Pasture production and water use measurements in the central waikato

In a sandy loam under pasture, water extraction occurred to about 100 cm with 50% of this water coming from the top 30 cm. Under well-watered conditions, the Priestley-Taylor estimate of the maximum, weather-dependent evapotranspiration (Emax) agreed closely with neutron probe measurements. At high soil water deficits, differences in the rate of evapotranspiration (E) relative to Emax were observed between 2 successive seasons, presumably resulting from changes in species composition. In 1981, yield reductions occurred at a smaller soil water deficit (35 mm) than did reductions in evapotranspiration (65 mm). In view of this result and also from considerations of the pattern of water extraction of the readily available moisture, it is argued that frequent, small irrigations should optimise the use of applied water. The difficulties of better defining water use and pasture production when either are limited by the availability of soil moisture are discussed.