NEWS low as 1 % in 2017 to 48 % in 1971 .
Years with the lowest rainfall were not always those with the lowest runoff or runoff ratio : seasonal distribution , rain intensity and the duration of a dry period made a difference . Several belowaverage rainfall years in a row were found to result in progressively declining runoff ratios as seen in 2008 – 2010 and 2016 – 2017 , such that a year with higher rainfall occurring later in a prolonged dry period can produce less streamflow than a lower-rainfall year occurring earlier in the drought .
Winter rainfall tended to produce a higher streamflow response , likely because evaporative demands are lower , as shown in Figure 4 illustrating monthly rainfall vs PET . The average winter runoff ratio was 20 % versus 13 % in summer . Years with more rainfall had higher runoff ratios ( R 2 = 0.52 , p < 0.1 ), but winter rainfall was a stronger predictor than total annual rainfall ( R 2 = 0.60 , p < 0.1 ). Although the long-term average showed a near-even split of rainfall between seasons , 67 % of the Kromme ’ s streamflow came in winter months .
The Mann-Kendall trend test ( Mann 1945 , Kendall 1975 ) was applied to both annual and seasonal rainfall , PET and runoff to determine if there is any statistical evidence of progressive change over time , over and above the interannual variability . Results indicate a statistically detectable declining trend in both annual rainfall ( MK test statistic z = -1.37 , p = 0.08 ) and winter rainfall ( z = -1.41 , p = 0.08 ) in the Kromme from 1960 – 2018 , with no indication of a trend over time in summer rainfall ( z = -1.0 , p = 0.3 ).
Figure 3 : Annual rainfall ( bottom ), runoff ( top ) and runoff ratio ( middle ) for the Kromme catchment indicating seasonal winter-summer split and long-term averages . NB : Displayed by water year , rainfall 1960-2018 , runoff and runoff ratio 1960- 2016 .
there is a winter streamflow peak because this would place the flow recession from a winter high into the next ‘ water year ’ ( Figure 2 ).
Climate and streamflow trends
Annual rainfall and runoff for the Kromme catchment by water year are shown in Figure 3 . The volume of streamflow is normalised by the catchment area to give a runoff depth value in millimetres for comparison to rainfall . The annual average rainfall for this period was 638 mm , but there is high variability with values ranging from 322 mm ( 50 % of average ) in 2008 to 1 035 ( 165 % of the average ) in 1971 .
On average , there is a fairly even split between rain falling in winter versus summer ( 52 % vs 48 %), but again this is highly variable , with winter rain ranging from 22 % to 77 % of the annual total in different years . The datasets used did not extend through the full 2019 water year , however catchment rainfall for the 2019 calendar year was 453 mm , well below average . The data shows that the current hydrological drought is not a product of the lowest rainfall years on record in recent decades , however , it is due to continuous below-average rainfall for multiple years in a row .
In the Kromme , streamflow is more variable than the rainfall . This is typical of semi-arid environments because they reach wetness thresholds for producing streamflow less often . The ratio of runoff to rainfall shows that on average , 18 % of annual rainfall becomes streamflow , with values for individual years ranging from as
PET showed clearer evidence of an increasing trend in annual , winter and summer values ( z values 3-5 , all p-values < 0.01 ). Annual and winter runoff totals showed detectable declining trends ( z = -1.9 and -2 , p = 0.03 , and 0.04 ), in keeping with the rainfall , as did the annual and winter runoff ratios ( z = -1.6 and -1.9 , p = 0.06 , and 0.08 ), in keeping with the increase in PET overall . No trends were detectable for summer runoff ( all p-values > 0.01 ).
These observed trends are in general agreement with climate model predictions for future decades for the region .
More to do , for everyone
Water managers and users are well aware of the problem given the Algoa Water Supply System has had supply shortfalls and needing use restrictions in 2009 – 2010 , 2017 – 2018 and again in 2020 . The current dam levels in the system are similar to the extremely low
19 Grassroots Vol 20 No 4 December 2020