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. River systems that support high biodiversity profiles are conservationpriorities worldwide. Understanding river ecosystem thresholds to low-flowconditions is important for the conservation of these systems. Whileclimatic variations are likely to impact the streamflow variability of manyriver courses into the future, understanding specific river flow dynamicswith regard to streamflow variability and aquifer baseflow contributions iscentral to the implementation of protection strategies. While streamflow isa measurable quantity, baseflow has to be estimated or calculated throughthe incorporation of hydrogeological variables. In this study, thegroundwater components within the J2000 rainfall–runoff model weredistributed to provide daily baseflow and streamflow estimates needed forreserve determination. The modelling approach was applied to theRAMSAR-listed Verlorenvlei estuarine lake system on the west coast of SouthAfrica, which is under threat due to agricultural expansion and climaticfluctuations. The sub-catchment consists of four main tributaries, KromAntonies, Hol, Bergvallei and Kruismans. Of these, Krom Antonies wasinitially presumed the largest baseflow contributor, but was shown to havesignificant streamflow variability attributed to the highly conductivenature of the Table Mountain Group sandstones and Quaternary sediments.Instead, Bergvallei was identified as the major contributor of baseflow. Holwas the least susceptible to streamflow fluctuations due to the higherbaseflow proportion (56 %) as well as the dominance of less conductiveMalmesbury shales that underlie it. The estimated flow exceedanceprobabilities indicated that during the 2008–2017 wet cycle average lakeinflows exceeded the average evaporation demand, although yearly rainfall istwice as variable in comparison to the first wet cycle between 1987 and 1996.During the 1997–2007 dry cycle, average lake inflows are exceeded 85 % ofthe time by the evaporation demand. The exceedance probabilities estimatedhere suggest that inflows from the four main tributaries are not enough tosupport Verlorenvlei, with the evaporation demand of the entire lake beingmet only 35 % of the time. This highlights the importance of low-occurrence events for filling up Verlorenvlei, allowing for regeneration oflake-supported ecosystems. As climate change drives increased temperaturesand rainfall variability, the length of dry cycles is likely to increaseinto the future and result in the lake drying up more frequently. For thisreason, it is important to ensure that water resources are not over-allocatedduring wet cycles, hindering ecosystem regeneration and prolonging thelength of these dry cycle conditions.

hydrology and earth system sciences

Distributive rainfall–runoff modelling to understand runoff-to-baseflow proportioning and its impact on the determination of reserve requirements of the Verlorenvlei estuarine lake, west coast, South Africa