Anticipating a Drought-Response Summer

Oh oh.

It's been reported that both 107.5FM radio and Channel 4TV this morning carried announcements that the Okanagan has the lowest recorded snowpack recorded of the past three decades...and to prepare for possible severe drought conditions this year.

So let's have a look at some of the pertinent statements and water demands that the Drought Management Plan identified (in no particular order):

Agricultural irrigation accounts for a large proportion of the total (estimated to be up to 80% during peak days). Metered residential water use accounts for approximately 10% of the total supply, while Industrial-Commercial-Institutional (ICI) water users account for only 2% of the total supply (based on peak day demand estimates).

Unaccounted for water, which includes leakage, hydrant-flushing, fire-fighting, inaccurate meters, theft, and other similar uses, accounts for approximately 6% of the total supply.

In the summer months, average daily water use is as high as 173 ML/day, which is almost seven times higher than the winter base water use (approximately 25 ML/day).

The data indicate that peak flow rates occur between mid-July and mid-August. Peak day water use in 2010 occurred on July 29 at 212 ML/day (total from all sources). The maximum withdrawal rate during the peak water demand period is considered to be between 2 to 3 m3/s. The data indicate that there is a consistent peak in water use between 8 pm and 10 pm, with peaks at approximately 9:30 pm. The peak day water usage is considered to be reflective of domestic irrigation through manual watering.

Annual residential water use is expected to increase to 20,651 ML by 2041, which is directly related to estimated population growth. Annual agricultural water use is expected to decrease from 55% of total to 41% of the projected total annual water use due to growth in urban demand.

Spring rains serve to replenish the upland reservoirs, if not already full, for future water supply. The spring rains are important from the perspective of drought potential because these rains can avert a drought situation if a less than average snow pack is present and/or the reservoirs are not full entering into the spring.

Precipitation in the Vernon area is characterized by wet, rainy May-June followed by dry July-August. Vernon Climate Station (Station 1128581) and Vernon Coldstream Ranch (Station 1128580) are situated at valley-bottom locations. Thus, do not necessarily reflect climate conditions in the higher elevation watershed.

One recent example of the importance of spring rain occurred in 2010. Entering into the spring of 2010, GVWU was faced with low reservoirs levels from the previous year, combined with a lower than average snowpack. Just as GVWU entered into a Stage 3 drought situation, the area received higher than average rainfall in May/June.  The spring rain was largely responsible for replenishing the reservoirs and raising the drought level (to Stage 2).

Summer climate conditions directly affect rates of water consumption and upland reservoir levels, which is a factor influencing the potential for drought conditions in the GVWU service area. During hot, dry summers (July - August) outdoor water use for irrigation increases, putting greater pressure on the water supply.

Rain during the early summer months accounts for a considerable proportion (40%) of the total annual precipitation (487 mm). Much is localized and of a convective nature, so the distribution across the watershed is variable. In comparison, the total annual precipitation in Vernon is low compared to
Vancouver (1110 mm).

Various factors, including summer climate and irrigation demand, affect reservoir levels in the fall. Fall reservoir levels are an important factor influencing the potential for drought in the subsequent year as these levels establish a pre-winter supply status.

The Duteau Creek water supply is vulnerable to changes in runoff attributed to various watershed factors (Kerr Wood Leidal and Dobson Engineering, 2008; M.J. Milne and Associates Ltd., 2011). Watershed factors including loss of forest cover due to fire, mountain pine beetle attack and forest harvesting have the potential to affect the Duteau Creek water supply.

Approximately 24% of the watershed is comprised of pine-leading stands that are susceptible to Mountain Pine Beetle (M.J. Milne and Associates Ltd., 2011) and a portion of these stands will be salvaged through timber harvesting. Given the future loss of forest cover, effects on snow accumulation, snow melt, runoff and streamflow are expected. With respect to the water supply situation, increases in
runoff will be attenuated by the upland reservoirs. However, with the advancement of the spring freshet, reservoir inflows will occur earlier in the spring, and water will need to be stored (perhaps for longer periods of time). Otherwise, earlier melt conditions may affect low flow conditions later in the year.

Excerpts from:  Greater Vernon Water Utility (GVWU) Drought Management Plan, Update Revised October 2011, by Clarke GeoScience, Kelowna, B.C.



Finally, a compelling graphic to show the effect of ground covers on rainfall and soil stability:

"Using an 18-foot deep slough for most of the area's water sounds like the worst idea in 100 years," offers Kia.