Archive Project News

  • WATCH Technical Report Number 40: Multi-model analysis of drought at the global scale: differences in hydrological drought between the first and the second part of the 20th Century Global hydrological drought for the twentieth century has been investigated based on the simulated total runoff (sum of the surface and sub-surface runoff) of two global hydrological models (WaterGAP and GWAVA), two land surface models (HTESSEL and Orchidee), and an ensemble mean derived from six global models. Global hydrological drought trend (two-tailed t-test, Mann-Kendall non parametric test) was analysed by splitting the 20th century in to the first part (1906-1957) and the second part (1958-2000) based on the different approaches used to generate the WATCH meteorological forcing data. Trend shifts in simulated total runoff between the two periods of the century are observed in nearly half of the land surface cells. Sliding time windows were used to determine thresholds to smooth the effect of these trend shifts and also to capture inter-decadal climatic variability. Percentage of the globe and the continents in drought was computed for the whole simulation period at a monthly time step for each model. Most of the world wide droughts, that are often associated with the ENSO phenomena, are identified by the four models as well as the multi-model ensemble mean though there exist differences in percentage of area in drought among models. Inter-comparing the two periods of the century, decreasing trends are observed in area of the globe in drought for the first part (1906 -1958) of the century, whereas increasing trends are observed for the second part(1958-2000) of the century among all models.
    Estifanos, S.T., van Huijgevoort, M.H.J., Hazenberg, P., van Lanen, H.A.J. & Weedon, G.P., August 25, 2011

  • WATCH Technical Report Number 39: Reference evapotranspiration with radiation-based and temperature-based method - impact on hydrological drought using WATCH Forcing Data Two different methods to calculate reference evapotranspiration were applied and resulting drought characteristics across the globe were compared. WATCH Forcing Data (WFD) have been used as input. One method is the so-called radiation based (ET0rad), and is the well-known Penman-Monteith equation with, among others, incoming short wave radiation as a variable. The other method is so-called temperature-based (ET0temp), the radiation term is replaced by an approximation of the radiation based on minimum and maximum air temperature. After calculating reference evapotranspiration ET0 with both methods, it is fed into a conceptual hydrological model that combines a soil water balance and a simple lumped groundwater model. The model generates daily actual evapotranspiration (ETA), groundwater recharge and discharge. Eventually, the simulated groundwater discharge was used to define periods of drought with a Variable Threshold (VT) method. Drought characteristics, like length and deficit, were identified. The model using the two different ET0 series, was applied to about 1500 cells (land grids) that well represent the five Koeppen-Geiger major climates across the world. There are clear differences between the two methods, and generally the radiation-based method leads to higher ET0 than the temperature-based method. As expected, the differences between ETArad and ETAtemp are significantly smaller than for the reference evapotranspiration. In general the radiation-based method still leads to higher ETA than the temperature-based method. Eventually, the different drought characteristics are compared. There are remarkable differences in drought characteristics, especially in deficit volume and intensity, but in general the differences are within ranges found in other literature that describes the impact of hydrological models or datasets with different diurnal forces on hydrological drought. The D and E-climates (snow-affected climates) show the largest differences.
    Melsen, L.A., van Lanen, H.A.J. Wanders, N., van Huijgevoort, M.H.J. & Weedon, G.P., July 27, 2011

  • WATCH Technical Report Number 38: Including climate feedbacks in regional water resource assessments Two important changes to the meteorological conditions can occur as a result of a change in the land-surface state: the precipitation can change (Koster et al, 2004, Wang et al, 2007) and the evaporative demand can change (Ek and Holtslag, 2004, Schubert et al, 2004). Precipitation is particularly difficult to predict as it can be affected by large-scale weather patterns or complex processes such as mesoscale circulations or convective processes. The physics involved and the scale and complexity of the processes means that complex numerical atmospheric models are usually necessary to quantify impact of the land surface on precipitation, although a simple, analytical model can sometimes be used to assess the effect of the land- surface state on the likelihood of triggering convective precipitation (e.g., De Ridder, 1997). Changes in the evaporative demand are equally important. For instance, Cai et al (2009) have demonstrated the role that the land-atmosphere feedbacks have had on the recent Australian drought. The feedback link between the land surface and the evaporative demand is through the Planetary Boundary Layer (PBL) and this feedback process lends itself to a fairly simple analysis. A framework to assess the impact of change in the land surface on the evaporative demand, and subsequent available regional water resources is proposed in this report.
    Eleanor Blyth and Cor Jacobs, July 20, 2011

  • WATCH Technical Report Number 35: Assessment of flood peak simulations by Global Hydrological Models With significant changes to flood frequency anticipated as a result of climate change it becomes important to investigate how global hydrological models process climate forcing data. This study has analysed the variability in extreme precipitation and flood events from three global hydrological models driven by Watch Forcing Data and Echam5 climate model data, in order to assess variation in extreme rainfall-runoff processes, model bias, and the impact of climate change on the probabilistic behaviour of floods. Global spatial patterns of variation in extreme rainfall-runoff processes differ between models, particularly in regions of extreme climate. Flood statistics from hydrological models run with the same forcing data differ significantly. Climate change simulations indicate some general agreement between models in the emerging spatial pattern of future changes to flood variability across Europe; however, some distinct regional and sub-regional differences in magnitude of change and spatial pattern are evident between models.
    James Miller, Thomas Kjeldsen, Christel Prudhomme, July 27, 2011

  • WATCH Technical Report Number 34: Improved hydrological processes in land surface models During the 4 years of WATCH the participating land-surface models progressed in their description of the continental water cycle. In particular 5 models participating in WATCH have been developed further in the course of the project. The main focus of the development of these models were the impact of human activities on the continental water budget. It was the comparison with global hydrological models during WATCH that lead land surface modelling groups to decide that the anthropogenic pressure on the water cycle was a major element missing in their models. Within WATCH some of the land-surface models introduced, for the first time, the impact of human activities on the continental water cycle. Model development and inclusion of anthropogenic influences will allow, in the future, to predict the evolution of the temperature of water bodies. An impact of climate change which will have important consequences for the ecology and industrial use of water bodies.
    Jan Polcher, Nathalie Bertrand, Hester Biemans, Douglas B. Clark, Marina Floerke, Nicola Gedney, Dieter Gerten, Tobias Stacke, Michelle Van Vliet, Frank Voss, July 27, 2011

  • WATCH Technical Report Number 33: Objective low flow catalogue for Europe Regional drought and high flow characteristics vary spatially and temporally throughout Europe, and therefore objective classifications of drought and high flows are preferred in determining the variable regional characteristics of hydrological extremes. This study applies objective methodologies for defining regional droughts (Regional Deficiency Index; RDI) and high flow episodes (Regional High Flow Index; RHFI) to observed daily river flow timeseries from 579 stations across Europe, over the period 1961-2005. Drought and high flow catalogues are presented for 23 homogeneous European regions, allowing an assessment of the regional variation in the extent, duration, seasonality and spatio-temporal evolution of hydrological extremes in Europe.
    Simon Parry, Christel Prudhomme, Jamie Hannaford, Jennifer Williamson, August 24, 2011

  • WATCH Technical Report Number 32: Revisiting GLACE: Understanding the role of the land surface in Land-Atmosphere coupling The Global Land-Atmosphere Coupling Experiment (GLACE) established a method for quantifying and comparing the influence of soil moisture on the atmosphere in AGCMs. The models included in the GLACE intercomparison displayed a wide range in the strength of this influence, with the Met Office Hadley Centre (MOHC) model, HadAM3, being one of the weakest. Applying the GLACE method to a much developed version of the MOHC model, HadGEM3-A, demonstrated this new model has a stronger coupling signal than its predecessor. Although this increased coupling strength cannot be attributed to changes in the land surface representation, the existence of the stronger signal enables an investigation of the signal's dependence on key land surface parameters. The GLACE method is applied to four HadGEM3-A experiment cases, with soil hydraulic parameters specified using two methods of calculation from two different underlying soil texture datasets.
    Ruth Comer and Martin Best, September 02, 2011

  • WATCH Technical Report Number 31: Propagation of Drought through the Hydrological Cycle Drought propagation is the propagation of an anomaly in the climate signal through the terrestrial part of the hydrological cycle. It reflects the processes involved in the development of hydrological drought from (several) meteorological drought(s). In this report, an overview of the results of studies on drought propagation on catchment scale (i.e. the WATCH test basins) and larger scales (i.e. Europe and Malawi) are presented. The methods used are analyses of observations and hydrological modelling results using various RBHMs and LSMs. The objective of this report is to obtain a better understanding of the processes underlying drought propagation. On catchment scale, we conclude that the main features of drought propagation, i.e. pooling, attenuation, lag, and lengthening, are reproduced by all models in all catchments. On larger scales, we conclude that some of the main features of drought propagation, i.e. pooling and lengthening of events, are reproduced in both regions studied. Another important feature of drought propagation, i.e. attenuation of the drought signal, is not captured well by the ensemble mean of LSMs, because storage is not always simulated well on larger scales.
    Anne F. van Loon, Henny A.J. van Lanen, Lena M. Tallaksen, Martin Hanel, Miriam Fendeková, Andrej Machlica, Gonzalo Sapriza, Aristeidis Koutroulis, Marjolein H.J. van Huijgevoort, Jorge Jódar Bermúdez, Hege Hisdal, Ioannis Tsanis, June 28, 2011

  • WATCH Technical Report Number 30: Water Assessment Report for Crete Basins The objective of this report is to analyze and assess the impact of global change to the water resources status for the island of Crete for a range of 24 different scenarios of projected hydro-climatological regime, demand and supply potential. The outcome of the analysis is useful for the comprehension of the role and consequently the priority of certain water resources related infrastructure developments. The quantitative impact of the projected changes on water availability can be substantial especially in a Mediterranean island like Crete.
    Ioannis K. Tsanis, Aristeidis G. Koutroulis / Technical University of Crete, June 17, 2011

  • WATCH Technical Report Number 28: Anthropogenic extreme weather events of the 20th Century Interest in attributing the risk of damaging weather-related events to anthropogenic climate change is increasing. Yet climate models used to study the attribution problem typically do not resolve the weather systems associated with damaging events such as the UK floods of October and November 2000. Occurring during the wettest autumn in England and Wales since records began in 1766, these floods damaged nearly 10,000 properties across that region, disrupted services severely, and caused insured losses estimated at £1.3 billion. Although the flooding was deemed a ‘wake-up call’ to the impacts of climate change at the time, such claims are typically supported only by general thermodynamic arguments that suggest increased extreme precipitation under global warming, but fail to account fully for the complex hydrometeorology associated with flooding. Here we present a multi-step, physically based ‘probabilistic event attribution’ framework showing that it is very likely that global anthropogenic greenhouse gas emissions substantially increased the risk of flood occurrence in England and Wales in autumn 2000.
    Pardeep Pall, Tolu Aina, Dáithí A. Stone, Peter A. Stott, Toru Nozawa, Arno G. J., June 02, 2011