1 INTRODUCTION
Pans are temporary aquatic formations which normally occur in arid and
semi-arid environments (Goudie, 1991). These temporary desert
depressions are extensively documented at a global scale (Goudie and
Wells (1995). The term ‘pan’ is of South African origin which refers to
a flat, sediment-filled depression that holds water periodically (Davies
and Day, 1998). This term will be used in this study to differentiate
rain pools of southern Africa from other types of temporary pools and
ponds.
The major drivers for pan formation in arid areas include; lack of
fluvial integration; deflation of the surface; limited sand;
accumulation and infilling; weathering of the surface, suitable stratum
and geological structure, (Marshall and Harmse, 1992; Goudie and Wells,
1995). Lancaster (1977) noted that dunes occur towards the south of the
majority of pans in the Southern Kalahari, Botswana. This adds another
dimension and needs to be explored in order to understand the impact the
pan has on dunes and vice versa. It was previously thought that pan
location is erratic and random in nature. However, Wormald et al (2003)
has shown that, on a larger scale, geological structural features play a
critical role in pan formation and location in southern Kalahari.
Verhagen (1990) proposed an ecohydrological model, which highlighted
erosion from animals as the major cause for pan formation in Botswana.
It should be noted that other ecological factors were not considered in
the past. Thus, the role that flora and fauna play in pan formation is
poorly understood. Therefore, the processes leading to pan formation are
not well known from an ecohydrological perspective.
Seasonal pans are the most common type of temporary pools in the lowveld
regions of Zimbabwe. Besides temporary aquatic habitats that are
directly connected to rivers during flooding, an important category of
temporary lentic water bodies is unconnected and has an autonomous
hydrology. This variation in dependency on rivers results in two main
groups of pans, namely; endorheic and floodplain pans. Endorheic pans
have an inward drainage system and depend on local rainfall and run-off
whilst floodplain pans are depressions along river banks that are
seasonally inundated during flooding. Basins with an autonomous
hydrology are defined as endorheic systems and are generally the most
abundant type of temporary pools. (Davies and Day, 1998).
The hydrological regime of the pans, and the processes related to their
formation, in the Save Valley conservancy are not little known. It
becomes critical to understand the hydrodynamics of this region, due to
its important role in ecological diversity (Nhiwatiwa et al., 2011).
This means that exploratory work is required in order to better
understand the processes dictating pan formation, pan evolution as well
as pan hydrodynamics. The ecology of these temporary pans has been
extensively studied and some environmental variables have been explored
by Nhiwatiwa et al. (2009). It has been shown that vegetation cover,
hydraulic connectivity between surface water bodies and pool depth are
major factors determining the biodiversity within pan ecosystems.