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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

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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,

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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 . . . . . . . . .

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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-

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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

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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.

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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

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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

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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. ~ ~ . ~ . ~ , ~ ~ . , ~ . _

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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

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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

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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

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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~.

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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

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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.

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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

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48 The Mono Basin Ecosystem 6360 ft above sea level as discussed in chapter 6) may be overestimated. REFERENCES Blaney, H. 1954. Consumptive-use requirements for water. Agric. Eng. 35:870-873, SS0. Box, E. P., and G. M. Jenkins. 1970. Time Series Analysis: Forecasting and Control. San Francisco, Calif.: Holden- Day. 553 PP. Goodridge, J. D. 1981. California Rainfall Summary: Monthly Total Precipitation 1849-1979. Sacramento, Calif.: California Department of Water Resources, Divi- sion of Planning. 55 pp. Hales, J. E., Ir. 1972. Surges of maritime tropical air northward over the Gulf of California. Mon. Weather Rev. 100:298-306. Hales, I. E., Jr. 1974. Southwestern United States summer monsoon source--Gulf of Mexico or Pacific Ocean? J. Appl. Meteorol. 13:331 -342. Houghton, J. G. 1969. Characteristics of Rainfall in the Great Basin. Ph.D. dissertation, University of Oregon, Eugene. 292 pp. Houghton, I. G., C. M. Sakamoto, and R. O. Gifford. 1975. Nevada's Weather and Climate. Reno, Nev.: Nevada Bureau of Mines and Geology and University of Nevada. 78 pp. Karl, T. R., R. E. Livezey, and E. S. Epstein. rat ~ ~ ~ _ . ~ ~ ~ 1984. ^~;~ll~ unusual mean winter temperatures across the contiguous United States. Bull. Am. Meteorol. Soc. 65:1302- 1309. Kiladis, G. N., and H. F. Diaz. 1986. An analysis of the 1977-78 ENSO episode and comparison with 1982-83. Mon. Weather Rev. 114: 1035- 1047. LaMarche, V. C., Jr. 1974. Paleoclimatic inferences from long tree-ring records. Science 183:1043- 1048. Lee, K. 1969. Infrared Exploration for Shoreline Springs: A Contribution to the Hydrogeology of Mono Basin, California. Ph.D. dissertation, Stanford University. 216 PP. Loeffler, R. M. 1977. Geology and hydrology. Pp. 6-38 in An Ecological Study of Mono Lake, California, D. W.

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Hydrology of the Mono Basin 49 Winkler, ed. Institute of Ecology Publication No. 12. Davis, Calif.: University of California, Institute of Ecology. Los Angeles Department of Water and Power. 1984. Back- ground Report on Geology and Hydrology of Mono Basin. Report of the Aqueduct Division, Hydrology Section. Los Angeles, Calif. Los Angeles Department of Water and Power. 1986. Report on Mono Lake Salinity. Los Angeles, Calif. Los Angeles Department of Water and Power. 1987. Mono Basin Geology and Hydrology. Los Angeles, Calif. Monteverdi, J. P. 1976. The single air mass disturbance and precipitation characteristics at San Francisco. Mon. Weather Rev. 104:1289- 1296. Pelagos Corporation. 1987. A Bathymetric and Geologic Survey at Mono Lake, California. Report prepared for Los Angeles Department of Water and Power. San Diego, Calif. Policansky, D. 1977. The winter of 1976-77 and the pre- diction of unlikely weather. Bull. Am. Meteorol. Soc. 58: 1073-74. Ramage, C. S. 1986. E1 Nino. Sci. Am. 254~6~:76-83. Reiter, E. R., and M. Tang. 1984. Plateau effects on diur- nal circulation patterns. Mon. Weather Rev. 112:638- 651. Tang, M., and E. R. Reiter. 1984. Plateau monsoons of the northern hemisphere: a comparison between North America and Tibet. Mon. Weather Rev. 112:617-637. Thornthwaite, C. W., and J. R. Mather. 1957. Instructions and Tables for Computing Potential Evapotranspiration and the Water Balance. Publications in Climatology 10~3~. Centerton, N.J.: Drexel Institute of Technology, Laboratory for Climatology. Todd, D. K. 1984. The Hydrology of Mono Lake: A Com- pilation of Basic Data Developed for State of California v. United States, Civil No. S-80-696, U.S.D.C., E.D. Cal. Berkeley, Calif.: David Keith Todd Consulting Engineers. Vorster, P. 1985. A Water Balance Forecast Model for Mono Lake, California. Master's thesis, California State University, Hayward. Earth Resources Monograph No. 10. San Francisco. Calif.: U.S. Forest Service, Region 5.