The soil water balance in a gragiaqualf and its effect on pasture growth in central New Zealand

Abstract

A water balance model for pasture is described, which takes into account the effect of soil water deficits on evapotranspiration. Data from small lysimeters were used to evaluate methods for the estimation of weather-controlled evapotranspiration from well-watered pasture. The relatively simple Priestley and Taylor method was found to be as accurate as the Penman method or the use of Class A pan data. Three years of computed water balance data were found to agree within 40 mm with the soil water deficits measured with a neutron moisture probe. The computed soil water balance was used to assess whether growth would occur on a given day, and to predict unirrigated pasture growth rates for the period December-April of each irrigation season. Predicted and measured pasture growth rates were in reasonable agreement, with the December-April growth rate on unirrigated pasture averaging only half that for irrigated pasture over the four years of the study.