Method
Streamflow History
The term ‘accumulated streamflow’ is defined as the runoff (i.e. stormflow plus baseflow) from a specific catchment plus the runoff from all catchment areas upstream of the specific Quinary catchment in question. From these, the accumulated streamflows were derived and mapped.
Daily values of local runoff and hence accumulated streamflows, were computed for all 1 401 Quinary catchments within the Western Cape with the daily time-step and process-based ACRU simulation model (Schulze, 1995; 2004 and updates). All computations utilised the 50 years of daily climate plus soils and land cover inputs from the Quinary Catchment Database (Schulze et al., 2010). A baseline land cover of natural vegetation was used, represented by Acocks’ (1988) Veld Types, the hydrological attributes of which were described by Schulze (2004), and using soils attributes from Schulze and Horan (2010).
Mean annual accumulated streamflows were calculated for the historical climate and simulated with the ACRU model for present (mid-1990s) climatic conditions using 6 bias-corrected CMIP5 GCMs used in a current (as yet unpublished) WRC Project at the Centre for Water Resources Research at the University of KwaZulu-Natal.
Maps
Map Information
The map, with its non-linear legend, shows the very wide range of mean annual streamflows in the region. These range from < 10 mm equivalent to ~ 400 mm under historical climatic conditions, with highest streamflows in the high-altitude mountains and in the major river systems such as the Olifants in the north-west and the Breede in the south.
Equally striking is the wide range in the inter-annual variability of streamflows shown as the CV (%) of accumulated annual streamflows in the second map, ranging from < 40% to > 200%. In general, an inverse relationship exists between the magnitude of streamflows and the magnitude of CVs, with low CVs in the high streamflow mountains and the major river systems, and high CVs in the north-west and in the Karoo, where annual streamflows are low. An interesting exception is the southern Cape, where annual streamflows are relatively high but a higher CV is experienced compared to the river systems in the west.
In the next two maps, the ACRU model-derived median annual streamflows are shown in mm equivalents under present (1990s) climatic conditions, and for the lowest flows in 10 years.