Surface erosion characteristics of three Manawatu soils : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Soil Science at Massey University
Sheet erosion is the most extensively mapped erosion type in New Zealand. With the current financial returns from pastoral framing, land which was previously unaffected by sheet erosion is being cultivated and therefore becoming more susceptible to sheet erosion. The main objective of this study was to assess quantitatively, under the same conditions of slope, cover, and rainfall, the erodibility of three soils which are suitable for either arable farming or market gardening. Whether the eroded sediment consisted of sand, silt or clay particles, or more predominantly aggregates of these primary particles was also determined. The three soils examined were from the Manawatu region and included the Kiwitea silt loam (Dystrochrept), Levin silt loam (Dystrochrept) and the Tokomaru silt loam (Fragiaqualf). A portable rainfall simulator was used to generate runoff and sediment from soil packed in 0.2 m2 trays. All "storms" were for 60 minutes in which 65 mm of rain was applied. Particle selectively was determined using pipette analysis methods and a settling tube. Quantitatively comparing the erodibility of the three soils, it was found that the Levin soil was the most erodible and the Kiwitea was the least erodible. If the same storm intensity and soil conditions were to occur over a large area, one hectare could produce 6.4, 17.6 and 10.3 tonnes of sediment from the Kiwitea, Levin and Tokomaru soils respectively. Soil particles and aggregates were selectively removed by rainsplash and overland flow. The proportion of sand particles present in the eroded sediment was always lower than the original soil due to the inability of sand particles to be entrained by overland flow. Silt particles were easily detached and were most commonly eroded as individual particles. Clay particles were eroded and transported in the form of aggregates, a result of their binding properties. The size distribution of eroded sediment became progressively coarser over the rainfall period. This was because initially there was insufficient runoff energy available to transport the larger particles. A vegetative cover severely reduces the volume of runoff and the amount of sediment eroded by cushioning the raindrop impact.