Report: TR 2009/18

Author: Matthew Taylor, Murray Mulholland and Danny Thornburrow

Abstract

There has been increasing pressure for conversion of forest to agricultural land within the upper Waikato catchment. Much of the soil mantle of this area consists of hydrological and erosion sensitive pumice soils. Changes in land use can impact on hydrological processes, e.g. interception losses from forest canopy are significantly greater than from pasture, resulting in significant changes to the overall water balance. Conversion from forest to pasture may also result in significant changes in the infiltration characteristics of the soil surface layer. These changes in infiltration characteristics may affect the frequency and magnitude of surface runoff generation under high intensity rainfalls, and thus may impact on the flooding characteristics of catchments subject to land use change. To enable a better understanding the impacts of land use change on flood risk, an assessment of the infiltration characteristics of soils in the upper Waikato under both pine forest and agriculture has been undertaken.

Infiltration characteristics of soil have been characterised by in situ measurements using a double ring infiltrometer at five paired sites, under agriculture and forestry, for 5 soils typical of the upper Waikato catchment. In addition to infiltration, the macroporosity of the soil surface layer was also measured in the laboratory from samples taken at each site.

The results from the in situ infiltration measurements showed infiltration capacity of soil under grazed pasture (between 3 and 99 mm h-1) was an order of magnitude less than that under pine forest (121-1207 mm h-1). Similar differences were observed for soil macroporosity, however total porosity does not seem to be greatly affected by land use. This suggests that intensified land use results primarily in a change in soil structure, rather than soil compaction.

The conversion of forest to agricultural land within the upper Waikato catchment is likely to result in increased flooding, with increased flood peak and intensity, and erosion and sedimentation. The degree of effect is yet to be quantified, but it is expected that the greatest effects will be felt for small catchments in high intensity, short duration storms, especially in summer due to dry antecedent conditions. Consequently there is likely to be a need for increased spending on soil conservation and river maintenance work. There are also potential effects on flood protection and drainage works downstream of Karapiro Dam, which require further investigation.

Infiltration measurements in this study were similar to literature values.

Texture class did not have a major influence on the infiltration rate for the soils used in this study, although the texture range is very narrow: (silt – sand).

Macroporosity appears to be a good predictor of infiltration capacity for the allophanic and pumice soils. It may also be a predictor for infiltration capacity of podzolic soils, although these soils often have a sub-surface pan restricting water movement. Macroporosity of any restricting pan or soil horizon may be a better predictor for infiltration capasity for podzolic soils though this requires more data to confirm.

Land use has a greater impact in determining soil infiltration capacity than soil type has within the Upper Waikato Catchment.

Recommendations:
1. Afforestation be encouraged, especially in erosion prone areas or to protect particularly sensitive areas.
2. Management techniques which maximise soil infiltration and minimise surface runoff from agricultural land are reported in the international literature, though their applicability to the Upper Waikato Catchment may require further investigation.

Some examples are:
a. New Zealand data on reducing hydrophobicity.
b. Investigate use of effluent irrigation to reduce hydrophobicity.
c. New Zealand data for Canterbury cropping land relief.
d. New Zealand data for relief of compaction in pumice soils.

Infiltration Characteristics of Soils Under Forestry and Agriculture in the Upper Waikato Catchment [PDF, 912 KB]