Physics and Chemistry of the Earth 114 (2019) 102805 Available online 8 November 2019 1474-7065/© 2019 Elsevier Ltd. All rights reserved. Quantifying the contribution of riparian total evaporation to streamflow transmission losses: Preliminary investigations along the Groot Letaba river S. Gokool a, * , E.S. Riddell a, b , J.M. Nel c , R. Raubenheimer d , T. Strydom e , A. Swemmer f , K. T. Chetty a a Centre for Water Resources Research, University of KwaZulu-Natal, Pietermaritzburg, South Africa b Conservation Management, South African National Parks, Skukuza, South Africa c Institute for Water Studies, University of the Western Cape, Western Cape, South Africa d Institute for Groundwater Studies, Faculty of Natural and Agricultural Sciences, University of the Free State, Free State, South Africa e Scientific Services, South African National Parks, Skukuza, South Africa f South African Environmental Observation Network (SAEON), Ndlovu Node, Phalaborwa, South Africa A R T I C L E INFO Keywords: Environmental water requirements Riparian total evaporation SEBS Subsurface water storage Transmission loss ABSTRACT The Groot Letaba River, situated in the semi-arid north-eastern region of South Africa is an example of a river system in which the uncertainty associated with transmission losses (TL) has limited the effective management of environmental water requirement (EWR) flows. TL along the river significantly impacts EWR flows, as it is often the case that specified EWR releases are not adequately received further downstream. Due to the limited un- derstanding of the magnitude of TL, as well as the dominant contributing processes to TL within the region, it remains a challenge to operate the river using downstream targets far from the source of operations. In an attempt to address this knowledge gap, detailed characterizations of hydrological processes were performed along the lower reaches of the river, which centred around the estimation of riparian total evaporation and quantifying the rapport between surface and subsurface water flow processes. Riparian total evaporation was estimated using the satellite-based surface energy balance system model, soil water evaporation measurements and open water evaporation estimates. Losses from the river to the adjacent aquifer were determined from the continuous monitoring of the groundwater phreatic surface and characterization of aquifer hydraulic properties. The results of these investigations indicated that present flows within the system are likely to be insufficient to satisfy gazetted median and extreme low flow targets. Overall, the study details key hydrological processes influencing TL along the river. It should, however, be noted that these observations only provide an under- standing of the system over a limited observation period. 1. Introduction Globally, water scarcity has been exacerbated by the effects of increasing population growth, socio-economic development and climate change (Molle et al., 2010; Pittock and Lankford, 2010). As a result, the availability of water to sustain the natural functioning of riverine eco- systems and the provision of ecosystem goods and services has fallen under threat (Pittock and Lankford, 2010). In a South African context, numerous perennial river systems have become severely constrained as water resources abstractions are close to exceeding or have exceeded the available supply and ecosystem resilience (Molle et al., 2010). According to Pollard and du Toit (2011), the environmental water availability in various catchments of the South African Lowveld have been on the decline during the latter periods of the 20th century. This has been attributed to the compounded effects of land-use change, as well as the improper management of water resources within these catchments. Consequently, the natural ecological functioning of the riverine ecosystems and surrounding environments have begun to steadily decline (Vlok and Engelbrecht, 2000; Everson et al., 2001; Pollard and du Toit, 2011; Shenton et al., 2012; Grantham et al., 2014; Overton et al., 2014), despite the EWR possessing the only ‘right’ to water, in addition to the Basic Human Needs reserve under South Afri- ca’s National Water Act (NWA, Act 36 of 1998). Intensive management of the EWR flows is therefore required to ensure that all water users * Corresponding author. University of KwaZulu Natal, Pietermaritzburg, 3201, South Africa. E-mail address: shaedengokool@gmail.com (S. Gokool). Contents lists available at ScienceDirect Physics and Chemistry of the Earth journal homepage: http://www.elsevier.com/locate/pce https://doi.org/10.1016/j.pce.2019.11.002 Received 25 February 2019; Received in revised form 28 July 2019; Accepted 6 November 2019