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Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


A. Introduction

Energy and water determine the nature of global plant cover and patterns of primary productivity (Starkel 1988; Breymeyer 1993). Plants, in turn, regulate over 80% of the energy flow through the ecosystem (Meagher 2000), affecting nutrient cycling, water retention, and water flow characteristics within overall the landscape (Baird and Wilby 1999).

One of the major consequences of an high rate of population growth and accelerated "development" has been the degradation of plant cover and freshwater resources. Elimination of, or a drastic reduction in, plant biomass within the landscape modifies temperature, hydrological conditions, and soil quality characteristics. These effects, often amplified by pollution, usually lead to a serious decline in water availability, agricultural productivity, and/or biodiversity.

Water is the medium that supports all life processes. Recently, overall global freshwater resources have been determined to have diminished (Shiklomanow 1998). As a consequence, the question emerges as to what should be a first step in reversing this dramatic trend? The answer is the quantification of ecological processes, through scientific analysis, and utilisation of this knowledge in sound ecological management. Thus, the river basin, where the water mesocycle and related biological processes are interlinked and can be measured, should be considered as the fundamental unit in sustainable water management.

There is an increasing body of evidence identifying the role of plant cover as a stabilising factor in the environment. Plant cover helps to moderate solar energy inputs, water dynamics, and biogeochemical cycles, both at the local scale as well as at the global scale. This role has positive consequences in reducing the frequency and intensity of catastrophic droughts and floods, and in improving the quality of freshwater resources (e.g., IGBP BAHC). The recent findings of Des Marais (2000) have suggested that, during the creation of the Earth, the appearance of oxygenic photosynthesis probably increased global organic productivity by at least two or three orders of magnitude.

The scientific basis for the use of plants as a controlling factor in ecological processes has been already identified and recognised. Examples of this recognition include the influences of plant communities on nutrient dynamics in land-water ecotones (Naiman and Decamps 1990), agricultural landscapes (Ryszkowski 1998), and phytoremediation (Rock 1997) and phytoextraction (Lasat et al. 1998) projects. This recognition extends to complementary measures, such as biomanipulation (Harper et al. 1999) However, in the face of the increasingly multidimensional forms of human impact, such measures need to be integrated into an holistic framework. This Chapter provides such a framework, linking energy, water, and plants as components of a fundamental ecosystem feedback mechanism. The development and implementation of this approach can assist in the restoration and management of ecological processes, leading to the achievement of sustainable development objectives.

B. The global pattern of biomass distribution and plant productivity

According to Zlotin and Bazilevich (1993), the global distribution of plant biomass covers a broad range, from less than 2 tons per hectare in desert and polar zones, to almost 1000 tons per hectare in tropical rain forests. In temperate zones, these value ranges from 300 to 400 tons per hectare, and, in boreal forests, from 50 to 300 tons per hectare over large portions of Africa, covered by savannahs, as well as parts of South and North America, Eastern Asia, and India. The pattern of primary productivity distribution on the Earth has been, to a certain extent, consistent with biomass distribution. However, in some regions - especially subtropical and tropical areas, primary production has been much higher than might have been expected. The maximum values, of up to 30 tons per hectare per year appear in over about one-half of South America, Central Eastern Africa, South-eastern and Central Asia, Eastern Australia, Southern Europe, Eastern North America, and Central East Africa. In India, the value is between 11 and 16 t/ha/y. This suggests that the potential upper limit can be achieved, under a given water /temperature soil regime, by restoring seminatural and natural plants communities through the use of phytotechnologies.

C. The historical consequences of degradation and modification of plant cover

From the point of view of thermodynamics, the evolution and functioning of the natural environment is based on three energy processes: flow, transformation, and accumulation. These processes are determined by long-term solar energy fluxes. Consequently, climatic changes arising from these long-term fluxes are a primary factor influencing water circulation, nutrient releases, and biotic succession and evolution. Water and energy, therefore, are the major abiotic factors that influence the evolution and succession of biota in any given of region of the earth (Figure 4.1).

Plant cover is the one of the most important factors in buffering the global heat balance (Ryszkowski and Kedziora 1999), stabilising water circulation within basins, reducing erosion, and controlling the transfer of nutrients from terrestrial to aquatic ecosystems. These functions, in turn, stabilise the quality of water resources, and enhance (or degrade) agricultural productivity and biodiversity.

Deforestation, brought about by human activity, modifies the structure of the soil and alters the water balance. The importance of these processes has been highlighted over the last ten thousand years. For example, the culture of ancient Greece evolved on the basis of biologically productive lands whereon degradation of plant cover and loss of soil productivity was reduced. As conditions changed, and plant cover was lost and soils degraded, the role of Greece in the economy and culture of Mediterranean region diminished.

On the other hand, there is evidence that highly efficient agriculture can replace natural plant cover, and can be maintained in a relatively stable manner in moist temperate climates if there is adequate water supply and artificial fertilisers are supplied (e.g., as is the case in Great Britain and other countries in Western Europe). However, there is a price for this stability, and it is an order of magnitude increase in the nutrient load to the freshwater ecosystems from agricultural lands (Maybeck 1998). These nutrient loads can cause eutrophication and serious water quality problems such as those related to toxic algal blooms (Figure 4.2).

This suggests that there is the potential to establish a new equilibrium between "water, bioproductivity, and biodiversity" in the most highly populated regions of the globe, even where the ecosystem structure and related processes have been seriously altered. To avoid potential negative side effects, such as declining water quality, the search for the point of equilibrium within a given watershed must be based on a scientific understanding of the multidimensional role of solar energy, water, plants, and soils in various types of landscapes.


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