Climate Change and...

Annotated Bibliography

Effects of Climate Change

Floods and Flood Frequency

P. C. D. Milly, R. T. Wetherald, K. A. Dunne, T. L. Delworth (2002). Increasing risk of great floods in a changing climate. Nature 415 (6871): 514-517

ABSTRACT: Radiative effects of anthropogenic changes in atmospheric composition are expected to cause climate changes, in particular an intensification of the global water cycle with a consequent increase in flood risk. But the detection of anthropogenically forced changes in flooding is difficult because of the substantial natural variability; the dependence of streamflow trends on flow regime further complicates the issue. Here we investigate the changes in risk of great floods—that is, floods with discharges exceeding 100-year levels from basins larger than 200,000 km2 —using both streamflow measurements and numerical simulations of the anthropogenic climate change associated with greenhouse gases and direct radiative effects of sulphate aerosols. We find that the frequency of great floods increased substantially during the twentieth century. The recent emergence of a statistically significant positive trend in risk of great floods is consistent with results from the climate model, and the model suggests that the trend will continue.

Pielke, R.A., Jr., M. W. Downton (2000). Precipitation and damaging floods: trends in the United States, 1932–97. Journal of Climate 13 (20): 3625–3637

ABSTRACT: The poor relationship between what climatologists, hydrologists, and other physical scientists call floods, and those floods that actually cause damage to life or property, has limited what can be reliably said about the causes of observed trends in damaging floods. It further limits what can be said about future impacts of floods on society based on predicted changes in the global hydrological cycle. This paper presents a conceptual framework for the systematic assessment of the factors that condition observed trends in flood damage. Using the framework, it assesses the role that variability in precipitation has in damaging flooding in the United States at national and regional levels. Three different measures of flood damage—absolute, per capita, and per unit wealth—each lead to different conclusions about the nature of the flood problem. At a national level, of the 10 precipitation measures examined in this study, the ones most closely related to flood damage are the number of 2-day heavy rainfall events and the number of wet days. Heavy rainfall events are defined relative to a measure of average rainfall in each area, not as absolute thresholds. The study indicates that the growth in recent decades in total damage is related to both climate factors and societal factors: increased damage is associated with increased precipitation and with increasing population and wealth. At the regional level, this study reports a stronger relationship between precipitation measures and flood damage, and indicates that different measures of precipitation are most closely related to damage in different regions. This study suggests that climate plays an important, but by no means determining, role in the growth in damaging floods in the United States in recent decades.

C. G. Pilling, J. A. A. Jones (2002). The impact of future climate change on seasonal discharge, hydrological processes and extreme flows in the Upper Wye experimental catchment, Mid-Wales. Hydrological Processes 16 (6): 1201-1213

ABSTRACT: Analysing the impact of future climate change on hydrological regimes is hampered by the disparity of scales between general circulation model (GCM) output and the spatial resolution required by catchment-scale hydrological simulation models. In order to overcome this, statistical relationships were established between three indices of atmospheric circulation (vorticity and the strength and direction of geostrophic windflow) and daily catchment precipitation and potential evapotranspiration (PET) to downscale from the HadCM2 GCM to the Upper Wye experimental catchment in mid-Wales. The atmospheric circulation indices were calculated from daily grid point sea-level pressure data for: (a) the Climatic Research Unit observed data set (1975-90); (b) the HadCM2SUL simulation representing the present climate (1980-99); and (c) the HadCM2SUL simulation representing future climate conditions (2080-99). The performance of the downscaling approach was evaluated by comparing diagnostic statistics from the three downscaled precipitation and PET scenarios with those recorded from the Upper Wye catchment. The most significant changes between the downscaled HadCM2SUL 1980-99 and 2080-99 scenarios are decreases in precipitation occurrence and amount in summer and autumn combined with a shortening of mean wet spell length, which is most pronounced in autumn. A hydrological simulation model (HYSIM) was calibrated on recorded flow data for the Upper Wye catchment and forced with the three downscaled precipitation and PET scenarios to model changes in river flow and hillslope hydrological processes. Results indicate increased seasonality of flows, with markedly drier summers. Analysis of extreme events suggests significant increases in the frequency of both high- and low-flow events.

Kiem, A.S., S.W. Franks, G. Kuczera (2003). Multi-decadal variability of flood risk. Geophysical Research Letters 30 (2): 1035

ABSTRACT: Recent research has highlighted the persistence of multi-decadal epochs of enhanced/reduced flood risk across New South Wales (NSW), Australia. Recent climatological studies have also revealed multi-decadal variability in the modulation of the magnitude of El Niño/Southern Oscillation (ENSO) impacts. In this paper, the variability of flood risk across NSW is analysed with respect to the observed modulation of ENSO event magnitude. This is achieved through the use of a simple index of regional flood risk. The results indicate that cold ENSO events (La Niña) are the dominant drivers of elevated flood risk. An analysis of multi-decadal modulation of flood risk is achieved using the inter-decadal Pacific Oscillation (IPO) index. The analysis reveals that IPO modulation of ENSO events leads to multi-decadal epochs of elevated flood risk, however this modulation appears to affect not only the magnitude of individual ENSO events, but also the frequency of their occurrence. This dual modulation of ENSO processes has the effect of reducing and elevating flood risk on multi-decadal timescales. These results have marked implications for achieving robust flood frequency analysis as well as providing a strong example of the role of natural climate variability.

Jain, S., U. Lall (2000). Magnitude and timing of annual floods: trends and large-scale climatic associations for the Blacksmith Fork River, Utah. Water Resources Research 36 (12): 3641-3651

ABSTRACT: The magnitude and timing of spring snowmelt floods reflects seasonal snow accumulation and spring temperature patterns. Consequently, interannual variations in regions such as the intermountain West, with snowmelt annual maximum floods, may be related to low-frequency variations in winter/spring large-scale climate variability. Changes in the seasonality of basin precipitation and temperature consequent to slow changes in the baseline climate state (e.g., owing to natural climate variations and/or potential global warming trends) may have significant impacts on such floods. A case study of the Blacksmith Fork River in northern Utah that explores such a hypothesis is presented here. Trends and associations in the magnitude and timing of annual maximum floods are documented, their impact on time-varying estimates of the 100 year flood is assessed, and relationships with known large-scale, quasi-oscillatory patterns of climate variability are explored. Evidence for structured low-frequency variation in flood timing and magnitude and its relation to winter/spring precipitation and temperature and to tropical (El Niño-Southern Oscillation) and extratropical (Pacific Decadal Oscillation) Pacific climate precursors is presented. Mechanisms for these ocean-atmosphere teleconnections to basin precipitation, temperature, and flood potential are discussed.

E. M. Douglas, R. M. Vogel, C.N. Kroll (2000). Trends in floods and low flows in the United States: impact of spatial correlation. Journal of Hydrology 240 (1-2): 90-105

ABSTRACT: Trends in flood and low flows in the US were evaluated using a regional average Kendall's S trend test at two spatial scales and over two timeframes. Field significance was assessed using a bootstrap methodology to account for the observed regional cross-correlation of streamflows. Using a 5% significance level, we found no evidence of trends in flood flows but did find evidence of upward trends in low flows at the larger scale in the Midwest and at the smaller scale in the Ohio, the north central and the upper Midwest regions. A dramatically different interpretation would have been achieved if regional cross-correlation had been ignored. In that case, statistically significant trends would have been found in all but two of the low flow analyses and in two-thirds of the flood flow analyses. We show that the cross-correlation of flow records dramatically reduces the effective number of samples available for trend assessment. We also found that low flow time series exhibit significant temporal persistence. Even when the serial correlation was removed from the time series, significant trends in low flow series were apparent, though the number of significant trends decreased.

J.C. Chatters, K.A. Hoover (1986). Changing late Holocene flooding frequencies on the Columbia River, Washington. Quaternary Research 26 (3): 309-320

ABSTRACT: Data from prehistoric fluvial deposits can be used to extend the flood history of a river valley beyond historical records, thus increasing our understanding of variability in large, low-frequency flood events and providing a valuable means for paleoenvironmental reconstruction. We have applied this form of analysis to fluvial deposits from an archaeological site on the upper Columbia River in the state of Washington dating from 120 A.D.* to 1948 A.D. It was our expectation that, had flood frequencies remained constant, sedimentation event frequency would conform to an exponential function derived from the Wolman and Leopold model of vertical floodplain accretion. Our findings deviate from this model, showing that flood frequencies comparable to those of the twentieth century existed prior to 1020 A.D.* and after 1390 A.D.* Large floods were three to four times more common during the intervening centuries. On the basis of field evidence, we can rule out changing channel geometry, leaving climatic conditions as the most probable factors controlling this variation in flood frequency.

Jacobeit, J., R. Glaser, J. Luterbacher, H. Wanner (2003). Links between flood events in central Europe since A.D. 1500 and large-scale atmospheric circulation modes. Geophysical Research Letters 30 (4): 1172

ABSTRACT: Based on documentary sources incidence variations of flood events can be reconstructed back to AD 1500 for several catchment areas in Central Europe. Links to atmospheric circulation modes, derived from reconstructed large-scale sea level pressure (SLP) grids for the last 500 years, have been identified on climatic time scales (monthly to seasonal). These relations are expressed in terms of several indices describing the particular importance of atmospheric circulation modes as a dynamical background for the varying incidence of flood events. During winter, the zonal circulation mode covers the largest part of these events, in relation to mode-frequency, however, other circulation modes become important during historical periods of increased flood frequency, e.g. modes characterised by Atlantic low and Russian high pressure centres. Different subtypes of this mode reached maxima in flood-association during different climatic periods of the past.

O'Connor, J. E., L. L. Ely, E. E. Wohl, L. E. Stevens, T. S. Melis, V. S. Kale, V. R. Baker (1994). A 4500-year record of large floods on the Colorado River in Grand Canyon, Arizona. Journal of Geology 102 (1): 1-9

ABSTRACT: A sequence of flood deposits left by the Colorado River in the Grand Canyon, Arizona, provides evidence of at least 15 floods with peak discharges greater than 5500 m3 sec-1 over the last 4500 yr. Ten floods during the last 2000-2300 yr had discharges greater than 6800 m3 sec-1 . One flood, 1600-1200 yr ago, had a discharge exceeding 14,000 m3 sec-1 , a flow rate more than twice the largest gaged flood. This record of flooding is one of the longest for a major U.S. river, and, combined with the gaged record of twentieth century floods, allows determination of the frequency and history of large floods that have affected key aspects of Colorado River geomorphology.

A. Schimmelmann, M. Zhao, C. C. Harvey, C. B. Lange (1998). A large California flood and correlative global climatic events 400 years ago. Quaternary Research 49 (1): 51-61

ABSTRACT: A gray silt layer 1–2 cm thick in the central Santa Barbara Basin, dated by varve counts to A.D. 1605 ± 5 yr, implies an intensity of precipitation, flooding of regional rivers, and transport of terrigenous detritus unmatched in the last 1000 yr. The inferred flood may correlate with the reported rare occurrence of a perennial lake (14 C dated to 390 ± 90 B.P.) in California's Mojave Desert, 300 km east of the area draining into the Santa Barbara Basin. The dating of the A.D. 1605 ± 5 yr flood event is consistent with tree-ring evidence for a wet and cold paleoclimate elsewhere in the region. Regional and global climate evidence indicates that much of the world also experienced rapid, intense cooling around A.D. 1605. This cooling was probably accompanied by an equatorward shift of prevailing wind patterns and associated storm tracks.

Woodhouse, C. A. (2001). A tree-ring reconstruction of streamflow for the Colorado Front Range. Journal of the American Water Resources Association 37 (3): 561-569

ABSTRACT: Water resource planning is based primarily on 20th century instrumental records of climate and streamflow. These records are limited in length to approximately 100 years, in the best cases, and can reflect only a portion of the range of natural variability. The instrumental record neither can be used to gage the unusualness of 20th Century extreme low flow events, nor does it allow the detection of low-frequency variability that may underlie short-term variations in flow. In this study, tree rings are used to reconstruct mean annual streamflow for Middle Boulder Creek in the Colorado Front Range, a semi-arid region of rapid growth and development. The reconstruction is based on a stepwise regression equation that accounts for 70 percent of the variance in the instrumental record, and extends from 1703-1987. The reconstruction suggests that the instrumental record of streamflow for Middle Boulder Creek is not representative of flow in past centuries and that several low flow events in the 19th century were more persistent than any in the 20th century. The 1840s to early 1850s period of low flow is a particularly notable event and may have coincided with a period of low flow in the Upper Colorado River Basin.

Knox, J. C. (1993). Large increases in flood magnitude in response to modest changes in climate. Nature 361 (6411): 430-432

ABSTRACT: Recent examinations of the possible hydrological response to global warming have emphasized changes in average conditions, rather than individual flooding events. Historical accounts suggest, however, that such events may have had a considerable regional impact even in the face of any relatively modest climate change. Here I present a 7,000-year geological record of overbank floods for upper Mississippi river tributaries in mid-continent North America, which provides concrete evidence for a high sensitivity of flood occurrence to changing climate. During a warmer, drier period between about 3,300 and 5,000 years ago, the largest, extremely rare floods were relatively small—the size of floods that now occur about once every fifty years. After ~3,300 years ago, when the climate became cooler and wetter, an abrupt shift in flood behaviour occurred, with frequent floods of a size that now recurs only once every 500 years or more. Still larger floods occurred between about AD 1250 and 1450, during the transition from the medieval warm interval to the cooler Little Ice Age. All of these changes were apparently associated with changes in mean annual temperature of only about 1–2 °C and changes in mean annual precipitation of 10–20%.

Enzel, Y., S. G. Wells (1997). Extracting Holocene paleohydrology and paleoclimatology information from modern extreme flood events: An example from southern California. Geomorphology 19 (3-4): 203-226

ABSTRACT: The extraction of paleohydrological and paleoclimatological information from a modern hydrological system, shown to represent unique and extreme hydroclimatological conditions, is illustrated by an example from the Mojave River drainage basin in southern California. The Mojave River allows only the most extreme floods to reach its terminal basin in the Silver Lake playa and to form ephemeral lakes. All the other floods are lost by transmission into the alluvial aquifer along its 200 km channel. This filtering out of regular floods by the river provides an essential tool in establishing a physical link between atmospheric and hydrologic conditions. We demonstrate such a link between anomalous, present-day atmospheric circulation patterns over the North Pacific Ocean, extreme storms in southern California that produced the heaviest precipitation on record, the largest floods of record in the Mojave River watershed, and ephemeral lakes in its terminal playa. This physical link determines the possible cause of the formations of perennial, short-duration, shallow lakes in Silver Lake playa during the late Holocene and characterizes the hydroclimatic conditions that prevailed during these lacustrine episodes. Hydrological simulations of this river and its filtering character demonstrate that these lakes could have formed only if the most extreme modern storms and floods were more frequent in at least an order of magnitude during specific time episodes. We conclude that such extreme hydroclimatic conditions occurred more frequent in past episodes during which the Holocene lakes formed. In turn, this conclusion indicates that the cause of these storms and floods, i.e. the anomalous atmospheric circulation pattern, must have been more frequent. This research outlines a way to extract information on Holocene climates in hydrologic settings that demonstrate a unique cause and effect relationship.

Ely, L. L. (1997). Response of extreme floods in the southwestern United States to climatic variations in the late Holocene. Geomorphology 19 (3-4): 175-201

ABSTRACT: A regional synthesis of paleoflood chronologies on rivers in Arizona and southern Utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations. The flood chronologies were constructed using fine-grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events. High-magnitude floods were frequent on rivers throughout the region from 5000 to 360014 C yrs BP (dendrocalibrated age = 3800-2200 BC) and increased again after 2200 BP (400 BC), with particularly prominent peaks in magnitude and frequency around 1100-900 BP (AD 900-1100) and after 500 yrs BP (AD 1400). In contrast, the periods 3600-2200 BP (2200-400 BC) and 800-600 yrs BP (1200-1400 AD) are marked by sharp decreases in the occurrence of large floods on these rivers.

In the modern record, storms that generate large floods ( 10-year) in the region fall into three categories: (1) winter North Pacific frontal storms; (2) late-summer and fall storms that draw in moisture from recurved Pacific tropical cyclones; and (3) summer storms, mainly convective thunderstorms. Winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study, and are the most probable causes of the paleofloods over the last 5000 years. Floods from both winter storms and tropical cyclones occur when deep mid-latitude troughs steer storm systems into the region. Composite anomaly maps of daily 700-mbar heights indicate that these floods are associated with a low-pressure anomaly off the California coast and a high-pressure anomaly over the Aleutians or Gulf of Alaska. A strong connection exists between the negative phase of the Southern Oscillation Index (often associated with El Niño conditions) and the large floods associated with winter storms and tropical cyclones.

The paleoflood records confirm the existence of centennial-scale variations in the conditions conducive to the occurrence of extreme floods and flood-generating storms in this region. The episodes with an increased frequency of high-magnitude floods coincide with periods of cool, wet climate in the western U.S., whereas warm intervals, such as the Medieval Warm Period, are times of dramatic decreases in the number of large floods. A positive relationship between the paleofloods and long-term variations in the frequency of El Niño events is evident over the last 1000 years. This relationship continues over at least the last 3000 years with warm coastal sea-surface temperatures indicative of El Niño-like conditions.

Hamlet, A. F., D.P. Lettenmaier (2007). Effects of 20th century warming and climate variability on flood risk in the western U.S.. Water Resources Research 43 (W06427): doi:10.1029/2006WR005099

ABSTRACT: Using precipitation and temperature data for the 20th century in combination with a macroscale hydrologic model, we evaluate changes in flood risk in the western U.S. associated both with century-scale warming and interannual climate variations. In addition, we examine the implications of apparent increases in precipitation variability over the region since the mid-1970s. We use detrended temperature data representing early and late 20th century climate to force the variable infiltration capacity hydrologic model and show that spatially homogeneous temperature changes over the western U.S. in the 20th century on the order of +1°C per century have resulted in substantial changes in flood risks over much of the region. Although changes specific to particular geographic areas are apparent in some cases, the overall changes due to observed warming trends are well categorized by midwinter temperature regimes in each watershed. Cold river basins where snow processes dominate the annual hydrologic cycle ( <−6°C average in midwinter) typically show reductions in flood risk due to overall reductions in spring snowpack. Relatively warm rain-dominant basins (> 5°C average in midwinter) show little systematic change. Intermediate or transient basins show a wide range of effects depending on competing factors such as the relative role of antecedent snow and contributing basin area during storms that cause flooding. Warmer transient basins along the coast in Washington, Oregon, and California, in particular, tend to show increased flood risk. While the absolute value of simulated changes in flood risk is affected by basin scale, the nature of the relationship of flood risk to basin temperatures in midwinter is largely scale-independent. Climate variations associated with Pacific Decadal Oscillation (PDO) and El Niño Southern Oscillation (ENSO) also have strong effects on flood risks. In contrast to the effects associated with 20th century warming, the climate variability signal is characterized by regional scale patterns related to the geographic distribution of cool season precipitation also identified in many previous studies. In general, the largest changes in simulated flood risks are associated with years when PDO and ENSO are “in phase,” particularly in the southwest. Changes in the variability of cool season precipitation after about 1973, the causes of which are uncertain, are shown to result in increased flood risk over much of the western U.S. in the simulations.

Ely, L. L., Y. Enzel, D.R. Cayan (1994). Anomalous North Pacific atmospheric circulation and large winter floods in the southwestern United States. Journal of Climate 7 (6): 977-987

ABSTRACT: Specific anomalous atmospheric circulation conditions over the North Pacific are conducive to the occurrence of the largest winter floods (10-yr return period) on rivers in six hydroclimatic subregions of Arizona and southern Utah, Nevada, and California. Composite maps of anomalies in daily 700-mb heights indicate that floods in all of the subregions are associated with a low pressure anomaly off the California coast and a high-pressure anomaly in the vicinity of either Alaska or the Aleutian Islands. Of these two major circulation features, the presence of the low is the controlling factor in determining whether large floods will occur. Shifts in the locations of the low and high pressure anomalies over the North Pacific appear to control which subregions experience floods, with high-elevation topographic features and proximity to air masses forming a major influence over the specific atmospheric circulation conditions that generate large floods in each hydroclimatic region. Concerning the interannual variability of flooding in the southwest, there is an increased frequency of large winter floods during multiple-year periods dominated by negative SOI and a virtual absence of large floods during the intervening periods. This suggests that global-scale climatic anomalies exert a strong influence on the occurrence of severe regional winter floods.

W. N. Engstrom (1996). The California storm of January 1862. Quaternary Research 46 (2): 141-148

ABSTRACT: The greatest storm in the written history of California struck the region in the winter of 1861–1862. The unusual weather began on Christmas Eve, 1861, and persisted for some 45 days as a series of middle-latitude cyclones made landfall along the California coast. Episodes of very cold and very warm temperatures occurred both during the storm and in the spring of 1862 as meridional flow prevailed. Heavy precipitation swelled the Santa Ana River to more than triple the highest estimated discharge in this century. High water levels in coastal streams between Los Angeles and San Diego persisted into the spring. Lakes were created in the Los Angeles Basin and the Mojave Desert. Arroyos were cut. Sediments from the flood may be preserved in offshore basins.

Jain, S., Lall, U. (2001). Floods in a changing climate: Does the past represent the future?. Water Resources Research 37 (12): 3193-3205

ABSTRACT: Hydrologists have traditionally assumed that the annual maximum flood process at a location is independent and identically distributed. While nonstationarities in the flood process due to land use changes have long been recognized, it is only recently becoming clear that structured interannual, interdecadal, and longer time variations in planetary climate impart the temporal structure to the flood frequency process at flood control system design and operation timescales. The influence of anthropogenic climate change on the nature of floods is also an issue of societal concern. Here we focus on (1) a diagnosis of variations in the frequency of floods that are synchronous with low-frequency climate state and (2) an exploration of limiting flood probability distributions implied by a long simulation of a model of the El Niño/Southern Oscillation. Implications for flood risk analysis are discussed.

F. J. Magilligan, B. E. Graber (1996). Hydroclimatological and geomorphic controls on the timing and spatial variability of floods in New England, USA. Journal of Hydrology 178 (1-4): 159-180

ABSTRACT: Numerous methods exist to express a region's flood regime, but because of the various controls on flooding, no salient method emerges. We have used directional (also known as orientation) statistics to express some of the hydroclimatological and geomorphic controls on flooding in New England and mapped these statistics for 36 gages to reveal the regional groupings of similarly responding basins. Directional statistics express the temporal variance of flood timings and characterize the seasonality of floods, and thus begin to demonstrate some of the climatic controls on flood timing and their spatial effects. Contour maps of the vector mean timing of floods indicate the strong control of coastal influences on the hydrologic regime, and multiple regression analyses indicate that both the surrogate variables for climate, latitude and distance to the coast, strongly explain the timing of the vector mean, although both the geomorphic variables, basin size and altitude, are also significant explanatory variables. An autumnal flood season exists along coastal locations but disappears moving both inland and northward through the region. For many of these coastal locations (especially in the south), the autumn mode reflects the occurrence of hurricane-induced floods which for many basins represent the largest flood in the annual series. The strength of the seasonality, as expressed by the mean resultant magnitude, is controlled primarily by basin size with distance to the coast and altitude also important. Finally, these results suggest that directional statistics are an appropriate method for depicting regional hydrologic regimes and in describing some of the hydroclimatological controls on flood occurrence.

Redmond, K. T., Koch, R. W. (1991). Surface climate and streamflow variability in the western United States and their relationship to large-scale circulation indices. Water Resources Research 27 (9): 2381-2399

ABSTRACT: A statistical analysis was undertaken to determine the nature and magnitude of the relationship of precipitation, temperature and streamflow in the western United States to large-scale atmospheric circulation patterns. The Southern Oscillation Index (SOI) was used as an indicator of the El Niño/Southern Oscillation (ENSO) and the PNA index as an indicator of the Pacific//North America pattern. These indices were correlated with surface climate data and split sample analyses were conducted to determine climate response during the extreme phases of each index. October-March precipitation was shown to be most strongly correlated with SOI averaged over the July-November period. The analysis showed that there are two centers of opposite association with the SOI. During low values of the SOI (ENSO events) precipitation is low in the Pacific northwest and high in the desert southwest. Correlations between SOI and temperature were greatest in the Pacific Northwest. The split sample analysis also revealed statistically significant differences in precipitation occurring during extremes of the SOI. The PNA pattern was related to precipitation and temperature over a concurrent time period. Especially strong associations were noted in the Pacific northwest for both precipitation and temperature. Streamflow showed associations with SOI similar to precipitation.

Ely, L. L., P.K. House (2003). Impact of climate variations on flood magnitude and frequency in three hydroclimatic regions of the western U.S.. unpublished: 13 pp.

ABSTRACT: Hydroclimatic variability over a wide range of spatial and temporal scales is directly linked to attendant variations in the magnitude and frequency of severe regional flooding events in the western U.S. Understanding this linkage is critical for improving flood-frequency forecasting and water resources management in this region. The spatial and temporal distributions of large floods in the western U.S. are largely controlled by persistent, anomalous patterns in hemispheric to global-scale atmospheric and oceanic circulation that directly influence flood-generating storm systems. Variability in flood frequency over short and long time scales thus provides an insight into variability in related larger-scale climatic phenomena over the same time scales.

This study employed paleoflood analyses to compare the influence of decadal- to millennialscale climatic variability on the magnitude and frequency of large floods on rivers in three distinct hydroclimatic regions of the western U.S: the Southwest, Northwest, and western Great Basin. This is the first study to construct regional paleoflood chronologies for rivers in the Northwest and Great Basin, and this aspect of the project alone will greatly increase the accuracy of flood-frequency forecasting in these areas. In addition, our comparison of paleoflood chronologies from three areas of the western U.S. will allow us to go one step further and determine whether there are consistent, predictable, long-term similarities or differences in the occurrence of large floods among these distinct hydroclimatic regions and investigate whether the regional differences in the timing and controls on floods in the short-term records hold true for the response of large floods in the three regions to longer-term climatic variations.

Dahm, C.N., M.A. Baker, D. I. Moore, J. R. Thibault (2003). Coupled biogeochemical and hydrological responses of streams and rivers to drought. Freshwater Biology 48 (7): 1219-1231

ABSTRACT: 1. Severe or extreme droughts occurred about 10% of the time over a 105-year record from central New Mexico, U.S.A., based on the Palmer Drought Severity Index.

2. Drought lowers water tables, creating extensive areas of groundwater recharge and fragmenting reaches of streams and rivers. Deeper groundwater inputs predominate as sources of surface flows during drought. Nutrient inputs to streams and rivers reflect the biogeochemistry of regional ground waters with longer subsurface residence times.

3. Inputs of bioavailable dissolved organic carbon to surface waters decrease during drought, with labile carbon limitation of microbial metabolism a byproduct of drought conditions.

4. Decreased inputs of organic forms of carbon, nitrogen and phosphorus and a decrease in the organic:inorganic ratio of nutrient inputs favours autotrophs over heterotrophs during drought.

5. The fate of autotrophic production during drought will be strongly influenced by the structure of the aquatic food web within impacted sites.

E. C. Carson, J. C. Knox, D. M. Mickelson (2007). Response of bankfull flood magnitudes to Holocene climate change, Uinta Mountains, northeastern Utah. Geological Society of America Bulletin 119 (9): 1066-1078

ABSTRACT: Long-term variations in Holocene flood magnitude were quantified from the bankfull dimensions of abandoned channels preserved on floodplain surfaces in the northern Uinta Mountains of northeastern Utah. Cross-sectional areas of abandoned channels were reconstructed, and relationships derived from the modern gage records were used to estimate bankfull discharges from bankfull cross-section areas. The results indicate systematic (nonrandom) variations of bankfull floods in the northern Uinta Mountains. Large floods, as much as 10%–15% greater than modern, dominated from 8500 to 5000 calendar yr B.P., and again from 2800 to 1000 cal yr B.P. Small floods, as much as 15%–20% less than modern, characterize the periods from 5000 to 2800 cal yr B.P., and from 1000 cal yr B.P. to near present.

The middle and late Holocene record of bankfull flood magnitude compares well with independent evidence for climatic variation in the area. The early Holocene record indicates that larger than modern bankfull floods coincide with warmer than modern mean annual temperature. We hypothesize that an increased range of magnitude for seasonal solar radiation during the early Holocene favored the accumulation and rapid melting of deep snowpacks in the high Uinta Mountains, thus producing large floods despite warmer mean annual temperatures. The episode of smaller than modern bank-full floods between 5000 and 2800 cal yr B.P. coincides with records of increased forest fire frequency in the northern Uintas. Larger than modern floods from 2800 to 1000 cal yr B.P. coincide with a local decrease in forest fire frequency and evidence for minor local glacial readvances. The decrease in flood magnitudes following 1000 cal yr B.P. corresponds to numerous local and regional records of warming during the Medieval Climatic Anomaly.

D. D. Breshears, N. S. Cobb, P. M. Rich, K. P. Price, C.D. Allen, R. G. Balice, W.H. Romme, J. H. Kastens, M. L. Floyd, J. Belnap, J.J. Anderson, O. B. Myers, C. W. Meyer (2005). Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences 102 (42): 15144-15148

ABSTRACT: Future drought is projected to occur under warmer temperature conditions as climate change progresses, referred to here as global-change-type drought, yet quantitative assessments of the triggers and potential extent of drought-induced vegetation die-off remain pivotal uncertainties in assessing climate-change impacts. Of particular concern is regional-scale mortality of overstory trees, which rapidly alters ecosystem type, associated ecosystem properties, and land surface conditions for decades. Here, we quantify regional-scale vegetation die-off across southwestern North American woodlands in 2002-2003 in response to drought and associated bark beetle infestations. At an intensively studied site within the region, we quantified that after 15 months of depleted soil water content, >90% of the dominant, overstory tree species (Pinus edulis, a piñon) died. The die-off was reflected in changes in a remotely sensed index of vegetation greenness (Normalized Difference Vegetation Index), not only at the intensively studied site but also across the region, extending over 12,000 km2 or more; aerial and field surveys confirmed the general extent of the die-off. Notably, the recent drought was warmer than the previous subcontinental drought of the 1950s. The limited, available observations suggest that die-off from the recent drought was more extensive than that from the previous drought, extending into wetter sites within the tree species' distribution. Our results quantify a trigger leading to rapid, drought-induced die-off of overstory woody plants at subcontinental scale and highlight the potential for such die-off to be more severe and extensive for future global-change-type drought under warmer conditions.

Sangoyomi, T. B., B. L. Harding (1995). Mitigating impacts of severe sustained drought on Colorado River water resources. Water Resources Bulletin 31 (5): 925-938

ABSTRACT: We evaluated the effects of institutional responses developed for coping with a severe sustained drought (SSD) in the Colorado River Basin on selected system variables using a SSD inflow hydrology derived from the drought which occurred in the Colorado River basin from 1579-1616. Institutional responses considered are reverse equalization, salinity reduction, minimum flow requirements, and temporary suspension of the delivery obligation of the Colorado River Compact. Selected system variables (reservoir contents, streamflows, consumptive uses, salinity, and power generation) from scenarios incorporating the drought-coping responses were compared to those from Baseline conditions using the current operating criteria. The coping responses successfully mitigated some impacts of the SSD on consumptive uses in the Upper Basin with only slight impacts on consumptive uses in the Lower Basin, and successfully maintained specified minimum streamflows throughout the drought with no apparent effect on consumptive uses. The impacts of the coping responses on other system variables were not as clear cut. We also assessed the effects of the drought coping responses to normal and wet hydrologic conditions to determine if they were overly conservative. The results show that the rules would have inconsequential effects on the system during normal and wet years.

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