NEWS
rainfall , what could this mean for water supply ?
The upper catchment of the Kromme River feeds the Kromrivier Dam ( also referred to as the Churchill Dam ), one of the major reservoirs in the Algoa Water Supply System , contributing close to 30 % of NMBM ’ s normal water allocation . The 360 km 2 catchment area lies in the Tsitsikamma and Suuranys mountains and is dominated by fynbos vegetation . It is also known for its palmiet wetlands , however much of the area is highly invaded with black wattle ( Acacia mearnsii ).
Figure 2 : Estimated average rainfall , potential evapotranspiration ( PET ) and runoff by month for the Kromme catchment based on data for 1960-2018 ( error bars indicate standard error of the mean ), shown for a water year from April-March . This alternative water year was selected over the typical October-September water year to account for the winter peak in runoff , keeping the recession period following it withing the same water year .
reconcile supply and demand have led to investment in river transfer infrastructure , bringing in water all the way from the Orange River via the Fish River into the Sundays River , at no small cost .
The reconciliation strategy also calls for addressing system leaks and water recycling in the NMBM and acknowledges the positive impacts of clearing of invasive alien plants in catchment areas of the reservoirs , an ongoing effort largely through the Working for Water Programme .
This , and other water supply-demand reconciliation strategies across the country , rely on assumptions about the level of climate variability that can be expected in areas feeding their supplies . There is growing recognition of the need to take climate change into account going forward , urgently underlined by the current prolonged dry periods .
The Algoa region lies in the a-seasonal rainfall zone of South Africa , between the winter rainfall zone to the west and the summer rainfall zone to the east . It has inherently high interannual climate variability , sometimes catching the edge of winter and / or summer weather systems and sometimes not . This poses an extra challenge for predictive climate modelling .
However , looking across the suite of downscaled global climate models ’ ( CMIP3 and 5 ) predictions across different greenhouse gas emissions scenarios ( RCP 4.5 and 8.5 ) presented on the Climate Information Platform of the University of Cape Town Climate Systems Analysis Group ( https :// cip . csag . uct . ac . za /), the majority of models suggest a future decrease in winter rainfall and an increase in summer rainfall on average , with increasing average temperatures and the likelihood of extremes ( 2040 – 2060 model predictions relative to 1980 – 2000 ). The magnitudes of predicted change vary notably across the models and scenarios , but there is less discrepancy about the direction of change .
Upper Kromme catchment and available data
Water supply shortfalls are the outcome of many factors which change over time . Prolonged dry and warm weather conditions will impact the supply-vsdemand scarcity differently depending on properties of the catchment areas that impact streamflow generation and groundwater recharge ( e . g . land cover and use ), the water supply infrastructure and management ( e . g . storage and leakage ) and the magnitude of the demand .
Analyses presented here for the Kromme catchment focus on the climate and the catchment response . They provide a preliminary exploration into a few key questions : How extreme is the current climatological drought compared to the long-term climate record for the area ? Is there evidence of a long-term trend in annual and seasonal rainfall and streamflow ? If there is a shift towards less winter and more summer
The area supports fruit orchards , dairy and small stock farming . There are a few rainfall and temperature gauges from the South African Weather Service ( SAWS ) and Department of Water and Sanitation ( DWS ) in operation in and around the catchment ( Figure 1 ), some with data going back to the 1950s or earlier .
Unfortunately , there is no long-term streamflow gauge in the catchment . However , the water balance from the Kromrivier Dam , i . e . recorded reservoir water levels , outflows , rainfall and evaporation available from DWS , can be used to back-calculate an estimate of the streamflow entering it starting in 1957 , with useable data until late 2017 when an outflow gauge malfunctioned .
Being so mountainous , rainfall is unevenly distributed over the catchment . Rainfall spatial surfaces derived from station data by Lynch , 2003 ( Figure 1 ), accounting for elevation , aspect , and distance from the sea , were used to upscale station data to estimate a catchment-scale average rainfall time-series for the Kromme for the 1960 – 2018 water years .
Temperature data was similarly scaled using surfaces from Schulze and Maharaj ( 2004 ) and was used to estimate potential evapotranspiration ( PET ) using the Hargreaves and Samani ( 1985 ) method . PET is a measure of the potential for water to evaporate or be transpired by plants if there is water available .
Data from SAWS , the Agricultural Research Council ( ARC ), DWS and SAEON were used to patch gaps in records for the long-term stations . ‘ Water years ’ are used instead of calendar years in hydrological analyses to avoid splitting seasons down the middle . In this case , an April to March water year ( i . e . April 1960 to March 1961 as the 1960 water year ) was applied because average streamflow was found to be higher in winter .
Starting the water year in October is more typical , but less appropriate when
Grassroots Vol 20 No 4 December 2020 18