5 Surface-water Allocation
This section presents analysis of surface-water takes relative to mean annual low flow and mean flow of individual catchments. As outlined in Section 2.2, the analysis is based on surface flow records from sites in the NIWA database and from regional authority records.
The objective was to determine surface-water allocations (direct takes) as a percentage of mean annual and mean annual low flow for the National catchment coverage. Surface water is allocated in 535 catchments with a combined weekly allocation of 387 m3/s (out of a total surface-water allocation of 405 m3/s). There were one or more flow monitoring sites within 128 of these catchments (128 catchments with mean flow and 113 catchments with mean annual low flow). However, there were issues and constraints for a number of sites, these included: location of sites in upper reaches or on tributaries which therefore did not provide a good measure of whole catchment flow; modified flow regime due to storage and hydropower generation; and limited duration of records. After screening of flow records, mean annual flow sites were selected for 113 catchments (from 128) and 78 catchments for mean annual low flow (from 113 catchments).
Bearing in mind the above constraints, the figures below provide an indication of levels of allocation (specific allocation calculated a weekly allocation per catchment area) and allocation as percentage of mean flow and mean annual low flow. Note un-shaded catchments in the figures are those for which flow records were unavailable or unsuitable.
Figure 5.1 shows the distribution of specific allocations (l/s/km2) for catchments with surface-water allocations. In the North Island, specific allocations, apart from a number of small urban catchments, are mostly less than 5 l/s/km2. In the South Island, there are a number of larger catchments (Waitaki, Rangitata, Rakaia and Waimakiriri) greater than 5 l/s/km2 reflecting the high allocations for irrigation.
Figure 5.2 shows the distribution of catchments by percentage allocation of mean flow. However, it should be recognised that, as discussed above, there are a number of limitations with the current analysis. In particular, where sites are not representative of the whole catchment flow, the percent allocation flow will be inflated. Nevertheless, the figure does show the relative allocation pressure on catchments.
Figure 5.3 shows the distribution of catchments by percentage allocation of mean annual low flow. The same limitations apply as discussed above.
Figure 5.1: Specific allocation for parent catchments
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figure at its full size (including the text description).
Figure 5.2: Allocation as percentage of mean flow
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figure at its full size (including the text description).
Figure 5.3: Allocation as percentage of mean annual low flow
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figure at its full size (including the text description).
