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

Abstract

Abstract. River systems that support high biodiversity profiles are conservation priorities worldwide. Understanding river ecosystem thresholds to low-flow conditions is important for the conservation of these systems. While climatic variations are likely to impact the streamflow variability of many river courses into the future, understanding specific river flow dynamics with regard to streamflow variability and aquifer baseflow contributions is central to the implementation of protection strategies. While streamflow is a measurable quantity, baseflow has to be estimated or calculated through the incorporation of hydrogeological variables. In this study, the groundwater components within the J2000 rainfall–runoff model were distributed to provide daily baseflow and streamflow estimates needed for reserve determination. The modelling approach was applied to the RAMSAR-listed Verlorenvlei estuarine lake system on the west coast of South Africa, which is under threat due to agricultural expansion and climatic fluctuations. The sub-catchment consists of four main tributaries, Krom Antonies, Hol, Bergvallei and Kruismans. Of these, Krom Antonies was initially presumed the largest baseflow contributor, but was shown to have significant streamflow variability attributed to the highly conductive nature of the Table Mountain Group sandstones and Quaternary sediments. Instead, Bergvallei was identified as the major contributor of baseflow. Hol was the least susceptible to streamflow fluctuations due to the higher baseflow proportion (56 %) as well as the dominance of less conductive Malmesbury shales that underlie it. The estimated flow exceedance probabilities indicated that during the 2008–2017 wet cycle average lake inflows exceeded the average evaporation demand, although yearly rainfall is twice 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 % of the time by the evaporation demand. The exceedance probabilities estimated here suggest that inflows from the four main tributaries are not enough to support Verlorenvlei, with the evaporation demand of the entire lake being met only 35 % of the time. This highlights the importance of low-occurrence events for filling up Verlorenvlei, allowing for regeneration of lake-supported ecosystems. As climate change drives increased temperatures and rainfall variability, the length of dry cycles is likely to increase into the future and result in the lake drying up more frequently. For this reason, it is important to ensure that water resources are not over-allocated during wet cycles, hindering ecosystem regeneration and prolonging the length of these dry cycle conditions.

Determining Hydrological Variability Using a Multi-Catchment Model Approach for the Western Cape, South Africa

“…Daily totals of precipitation and solar radiation, mean daily relative humidity, temperature, windspeed and maximum and minimum daily temperature were collected for the period 1988-2002 (including a 2-year initialization period). The data were attained from the World Meteorological Organisation (WMO) as global surface summary of the day (GSOD) data [23], as well as the Agricultural Research Council (ARC), the South African Weather Services (SAWS), the Department of Water Affairs and Sanitation, South Africa (DWS), the South African Earth Observation Network [45] and previous JAMS/J2000 model data [18,[46][47][48][49][50]. In total, 97 precipitation, 24 temperature, 12 relative humidity, 15 windspeed and seven solar radiation stations were used (Table 2).…”

“…The model calibration was performed using a semi-automated procedure (e.g., [66]), which utilized the non-dominated sorting genetic algorithm (NSGA-II) [67] to explore the 'optimal' parameter space and model expertise from other applications [18,46,48,49,68,69] to finally select quantifiable model parameters for the region. For the calibration and validation of the Velorenvlei, Berg, Bot and Breede catchments, the time series was split into two (e.g., [70]), with (1) calibration from 01-01-1990 to 31-12-1995 and (2) validation from 01-01-1996 to 01-01-2002.…”

Coupling ecological water requirement to optimize water resources under changing climatic conditions

“…The nonstationary features of ecological flow arise from fluctuations in watershed runoff. Previously, watershed runoff, which is correlated with rainfall, has generally been analyzed using rainfall-runoff models (Watson et al, 2019). Traditionally, climate change effects are described into multiple downscaled climate-change scenarios related to future rainfall and temperature.…”

Mapping Interflow Potential and the Validation of Index-Overlay Weightings by Using Coupled Surface Water and Groundwater Flow Model

“…However, the validation of the interflow potential might require interflow observations, and the direct observations are usually limited. In many studies, the observations of interflow are based on the calculation of water balance in different reaches of the rivers [23][24][25]. The river discharge values across different river sections must account for the discharge differences of interflow, specifically between two river sections.…”