plots).Stream networks derived from DEMs can also provide valuable information on landscape organisation (Seibert and McGlynn, 2005). One important observation is that most catchment area is enters the stream network in rather small headwater catchments. This has implications for catchment management as riparian buffer zones might be more important along these small streams than along larger streams. McGlynn and Seibert (2003) examined the variability in, and controls on, hillslope inputs to stream networks and the potential for riparian zones to regulate hillslope inputs and thereby both quantitatively and qualitatively buffer, or modify, stream responses to hillslope hydrology. They found that the ratio of riparian zone storage to hillslope inputs was the most important plot-scale measure of the buffering capacity of the riparian zone. One particularly important finding was that the catchment-wide proportion of the riparian area might be misleading. At the 280 ha Maimai research area that ratio was 0.14. When this ‘buffer capacity’ was calculated for each 20 m stream reach along the stream network, the values were below 0.14 for 75% of the stream length and the median was 0.06. Using the catchment-wide ratio would thus significantly overestimate the ‘effective’ riparian-to-hillslope-area ratio.McGlynn et al (2003) and McGuire et al. (2005) both demonstrated that topography and landscape structure is a major control on catchment transit times for water. McGlynn et al. (2003) found the median subcatchment size to be correlated to mean water transit times for the Maimai catchment in New Zealand. These results were confirmed by new results from the Krycklan catchment in Northern Sweden where winter baseflow 18O was strongly correlated to median subcatchment sizes. The median subcatchmnet size is computed as the median of the accumulated catchment areas in all upstream stream cells. Typically the median subcatchment area first increases along the stream network (i.e., increases with catchment area) but than reaches some level where it remains constant even with largely increased catchment area. The median subcatchment area is a measure of stream network organisation, values are smaller for tree-like network patterns and larger when the network is less dissected. McGuire et al. (2005) used methods developed by Seibert and McGlynn (2003) and found catchment-wide median flowpath lengths and gradients towards the stream network to be best correlated with residence time.The TWI pattern is constant in time because the index is a static representation of the landscape. Grabs et al. (2007) examined the predictions of saturated areas using this static topographic wetness index and compared the spatial predictions with temporally aggregated simulations of a distributed hydrological model. The model was calibrated against discharge measured in two small subcatchment of the Krycklan catchment in northern Sweden. After calibration the model was applied to the entire larger 68 km2 catchment. The dynamic groundwater level simulations of this model were temporally averaged and provided, thus, alternative indices of wetness distribution (dynamic indices). These indices were compared to the static topographic wetness index (TWI). Grabs et al. (2007) used the ability to spatially predict the occurrence of wetlands as a validation of the static and dynamic indices. In the lower sedimentary part of the catchment both approaches overestimate the wetness due to deeper and well drained soils. These areas were, thus, excluded from the analyses. For the moraine part of the catchment first results indicate that the dynamic approach is superior to the static TWI. One explanation was that the dynamic models allows considering temporally varying hydraulic gradients, which do not equal the surface gradients. While the use of dynamic models might be restricted to smaller areas (compared to the computation of static indices such as TWI), the results of such models might help to guide how to improve the calculation of topographic indices.Above different examples on how to use digital elevation data for hydrological landscape analysis are discussed. While obviously other variables than topography are important for the spatial variation of hydrological processes, elevation data is usually the information which is most accessible. High-resolution elevation data, which becomes more and more available, provides additional opportunities for hydrological landscape analysis.ReferencesBeven, K. J., and Kirkby, M. J., 1979. A physically based, variable contributing area model of basin hydrology. Hydrological Sciences Journal, 24, 43-69.Grabs, T.; Seibert, J.; Laudon, H. (2007). Modelling spatial patterns of saturated areas: a comparison of the topographic wetness index and a distributed model (oral presentation), Geophysical Research Abstracts, Vol. 9, 00894, 2007, SRef-ID: 1607-7962/gra/EGU2007-A-00894, European Geosciences Union, General Assembly 2007, Vienna, Austria, 15 – 20 April 2007.Quinn, P.F., Beven, K.J. , Chevallier, P. and Planchon, O., 1991. The prediction of hillslope flowpaths for distributed modelling using digital terrain models, Hydrological Processes, 5, 59-80.McGlynn, B.L., J.McDonnell, M.Stewart and J. Seibert, 2003, On the relationships between catchment scale and streamwater mean residence time, Hydrological Processes, 17: 175-181McGlynn, B.L. and J. Seibert, 2003. Distributed assessment of contributing area and riparian buffering along stream networks, Water Resources Research 39(4), 1082, doi:10.1029/2002WR001521Hjerdt, K. N., J. J. McDonnell, J. Seibert, and A. Rodhe, 2004. A new topographic index to quantify downslope controls on local drainage, Water Resour. Res., 40, W05602, doi:10.1029/2004WR003130.McGuire, K.J., J.J. McDonnell, M. Weiler, C. Kendall, B.L. McGlynn, J.M. Welker, J. Seibert, 2005. The role of topography on catchment-scale water residence time, Water Resour. Res., Res., 41, W05002, doi:10.1029/2004WR003657.Zinko, U., J. Seibert, M. Dynesius, C. Nils
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| 19. |
- Seibert, Jan, et al.
(författare)
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Linking soil- and stream-water chemistry based on a Riparian Flow-Concentration Integration Model
- 2009
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Ingår i: Hydrology and earth system sciences. - : Copernicus Publications. - 1607-7938 .- 1027-5606. ; 13:12, s. 2287-2297
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Tidskriftsartikel (refereegranskat)abstract
- The riparian zone, the last few metres of soil through which water flows before entering a gaining stream, has been identified as a first order control on key aspects of stream water chemistry dynamics. We propose that the distribution of lateral flow of water across the vertical profile of soil water chemistry in the riparian zone provides a conceptual explanation of how this control functions in catchments where matrix flow predominates. This paper presents a mathematical implementation of this concept as well as the model assumptions. We also present an analytical solution, which provides a physical basis for the commonly used power-law flow-load equation. This approach quantifies the concept of riparian control on stream-water chemistry providing a basis for testing the concept of riparian control. By backward calculation of soil-water-chemistry profiles, and comparing those with observed profiles we demonstrate that the simple juxtaposition of the vertical profiles of water flux and soil water chemistry provides a plausible explanation for observed variations in stream water chemistry of several major stream components such as Total Organic Carbon (TOC), magnesium, calcium and chloride. The "static" implementation of the model structure presented here provides a basis for further development to account for seasonal influences and hydrological hysteresis in the representation of hyporheic, riparian, and hillslope processes.
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| 20. |
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| 21. |
- Sundqvist, Hanna S., et al.
(författare)
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Understanding conditions behind speleothem formation in Korallgrottan, northwestern Sweden
- 2007
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 347:1-2, s. 13-22
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Tidskriftsartikel (refereegranskat)abstract
- In this study we investigate and characterise the environmental factors that control active speleothem growth in Korallgrottan, northwestern Sweden, in order to get a better understanding of seepage processes in karst areas and to determine whether the fossil speleothems from this site are suitable as palaeoclimatic archives. The drip rates from fast-dripping stalactites (>100 ml/day) vary substantially with the season and the snow regime. Comparisons with measurements of river discharge and simulated ground water recharge show that the drip rate from fast-dripping stalactites can be used as an estimation of the weekly to monthly ground water recharge. Slow-dripping stalactites however, have a steadier drip rate, with almost no seasonal variations. The δ18O composition of the drip water from both fast- and slow-dripping stalactites show some seasonal variation (±1.2‰), but is fairly stable compared to outside precipitation (±11.1‰). The δ18O signal from fast-dripping stalactites is biased towards summer conditions, while the signal is dampened at slow-dripping sites and an annual or even longer signal is evident. This holds true even though calcite precipitation may not occur continuously throughout the year. Similarly, the trace elemental composition of drip water is more stable in the slow-dripping stalactites, reflecting mean annual values or longer. Generally the drip water reaches the highest saturation level during the summer and autumn when biological activity in the soil zone is most intense, and the partial pressure of carbon dioxide, which controls limestone dissolution, is high.
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| 22. |
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| 23. |
- Sörensen, Rasmus, et al.
(författare)
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Forest Harvest Increases Runoff Most during Low Flows in Two Boreal Streams
- 2009
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Ingår i: Ambio. - : Royal Swedish Academy of Sciences. - 0044-7447 .- 1654-7209. ; 38:7, s. 357-363
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- To understand how forest harvest influences the aquatic environment, it is essential to determine the changes in the flow regime. This paper presents changes in the hydrological regime during the first 2 y after harvest in two catchments of the Balsjo Catchment Study in Sweden. The changes were judged relative to a reference catchment, calibrated during an 18-mo pretreatment period starting in September 2004. From August 2006 through March 2008, there was an average of 35% more runoff from the harvested catchments relative to the reference. The flow increased most during the growing seasons and at base flows (< 1 mm d(-1); 58-99% increase), followed by dormant season and intermediate flows (30-43%). No significant changes were observed during the highest flows (over 5 mm d(-1)), except for the spring flood a few weeks after harvest, which was delayed and attenuated. Large relative changes in low flow may influence the ecosystem by altering the aquatic habitat
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| 24. |
- Sörensen, Rasmus, et al.
(författare)
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On the calculation of the topographic wetness index: evaluation of different methods based on field observations.
- 2006
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Ingår i: Hydrology and Earth System Sciences. - 1027-5606 .- 1607-7938. ; 10:1, s. 101-112
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Tidskriftsartikel (refereegranskat)abstract
- The topographic wetness index (TWI, ln(a/tan beta)), which combines local upslope contributing area and slope, is commonly used to quantify topographic control on hydrological processes. Methods of computing this index differ primarily in the way the upslope contributing area is calculated. In this study we compared a number of calculation methods for TWI and evaluated them in terms of their correlation with the following measured variables: vascular plant species richness, soil pH, groundwater level, soil moisture, and a constructed wetness degree. The TWI was calculated by varying six parameters affecting the distribution of accumulated area among downslope cells and by varying the way the slope was calculated. All possible combinations of these parameters were calculated for two separate boreal forest sites in northern Sweden. We did not find a calculation method that performed best for all measured variables; rather the best methods seemed to be variable and site specific. However, we were able to identify some general characteristics of the best methods for different groups of measured variables. The results provide guiding principles for choosing the best method for estimating species richness, soil pH, groundwater level, and soil moisture by the TWI derived from digital elevation models.
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| 25. |
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| 26. |
- Temnerud, Johan, et al.
(författare)
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Spatial variation in discharge and concentrations of organic carbon in a catchment network of boreal streams in northern Sweden
- 2007
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 342:1-2, s. 72-87
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Tidskriftsartikel (refereegranskat)abstract
- Total organic carbon (TOC) is an important feature of water quality in Fenno-Scandia's till catchments. In this study we estimated the contribution of headwater streams to downstream TOC, tested the hypothesis that TOC-concentration decreases downstream and explored mechanisms for the observed patterns. The drainage network of a boreal catchment (66 sites) in northern Sweden, with subcatchment sizes 0.1178 km(2), was sampled in August 2002. In the headwaters there was a large variation in TOC-concentration (4-66 mg l(-1)) as well as other chemical parameters and specific discharge (0.13-8.21 s(-1) km(-2)). Further downstream there was less variation in both chemistry and specific discharge. Both flow and chemistry stabilized at catchment areas larger than 5 km2. No clear indication of in-stream processing effects on downstream TOC was observed, though there was TOC loss at takes and some stream junctions. To test whether the observed downstream decrease is different than that expected from conservative mixing along the stream network, we used a Monte Carlo approach to simulate downstream conservative mixing. The observed spatial variability was higher than the simulated, indicating that landscape-scale patterns are more than conservative mixing of random inputs. While the importance of in-stream processes that after TOC- concentrations cannot be ruled out, and loci of TOC loss do exist, we propose that headwater/ downstream patterns in TOC, and related parameters depend largely on the mosaic of landscape elements (mires, takes and forest) together with specific discharge. (c) 2007 Elsevier B.V. All. rights reserved.
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| 27. |
- Thomas, Grabs, et al.
(författare)
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Distributed runoff modelling : Wetland runoff and its importance for spring-flood predictions
- 2007
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Rapport (populärvet., debatt m.m.)abstract
- The focus of this study was on the development and testing of a new fully-distributed modelling approach to improve simulations of stream discharge and spatial-temporal patterns of snow and soil moisture at the catchment scale. In this report we describe the implemented hydrological routines in detail and the novel conceptualisation of soil water dynamics. The latter routine was also compared to the commonly-used soil routine in the HBV model. Additionally we describe a new approach for simulating the effect of forests on snow accumulation using a GIS forest database available for all of Sweden. The basic utilized model concepts are introduced: Interception, snow accumulation and –melt, two alternative soil- and groundwater storage conceptualisations, potential and actual evaporation and routing of water flows. Furthermore the processing of meteorological data is described and regionalisation techniques for model parameters are discussed especially in regard to distributed modelling of snow accumulation. Techniques and general problems associated with the solution of ordinary differential equations are brought to mind and the solution strategy in the current modelling context is elucidated. The first part of this document is concluded by a brief note on the way the model was implemented and on its interactions with the user and third party software. Calibration and evaluation of the model was performed based on data from the Krycklan research catchment in Northern Sweden. The model was first calibrated on discharge data recorded at two internal points within the Kallkälsbäcken catchment (which is a nested subcatchment of the Krycklan catchment) and evaluated against discharge registered at the subcatchment outlet. Spatial predictions of soil moisture were compared to mapped wetlands by applying the model to the entire 68 km2 large Krycklan catchment. It was found that the new soil routine performed better than the standard HBV routine when wetland and forest discharges were to be simulated simultaneously using the same set of parameters. Spatial wetness patterns produced by the dynamic model compared well to mapped wetland and outperform the results achieved using the common static topographic wetness, TWI (Beven and Kirkby, 1979). The distributions of simulated water storage values were compared for wetlands and forested areas. This analysis also indicated that the new modelling approach might be more appropriate than the traditional HBV formulation.
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| 28. |
- Viney, N. R., et al.
(författare)
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Assessing the impact of land use change on hydrology by ensemble modelling (LUCHEM) II : Ensemble combinations and predictions
- 2009
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Ingår i: Advances in Water Resources. - : Elsevier BV. - 0309-1708 .- 1872-9657. ; 32:2, s. 147-158
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports on a project to compare predictions from a range of catchment models applied to a mesoscale river basin in central Germany and to assess various ensemble predictions of catchment streamflow. The models encompass a large range in inherent complexity and input requirements. In approximate order of decreasing complexity, they are DHSVM, MIKE-SHE, TOPLATS, WASIM-ETH, SWAT, PRMS, SLURP, HBV, LASCAM and IHACRES. The models are calibrated twice using different sets of input data. The two predictions from each model are then combined by simple averaging to produce a single-model ensemble. The 10 resulting single-model ensembles are combined in various ways to produce multi-model ensemble predictions. Both the single-model ensembles and the multi-model ensembles are shown to give predictions that are generally superior to those of their respective constituent models, both during a 7-year calibration period and a 9-year validation period. This occurs despite a considerable disparity in performance of the individual models. Even the weakest of models is shown to contribute useful information to the ensembles they are part of. The best model combination methods are a trimmed mean (constructed using the central four or six predictions each day) and a weighted mean ensemble (with weights calculated from calibration performance) that places relatively large weights on the better performing models. Conditional ensembles. in which separate model weights are used in different system states (e.g. summer and winter, high and low flows) generally yield little improvement over the weighted mean ensemble. However a conditional ensemble that discriminates between rising and receding flows shows moderate improvement. An analysis of ensemble predictions shows that the best ensembles are not necessarily those containing the best individual models. Conversely, it appears that some models that predict well individually do not necessarily combine well with other models in multi-model ensembles. The reasons behind these observations may relate to the effects of the weighting schemes, non-stationarity of the climate series and possible cross-correlations between models.
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| 29. |
- Wrede, Sebastian, et al.
(författare)
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Distributed conceptual modelling considering sub-grid variability of land use in a mesoscale lowland catchment in Sweden
- 2007
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A distributed version of a conceptual runoff model, the HBV model, was applied in the Fyrisån catchment in Sweden using 50-by-50 m grid cells and a daily time step. Key elements for the spatial distribution were obtained from the TACD model and separate parameterisations were used in the runoff generation routine for dominant land use classes. To account for small-scale landscape features of the catchment the model was equipped with a sub-grid parameterisation scheme and simple conceptual lake and flow distribution routines. Calibration of the model was achieved against measured runoff at two stations by coupling the model to the automated parameter estimator PEST. Model validation included split-sample test, proxy-basin test, evaluation against synoptic runoff measurements and the comparison with a lumped HBV model. The evaluation revealed an overall good model performance and a conceptual reasonable simulation of spatial distributed hydrologic conditions in the Fyrisån catchment, but also indicated weaknesses of the model. The sub-grid parameterisation scheme allowed a correct representation of small scale land use patterns, such as wetlands, within the catchment while the lake and flow distribution routines captured the runoff dynamics adequately. The results further demonstrated the importance of a thorough model evaluation procedure and showed that the comparison with a hydrologic benchmark models, such as the lumped HBV model, was crucial to gain insight in the model performance. The additional use of synoptic data allowed to add a spatial component to the validation process and proofed to be a feasible method in a multi-scale validation strategy.
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| 30. |
- Zinko, Ursula, et al.
(författare)
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Plant species number predicted by a topography based groundwater-flow index
- 2005
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Ingår i: Ecosystems (New York. Print). - New York, N.Y. : Springer. - 1432-9840 .- 1435-0629. ; 8:4, s. 430-441
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Tidskriftsartikel (refereegranskat)abstract
- The lack of a clear understanding of the factors governing the often-great variation of species numbers over entire landscapes confounds attempts to manage biodiversity. We hypothesized that in a topographically variable boreal forest landscape the availability of shallow groundwater is a major determinant of plant species numbers. We then developed a topographically derived hydrologic index based on multidirectional flow algorithms to account for the variation in availability of such groundwater in the landscape. We found a positive correlation between species numbers of vascular plants in plots ranging from 0.01 to 200 m2 and the hydrologic index. Generally, the landscape was relatively dry and species-poor, but interspersed patches with shallow groundwater had high species numbers and high proportions of regionally uncommon plant species. The index explained 30% of the variation in vascular plant number and correlated quite well (rs = 0.50) with groundwater level, but not as well with a community H+concentration value (instead of community pH, rs = −0.31), based on species composition. In addition, we found a very strong correlation between species number and the community H+ concentration value (rs−0.84). The hydrologic index is a useful tool for the identification of spatial of species number patterns across entire landscapes. This is an important step in identifying the areas most in need of protection or restoration, designing survey techniques, and understanding the fundamental processes that control the spatial distribution of species.
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| 31. |
- Zinko, Ursula, et al.
(författare)
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The role of soil pH in linking groundwater flow and plant species density in boreal forest landscapes
- 2006
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Ingår i: Ecography. - : Wiley. - 0906-7590 .- 1600-0587. ; 29:4, s. 515-524
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Tidskriftsartikel (refereegranskat)abstract
- In hilly boreal landscapes topography governs groundwater flow which strongly influences soil development, and thus vegetation composition. Soil pH is known to correlate well with plant species density and composition, but in boreal forests this relationship has been little studied. Previously, we successfully used a topography-based hydrological index, the topographical wetness index (TWI), as an approximation of the variation in groundwater flow to predict local plant species density in a boreal forest landscape. Data on species indicator values demonstrated that soil pH can be an important soil variable linking groundwater flow and plant species density. In the present paper we explore this link by relating measured soil pH to species numbers of vascular plants and TWI in 200-m2 plots within two boreal forest landscapes, differing in average soil pH. The two landscapes showed almost identical relationships between plant species number and soil pH, implying that this relationship is robust. The landscapes also had similar relationships between soil pH and TWI as well as between plant species number and TWI except at high TWI values, which indicate groundwater discharge areas. In these areas soil pH and plant species numbers were higher in the high-pH landscape at any given TWI value. We conclude that for predictive mapping of the species density of vascular plants in boreal forests, soil pH is a major factor. However, TWI as a measure of groundwater flow is a practical alternative predictor.
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| 32. |
- Ågren, Anneli, et al.
(författare)
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Seasonal and runoff-related changes in total organic carbon concentrations in the River Öre, Northern Sweden
- 2008
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Ingår i: Aquatic Sciences. - : Springer Science and Business Media LLC. - 1015-1621 .- 1420-9055. ; 70:1, s. 21-29
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Tidskriftsartikel (refereegranskat)abstract
- The impact of runoff on allochthonous organic carbon was studied in the River Ore, Northern Sweden, using extensive TOC (total organic carbon) and runoff measurements. No relationship existed between TOC concentration and runoff on an annual basis. However, positive correlations between TOC concentration and runoff were found when observations were divided into three different seasons (winter, spring and summer/autumn). During these seasons runoff explained 62-70% of the TOC variation. Differences in these seasonal relationships indicated that the TOC concentration was restricted by the soil TOC pool during snowmelt, while the pool of TOC in the soil or its availability never limited the TOC export during the rest of the year. Two sets of data were used, a detailed study over 2 years and a long-term study over 14 years. Both showed similar results which indicated that the seasonal variation in the relationship between TOC and runoff is similar from year to year. The chemical variation usually decreases downstream in large rivers due to mixing of water from different sources. Our study, however, showed a strong correlation between TOC and runoff even in a large river like the River Ore. This result indicated that the general pattern of the TOC concentrations was to a large extent determined by the hydrology and climate conditions.
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