WQ2000: DEVELOPMENT OF AN INTERACTIVE SURFACE WATER QUALITY INFORMATION AND EVALUATION SYSTEM FOR SOUTH AFRICA
Report No: 950/1/04
February 2004

EXECUTIVE SUMMARY

INTRODUCTION

Much of South Africa is water stressed resulting in the continual need to develop water resources. Increasing reliance has to be placed on effluent return flows. Pollutant loads are also added by mining and industrial development, with evaporative concentration of salts in impoundments and irrigation schemes. This, together with the general aridity of much of the country, has lead to increasing salinity problems. Meaningful evaluation of water resource development schemes therefore requires assessment of water quality. Salinity has long been recognised as the most intractable and economically important water quality problem.

For the past two decades the tools have been in place to integrate salinity into the water resource planning process. However, high cost and time restraints have prevented this from taking place in all but the biggest development schemes. Even in these instances water quality evaluation has tended to be addressed only once the most favoured options have been chosen on quantitative criteria.

This raises the need for a tool that will enable planners to make rapid low cost, but realistic, initial assessments of water quality impacts at the earliest planning stage. The WQ2000 interactive system is aimed at providing just such a planning tool.

The initial focus was on developing and testing the system for the Vaal River catchment. This would be followed by refinement of the methodology and extension to include the remaining South African hydrological regions.

WQ2000 allows the user to:

• Rapidly assess the salinity implications of anticipated developments
• Provide support for more detailed assessments
• Overview the salinity status of resources
• Support extension of the WRC's Water Resource 90 (WR90) resource quantity assessment to other Southern African countries
• Provide input to the DWAF's Water Situation Assessment Model (WSAM).

Three project phases are envisaged:

• Phase 1: Development of and testing of methodology
• Phase 2: User testing and model refinement
• Phase 3: Extension to remainder of South Africa.

MODEL LAYOUT

The WQ2000 model layout is shown in Figure 1.

Figure 2.1: WQ2000 Model structure

WQ2000 provides the interface between the user, a database containing a large amount of data for each quaternary catchment, the monthly-time step WQT hydro-salinity model and the DWAF's GIS Viewer.

This enables the user to run the WQT model for a selected quaternary catchment and view the results for four different conditions:

• Virgin conditions for the specified quaternary catchment
• Virgin conditions for the entire cumulative system
• Developed conditions for the specified quaternary catchment
• Developed conditions for the entire cumulative system.

The user can elect to use default present day values, or to modify them via the user-friendly screen interface.

The GIS Viewer provides a means to present the results for a number of quaternaries.

HYDRO-SALINITY MODEL

The WQT hydro-salinity model represents the catchment system by means of user defined sub-system elements connected by flow routes. The following five discrete WQT sub-model elements are used in WQ2000:

• Catchment salt washoff sub-model
• Channel reach sub-model
• Irrigation sub-model
• Junction sub-model
• Reservoir sub-model.

The selected quaternary catchment is represented by two catchments, namely the quaternary catchment itself and the upstream catchment area.

MODEL CALIBRATION

The following factors prompted the selection of the Vaal River catchment as the study area:

• Strategic importance of the catchment
• Severe water quality problems
• Availability of an extensive database and recent calibration of the WQT model
• Familiarity of researchers with catchment
• Initiatives to extend the DWAF's WSAM model to include salinity
• The comprehensiveness of the study area.

DATA PREPARATION

A large amount of data had to be prepared for inclusion in the WQ2000 model. The default values reflected in the menu sheets are drawn from this data.

Three basic types of data are stored. These comprise:

• Quaternary catchment physical data
• WQT model parameter values
• Monthly data files.

The default values can be changed to reflect new or planned catchment developments. Permissible changes can be made to:

• Paved urban area
• Irrigation area supported by farm dams and opportunistic irrigation area
• Irrigation area from main stem river supported from upstream major dams
• Wetland area
• Channel bed loss
• Catchment runoff proportions upstream of minor and major dams
• Major or minor dam full storage capacities and full storage areas
• Dam basin shape
• Effluent and mining quantity and quality
• Inflow from inter-basin water transfer
• Water requirement from minor and major dams
• Compensation release from the major dam
• WQT catchment salt washoff module calibration parameters.

Any number of such changes can be made simultaneously for the selected quaternary catchment, and can be changed again in subsequent runs. Simulated quaternary catchment outflows can be stored as defined files. Such defined flows are used In subsequent runs for downstream quaternaries, obviating the need to re-simulate all the upstream quaternaries.

The main data sources used to populate the WQ2000 database include:

MODEL CAPABILITIES

Standard runs: WQ2000 allows simulation of time series of monthly flows and TDS concentrations for any selected quaternary catchment for natural or present day development conditions using the standard default values. The model then automatically generates a result summary that contains the following information for natural and present day conditions with and without the effect of inflows from upstream catchments:

• Quaternary outflow TDS concentration (average, median, 95 and 98 percentiles and flow-weighted average)
• Quaternary average outflow volume
• Catchment runoff before alteration by channel and reservoir storage, irrigation, point inflow or abstraction (average volume and TDS concentration)
• Average TDS concentration in major dam and flow-weighted average TDS concentration of spillage.

User defined changes: One or more of the default parameter values may be changed. This powerful feature allows the user to play the "what if?" game. For example, the effect of a new dam, changed effluent discharge or effluent quality may be tested.

Mapping: WQ2000 allows the user to generate, display and print maps.

MODEL LIMITATIONS

WQ2000 is intended primarily to provide a rapid assessment of the expected salinity implications of a planned development, or to prepare an overview of the regional salinity status. This is a powerful and flexible tool. However, it does have limitations that need to be observed.

• System simulation: WQ2000 is not a large system simulation model. In particular it is not intended to simulate month by month variations in water release between dams in highly regulated river reaches involving more than two major dams.
• Feedback loops: WQ2000 does not cater for feedback loops.
• Detailed evaluation: In many instances the standard WQ2000 system layout will be sufficiently accurate to yield the final results required to support the investigations. However, more detailed evaluation of some development options may require a custom made system layout to more accurately describe the catchment features.

USER MANUAL

A user manual has been prepared. This is available in electronic form via the WQ2000 help menus.

FURTHER DEVELOPMENT

Two types of further development are proposed. These comprise extension of the WQ2000 model to include the rest of South Africa and improvements aimed at increasing the versatility the system.

Envisaged enhancements include:

• Graphical display: Inclusion of routines to plot time series and duration curves of monthly flow, TDS concentration and TDS load.
• Comparison with water quality targets: Tabular and graphical comparison of simulation results with water quality standards and user modifiable water quality targets.
• Irrigation model parameters: Inclusion of menu tabs to adjust WQT irrigation module parameter values.
• Future time horizons: Provision of scenarios of future development conditions and routines to calculate appropriate starting catchment and irrigation area salt storages.
• Other water quality variables: Extension to include other water quality variables.
• System modelling: Setting up of a large-scale coarse system model to simulate the monthly releases from the major system dams.

CONCLUSIONS

Attainment of project aims: The WQ2000 model has been successfully developed and applied to the Vaal River catchment. The study area includes both highly developed and undeveloped catchment areas. In many instances model calibrations were interpolated to cover large incremental catchments.

Model uses: WQ2000 provides a powerful means to rapidly assess catchment water quality using a sophisticated monthly time step hydro-salinity model, without the user needing any in-depth knowledge of the WQT model or its calibration. WQ2000 can be used to:

• Obtain a rapid overview of the current water quality status
• Obtain present day (or natural) WQT model calibration parameter values.
• Rapidly test a wide variety of development options, including:
• - Change in urban area
• Change in irrigated area
• Change wetland area
• Introduce new dams, or change the sizes of existing dams
• Change mine pumpage flow rate and concentration
• Add new effluent sources or alter flow rate and TDS concentration
• Alter water importation flow rate and salinity
• Change or introduce channel bed loss (usually associated with mining)
• Change water demand on minor dams or major dams
• Change the major dam minimum water release.
• Compliment to WR90: WQ2000 is a valuable complement to the existing WR90 and could serve as a useful prototype for its proposed extensions.

RECOMMENDATIONS

The following recommendations arise from the project:

Application of WQ2000: The WQ2000 model should be put to use by DWAF, consultants and Water Board practitioners as soon as possible. This will permit taking account of water quality at all levels of scheme development. The speed and ease of use of WQ2000 makes it feasible to carry out water quality assessments for even the smallest schemes and at the very earliest stages of project development.

Identification and solution of problems: It is anticipated that use of the model may reveal some shortcomings. It is strongly recommended that such problems are brought to the attention of the developers so that appropriate correction can be made. Provision should be made to issue a new distribution CD-ROM after one year's use and to set up a Web Site where new revisions can be posted.

Model improvements: Model enhancements that have already been identified should be implemented at an early stage. These include:

• Graphical display of monthly time series and TDS duration curves
• Comparison of simulation results with water quality targets
• Future time horizon development scenarios and routines to calculate starting catchment and irrigation area salt storages
• Irrigation model parameter adjustment
• Inclusion of other water quality variables (after geographical extension of the existing system)

Extension to rest of South Africa: The WQ2000 model database should be extended to cover the remaining Water Management Areas (WMAs).

Recommended program: A phased approach is recommended:

• Phase 1: Model development This is the initial development phase, which has been completed with this report and its products.
• Phase 2: Model testing and enhancement A one-year phase is envisaged.
• Phase 3: Extension to remainder of Country It is anticipated that this would require 2 to 3 years. It could best be co-ordinated with the revision of WR90.