Estimated Sustainable Discharge Rate
Based
on 1992 -1995 Water Levels in Mountain Lake
This analysis examines the weekly records of the water level in
Mountain Lake
from 1992 - 1995:
We thank the employees at Moran State Park for providing this data as
part of the Mountain Lake Management
and Operations Agreement.
Click on the image for a larger view of the wide chart, or for one
specific year: 1992 1993 1994 1995
Read the chart carefully, the horizontal lines are at 5" steps in the
water level, the vertical lines vary,
and are not months.
The raw text data will be digitized and put on the web later.
The graph shows that the lake only filled to capacity once during the
four year period for which we have detailed records.
The graph shows the level rises when the recharge from rainfall and
inflow exceeds the discharge.
When there is little or no rainfall, the rate of drop of the water
level can be converted
into a flow rate based on the area of Mountain Lake.
A steady discharge of 3 CFS (= 361.3 acre ft/month) corresponds to a
steady drop of 5"/wk given the area of Mountain Lake is 198
acres.
This steep slope of 5"/wk does not show up on the graphs even once.
Based on the rate of fall the following rates of discharge have been
calculated:
|
|
Level (inches) |
Drop in feet |
Acre ft |
Duration(days) |
Acre ft/yr |
| 08/19/9 |
14.4
|
|
|
|
|
| 12/03/93 |
1.2 |
1.1 |
217.8 |
106 |
750 |
|
|
|
|
|
|
| 05/06/94 |
7.2 |
|
|
|
|
| 07/11/94 |
1.2 |
0.5 |
99 |
66 |
548 |
|
|
|
|
|
|
| The
discharge rate from Mountain before the
93/94 rainy season was approximately 748
acre ft/yr |
| The
discharge rate from Mountain after the 93/94 rainy season
was approximately 550 acre ft/yr |
The
net loss in storage from Mountain Lake was 238 acre ft.
|
Without depleting the storage during these non drought years, using an
average93/94 discharge rate,
the sustainable discharge is
estimated at 411 acre ft/year.
The rate of withdrawal from Mountain Lake that can be sustained if the
highest level is allowed to fluctuate from year to year is
probably of the order of 500 acre ft/year. This could be estimated with
better accuracy if the weekly rainfall records were correlated with the
change in lake levels, and the 4 years of records were compared to the
100 years of data available from NOAA.
The onset of the rainy season is usually quite pronounced. The end is
more tapered:
(Click on the graph to go to the Western Regional Climate Center for
more options)
Unfortunately there are no detailed records of how or when the flow out
of Mountain Lake was adjusted.
Ongoing instream flow measurements will provide some useful data,
however the data must be used carefully since the volume of water
diverted at the Rosario diversion is not accurately measured, and
there is a substantial return flow, (often 100% while the initial
diversion at or less than 2 CFS.)
Sandy Taylor
Olga Water Users Inc.