If the vapour pressure at 0900 hours from the SILO Patched Point Dataset was used as the actual daily mean vapour pressure, the accuracy of daytime VPD estimation was further improved. Despite the poor estimation of vapour pressure, daytime mean VPD was predicted reasonably well using daily maximum and minimum temperatures. Vapour pressure at 0900 hours was a better estimate of daily mean vapour pressure. Thus the prediction of vapour pressure was poor. Daily minimum temperature was found to be a poor estimate of dew point temperature, being higher than dew point in summer and lower in winter. Daytime mean VPD ranged from 0 to 5.3 kPa. For more than 75% of the time its variation was less than 20%, and the maximum variation was up to 50%. Actual vapour pressure of the air ranged from 0.5 to 2.5 kPa. The accuracy of such VPD estimations was assessed using data collected every 15 min near Wagga Wagga in New South Wales, Australia. In APSIM (Agricultural Production Systems Simulator), VPD is estimated from daily maximum and minimum temperatures with the assumption that the minimum temperature equals dew point, and there is little change in vapour pressure or dew point during any one day. Data required to calculate the mean VPD on a daily basis are rarely available, and most models use approximations to estimate it. Vapour pressure deficit (VPD) has a significant effect on the amount of water required by the crop to maintain optimal growth.
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