Effects of nitrogen deficiency and soil moisture stress on growth of pasture grasses at Samford, south-east Queensland. 2. Calculation of the expected frequency of dry periods by a water budget analysis

A water-budget analysis was carried out using the Penman formula to assess the long-term effectiveness of rainfall at Samford. The analysis successfully predicted the occurrence of every important period of soil moisture stress that was recorded by direct measurement during a field experiment. It tended to over-estimate the length of dry periods, but the calculated drought days were related reasonably closely to effects on grass growth. Analysis of 100 years' rainfall, using the records for Brisbane and Samford, predicted that periods of soil moisture stress will occur during the months of October, November and December in slightly more than half the years at Samford. It was calculated that the average effect of dry weather between September 1 and December 1 would be to reduce yields of nitrogen-fertilized Rhodes grass (Chloris gayana Kunth.) by about 20 per cent. A major depression of yield by, drought, i.e. a reduction to less than 50 per cent of yield with adequate water, is expected with an average frequency of slightly less than 1 year in 10. Under the climatic conditions of Samford there seems to be little justification for supplementary irrigation of nitrogen- fertilized grasses. It is suggested that water resources could be better used on pasture legumes such as white clover.