Author: Murray Mulholland
Due to changes in economic returns from existing land uses, there is increasing pressure for conversion of forest to pastoral farming in the Central North Island. These areas consist of pumiceous soils, whose hydrological and erosion characteristics are sensitive to changes in land use. Conversion of forested areas to pasture within the Waikato region is currently unregulated by statutory planning instruments and can occur ‘as of right’.
A large block of plantation forest between Taupo and Atiamuri (the Wairakei Pastoral Block, or WPL) is being converted from exotic forestry to pasture . This work is programmed to occur over 15 years and is already well advanced. This study is aimed at estimating the effect of the change of land use for this 22,500 ha Pastoral Block on the flood hydrology of the Waikato catchment. The bulk of the Wairakei Pastoral conversion (19,900 ha) falls within four major catchments which have a combined area of 34,900 ha. This equates to 57% of the total catchment area within these four catchments being converted to pasture. The quantitative assessment in this study focuses on the effects on flood hydrology of the conversion of this 19,900 ha area to pasture within these catchments. Other potential effects of this conversion are addressed elsewhere.
In addition to the WPL land, large areas of the Carter Holt Harvey Kinleith forest are being sold and converted to pasture (mainly for dairying). The effects of that process would be additional to those addressed in this report, but for a number of reasons have not been taken into consideration here.
As part of the investigation, a review has been undertaken of the existing literature regarding the effect of conversion from forest to pasture on flood hydrology. This literature indicates that changes for pumice soils can potentially be in excess of an order of magnitude in terms of both flood peaks and flood volumes. For non-pumice country, flood peaks for pasture are about twice those from forest.
This study investigates the effect that the changes in land use on the Wairakei Pastoral land may have on the flood hydrology of the main catchments involved. The flood hydrology of the pumice soils of the central North Island is known to be particularly sensitive to changes in land use (Selby 1972).
The Effect of Land Use Change on the Flood Hydrology of Pumice Catchments
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|3. Existing knowledge||6|
|3.1 Catchment studies||6|
|3.1.1 Selby (1972)||6|
|3.1.2 Jackson (1973)||6|
|3.1.3 Rowe (2003)||6|
|3.1.4 Taupo Bay flood and erosion study (1978)||7|
|3.2 Analytical studies||7|
|3.2.1 Jackson (1999)||8|
|4. Additional analysis||9|
|4.1 Regional flood frequency analysis||9|
|4.2 Infiltration and unit hydrograph analysis||14|
|4.2.1 Infiltration analysis||16|
|4.2.2 Unit hydrograph analysis||19|
|4.2.3 Application of results to known catchments||23|
|4.2.4 Application of results to Wairakei pastoral catchments||27|
|4.2.5 Changes in runoff volume||31|
|6. Impacts of potential changes||33|
|7. Means of addressing potential changes||34|
|8. Summary and conclusions||35|
|Appendix A Analysed Flood Hydrographs||38|
|A.1 Purukohukohu at Puriki in Pasture||38|
|A.2 Purukohukohu at Puruki in Forest||42|
|A.2 Waiotapu Stream at Reporoa||47|