Aspect influences on pasture microclimate at Coopers Creek, North Canterbury

Abstract

Rainfall, soil moisture and soil and air temperatures were measured over 5 yr on north and south aspects (about 25.degree. slope) of a grazed pasture and on the exposed intervening ridge crest at Cooper''s Creek, North Canterbury (43.degree.15''S), at about 380 m a.s.l. [above sea level]. Solar radiation and wind were measured only on the ridge, and radiation received on north and south slopes was derived from ridge measurements. Potential evapotranspiration on all aspects was calculated from solar radiation and air temperature according to the Priestley and Taylor formula. Daily mean solar radiation on the ridge averaged 332 langleys/day with a maximum of 591 langleys/day in Dec. and a minimum of 118 langleys/day in June. Over a year the north aspect received 80% more radiation than the south. Northwest winds prevailed; windrun at 2.1 m above ground was usually over 200 km/day and occasionally was over 2000 km/day. Total potential evapotranspiration on the ridge averaged 1093 mm/yr with a maximum of 5.9 mm/day in Dec. and a minimum of 1.0 mm/day in June. Over a year potential evapotranspiration on the north aspect was almost double that on the south aspect. Average annual rainfall was about 1400 mm and was distributed evenly over all aspects, although the catch from individual rain periods varied by up to 80% among aspects. The north aspect soil was always drier than the south aspect soil through the growing season and sometimes approached wilting point between Dec. and March. Moisture content in the ridge soil was intermediate between north and south aspects. Mean annual Stevenson screen air temperatures (1.2 m above ground) were measured as 10.5.degree. C on the ridge aspect and calculated as 13.7.degree. C and 9.9.degree. C on the north and south aspects respectively. These are equivalent to a latitudinal displacement of about 9.degree. within New Zealand. Soil temperatures on the north aspect were always warmer than on south or ridge aspects under both clear and overcast skies. Ridge temperatures were similar to those on the south aspect. Mean annual temperatures at 10 cm depth were 7.7.degree. C on the ridge aspect and 13.8.degree. C and 8.2.degree. C on north and south aspects, respectively. On the north aspect, soil maximum temperatures under short grass (2-4 cm) were higher than those under tussock (30-40 cm) and soil minima were lower. On the ridge, soil maxima and minima under short grass were lower than those beneath tussock.