Optimal allocation of water resource in irrigated farming at the Ramah Canal Vanderkloof Dam

[摘要] 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 preparedto respond to limited water by becoming efficient in water use. Increase in efficiencyrequires that the demand and supply management by individual water users be optimisedand 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 ascertainthe current farming situation and to determine whether economies of size existed in thearea. Irrigated farms in the area were classified based on irrigation water rights intothree average farm sizes of 75, 180 and 240 ha. Income and balance sheet statementswere compiled to determine the financial situation of the three farm groups. From thestatements, different financial ratios including solvency, liquidity, profitability andefficiency were calculated. The financial analysis showed that 180 ha farm group hadthe best solvency, liquidity, profitability and efficiency ratios. In the second position was240 ha farm group. The analysis indicated that economies of size exists between farmgroups with 180 ha farm being the optimal farm size to operate and 75 ha being the leastefficient farm group.In the second part of the study, optimal cropping mixes at the Ramah Canal weredetermined under constrained and unconstrained irrigation water supply. Five cropmixes were formulated for each farm group. Crops under investigation were maize,wheat, lucerne, groundnuts, cotton and potatoes. A Linear Programming (LP) modelwas developed to determine optimal cropping mix that gives maximum returns underunconstrained water supply (100 percent). The objective function of the model was tomaximise total gross margin subject to the following constraints: total available waterand land during summer and winter seasons, maximum area under each crop, labour andtractor power required by the crop mixes.From the LP results, the total value product (IVP) functions presented as linearsegments showing gross margin as a function of water applied were developed for eachcrop mix. The TVP functions indicated the sequence by which crops would be irrigatedbased on their contribution in maximising gross margin. Results showed that in summerseason, potatoes would be irrigated first because of high profitability relative to othercrops. As irrigation water becomes abundant, groundnuts, cotton, lucerne and maize willbe irrigated in that order. Wheat was the only winter crop dealt with. From the TVPfunctions, 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 andpotatoes, respectively.Sensitivity analysis was carried out by reducing the full water application level to 75, 50and 25 per cent water availability to determine the response of different crop mixes underrestricted irrigation water supply. Results showed that in summer season, maize is thefirst 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 severewater restrictions, farmers could lose more than half of their potential income.Water management strategies which farmers would follow in future when irrigationwater is limited were determined. Farmers in 75 and 180 ha groups indicated that theywould completely change crop mix under severe water restrictions. Farmers in the 240ha group with lots of farm investments, are very sensitive to reductions in water supplyand are prepared to quit farming if water limitation persists.In conclusion, the study provided information and guidelines for choosing the bestcropping strategies based on available irrigation water and other production resources.It is recommended that the study be done for a reasonable period of time sinceproduction is a continuous process. Furthermore, the potential of the area in producinghigh value crops should be investigated.

[发布机构] University of the Free State