 | This historical analysis
assumes Canyon Lake existed and was full on January 1,1950.
|
| The volume of water in the lake
is adjusted on a daily basis
using an EXCEL spreadsheet. (There are 3650 lines
of data for each decade) The daily flow from the Guadalupe River entering the lake was
determined from the historical record of USGS Gage 08167500
near Spring Branch, TX
|
| Water flowing into the lake when the elevation is over 909 feet
must be released for flood control so no curve exceeds
that elevation.
|
| 104 cfs (cubic feet per second) is released downstream unless the
average daily flow for the year is less than that number.
For instance the average daily flow in 1956 was 11 cfs at Spring Branch. By comparison the flow at Spring Branch for the
month of March, 2001 has been between 800 an 900 cfs.
The flow at Spring Branch in the summer of 2000 dipped to less than 5 cfs and the
river was dry at the Rebecca Creek Rd. bridge above Canyon Lake. The values of 104
cfs is based on the 1984 year when the lake was allowed to drop about 10 feet, the
Guadalupe River went dry above the lake, and more water was released downstream than
passed the Spring Branch gage. In other words, this analysis assumes that GBRA must
allow a minimum flow rate to pass the dam as though the dam did not exist. In fact,
50 cfs was released on average in the summer of 2000 when there was no flow into the lake.
GBRA was delivering about 10,000 acre ft/yr during those years so the
equivalent 13 cfs is part of that 50 cfs release.
|
| Evaporation is taken at 60 inches per year. No credit is give for precipitation since rain
during periods of drought is rare and there are loses
from the lake from pumping adjacent to the lake that more than make up for
rainfall. The
inflow consumed by evaporation was proportional to the lake area based on the
previous days volume. Lake area was calculated as area = 1.2107(volume)0.6867
|
| If the lake runs dry, no water is withdrawn for use until the
lake is 1/4 full.
|
The yield of Canyon Lake was determined in a study by the Corps of
Engineers. The following statement was
received in response to an inquiry:
A recent detailed yield study indicates that the yield of Canyon
Lake is 134.7 cfs which translates to 97,600 acre feet per year. The critical period in
the analysis begins in July 1947; maximum
drawdown occurs in the analysis in February 1957; and the computed refill of the
conservation pool occurs in December 1957.
The Corps
analysis assumes no release of water downstream.
This would make a big difference in
sustainable withdrawal. In 1984 all water flowing into the lake was
in fact released and the lake
level was allowed to drop about 10 feet. The
downstream release of 104 cfs is the average flow past Spring Branch for 1984. This means that an amount equal to evaporation is lost to
recharge from the river below Spring Branch and the lake itself. Our analysis is non
conservative as it did not consider this additional natural loss of water.
The Corp never released its report.
GBRA has
stated publicly that since the Guadalupe River runs dry periodically, no downstream
release is necessary. (This may not be policy as we
have never seen it happen) If no water is released
from the lake the curves on the following pages would change. However, the river would be dry between Canyon dam
and the Comal River. We have inquired of the GBRA how the lake reservoir will be operated
when the lake level is below 909 feet and have received no response. GBRA has also stated
that all rights below Canyon dam are satisfied by Comal Springs. We would like to
see a simple written procedure. Water rights are important only in determining who
gets the water and in what sequence. Water rights do not have to be considered in a
mathematical analysis. The amount of water withdrawn from the lake by diversion or
release downstream is the only quantity that matters.
The author
does not challenge the legal right of the GBRA to take the water from Canyon Lake at the
expense of recreation. We simply believe that
people living in the area around and downstream of Canyon Lake have the right to know that the
lake will see severe fluctuations at least once per decade.
Only then they can judge what effect this will have on their businesses,
recreational facilities and quality of life around the lake.
In
addition, our analysis shows that the quantities of water desired by GBRA will not be
available if the worst drought of record reoccurs. Further, it is irresponsible to
assume that the worst drought of record is the worst that can occur. A ten year
drought is a normal assumption in other western states.
Top of Page
How to
Read the Graphs
| Each graph is a plot of the remaining water in the lake (storage
reserve) versus time. There are 3 curves on each graph:
 | Normal evaporation and downstream release of water of 104 cfs or
average daily flow for the year if less than 104- no water
delivered by GBRA
|
| GBRA delivery of
50,000 acre feet per year – Original permit
|
| GBRA delivery of 90,000 acre feet per year –
New permit.
|
|
| Each line shows the decrease or increase in water
in Canyon Lake as water flows in from the Guadalupe River and as water
is delivered by GBRA. Prior
to the year 2000 all water delivered by GBRA was released downstream through
the gates of Canyon Dam. Today GBRA diverts water at points upstream and
from the lake to water supply companies in addition to that released down
stream.
|
| The elevation of the lake level is shown on the graph for
different volumes of water. At elevation 909 feet the lake is full and contains
386,200 acre feet of water. Any water over that amount is released downstream to
allow for flood control. When the lake level is 770 feet the lake is empty.
|
| Example - in the graph for 1950 - 1959 follow the graph for sale
of 90,000 acre feet per year. Note that sometime in 1954 the lake goes dry.
There is not enough rainfall to raise the lake level until 1957. What will 300,000
people do for water for those dry years if they are dependent on Canyon Lake. We
cannot haul that much water. People will have to
leave the area and wait until it rains much as people must flee a flood and
wait for the water to recede.
|
