When we make large scale changes in land use, such as modifying the land surface or vegetation cover, we can significantly change the annual water yield, flood and low flows in the associated catchment.

Land uses that decrease water yield, reduce the amount of water available for fish and other aquatic life, and for human use. Other land use changes can increase runoff, increasing flood flows and potential flood damage.

The Cause

Afforestation can cause significant reductions in water flows, with afforestation of developed pasture catchments reducing yield by average of 50 percent. Significant increases in the extent of forestry plantings are anticipated in Canterbury and, while small blocks planted in the catchments of the major rivers are unlikely to be of immediate concern, extensive plantings in small catchments could increase the competition for an already limited resource.

Conversely, logging forests can significantly increase flood peaks during small to medium sized rain storms. Continued urbanisation in the Heathcote and Halswell catchments, by reducing the rate of which water infiltrators the soil, will reduce groundwater recharge, and increase both the occurrence of low flows and the size of flood flows.

The Options

Monitor the occurrence of high impact land uses in water short areas as an early warning measure of flow changes, with "trigger levels" set where the need for land use controls will be considered.

This will provide a better basis for targeted management, but results may be too late to influence land use changes.
Where high-impact land uses require a consent from a district council request (through submissions and liaison) that effects of the land use on water quantity be part of the consent assessment requirements.

This will reduce the duplication of consent requirements, but will need the cooperation of district councils.
Encourage land-use practices to protect or enhance water resources, such as by off-stream storage, and have property plans identify appropriate land use, such as planting trees in major river catchments instead of in sensitive foothill catchments.

This could be extremely effective in the minimising the effects of land use changes, but it requires a cooperation from the landholder and individual plans may not cover all of a catchment.
Promote tailoring of the national Forest Code of Practice to the Canterbury Region, including identification of areas sensitive to land use changes and forest management practices that minimise reductions in water yield.

This could encourage development in less vulnerable areas, but compliance would be voluntary.
Prepare regional rules to require consent for high impact land uses in sensitive (e.g. water short) areas.

This enables assessment of the risk and restriction of potential impacts, but may limit land use choices in some areas.