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OCR for page 22
2
Hydrology of the Mono Basin
INTRODUCTION
Understanding the hydrology of the Mono Basin is
important both as a basis for constructing water balance
models to predict future lake levels and as a means for
assessing potential changes in the availability and salinity
of water that might affect the ecosystem of the basin.
For example, lake levels directly control the position of
the shallow water table around the lake and thus the
availability of shallow groundwater for nearshore vegeta-
tion. Similarly, salinity and its consequences for wildlife
are determined by the amount of water that flows into the
lake.
This chapter gives an overview of the hydrologic pro-
cesses in the Mono Basin and the models used to predict
future lake levels and salinity. The discussion has four
parts: (1) a general review of the meteorology and cli-
matic influences in the region; (2) a description of hydro-
logic processes in the basin and a brief assessment of the
available data; (3) a review of currently used water balance
models for the lake or basin; and (4) a description of pre-
dicted lake levels and salinities with these models.
HYDROMETEOROLOGY
The hydrology of the Mono Basin is principally
controlled by the amount and distribution of precipitation
22
OCR for page 23
Hydrology of the Mono Basin
23
it receives, which in turn is a function of the meteorology
of the Great Basin. The following discussion, providing
background information on the hydrometeorology of the
region, is thus based in large part on works that consider
the meteorology of the Great Basin as a whole.
Synoptic-Scale Weather Systems and Air Masses
Synoptic-scale, or large-scale, weather systems are re-
sponsible for most of the precipitation around the Mono
Basin, and variability in those systems causes variability in
the precipitation. In his investigation of precipitation
characteristics of the Great Basin, Houghton ( 1969) iden-
tified three principal regimes, all of which occur through-
out the Great Basin. Each of these regimes is dominant
in different sectors of the region and has recognized cir-
culation patterns and air mass trajectories, each of which
brings precipitation to the Mono Basin in different seasons.
The Pacific component, (including polar and subtropical
flows), has a winter precipitation maximum and is predomi-
nant in the western, northern, and southern sectors; the
continental component, with a spring precipitation max-
imum, is predominant in the central and eastern sectors;
and the Gulf component, with a summer precipitation max-
imum, is predominant in the southeastern sector. The
source regions and trajectories of the air masses are shown
in Figure 2.1.
Precipitation from Pacific storms, which is the majority
of the precipitation in the area, is associated with ascend-
ing air in frontal zones and associated upper troughs, and
with orographic lifting over the Sierra Nevada and other
mountain ranges. The Sierra Nevada acts as a barrier to
the moisture. Most of the precipitation from these storms
falls in the high elevations, with very little reaching the
east side of Mono Lake.
A substantial amount of the precipitation that falls over
the Great Basin is associated with nonfrontal cyclones in-
volving modified polar air (Houghton et al., 1975;
Monteverdi, 1976~. Such circulations (known in Nevada as
"Tonopah Lows") are important for precipitation in eastern
California in general and the Mono Basin in particular,
OCR for page 24
24
soon
Rawinsonde Stations shown are:
MFR Medford, OR
OAK Oakland, CA
VBG Vandenburg AFB, CA
SAN San Diego, CA
WMC lI\finnemucca, NV
ELY Ely, NV
The Mono Basin Ecosystem
~ J GONE
NO BOI
TO sac
\UCC ~
\ INW
o
~ (MFR
-
..__ I
\\iAN J ) >
UCC Yucca Flat, NV
BOI Boise, ID
SLC Salt Lake City, UT
INW Winslow, AZ
YUM Yuma, A2
FIGURE 2.1 Major air flow patterns and air mass types
affecting California and the Great Basin.
including heavy snowfall around the
White Mountains (LaMarche, 1974~.
entrain maritime air from the Pacific
southern California.
eastward, bringing moderate to heavy precipitation to the
eastern Sierra and Mono Basin.
Summer rainfall in Arizona and New Mexico, as well as
in adjacent areas of California (including the Mono Basin),
Nevada, and Colorado, is mainly dependent on air mass
Owens Valley and
Often these storms
west and south of
The cyclone may then move slowly
. . . . . . .
. .
OCR for page 25
Hydrology of the Mono Basin
25
thunderstorms or organized synoptic-scale convective
storms involving air from the tropical Pacific, the Gulf of
California, or the Gulf of Mexico (Hales, 1972, 1974~. In
the summer, solar heating of the Southwest Plateau favors
the development of anticyclonic flow over the Great Basin
(Reiter and Tang, 1984; Tang and Reiter, 1984~. At the
same time, dry Pacific air is involved in a diurnal monsoon
or large-scale sea breeze across California and the Sierra.
. .
Where the westerly flow meets the cyclonic southerly or
southeasterly flow of air from Arizona and Baja California,
it forms a shear line or line of convergence. This shear
line moves back and forth over southeastern California and
western Nevada and is often coincidental with climatologi-
cally important phenomena such as lightning, blowing dust,
hailstorms, and flash floods that affect Mono Lake and the
surrounding mountains.
During anticyclonic conditions in winter, fog often
forms over Mono Lake. With the absence of wind, this fog
prevents further evaporation. The effects of cloud cover
on evaporation are more complex because clouds are often
accompanied by strong winds. It is conceivable that a
series of wet seasons such as 1982-1983 with relatively
short, cool summers could account for a decrease in evapo-
ration, contributing to a rapid increase in size of Mono
Lake and other Great Basin lakes.
Precipitation Patterns Related to the Fall and Rise
of Great Basin Lakes
During the most recent years of the historical period
(1975 to 1986), two related hydrometeorological phenomena
have attracted attention: the increased incidence of
extreme weather events, including extremely wet and
extremely dry periods (Policansky, 1977; Goodridge, 1981;
Karl et al., 1984) and the rise in level of Great Salt Lake,
Pyramid Lake, Walker Lake, and Mono Lake.
To review the temporal and spatial extent of both
droughts and periods of greater than normal precipitation
in the Great Basin, the average monthly and annual precip-
itation at eight locations (Elko, Ely, Las Vegas, Reno, and
Winnemucca in Nevada; Milford and Salt Lake City in Utah;
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26
The Mono Basin Ecosystem
and Bishop, California; see Figure 2.2) for the period 1951
to 1980 were analyzed (H. Klieforth, University of Nevada,
personal communication, 1986~. The elevations range from
2162 ft at Las Vegas to 6253 ft at Ely. At Fly, Milford,
and Salt Lake City, spring is usually the wettest season; at
Las Vegas, winter and summer are the wettest; and for the
other four, winter is the wettest season.
In the historical record of extreme precipitation events,
the contrasting 2-year periods of 1975 to 1977 and 1981 to
1983 are outstanding. The former years were extremely
dry and the latter extremely wet. In 100 years of rainfall
records in California, including the Sierra Nevada, the oc-
currences of two consecutive extremely dry years and two
consecutive extremely wet years were unprecedented
(Goodridge, 1981~.
The rise in Great Basin lakes during the 1 980s is well
documented. In June 1986 Pyramid Lake had risen to an
elevation of 3817 ft above sea level, higher than it had
been since 1944, and Mono Lake had risen ~ ft since 1982.
While the levels of Mono and Pyramid lakes and the flow
of the Truckee River are affected by releases from dams
upstream and by various diversions for irrigation, there is
nevertheless a close correlation between their recorded
levels and the record of precipitation, particularly that at
higher elevation.
It is apparent from the recent historical record that
transitions from dry to wet regimes and back are relatively
abrupt. These shifts may be related to preferred wave-
lengths in the upper air flow and these in turn to major
physiographic features and to variable thermal influences
such as sea surface temperatures.
During the recent extreme events of the 1 980s, a search
for causes focused attention on the E1 Nino-Southern
Oscillation (ENSO) phenomenon (Kiladis and Diaz, 1986~. A
strong ENSO development leads to extreme events of oppo-
site character in various parts of the world, including dev-
astating droughts in some regions and excessive rainfall in
other regions. There were E1 Nino events in both 1976-
1977 (dry in California) and in 1982-1983 (wet in Califor-
nia) (Ramage, 1986~. An additional likely cause of climate
change is the global warming expected to result from in-
OCR for page 27
Hydrology of the Mono Basin
OREGON / _
__1 _
CALIFORNIA
l
I .
.
A-- ~_~AHO
~1
\l RNO
~0
at\
at'''\
BIH - Bishop
EKO - Elko
ELY- Ely
lAS - Las Vegas
\
27
l
A_
_'
|` WYOMING
SLC _ _
o
WMC
O EKO
o
GREAT BASIN I
IELY
o
MLF
o
\ LAS /~
~ \` /~
hi\ ~
UTAH
ARIZONA
MLF- Milford
RNO - Reno
SLC - Salt Lake City
WMC - Winnemucca
/
FIGURE 2.2 Great Basin region and eight weather stations
selected for precipitation study.
creased amounts of carbon dioxide and other spectrally
active trace gases in the atmosphere.
HYDROLOGIC PROCESSES
The most basic concept in hydrology is the hydrologic
cycle--the continuous transfer of water between the sur-
face (e.g., oceans and lakes), the atmosphere, and the
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28
The Mono Basin Ecosystem
subsurface (i.e., groundwater). Water balance models at-
tempt to quantify components of the hydrologic cycle for a
specified region using conservation of mass for inflows,
outflows, and changes in storage.
Mono Basin is a closed basin, the only natural outflows
being evaporation from the lake and soil and evapotranspi-
ration from the sparse vegetation. Moisture input to the
basin occurs as snow and rainfall. Water derived from
melting snow and rainstorms reaches ~tr~.nm~ hv ov~rl~nr1
flow and groundwater seepage. Thus, the dominant proces-
ses controlling the distribution of water within the basin
are precipitation, surface runoff in streams, groundwater
discharge to the lake, lake evaporation, and terrestrial
evapotranspiration. The following sections describe these
processes in more detail and discuss data that are available
to estimate each as a component of the moisture budget
for the basin.
_ _ _ is, _ . _,, in,, ~
Precipitation
The average annual precipitation in the Mono Basin
varies from about 6 in. at the east side of the lake to
about 50 in. at higher elevations in the Sierra Nevada.
Although intense, localized thunderstorms occur in the
summer, the greatest amount of precipitation falls in the
winter. Approximately 75 percent of the annual precipita-
tion occurs between October and March (Vorster, 1985~.
The Sierra snowpack is the principal source of surface
runoff in the basin. Snowfall occurs year-round at high
elevations and begins to accumulate in middle to late Octo-
ber. Snowmelt begins in April and continues through May,
with maximum amounts in May and June. Vorster (1985)
suggests that about 77 percent of the average annual pre-
cipitation at elevations above 8500 ft is contained in the
snowpack on April 1. This is not an unreasonable assump-
tion, but in years of particularly heavy summer and fall
rains the nonsnow precipitation may contribute more than
23 percent of the annual precipitation.
Although few precipitation gages with continuous rec-
ords are present in the basin, mean annual precipitation
OCR for page 29
Hydrology of the Mono Basin
29
appears to be adequately known relative to other hydro-
logic components. Table 2.1 shows average precipitation
for the eight precipitation stations in the basin. Locations
of the stations are shown in Figure 2.3.
In addition to rainfall measurements, snow-course data
are available from nine locations in or adjacent to the
basin. A summary of rainfall at gaging stations and iso-
hyetal maps of mean annual precipitation over the basin
are given by Vorster (1985) and LADWP (1987~. The posi-
tions of the isohyetals differ, particularly at high eleva-
tions where Vorster utilized snow-course data and in the
eastern side of the basin where few gaging stations are
located. Nevertheless, both studies estimate the mean an-
nual precipitation over the lake to be approximately ~ in.
Surface Runoff
Much of the surface runoff in the basin originates in
the Sierra Nevada, where most precipitation occurs. Five
major Sierra Nevada streams (Rush, Lee Vining, Mill,
Walker, and Parker), as well as a number of smaller
streams, drain into the basin from the Sierra and other
surrounding hills. Because runoff from the Sierra is fed
primarily by snowmelt, streamflows are highly seasonal,
with one-half to two-thirds of the total annual flow oc-
curring in May, June, and July (Vorster, 1985~.
Locations and descriptions of surface runoff gaging sta-
tions are given in detail by both Vorster (19X5) and
LADWP (1987~. The total mean gaged surface runoff in the
principal Sierra streams is approximately 150,000 acre-ft/yr.
This represents about 75 to 85 percent of total surface and
subsurface inflows to the basin. Approximately 75 percent
of the gaged runoff is measured on the two largest
streams, Rush and Lee Vining creeks. LADWP estimated
total unmeasured flows to be about 25,000 acre-ft/yr, while
Vorster (1985) gives a higher estimate as the sum of two
components, unmeasured Sierra runoff of approximately
17,000 acre-ft/yr, and non-Sierran runoff of about 20,000
acre-ft/yr. The most significant creeks that are not gaged
and reported on a regular basis are Wilson, Bridgeport,
Cottonwood, and Post Office creeks.
OCR for page 30
30
The Mono Basin Ecosystem
TABLE 2.1 Average Annual Precipitation at Gaging Sta-
tions in Mono Basin (LADWP, 1987)
Average Precipitation (in.)
Period of Elevation Period of Period of
Station Record (ft) Record 1941-1985
Bodie 1965-1968 8370 19.2 --
Cain Ranch 1931-1932 to 1984-1985 6850 11.44 11.34
East Side
Mono Lake 1975-1976 to 1984-1985 6840 5.70 --
Ellery Lake 1925-1926 to 1984-1985 9645 25.68 20.42
Gem Lake 1925-1926 to 1984-1985 8970 21.81 20.91
Mark Twain
Camp 1950-1955 7230 6.80 --
Mono Lake 1951-1968 6450 12.50 --
Rush Creek
Power House 1957-1979 7235 25.20 --
Estimates of ungaged runoff will clearly introduce inac-
curacies into computation of the moisture budget for the
basin. Another source of error in the calculations is the
fact that the actual surface runoff into the lake is
unknown. Most stream gaging stations are located above
LADWP diversion points, 4 to ~ mi frown the lake. Releases
past these points flow over porous channel beds to under-
lying aquifers. Thus surface runoff inflows to the lake,
distinct from groundwater inflows, cannot be estimated
accurately.
Groundwater Occurrence and Movement
The movement of groundwater is strongly controlled by
the geology of the basin. As discussed in chapter 1, the
basin is filled with layers of interfingered glacial, fluvial,
lacustrine, and volcanic deposits. These basin sediments
form a complex series of confined and semiconfined aqui-
fers and aquitards, which are recharged by precipitation in
adjacent hill and mountain areas. The water table is gen-
erally within 50 m of the ground surface throughout the
OCR for page 31
Hydrology of the Mono Basin
31
Mark Twain By. .
I · Preclpltatlon 1
| stations l
|N
FIGURE 2.3 Locations of precipitation gaging stations in
the Mono Basin (LADWP, 1987).
basin and occurs at much shallower depths near the lake-
shore (Lee, 1969~. Mono Lake acts as a regional ground-
water sink; groundwater moves toward the lake, discharging
at discrete springs and zones of diffuse seepage along the
lakeshore and beneath the lake.
Nearshore Groundwater Flow
From the perspective of the Mono Basin ecosystem, the
most important aspect of groundwater flow is the seepage
OCR for page 32
32
The Mono Basin Ecosystem
at the lake-sediment interface and in nearshore marshes
and salt flats. Several studies have described the locations
of springs and shallow groundwater gradients around the
lake (Lee, 1969, Loeffler, 1977; Vorster, 1985; LADWP,
1987~. Locations of the largest springs, as mapped by
LADWP (1987), are shown in Figure 2.4. Zones of seepage
on the north and east sides of the lake can be seen on
infrared photos as strips of vegetation (see back cover).
To supplement and update the information in previous
reports, this committee, in conjunction with LADWP, in-
stalled 23 shallow piezometers along four transects around
the lake in September 1986 (see Figure 2.5~. At each loca-
tion at least one test hole was drilled by LADWP to a
depth of more than 20 ft using a water jet rig. With this
method, a small-diameter steel casing, slotted over the
lower 5 ft. was installed during drilling. Other shallow
test holes were hand augered and cased with 1.25-in.-diam-
eter P9C that was capped at the bottom and slotted over
the lower 4 to 6 in. of casing. Each hole was filled with
sand around the slotted section, and then backfilled to the
ground surface with sediments from the angered hole.
Water levels and specific conductivity were measured about
3 to 7 days after installation when water levels in the test
holes had equilibrated. LADWP has monitored water levels
in the wells at monthly intervals since the installation.
Results from these test holes are summarized in Table 2.2.
Shallow groundwater gradients to the lake, rates of
~rou''uwater row, ana sprlng~low chemistry vary greatly
around the lake. Gradients are highest and total dissolved
solids lowest at the west side of the lake, where ground-
water inflow from the Sierra Nevada is greatest. In con-
trast, gradients are very low on the north and northeast
side, where little groundwater inflow occurs. Nearshore
shallow groundwater in this area has high total dissolved
solids and is either residual lake water, remaining after the
lake elevation declined, or lake water drawn into the sedi-
ments by evaporation in the salt flats. In areas of exten-
S;~; iala ci~v~iopment, such as fine Ala Marina, Simon's
Spring, and Warm Springs, gradients are erratic and the
locations of springs are controlled by fractures and tufa
ridges.
~ ~ . ~ . · ~
, ~ ~ . , ~ . _
OCR for page 39
Hydrology of the Mono Basin
39
terrestrial evapotranspiration, excluding that from xero-
phytes, which utilize local precipitation, appears to be a
relatively minor component in the basin moisture budget.
DESCRIPTION AND ASSESSMENT OF WATER
BALANCE MODELS
Although the water balance approach to hydrologic
studies is conceptually simple, accurate water balance
models are difficult to construct. Measurements of the
components are rarely complete, and measurement errors
may be large. The instrumentation required to adequately
describe the individual components of such a model is ex-
tensive. Very few watersheds in the United States are
monitored in sufficient detail to describe all the processes.
For this reason, water balance models are generally
designed to derive maximum information from the available
data. This is the case for past and present models for
Mono Lake and the Mono Basin.
Vorster (1985) surveyed and evaluated previously devel-
oped water balance models for the Mono Lake and Basin.
In addition, Todd (1984) reviewed the two most recent
models by Vorster and LADWP in the context of previous
studies. Because these reviews are available and because
Vorster's model (1985) and LADWP's model (1984 and 1987)
are the most extensive and complete, the following discus-
sion is limited to these two models. Vorster's model was
developed as a master's thesis in geography at the Califor-
nia State University, Hayward. The LADWP model, first
published in November 1984, has been revised and updated
as more data have become available. The most recent ver-
sion was published in January 1987.
The basic equation for a hydrologic mass balance model
states the conservation of mass over a specified region:
I - 0 = AS + ER
where I represents inflows to the solution domain, O is
outflows from the domain, AS is the change in storage, and
ER represents residual errors due to measurement errors
OCR for page 40
40
The Mono Basin Ecosystem
and inaccuracies and unknown or unmeasured components.
While a complete model might include all of the compo-
nents shown in Figure 2.6, often the decision of which
individual terms are included in inflow and outflow esti-
mates is determined by available data. Selection of the
solution domain or the boundaries of the study region is of
fundamental importance because it also affects components
that must be defined in order to estimate inflows and
outflows. Thus data availability as well as physical setting
should be considered in the selection of the study region
boundary.
In the case of Mono Lake, at least three choices of
boundary location and moisture balance equation are pos-
sible (Table 2.3~. The solution of any one of these equa-
tions would provide an estimate of changing lake volumes
or elevations if the other terms in the moisture balance
~ . Selection of the
most appropriate boundary to use is determined by which
equation can be solved most accurately using available data.
· .
equation can be estimated accurately
For example, Case I represents a traditional approach to
hydrologic water balance models. Here surface water in-
flows need not be estimated because the choice of the
problem domain coincides with surface water drainage
divides. On the other hand, this treatment requires quan-
tification of snowmelt and snowpack storage, as well as of
evapotranspiration and changes in groundwater storage,
components that are difficult to measure accurately.
In Case II, the study region boundary is located at the
contact between unconsolidated basin fill sediments and
lower permeability glacial tills and rocks of the Sierra.
This boundary was used by Vorster (1985), who further
expanded the moisture balance equation to include 18
terms. The major advantage to this approach is that in-
flows to the system are relatively well-defined. Assuming
groundwater inflows into the valley sediments are small,
inflow is defined by precipitation on the lake and basin fill
and streamflows across the boundary of the problem domain
where approximately 75 to 85 percent of the surface runoff
is gaged. Vorster did not treat groundwater inflow as a
distinct term, out Instead combined unknown groundwater
inflows and unmeasured surface runoff inflows into a single
term. Losses, due to export, lake evaporation, anti
OCR for page 41
Hydrology of the Mono Basin
Subilmatlon
L
..
Precipitallon ~ e
Snow | Rain
. .
| Snowm ~L:
Runoff _ Flow
1 1 .
t ~ 1e
EXPORTS Grant ~ Stream Channel _
lake Runoff
l - 1
IPr~ipl~tlonh
e _' ~ .
Mono ~ Springs
Lake L
e- evaporation
ET = evapo~nspiraUon
41
~ IMPORTS
~ Ate
Inflltratlon and _
Soll Moisture Storage _
' 1
L
Groundwater
Storage |
1
1 ~ 1
FIGURE 2.6 Components of hydrologic mass balance model
for Mono Lake.
terrestrial evapotranspiration, are less well-known than
inputs, but generally must be specified in any water budget
model. Changes in storage occur as changes in soil mois-
ture storage, surface runoff storage, eroundwater storage.
and lake storage. For an annual time interval,
lake storage is probably the most significant of these and
is relatively well-known from historic measurements of lake
levels.
Case III, used by LADWP (1984, 1987), considers the
water budget of the lake only. This is the most straight-
forward approach to a moisture budget model for predicting
lake levels in the sense that it includes the smallest num-
ber of components. However, several of these components
are poorly known. Inflows include surface flow into the
lake, groundwater inflows, and precipitation on the lake.
storage
groundwater storage,
· . ~ ~ -
cnange In
these
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42
The Mono Basin Ecosystem
TABLE 2.3 Moisture Balance Equations for Mono Lake for
Three Possible Solution Domains
Case I: A boundary that encompasses the entire Mono Basin watershed. The governing
equation is:
PW + IM - EX - EWL - EML - EWT = ASML + BASILS + ~SWGW + SWUM + ~SWSR
where: PW= precipitation on the entire Mono Basin
IM= imports to the Mono Basin
EX= exports from the Mono Basin including groundwater seepage into the
LADWP tunnel
EWL = evaporation from all of the lakes within the basin excluding Mono
Lake
EML = evaporation from Mono Lake
EWT= terrestrial evapotranspiration
BASIL= change in storage of Mono Lake
ASWLS = change in storage of all watershed lakes
except Mono Lake
xSWGW = change in storage of groundwater
throughout the watershed
SWUM = change in storage in soil moisture
throughout the watershed
SWSR = change in storage of surface water
runoff
Case II: A boundary that includes Mono Lake and the surrounding groundwater storage
area (basin fill). The governing equation is:
PB + ISWF + IGWF - EBX - EML - EBT = ~SML + ~SBGW + ~SBSM + /`SBSR
where previously undefined components are:
PB= precipitation on the basin fill and Mono Lake
ISWF= surface water inflows to the basin fill
IGWF = groundwater inflows to the basin fill
EBX = exports from study region
EBT = evapotranspiration from basin fill including
losses from lakeshore vegetation
SBGW = change in groundwater storage in the basin fill
SBSM = change in soil moisture storage in the basin fill
ASBSR = change in storage of surface runoff within the basin fill
Case III: A boundary at the lakeshore. Study region includes only Mono Lake. The
. . .
gOVerIllIlg eqUatlOI1 IS:
PL + ISWL + IGWL - EML = /`SML
where previously undefined components are:
PL= precipitation on Mono Lake
ISWL = surface water inflow to Mono Lake
IGWL = groundwater inflow to Mono Lake
OCR for page 43
Hydrology of the Mono Basin
43
Because the stream gaging stations are located several
miles from the lakeshore, both surface water and ground-
water inflows are unknown and must be lumped with the
residual error term in the model. Outflows are due
entirely to evaporation from the lake surface, a poorly
measured process. The change in storage is the relatively
well-known change in lake storage estimated from lake
level measurements.
Vorster's model is
been developed for the Mono Lake or Basin and is con-
structed to take advantage of available hydrologic data.
However, the reliability and accuracy of all current hydro-
logic models are limited by poor estimates of major com-
ponents of the hydrologic cycle. Models by both Vorster
and LADWP require estimates of mean annual precipitation
over the lake, surface and subsurface inflows, and lake
evaporation.
Estimated values for these parameters are highly vari-
able. For example, estimates of the average annual pre-
cipitation rate on Mono Lake range from 5.3 to 12.0 in./yr.
Estimates of average annual inflows from ungaged water-
sheds vary from 0 to 1 13,000 acre-ft/yr, and lake evapo-
ration estimates range between 37.4 to 78.S in./yr (Vorster,
1985~.
Vorster performed a sensitivity analysis to assess the
effect of uncertainty in the data on the ability of his
model to predict observed lake levels. As expected, uncer-
tainty in lake evaporation estimates had the greatest influ-
ence on model results. Depending on the volume of sur-
face water exports, a change of +S percent in estimates of
lake evaporation rates resulted in a variation of 2 to 14 ft
in projected long-term lake levels.
been performed on the LADWP model.
Limitations in available data are the major large source
ot error in the moisture budget models. Of most impor-
tance is the need for more accurate measurements of lake
evaporation. In addition, estimation of groundwater inflows
to the lake, monitoring of major ungaged streams, and
measurements of precipitation on the east side of the lake
would improve the reliability and accuracy of the model
simulations.
the most detailed model that has
No error analysis has
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44
The Mono Basin Ecosystem
MODELING OF MONO LAKE LEVELS AND SALINITY
For the purposes of this report, two relationships pre-
dicted by the hydrological models need to be determined
(1) the relationship between elevations of Mono Lake and
streamflow at the points of the diversions and (2) the rela-
tionship between lake elevation and salinity. The former is
required for describing the effects of changes in lake level
on the riparian (stream) systems.
~ . .
The latter is required
for predicting the lake levels at which the aquatic biota
will be affected bY increased salinity {see chanter 61.
~ .
The two available models of the hydrology of the Mono
Basin (Vorster, 1985; LADWP, 1987) were modified for this
report by using synthetically generated sequences of
streamflows as inputs rather than historical streamflows.
The historical streamflows of approximately 40 years' dura-
tion were extended to 2000 years by using an autoregres-
sive moving average model (Box and Jenkins, 1970~. Syn-
thetic streamflows were used because they preserve the
statistical properties of the original record (mean, variance,
and skewness) while allowing equally likely hydrologic
events to be input to the models. This approach minimizes
the cyclical modeling results of the brief 40-year historical
data set user! in the Vorster and LADWP models. The gen-
eration of 2000 years of data is arbitrary but considered
sufficient for the required modeling results.
For standardization, an evaporation rate of 42 in./yr
was used in both models. The LADWP and Vorster models
use different values of freshwater evaporation as inouts to
their models.
uses 45 in./yr.
·
~ ~ _
The former uses 40.S in./yr, and the latter
Because of the large uncertainties in the
evaporation data, and in order to eliminate an excessive
number of modeling results, an approximate freshwater rate
of 42 in./yr was used for this report as an input value for
both models. This annual freshwater evaporation rate is
converted to an annual saline water evaporation rate inde-
pendently as a function of lake volume and specific gravity
by each model. Vorster's model was also adjusted to in-
clude the most recent bathymetric data from Pelagos Cor-
poration (1987~. Both models were then used to simulate
the elevation of Mono Lake as a function of flow at the
diversion points (Figure 2.7~.
OCR for page 45
Hydrology of the Mono Basin
6.40
6.39
6.38
6.41
s
o
In
3
o
i; 6.37 I, vorster
go 6.35 ~ ''
fir 6.34 _ ''
6.33 A''
6.32 1 1 1 !
10 30 50 70 90
45
LADE
,'
RELEASE (thousands of acr~fVyr)
FIGURE 2.7 Calculations of equilibrium lake level versus
flow at diversion points for an evaporation rate of 42
in./yr.
For any given amount of water exported from the basin,
the lake will not attain its equilibrium level (level at which
inflow of water equals outflow from evaporation) for more
than 100 years. Figures 2.X and 2.9 show the results of
the Vorster and LADWP models for predicting lake level as
a function of time for releases (controlled releases past
LADWP diversion structures on Rush, Parker, Walker, and
Lee Vining creeks) to Mono Lake of 10~000, 25,000, 50,000,
75,000, and 100,000 acre-ft/yr.
~ , . . .
In both these figures, the
early fluctuations (before 200 years) are due to oscillations
in the climatic data used as input to the models. After
approximately 200 years the lake levels stabilize. Small
fluctuations in lake level still occur due to oscillation in
the climatic data.
A comparison of the results of Figures 2.7, 2.S, and 2.9
indicates that the Vorster model predicts lower lake eleva-
tions for the same values of releases to Mono Lake. For
OCR for page 46
46
The Mono Basin Ecosystem
6.41
6.40
o
6.39
6.38
o
e; 6.37
6.36
6.35
6.34
6.33
- 100,000 ~
m
m
u,
o
o
o
10,000 D
1 1 1 ~ I I I ~
240 280
/~ ~ 75,000
50,000
it. ^_ _ .~
0 40 80 120 160 200
TIME (years)
~ 25,000
FIGURE 2.8 Lake level versus year predicted from LADWP
model (1987) using an evaporation rate of 42 in./yr.
6.41
A, ~
~ \~/
6.31
6.40
6.39
6.38
6.37
6.36
6.35
6 sa
~ ~
_ _.__ ~ 10O,OOO m
0 6.39 _ ~ ~
~ 638 ~ _ 75,000 0
=> 635~\ ~50,000
' 6.34 ~ ~ 25000
_
120 160 200 240 280
0 40 80
TIME (years)
FIGURE 2.9 Lake level versus year predicted from Vorster
model (1985) using an evaporation rate of 42 in./yr.
OCR for page 47
Hydrology of the Mono Basin
47
example, for a release of 10,000 acre-ft/yr, the LADWP
model predicts a lake elevation of approximately 6349 ft
while the Vorster model predicts a lake elevation of
approximately 6328 ft. This difference of 21 ft is the
largest that occurs. For a release of 100,000 acre-ft/yr,
the Vorster mode] predicts a value that is only 4 ft below
that predicted by the LADWP model. There are several
reasons for these differences. Given that the two models
use different boundary locations, time bases for model cal-
ibration, procedures for terrestrial evapotranspiration, and
estimates of ungaged surface water runoff, the differences
in the results of the two models are not unexpectecl.
To improve the modeling capability for the Mono Basin,
a new set of models with a monthly time increment, based
on a comprehensive surface water and groundwater hydro-
logic data collection network, is needed. This data collec-
tion network would need to focus on the components with
limited or missing data,
such as nearshore and deep
groundwater, the ungageo surface runoff areas, lake evapo-
ration, and terrestrial evapotranspiration.
Both Vorster (1985) and LADWP (1987) calculated the
relationship between salinity and lake level by assuming a
constant amount of salt in the lake. ~~ ~ '
taken to be a linear function or lake
value used by LADWP (1987) for the
the lake is an average of summations of the major solutes
analyzed separately in 11 surface samples obtained from
1940 to 1980. This number (285 x 106 tons) is similar to
the average of gravimetric determinations made for several
stations and depths in 1982 (288 x 106 tons) (B. White, Los
Angeles Department of Water and Power, personal commu-
nication, 1987~.
The assumption of a constant amount of salts in the
lake appears justified for salinities below approximately 125
g/l. Above this salinity, minerals will begin to precipitate
and will remove some ions from solution, as discussed in
chapter 3. However, the geochemistry is not well enough
understood to precisely estimate the relationship between
lake level and salinity for salinities above 125 g/1. The
committee adopted, for this report, the values calculated by
LADWP (1986), recognizing that values for salinity above
125 g/1 (corresponding to a lake level of approximately
~ . . ~ . .
Therefore, salinity Is
volume. The actual
total salt content of
OCR for page 48
48
The Mono Basin Ecosystem
6360 ft above sea level as discussed in chapter 6) may be
overestimated.
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Hydrology of the Mono Basin
49
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Representative terms from entire chapter:
mono lake
