Yellow-brown pumice soils cover a large area of the central North Island of New Zealand. In the Waikato River basin these soils occupy over 7,000km². Since land development started in the mid-1930s approximately 4,000km² have been converted from the native vegetation to exotic forests (1,243km²) and pasture (2,740km²), and a further 500km² could be developed. Since about 1959 gully erosion has become more common and widespread. The causes of this erosion were not known, although many hypotheses attempting to account for erosion have been put forward. The research reported in this thesis was undertaken in an attempt to isolate the most important causes of erosion. Three experiments have been completed. (1) A study of runoff from plots placed in areas of pasture grass, ungrazed grass and scrub vegetation has been made. Climatic, soil, vegetation, and slope variables were studied and as a result of statistical analysis it is concluded that surface water runoff is greater from developed land in pasture and less from areas covered by scrub and ungrazed grass vegetation. The major causes of runoff from pastures are very intense rainfall on a soil with low moisture content. (2) Infiltration studies with an infiltrometer, designed and built for the purpose, reinforce the conclusions drawn from runoff studies. They also show that modifications of soil properties, especially compaction caused by animals and vehicles, decrease infiltration and hence promote runoff. (3) Flume studies of the erodibility of pumice soils indicate that soil particles are easily entrained by running water, but that plant roots inhibit this process. Analysis of data from the three experiments indicates that land development should be carried out so that: (a) a close vegetation cover is kept on all soils; (b) channel development is avoided; (c) vulnerable areas such as valley floors and steep valley sides are kept as water absorption areas; (d) animals and vehicles are excluded from water absorption areas to prevent soil compaction; (e) plants with strong and dense root systems are used to protect surface soils.