|
For croplands, a somewhat different method was used, because they are located in a wide range of climatic regions (from tropic to boreal and from arid to humid), vary highly in production intensities, and there are detailed data on production (yield x surface area) and water requirements of production. By crop production, we refer to actual harvest and not to potential crop production. Biomass production in croplands is roughly linearly proportional to ET for constant hydroclimatic conditions, when water is not limiting growth (Sinclair et al. 1984). The slope of the relationship between biomass growth and ET is defined as the water use efficiency (WUE). WUE has, however, been defined in various ways in the literature, commonly as the amount of transpired water per yield unit, or the amount of water applied (through irrigation) per yield unit.
We have used this relationship in the calculation of water vapor flow from croplands, multiplying the annual crop production (in megagrams per year of harvested economic biomass; 1 Mg = 1 ton) by WUE estimates (cubic meters per megagram). The WUE data for each subgroup in Table 1 derive from a broad number of sources (n in Table 1; fully specified in Appendix 1).
The actual water vapor flow for each subsystem will vary in space and over time, due to climatic fluctuations, different biotic and abiotic conditions, and different land management practices. We have taken into account the effects of such variations on water vapor flows by calculating a high and low estimate for each subgroup, based on the lowest and the highest ET or WUE data.
Grasslands, wetlands, woodlands, and forests
Surface extensions of the biomes shown in Table 1 were derived from those in Carbon in Live Vegetation of Major World Ecosystems (Olson et al. 1983), except for wetlands, for which we used Matthews' (1983) Global Database on Distribution, Characteristics, and Methane Emission of Natural Wetlands.
Grasslands include all noncultivated formations with <10% tree canopy
cover, thus including natural grazing land, pastures, and shrubland. Woodland
is a wide description, with various densities of trees and tree canopy coverage
between 10% and 99%. Forests are defined by tree canopy coverage of 100% (Olson
et al. 1983). Wetlands include bogs/fens and swamps/marshes, and are here defined
as permanently or seasonally inundated areas, forested or nonforested.
Based on the subclasses from Olson et al. (1983), some reclassifications were made
(Table 1). In wetlands and forests/woodlands, vegetation type and climate interact
in the generation of ET (Mitch and Gosselink 1983, Nulsen et al. 1986); these biomes were thus classified in subgroups according to those variables. For grasslands, we have assumed that total ET depends primarily on climatic factors rather than on vegetation cover, although the relation between evaporation and transpiration can vary (Penning de Vries and Djitèye 1982, Liang et al. 1989). For warm and hot grasslands with annual precipitation of P < 600 mm/yr, we assume that ET = P (i.e., that there is no liquid water flow). This assumption is valid for dry grasslands on a large spatial scale (le Hourerou 1984). For grassland systems with P > 600 mm/yr, 20% runoff is assumed. The ET data for each subgroup derive from a broad number of peer-reviewed sources (indicated under n in Table 1 and fully specified
in Appendix 2).
Croplands
Agricultural ET was estimated from mean crop production data over a period
of five years (1992 -1996) using individual crop data from FAO (Faostat 1997).
The time span was included to reduce the effects of interannual yield fluctuations.
WUE data were collected for each major food crop. All crops were classified
into 16 subgroups according to key parameters influencing WUE, i.e., hydroclimate,
plant community, and the harvested part of plant (grain, fiber, fruit, etc.).
Special attention was given to ensure that the WUE values corresponded
to the economic yield registered in Faostat. WUE data from several research
sites were included for each major crop and subgroup in order to reflect the
variability in ET for different agricultural settings (see Appendix 1).
These calculations for croplands cover ET requirements to produce the harvested economic yield. Added to this flow is the ET water from other non-economic vegetation in agricultural lands. Here, non-economic vegetation includes weeds and vegetation in open drainage ditches, green enclosures,
and wind breaks. This vegetation can, however, support ecological services in that it can, for example, contribute to nutrient retention in the landscape and provide
a habitat for insects that may be important for pollination and predation of
pests (Matson et al. 1997). Earlier efforts at estimating this share of the
water cycle are very rudimentary. For example, the total net primary production
(NPP) in croplands used in Postel et al. (1996) and Postel (1998), and based
on Vitousek et al. (1986) and Ajtay et al. (1979), includes only NPP from crops
grown for harvest. The assumption that the total annual ET, based on this NPP multiplied with a global average WUE, would reflect the actual ET from the world's croplands seems to be a very rough estimate. We have not found any global estimate of NPP in croplands coming from weeds, drains, ditches, etc., nor an estimate that covers the ET from this production. Thus, we assume that 10% of the average annual rainfall over land surfaces (834 mm/yr), i.e., roughly 80 mm/yr, supports non-economic biomass growth in agricultural lands. Even though our estimate, based on the assumption that 10% of the precipitation on croplands supports such production,
is crude, it seems more reasonable than previous estimates.
Results of water vapor estimates of major terrestrial biomes
The estimate resulted in a total water vapor flow from forests, woodlands,
wetlands, grasslands, and croplands of 63,200 km3/yr (Table 2). We estimated the total water vapor flow from grasslands to be 15,100 km3/yr (range 9300 to 21,700 km3/yr); from forests and woodlands to be 40,000 km3/yr (range 35,300 - 45,000 km3/yr); and from wetlands to be1400 km3/yr (range of 1100 - 1700 km3/yr)
(Table 1). The total water vapor flow in the world's croplands
for crop production was estimated as 5400 km3/yr,
with low/high values ranging from 3600 to 8400 km3/yr.
Adding ET for non-economic plant growth on agricultural lands of 1300
km3/yr gives a mean water vapor flow of 6700 km3/yr, ranging from 4900 to 9800 km3/yr.
Table 2. Bottom-up estimate of global water vapor flows from the continents.
| Water vapor source |
Our estimates (km3/yr) |
Earlier estimates (km3/yr) |
References |
| Major terrestrial biomes |
|
|
|
|
| Croplands |
|
6800 |
2285 - 5500 |
Postel et al. (1996), Shiklomanov (1996), Postel (1998) |
| Temperate and tropical grasslands |
|
15,100 |
5800 |
Postel et al. (1996) |
| Temperate and tropical forests, woodlands, and taiga |
|
40,000 |
6800 |
Postel et al. (1996) |
| Bogs, fens, swamps, and marshes |
|
1400 |
|
|
| |
Subtotal |
63,200 |
14,885 - 18,100 |
|
| Other systems |
|
|
|
|
| Green areas in urban settlements |
|
100 |
100 |
Postel et al. (1996) |
| Upstream rural water use |
|
210 |
|
|
| Lake evaporation |
|
600 |
600 |
L´vovich (1979) |
| Evaporation from large reservoirs (>100 x 106 m3) |
|
130 |
130 |
L´vovich and White (1990) |
| Evaporation from small reservoirs (<100 x 106 m3) |
|
30 |
|
|
| Tundra and deserts |
|
5700 |
|
|
| |
Subtotal |
6800 |
830 |
|
| |
Total |
70,000 |
15,715 - 18,930 |
|
|
We believe our estimates to be conservative, especially for agriculture. Our ET estimates for crop production only relate to harvested yield after reduction for threshing and post-harvest losses (which can amount to > 20% of the ET-demanding crop on the farmer's field). The WUE data used in this article originate from research stations that generally have more favorable cultivation conditions than does the farmer, which results in higher WUE values than under on-farm conditions. Our data show that, on average, some 1400 m3 of ET flow is needed to produce 1 Mg of cereal grain in the tropics. There are, however, many research findings suggesting that WUE is much lower in farmers' fields, often amounting to some 3000 - 6000 m3/Mg (Dancette 1983, Rockström et al. 1998). This is explained by relatively lower soil fertility, higher runoff losses, and less advanced land management practices on-farm, and will result in lower yields (< 1000 kg/ha in sub-Saharan Africa) and higher soil evaporation losses. This low WUE in agriculture is reflected by the high estimate in Table 1 of 8427 km3/yr for crops.
The aggregate estimate in Table 1 ranges from 49,000 to 77,000 km3/yr,
which is roughly a deviation of 14,000 km3/yr from the mean. The large fluctuations in water vapor flow within the subgroups mainly reflect four different sources of variation: location, climatic fluctuations, land management, and random error. It is worth mentioning that the fluctuation of annual rainfall over land surfaces is of the same order of magnitude as
our estimated water vapor fluctuations, and varies between 90,000 and 120,000
km3/yr. The considerable variations in water vapor use suggest that mean water vapor estimates, especially for agriculture, are of limited interest in assessing regional and global freshwater needs. The range includes parameters that we cannot influence (e.g., soil properties and hydroclimatic fluctuations), but also factors that we can influence through integrated land and freshwater management.
The large range also indicates that there is an important potential for improving WUE in agriculture. Crop management, such as choice of cultivars, planting
density, crop protection, and soil and water management, will affect the ratio
between ET and yield and, thereby, WUE. In soil and water management,
care must be taken with nutrients and soil structure in order to minimize the
effects of erosion and runoff. Variations in WUE for a specific crop species
also illustrate the capacity of a certain crop to grow in a spectrum of hydroclimates
(e.g., maize from humid to semiarid tropics).
Postel et al. (1996) estimated the freshwater requirements for the annual human
appropriation of net primary production of grasslands to be 5800 km3/yr, and of harvested forest products to be 6800 km3/yr. Postel (1998) also estimated the annual human appropriation for total food production (including croplands, grazing lands, irrigation water losses, and aquaculture) to be 13,800 km3/yr.
In summary, earlier estimates suggest that humans depend on some 14,900-15,800 km3/yr (Table 2) of water vapor to support human-appropriated primary
production. This corresponds to 21-22% of the top-down estimate by L'vovich and White (1990) of water vapor flow from continents (72,075 km3/yr). Our results indicate that the major terrestrial biomes appropriate as much as 88% of this water vapor flow.
ESTIMATING TOTAL WATER VAPOR FLOWS FROM CONTINENTS
As shown in Table 2, our estimated average water vapor flow from croplands,
forests, woodlands, grasslands, and wetlands amounts to 63,200 km3/yr.
By adding water vapor flows from remaining continental systems, we perform,
to our knowledge, the first bottom-up calculation of total water vapor flows
from continents. The estimate is generalized from field studies of water vapor
flows from different biomes.
Evapotranspiration from green areas in urban settlements has been estimated
at 100 km3/yr (Postel et al. 1996), and vapor flows
from lakes account for an estimated 600 km3/yr (L'vovich
1979). Added to this is the complex grey zone of domestic water use by rural
societies. The magnitude of this upstream rural water evaporating after use
is difficult to estimate. If 82% of the population in developing countries
(estimated from WRI 1994 and FAO 1995) is assumed to have a daily need, for
domestic purposes, of 150 l p/d, an estimated 180 km3/yr is appropriated. The suggested domestic daily water use of 150 l p/d is taken as an aggregate of
Shuval's estimate of roughly 25 m3 p-1 yr-1
(= 68 l p-1 d-1) needed for basic small-scale production
of legumes, livestock, and chicken around homesteads in arid regions (Lundqvist
and Gleick 1997), and Gleick's suggested basic household need of water amounting
to 50 l p/d (Gleick 1996). 20 l p/d were added in order to reflect the water demand for animals in pastoral communities and large-scale livestock raising.
L'vovich and White (1990) estimated that the volume of water in small reservoirs amounts
to some 5% of the volume in large reservoirs (about 5500 km3 when full). Based on this, we have estimated the vapor flow from small reservoirs as 30 km3/yr, by assuming an average depth of small reservoirs to 3 m and a vapor flow of 400 mm/yr. In Table 2, we include the vapor flow from small reservoirs in upstream rural water use. Large reservoirs (with a storage capacity > 100 x 10 6 m3) return an estimated 130 km3/yr of vapor flow to the atmosphere (L'vovich and White 1990).
Tundra and deserts, covering some 31 x 10 6 km2 of land (Olson
et al. 1983), with an average annual ET of 180 mm (Frank and Inouye 1994),
return approximately 5730 km3 water to the atmosphere each year.
These biomes play a role in global climate and support local human populations and biota.
Adding evaporation from lakes, large and small reservoirs, and ET flow from green areas in human settlements, tundra, and deserts, and upstream rural water use gives a total water vapor flow of about 70,000 km3/yr (Table 2). This implies that our estimate generalized from field data of water vapor flows from a diversity of systems has captured 97% of previous global top-down and indirect ET estimates from continents. It should be noted, however, that this range might vary between 56,000 and 84,000 km3/yr (51-76% of annual mean rainfall) just by taking into account the variation of the major biomes (Table 1).
How much of this freshwater flow does humanity depend upon for terrestrial
ecosystem services? Because ecosystems are complex systems linked dynamically
across spatial and temporal scales, it is difficult to judge human water vapor
dependence on a global level. There are those who believe that such a dependence
should only be attributed to a particular service or to marginal changes in
freshwater requirements between services and other human uses of freshwater.
There are others who would argue that the water vapor requirement of the whole
ecosystem is necessary for the generation of ecosystem services, at least in
a longer term and sustainability perspective. In the following section, we will discuss
interrelations between freshwater and terrestrial ecosystem services, and illuminate
the many welfare-supporting ecosystem services that depend on complex ecosystem dynamics, which, in turn, depend on the bloodstream of the biosphere.
INTERRELATIONS BETWEEN WATER VAPOR FLOWS AND TERRESTRIAL ECOSYSTEM SERVICES
Physical and chemical processes provided by freshwater are fundamental. Water
constitutes an essential building block in all terrestrial production, contributes
to the processes that generate ecosystem services, and provides crucial interconnections
within and between ecosystems. It works as a carrier of solutes, plays a key
role in global, regional, and local climate regulation, and sets the ecohydrological
conditions for biological diversity in any habitat.
Freshwater availability is a prerequisite in the production (e.g., crops,
timber, cattle), information (e.g., nature experiences, aesthetic information),
and regulation (e.g., formation of topsoil, sequestering of CO2,
assimilation of nutrients) functions of the environment (de Groot 1992). These
functions are defined as ecosystem services and include ecological processes that produce, directly or indirectly, goods and services from which humans benefit (Daily 1997).
Crops, trees, cattle, and other biomass production depend on accessible renewable
freshwater. Nature requires water for food web support to wildlife and for
maintenance of habitats in which they live. The processes of topsoil formation
in forests and croplands and nutrient retention in wetlands involve water.
Grassland systems develop patchy dynamics that respond to water availability
by redistributing water and nutrients in the landscape for improved performance
(Walker 1993). Ecosystem services of tropical rain forests depend both on water
transpired by vegetation and on evaporation that supports species adapted to
a moist environment.
Ecosystems are interconnected by liquid water and water vapor flows. Forests
are linked to other systems such as grasslands and wetlands, both directly and
indirectly, in ways in which freshwater plays a critical role. Freshwater directly
transports mineral nutrients and organic matter between systems. Indirectly,
freshwater supports services across ecosystems, such as the spreading of seeds,
both directly by water and indirectly as water is needed to sustain a habitat
for mobile organisms that spread seeds, and to sustain a habitat for bees and
other insects that are important for pollination. The biota play an important
role in the regulation of atmospheric water by redirecting liquid water to
water vapor flow, thereby recycling it to local rainfall. This can be of great
significance, e.g., in the Sahel region where > 90% of the rainfall
appears to be attributed to ET flow from vegetated land surfaces (Savenije
1995). Furthermore, terrestrial ecosystems contribute to freshwater quality
through biochemical processes such as denitrification and other forms of microbiological
activity, and by facilitating infiltration, thereby moderating river flow seasonality,
erosion, and flooding.
Freshwater is also required for ecosystem resilience. Resilience is the buffer
capacity to disturbance performed by functional groups of species linked in
complex temporal and spatial webs of interactions (Peterson et al. 1998). Dynamics
of ecosystems (Holling 1986) and variability in water flow patterns can interact
and respond to each other with feedback mechanisms at different temporal and
spatial scales (Mitch and Gosselink 1983, Swank et al. 1988). Forest fires can
cause huge runoff increases that may impact on downstream systems, as experienced
in Australia (E. O'Laughlin, Canberra, Australia, personal communication). Resilience
makes it possible for a forest to absorb a fire and maintain the potential to reorganize and recover, thereby continuing to supply ecosystem services essential to society, and also to reduce negative effects on downstream water-ecosystem services for other human uses. Similarly, grasslands have adapted to disturbances such as invasion of grazers or insects, fire, and periods of flooding or drought, and need the dynamic interactions of biological diversity to respond
in a resilient fashion to these disturbances (Walker 1993). Freshwater is a
key driverin these dynamics.
Putting freshwater in such an ecological context and in the light of data in Tables 1 and 2 suggests that the degrees of freedom for production of life support for the expanding world population is limited. There will be fundamental trade-offs between food production and other welfare-supporting ecosystem services in terms of available freshwater.
FRESHWATER, FOOD, AND ECOSYSTEM SERVICES FOR A GROWING HUMAN POPULATION
The per capita dependence on water vapor for production of food in croplands
is roughly 1180 m3/yr, based on a population of 5.7 billion people in 1995 (UN 1997). Recognizing that the human population probably will reach 6 billion within the next few months, we used data from 1995, as they can be compared with the data on crop production that we have used, which refer to the years 1992-1996. Future demand for food will involve an increased
appropriation in terms of additional water vapor flow for crop production. (Grasslands also provide food in terms of animal protein. However, because grazing is only one of multiple functions in the grassland system, and is also a process within the system, it would be misleading to try to estimate how much of the 2650 m3 p-1 yr-1 of water vapor estimated here from grasslands is attributed to cattle production).
L'vovich and White (1990) have estimated the changes in runoff during the past
300 years (1680-1980) caused by redirections of liquid water to water vapor
flows through irrigation. Their results suggest that the water vapor flows
have increased from 86 km3/yr to 2570 km3/yr during this period. In the coming 100 years, they estimate a further doubling in response to food production needs. Considerable changes in water vapor and liquid water flow patterns seem unavoidable.
Using the human population increase reported by the United Nations (UN 1997), i.e., an increase
of 2.6 to 8.3 billion in 2025, and assuming a current per capita water vapor use for crop production, we calculate an additional water need of 3100 km3/yr in 2025. This would imply a total crop water demand in 2025 of about 9,800 km3/yr, a 31% increase in freshwater demand for crop production. Could we appropriate this amount of freshwater in a trade-off-free manner toward other terrestrial biomes? We have identified three possible options.
The first option, propagated by international organizations (e.g., FAO, UNDP,
IIMI), is to increase irrigated agriculture. According to Shiklomanov (1997),
the increase in ET in irrigated agriculture by 2025 would amount to 425
km3/yr, or about 14% of the additional freshwater demand.
Because increased irrigation implies liquid-to-vapor redirection of freshwater
and, thereby, a continuation of river depletion, the scope for solving future
food shortages through irrigation alone, without causing severe impacts elsewhere,
seems limited (Leah 1995).
The second option is to improve rain-fed agriculture (Falkenmark et al. 1998).
There seem to be two major avenues. The first is to improve water-use-efficiency
in crop production by redirecting in-field evaporation to transpiration within
croplands, i.e., increasing the yields with the same amount of water vapor flow.
It seems reasonable to assume a 10% overall increase in WUE as a result of
e.g., better crop varieties, improved farming practices, soil fertility management,
and soil and water conservation measures. This would diminish the future water
needs by about 300 km3/yr. The second avenue is to redirect evaporating surface runoff for use in croplands. This option concerns water that now runs off from croplands and evaporates in areas of low biomass productivity and degraded lands, predominantly in semiarid and
arid regions, i.e., water that never reaches rivers and does not contribute to the generation of ecosystem services. This water could be captured by surface-water harvesting and used for supplementary irrigation during dry spells (Rockström and Valentin 1997). This measure would not only conserve water but alsowould conserve soil by diminishing erosion caused by surface water runoff. A first-cut estimate of this option is arrived at by a comparison between surface runoff on a local scale from croplands vs. runoff on a continental scale, assuming an
even distribution of croplands globally. The amount of water available for
redirection from evaporating surface runoff in semiarid and arid regions
for use in croplands is hard to estimate. We assumed that the difference in
surface runoff coefficients between field scale and continental scale for croplands
in Africa, Asia, and South America is attributed to evaporating surface runoff.
An even distribution of croplands on the different continents was assumed.
Croplands cover 10.5% of the global terrestrial area. The runoff water from
croplands available for surface water harvesting in Africa, Asia, and South
America would then be roughly 300 km3/yr. See Appendix
3 for data and references.
Thus, it may very well be that developments in irrigated and rain-fed agriculture
cannot cover the full need of increased water appropriation for food production,
actually only about one-third or 1000 km3/yr out of 3100 km3/yr, according to our first-cut estimate. Desalinization of seawater for food production is not a viable solution because the costs would be several factors higher than the price of the crops.
It seems as though the final option to feed another 2.6 billion world inhabitants
until AD 2025, is to redirect substantial amounts of water vapor flows from
other biomes to croplands. Intensifying the conversions of forests, woodlands,
and, to some extent, grasslands and wetlands, to croplands in the tropics and
subtropics is a likely development scenario. Assuming that the main part of
the remaining freshwater demand would be appropriated from tropical/subtropical
systems, their water vapor flows would decrease by 5.5% in only 25 years. Because
most of the population growth will occur in the tropical region, this is also where
the increase in food production primarily will occur. Thus, we divided the 2100
km3/yr of additional water vapor needed by the total water
vapor from our estimates in tropical grasslands, forests, woodlands, and wetlands,
which amounts to 38,000 km3/yr, thus resulting in a 5.5% increase.
There is a severe risk that further land use change to capture freshwater for
crop production will lead to increasingly fragile, less diverse systems with
lower resilience, and will cause subsequent erosion of ecosystem services. Will such
redirections of water vapor increase or decrease total human well-being? The results of our estimate, in the light of an expanding human population and escalating globalization, illustrate that we are facing major challenges in freshwater-land use management. Management must explicitly deal with what we call the increasing water vapor-related scarcity. This "new scarcity," which concerns the critical trade-off between water vapor for ecosystem services
generated by terrestrial biomes and water vapor for food production, has not been sufficiently addressed in freshwater assessments.
INTENTIONAL ECOHYDROLOGICAL LANDSCAPE MANAGEMENT
The critical trade-off between use of water vapor for food production to a
growing world population or for welfare-supporting ecosystem services must be addressed in a conscious way. Proper attention must be paid to side effects generated by land use change. Modifications of ecosystems will alter water flows, and redirection of water flows will modify ecosystem services. There are numerous intentional local and sector-based land use decisions that have caused unintentional ecologically and water-driven side effects. Such effects
are generally discussed under the term "environmental impacts," without perception
of the causes behind them.
Ecologically driven side effects of land use conversion, such as shifts
in key functional groups of species or loss of resilience, can change ecological
and hydrological preconditions for the generation of ecosystem services. For
example, movements of organisms in the landscape may change, and thereby impact
on ecosystem services such as pest control, pollination, and seed dispersal by
birds, bats, mammals, and insects (Baskin 1997, Bisonette 1997). Ecologically
driven side effects can impact on processes of significance to the surrounding region (such as denitrification by wetlands), or processes performed on a local scale, but valued at a global scale
(such as sequestration of CO2 by forests). These side effects may accumulate and transfer to the landscape and further, to a regional and even to a global scale (Holling 1994).
Freshwater-driven side effects of human activities caused by land use conversions can
also change ecological and hydrological preconditions for the generation of
ecosystem services. Such side effects are linked to interventions with the
water partitioning process, and are propagated downstream or downwind by the water
cycle. They may involve river depletion, altered relations between storm flow
and low flow, and consequences for water-dependent downstream activities such
as direct water uses, or ecosystem services generated by riparian wetlands and aquatic ecosystems. For example, land-clearing in southwestern Australia caused a
rising water table and a threat of saline groundwater seepage into ephemeral
watercourses that fed drinking water reservoirs. In the Murray Darling basin
and the Hungarian Great Plain, deforestation caused widespread water-logging.
Land conversion may also have atmospherically transferred consequences on downwind
rainfall (Savenije 1995).
Our scenario of freshwater needs for food production for the additional world
population indicates that substantial amounts of freshwater will have to be
redirected to croplands from other terrestrial biomes. Increased irrigation
and land conversions will produce costly side effects on the capacity of both
aquatic (Postel and Carpenter 1997) and terrestrial ecosystems to generate
ecosystem services. With a sectoral management and a business-as-usual approach,
regional conflicts will probably grow rapidly. Instead of passively allowing
unintentional impacts to develop, as in the past, an ability to manage the
overall catchment, or the ecohydrological landscape, in an intentional
manner must be developed.
A few cases of intentional ecohydrological landscape management have been reported from Australia and South Africa, recognizing the interdependence among liquid/vapor freshwater flows, ecosystems services, and human well-being. In Australia, an agreement has been signed between a forest firm and Melbourne City on increasing the rotation time in an upland forest to improve the water source for the city (Jayasuria 1994). In South Africa a permit system has been in operation for several decades, by which the "costs" of afforestation, in terms of river depletion, are estimated (van der Zel 1997). Moreover, the South African fynbos restoration project involves systematic reduction of the invasion of highly water-consuming alien vegetation. The fynbos catchment is seen as an integrated whole, and governance rests on combined ecological and hydrological knowledge and understanding (van Wilgen et al. 1996).
CONCLUSIONS
We have estimated the total water vapor flow from continental ecosystems to be
70,000 km3/yr, based on generalized field data. Our result captures 97% of the evapotranspiration branch (72,075 km3/yr) of global freshwater budgets (L'vovich and White 1990). A large part of our water vapor flow (63,200 km3/yr, or 90%) is attributed to forests, woodlands, wetlands, grasslands, and croplands. These terrestrial biomes sustain society with essential welfare-supporting ecosystem services, including food production.
We do not know the actual freshwater requirements for generating key terrestrial ecosystem services appropriated by the present global human population. To what extent freshwater can be used more efficiently in existing ecosystems is also an open question. Future understanding of complex behavior and interactions within and between ecosystems and freshwater flows may improve this knowledge. We can, however, conclude that earlier global freshwater assessments,
which have focused their analysis on the runoff branch of freshwater (e.g., Gleick 1993, UN 1997), have seriously underestimated the human dependence on renewable freshwater flows. Water perceived as unused or even invisible on a human-dominated planet, to a large extent, is already in use for ecosystem support and services to social and economic development.
What are the implications of our results for the management of freshwater, food production, and terrestrial ecosystem services in a world of an expanding human population, intensification in global affairs, and ecological systems undergoing rapid change? Obviously, a shift in perception and approach to water management is necessary. Water is not just an economic commodity to be engineered as input in food production or industrial activities. Water is a fundamental force in ecological life-support systems on which social and economic development depend. Freshwater flows, crop production, and other terrestrial ecosystem services are interconnected and interdependent. Therefore, water appropriation for crop production to a growing human population should no longer be viewed in isolation from potential impacts of freshwater re-directions. It may lead to erosion of critical and welfare-supporting ecosystem services in both terrestrial and aquatic systems, and potential conflicts between upstream and downstream
users.
Land use choices are also water choices, and will always lead to alterations
in the flow of freshwater and ecosystem services elsewhere. This trade-off
is made explicit in our scenario of freshwater for crop production to support a growing
human population. It has to become embedded in the management of dynamic freshwater ecosystem
linkages, in what we call the ecohydrological landscape. The challenge is immense
and will require co-management at catchment levels, often crossing administrative
and even national boundaries.
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Acknowledgments:
This article is the product of an interdisciplinary team work, where the
contributions of the authors successively grew out from their starting
contributions (Falkenmark's and Folke's conceptual bridge building,
Rockström's methodology development and Gordon's and Engvall's in depth
data analysis). Gordon's work is supported by the Swedish Council for Forestry
and Agricultural Research (SJFR), and Folke's partly by the Pew Scholars
program of The Pew Charitable Trusts.
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