Predicted frequency of wet and dry soil conditions in Tasmanian dairy regions under future climate scenarios

Durations of wet and dry periods are significant factors that influence pasture management on dairy farms. Historical and future simulated daily climate data for two dairy regions of Tasmania (Flowerdale and Ringarooma) was accessed from the ‘Climate Futures for Tasmania’ project and used to simulate a perennial ryegrass sward growing on a clay loam soil. The simulated soil moisture content to depth of 400mm was used as the criteria for determining number of wet (soil moisture > 160mm) and dry (soil moisture <120mm) days per annum. The mean number of dry and wet days for each region for six general circulation models (GCMs; CSIRO-Mk3.5, ECHAM5, GFDL 2.0, GFDL 2.1, MIROC3.2 and UKHad), was computed for the baseline period (years 1971 to 2000) and three future climate periods (years of 2001 to 2030, 2031 to 2060 and 2061 to 2090). By years 2061 to 2090, the mean number of wet days at Flowerdale and Ringarooma is predicted to decline by 8.5 and 4.5 days per annum, respectively. In contrast, the mean number of dry days for Flowerdale and Ringarooma is predicted to increase by 7.5 and 4.8 days, respectively. While there was little change in the mean number of wet and dry days when averaged over each GCM, there was substantial variation between GCMs for any particular period and region. This paper discusses the implication of these results and also highlights the influence of inter-annual climate variability on dairy farming systems.