THE DISA HYDROSALINITY MODEL
REPORT NO. 369/1/01
The Breede River is one of South Africa's primary vine and deciduous fruit growing areas. Although irrigation occurs throughout the catchment, the greater portion of the irrigated lands, comprising some 45 000 ha, is situated between Worcester and Bonnievale. This irrigation occurs along the main stem of the Breede River, as well as along its tributaries. The majority of these lands are supplied with water from the Greater Brandvlei Dam as part of the Greater BrandvIei Dam Government Water Scheme (GBDGWS). Relatively small volumes of water are supplied from groundwater and small dams on tributary rivers. The bulk of the Brandvlei Dam water is either released into the Breede River channel from where it is diverted into canals or pumped, or released directly into the Le Chasseur canal for downstream distribution. The main channel of the Breede River, however, also acts as a drain for saline irrigation return flows, the effect of which requires amelioration in the form of freshening releases from Brandvlei Dam.
During the 1980s irrigated areas were expanding steadily, albeit at a relatively low rate. The political and constitutional reforms set in motion in the late 1980s created expectations of increased export opportunities for products from this area, leading to a distinct likelihood of increased irrigation expansion in the 1990s. Concerns arose that the concomitant increases in future saline return flows would render certain stretches of the Breede main channel unuseable as a supply conduit.
Against this background the then Department of Water Affairs (now known as the Department of Water Affairs and Forestry (DWAF)) appointed Ninham Shand in 1988 to develop and apply a computer model, capable of predicting the impact of irrigation development in the GBDGWS supply area on river flow and salinity. During 1990, the DISA (Daily Irrigation and Salinity Analysis) model was completed and implemented to examine the potential impacts of a number of planning scenarios for the GBDGWS supply area. Aspects addressed included future salinity and flow patterns in the Breede River, as well as requirements for future freshening releases from BrandvIei Dam.
In 1987, an intensive five year hydro-salinity field research programme to support the development of the DISA model was initiated and conducted by the Hydrological Research Institute (HRI) in the Breede River Valley. This research was based on an intensive monitoring programme at measurement points in the main river, tributaries, canals, deep and shallow boreholes, piesometers in alluvial soils, tensiometer fields and rainfall stations. By 1990, Government spending cuts and subsequent Departmental budgetary constraints necessitated a curtailment of some longer term projects, which included a premature end to the HRI's Breede River Salination Research Programme. The time table for this termination required the conclusion of all Breede River field studies, including intensive monitoring, by the end of the 1989/1990 irrigation season. This timetable meant that certain ongoing aspects related to the model development had to be regarded as provisional, as continued field research and monitoring was required for further model verification.
To ensure that the full potential of the Breede River Salination Research Programme be realised, an agreement was subsequently reached whereby the Water Research Commission would provide funding to ensure that certain actions relating to the original project specifications could be finalised. These included a further year of intensive monitoring, refinements to certain aspects of the DISA model, and effective technology transfer. This report describes the refined version of the DISA model, including model verification and sensitivity analyses. It also serves to upgrade the existing documentation, which is in the form of consulting reports to the DWAF, to a research oriented communication.
In January 1991, an agreement was reached whereby the Water Research Commission would fund a project to complete research related to the DISA model. The aims of this project, which culminates with the publishing of this report, were to:
DEVELOPMENT OF THE DISA MODEL
The following comments summarize the modelling philosophy which governed the development of the DISA model:
The DISA model is imbedded in a user-friendly software environment, and was developed with the Turbo Pascal programming language. The software consists of four components:
It can be appreciated that the modelling approach implemented by DISA, which is both physically based and highly organised in terms of the physical structure of the modelled area, is extremely data demanding. The data input to the model consists of two types, viz. time series data and physical data parameters.
Time series data serve as external input to the modelled catchment area. These include daily rainfall and evaporation records within the catchment, as well as daily flow (m3/s) and salinity (mg/1) records in rivers and tributaries entering and leaving the catchment
Every return flow cell as well as every sub-model in the water distribution network has to be defined in the model in terms of parameters or physical process constants. To obtain these parameters, a detailed knowledge of the system is required in terms of layout, land-use, soil characteristics, river channel, canal and farm dam geometry and abstraction rules implemented by farmers.
The final stage of model development involved the verification of the model to demonstrate the applicability and accuracy thereof. The model was verified on the Breede River system as well as the Vaalharts Irrigation system.
The Breede River System
Model verification for the Breede River system was completed in two phases, namely an internal verification and an external verification. Three database years were used, viz. 1985/86, 1986/87 and 1987/88. Verification of the model was extended to cover two additional years, namely 1988/89 and 1989/90. Due to gaps in either inflowing datasets or downstream datasets, verification on these two years was only partially successful. The internal verification was done by evaluating alluvial aquifer behaviour, with the groundwater levels in the alluvial return flow cells used as an indicator of "acceptable" behaviour. Groundwater levels in the aquifers behaved as expected, with simulated water levels fluctuating between approximately 300 mm below the surface and 500 mm above the bottom of the lowest layer. Salinity concentrations in a few aquifers were not in equilibrium. In line with the "no calibration" approach, it was decided that overall model performance was not adversely affected by this and that no rectifying measures were required.
External verification was evaluated by comparison of observed and modelled river flow and salinity time series at two different flow gauging stations. It was found that seasonal trends and magnitudes of river flow were simulated satisfactorily.
The Vaalharts Irrigation System
During 1995 the DISA model was applied to the Vaalharts Irrigation system. This was done to evaluate the applicability of DISA to an irrigation system situated in a summer rainfall region, as well as to evaluate the performance of various additional processes incorporated into DISA, e.g. surface runoff, artificial drainage and deep percolation.
Only an external verification was completed. A period of five seasons between October 1988 and April 1991 was considered and included three summer and two winter seasons. Observed and modelled river flow and salinity time series at a weir situated at the downstream end of the system were compared.
Overall, the time series and percentile curves indicated that during certain months, DISA underestimated the salt concentrations, while simultaneously overestimating flow. This was attributed to the fact that the simulated volume of tailwater (water which is not abstracted from the canal system for irrigation) is too high. As this water is relatively fresh, it leads to low TDS concentrations and an accompanying overestimation of river flow. This was caused by incorrect monthly abstraction volumes for irrigation. The situation was remedied by modification of the monthly distribution of irrigation supply, which led to a much better fit between observed and simulated flows and especially salinities.
This application of the DISA model confirmed that about 80% of the TDS load in the incoming irrigation water is retained in deep groundwater bodies underneath the Vaalharts Scheme.
The DISA model was subjected to two series of sensitivity analyses performed during different stages of the model development. Five model components for which inadequate or no field data existed, were identified during the verification phase of model development. The need for sensitivity testing of a further five model components came to the fore during model application for a planning study of the Breede River System. The full set of components which were tested are:
The configuration of the Breede River system was used to perform the sensitivity analyses. In order to assess model response, the return flow sub-model behaviour (internal) and model response at the downstream end of the modelled area (external) were monitored. Internal model response was evaluated by monitoring changes in the groundwater levels of four representative alluvial return flow cells, while external model behaviour was assessed by comparison of modelled outflows and associated salinities with observed values at a downstream weir.
Several scenarios were investigated for each component under consideration and where necessary, refinements were made to the relevant process variables.
CONCLUSIONS AND RECOMMENDATIONS
The DISA model provides a useful tool for anticipating the effects of various irrigation and operational planning scenarios on river salinity. The principal conclusions that were reached during model development and subsequent verification are:
The extended surface and groundwater monitoring exercise, undertaken by MBB Consulting Engineers on behalf of Ninham Shand, and taken over from the Department of Water Affairs and Forestry's Hydrological Research Institute (HRI) in May 1990, was completed in April 1991.
The original dataset for 1990/91 contained a number of problems, the most notable of which was a downward drift in electrical conductivity (EC) readings, due to progressive under-registration of the reading instrument. Based on laboratory tests of the EC probes used in the monitoring, conducted by Prof H Moolman of the University of Stellenbosch, the original datasets were adjusted to compensate for the instrument error. Adjusted datasets were handed over to the HRI for archiving.
During verification of the DISA model, it became clear that further improvements could be made to either facilitate its application or improve its accuracy. These improvements include :