Environmental Engineering Reference
In-Depth Information
found in much higher densities than in arid regions (Mortimore, 1998).
The variation in dei nition means that areas are included or excluded from
the semi-arid zone depending on the dei nition used and the type of vegeta-
tion associated with semi-arid areas.
In Tanzania, a restricted dei nition based on the FAO agroclimatologi-
cal stratii cation, or an upper annual rainfall limit of 900mm, would limit
the area considered semi-arid to Somalia-Maasai vegetation, which forms
a strip of arid land running from the north to a tip at the junction of the
Tanganyika, Nyasa and Eastern Arc rifts (Figure 9.2). The DFID dei ni-
tion of semi-arid regions, on the other hand, where the mean monthly tem-
perature is above 18°C, evapotranspiration exceeds precipitation in one or
more seasons and mean annual rainfall is between 400mm and 1200mm,
covers a more extensive area of land. This dei nition covers an area that
includes the Zambezian vegetation zone as well as the Somalia-Maasai
vegetation zone (see Figure 9.3). For Tanzania the semi-arid region in this
context covers approximately 80 per cent of the land area of the country,
an area of approximately 704 800km
2
.
The role of people and livelihoods
In the same way that climate and resources are variable in semi-arid
regions, so are the people who live and maintain their livelihoods in them.
Mortimore (1998, p. 10) refers to semi-arid regions as the 'heart of the dry-
lands' in Africa. They are extremely important for people, livestock and
agriculture. In order to understand semi-arid regions it is not enough to
understand the ecology in isolation. It is also necessary to understand the
communities who live in these regions. These communities have had, and
continue to have, an impact on the ecology of where they live. Equally,
the ecological conditions in which these people live have impacted, and
continue to impact, on them. It is this that constitutes the reciprocal
relationship between people and their environment in semi-arid regions.
This relationship has not always been recognized and as a result attempts
at sociopolitical and ecological control, i rst by the colonial governments
and later by the independent socialist government, have often led to a
decoupling of people and the environment.
Many of the ecological conditions observed today are the result of past
people-environment interactions and the subsequent decoupling of the
people-environment relationship. For example, savanna ecosystems may
be the result of long-term interactions between humankind and the envi-
ronment while the distribution of tsetse l y may be the result of depopula-
tion and subsequent woodland encroachment in areas once inhabited and
cleared for livestock or cultivation (Ilif e, 1979).
East Africa is recognized as important to human history. Archaeological
