Michael Bruen

Modeling root water uptake in hydrological and climate models

Abstract From 30 September to 2 October 1999 a workshop was held in Gif–sur–Yvette, France, with the central objective to develop a research strategy for the next 3–5 years, aiming at a systematic description of root functioning, rooting depth, and root distribution for modeling root water uptake from local and regional to global scales. The goal was to link more closely the weather prediction and climate and hydrological models with ecological and plant physiological information in order to improve the understanding of the impact that root functioning has on the hydrological cycle at various scales. The major outcome of the workshop was a number of recommendations, detailed at the end of this paper, on root water uptake parameterization and modeling and on collection of root and soil hydraulic data.

A simple model for estimating the sensitivity of runoff to long-term changes in precipitation without a change in vegetation

Abstract Forecasts of changes in precipitation (P) and potential evaporation (PE) can be applied to hydrologic models calibrated on existing conditions to obtain predictions of changes in runoff. This study describes an alternative approach, which uses a simple soil-moisture accounting model with a small number of independent and physically based parameters to explore the sensitivity of runoff to climate change for three simplified climates. The climate types chosen initially are those for which a piecewise analytical solution can be obtained so that computer programmes involving numerical solutions can be verified before being applied to field data. Sensitivity factors are calculated for the various cases and their relationships with climatic conditions and soil conditions are explored. Breakpoint values were determined for each type of climate studied. These correspond to situations in which the soil becomes momentarily saturated once during the seasonal cycle but does not remain saturated for any finite duration. For humidity ratios greater than the breakpoint, the sensitivity of runoff to precipitation increases abruptly. For the climates studied, the sensitivity factor approaches the value of the soil parameter c as the humidity index approaches zero. The other climates studied exhibit the same sensitivity at this limit. A particular feature of the model is that analytical solutions can be determined in many cases to check and confirm the results of the numerical simulations.

Choosing realistic values of indifference, preference and veto thresholds for use with environmental criteria within ELECTRE

Abstract The ELECTRE III outranking model is particularly suited to aiding the choice between project alternatives on the basis of mainly environmental criteria. The model requires values of three criterion thresholds, the indifference threshold (q), the preference threshold (p) and the veto threshold (v). These allow the uncertainties inherent in the criteria valuations to be incorporated into the decision process. There is, at present, a high degree of subjectivity involved in determining these thresholds, which are expressed in terms of the error/uncertainty associated with the valuations of each of the criteria under scrutiny. If, however, the ELECTRE III outranking model is to be used within a formal environmental appraisal system, the thresholds which govern the outranking relationship of one project option over another must take account of the effect on human beings of the difference between any two criterion scores. The authors suggest a new method for applying the standard ELECTRE III model to decision-aid problems within the formal mechanism of environmental impact assessment. This involves a new, more comprehensive approach for specifying realistic limits for p, q and v, within the context of an environmental appraisal, where both criterion error/uncertainty and human sensitivity to differing levels of the criterion are taken into account. Threshold valuations for noise impacts from a highway project are used to illustrate the proposed method.

Modelling stream -aquifer seepage in an alluvial aquifer: an improved loosing-stream package for MODFLOW

Abstract Seepage from a stream, which partially penetrates an unconfined alluvial aquifer, is studied for the case when the water table falls below the streambed level. Inadequacies are identified in current modelling approaches to this situation. A simple and improved method of incorporating such seepage into groundwater models is presented. This considers the effect on seepage flow of suction in the unsaturated part of the aquifer below a disconnected stream and allows for the variation of seepage with water table fluctuations. The suggested technique is incorporated into the saturated code MODFLOW and is tested by comparing its predictions with those of a widely used variably saturated model, SWMS_2D simulating water flow and solute transport in two-dimensional variably saturated media. Comparisons are made of both seepage flows and local mounding of the water table. The suggested technique compares very well with the results of variably saturated model simulations. Most currently used approaches are shown to underestimate the seepage and associated local water table mounding, sometimes substantially. The proposed method is simple, easy to implement and requires only a small amount of additional data about the aquifer hydraulic properties.

Technical assessment and evaluation of environmental models and software

This letter details the collective views of a number of independent researchers on the technical assessment and evaluation of environmental models and software. The purpose is to stimulate debate and initiate action that leads to an improved quality of model development and evaluation, so increasing the capacity for models to have positive outcomes from their use. As such, we emphasize the relationship between the model evaluation process and credibility with stakeholders (including funding agencies) with a view to ensure continued support for modelling efforts.Many journals, including EM&S, publish the results of environmental modelling studies and must judge the work and the submitted papers based solely on the material that the authors have chosen to present and on how they present it. There is considerable variation in how this is done with the consequent risk of considerable variation in the quality and usefulness of the resulting publication. Part of the problem is that the review process is reactive, responding to the submitted manuscript. In this letter, we attempt to be proactive and give guidelines for researchers, authors and reviewers as to what constitutes best practice in presenting environmental modelling results. This is a unique contribution to the organisation and practice of model-based research and the communication of its results that will benefit the entire environmental modelling community. For a start, our view is that the community of environmental modellers should have a common vision of minimum standards that an environmental model must meet. A common vision of what a good model should be is expressed in various guidelines on Good Modelling Practice. The guidelines prompt modellers to codify their practice and to be more rigorous in their model testing. Our statement within this letter deals with another aspect of the issue - it prompts professional journals to codify the peer-review process. Introducing a more formalized approach to peer-review may discourage reviewers from accepting invitations to review given the additional time and labour requirements. The burden of proving model credibility is thus shifted to the authors. Here we discuss how to reduce this burden by selecting realistic evaluation criteria and conclude by advocating the use of standardized evaluation tools as this is a key issue that needs to be tackled.

An efficient and robust method for estimating unit hydrograph ordinates

Abstract The coefficient matrix for the “normal equations” for least-squares estimates of discrete unit hydrographs (DUH) is symmetric Toeplitz. This paper describes how to take advantage of this matrix structure to reduce both the data storage and arithmetic computation requirements of computer programs for estimating DUHs. The method is particularly useful for small computers when memory space is limited or when large amounts of data are involved. The algorithm can be used to provide smoothed least-squares estimates of the DUH using a single extra arithmetic addition. Estimates of DUHs for multiple-event data can easily be calculated. Computer programs are presented for both the single-event and multiple-event case. The utility of the smoothed estimates is illustrated using real data which would give unrealistic ordinary least-squares estimates.

Sensitivity of surface fluxes to the number of layers in the soil model used in GCMs.

Many GCM land surface schemes were recently developed in order to take into account the physical processes of soil water flow by resolving Darcy equation. But numerical resolution of the Darcy equation requires care because of its high degree of non-linearity. This paper presents a sensitivity experiment of the numerically computed surface fluxes to the vertical soil resolution. It is shown that too coarse resolutions drastically affects the ability of the scheme to give physically based representations of moisture and energy fluxes.

Reuse of aluminium-based water treatment sludge to immobilize a wide range of phosphorus contamination : equilibrium study with different isotherm models

Abstract The adsorption equilibrium of a wide range of phosphorus species by an aluminum‐based water treatment sludge (Al‐WTS) was examined in this study. Four kinds of adsorption‐isotherm models, namely Langmuir, Freundlich, Temkin, and Dubinin‐Radushkevich, were used to fit the adsorption equilibrium data. In order to optimize the adsorption‐isotherm model, correlation coefficient (R2) and four error functions were employed to facilitate the evaluation of fitting accuracy. Experiments have demonstrated that the Al‐WTS may be an excellent raw material to adsorb P in a polluted aqueous environment with adsorption ability in the order of KH2PO4 (ortho‐P)>Na(PO3)6 (poly‐P)>C10H14N5O7P · H2O (organic‐P). More importantly, this study provides an entire comparison of the four isotherms in describing the P adsorption behavior. By considering both the standard least‐square based R2 and the results of four error functions analysis, this study reveals that the Freundlich isotherm appears to be the best model to fit the experimental equilibrium data. Langmuir and Temkin isotherms are also good models in current experimental conditions while the Dubinin‐Radushkevich isotherm poorly described the adsorption behavior. The error analysis in this study provides vital evidence to reflect its role in facilitating the optimization in the adsorption isotherm study. Obviously, R2 seems inadequate in optimizing multi‐isotherm models due to its inherent bias resulting from the least‐squares linearization.

A regional examination of episodic acidification response to reduced acidic deposition and the influence of plantation forests in Irish headwater streams

Episodic surface water acidification is common in many regions worldwide; the driving processes are dependent on a variety of physicochemical and climatic characteristics, and acid deposition pressures, which have changed significantly over the last two decades. This study provided a unique opportunity to re-examine the drivers of acidity in an environment of low anthropogenic input. In three geologically distinct acid-sensitive regions of Ireland during 2009-2010, 34 headwater streams were evaluated in peat-dominated catchments draining moorlands without forest, 20-50% (low) forest cover and >50% (high) forest cover. Results indicated episodic acidity/alkalinity loss in headwater streams, despite significant reductions in acid deposition. Both the differences in pH between base and storm-flow (∆pH) and the number of pH events≤5.5 were higher in forested streams. Dissolved organic carbon and inorganic aluminium concentrations were also higher in forested catchments. The primary driver of acidity was strong organic anions, which generally increased with increasing forest cover. Base-cation dilution was also prominent in west and southern regions, while surprisingly chlorine anion acidity from sea-salts had little or no influence on stream acidity. The contributions of excess non-marine sulphate (xSO(4)) and nitrate (NO(3)) to storm-water were low, with no observed increases in xSO(4) with increasing forest cover, although contributions of NO(3) were higher in forested catchments in the east. The results suggest that episodic acidification in Ireland is primarily driven by organic acids. However in peat dominant catchments, plantation forest, climate change and/or reductions in xSO(4) appear to also be having an effect on stream pH from increased DOC, with some forested streams previously unaffected by deposition now showing low pH (<5.5) during storm-flow. As quantified from this study, observed changes in stream acidification in Ireland may provide a better understanding of future chemical responses to declining acid deposition and climate change elsewhere.

Environmental consequences of a power plant shut-down: a three-dimensional water quality model of Dublin Bay.

A hydro-environmental model is used to investigate the effect of cessation of thermal discharges from a power plant on the bathing water quality of Dublin Bay. Before closing down, cooling water from the plant was mixed with sewage effluent prior to its discharge, creating a warmer, less-saline buoyant pollutant plume that adversely affects the water quality of Dublin Bay. The model, calibrated to data from the period prior to the power-plant shut-down (Scenario1), assessed the water quality following its shut-down under two scenarios; (i) Scenario2: continued abstraction of water to dilute sewage effluents before discharge, and (ii) Scnenario3: sewage effluents are discharged directly into the Estuary. Comparison between scenarios was based on distribution of Escherichia coli (E. coli), a main bathing quality indicator. Scenarios1 and 2, showed almost similar E. coli distribution patterns while Scenario3 displayed significantly higher E. coli concentrations due to the increased stratification caused by the lack of prior dilution.