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D. The application of phytotechnologies and ecohydrology in temperate and tropical catchments

As may be inferred from the foregoing discussion, there are both similarities and differences between the application of phytotechnolgies and ecohydrology in the temperate zone and in the tropics. Aside from the more obvious differences of specific plant materials appropriate to each climatic zone, length of the growing season, quality of the soils, and periodicity in rainfall noted above, the general approach to implementing phytotechnologies in both climatic zones is not dissimilar. The following discussion applies equally to both temperate and tropical regions, with the exceptions as noted. Most often, these exceptions are more a matter of degree than of substance, but they do need to be recognised and accounted for in the implementation of remedial measures (Thornton et al. 1999).

Planting strategies

The ecohydrological approach and phytotechnologies have to be used at the catchment scale from the headwaters of the catchment downstream. Based upon a theoretical tropical catchment, which flows from a forested area in the upper part of the catchment, to a semi-arid region in the lower part, a complex series of solutions have to be applied (Figure 2.1).

Due to heavy rains in the upper, forested watershed, an excess of water and vigorous catchment development processes may create danger of erosion of fertile surface of soil. In parallel, mineral and organic matter transported by erosion is deposited in a downstream reservoir, reducing its capacity, water reserve, and potential for energy generation. One method of stopping the extensive erosion is the planting of deforested areas. Such reforestation is shown as intermediate or patchy reforestation (Figure 2.1). Patches of fast growing, pioneering herbaceous vegetation are mixed with patches of trees having strong root systems. The consequent, rapid development of a diversified herbaceous structure comprised of plants with different rates of growth will reduce erosion and generate sufficient plant biomass to stabilise the heat budget of a given area. This, in turn, will maintain conditions for sustainable agriculture.

In the arid and semi-arid regions of the watershed, the major problem is limited plant biomass within the landscape. This limits the ability to stabilise the heat budget of the area, limiting agriculture to irrigated operations. To improve conditions for sustainable agriculture in this area, there is a need to increase plant biomass. The often observed failure of trees planted in semi-arid regions is due to uncontrolled grazing and the lack of adjustment of the spatial distribution of the plantings to the recent water and heat budget conditions. In this case, the solution is the sequential planting, starting at the land-water interface with protection against overgrazing. Where the plants are protected, and grow enough to provide the shade and maintain moisture in the soil, the plantings should be successful. For each plant community, there is a critical standing crop that needs to be maintained in order to provide sufficient biomass for a self-sustaining population to develop. High insolation in the tropics, combined with deforestation, often creates conditions that are difficult to reverse, and can lead to irreparable loss of plant cover. This is a consequence of loss of organic matter, rapid mineralisation, and water and wind erosion. The restoration of these areas using phytotechnologies in such case must be done using sequential planting (Figure 2.1). This process is implemented as follows:

• First, the land-water ecotones are restored.
• Second, plantings are used to stabilise the local heat budget and improve the maintenance of soil water conditions.
• Third, sequential plantings commence in stages, beginning with the upslope portions of the catchment, and continuing downslope after the establishment solid planted patches.

Trapping systems for eroded materials

The complete elimination of erosion from cultivated lands is impossible. Therefore, a need exists for a means of trapping and reusing eroded materials. In the case of small streams, dams of stones and brush, which overflow during periods of all but low intensity rainfall, can be an efficient means of trapping eroded soils. Other alternatives include the use of sand bags where such erosion is limited in intensity, and the use of dikes or other larger structures. However, the valuable organic fraction of the soil may not be retained by such small structure. This fraction is more efficiently trapped by shallow areas covered with vegetation. That is why, especially in case of large reservoirs, construction of a shallow, preimpoundment reservoir, with a portion of its area covered by vegetation, should be considered a priority.

Fig. 2.2. The scheme of nutrient trapping by constructed wetland

Such area can be periodically exploited as pasture lands for animals and the soils periodically excavated as a source of natural fertilisers for agricultural use (Figure 2.2).

Water quantity and quality maintenance

Key elements to be considered in assessing the suitability of phytotechnologies for use in watershed management programs typically relate to the influence of plants on the water balance within the drainage system. In temperate areas, these considerations are generally less significant than in the arid and semi-arid tropics, where water use by shoreland vegetation may be considered to be in competition with water use by crops and for other economic purposes.

Vegetation influences the hydrological cycle in several ways. These include:

• Interception of rainfall water by the vegetative canopy.
• Increased soil-water absorption capacity due to higher levels of organic matter.
• Uptake of seepage water by root systems and retention within plant biomass.
• Evapotranspiration of water from the soil to the atmosphere.
• Reduced overland flow as a result of vegetative water use and reduced rates of overland flow in areas covered by organic debris.
• Moderation of flood peaks due to the accumulation of woody debris in low order stream channels.

In addition, maintenance of shoreland vegetation along stream corridors can provide a degree of compensation for flood flows in larger rivers due to their role in floodplain protection.

Clearly, the role of vegetation, as summarised above, can be viewed as both a positive influence and as a negative influence depending upon the magnitude and frequency of rainfall events. As noted, in the temperate zone where rainfall periodicity is less apparent, the function of shoreland vegetation, and, indeed, vegetation with the watershed, is generally seen to be positive in that the ability of vegetation to retard overland flows reduces erosion, increases groundwater reservoirs through infiltration, and provides for flood storage. In the temperate zone, the relative volume of water retained within the vegetation and/or lost to the atmosphere by evapotranspiration is minor compared with the overall volume of precipitation. Likewise, in the humid tropics, diminution of flood flows and retention of water within vegetative biomass generally results in positive benefit. In the arid tropics, however, the interception and evapotranspiration functions of plants, when superimposed upon the greater periodicity in rainfall frequency and duration, often results in water use conflicts.

Recognising this conflict, the government of South Africa has recently adopted a new water law that recognises the desirability of maintaining the natural ecosystem. Under the provisions of this new legislation, water use by the natural environment has been assigned a water use allocation, in similar manner as other water users are allocated water rights. In addition to allocating water for use by wildlife and their habitats - which has positive economic benefit from ecotourism, this allocation has been used to protect and maintain the internal deltaic floodplains characteristic of the larger river systems of sub-Saharan Africa. These floodplains not only provide much of the remaining available wildlife habitat in the countries of sub-Saharan Africa, but also serve to protect downstream developed areas from flooding. Thus, even under conditions where shoreland and natural vegetation has traditionally been viewed as a competing water use, mechanisms exist whereby phytotechnologies can be used in sustainable watershed management.

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