A management-oriented water quality model for data scarce catchments A.R. Slaughter * , D.A. Hughes, D.C.H. Retief 1 , S.K. Mantel Institute for Water Research, Rhodes University, Grahamstown, South Africa article info Article history: Received 18 November 2016 Received in revised form 14 April 2017 Accepted 25 July 2017 Keywords: Water quality modelling Requisite simplicity WQSAM Olifants river abstract Due to the degeneration of water quality globally, water quality models could increasingly be utilised within water resource management. However, a lack of observed data as well as financial resources often constrain the number of potential water quality models that could practically be utilised. This study presents the Water Quality Systems Assessment Model (WQSAM). WQSAM directly utilises flow data generated by systems models to drive water quality simulations. The model subscribes to requisite simplicity by constraining the number of variables simulated as well as the processes represented to only those most important to water quality management, in this case, nutrients and salinity. The model application to the upper Olifants River catchment in South Africa is described. WQSAM was able to use the limited observed data to simulate representative frequency distributions of water quality, and the approach used within WQSAM was shown to be suitable for application to data scarce catchments. © 2017 Elsevier Ltd. All rights reserved. Software and/or data availability WQSAM is run from the SPATSIM modelling framework, which is freely downloadable from http://iwr.ru.ac.za/iwr/software/ spatsim.php. To obtain WQSAM, contact Dr Andrew Slaughter, Institute for Water Research, Rhodes University, PO Box 94, Gra- hamstown, 6140, South Africa Email: a.slaughter@ru.ac.za. SPATSIM is windows-based and will run under recent and latest versions of Microsoft Windows on a desktop PC. 1. Introduction Degradation of the water quality of fresh surface waters has become a global problem, particularly within developing countries which typically have less resources available to implement man- agement of water quality (Zimmerman et al., 2008). This situation indicates the urgent need for water quality models as management tools. However, water quality models for managing water quality generally show less maturity than those for managing quantity. This is particularly true in South Africa, which has a relatively long and rich research history related to hydrological and systems models, having yielded two major hydrological models, namely the Agricultural Catchments Research Unit (ACRU) model (Schulze, 1989) and the Pitman Model (Pitman, 1973), their refinements and extensions (for example Hughes, 2004a,b; Hughes et al., 2010), as well as two systems models, the Water Resources Modelling Platform (WReMP) (Mallory et al., 2011) and the Water Resources Yield Model (WRYM) (Basson et al., 1994). The determination of the ecological Reserve for rivers in South Africa, which is the water quantity and quality that should be ‘reserved’ to maintain the aquatic ecosystem, has been facilitated from a quantity point of view through relatively sophisticated tools and methodologies, such as that developed by Hughes (2004a,b). In comparison, research on water quality modelling tools specific for use in South Africa is a relatively young and emerging science, and although some initial progress has been made, using mostly statistical regression relationships between flow and water quality, for example Malan and Day (2002), no mechanistic water quality models have gained traction within water resource management in South Africa as yet. Key to management of water quality in South Africa is understanding the relationship between flow and water quality. This is because flow is the primary driver of water quality, and directly affects water quality as a transporting mechanism of non-point source water quality loads from the catchment, by diluting water quality instream and by driving the residence time of water quality loads in surface waters. The relationship between flow and water quality in South Africa in particular is very * Corresponding author. E-mail address: a.slaughter@ru.ac.za (A.R. Slaughter). 1 Current address: The Association for Water and Rural Development, Sunset View Office Park, Corner Buffel Street and Koedoe Street, Hoedspruit, 1380. Contents lists available at ScienceDirect Environmental Modelling & Software journal homepage: www.elsevier.com/locate/envsoft http://dx.doi.org/10.1016/j.envsoft.2017.07.015 1364-8152/© 2017 Elsevier Ltd. All rights reserved. Environmental Modelling & Software 97 (2017) 93e111