Hydrological drought propagation and societal feedbacks in the Incomati River basin

Human influences have become an important part in the drivers of drought in the Anthropocene. Activities such as land use change, damming of rivers, expansion of irrigated agriculture and the cultivation of high water consuming exotic trees can modify the hydrology of catchments and lead to unintended consequences such as the exacerbation of droughts. The measures that societies implement in response to drought such as the excessive abstraction of groundwater; increase of the storage capacity of reservoirs and increase in irrigation efficiency have also recently been observed to have feedbacks on the occurrence of drought. Recent observations indicate that in some river basins of the world, have unintentionally intensified droughts. Understanding the feedbacks that exist between society and drought propagation is essential in designing sustainable drought response plans. However, the understanding of how drought propagates from meteorological drought through to hydrological drought is still limited. The human influence on the propagation of hydrological drought still needs to be studied in the majority of river basins. This research aims to widen the scope of understanding by studying the influence of society on the propagation of hydrological drought, with the Incomati River basin in South Africa as a case study. The main approach used is the pairing of catchments that are heavily regulated due to for example dams to relatively natural catchments. In some cases comparison between sites upstream and downstream of a dam has been employed. Drought analysis was mainly based on the variable monthly exceedance thresholds of 70 – 95% for both precipitation and discharge. Drought events identified through these thresholds were compared with droughts identified using the standard indices of SPI and SRI at timescales of 1 month, 3 months, and 6 months. Drought characteristics were determined over the entire analysis period of 1981-2018 for ten selected catchments. Propagation characteristics in the relatively natural catchments were compared with those in the heavily regulated catchments. This comparison enabled the identification of feedbacks that were later discussed in a wider framework of socio-hydrology to identify certain feedback loops that are being effected by the conditions of human influence. Results show that for the Incomati River basin, drought management interventions through the operation of dams that support water supply in catchments downstream of the dam, or inter-basin transfers are increasing the frequency of moderate droughts downstream, though the duration of these droughts decreased. The opposite is true for the catchments whose dams have no external demands for water supply. Drought management interventions therefore need to be planned with care because of their mixed feedbacks. This research recommends that the existing operation rules for Vygeboom dam and Nooitgedacht dam be revised to minimise the frequency of both moderate and severe droughts which are being aggravated in the Komati Basin due to the inter-basin water transfers from the dams to the Orifants catchment