Integrated modelling for sustainable management of salinity in the lower Vaal and Riet river irrigation areas

English: This thesis is the culmination of salinity economics research conducted for the South African Water Research Commission. The contribution of this thesis to science is not only in the field of Agricultural Economics. but also in other fields involved in irrigation salinisation research. It integrates the diverse mono-disciplinary spatial and temporal dimensions of the various disciplines of hydrology, agronomy, soil science and agricultural- and macro-economics, into an economic base model, to test scenarios and evaluate the economic, social and environmental sustainability of irrigated areas subject to salinisation. Problem Statement and the Study Area: Salinisation of irrigation schemes has become a problem in various schemes in South Africa. One such area that experiences salinisation problems selected for this research is the Lower Vaal and Lower Riet irrigation areas, upstream from where these two rivers converge and flow into the Orange River. By understanding the dynamics and interactions between irrigation water quality and the soil salinity status on crop yield over time, mistakes made in the past by choosing unsustainable irrigation sites and practices can be prevented in the future. Furthermore the impact of various natural or artificial (e.g. policy mechanism) scenarios on existing schemes can be more accurately modelled, leading to increased economic efficiency and sustainability of the irrigation industry, together with its primary and secondary linkages, as a whole. Aims: The overall aim of the WRC study on which this thesis is based was to develop and integrate multi-dimensional models for sustainable management of water quantity and quality in the Orange-Vaal-Riet (OVR) convergence system. More specifically the following sub-objectives had to be addressed: 1. To better understand the polluting chemical processes and interactions in and in-between the plant andsurface-, vadase zone- and ground- water, to achieve efficient and sustainable water quality management 2. To develop new economic models at both, a. Micro level, namely dynamic long term simulation models, and at b. Macro level, using a regional dynamic Input / Output model' 3. To integrate these new economic models with models from the other disciplines of: a. Hydroloqy" (incorporating a salt mass balance and flow), and b. Agronomy (crop growth in the presence of salinity model) 4. To determine and prioritise best management practices at: a. Micro level, (i.e. per hectare and irrigation block level) and at b. Regional level. 5. Through a better understanding of the multi-dimensional interactions, to enhance water use efficiency as the quantity and quality of water available for agriculture inevitably decreases 6. To develop policy guidelines to ensure social, environmental and economic sustainability 7. To achieve all these aims based on using the complex OVR convergence system as a study area, but developing a method and models that can be applied elsewhere with relative ease. This thesis however only covers the micro-economic aspect of the WRC project conducted by the author, and how it is driven by the hydrological and bio-physical processes and how it links and translates to the macroeconomic (regional) impact. Model: The economic base model of the integrated model uses hydrology and biophysical data and algorithms as inputinto the monthly time-step, per hectare Crop Enterprise Budget based, MSExcel simulation model (SMsim) to generate the base data. The resulting steehastic and spatially differentiated data set of per hectare total gross margin above specified costs data is then converted to sub-WUA, WUA, combined WUA and regional area level data for comparison and interpretation at these various levels and for input into the macro-economic regional level model (ISIM) and the index for socio-economic welfare (ISEW) for sustainability evaluation between alternative scenarios. Results: The results of this thesis inter alia show that the installation of irrigation drainage to facilitate leaching is a far better option than planting more salt tolerant crops. In the WRC project on which this thesis is based the results of a macro-economic analysis based on the micro-economic results from this thesis show that although at sub- WUA level it may not be financially feasible to install drainage in some sub-WUA areas, the secondary and regional socio-economic and environmental impacts justify the spending of government grants for drainage installation as the secondary benefits on the regional economy exceed the costs of the drains.