Downslope movement of nutrients in hill pasture, Taita, New Zealand

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Abstract
Surface water was collected over one year in a study of downslope nutrient movement in hill pasture. The areas above the collectors were unconfined by artificial boundaries and extended about 80 m to the nearest watershed boundary. Rain = 5 mm usually produced downslope movement but in most events water entered the collectors from only a few metres upslope. Overland flow was seen in only 3 out of 44 rain events and these accounted for about 3 times more runoff than all the others combined during the year. It was estimated that downslope transfer per rain event from the areas above the collectors varied from 0.02–108 m3/ha (geometric mean 0.38 m3/ha) or 0.02–12% (mean 0.15%) of the rainfall. During most rain events, the pasture area studied did not appear to contribute surface runoff to the stream 30 m away. A multiple regression equation showed that the variables: durations of rainfall at a range of intensities, soil moisture, and days since previous rain, explained 76% of the variation in runoff volume in rain events with intensities < 8.4 mm/h. Saturated soil moisture conditions enhanced runoff but the potential for runoff apparently increased also when the soil surface was dry. Inclusion of the number of days since grazing, mean grass length, and area of the bare soil in the regression equation explained only a further 4% of variation. However, grazing clearly resulted in increased runoff volume in several spring rain events. The significance of these results to the study of downslope nutrient movement is discussed.

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