Jan Seibert

The role of topography on catchment-scale water residence time

62.4 km 2 ) that represent diverse geologic and geomorphic conditions in the western Cascade Mountains of Oregon. Our primary objective was to determine the dominant physical controls on catchment-scale water residence time and specifically test the hypothesis that residence time is related to the size of the basin. Residence times were estimated by simple convolution models that described the transfer of precipitation isotopic composition to the stream network. We found that base flow mean residence times for exponential distributions ranged from 0.8 to 3.3 years. Mean residence time showed no correlation to basin area (r 2 < 0.01) but instead was correlated (r 2 = 0.91) to catchment terrain indices representing the flow path distance and flow path gradient to the stream network. These results illustrate that landscape organization (i.e., topography) rather than basin area controls catchment-scale transport. Results from this study may provide a framework for describing scale-invariant transport across climatic and geologic conditions, whereby the internal form and structure of the basin defines the first-order control on base flow residence time.

Regionalisation of parameters for a conceptual rainfall-runoff model

The HBV model, a conceptual rainfall-runoff model, was applied to 11 catchments within the NOPEX area. The catchment areas ranged from 7 to 950 km 2 with between 41 and 87% covered by forest. The aim was to relate the different model parameters to physical catchment characteristics. Such relationships would allow simulating runoff from ungauged catchments and could be used to discuss the physical basis of the model. Using a 9-year calibration period the best parameter sets were determined for each catchment. A Monte Carlo procedure and two different criteria were used for the optimisation: the common efficiency and a fuzzy measure that combined different objective functions and was found to reduce parameter uncertainty. The runoff simulations of the model agreed well with the observed runoff series and relationships to catchment characteristics could be found for six of the 13 parameters. The goodness of runoff predictions using derived regional parameter sets was tested with variable results. Some relationships between lake percentage and soil parameters called the physical basis of the model into question as they could not be explained by the physical processes in the soil but by the dominating effect of lakes to runoff variations. On the other hand, relationships between forest percentage and snow parameters supported the physical basis of the model. # 1999 Elsevier Science B.V. All rights reserved.

A new triangular multiple flow direction algorithm for computing upslope areas from gridded digital elevation models

[1] Gridded digital elevation data, often referred to as DEMs, are one of the most widely available forms of environmental data. Topographic analysis of DEMs can take many forms, but in hydrologic and geomorphologic applications it is typically used as a surrogate for the spatial variation of hydrological conditions (topographic indices) and flow routing. Here we report on a new flow routing algorithm and compare it to three common classes of algorithms currently in widespread use. The advantage of the new algorithm is that unrealistic dispersion on planar or concave hillslopes is avoided, whereas multiple flow directions are allowed on convex hillslopes. We suggest that this new triangular multiple flow direction algorithm (MD1) is more appropriate for a range of flow routing and topographic index applications.

Prediction uncertainty of conceptual rainfall–runoff models caused by problems in identifying model parameters and structure

Abstract The uncertainties arising from the problem of identifying a representative model structure and model parameters in a conceptual rainfall-runoff model were investigated. A conceptual model, the HBV model, was applied to the mountainous Brugga basin (39.9 km”) in the Black Forest, southwestern Germany. In a first step, a Monte Carlo procedure with randomly generated parameter sets was used for calibration. For a ten-year calibration period, different parameter sets resulted in an equally good correspondence between observed and simulated runoff. A few parameters were well defined (i.e. best parameter values were within small ranges), but for most parameters good simulations were found with values varying over wide ranges. In a second step, model variants with different numbers of elevation and landuse zones and various runoff generation conceptualizations were tested. In some cases, representation of more spatial variability gave better simulations in terms of discharge. However, good results could...

Wetland occurrence in relation to topography : a test of topographic indices as moisture indicators

Information about the spatial distributions of soil moisture or groundwater levels is needed for aggregation of soil- vegetation-atmosphere-transfer (SVAT) models. The possibility of predicting wetness distributions in catchments from topographic data was investigated using topographic indices, notably the TOPMODEL index. The indices were calculated from commercially available gridded data (50 50 m 2 ) over two areas with contrasting topography: a catchment (Nasten) in the low-relief NOPEX region in southern Sweden and a group of catchments in a hilly area (Kassjoan) in central Sweden. The occurrence of mires, assumed to represent the extreme wetness end of the wetness spectrum, was used as field data. The frequency distributions of topographic indices for mire and non-mires were clearly different in Kassjoan, although there was a large overlapping, whereas the two distributions were very similar in Nasten. Prediction of mires from topographic indices was meaningful only in Kassjoan. Although it gave poor results in terms of fractions of successfully predicted mire cells out of the observed mire cells, the general spatial patterns of mires were fairly well simulated. One important reason for the failure of the indices to predict mires in Nasten, and probably also to predict other wetness classes, is that the spatial resolution in the index calculation was coarser than typical length scales of the topographic features in this catchment, being only a few tenths of meters. The importance of geologic conditions in modifying the topographic control over the wetness is exemplified from the obtained relationship between topographic indices and mire occurrence. # 1999 Published by Elsevier Science B.V. All rights reserved.

Plant Species Numbers Predicted by a Topography-based Groundwater Flow Index

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.

Multi-criterial validation of TOPMODEL in a mountainous catchment

The need for powerful validation methods for hydrological models including the evaluation of internal stages and spatially distributed simulations has often been emphasized. In this study a multi-criterial validation scheme was used for validation of TOPMODEL, a conceptual semi-distributed rainfall–runoff model. The objective was to test TOPMODELs capability of adequately representing dominant hydrological processes by simple conceptual approaches. Validation methods differed in the type of data used, in their target and in mode. The model was applied in the humid and mountainous Brugga catchment (40 km2) in south-west Germany. It was calibrated by a Monte Carlo method based on hourly runoff data. Additional information for validation was derived from a recession analysis, hydrograph separation with environmental tracers and from field surveys, including the mapping of saturated areas. Although runoff simulations were satisfying, inadequacies of the model structure compared with the real situation with regard to hydrological processes in the study area were found. These belong mainly to the concept of variable contributing areas for saturation excess overland flow and their dynamics, which were overestimated by the model. The simple TOPMODEL approach of two flow components was found to be insufficient. The multi-criterial validation scheme enables not only to demonstrate limitations with regard to process representation, but also to specify where and why these limitations occur. It may serve as a valuable tool for the development of physically sound model modifications. Copyright

Land-cover impacts on streamflow: a change-detection modelling approach that incorporates parameter uncertainty

Abstract The effect of land-use or land-cover change on stream runoff dynamics is not fully understood. In many parts of the world, forest management is the major land-cover change agent. While the paired catchment approach has been the primary methodology used to quantify such effects, it is only possible for small headwater catchments where there is uniformity in precipitation inputs and catchment characteristics between the treatment and control catchments. This paper presents a model-based change-detection approach that includes model and parameter uncertainty as an alternative to the traditional paired-catchment method for larger catchments. We use the HBV model and data from the HJ Andrews Experimental Forest in Oregon, USA, to develop and test the approach on two small (<1 km2) headwater catchments (a 100% clear-cut and a control) and then apply the technique to the larger 62 km2 Lookout catchment. Three different approaches are used to detect changes in stream peak flows using: (a) calibration for a period before (or after) change and simulation of runoff that would have been observed without land-cover changes (reconstruction of runoff series); (b) comparison of calibrated parameter values for periods before and after a land-cover change; and (c) comparison of runoff predicted with parameter sets calibrated for periods before and after a land-cover change. Our proof-of-concept change detection modelling showed that peak flows increased in the clear-cut headwater catchment, relative to the headwater control catchment, and several parameter values in the model changed after the clear-cutting. Some minor changes were also detected in the control, illustrating the problem of false detections. For the larger Lookout catchment, moderately increased peak flows were detected. Monte Carlo techniques used to quantify parameter uncertainty and compute confidence intervals in model results and parameter ranges showed rather wide distributions of model simulations. While this makes change detection more difficult, it also demonstrated the need to explicitly consider parameter uncertainty in the modelling approach to obtain reliable results. Citation Seibert, J. & McDonnell, J. J. (2010) Land-cover impacts on streamflow: a change-detection modelling approach that incorporates parameter uncertainty. Hydrol. Sci. J. 55(3), 316–332.

Continuous long-term measurements of soil-plant-atmosphere variables at a forest site

It is a major challenge in modern science to decrease the uncertainty in predictions of global climate change. One of the largest uncertainties in present-day global climate models resides with the understanding of processes in the soil-vegetationatmosphere-transfer (SVAT) system. Continuous, long-term data are needed in order to correctly quantify balances of water, energy and CO2 in this system and to correctly model it. It is the objective of this paper to demonstrate how a combined system of existing sensor, computer, and network technologies could be set up to provide continuous and reliable long-term SVAT-process data from a forested site under almost all environmental conditions. The Central Tower Site (CTS) system was set up in 1993‐1994 in a 25 m high boreal forest growing on a highly heterogeneous till soil with a high content of stones and blocks. It has successfully monitored relevant states and fluxes in the system, such as atmospheric fluxes of momentum, heat, water vapour and CO2, atmospheric profiles of temperature, water vapour, CO2, short-and long-wave radiation, heat storage in soil and trees, sap-flow and a variety of ecophysiological properties, soil-water contents and tensions, and groundwater levels, rainfall and throughfall. System uptime has been more than 90% for most of its components during the first 5 years of operation. Results from the first 5 years of operation include e.g., budgets for energy, water and CO2, information on important but rarely occurring events such as evaporation from snow-covered canopies, and reactions of the forest to extreme drought. The carbon budget shows that the forest may be a sink of carbon although it is still growing. The completeness of the data has made it possible to test the internal consistency of SVAT models. The pioneering set-up at the CTS has been adopted by a large number of SVAT-monitoring sites around the world. Questions concerning tower maintenance, long-term calibration plans, maintenance of sensors and data-collection system, and continuous development of the computer network to keep it up to date are, however, only partly of interest as a research project in itself. It is thus difficult to get it funded from usual researchfunding agencies.

Robust changes and sources of uncertainty in the projected hydrological regimes of Swiss catchments

Projections of discharge are key for future water resources management. These projections are subject to uncertainties, which are difficult to handle in the decision process on adaptation strategies. Uncertainties arise from different sources such as the emission scenarios, the climate models and their postprocessing, the hydrological models, and the natural variability. Here we present a detailed and quantitative uncertainty assessment, based on recent climate scenarios for Switzerland (CH2011 data set) and covering catchments representative for midlatitude alpine areas. This study relies on a particularly wide range of discharge projections resulting from the factorial combination of 3 emission scenarios, 10–20 regional climate models, 2 postprocessing methods, and 3 hydrological models of different complexity. This enabled us to decompose the uncertainty in the ensemble of projections using analyses of variance (ANOVA). We applied the same modeling setup to six catchments to assess the influence of catchment characteristics on the projected streamflow, and focused on changes in the annual discharge cycle. The uncertainties captured by our setup originate mainly from the climate models and natural climate variability, but the choice of emission scenario plays a large role by the end of the 21st century. The contribution of the hydrological models to the projection uncertainty varied strongly with catchment elevation. The discharge changes were compared to the estimated natural decadal variability, which revealed that a climate change signal emerges even under the lowest emission scenario (RCP2.6) by the end of the century. Limiting emissions to RCP2.6 levels would nevertheless reduce the largest regime changes by the end of the century by approximately a factor of two, in comparison to impacts projected for the high emission scenario SRES A2. We finally show that robust regime changes emerge despite the projection uncertainty. These changes are significant and are consistent across a wide range of scenarios and catchments. We propose their identification as a way to aid decision making under uncertainty.