Water use, irrigation, and pasture production on stratford silt loam

Abstract

In a deep allophanic volcanic ash soil under pasture, neutron probe measurements showed that water extraction occurred to a depth of about 1.8 m in the summers of 1981–82 and 1982–83. When the soil water deficit reached about 60 mm, pasture growth became limited by soil water, and when the deficit was about 125 mm, non-irrigated yields were reduced to half of the irrigated yields. At this deficit (125 mm) about 50% of the water had been extracted from the top 0.25 m of soil. Pasture growth continued slowly through the summer even when deficits of 140 mm were present. Although the maximum soil water deficit was about 150 mm (February 1982) leaf extension measurements showed that regrowth recommenced rapidly with significant autumn rain. When the soil water deficit was 125 mm, the top 0.40 m of soil was at a matric potential of about -15 bar, and at depths of 0.40–0.90 m the potential was about -I bar. These data, together with the observed root depth, can be used to predict the soil water available for pasture growth. A simple model using available soil water, monthly rainfall, Penman potential evapotranspiration data, and mean irrigated pasture accumulation rates gave reasonable predictions of non-irrigated pasture accumulation rates for the summer months. © 1985 Taylor & Francis Group, LLC.