5. Identification of methodological issues
5.1 Identifying Irrigated Area, Waster Use and Land Use
There is a lack of information generally about water use, the area irrigated and the land use. The assumptions in this report are “best guesses”, but should be updated as new information becomes available. The figures contained should be interpreted as being “in the order of” rather than absolute. AgriBase is a farm type database, and as such tends to understate arable and vegetable land uses.
5.2 Assigning Value
The analysis provides estimates of farmgate GDP attributable to irrigation from the waterbodies identified. It does not measure the return on the investment in irrigation or the comparable value of other uses. However, the basic information is similar, and the accompanying spreadsheets are formulated to enable variations on the method to be investigated and sensitivity analysis to be carried out.
5.3 Flowon Impacts
There will be some additional flow-on positive impacts on GDP in other sectors of the economy (e.g., processing and servicing industries) due to irrigation. However, this flow-on impact is not included in this calculation. To do so properly would require the development of a model of the economy without irrigation. While approximations could be made from standard Input/Output tables, the results would be based on some very broad assumptions. Given the magnitude of possible errors associated with assumptions about land use, productivity, costs and returns inherent in this study, estimating the flow-on impacts through the economy, real as they are, would not provide useful information without more detailed modelling than has been undertaken in this study
5.4 Marginal Return vs Average Return
The method used takes no account of water use and therefore measures average returns rather than marginal returns. For example, it may be that irrigation abstraction can be reduced in some waterbodies and re-allocated to other developments or other uses with no reduction in the value of the use.
5.5 Sharing of Water
Some irrigation schemes use water from more than one waterbody. In the analysis, the value is shared among the waterbodies in the ratio that they supply the water, where known. It must be remembered that without either waterbody the scheme would operate considerably less efficiently. Groundwater replenishment due to scheme irrigation above is also not factored into the analysis. For example, the Rangitata River supplies a large area directly and supplements the Mid Canterbury groundwater body as well, although the latter is treated as a separate waterbody in the analysis.
5.6 Legislative Impediments to Potential
The potential of waterbodies to supply irrigation water in some cases is limited not by their flows but by regulatory instruments e.g., water conservation orders. For example, the Rakaia River has enough water to irrigate considerably more than the area included in this analysis. This situation undervalues the potential of these larger waterbodies in two ways. Firstly, new areas of irrigation may be considered from that source. Perhaps of more importance is the likely substitution of the source of irrigation that would occur. This would be of benefit to the in-stream values of smaller waterbodies that are under abstractive pressure.
5.7 Unknown Potential of Private Development
The analysis doesn’t include future private irrigation development potential. This particularly applies to groundwater waterbodies and to individual takes from rivers and streams. In many districts there are restrictions emerging on new abstractions from these waterbodies, e.g., the Canterbury groundwater systems are largely under “red zones” where applicants for new takes need to show the impact on existing takes and the environment are minor. The exception is the Rautaniwha Plains in Hawke’s Bay, where the Regional Council has identified considerable potential for new abstraction. This area is therefore included in this analysis.
Contact for Enquiries
Water Programme of Action
Ministry for the Environment
PO Box 10-362
Wellington
or
Water Programme of Action
Ministry of Agriculture and Forestry
PO Box 2526
Wellington
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