Optimal allocation of water resource in irrigated farming at the Ramah Canal Vanderkloof Dam
Mahlaha, Jacinta Mamaleke
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The flood plain in the Orange River at Vanderkloof Dam is classified as semi-arid. Natural rainfall in the area is very low and cannot support crop production. Therefore, the feasible way of producing crops is through irrigation. Agriculture must be prepared to respond to limited water by becoming efficient in water use. Increase in efficiency requires that the demand and supply management by individual water users be optimised and the value of water derived as measures to achieve efficiency in water use. The first part of the study involved a survey conducted at the Ramah Canal to ascertain the current farming situation and to determine whether economies of size existed in the area. Irrigated farms in the area were classified based on irrigation water rights into three average farm sizes of 75, 180 and 240 ha. Income and balance sheet statements were compiled to determine the financial situation of the three farm groups. From the statements, different financial ratios including solvency, liquidity, profitability and efficiency were calculated. The financial analysis showed that 180 ha farm group had the best solvency, liquidity, profitability and efficiency ratios. In the second position was 240 ha farm group. The analysis indicated that economies of size exists between farm groups with 180 ha farm being the optimal farm size to operate and 75 ha being the least efficient farm group. In the second part of the study, optimal cropping mixes at the Ramah Canal were determined under constrained and unconstrained irrigation water supply. Five crop mixes were formulated for each farm group. Crops under investigation were maize, wheat, lucerne, groundnuts, cotton and potatoes. A Linear Programming (LP) model was developed to determine optimal cropping mix that gives maximum returns under unconstrained water supply (100 percent). The objective function of the model was to maximise total gross margin subject to the following constraints: total available water and land during summer and winter seasons, maximum area under each crop, labour and tractor power required by the crop mixes. From the LP results, the total value product (IVP) functions presented as linear segments showing gross margin as a function of water applied were developed for each crop mix. The TVP functions indicated the sequence by which crops would be irrigated based on their contribution in maximising gross margin. Results showed that in summer season, potatoes would be irrigated first because of high profitability relative to other crops. As irrigation water becomes abundant, groundnuts, cotton, lucerne and maize will be irrigated in that order. Wheat was the only winter crop dealt with. From the TVP functions, Marginal Value Product (MVP) for water was derived. The MVPs were RO.09, RO.18, RO.25, RO.38, RO.39 and R3.64 for maize, lucerne, cotton, groundnuts, wheat and potatoes, respectively. Sensitivity analysis was carried out by reducing the full water application level to 75, 50 and 25 per cent water availability to determine the response of different crop mixes under restricted irrigation water supply. Results showed that in summer season, maize is the first to be affected by water limitations. Next is lucerne, then cotton, and groundnuts. Potatoes are the last to be affected by water restrictions. Furthermore, under severe water restrictions, farmers could lose more than half of their potential income. Water" management strategies which farmers would follow in future when irrigation water is limited were determined. Farmers in 75 and 180 ha groups indicated that they would completely change crop mix under severe water restrictions. Farmers in the 240 ha group with lots of farm investments, are very sensitive to reductions in water supply and are prepared to quit farming if water limitation persists. In conclusion, the study provided information and guidelines for choosing the best cropping strategies based on available irrigation water and other production resources. It is recommended that the study be done for a reasonable period of time since production is a continuous process. Furthermore, the potential of the area in producing high value crops should be investigated.