Geology topics

Michael D. Dettinger

Publications and source records attributed to Michael D. Dettinger.

62 records · Page 4Linked to original sources

USGS2003

Climate effects of pacific decadal oscillation on streamflow of the Feather River, California

The timing of maximum monthly-mean streamflow for the Feather River in northern California has come earlier in the year in recent decades (since the 1950s), as have timings in most rivers throughout California and the western United States. Much of the timing shift in the Feather River basin appears to coincide with interdecadal changes in the North Pacific climate regime. The coincident timing changes are seen as a shift in the month of maximum streamflow from April-May during the cooler Pacific Decadal Oscillation (PDO) phase to March-April during the warmer phase. The change in streamflow timing in the Feather River basin became an issue during the testing of a new set of watershed models of inflow to Lake Oroville, because model performance degraded in simulations of recent years (1998-2001). The model calibration period (1971-97) was dominated by the warmer (1977-98) PDO phase. However, the 1998-2001 period mostly corresponds to a newly reestablished cool PDO (beginning late 1998). Simulations during 1998-2001 failed to reproduce streamflow as well as simulations of the calibration period, probably because some model parameters, like those associated with rain-snow mixes or temperature and precipitation distributions, are not calibrated for climatic conditions that occur during a cool PDO.

California↗

USGS2003

Linking diurnal cycles of river flow to interannual variations in climate

Many rivers in the Western United States have diurnal variations exceeding 10% of their mean flow in the spring and summer months. The shape and timing of the diurnal cycle is influenced by an interplay of the snow, topography, vegetation, and meteorology in a basin, and the measured result differs between wet and dry years. The largest interannual differences occur during the latter half of the melt season, as the snowline retreats to the highest elevations and most shaded slopes in a basin. In most basins, during this period, the hour of peak discharge shifts to later in the day, and the relative amplitude of the diurnal cycle decreases. The magnitude and rate of these changes in the diurnal cycle vary between years and may provide clues about how long- term hydroclimatic variations affect short-term basin dynamics.

Western United States↗

USGS2001

Comments on “ Changes in the onset of dpring in the western United States ” reply

We thank Dr. Lilly for his comment (Lilly 2001) concerning the possibility that the trend toward early lilac and honeysuckle bloom that we noted in our recent (Cayan et al. 2001) study may have been caused by urbanization. For some individual lilac or honeysuckle bushes, it is very possible that such an urban influence could be operating. Unfortunately, because the phenological network has vanished after it was retired in 1994, it is virtually impossible to examine this directly from site inspections. However, the circumstantial evidence that this trend is a symptom of regional and larger-scale climate fluctuations or changes, not from local urban effects, is quite strong, as explained below.

Bulletin of the American Meteorological Society↗

USGS2000

Streamflow changes in the Sierra Nevada, California, simulated using a statistically downscaled general circulation model scenario of climate change

Simulations of future climate using general circulation models (GCMs) suggest that rising concentrations of greenhouse gases may have significant consequences for the global climate. Of less certainty is the extent to which regional scale (i.e., sub-GCM grid) environmental processes will be affected. In this chapter, a range of downscaling techniques are critiqued. Then a relatively simple (yet robust) statistical downscaling technique and its use in the modelling of future runoff scenarios for three river basins in the Sierra Nevada, California, is described. This region was selected because GCM experiments driven by combined greenhouse-gas and sulphate-aerosol forcings consistently show major changes in the hydro-climate of the southwest United States by the end of the 21st century. The regression-based downscaling method was used to simulate daily rainfall and temperature series for streamflow modelling in three Californian river basins under current-and future-climate conditions. The downscaling involved just three predictor variables (specific humidity, zonal velocity component of airflow, and 500 hPa geopotential heights) supplied by the U.K. Meteorological Office couple ocean-atmosphere model (HadCM2) for the grid point nearest the target basins. When evaluated using independent data, the model showed reasonable skill at reproducing observed area-average precipitation, temperature, and concomitant streamflow variations. Overall, the downscaled data resulted in slight underestimates of mean annual streamflow due to underestimates of precipitation in spring and positive temperature biases in winter. Differences in the skill of simulated streamflows amongst the three basins were attributed to the smoothing effects of snowpack on streamflow responses to climate forcing. The Merced and American River basins drain the western, windward slope of the Sierra Nevada and are snowmelt dominated, whereas the Carson River drains the eastern, leeward slope and is a mix of rainfall runoff and snowmelt runoff. Simulated streamflow in the American River responds rapidly and sensitively to daily-scale temperature and precipitation fluctuations and errors; in the Merced and Carson Rivers, the response to the same short-term influences is much less. Consequently, the skill of simulated flows was significantly lower in the American River model than in the Carson and Merced. The physiography of the three basins also accounts for differences in their sensitivities to future climate change. Increases in winter precipitation exceeding +100% coupled with mean temperature rises greater than +2°C result in increased winter streamflows in all three basins. In the Merced and Carson basins, these streamflow increases reflect large changes in winter snowpack, whereas the streamflow changes in the lower elevation American basin are driven primarily by rainfall runoff. Furthermore, reductions in winter snowpack in the American River basin, owing to less precipitation falling as snow and earlier melting of snow at middle elevations, lead to less spring and summer streamflow. Taken collectively, the downscaling results suggest significant changes to both the timing and magnitude of streamflows in the Sierra Nevada by the end of the 21st Century. In the higher elevation basins, the HadCM2 scenario implies more annual streamflow and more streamflow during the spring and summer months that are critical for water-resources management in California. Depending on the relative significance of rainfall runoff and snowmelt, each basin responds in its own way to regional climate forcing. Generally, then, climate scenarios need to be specified — by whatever means — with sufficient temporal and spatial resolution to capture subtle orographic influences if projections of climate-change responses are to be useful and reproducible.

California↗

USGS1997

Effects of El Nino on streamflow, lake level, and landslide potential

One of the most important sources of year-to-year climate variation in the Southwest is the El Niño phenomenon of the tropical Pacific Ocean. El Niño is a natural but largely unpredictable condition that results from complex interplay among clouds and storms, regional winds, oceanic temperatures, and ocean currents along the equatorial Pacific. Under "normal" conditions, the tropical trade winds blow from east to west, Figure 1. Schematic diagram of normal and El Niño conditions in the Pacific Ocean. From NOAA El Niño website. ponding up warm water in the western Pacific. In the eastern Pacific, the trade winds pull up cold, deep, nutrient-rich waters along the equator from the Ecuadorian coast to the central Pacific. The warmth of the western Pacific results in a particularly vigorous hydrologic cycle there with towering cumulus clouds and tropical storms that "radiate" atmospheric waves and disturbances across vast regions of the globe. Heat and moisture lofted into the upper atmosphere by the clouds and storms are distributed by high-altitude winds across vast regions of the globe. During an El Niño, this situation is disrupted and the trade winds weaken, thus reducing the upwelling of cool waters in the eastern Pacific and allowing the pool of warm water in the west to drift eastward toward South America. As the central and eastern Pacific warms, atmospheric pressure gradients along the equator weaken, and the trade winds diminish even more. These changes in sea-level pressure of the atmosphere are characteristic of the strongest El Niño and were identified as the "Southern Oscillation" of the global atmosphere by Sir Gilbert Walker in the early decades of this century. A chicken-and-egg relation exists between the changes in ocean temperatures and changes in winds (and atmospheric pressure gradients); the two sets of changes reinforce and drive each other but neither is clearly or universally "the" initiator of El Niño. Ocean temperatures and surface winds interact to form the complex process, El Niño-Southern Oscillation (ENSO). The interactions can be set off by subtle changes in one or the other, by buffeting from other parts of the tropics, or from regions beyond the tropics. Such a complex interplay and its uncertain (and variable) origins are the primary limitations on our ability to predict El Niño. As the waters of the central and eastern Pacific warm, the powerful tropical Pacific storms begin to form farther east than usual (Fig. 1). As the distribution of storms spreads east along the equator, their influence on global weather systems also changes. Most notably, for our purposes, the jet stream over the North Pacific Ocean is invigorated and pulled farther south than normal, where it collects moisture and storms and carries them to the southwestern United States and northern Mexico. During an El Niño, the trade winds are too weak to cause upwelling of nutrient-rich waters off the coasts of Ecuador and Peru. Generations of South American fisherman thus have recognized these conditions by the disappearance of their standard catch, commonly during December and January, every three to seven years. Because of the near coincidence in timing between these conditions and Christmas, the fishing communities have called the phenomenon "El Niño", for the Christ child. The geologic record suggests that El Niño conditions have been a part of earth's climate for at least several thousand years. An El Niño event usually lasts for several seasons, and, along with its other effects, represents an interruption of the "normal" seasonal cycle of the tropical climate. After a few seasons, and usually during spring time (in the Northern Hemisphere), the seasonal cycle reasserts itself and the tropical ocean cools back to the normal east-to-west sea-surface temperature gradients. Sometimes the warm El Niño events give way to unusually cold sea-surface temperatures and unusually strong trade winds, a condition now called La Niña. On other occasions, La Niñas may begin on their own, without an immediately preceding El Niño. The effects of the El Niño and La Niña on global climate are, in part, mirror images of each other. For example, drought is a common occurrence in the southwestern United States during La Niña, in contrast to the wet years associated with El Niño.

Arizona, California, Nevada, New Mexico, Oregon, U↗

USGS1996

Ground-water conditions in Las Vegas Valley, Clark County, Nevada: Part 2, hydrogeology and simulation of ground-water flow

In sedimentary deposits beneath arid Las Vegas Valley, ground-water levels have declined as much as 280 feet since 1912 in deeper aquifers due to pumping associated with urban development. Accompanying land subsidence has been as great as 5 feet. Predictive simulations show that by maintaining pumpage and recharge at 1980 levels and using municipal wells only during periods of peak water demand, rates of water-level decline and land subsidence will be reduced.

Nevada↗

USGS1995

Large-scale atmospheric forcing of recent trends toward early snowmelt runoff in California

Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, which are sensitive to changes in mean winter temperatures. Such basins have broad areas in which winter temperatures are near enough to freezing that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of California, a declining fraction of the annual runoff has come in April–June. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year. Weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that runoff timing responds equally to the observed decadal-scale trends in winter temperature and interannual temperature variations of the same magnitude, suggesting that the temperature trend is sufficient to explain the runoff-timing trends. The immediate cause of the trend toward warmer winters in California is a concurrent, long-term fluctuation in winter atmospheric circulations over the North Pacific Ocean and North America that is not immediately distinguishable from natural atmospheric variability. The fluctuation began to affect California in the 1940s, when the region of strongest low-frequency variation of winter circulations shifted to a part of the central North Pacific Ocean that is teleconnected to California temperatures. Since the late 1940s, winter wind fields have been displaced progressively southward over the central North Pacific and northward over the west coast of North America. These shifts in atmospheric circulations are associated with concurrent shifts in both West Coast air temperatures and North Pacific sea surface temperatures.

California↗

USGS1995

Seasonal/yearly salinity variations in San Francisco Bay

The ability of resource agencies to manage fish, wildlife and freshwater supplies of San Francisco Bay estuary requires an integrated knowledge of the relations between the biota and their physical environment. A key factor in these relations is the role of salinity in determining both the physical and the biological character of the estuary. The saltiness of the water, and particularly its seasonal and interannual patterns of variability, affects which aquatic species live where within the estuary. Salinity also determines where water can and cannot be diverted for human consumption and irrigated agriculture, and plays a role in determining the capacity of the estuary to cleanse itself of wastes. In short, salinity is a fundamental property of estuarine physics and chemistry that, in turn, determines the biological characteristics of each estuary. Freshwater is a major control on estuarine salinity. Most freshwater supplied to the Bay is from river flow through the Delta, which is primarily runoff from the Sierra Nevada. Most contaminants in San Francisco Bay are from the Sacramento/San Joaquin Valley and the local watershed around the Bay rather than the sea or atmosphere. Land is the primary source of freshwater and freshwater serves as a tracer of land-derived substances such as the trace metals (copper, lead and selenium), pesticides and plant nutrients (nitrate and phosphate). The U.S. Geological Survey is collaborating with other agencies and institutions in studying San Francisco Bay salinity using field observations and numerical simulations to define the physical processes that control salinity. The issues that arise from salinity fluctuations, however, differ in the northern and southern parts of the bay. In North Bay we need to know how salinity responds to freshwater flow through the Sacramento/San Joaquin Delta; this knowledge will benefit water managers who determine how much delta flow is needed a) to protect freshwater supplies for municipal water use and b) modulate salinity for a healthy estuary. In South Bay we need to know where the freshwater comes from (the distant Delta or local streams) to sort out the sources of a) contamination or b) dilution.

California↗