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The late Holocene histories of Walker Lake and the Carson Sink were reconstructed by synthesizing existing data in both basins along with new age constraints from key sites, supplemented with paleohydrologic modeling. The repeated diversions of the Walker River to the Carson Sink and then back to Walker Lake caused Walker Lake–level fluctuations spanning ± 50 m. Low lake levels at about 1000, 750, and 300 cal yr BP are time correlative to the ages of fluvial deposits along the Walker River paleochannel, when flow was directed toward the Carson Sink. The timing and duration of large lakes in the Carson Sink were further refined using moisture-sensitive tree-ring chronologies. The largest lakes required a fourfold to fivefold increase in discharge spanning decades. Addition of Walker River flow to the Carson Sink by itself is inadequate to account for the required discharge. Instead, increases in the runoff coefficient and larger areas of the drainage basin contributing surface runoff may explain the enhanced discharge required to create these large lakes.
A new lake-level curve for Pyramid and Winnemucca lakes, Nevada, is presented that indicates that after the ~15,500 cal yr BP Lake Lahontan high stand (1338 m), lake level fell to an elevation below 1200 m, before rising to 1230 m at the 12,000 cal yr BP Younger Dryas high stand. Lake level then fell to 1155 m by ~10,500 cal yr BP followed by a rise to 1200 m around 8000 cal yr BP. During the mid-Holocene, levels were relatively low (~1155 m) before rising to moderate levels (1190–1195 m) during the Neopluvial period (~4800–3400 cal yr BP). Lake level again plunged to about 1155 m during the late Holocene dry period (~2800–1900 cal yr BP) before rising to about 1190 m by ~1200 cal yr BP. Levels have since fluctuated within the elevation range of about 1170–1182 m except for the last 100 yr of managed river discharge when they dropped to as low as 1153 m. Late Holocene lake-level changes correspond to volume changes between 25 and 55 km3 and surface area changes between 450 and 900 km2. These lake state changes probably encompass the hydrologic variability possible under current climate boundary conditions.
Reconstruction of lake-level fluctuations from landform and outcrop evidence typically involves characterizing periods with relative high stands. We developed a new approach to provide water-level estimates in the absence of shoreline evidence for Owens Lake in eastern California by integrating landform, outcrop, and existing lake-core data with wind-wave and sediment entrainment modeling of lake-core sedimentology. We also refined the late Holocene lake-level history of Owens Lake by dating four previously undated shoreline features above the water level (1096.4 m) in AD 1872. The new ages coincide with wetter and cooler climate during the Neopluvial (~3.6 ka), Medieval Pluvial (~0.8 ka), and Little Ice Age (~0.35 ka). Dates from stumps below 1096 m also indicate two periods of low stands at ~0.89 and 0.67 ka during the Medieval Climatic Anomaly. The timing of modeled water levels associated with 22 mud and sand units in lake cores agree well with shoreline records of Owens Lake and nearby Mono Lake, as well as with proxy evidence for relatively wet and dry periods from tree-ring and glacial records within the watershed. Our integrated analysis provides a continuous 4000-yr lake-level record showing the timing, duration, and magnitude of hydroclimate variability along the south-central Sierra Nevada.
We have obtained estimates of the solar internal rotational velocity from measurements of the frequency splittings of p-mode oscillations. Specifically, we have analyzed a 10-day time series of full-disk Dopplergrams obtained during July and August 1984 at the 60-Foot Tower Telescope of the Mt. Wilson Observatory. The Dopplergrams were obtained with a Na magneto-optical filter and a 244 × 248-pixel CID camera. From the time series we computed power spectra for all of the prograde and retrograde sectoral p-modes from ℓ = 0 to 200 and for all of the tessaral harmonics up to ℓ = 89. We then applied a cross-correlation analysis to the resulting sectoral power spectra to obtain estimates of the frequency splittings. From ℓ = 4 to ℓ = 30 we obtained a mean value of the frequency spitting of roughly 450 nHz (sidereal) in close agreement with most previously published results, while from ℓ = 40 to ℓ = 140 we obtained a mean value of about 470 nHz. We believe that the latter value is slightly higher than the surface rotational splitting of 461 nHz because of possible confusion due to the temporal sidelobes introduced by the day/night observing cycle. Confirmation of this possibility will have to await our computation of tesseral power spectra for degrees greater than our current limit of 89. Finally, for degrees between 140 and 200, the frequency splittings are indistinguishable from the surface rotation rate.
We present here the first full-disk solar Dopplergram obtained with the new 1024 × 1024-pixel CCD camera which has recently been installed at the 60-Foot Tower Telescope of the Mt. Wilson Observatory. This Dopplergram has a spatial resolution of 2.2 arcseconds and was obtained in less than one minute of time. The Dopplergram was obtained with a magneto-optical filter which was designed to obtain images in the two Na D lines. The filter and the camera were operated together as part of the development of a Solar Oscillations Imager (SOI) esperiment which is currently being designed at JPL for the joint NASA/ESA Solar and Heliospheric Observatory (SOHO) mission.
In this paper we describe a new observing system which is currently nearing completation at the Mount Wilson Observatory. This system has been designed to obtain daily measurements of solar photospheric and subphotospheric rotational velocities from the frequency splitting of non-radial solar p-mode oscillations of moderate to high degree (i.e. l > 150). The completed system will combine a 244 x 248 pixel CID camera with a high-speed floating point array processor, a 32-bit minicomputer, and a large-capacity disc storage system. We are integrating these components into the spectrograph of the 60-foot solar tower telescope at Mount Wilson in order to provide a facility which will be dedicated to the acquisition of oscillation data.
A brief summary is given of a program which is currently being carried out with the McMath telescope of the Kitt Peak National Observatory in order to study high-degree (l ≳ 150) solar p-mode oscillations. This program uses a 244 x 248 pixel CID camera and the main spectrograph of the McMath telescope to obtain velocity-time maps of the oscillations which can be converted into two-dimensional (kh – ω) power spectra of the oscillations. Several different regions of the solar spectrum have been used in order to study the oscillations at different elevations in the solar atmosphere. The program concentrates on eastward- and westward-propagating sectoral harmonic waves so that measurements can be made of the absolute rotational velocities of the solar photospheric and shallow sub-photospheric layers. Some preliminary results from this program are now available. First, we have been unable to confirm the existence of a radial gradient in the equatorial rotational velocity as was previously suggested. Second, we have indeed been able to confirm the presence of p-mode waves in the solar chromosphere as was first suggested by Rhodes et al. (1977). Third, we have been able to demonstrate differences in photospheric and chromospheric power spectra.