The total Mean Annual Runoff (MAR) of the Gouritz water management area is 1 680 million m3/a. Approximately 20% (332 million m3/a) of the total surface water runoff originates from the Great Karoo (Gamka and Groot River catchments), 14% (229 million m3/a) from the Olifants River catchment and 66% (1 118 million m3/a) from the rivers of the coastal belt. Of the MAR within the coastal belt, the catchments to the east of Mossel Bay  generate almost 70% (771 million m3/a) of the 1 118 million m3/a.  The runoff from the Great Karoo and Olifants River catchment is very erratic. Consequently some dams take up to 10 years to fill, only reaching full capacity after major flood events. Thereafter storage levels decrease significantly over periods of up to 3 years, and fluctuate at low levels until the next flood event of sufficient size allows the dam to fill again. (Ref 1)

In the interior catchments of the Karoo and Olifants River, runoff from many of the catchments in the Swartberg, the Outeniqua and Langeberg Mountains is perennial and the normal flows are  diverted into farm dams or into earth canals for run-of-river irrigation on a shared basis. Flood runoff from these mountains and from the Great and Little Karoo is also used for opportunistic run-of-river irrigation, but most is stored in dams for later use by irrigators, the largest of these being the Kammanassie, Stompdrift and Gamkapoort Dams. Lucerne is the dominant crop irrigated by run-of-river, whilst the Gamkapoort and Calitzdorp Dams supply water primarily for vineyards and fruit production. Groundwater is used extensively for water supply to the urban sector, and for rural domestic use, stock watering and irrigation to a lesser extent.  (Ref 1)

Groundwater plays an important current and potential role as a source of supply, particularly in the drier regions of the Karoo and Little Karoo, where surface water runoff is predominantly in the form of flash floods, and there is very limited base flow. Groundwater supplies in these areas have a higher assurance of supply than surface water even when this is stored in dams, because of the irregular nature of flood runoff and the high evaporation rates, which impact assurance of supply.  (Ref 1)