WATER-BALANCE OF SNOW TUSSOCK GRASSLAND IN NEW-ZEALAND

Measurements of rainfall, drainage, and weight change of a 6-m2 surface area cylindrical lysimeter containing nine snow tussocks (Chionochloa rigida), were combined in a simple water balance equation to calculate evaporation on an hourly basis for 16 months in Otago, New Zealand. For most of the period it was possible to distinguish evaporation from dry foliage (transpiration), and wet canopy evaporation (interception loss), on the basis of leaf wetness. Mean daily transpiration rates ranged from 0.55 mm day-1 in June, to 3.32 mm day-1 in January; intercepted water evaporated at a median rate of 0.04 mm h-1 in winter and on summer nights, and 0.08 mm h-1 during daylight in summer. Interception storage capacity is about 0.5 mm but relative interception losses are high when there are many small rain events. Horizontal interception of water from fog or cloud contributed about 1% of precipitation for the period examined. For the April-March year, total evaporation was 622 mm and transpiration exceeded interception loss in each month. Annual interception loss was 222 mm, or 21.3% of the total precipitation of 1042 mm. Monthly interception loss varied between 10% and 45% of precipitation and was very dependent on storm size and frequency. Snow tussock may be the optimal vegetation for water yield in this environment: transpiration rates are lower than for pasture and interception losses are less than for forests. © 1990.