Figure 1. 

 The Meteorological Station, Yudnapinna. 



Summer temperatures are high, and the daily mean temperature from November to 

 March exceeds 70°F. The rate of evaporation from a free water surface is high during 

 summer, with a maximum of 14.5 inches in January; evaporation falls to a minimum 

 of 2.6 inches in June. (Table 1.) 



Due to lower evaporation, winter rains are more effective for plant growth than 

 summer rains of equal magnitude and use has been made of the index P/E*^ (Pres- 

 cott, 1949) to define the minimum influential rainfall for each month. On the basis 

 of observations on plant response and monthly records of soil moisture in the root 

 zone, a monthly value of P/E^-^^ = 0.2 has been found to indicate the lower limit 

 above which soil moisture tends to become available, and new growth therefore pos- 

 sible. (Table 1.) Rainfall in excess of O.^EO-75' constitutes water available for trans- 

 piration, to which the amount of herbage growth can be related. 



The pattern of influential rainfall for the period 1885- 1951 (Fig. 2.) indicates 

 clearly the predominance of favourable growing conditions during the restricted period 



* For the month of June, the value of 0.41 derived from the above expression has been in- 

 creased to 0.5 to allow for the more frequent light falls of only a few points each in this 

 moDth. 



130 



