Tuesday, April 3, 2012

Raise North Pool Five Feet: Less Than Amount Lost to Annual Evaporation

Councilman Paul Alexander shared the city's study on pumping the total volume of the South Pool into the North Pool at Twin Buttes.  It turns out the North Pool will rise by 5.15 feet, while annual evaporation is 5.25 feet, the vast majority of which occurs during the summer. If the whole amount of the transfer stands to be evaporated over a year's time, what are the savings? 

April 2, 2012 Evaporation savings by moving water from South Pool to North Pool at Twin Buttes

1) current levels of both pools as of 4/2/2012,
2) no loss of water in the transfer from the south to north pools.

Twin Buttes Reservoir
North Pool 1890.65’ 882 8,170
South Pool 1925.78’ 520 5,028

Total 1,402 13,198

Projected with observed south pool transferred to north pool

North Pool 1895.8’ 1,093 13,198

Projected change in water surface area (1402-1093) = 309 acres

Approximate annual average evaporation is 63 inches/year=5.25 feet

Change in water evaporated: 309 Acre Feet * 5,25 feet = 1622 acre feet per year.

These numbers are approximate and apply with the assumptions made.

The reality is the dry 211 acres in the North Pool will absorb some amount of South Pool water.  How much thirsty vegetation grew on those 211 acres?  This came from a Twin Buttes Brush Control Study from April 2010:

One acre of saltcedar on the Pecos River of Texas was estimated to use 5-7 acre-ft. of water every year (Hart 2003). The longer saltcedar occupies an area, the drier the area becomes. Saltcedar currently dominates 1,092 acres in the project area (Twin Buttes).

Councilman Alexander believes a few modest rains will rapidly refill the South Pool.  We've had modest rains since October and the South Pool is up about two feet.  What happens when the South Pool is completely drained, as in the city's assumption?  Will the spring fed South Concho keep its flow?  How much thirsty vegetation will grow on the South Pool lake bed?  How will that impact a "modest rain's" ability to refill the South Pool.

The City frequently offers crude analysis to push their desired strategies.  This got play in August last year, then became embedded in a consent agenda.  The public, especially users of the South Pool, deserve better.

I understand the city needs water.  It should pump what it needs to in the most efficient and effective way possible.  If it can build a pipeline to pump Hickory water, it should be able to do the same from the South Pool.  Rather than assume no loss, it should guarantee such.

This issue arose in 2004.   Why didn't San Angelo's Water Czar make recommendations to utilize South Concho River water effectively in a time of drought?  What impact did agricultural commitments have on such plans?

Much of the stored water is used to irrigate 10,000 acres of crop and rangeland (Reclamation 1994).

How did the Water Czar's "hobby farm," which draws water for irrigation from the South Pool, enter into consideration?  The public won't know.  That topic is off limits.

Update 4-7-12:  Councilman Alexander's medium rain may be in the offing.

1 comment:

  1. The average depth of the South Pool at April 2nd levels is 11 feet. As I said before, the South Pool is shaped like a bowl, the North Pool's topography is varied.

    I'd like to keep as much water as possible in the South Pool, while meeting our critical water needs.