Tussock grasslands and high water yield : a review of the evidence

Two hypotheses have been proposed to account for sustained high water yields in tussock grassland of upland east Otago: low evaporation and fog interception. We examine the evidence for the relative importance of the two mechanisms. Weighing lysimeter studies and those measuring snow tussock transpiration all suggest that restricted transpiration is responsible for high water yields from tussock grassland, despite the potential for tussocks to transpire more freely. Snow tussock appears to start controlling its transpiration rate through shutting stomata as the atmospheric demand for water vapour increases (i.e., an increase in saturation vapour pressure deficit). When this is combined with modest wet leaf evaporation, or canopy interception loss, it is clear that snow tussock is conservative in its use of water. A recent analysis of the isotopic signatures of rainfall, fog and water collected at the base of lysimeters suggests that water draining from the soil is a mix of rain and fog in sub-equal proportions. We offer other, equally valid, explanations for the same results that do not lead to the conclusion that fog deposition makes a substantial contribution to water yield. There is no evidence from catchment studies undertaken in the east Otago uplands to suggest that fog deposition is capable of augmenting water yield from tussock grasslands. We conclude that low evaporation from tussock grassland, especially during dry periods, is much more likely to explain the high water yields at both the plant and catchment scale.