Howard S. Wheater

Ensemble predictions of runoff in ungauged catchments

A new approach to regionalization of conceptual rainfall-runoff models is presented on the basis of ensemble modeling and model averaging. It is argued that in principle, this approach represents an improvement on the established procedure of regressing parameter values against numeric catchment descriptors. Using daily data from 127 catchments in the United Kingdom, alternative schemes for defining prior and posterior likelihoods of candidate models are tested in terms of accuracy of ungauged catchment predictions. A probability distributed model structure is used, and alternative parameter sets are identified using data from each of a number of gauged catchments. Using the models of the 10 gauged catchments most similar to the ungauged catchment provides generally the best results and performs significantly better than the regression method, especially for predicting low flows. The ensemble of candidate models provides an indication of uncertainty in ungauged catchment predictions, although this is not a robust estimate of possible flow ranges, and frequently fails to encompass flow peaks. Options for developing the new method to resolve these problems are discussed.

Modelling of British rainfall using a random parameter Bartlett-Lewis rectangular pulse model

Abstract The performance of a stochastic model, in which Poisson cluster processes represent the arrival of rain cells within a rainstorm, is assessed for hourly point rainfall data from Birmingham, UK. The Bartlett-Lewis Rectangular Pulse Model is extended to include random cell duration, and its performance is compared with that of the basic model analysed previously. Both models perform well in reproducing statistics of the rainfall depth distribution at different time-scales for individual months, but the randomised model also enables a good reproduction of the proportions of dry periods of different durations. Performance is improved by an optimisation method which addresses issues of parameter identification. However, some deficiencies remain with respect to extreme values. Finally, the seasonal variability of parameters is represented on a daily, rather than monthly basis, with comparable results.

Progress in and prospects for fluvial flood modelling

Recent floods in the UK have raised public and political awareness of flood risk. There is an increasing recognition that flood management and land–use planning are linked, and that decision–support modelling tools are required to address issues of climate and land–use change for integrated catchment management. In this paper, the scientific context for fluvial flood modelling is discussed, current modelling capability is considered and research challenges are identified. Priorities include (i) appropriate representation of spatial precipitation, including scenarios of climate change; (ii) development of a national capability for continuous hydrological simulation of ungauged catchments; (iii) improved scientific understanding of impacts of agricultural land–use and land–management change, and the development of new modelling approaches to represent those impacts; (iv) improved representation of urban flooding, at both local and catchment scale; (v) appropriate parametrizations for hydraulic simulation of in–channel and flood–plain flows, assimilating available ground observations and remotely sensed data; and (vi) a flexible decision–support modelling framework, incorporating developments in computing, data availability, data assimilation and uncertainty analysis.

Improvements to the modelling of British rainfall using a modified Random Parameter Bartlett-Lewis Rectangular Pulse Model

Abstract The Random Parameter Bartlett-Lewis Rectangular Pulse Model (RPBLRPM) was shown in a previous paper to provide generally good results when applied to a 38.5 year record of hourly rainfall from the Elmdon raingauge, Birmingham, UK: second-order properties of the depth and the interevent time distributions and the proportion of dry periods for different time-scales were on the whole well reproduced by the optimized parameters. However, two main problems arose, namely an overestimation of the autocorrelations for daily data in particular, and relatively poor performance in reproducing the extreme values of the depth distribution. The first of these problems is here addressed by superposition of a jitter process on the rectangular pulse, thus providing a more realistic representation of the continuous time process. Applying the jitter to each rainfall cell appears more effective than applying it to the instantaneous rainfall depth and enables a more satisfactory reproduction of the autocorrelations. The use of a gamma variable to replace the exponential distribution of the cell intensity provides a good reproduction of the extreme rainfall depths at different time-scales. Moreover, the values of the parameters providing an optimal reproduction of the extreme values of the depth distribution are in most cases close to those obtained by optimizing on the interevent time features, which provides a method for estimation and ensures that the two objectives do not conflict.

Multivariate rainfall disaggregation at a fine timescale

[1] A methodology for spatial-temporal disaggregation of rainfall is proposed. The methodology involves the combination of several univariate and multivariate rainfall models operating at different timescales, in a disaggregation framework that can appropriately modify outputs of finer timescale models so as to become consistent with given coarser timescale series. Potential hydrologic applications include enhancement of historical data series and generation of simulated data series. Specifically, the methodology can be applied to derive spatially consistent hourly rainfall series in rain gages where only daily data are available. In addition, in a simulation framework the methodology provides a way to take simulations of multivariate daily rainfall (incorporating spatial and temporal nonstationarity) and generate multivariate fields at fine temporal resolution. The methodology is tested via a case study dealing with the disaggregation of daily historical data of five rain gages into hourly series. Comparisons show that the methodology results in good preservation of important properties of the hourly rainfall process such as marginal moments, temporal and spatial correlations, and proportions and lengths of dry intervals as well as a good reproduction of the actual hyetographs.

Flood hazard and management: a UK perspective

This paper discusses whether flood hazard in the UK is increasing and considers issues of flood risk management. Urban development is known to increase fluvial flood frequency, hence design measures are routinely implemented to minimize the impact. Studies suggest that historical effects, while potentially large at small scale, are not significant for large river basins. Storm water flooding within the urban environment is an area where flood hazard is inadequately defined; new methods are needed to assess and manage flood risk. Development on flood plains has led to major capital expenditure on flood protection, but government is attempting to strengthen the planning role of the environmental regulator to prevent this. Rural land use management has intensified significantly over the past 30 years, leading to concerns that flood risk has increased, at least at local scale; the implications for catchment-scale flooding are unclear. New research is addressing this issue, and more broadly, the role of land management in reducing flood risk. Climate change impacts on flooding and current guidelines for UK practice are reviewed. Large uncertainties remain, not least for the occurrence of extreme precipitation, but precautionary guidance is in place. Finally, current levels of flood protection are discussed. Reassessment of flood hazard has led to targets for increased flood protection, but despite important developments to communicate flood risk to the public, much remains to be done to increase public awareness of flood hazard.

An Analysis of Daily Maximum Wind Speed in Northwestern Europe Using Generalized Linear Models

Abstract The basic climatological pattern and recent trends in daily maximum wind speed (DMWS) for the region 47.5°–65°N and 12.5°W–22.5°E are studied using gamma distributions within a generalized linear model. Between 1958 and 1998, DMWS has increased over the ocean in winter but weakened over continental Europe in summer. Large-scale circulation changes such as those of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) account for the strengthening wind over the ocean. Global warming may have impacted the regional wind climate. In particular, Southern Hemisphere temperature exhibits a significant effect on the distinct oceanic and continental trends in DMWS. It is suggested that the steady warming of the Southern Hemisphere during the last few decades may have forced the North Atlantic storm track to shift in such a way that storms are enhanced toward the northwestern oceanic area, but weakened throughout most of the European continent.

Developing interdisciplinary science for integrated catchment management : the UK lowland catchment research (LOCAR) programme

Across the European Union, the Water Framework Directive is a major driver for change in river basin management. However, its focus on integrated management and, in particular, on ecological quality raises major scientific and technical questions. In the UK, the focus of experimental hydrology has been on the uplands, and at small catchment scale (< 10 km2), whereas major management pressures lie in the lowlands, and for catchment management units of about 300–400 km2. Particular problems arise for permeable lowland catchments: the scientific understanding of the major UK aquifers (the Chalk and the Triassic Sandstone) is poor, and tools for the integrated modelling of surface water–groundwater interactions are limited. In response to these factors, the LOwland CAtchment Research programme (LOCAR) was conceived. A major objective of the programme is to develop new interdisciplinary science and improved modelling tools to meet the challenges of integrated catchment management. The paper describes the research programme and addresses the issues raised in designing and implementing a major interdisciplinary research initiative.

Water security in the Canadian Prairies: science and management challenges

In this paper, we discuss the multiple dimensions of water security and define a set of thematic challenges for science, policy and governance, based around cross-scale dynamics, complexity and uncertainty. A case study of the Saskatchewan River basin (SRB) in western Canada is presented, which encompasses many of the water-security challenges faced worldwide. A science agenda is defined based on the development of the SRB as a large-scale observatory to develop the underpinning science and social science needed to improve our understanding of water futures under societal and environmental change. We argue that non-stationarity poses profound challenges for existing science and that new integration of the natural sciences, engineering and social sciences is needed to address decision making under deep uncertainty. We suggest that vulnerability analysis can be combined with scenario-based modelling to address issues of water security and that knowledge translation should be coupled with place-based modelling, adaptive governance and social learning to address the complexity uncertainty and scale dynamics of contemporary water problems.

Water security and the science agenda

The freshwater environment is facing unprecedented global pressures. Unsustainable use of surface and groundwater is ubiquitous. Gross pollution is seen in developing economies, nutrient pollution is a global threat to aquatic ecosystems, and flood damage is increasing. Droughts have severe local consequences, but effects on food can be global. These current pressures are set in the context of rapid environmental change and socio-economic development, population growth, and weak and fragmented governance. We ask what should be the role of the water science community in addressing water security challenges. Deeper understanding of aquatic and terrestrial environments and their interactions with the climate system is needed, along with trans-disciplinary analysis of vulnerabilities to environmental and societal change. The human dimension must be fully integrated into water science research and viewed as an endogenous component of water system dynamics. Land and water management are inextricably linked, and thus more cross-sector coordination of research and policy is imperative. To solve real-world problems, the products of science must emerge from an iterative, collaborative, two-way exchange with management and policy communities. Science must produce knowledge that is deemed to be credible, legitimate, and salient by relevant stakeholders, and the social process of linking science to policy is thus vital to efforts to solve water problems. The paper shows how a large-scale catchment-based observatory can be used to practice trans-disciplinary science integration and address the Anthropocenes water problems.