Environmental Engineering Reference
In-Depth Information
on a larger spatial scale they can overlook the importance of smaller-scale
heterogeneity in vegetation distribution. East Africa has been a region of
large geological upheaval with volcanoes, block faulting and erosion creat-
ing plains, mountains and rift valleys and a variety of soils (Grii ths, 1993)
so that vegetation composition can change rapidly over short distances
(Van Keulen and Seligman, 1992; Snyman and Fouche, 1993; Armitage,
1996). Water availability is also an important factor for vegetation distri-
bution in semi-arid regions. While rainfall is seasonal and variable in low-
lying areas, rainfall in the mountains is more consistent and provides an
important source of water (Quinn et al., 2001). As a result, gallery forests
along rivers and wetlands can be found in semi-arid regions and provide
important variations in vegetation as well as sources of water in the dry
season. All these factors of seasonality and geology have created a system
in which resources such as grasslands, forests and water vary both spa-
tially and temporally. Primary production tracks rainfall patterns so that
not all areas will be productive at the same time over the region.
The reliance on climate as the dei ning factor for semi-arid regions has
posed dii culties in producing an accurate dei nition. A review of govern-
mental and international organizations working in semi-arid regions has
revealed four main dei nitions of semi-arid regions. The UK Department
for International Development (DFID) refers to semi-arid zones in its
Renewable Natural Resources Research Strategy as one of six produc-
tion systems (ODA, 1994). In the DFID dei nition a semi-arid zone is one
with a mean monthly temperature above 18°C, where evapotranspiration
exceeds precipitation in one or more seasons and mean annual rainfall
is between 400mm and 1200mm. The term 'semi-arid' is also used in the
Food and Agriculture Organization (FAO) agroclimatological stratii ca-
tion and dei nes an area with a growing period of between 75 and 119
days (Bourn and Blench, 1999). This does not correspond directly with the
DFID dei nition as the DFID rainfall parameters cover a growing season
of 75 to 180 days (Bourn and Blench, 1999). The FAO agroclimatological
dei nition is also more restrictive than the arid and semi-arid tropics and
subtropics dei nition that the FAO uses in its World Livestock Productions
Systems (growing season less than 180 days) (Sere et al., 1995). In addi-
tion a fourth dei nition is used by UNEP (United Nations Environment
Programme) in its studies of desertii cation (Mortimore, 1998). In many
empirical studies the semi-arid region has been dei ned by a minimum
rainfall of 400-500mm a year to exclude arid regions, which come with
their own set of particular management issues related to extremely low
rainfall and low population densities (Pratt and Gwynne, 1977; Mwalyosi,
1995; Mortimore, 1998). Above 400-500mm of rainfall a year rain-fed
agriculture, although marginal, is possible and livestock and people are
