A. M. Ireson

Precipitation downscaling under climate change: Recent developments to bridge the gap between dynamical models and the end user

Precipitation downscaling improves the coarse resolution and poor representation of precipitation in global climate models and helps end users to assess the likely hydrological impacts of climate change. This paper integrates perspectives from meteorologists, climatologists, statisticians, and hydrologists to identify generic end user (in particular, impact modeler) needs and to discuss downscaling capabilities and gaps. End users need a reliable representation of precipitation intensities and temporal and spatial variability, as well as physical consistency, independent of region and season. In addition to presenting dynamical downscaling, we review perfect prognosis statistical downscaling, model output statistics, and weather generators, focusing on recent developments to improve the representation of space-time variability. Furthermore, evaluation techniques to assess downscaling skill are presented. Downscaling adds considerable value to projections from global climate models. Remaining gaps are uncertainties arising from sparse data; representation of extreme summer precipitation, subdaily precipitation, and full precipitation fields on fine scales; capturing changes in small-scale processes and their feedback on large scales; and errors inherited from the driving global climate model.

Hydrogeological processes in seasonally frozen northern latitudes: understanding, gaps and challenges

The groundwater regime in seasonally frozen regions of the world exhibits distinct behavior. This paper presents an overview of flow and associated heat and solute transport processes in the subsurface, from the soil/vadose zone, through groundwater recharge to groundwater discharge processes in these areas. Theoretical developments, field studies and model development are considered. An illustrative conceptual model of the system is presented. From a groundwater perspective, the dominant effect is the extent of hydraulic isolation between the water above and that below the near-surface frozen zone. The spatial and temporal occurrences of this isolation are seasonally variable and may also be modified under a future changing climate. A good qualitative conceptual understanding of the system has been developed over numerous decades of study. A major gap is the inability to effectively monitor processes in the field, particularly unfrozen water content during freezing conditions. Modeling of field-scale behavior represents a major challenge, even while physically based models continue to improve. It is suggested that progress can be made by combining well-designed field experiments with modeling studies. A major motivation for improving quantification of these processes derives from the need to better predict the impacts of a future changing climate.RésuméLe régime de l’eau souterraine dans les régions gelées de façon saisonnière fait apparaître différents comportements. Cet article présente une vue d’ensemble du flux et des processus de transport de chaleur et de solutés en sub-surface, depuis la zone sol/vadose, à travers des processus de recharge-décharge dans ces zones. Des développements théoriques, études de terrain et développement de modèles sont considérés. Un modèle conceptuel est présenté pour illustrer. Si l’on considère l’eau de nappe, l’effet dominant est l’importance de l’isolation hydraulique entre l’eau sur et l’eau sous la sub-surface gelée. Dans le temps et dans l’espace cette isolation varie avec les saisons et pourra aussi être modifiée par un futur changement du climat. Une bonne compréhension conceptuelle qualitative du système a été développée durant de nombreuses décades d’étude. Une lacune majeure est l’impossibilité de contrôler effectivement le processus sur le terrain, particulièrement la teneur en eau libre durant les périodes de gel. La modélisation du comportement à l’échelle du terrain représente un défi majeur, même si les modèles à base physique continuent de s’améliorer. On suggère que des progrès peuvent être faits en combinant des expériences de terrain bien conçues et des études de modélisation. Une motivation majeure pour améliorer la quantification de ces processus dérive du besoin de mieux prévoir les impacts d’un futur changement climatique.ResumenEl régimen de agua subterránea en regiones estacionalmente congeladas del mundo exhibe comportamientos distintivos. Este trabajo presenta un panorama del flujo y los procesos asociados de transporte de calor y soluto en el subsuelo, a partir de la zona vadosa / suelo, a través de los procesos de recarga a la descarga del agua subterránea en estas áreas. Se consideran los desarrollos teóricos, los estudios de campo y los modelos desarrollados. Se presenta un modelo conceptual ilustrativo del sistema. Desde la perspectiva del agua subterránea, el efecto dominante es el grado de aislamiento entre el agua por encima y por debajo de la zona congelada próxima a la superficie. La presencia espacial y temporal de este aislamiento es variable estacionalmente y pueden también ser modificadas bajo un futuro cambio climático. Un buen entendimiento cualitativo conceptual del sistema se desarrolló durante numerosas décadas de estudio. Una dificultad mayor es la incapacidad para monitorear efectivamente los procesos en el campo, particularmente el contenido de agua no congelada durante las condiciones de congelamiento. El comportamiento modelado a escala de campo representa un desafío mayor, aun cuando los modelos de bases físicas pueden seguir mejorando. Se sugiere que se puede avanzar mediante la combinación de experimentos de campo bien diseñados con estudios de modelos. Una motivación mayor para mejorar la cuantificación de estos procesos deriva de la necesidad de predecir mejor los impactos de un futuro cambio climático.摘要世界上季节性冻土区的地下水动态呈现出不同的行为特征。本文对这些地区的地面以下,从土壤/渗流区通过地下水的补给到地下水的排泄过程的水流运动和相关的热、溶质运移过程作了一个概述。本文综合考虑了理论的发展,场地的研究和模型的发展。另外,文中还展示了一个解释说明性的系统概念模型。从地下水的角度来说,占优势的效应是近地表的冻土区上、下的地下水间的水力隔绝程度。时间、空间上这种隔绝的存在是随着季节变化的,可以在未来变化的气候下进行模拟。通过几十年的研究,对季节性冻土系统的较好的定性概念性认识已经建立起来了。一个主要的不足在于无法有效地监测场地的过程,特别是在严寒条件下不冻水的成分。甚至当基于物理的模型在继续向前发展时,场地尺度上的行为模拟仍是一个主要的挑战。文中指出,可以通过将设计精良的场地试验与模型研究相结合来获得突破。提高这些过程的定量化的主要动机来源于更好地对未来气候变化影响的预测的需求。ResumoO regime de águas subterrâneas em regiões do mundo sazonalmente geladas exibe comportamento distinto. Este artigo apresenta uma visão geral dos processos subsuperficiais de fluxo e de transporte de calor e de soluto associados, desde o solo/zona vadosa, através dos processos de recarga de águas subterrâneas, até à sua descarga. São considerados desenvolvimentos teóricos, estudos de campo e desenvolvimento de modelos. É apresentado um modelo concetual ilustrativo do sistema. Da perspetiva das águas subterrâneas, o efeito dominante é a extensão do isolamento hidráulico entre as águas acima e abaixo da zona gelada próxima da superfície. As ocorrências espaciais e temporais destes isolamentos são variáveis sazonalmente e podem também ser modificadas num clima futuro em mudança. Ao longo de numerosas décadas de estudo, desenvolveu-se uma boa compreensão concetual qualitativa do sistema. Uma grande lacuna é a incapacidade de monitorizar eficazmente os processos no terreno, particularmente o teor de água não solidificada durante condições de congelação. A modelação do comportamento à escala do terreno representa um desafio maior, mesmo enquanto os modelos físicos continuam a melhorar. Sugere-se que se pode conseguir uma boa progressão combinando experiências de terreno bem concebidas com estudos de modelação. Uma motivação principal para melhorar a quantificação destes processos deriva da necessidade de prever melhor os impactes de alterações climáticas futuras.

Recent advances in modelling nitrate transport in the Chalk unsaturated zone

The Chalk unsaturated zone is crucial in controlling the delivery of nitrate to Chalk streams and groundwater abstraction wells. In this paper, results from a dual-permeability numerical model of the Chalk unsaturated zone are used to illustrate the relative roles of matrix and fracture flow. A major challenge arises in representing the Chalk unsaturated zone within catchment-scale models for nutrient management. These have generally been based on simple conceptual reservoirs or compartments to represent soils and groundwater. A more appropriate conceptualization has recently been developed and applied to the Lambourn catchment within a catchment-scale nutrient model (INCA-Chalk). Preliminary results from this work are discussed, which clearly illustrate the decadal time scales that need to be considered in the context of nutrient management and the Water Framework Directive.

Advances in modelling groundwater behaviour in Chalk catchments

Abstract Groundwater in Chalk catchments is a major resource that also helps support internationally important habitats and ecosystems. Its dual porosity and dual permeability properties, coupled with large-scale structural features (such as hard rock layers and marls), produce a highly complex hydrogeological system. Recent impacts from groundwater flooding as well as vulnerability to drought have raised questions over the ability of traditional approaches to model these aquifers. Current work on near-surface hydrological processes has highlighted the importance of the soil and weathered zone for controlling recharge rates. In addition, karst-like features, sedimentary deposits and valley bottom processes govern stream–aquifer interaction and present a challenge in their representation in any modelling system. Methods that have, and are being, developed to incorporate these features, and their use in modelling Chalk catchments, are described. These are required in order to address major challenges, such as groundwater flooding and drought impacts, both of which could become more frequent and intense as a result of climate change.

Water Vapor Transport in Soils from a Pervaporative Irrigation System

AbstractA novel method for irrigation with saline water uses a polymer membrane, formed into a tube, to treat and distribute the water simultaneously. The flux of water across the membrane occurs by the process of pervaporation, during which a phase change from liquid to vapor occurs. Thus, water arrives in the soil in the vapor phase. The experimental results presented in this paper demonstrate that, contrary to previous assumptions, soil vapor flows are a significant transport mechanism during pervaporative irrigation in dry soils. The soil water sorption properties affect the rate of condensation in the soil, which in turn affects both the water distribution in the soil and the loss of water vapor to the atmosphere. The flux from the tube becomes limited by high humidities adjacent to the external surface of the membrane. Thus, enhancing condensation in the soil or increasing diffusion through the soil increases flux from the system. These findings highlight the need to consider how plants might intera...

Field-scale water balance closure in seasonally frozen conditions

Hydrological water balance closure is a simple concept, yet in practice it is uncommon to measure every significant term independently in the field. Here we demonstrate the degree to which the field-scale water balance can be closed using only routine field observations in a seasonally frozen prairie pasture field site in Saskatchewan, Canada. Arrays of snow and soil moisture measurements were combined with a precipitation gauge and flux tower evapotranspiration estimates. We consider three hydrologically distinct periods: the snow accumulation period over the winter, the snowmelt period in spring, and the summer growing season. In each period, we attempt to quantify the residual between net precipitation (precipitation minus evaporation) and the change in field-scale storage (snow and soil moisture), while accounting for measurement uncertainties. When the residual is negligible, a simple 1-D water balance with no net drainage is adequate. When the residual is non-negligible, we must find additional processes to explain the result. We identify the hydrological fluxes which confound the 1-D water balance assumptions during different periods of the year, notably blowing snow and frozen soil moisture redistribution during the snow accumulation period, and snowmelt runoff and soil drainage during the melt period. Challenges associated with quantifying these processes, as well as uncertainties in the measurable quantities, caution against the common use of water balance residuals to estimate fluxes and constrain models in such a complex environment.