4 DISCUSSION
We performed a preliminary study to better understand pan formation in
Save Conservancy in Zimbabwe. We studied the landscape setting of
natural pans and followed up a suspected pan in formation. The presence
and orientation of trees at most pans suggests a distinct relationship
between trees and pans. The orientation of pans in relation to trees
highlights the importance of shade in pan formation as it minimises
evapotranspiration (de Vries et al., 2011).
Our fieldwork data point towards preferential infiltration occurring
during the initial stages of pan formation. The shape of the
preferential flow area is distinctly similar to that of a pan. The
location of pans in tree shadows further alludes to the importance of
trees in pan formation. Shaded areas beneath trees are also vital for
animal congregation in the late afternoon towards east of trees. Goudie
and Wells (1995) show that animals play a major role in pan formation in
arid regions of the world by aiding deflation of surface or erosion.
Our field observations in the study area indicate that the following
prerequisites are critical for pan formation:
- One or more trees are required in order to provide shade, particularly
in the late afternoons towards the east.
- Animals that act as eroding agents in the immediate vicinity of the
tree, particularly by congregating to the east of trees in the late
afternoon.
- The eroded area needs to fall within the drainage system in order for
water to be supplied to form the pan. In addition, there is
possibility of groundwater discharging into the pan.
- The drainage system transports sediments along channels and deposits
them along preferential pathways towards the pans.
- Fine sediments form layers on the base of the pan limiting
infiltration and promoting ponding of water on temporary basis.
In contrast to the last point (number 5), our measurements revealed the
forming depression in the shaded zone as a point of preferential
infiltration (Figure 3). A prolonged period of time might be needed in
order for the deposition of clay to accumulate to levels where
infiltration is minimized and ponding can occur. This is important as
clays have the lowest infiltration rate among all soil types (Davie,
2008). Shaw (1988) has shown that most soil moisture stems from
rainfall. Thus, amount of rainfall has major implications on runoff,
infiltration and surface processes, including the deposition of clay
(Davie, 2008).
This distinct roles of fauna, flora and soil and hydrological processes
are critical for the formation of pans. Thus, their role in pan
formation needs to be understood and studied in order to verify this
hypothesis on a larger scale. To understand better the spatial
distribution of major hydrological processes and connectivity between
pans, we recommend that isotopic finger printing of water and soil in
pans, rivers as well as in terrestrial environments needs to be done.
Observations should be done in the initial phases of the wet season in
order to minimise the effect of evaporation on isotopic signatures.
Sampling exercise should be completed over multiple years to cover the
temporal dynamics of the system.
The use of remote sensing could also aid in better understanding of pan
formation at a large spatial scale. Goudie (1991) has shown how this
technique is critical in understanding large scale regional hydrological
process dynamics which contribute to pan formation and maintenance.
Wormald et al. (2003) have also shown how spatial analysis, in
conjunction with remote sensing, aid in gaining insights into
emplacement mechanisms of pans. Hence, multiple remote sensing tools be
used to gain insights into spatial and temporal dynamics of pan
formation (Meijerink, 2007). This will include, but not be limited to:
soil moisture; evapotranspiration; surface deflation; digital elevation
modeling; gravity; and surface hydrology.
A multitude of tools exist in order to complete a study of this nature.
One such tool is the use of Gravity Recovery and Climate Experiment
(GRACE) satellite data which provide long term total groundwater water
storage over large areas. When using satellite products, a major
challenge is to select the correct tool/tools taking into consideration
the resolution, as well as spatial and temporal variability of the
chosen satellite sensor. Despite all these issues, it is still an avenue
that needs to be explored, due to hydrological data scarcity in arid and
semi-arid environments like Save Valley Conservancy. The use unmanned
aerial vehicles (UAVs) to target areas of interest, or where a higher
resolution is required is a good option.