128 E. T. Quayle: 



cause of the small rainfall. But when cleared and large blocks- 

 of growing wheat or of green grass alternate with heat absorb- 

 ing areas of fallowed land, we have a totally different effect. 

 The uniformity of cloud stratification is broken by columns of 

 heated air supplied with moisture from the green vegetation, 

 and these tend to set up various convection centres which, when 

 reaching the cloud level, draw upon the moist air of the cloud 

 stratum, and cause local showers over areas which might other- 

 wise be passed over. Thunder showers should also be of more 

 frequent occurrence. 



The same effect must, of course, result from irrigation, but 

 with the advantage of not being limited to any season. It is 

 quite probable, too, that these disturbances to the cloud layers 

 rising from beneath cause. local precipitation out of all propor- 

 tion to the amount of moisture contributed by the evaporation 

 areas. 



(3) Persistent Increase of Monthly Rainfall during 



1919 (a year of record irrigation) in Lee of 



Irrigation Area. 



The monthly rain maps for 1919 seemed to indicate in a very 

 striking way increased rainfalls due to irrigation, water con- 

 servation, and possibly previous flooding. The monthly isohyets 

 showed a most persistent tendency towards increased rainfall 

 beginning somewhere in the neighbourhood of Wentworth and 

 Mildura, but most marked about Swan Hill and Kerang. Now, 

 as both districts are the scenes of considerable irrigation develop- 

 ment, and the latter of large water storages as well, a chain of 

 lakes being kept filled for irrigation and water supply north- 

 west from Kerang, we have an apparent cause of increased rain- 

 fall. The annual rain map for 1919 showed a strip of well- 

 rained-on country from Euston to Mitiamo generally following 

 the Murray, but diverging a little to southwards after passing 

 Swan Hill. The year's rainfalls of Kerang and Mitiamo were ac- 

 tually above average, although the year was in general so dry that 

 both N.E. and S.W. of this strip the rainfalls declined to 20, and 

 even 30, per cent, below average. That this was no mere chance 

 effect may be assumed from the fact that the monthly isohyets 

 showed the same tendency in at least seven cases. The effect was 

 possibly helped by heavy thunderstorm rains from Swan Hill to» 

 Kerang in February. 



