Preface
Water resources and climate change processes
There is global concern for water resources as we begin the twenty ®rst century. This is in light of the growing likelihood of climate change [9,11], which if realised may aect the global distribution of precipita-tion and hence water resources. Europe may be con-sidered as representative of the changing patterns on earth, aecting hydrological and climate processes. In Northern Europe, it is now documented [1,22,25] that annual precipitation is increasing, while in some areas of Southern Europe it appears that annual precipitation is decreasing. In Eastern Europe there are con¯icting sig-nals, with some areas experiencing increased precipita-tion while others areas show decreases. The cause of increasing precipitation may be related to the changing patterns of the North Atlantic Oscillation and perhaps also to other teleconnection phenomena. The eects of changing precipitation patterns are increases in fre-quency of ¯ooding or droughts, and hence changes in the availability of river and groundwater resources. Furthermore, there may be impacts on the other com-ponents of the energy and water cycle [3,4]. Increases in ¯ood ¯ows and decreases in low ¯ows appear to be as-sociated with climate warming [11,22]. The impact of temperature and precipitation increases on evaporation, cloud cover and on the amount and distribution of soil moisture remains unclear. Furthermore the feedbacks between hydrological and climate processes are less well explored. While we may have some general under-standing of the impacts of climate changes over the continents, our understanding of the impacts at the re-gional scale is limited. It is prudent to investigate not just the physical manifestations of change, (e.g., the degree of warming) but also the processes of change (e.g., evapotranspiration, stream¯ow, soil moisture and process feedbacks).
The clear research and graduate education needs re-lated to this group of problems motivated the call for the Advanced Summer School. The European Union sup-ported the summer school on Water Resources and Climate Change Processes, which was held at University College Cork, Ireland from 15 June±25 June 1997. This special issue reports to the broader community many of the ®ndings communicated during this event.
The most powerful tools available with which to as-sess future climates are coupled models which include 3-D representation of the atmosphere, ocean, cryosphere
and the land surface. General Circulation Models (GCMs) are most accurate/relevant at large space scales (hemisphericale or continental) and less robust at re-gional scales [11,16]. The ®rst two contributions in this special issue [2,15] use GCMs to show the potential impacts of increasing greenhouse gases on climate sys-tem variables. Simulations with the MPI model (Max Planck Institute, Hamburg) of [2], assuming further in-creases of anthropogenic greenhouse gases, show clear trends in precipitation and temperature. The trends that may signi®cantly aect human activity in Eurasia in-clude: a further increase of the sea level of the Caspian Sea and less water in the Rhine and Danube. Ref. [15] shows that models used to compute scenarios of future climate change suer from important uncertainties which render illusive the detailed description of regional impacts. He describes the manifestation of water vapour and cloud feedbacks in present GCMs and shows that satellite data may constitute an important source of data to improve model eciency.
Several observations and GCM modelling results in-dicate an increase in precipitation intensity, suggesting a possibility for more extreme rainfall events [5,10±12,24]. Two papers in this special issue [13,14] investigate the eect of climate change on precipitation. Katz [3] uses extreme value theory for the maximum of a time series of daily rainfall to examine how the eective return period for extreme events would change as the param-eters of the chain-dependent process changes. This paper shows that the extreme events are most sensitive to the scale parameter of the intensity distribution. Kiely [14] examined long-term observations of precipitation and stream¯ow in Ireland and found that the daily, monthly and annual averages of both precipitation and stream-¯ow increased signi®cantly since 1975. Extreme values of precipitation and stream¯ow have also increased. The increases were shown to be associated with simultaneous changes in the North Atlantic Oscillation index in 1975, causing an increased westerly circulation over the island. In CO2 doubling experiments there is evidence of an enhanced hydrologic cycle [3]. This translates into prospects for more severe droughts and/or ¯oods in some places and less severe droughts and/or ¯oods in other places. Increasing precipitation may lead to more ¯ood days and longer ¯ood events with larger ¯ood volumes [7,18]. Ref. [8] in this special issue, use a simple Advances in Water Resources 23 (1999) 101±103
soil-moisture accounting model with a small number of independent and physically based parameters to explore to sensitivity of runo to climate change for three dif-ferent climates. Sensitivity factors were calculated for the various cases and their relationships with climate conditions and soil conditions were explored. Threshold values were determined for each climate type and these correspond to situations in which the soil is momentarily saturated once during the seasonal cycle but does not remain saturated. For humidity ratios greater than a speci®c threshold, the sensitivity of runo to precipita-tion increases abruptly.
Sensitivity studies with GCMs have shown that the treatment of land surface processes in climate models has major eects on the model climate, especially near the land surface [6,16,21]. Important elements of the land surface schemes are their storage reservoirs and their mechanisms for the exchange with the atmosphere of water and thermal energy. Water storage and its time history in the soil reservoir is the key to advancing the modelling eort of soil-vegetation-atmosphere-transfer (SVAT) schemes. Mengelkamp et al. [17], in this special issue, describe an SVAT model, with emphasis on the soil processes linking the hydrologic and atmospheric system. The model (SEWAB) represents very well the sensible and latent heat ¯uxes of the FIFE data set. Reasonable representation of surface runo was pro-duced over the long term, but the high frequency surface runo events were less well represented.
Watershed models can be used for watershed impact studies to understand the impact of climate change scenarios. Szilagyi and Parlange [23], in this special is-sue, present a semi-distributed watershed model that can be used for watershed impact studies. A signi®cant contribution of this model is the inclusion of a physically based groundwater±stream interaction component. Combining climate scenarios with regional hydrologic models have the potential to be applied to climate change studies, helping to further advance our under-standing of stream¯ow process at regional scales. Suc-cess in this area [9] should lead to better de®nition of available water resources (river ¯ows and groundwater). The understanding of the dynamical water movement within the unsaturated zone is still beyond our complete understanding. The complexities of this zone with its pores and macropores of unknown size and varying distribution, is a daunting challenge. However, recent progress in ®eld instrumentation allowing high fre-quency soil moisture time series and remote sensing of surface soil moisture are likely to lead to better under-standing. Soil moisture dynamics in space and time constitutes a most important process in the study of the hydrologic phenomena and soil-atmosphere interaction [20]. Parlange et al. [19], in this special issue, describe an improved solution of RichardsÕ equation which im-proves the understanding of the physical processes of
in®ltration and ponding. The approach is applied to analyse the standard hydrologic tool of Time Com-pression Approximation (TCA). The new approximate method can be used directly analyse in®ltration data or assess the reliability of other techniques.
Acknowledgements
The EU Environment and Climate Programme funded the Advanced Study Course. We thank Panagi-otis Balabanis and his colleagues at the EU for their support. Several individuals contributed to the success-ful management of the course particularly Charles Do-lan, Anne-Marie Aherne and Margaret Murphy. Special thanks also to Markus Pahlow and Corinna Moehrlen for help in preparing the documentation, Abstracts and Special Issues. We would like to gratefully thank all the reviewers. We extend our very special thanks to all the speakers and to Professor Con Cunnane and Professor Philip OÕKane for their support. Finally we would like to thank Majid Hassanizadeh, Editor of Advances in Water Resources, who encouraged the production of this Special Issue.
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Gerard Kiely Department of Civil and Environmental Engineering, University College Cork, Cork Ireland E-mail: gkiely@ucc.ie
Marc Parlange Geography and Environmental Engineering, Johns Hopkins University, Baltimore, MD, USA E-mail: mbparlange@jhu.edu
John Albertson Environmental Sciences, University of Virginia, Charlottesville, VA, USA E-mail: jdalbertson@virginia.edu
