Modifying Climate. 129 



(4) Modification of Drought through Murray Floods. 



To see whether there was any evidence of the Murray floods- 

 causing subsequent rainfall improvement, I selected Murray 

 Downs, an old and, I believe, very reliable rain station near the 

 Murray on the opposite side from Swan Hill. This station has. 

 for 20 or 30 miles north-west from it a good deal of country 

 apparently either marshy or well supplied with lakes, filled when 

 the Murray is in high flood. As it is probably from this direction,, 

 or from west-north-west that the storms come in dry years in- 

 stead of, say, from north-north-west, as in wet years, Murray 

 Downs should, in dry years following flood, feel some effect. 

 Having graphed the flood levels at Torrumbarry, I selected all 

 the dry years immediately following one of heavy flooding in. 

 its latter half. There were 22 of these, beginning with 1868. 

 Taking annual rain totals only, and comparing with those of 

 stations presumably less favourably placed, Murray Downs 

 should, in these years, receive a better percentage of the average- 

 fall than should, say, Wentworth, beyond which the river trends- 

 westerly, or Moulamein and Balranald, which are on the north 

 side of the frequently flooded areas, or Tyrrell Downs, which 

 is to southward of them. Using the records available, compari- 

 son with Wentworth shows 14 out of 18 years favouring Murray 

 Downs, with Moulamein 8 out of 10, with Balranald 11 out of 



15, and with Tyrrell Downs 8 out of 11 in favour of Murray 

 Downs. These necessarily give much weight to the Murray 

 Downs records, but these are well supported by those of Swan 

 Hill. This peculiarity of Murray Downs is not as fully felt 

 at stations further south-east, Kaarimba being less favoured than 

 Murray Downs in only 17 cases out of 26, Numurkah 12 out of 



16, and Yarrawonga 9 out of 15. It is, of course, probable that 

 these stations also gain from the evaporation from the Murray 

 valley. I may say that monthly comparisons do not show any 

 marked increased frequency of benefit at Murray Downs over 

 these south-eastern stations, although it does with the others. 

 This was unexpected, but may point to the principal way in 

 which the rainfall is increased, namely, by the production of 

 convectional centres in ordinary rain storms, or by intensifying: 

 thunderstorm action, in which case the total annual benefit at 

 any one station might be due to a few exceptional showers. 



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