Drought

Science and Policy

Ana Iglesias, Dionysis Assimacopoulos, Henny A.J. Van Lanen, Ana Iglesias, Dionysis Assimacopoulos, Henny A.J. Van Lanen

• Publisher: Wiley-Blackwell
• Year: 2018
• Print ISBN: 9781119017202
• eBook ISBN: 9781119017172
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geography
• Index: Geography

Comprehensive coverage of understanding, prevention, and risk management of extreme drought events, with examples of approaches followed in water-stressed regions

This book describes the progress made in our understanding of severe drought and explains how we can deal with—and even avoid—complete devastation brought on by such punishing events. It brings forward advanced knowledge on drought hazard analysis and management, particularly from EU-funded research projects, to assist in the development of the corresponding drought management plans. In addition, this book addresses issues of social vulnerability to drought and science-policy interfaces, which are important elements of drought management.

Divided into three sections, this book covers the diagnosis of physical processes, historic drought and the trends in historic drought, and perspectives of future drought. It takes an academic approach to risk evaluation, including characterization of drought episodes, development of indicators of risk in hydrological and agricultural systems, and analysis of the role of socio-economic instruments for risk mitigation. It also discusses the interactions that have resulted in the complex institutional framework, and highlights the importance of stakeholder involvement and awareness building for successful drought management. In addition, Drought: Science and Policy features a collection of case studies that include the description of effective measures taken in the past. 

• Addresses the growing issue of drought preparedness planning, monitoring, and mitigation
• Teaches methodologies and lessons focused on specific, drought-prone regions so the applications have more significance
• Provides examples of approaches followed in water-stressed regions (river basin and national scale) with drought analyses at the pan-European scale

Drought: Science and Policy will be an invaluable reference for researchers and practitioners in the field as well as Masters students taking relevant courses in drought management and natural disaster management.

Part One
Understanding Drought as a Natural Hazard

1.1
Diagnosis of Drought‐Generating Processes

Henny A.J. Van Lanen¹ Anne F. Van Loon² and Lena M. Tallaksen³

¹Wageningen University, Wageningen, The Netherlands

²University of Birmingham, Birmingham, United Kingdom

³University of Oslo, Oslo, Norway

1.1.1 Introduction

It is well known that precipitation deficits initiate drought development. In cold climates (snow and glaciers), temperature anomalies (both high and low) too may contribute to drought generation. An in‐depth understanding, that is, drought diagnosis, of how climate drives precipitation and temperature anomalies, and subsequently how these anomalies propagate into soil moisture, groundwater, and streamflow deficits (catchment control), is a prerequisite for reducing the immense socioeconomic and environment impacts of droughts (e.g. Stahl et al., 2016). Comprehensive overviews on drought are provided by Wilhite (2000); Tallaksen and Van Lanen (2004); Mishra and Singh (2010); Sheffield and Wood (2011); and Van Loon (2015). This chapter builds upon these overviews and complements them by synthesising knowledge from recently finished EU projects.

The chapter starts with an introduction about key hydroclimatological processes controlling drought generation – that is, how precipitation and temperature drive drought, and which stores and fluxes are affected. Different drought types are explained, and the main drought indices that are used in this chapter are briefly described (Section 1.1.2). Section 1.1.3 gives an overview of the main atmospheric and oceanic drivers for meteorological drought (deficit in precipitation, temperature anomalies), supplemented with a detailed description of the drivers of the 2015 summer drought in Europe, and a discussion of the influence of climate change on the meteorological drought in Europe. Section 1.1.4 continues with a more comprehensive description of the influence of meteorological drought on soil water, followed by the influence on groundwater and streamflow (Section 1.1.5). Both sections focus on key processes controlling the development of droughts in the hydrological system – that is, soil moisture, streamflow, and groundwater drought – followed by the role of human influence in modifying the drought signal. Section 1.1.6 addresses how these different droughts propagate in the hydrological system – that is, how precipitation deficits and temperature anomalies affect snow accumulation and smelt, propagating into soil water, groundwater, and streamflow (drought propagation). A recently developed hydrological drought typology explains how climate and catchment controls determine drought propagation (Section 1.1.6). The focus in this chapter is on natural processes; however, at the end of each section, human interferences and their feedbacks are briefly touched upon (Sections 1.1.3–1.1.6). Finally, the concluding remarks are given in Section 1.1.7.

1.1.2 Background

The climate in the centre and north of Europe is influenced by the westerlies of the mid‐latitudes during the whole year, bringing moisture from the Atlantic Ocean. The Mediterranean region lies in a transitional climate zone, influenced by the Subtropical High‐Pressure Belt during summer and the mid‐latitude westerlies during winter. Hence, two main climate regions can be distinguished: a temperate climate with a dry summer season in the Mediterranean; and a temperate climate and a cold climate without any dry season in the centre and north of Europe, respectively. Within these regions, climate is modified by numerous other permanent or temporally variable global, regional, or local factors, such as soil moisture, oceanic currents, and topography. Blocking situations disturb the common eastwards movement of the mid‐latitude pressure systems, that is, the westerlies. During a blocking phase, an extended, persistent, high‐pressure system develops in the eastern Atlantic Ocean at the mid‐latitudes that does not move eastwards or moves only very slowly (Stahl and Hisdal, 2004). As a consequence, the moisture‐bringing pressure systems divert moisture to Northern Africa and Northern Fennoscandia, causing an extended dry period (precipitation deficits) in mainland Europe (Section 1.1.3). During a dry and warm summer, feedbacks between the land surface and the atmosphere may amplify the drought signal. As the soil dries out, less energy is used for evapotranspiration (latent heat flux), and the partitioning of incident solar energy changes as more energy is used for heating the air (sensible heat flux). Heat waves thus frequently accompany major droughts, as reported for Europe by Ionita et al. (2017).

The lower‐than‐normal precipitation, usually combined with higher temperature and associated larger potential evapotranspiration (PET), leads to a decreased net precipitation (gross precipitation minus evaporated interception water), and hence infiltration into the topsoil of the vegetated surfaces (Figure 1.1.1). In places without vegetation, the lower gross precipitation directly results in lower infiltration. Evaporation of intercepted water, especially in forests, and overland flow on sloping land is also lower.

Figure 1.1.1 Water stores and fluxes affected by drought.

Source: Derived from Van Lanen et al. (2004a).

The reduced soil infiltration, together with the often‐higher atmospheric water demand (increased PET), causes a larger depletion of the soil moisture storage than normal. Consequently, lowered soil evaporation and lessened soil water uptake by vegetation results in reduced evapotranspiration in many cases. Another important effect of the more depleted soil moisture store is the lower recharge to the underlying aquifer. In catchments where interflow takes place (e.g. soils with contrasting hydraulic conductivities, slopes), the aquifer also receives less water. This leads to lower groundwater levels, and hence reduced groundwater discharge to streams and lakes. Deeper, regional aquifers also receive less water input (lower leakage), which may have long‐lasting impacts in a wide area. Lakes can mitigate the effects of a downstream drought, because their natural role is to store surface water during wet periods and to release it during a dry period (upstream–downstream differences in streamflow). The changes in hydrological processes, which also have different time delays (i.e. response times), are more elaborated in the following sections (Sections 1.1.4 and 1.1.5).

In cold climates, irrespective of precipitation amounts, below‐normal temperatures can result in earlier snow accumulation at the start of the cold season, which might lead to lower inflow to the streams. This also takes place when the cold season is longer than normal (delayed snow melt peak). In glaciated regions, cold temperature anomalies also lead to below‐normal streamflow. Higher‐than‐normal winter t...

Table of contents

• Cover
• Table of Contents
• Series Preface
• The Series Editor – Philippe Quevauviller
• List of Contributors
• Part One: Understanding Drought as a Natural Hazard
• Part Two: Vulnerability, Risk, and Policy
• Part Three: Drought Management Experiences and Links to Stakeholders
• Index
• End User License Agreement