Sally Kilburg, chair of the Okanagan Similkameen Conservation Alliance, states that, “Spring runoff supplies much of the water used in the valley.”
“If we use more than that, then we use the lake … the lake really only holds about 50 or 60 years of our annual use and it would be gone.”
This latter statement does not appear to be correct.
The Okanagan Basin Water Board estimates that, on average, 219 million cubic meters of water is used for human needs (domestic indoor and outdoor, agricultural, commercial, golf courses, etc.) in the Okanagan Valley each year (http://www.obwb.ca/wsd/key-findings/water-use).
Okanagan Lake has a volume of 24,644,000,000 cubic meters (http://www.ilec.or.jp/database/nam/dnam51.html), which means it would take 113 years (not 50-60 years as Kilburg claims) to “use” all the water in the lake at our average annual use rates.
This statistic assumes there is no inflow to the lake over this period — in other words, no precipitation falls in the valley at all for the next 113 years.
Furthermore, as we have previously shown (http://precedings.nature.com/documents/4946/version/1), the average annual flow in the Okanagan River at Oliver has not changed over the past century (and may even be increasing).
This hydrometric station at Oliver integrates all watershed activities upstream in the Okanagan Valley, be they natural or man-made uses or productions of water.
Consequently, it appears that despite the large increases in population and agricultural land in the valley over this time, the net amount of water “left over” and leaving the Okanagan Valley via the Okanagan River at Oliver each year (i.e., the annual water yield) has not changed significantly.