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2. SUSTAINABLE MANAGEMENT OF WATER RESOURCES AT THE BASIN SCALE THROUGH ECOHYDROLOGY AND THE APPLICATION OF PHYTOTECHNOLOGIES IN TEMPERATE AND TROPICAL WATERS

A. Introduction

The application of phytotechnologies requires conditions that are conducive to plant growth. While temperate systems are subjected to intense seasonality, tropical ecosystems are characterised by high rates of plant growth throughout much of the year. Under such conditions, given adequate availability of water, the range of phytotechnologies that are employed in tropical systems can be more extensive than in temperate climates. However, such systems also often suffer from an high vulnerability to disturbance due to changing heat budgets and low levels of organic matter in the soils, as well as more extreme periodicity in rainfall. In addition, the growth of human populations and their aspirations in the absence of a sustainable development strategy has severely stressed freshwater ecosystems, and has been the major reason that water has become a non-renewable resource, in tropics. Analysing the major reasons for the decline in available water resources at the global scale, two major factors become evident.

First, water management has been focused on mechanistic/hydrotechnical approaches, roughly expressed through contradictory types of actions - actions that accelerate river flows due to stream channelisation, and actions that increase the retention of nutrients and minerals as an effect of the impoundment of rivers by dams.

Second, the watersheds in which the aquatic systems are located have been increasingly subject to progressive degradation of plant cover. Such degradation limits this most dynamic and vulnerable regulatory component of the water cycle within catchments, and, additionally, amplifies the effects of floods, droughts, and erosion.

The lack of understanding of the role and consequences of such degradation of the biotic component of the water cycle, within a catchment and its component freshwater ecosystems, leads to the point at which an ecosystem can no longer function in a manner which is considered beneficial by humankind. At this time, the resources of the catchment and its component ecosystems have limited human value. In reality, a fundamental element of the management of the water cycle at the basin scale must be an understanding the of biogeochemical evolution of the system. Therefore, the key to maintaining an increasing human population, especially in tropics, is the knowledge of the interplay between abiotic ecosystem elements, hydrology, and biological processes at a range of scales, in the face of increasing evidences of global climatic changes.

B. Ecohydrology and phytotechnology in integrated catchment management in the temperate zone

The temperate zones of the world are characterised by extremes in temperature and solar insolation that limit growing seasons, and, hence, the opportunity to utilise phytotechnologies on a year-round basis. This is not to say that phytotechnologies are not applicable, because they are effective means for protecting waterways and waterbodies at those times when they are most vulnerable to disturbances. These times are generally during open water periods, when temperatures and insolation are at their highest values - typically during the summer months. During the period from spring to autumn, the growth rates of vegetation are also at their highest levels. Thus, promoting and protecting the integrity of environmental corridors, including stream corridors, can benefit the natural resource base in a sustainable manner.

Temperate waterbodies can be characterised by watersheds having organic-rich soils, relatively stable hydrologic conditions, and fairly constant levels of rainfall throughout the growing season. Rates of plant growth are moderately high during this period, making phytotechnologies a feasible alternative for watershed management. However, the summer months tend to be the period where waterbodies are most stressed by human demands, which range from demands for potable and process water for economic activities (e.g. field irrigation), to demands for high quality water resources to support recreational activities that include both active (e.g., boating, swimming, angling, etc.) and passive (e.g., picnicking, wildlife viewing, etc.) uses. This competition for water resources often results in less water availability for environmental purposes, which may be perceived as having less economic value than other consumptive uses. (In this way, the experiences of the temperate zone mirror those of the tropics, even though the temperate zone may be considered water-rich compared with some of the more arid tropical regions.)

C. Ecohydrology and phytotechnology in integrated catchment management in the tropics

One of the basic factors governing the growing season, other than sufficient water availability in the catchments of tropical rivers, is temperature. Tropical catchments represent broad range of hydrological conditions, ranging from areas of intensive rainfall where the persistent water supply generates intensive and consistent outflows to the basins where a large part of catchment is comprised of arid and dessert conditions. In both types of tropical ecosystems, there are different problems to be solved and a different hierarchy of goals and ecohydrological methods to be considered.

Highly vulnerable water resources in the tropics can be categorised by several factors: high temperatures, variable intensities of rainfall, and low organic contents in the soil. On the other hand, there is an high rate of plant growth and the potential for adapting a range of ecosystem components that complement the biodiversity within a broad range of ecological conditions. This allows the application of phytotechnologies for the restoration of the water cycle and attendant ecological processes. However, the use of this potential depends on available knowledge, as well as socio-economic and demographic conditions. Also, lessons learned from the past should be considered (see, for example, Figure 4.3).

The most important factor determining climatic evolution, and the consequent changes in the forest:grassland ratio, is the amount of solar energy reaching the Earth. This varies as a result of changes in the orbit of the Earth. During cooler periods, there is generally a decline in rainfall amounts, and the forests diminish while grassland increase in area. Such a dynamic has to be considered, in light of global change scenarios, when evaluating the potential for implementing phytotechnologies. Considering ecohydrology and phytotechnology as proactive tools, the sustainable management of tropical water resources has to be based on an understanding of the full range of interplay between hydrologic processes and biota, including the climatic evolution.

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