192 



Report of the Commissioners on the 



character of the vegetation in the ranges, consisting of im- 

 mense gum trees, 250 feet high, tree-ferns, thick scrub, and 

 other plants, whose growth and luxuriance indicated a great 

 amount of moisture, as also a thick vegetable soil, capable of 

 holdino- a large quantity of water ; but, as this increased 

 vegetation appears to be the elFect of the increased moisture, 

 and hence, consumes as nutriment a large proportion of it, it 

 not all, we are hence not prepared to assert that any portion 

 of this additional moisture over Mount Disappointment swells 

 the supply already estimated at thirty-one inches over the 

 whole basin, which depth of rainfall we therefore adopt. 



Havino-, therefore, taken the rainfall, we have next to 

 estimate "what proportion of it will be delivered into the 

 Plenty, or that amount which is left and flows over, after 

 surface absorption and evaporation, , t • 



This amount will materially depend upon the dechvity ot 

 the surfece, combined with the imperviousness of the soil, and 

 the number and duration of the rainfalls. In the absence of 

 data on this subject, in this country, we are obliged to fall 

 back upon EngHsh data, although obtained under difterent 

 conditions. In England, according to G. D. Dempsey's work 

 on Drainage of Districts and Lands, in Wheale's Series, the 

 mean annual amount evaporated from the surface ot tfie 

 o-round, is 57-6 per cent, of the rainfall, leaving 42-4 per cent, 

 as available for collection ; assuming therefore that 58-6 is the 

 correct per centage of evaporation for England, we propose 

 to take such a comparative view of its conditions m England, 

 relatively with those in the Plenty basin, as shall enable us to 

 form a practical judgment as to the apphcability of this per 

 centage of evaporation to the Plenty basm. _ _ 



This Enc^lish per centage of loss of 57-6, is m a country 

 highly favourable to evaporation, owing to its cultivated surface 

 exposing a loose spongy soil, capable of holding a large amount 

 of rainwater while being evaporated by the sun, also to the 

 sli-htly undulating charac^ter of the country, not involving so 

 rapid delivery of its rainfalls into the rivers, and finally from 

 the differences of the rainfalls producing milder showers a 

 a greater proportion of which must necessarily be evaporated 



from longer exposure. . . j +1 



On the other hand, the basin m question is composed mostly 

 of steep ranges and hills, presenting only a few square miles 

 of fiat land, and hence capable of dehvering its surplus waters 

 with rapidity into the Plenty; it also has, for the most part, 

 a close impervious surface, undisturbed by agriculture, and 

 only capable of surface absorption, which character ot 



