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:
  1. One or more trees are required in order to provide shade, particularly in the late afternoons towards the east.
  2. 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.
  3. 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.
  4. The drainage system transports sediments along channels and deposits them along preferential pathways towards the pans.
  5. 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.