Disaggregating the components of a monthly water resources system model to daily values for use with a water quality model
Introduction
Monthly rainfall-runoff (Hughes, 2013) and water resources system yield (Basson et al., 1994, Mallory et al., 2008) models have been in practical use in southern Africa for many years. The Pitman (1973) monthly rainfall-runoff model was first used in the early 1970s and has been in continuous use with some modifications (Hughes, 2013) ever since. It has formed the basis of several national water resources assessments of South Africa, including the most recent update (http://waterresourceswr2012.co.za/: accessed on 2 April 2015). However, the use of a monthly time step has attracted criticism during some recent local conferences and workshops (not published information) for not being able to generate daily data that are required for some water resources decision-making objectives. One such objective is the integration of water quantity and quality modelling, while others might include the quantification of environmental flows and detailed routing of reservoir releases for downstream water users in semi-arid areas. There are many different options available to fill the gap in the availability of appropriate modelling tools that can generate simulated daily flows. These include an updated development of a daily version of the same rainfall-runoff model (Pitman, 1976), the use of an existing daily rainfall-runoff model that has been designed for the region (Warburton et al., 2010), the development of an entirely new daily rainfall-runoff model or the use of a tried and tested internationally available model (e.g. the GR4J or HBV models; Perrin et al., 2003, Staudinger et al., 2011). With respect to the objective of integrating quantity and quality, a further option would be to use an existing integrated model (Lindström et al., 2010).
All of these options would have to be associated with the development of a daily version of a water resources system yield model that is appropriate, and aligned to existing South Africa approaches for system yield analysis and planning (Basson et al., 1994 and many Department of Water and Sanitation internal reports available on the DWS website (https://www6.dwa.gov.za/DocPortal/AllDocuments.aspx). This statement is based on the assumption that it would be extremely poor water resources management practise to use a water quality model that was forced by different hydrological simulations than those used for water allocation and system yield management. The consequences, for all of the modelling options referred to above, would be that existing model setups would have to be reproduced within a new modelling environment and that practitioners experienced in the use of both the Pitman rainfall-runoff model and existing yield models would require re-training. Given that there are a large number of such model setups that are in current practical use by the DWS, this is likely to be totally impractical and not very popular with many water resources engineering practitioners. This paper refers to the water quantity components of an alternative approach that is currently under development and that involves the disaggregation of simulated monthly flow volumes (either existing or from future applications of the existing models) into daily sequences, which are then used to force a water quality model.
Section snippets
Disaggregation approaches
The first important key design issue is that the monthly water volumes of all of the components included within the water resources system model must be preserved within the disaggregated daily flow time series. While it is noted that, in some situations, the monthly systems model may not simulate the real flows very well (Hughes and Slaughter, 2015), it is nevertheless important to maintain consistency in the monthly water mass balance between the two models. Any problems with poor simulation
Discussion and conclusions
While some examples of the application of the disaggregation algorithms have been provided, many of them are difficult to test and validate, given the observed information that is available within South Africa. Slaughter et al. (2015) and Hughes and Slaughter (2015) provide additional examples of the main incremental flow disaggregation approach, and it is clear that the main sources of uncertainty are associated with the quality and validity of the monthly volume simulations and the rainfall
Software availability
The disaggregation model and the associated water quality model are being developed as part of the SPATSIM integrated modelling framework that is freely available from the website of the Institute for Water Research, Rhodes University (http://www.ru.ac.za/static/institutes/iwr/software/spatsimupdate.php).
Acknowledgements
The water quality model is being developed as part of a previous Water Research Commission of South Africa project (K5/2237) from 2012 to 2015 and a continuation project (K5/2448) from 2015 to 2018, which are managed by the second author.
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