The impacts of degraded vegetation on water flows: a case study in the Mzimvubu catchment.

Abstract

The Mzimvubu River is the largest undeveloped river course in South Africa, with the Mzimvubu catchment set to undergo high levels of both social and economic development. A study was undertaken for the catchment with the aim being to determine the impacts of different land use management scenarios on the catchment water flows through the use of the ACRU model. The verification stage of the study involved the modelling of the baseline scenarios of two preselected catchments, viz. T35C and T32A/B/C, in order to perform statistical comparisons of both simulated and observed streamflow. Whilst a number of the desired statistics were out of the ±15% confidence range, the differences between observed and simulated variances and standard deviations were well within the range and the R2 and Nash-Sutcliffe Efficiency Index (Ef) factors, though not exceeding 0.7, were deemed acceptable. The verification of the two Mzimvubu catchments was not ideal, and it was hypothesised that this may have been due, in part, to the parameterisation of degraded areas in the ACRU model configuration Degradation of vegetation can be considered in a number of different ways (from loss of cover through to bush encroachment and poor burning practice), although in ACRU it has only been modelled as a pure loss of vegetative cover. A methodology for determining vegetation parameters was thus determined from Leaf Area Index (LAI) data for 2008-2017 for sites within degraded areas and pristine veld areas within protected sites, and included calculation of crop coefficient, interception and percentage surface cover parameters that were then used within ACRU as the degraded vegetation parameters. These parameters were then input into the model, with simulations being run for both study catchments using both the Kristensen and FAO dual crop coefficients, as well as a set of simulations using degraded parameters that were calculated by using a percentage change (between 10 and 15 % difference) on the existing Acocks veld parameters within the model. This percentage change yielded very minor changes to the initial verification simulations; however, the two other sets of runs using the different crop coefficients both made significant changes to the verification simulations. The T32A/B/C simulation improved by almost 20 % and was only just outside the range of ±15% for the Kristensen set of runs. The T35C simulation, on the other hand, worsened although a challenge existed insofar as only the natural and degraded vegetation Hydrological Response Units (HRUs) had updated parameters – the large amount of commercial forestry,a known streamflow reduction activity (SFRA), within the catchment could have played a role in the under simulation of all the catchment’s model runs. Lastly, land use change scenarios were then modelled by changing both vegetative parameters and the area of different HRUs within both the T35C and T32A/B/C catchments. The scenarios modelled considered land degradation in its many forms, from the degradation of natural vegetation and subsequent rehabilitation, the increase in bush encroachment, differing severities and timing of burning, changes in areas under irrigated and dryland agriculture, and the conversion of traditional dryland crops to biofuel crops. These different scenarios proved to have different sensitivities to change, although all scenarios showed a lessening in the sensitivity as the area under change increased. Given the problems with both rainfall and streamflow records, further research on remote sensing and satellite imagery could provide another source of data for both climatic and land use. Further to this, the methodology used to determine the degraded vegetation parameters using remotely sensed data was shown to be an explicit and repeatable method and can be extended to incorporate the calculation of the parameters of other land uses, such as forestry and agricultural practices. This could be done in conjunction with in situ studies to test whether the methodology works for all types of land use.