CALIBRATION AND PERFORMANCE OF THE SINGLE-LAYER SOIL-WATER BALANCE MODEL FOR PASTURE SITES

Two single-layer models of soil water balance were tested against fort nightly neutron probe field measurements of soil water at four North I sland sites. The first model assumed constant evapotranspiration from field capacity to wilting point (the ''Veihmeyer'' approach), whereas the second model simulated a ''two-phase'' accounting of the soil mois ture balance-potential evapotranspiration above a critical water defic it and declining evapotranspiration below this deficit. The Veihmeyer model was found to be the better choice for modelling soil water in la yers of 0-50 and 0-70 cm, corresponding to the zone of most pasture ro ot activity, but the two-phase ap ach had advantages when modelling th e full depth of sampled soil profile (about 120 cm). The available wat er capacities used in the models were derived from the neutron probe m easurements. Soil moisture profiles were measured fortnightly from Sep tember 1986 to September 1988. Regression of predicted moisture defici t against measured deficit explained between 88 and 97% of the varianc e in most model runs for three of the four sites, with soil moisture d eficit errors typically in the range of 10-20 mm. Overall, at these si tes the accuracy figures for the two-phase model runs were similar to those for the Veihmeyer model runs. The fourth soil profile was distur bed and therefore atypical. In general, there was little advantage to be gained in using daily evapotranspiration data over monthly mean eva potranspiration data. Results from this study suggest that for modelli ng pasture soil moisture on time scales of 2 weeks or more, the simple r Veihmeyer model is as good as the two-phase model and can be applied with confidence to many pastoral agricultural applications such as ir rigation planning and drought assessments.