Balancing Water for Humans and Nature

The New Approach in Ecohydrology

Malin Falkenmark, Johan Rockstrom

• Éditeur: Routledge
• Année: 2013
• ISBN:
  9781136571411
• Édition: 1
• Sujet: Law
• Sous-sujet: Environmental Law

Balancing Water for Humans and Nature, authored by two of the world's leading experts on water management, examines water flows - the 'blood stream' of both nature and society - in terms of the crucial links, balances, conflicts and trade-offs between human and environmental needs. The authors argue that a sustainable future depends fundamentally on our ability to manage these trade-offs and encourage long-term resilience. They advocate an ecohydrological approach to land/water/environmental problems and advance a strong, reasoned argument for viewing precipitation as the gross fresh water resource, ultimately responsible for sustaining all terrestrial and aquatic ecosystem services.

This book makes the most coherent and holistic argument to date for a new ecological approach to understanding and managing water resources for the benefit of all. Basing their analysis on per capita needs for an acceptable nutritional diet, the authors analyse predictions of the amounts of water needed for global food production by 2050 and identify potential sources. Drawing on small-scale experiences in Africa and Asia, they also cover the vulnerability of the semi-arid tropics through a simplified model of green and blue water scarcity components.

Part 1

Water Functions in the
Life-support System

1

Water – The Bloodstream of the Biosphere

Water is always on the move in the hydrological cycle. It is the very foundation for all biological life on Earth, and the basic link between the biosphere and the anthroposphere.

Water as the Liquid of Life

Water is the most essential component for the life of all beings. Hardly any economic activity can be sustained without water (Haddadin, 2001).

The water cycle links human society with ecosystems

In view of past failures, a better idea is now needed of the roles and functions of water in the global life support system. For example, the conventional conceptions of water related to plant production are increasingly unsatisfactory from a hydrological perspective. Through its physical, chemical and biological involvement, water has fundamental balancing functions in the water cycle (Ripl, 1995). It dissipates solar energy variations in space and time. This is performed through three main interactive processes with mutually balancing components:

1. Physical processes. The interaction between evaporation and condensation is of major importance for the distribution of energy around the planet.
2. Chemical processes. The interaction between crystallization and dissolution is of fundamental importance for the distribution of soluble substances around the planet.
3. Biological processes. The interaction between splitting water molecules in the first step of the photosynthesis process and their later re-assemblage through respiration creates sugar and oxygen in the process.

Figure 1.1 illustrates in a schematic way the linkages between the freshwater cycle, human livelihoods and ecosystems. The terrestrial system, fed by precipitation, returns green water to the atmosphere through the biomass production process. Aquatic ecosystems thrive in the habitats formed by the liquid blue water. Human society benefits from freshwater services for settlements and industries, and from crop irrigation. When linking this figure to Ripl’s balancing processes (1995), one finds that the different branches of the water cycle are basically dominated by different key functions. The physical functions in the atmospheric green water branch of the hydrological cycle involve a balancing of evaporation from land and water surfaces and condensation of vapour into water droplets that combine and form rainfall. The lower left section of Figure 1.1 shows the blue water branch of the hydrological cycle with its biological and chemical functions. Blue water flows are important determinants of aquatic habitats. Biological functions in terrestrial ecosystems are closely linked to green water flow. Blue water derives its quality from interactions within terrestrial ecosystems, and carries out chemical functions. It is important to realize that the planetary life support system is composed of both a non-negotiable biological sphere, which includes the interactions between the biosphere and the water cycle, and a negotiable socioeconomic sphere, which includes human activities (FAO, 2000).

BOX 1.1 WATER – A MOST REMARKABLE SUBSTANCE

Water is a most remarkable substance. Water molecules have pervasive effects on macromolecules of proteins and nucleic acids (Chaplin, 2001). The water molecule is among the smallest of all molecules, but in spite of this it has highly complex properties, such as a large heat capacity and a high latent heat of evaporation. Water is an excellent solvent due to its polarity, high dielectric constant and small size. The complexity of its properties seems to fit ideally into the requirements of the carbon-based life on this planet as no other molecule can. Chaplin (2001) therefore stresses that we should always be alert to the central role that water plays in the rich diversity of biological processes.

The water cycle is an essential part of the life support mechanism of the planet. The interaction with the biota adds water vapour – an active greenhouse gas – to the atmosphere. The result is that the planet’s temperature is kept 30°C higher than it would be without water in the atmosphere. Carbon dioxide (CO₂)contributes another 3°C. The resulting temperature rise allows liquid water to exist on Earth, something that makes this planet exceptional among all the planets in the solar system.

The human body is 65–70 per cent composed of water. None of this water is stagnant. Water is the body’s busiest substance (Leopold and Davis, 1968). All the water molecules in one part of the body at any moment are somewhere else seconds later, and have been replaced by new molecules. Although the molecular formula is H₂O, the hydrogen atoms are constantly exchanged between neighbouring oxygen atoms. At pH7 the average time that a water molecule exists between gaining and losing a proton is only a millisecond.

The unique hydration properties of water to a large extent determine the three-dimensional structure of biological macromolecules, and hence also their functions in solution. Water networks around proteins link secondary structures and so determine fine details of a protein’s structure. Hydrophobic interactions with surfaces to which water cannot form hydrogen bonds encourages a surface minimization that drives protein folding.

By introducing the concept of ‘ecosystem goods and services’ the ecological research community has increased the attention given to ecosystems among groups concerned with natural resource management. Gretchen Daily (1997) defined ecosystem services as ‘the conditions and processes through which natural ecosystems, and the species that make them up, sustain and fulfil human life’. We also include in this definition the various forms of agricultural land, which can be seen as ecosystems manipulated by humans. Ecosystems provide a wide spectrum of essential ‘goods’ for supporting life, such as timber, food and biodiversity, as well as ‘services’, such as maintenance of genetic resources, partitioning of rainfall, and pollination for our food consumption needs. Some ecological services are evident, but others are not immediately obvious. Based on a systematic approach, we may structure the ‘services’ as follows (FAO, 2000):

• Physical services, such as phosphorus absorption in the soil, erosion and sedimentation of silt, interception of rainfall, and facilitating rainwater infiltration into the soil.
• Chemical services, such as oxygen production and CO₂ uptake in the photosynthesis process, denitrification, and nutrient release through biodegradation.
• Biological services, such as photosynthesis, pollination, seed dispersal, pest control, production of biomass, and macropore creation in the soil.

Note: The terrestrial system is fed by the water cycle, aquatic ecosystems thrive in the habitats formed by the water, and society withdraws water for its needs and returns the water either as a (polluted) return flow of wastewater or as a green water flow/evaporation.

Figure 1.1 Schematic illustration of links between the cyclic movement of fresh water, the terrestrial and the aquatic ecosystems, and human society

Distinguishing between green and blue water flows

We have already used the terms blue and green water, so this would be a convenient place to explain more fully what is represented by these concepts. The picture of the principal partitioning of rainfall into green and blue water flows at the soil surface is shown in Figure 1.2.

Blue water flow is the visible liquid water flow moving above and below the ground as surface or sub-surface runoff, respectively (FAO, 1995b, 1997). Blue water flow can thus be in the form of surface runoff in rills, gullies and rivers, or water flowing underground, recharging water tables and aquifers. Green water flow is defined as the invisible flow of vapour to the atmosphere (FAO, 1995, 1997). Productive green water flow is defined as the transpiration from plants or trees, which directly contributes to biomass growth. Non-productive green water flow consists of the evaporation flows from soil (direct evaporation from water puddles on the soil surface and evaporation of water from the soil), and evaporating interception (rainfall trapped in the canopies of trees and plants). Green water flow thus equates to the commonly used term evapotranspiration, which combines productive and non-productive vapour flows in one term. We will return to this discussion in Chapter 2.

Figure 1.2 Rainfall partitioning – the vertical green water branch of water vapour and the semi-horizontal blue water branch of liquid water

Both blue and green water flows support ecological functions and delivery of ecosystem goods and services. Both are also a precondition for human survival and societal development.

Another important difference between green and blue water flows is that green water flow (evaporation + transpiration) always involves a consumptive use of water, which is not the case with blue water use. Green water cannot be used again further downstream. Blue water, on the other hand, can be recycled and reused again. For example, because irrigation is so inefficient, only a fraction of the blue water withdrawn from the system is consumed as green water flow. A large portion percolates and recharges groundwater, and can therefore be reused further downstream.

As will be seen later, the distinction between green and blue water flows is useful in hydrologically based projections of water needs for food production for a growing world population. The idea of dividing water flows into green and blue water is a new concept and the terms will therefore be used consistently throughout this book. This will allow a discussion in later chapters of the consumptive use of water for crop production compared to the water involved in biomass production in the main biomes of the world.

The green/blue concept will also be useful in a closer analysis of water balance alterations linked to land-use changes and referred to as ‘blue/green redirections’. We will show through examples that a land-use decision is also a decision about water. Consumptive use phenomena have in the past often been subsumed under the term ‘environmental impacts’. Such phenomena need to be a topic for discussion so that incompatible water uses can be properly analysed. Serious conflicts of interest can be foreseen between human activities and biomass production upstream in a river basin and the activities downstream. The conflicts will be particularly difficult in basins with a rapid population growth and escalating food needs.

Society and ecosystems share the same water

If we look at the planetary life support system on the landscape scale, there is only one water source and that is precipitation. Since we will not be discussing the peculiarities of cold climates and snow in this book, we will speak of precipitation solely as rainfall. On sloping land, all water-dependent human activities and all the ecosystems are enclosed in nature’s own wate...