Newsletter and Technical Publications
Freshwater Management Series No. 5
Guidelines for the Integrated Management of
- Phytotechnology and Ecohydrology -
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).
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
|Fig. 2.1. The application of
Phytotechnologies for the restoration of water,\ and nutrients cycles in basion
scale according to Ecohydrology Principles. (lager
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
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
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.
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.