Lena M. Tallaksen

A review of baseflow recession analysis

Abstract Recession analysis is a well-known tool in hydrological analysis. Its application, however, poses many methodical questions, and throughout the literature numerous solutions have been sought. The quantification of the recession curve involves the selection of an analytical expression, derivation of a characteristic recession and optimization of the recession parameters. A major problem is the high variability encountered in the recession behaviour of individual segments. The segments represent different stages in the outflow process, and a physically based short-term or seasonal influence on the recession rate adds to the problem of deriving a characteristic recession. This paper discusses these elements of recession analysis and reviews different ways of characterizing the baseflow recession rate.

On the definition and modelling of streamflow drought duration and deficit volume

Abstract The threshold level approach is used to define drought characteristics, i.e. drought duration and deficit volume from time series of daily streamflow. Three different procedures for pooling dependent droughts are compared: a method based on an inter-event time and volume criterion (IC), a moving average procedure (MA), and a method based on the sequent peak algorithm (SPA). The extreme values of drought duration and deficit volume are analysed using both an annual maximum series (AMS) and a partial duration series (PDS) approach. Two Danish catchments with very different flow regimes were used in the study. The IC and MA methods provided virtually the same sample statistics of the AMS of drought duration and deficit volume for all thresholds considered. The results of the SPA method differed significantly from the other two methods for high thresholds due to the presence of multi-year droughts. For analysis of seasonal droughts the SPA method is restricted to low thresholds. The occurrence of a l...

Estimation of regional meteorological and hydrological drought characteristics: a case study for Denmark

Abstract Information on regional drought characteristics provides critical information for adequate water resource management. This study introduces a method to calculate the probability of a specific area to be affected by a drought of a given severity and demonstrates its potential for calculating both meteorological and hydrological drought characteristics. The method is demonstrated using Denmark as a case study. The calculation procedure was applied to monthly precipitation and streamflow series separately, which were linearly transformed by the Empirical Orthogonal Functions (EOF) method. Denmark was divided into 260 grid-cells of 14×17 km, and the monthly mean and the EOF-weight coefficients were interpolated by kriging. The frequency distributions of the first two (streamflow) or three (precipitation) amplitude functions were then derived. By performing Monte Carlo simulations, amplitude functions corresponding to 1000 years of data were generated. Based on these simulated functions as well as interpolated mean and weight coefficients, long time series of precipitation and streamflow were simulated for each grid-cell. The probability distribution functions of the area covered by a drought and the drought deficit volumes were then derived and combined to produce drought severity–area–frequency curves. These curves allowed an estimation of the probability of an area of a certain extent to have a drought of a given severity, and thereby return periods could be assigned to historical drought events. A comparison of drought characteristics showed that streamflow droughts are less homogeneous over the region, less frequent and last for longer time periods than precipitation droughts.

WATCH: Current Knowledge of the Terrestrial Global Water Cycle

AbstractWater-related impacts are among the most important consequences of increasing greenhouse gas concentrations. Changes in the global water cycle will also impact the carbon and nutrient cycles and vegetation patterns. There is already some evidence of increasing severity of floods and droughts and increasing water scarcity linked to increasing greenhouse gases. So far, however, the most important impacts on water resources are the direct interventions by humans, such as dams, water extractions, and river channel modifications. The Water and Global Change (WATCH) project is a major international initiative to bring together climate and water scientists to better understand the current and future water cycle. This paper summarizes the underlying motivation for the WATCH project and the major results from a series of papers published or soon to be published in the Journal of Hydrometeorology WATCH special collection. At its core is the Water Model Intercomparison Project (WaterMIP), which brings togeth...

Derivation of low flow distribution functions using recession curves

Abstract Recession analysis and low flow frequency analysis have been treated by tradition as separate problems, although both are related to low flow conditions. The shape and analytical expression of the streamflow decay during rainless periods might give an insight into the behaviour of the tails of low flow distribution functions. Furthermore, models for streamflow recession have, as a rule, a more or less straightforward physical background and this might be of help in the interpretation and regionalization of parameters of low flow distribution functions. The present paper is an attempt to combine results from these two separate fields in order to use recession equations to derive theoretical expressions for low flow distribution functions. The derived distribution approach together with the concept of compound distributions are used to deduce expressions for a family of low flow distributions related to linear and non-linear recession models. Approaches for parameter estimation are elaborated and exemplified on low flow data from the Nordic countries.

Distribution of soil moisture and groundwater levels at patch and catchment scales

This study is a contribution to the northern hemisphere climate processes land surface experiment (NOPEX). Its purpose is to investigate the spatial variability of groundwater levels and soil moisture content at different scales in a landscape dominated by boreal forest and till soils, which is characteristic of the Nordic countries. The analysis of data from the NOPEX area are based on a review of previous studies on the spatial distribution of these state variables and their significance for runoff formation. Soil moisture content in the unsaturated zone and depth to the groundwater table show characteristic patterns which are related to the landscape elements (patches) of the drainage basins. Similar behaviour is observed in different parts of the NOPEX region. The variability of average values between areas decreases to a minimum for catchments with size larger than 1 km 2 . It can therefore be concluded that the main part of the spatial variability of soil moisture content and depth to the groundwater level in the till soils of the NOPEX area is found within small drainage basins. Based on a physical description of the soil, distribution functions of soil moisture content conditioned on the depth to the groundwater table have been developed, both for the patch scale and the catchment scale. # 1999 Elsevier Science B.V. All rights reserved.

Spatial and temporal patterns of large‐scale droughts in Europe: Model dispersion and performance

This study explores the performance of a suite of off-line, global (hydrological and land surface) models in mapping spatial and temporal patterns of large-scale hydrological droughts in Europe from simulated runoff in the period 1963–2000. Consistent model behavior was found for annual variability in mean drought area, whereas high model dispersion was revealed in the weekly evolution of contiguous area in drought and its annual maximum. Comparison with nearly three hundred catchment-scale streamflow observations showed an overall tendency to overestimate the number of drought events and hence underestimate drought duration, whereas persistence in drought-affected area (weekly mean) was underestimated, noticeable for one group of models. The high model dispersion in temporal and spatial persistence of drought identified implies that care should be taken when analyzing drought characteristics from only one or a limited number of models unless validated specifically for hydrological drought.

Streamflow Data from Small Basins: A Challenging Test to High-Resolution Regional Climate Modeling

AbstractLand surface models and large-scale hydrological models provide the basis for studying impacts of climate and anthropogenic changes on continental- to regional-scale hydrology. Hence, there is a need for comparison and validation of simulated characteristics of spatial and temporal dynamics with independent observations. This study introduces a novel validation framework that relates to common hydrological design measures. The framework is tested by comparing anomalies of runoff from a high-resolution climate-model simulation for Europe with a large number of streamflow observations from small near-natural basins. The regional climate simulation was performed as a “poor man’s reanalysis,” involving a dynamical downscaling of the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) with the Danish “HIRHAM5” model. For 19 different anomaly levels, two indices evaluate the temporal agreement (i.e., the occurrence and frequency of dry and wet events based on daily anomalies), ...

European-Scale Drought: Understanding Connections between Atmospheric Circulation and Meteorological Drought Indices

AbstractQuantification of large-scale climate drivers of drought is necessary to understand better and manage these spatially extensive and often prolonged natural hazards. Here, this issue is advanced at the continental scale for Europe. Drought events are identified using two indices—the 6-month cumulative standardized precipitation and standardized precipitation evapotranspiration indices (SPI-6 and SPEI-6, respectively)—both calculated using the gridded Water and Global Change (WATCH) Forcing Dataset for 1958–2001. Correlation of monthly time series of the percentage of European area in drought with geopotential height for 1958–2001 indicates that a weakening of the prevailing westerly circulation is associated with drought onset. Such conditions are linked to variations in the eastern Atlantic/western Russia (EA/WR) and North Atlantic Oscillation (NAO) atmospheric circulation patterns. Event-based analysis of the most widespread European droughts reveals that a higher number are identified by the SPE...

Kinematic wave approximations to hillslope hydrological processes in tills.

This work has been carried out within the framework of NOPEX - a NOrthern hemisphere climate Processes land surface EXperiment. Its purpose is to describe the spatial variability of groundwater lev ...