 Since 1975 long @-@ term average <unk> at the dam wall have decreased by 20 percent and <unk> into the catchment by approximately 60 percent — the average annual inflow between 1948 and 1974 was 52 gigalitres ( 1 @.@ 8 × 109 cu ft ) which had reduced to 22 gigalitres ( 780 @,@ 000 @,@ 000 cu ft ) between 1975 and 2004 . 
