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Ancient blue oak trees are still widespread across the foothills of the Coast Ranges, Cascades, and Sierra Nevada in California. The most extensive tracts of intact old-growth blue oak woodland appear to survive on rugged and remote terrain in the southern Coast Ranges and on the foothills west and southwest of Mt. Lassen. In the authors' sampling of old-growth stands, most blue oak appear to have recruited to the canopy in the middle to late nineteenth century. The oldest living blue oak tree sampled was over 459 years old, and several dead blue oak logs had over 500 annual rings. Precipitation sensitive tree-ring chronologies up to 700 years long have been developed from old blue oak trees and logs. Annual ring-width chronologies of blue oak are strongly correlated with cool season precipitation totals, streamflow in the major rivers of California, and the estuarine water quality of San Francisco Bay. A new network of 36 blue oak chronologies records spatial anomalies in growth that arise from latitudinal changes in the mean storm track and location of landfalling atmospheric rivers. These long, climate-sensitive blue oak chronologies have been used to reconstruct hydroclimatic history in California and will help to better understand and manage water resources. The environmental history embedded in blue oak growth chronologies may help justify efforts to conserve these authentic old-growth native woodlands.

Summary This work presents the first tree-ring reconstructions of hydroclimate for the Upper Klamath River basin, which stretches from northern California into southern Oregon. The extended record provides a centuries-long perspective on the region’s hydroclimatic variability and context for water-related political issues that have erupted in recent years. Reconstructions of water-year precipitation for Klamath Falls, Oregon (extending 1564–2004 and 1000–2010 CE) were developed to compare past drought severity with drought severity of the instrumental record (extending 1896–2011). The reconstructions suggest that variability exhibited during the instrumental period captures extremes of moderate-to-long-duration (6-, 10-, and 20-year) droughts, but not of short (single-year and 3-year) and very long (50-year) droughts, which were more severe during the 11th–13th centuries. The late-16th-century “mega drought” is present in the Klamath River basin, though with less strength than in the neighboring Sacramento River basin. Cool-season storm tracks appear to be a direct driver of hydroclimatic variability, leading to instances of see-saw like relationships with neighboring regions, such as in the mid-14th century. In contrast, the larger area of drought in the 12th century is suggestive of a long-term northward shift in cool-season storm tracks.

[1] The North American monsoon is a major focus of modern and paleoclimate research, but relatively little is known about interannual- to decadal-scale monsoon moisture variability in the pre-instrumental era. This study draws from a new network of subannual tree-ring latewood width chronologies and presents a 470-year reconstruction of monsoon (June–August) standardized precipitation for southwestern North America. Comparison with an independent reconstruction of cool-season (October–April) standardized precipitation indicates that southwestern decadal droughts of the last five centuries were characterized not only by cool-season precipitation deficits but also by concurrent failure of the summer monsoon. Monsoon drought events identified in the past were more severe and persistent than any of the instrumental era. The relationship between winter and summer precipitation is weak, at best, and not time stable. Years with opposing-sign seasonal precipitation anomalies, as noted by other studies, were anomalously frequent during the mid to late 20th century.

[1] Agriculture and ranching in semiarid regions often rely on local precipitation during the growing season as well as streamflow from runoff in distant headwaters. Where snowpack and reservoir storage are important, this pattern of reliance leads to vulnerability to multiseason drought. The lower Rio Grande basin in New Mexico, used as a case study here, has experienced drought conditions over the past 12 years characterized both by low local summer monsoon precipitation and by reduced availability of surface water supplies from the upper Rio Grande. To place this drought in a long-term context, we evaluate the covariability of local warm-season and remote cool-season hydroclimate over both the modern period and past centuries. We draw on a recently developed network of tree-ring data that allows an assessment of preinstrumental warm-season variations in precipitation over the southwest. Both instrumental and paleoclimatic data suggest that low runoff followed by a dry monsoon is not unusual, although over the full reconstruction period (1659–2008), years with wet or dry conditions shared in both seasons do not occur significantly more often than unshared conditions. Low flows followed by dry monsoon conditions were most persistent in the 1770s and 1780s; other notable periods of shared seasonal droughts occurred in the 1660s and 1950s. The recent drought does not yet appear to be unusually severe in either the instrumental or paleoclimatic context.

We use the process-based VS (Vaganov-Shashkin) model to investigate whether a regional Pinus halepensis tree-ring chronology from Tunisia can be simulated as a function of climate alone by employing a biological model linking day length and daily temperature and precipitation (AD 1959–2004) from a climate station to ring-width variations. We check performance of the model on independent data by a validation exercise in which the model's parameters are tuned using data for 1982–2004 and the model is applied to generate tree-ring indices for 1959–1981. The validation exercise yields a highly significant positive correlation between the residual chronology and estimated growth curve (r=0.76 pn=23). The model shows that the average duration of the growing season is 191 days, with considerable variation from year to year. On average, soil moisture limits tree-ring growth for 128 days and temperature for 63 days. Model results depend on chosen values of parameters, in particular a parameter specifying a balance ratio between soil moisture and precipitation. Future work in the Mediterranean region should include multi-year natural experiments to verify patterns of cambial-growth variation suggested by the VS model.

Summary Streamflow variability on timescales of decades to centuries becomes increasingly important as water managers grapple with shortages imposed by increasing demand and limited supply, and possibly exacerbated by climate change. ► Two applications of dendrochronology to the study of flow variability are illustrated for an existing 1244-yr reconstruction of annual flows of the Colorado River at Lees Ferry, Arizona, USA: (1) identification and climatological interpretation of rare flow events, and (2) assessment of vulnerability of water-supply systems to climatic variability. ► Analysis centers on a sustained drought of the mid-1100s characterized by persistent low flows on both the Colorado and Sacramento Rivers. ► Analysis of geopotential height anomalies during modern joint-droughts suggests more than one mode of circulation might accompany joint-drought in the two basins. ► Monte Carlo simulation is used to demonstrate that a drought as severe as that in the 1100s on the Colorado River might be expected about once in every 4–6 centuries by chance alone given the time-series properties of the modern gaged flows. Application of a river-management model suggests a mid-1100s-style drought, were it to occur today, would drop reservoir levels in Lake Mead to dead-pool within a few decades. ► Uncertainty presents challenges to accurately quantifying severe sustained droughts from streamflow reconstructions, especially early in the tree-ring record. ► Corroboration by multiple proxy records is essential. ► Future improvements are likely to require a combination of methodological advancements and expanded basic data.

Tree-ring studies have demonstrated that conifer latewood measurements contain information on long-term North American monsoon (NAM) variability, a hydroclimatic feature of great importance to plants, animals, and human society in the US Southwest. This paper explores data-treatment options for developing latewood chronologies aimed at NAM reconstruction. Archived wood samples for five Douglas-fir (Pseudotsuga menziesii, Mirb. Franco) sites in southeastern Arizona are augmented with new collections. The combined dataset is analyzed along with time series of regionally averaged observed precipitation to quantify the strength of regional precipitation signal in latewood time series and to identify ways of increasing the signal strength. Analysis addresses the signal strength influences of including or excluding “false” latewood bands in the nominal “latewood” portion of the ring, the necessary adjustment of latewood width for statistical dependence on antecedent earlywood width, and tree age. Resul...

Drought is a recurring phenomenon in the American Southwest. Since the frequency and severity of hydrologic droughts and other hydroclimatic events are of critical importance to the ecology and rapidly growing human population of this region, knowledge of long-term natural hydroclimatic variability is valuable for resource managers and policy-makers. An October–June precipitation reconstruction for the period AD 824–2007 was developed from multi-century tree-ring records of Pseudotsuga menziesii (Douglas-fir), Pinus strobiformis (Southwestern white pine) and Pinus ponderosa (Ponderosa pine) for the Jemez Mountains in Northern New Mexico. Calibration and verification statistics for the period 1896–2007 show a high level of skill, and account for a significant portion of the observed variance (>50%) irrespective of which period is used to develop or verify the regression model. Split-sample validation supports our use of a reconstruction model based on the full period of reliable observational data (1896–2007). A recent segment of the reconstruction (2000–2006) emerges as the driest 7-year period sensed by the trees in the entire record. That this period was only moderately dry in precipitation anomaly likely indicates accentuated stress from other factors, such as warmer temperatures. Correlation field maps of actual and reconstructed October–June total precipitation, sea surface temperatures and 500-mb geopotential heights show characteristics that are similar to those indicative of El Nino–Southern Oscillation patterns, particularly with regard to ocean and atmospheric conditions in the equatorial and north Pacific. Our 1184-year reconstruction of hydroclimatic variability provides long-term perspective on current and 20th century wet and dry events in Northern New Mexico, is useful to guide expectations of future variability, aids sustainable water management, provides scenarios for drought planning and as inputs for hydrologic models under a broader range of conditions than those provided by historical climate records. Copyright © 2010 Royal Meteorological Society

The North American Monsoon (NAM) contributes critical summer moisture to the U.S. Southwest from July through September, but instrumental records of monsoon precipitation are limited to 100 years or less. Tree-ring investigation offers a means of improving our understanding of its long-term spatial and temporal variability. Available evidence indicates the stable-carbon isotopic composition (δ 13 C) of tree rings in this region is strongly linked to moisture. In addition to latewood width as a precipitation proxy, the δ 13 C of latewood also appears to be a strong proxy, largely manifesting water stress effects on stomatal conductance and their consequence to isotopic discrimination against 13 CO 2 . In one promising study, the δ 13 C of 11 years of latewood from 8 sites regressed against their corresponding precipitation exhibited a coefficient of −0.061‰ per cm of July + August + September precipitation ( r 2 = 0.41). Long latewood δ 13 C chronologies are currently being developed from tree rings of ponderosa pine and Douglas-fir at several sites in NAM core regions in the Southwest to evaluate its usefulness in supplementing precipitation reconstructions derived from latewood widths. Among planned outcomes, the improved monsoon precipitation records can be used to better evaluate natural variability of NAM precipitation and its linkage to winter precipitation, document the character of monsoon season droughts, and test accuracy of regional climate models.

Long-term information on gradients in precipitation-anomaly over tens to hundreds of km is important to hydroclimatology for improved understanding of the spatiotemporal variability of moisture-delivery systems and runoff. Site-centered reconstructions of cool-season (Nov–Apr) precipitation at 36 Quercus douglasii tree-ring sites in the Central Valley of California, USA, are generated, regionalized, and evaluated for ability to track north–south gradients in precipitation-anomaly. Event series are constructed for overall-wet (W), overall-dry (D), wetter-to-north (W/D) and wetter-to-south (D/W) conditions, 1557–2001. Interesting features of the event series are clustering of W events in the 1780–1790s and a three-year run of D/W events in 1816–1818 (coincidentally following the eruption of Tambora in 1815). The most recent 25 years of the event series stand out for a high frequency of W and D events and low frequency of events associated with strong gradients in precipitation-anomaly. The five strongest W events in this period and seven of the nine W events since 1934 match El Nino years. Recent changes in the event series may be a Central-Valley footprint of a well-documented post-1976 change in the atmosphere-ocean climate system over the North Pacific. Similar studies may prove useful in other geographical areas where networks of tree-ring data sufficiently sensitive to precipitation are available.

Drought is among the most costly natural hazards affecting the United States, averaging $6 to $8 billion annually in damages, primarily in crop losses. Mitigating the impacts of drought through planning and preparedness has the potential to save billions of dollars. We used a new long tree-ring chronology developed from the central U.S. to reconstruct annual drought and characterize past drought duration, frequency, and cycles in the U.S. Corn Belt region during the last millennium. This is the first paleoclimate reconstruction achieved with subfossil oak wood in the U.S. and increases the current dendroclimatic record in the central U.S. agricultural region by over 500 years. A tree ring-width drought response function was calibrated and verified against monthly instrumental Palmer Hydrologic Drought Index (PHDI) during the summer season (JJA). Separate reconstructions tailored to emphasize high-frequency and low-frequency variations indicate that drought conditions over the period of instrumental records (since 1895) do not exhibit the full range of variability, severity, or duration of droughts during the last millennium. For example, three years in the last millennium were drier than 1934, a classic Dust-Bowl year and the driest year of the instrumental period. Thirteen decadal to multidecadal droughts (i.e., ≥10 years) occurred during the last millennium – the longest lasting sixty-one years and centered on the late twelfth century. Reconstructions exhibited quasi-periodicity at bidecadal and century-scale periods. Significant rhythms in drought were identified near 20-yr and 128-yr periods. The tree-ring drought reconstruction shows promise in providing new information about long-term climate variability in the agricultural regions that could potentially span multimillennia. We postulate that tree-ring chronologies (i.e., tree growth), thus far under-utilized in agricultural applications, have the potential to match contributions of instrumental climate data.

Summary The upper Santa Fe River provides up to 50% of the water supply for the growing population of Santa Fe, NM. Recent droughts have dramatically lowered reservoir levels and raised concern about the future of the water supply, particularly when combined with projections of a warmer and drier future climate. In this study, new and updated tree-ring chronologies are used to reconstruct annual discharge for the upper Santa Fe River and place the short period of gaged flows, 1914–2007, in a long-term context. Principal components analysis and forward stepwise multiple linear regression were used to produce two reconstructions: (1) a better fit, “short reconstruction” (adjusted R2 = 0.62, 1592–2007) and (2) a less robust, “long reconstruction” (adj. R2 = 0.50, 1305–2007). Both reconstructions indicate that recent extreme low flow events (e.g., 2002) are rare (5th percentile) in the long-term records and that the 1950s drought contained the lowest 7-year mean flows over the past 400–700 years. However, longer, multi-decadal dry periods not present in the gaged flows occurred in the past. For example, the 40-year mean for 1544–1583 is estimated at just 86% of the 1914–2007 mean. During extended dry periods in the 16th and 18th centuries the probability that annual flow would not meet the current surface water allocation and instream flow target (7.52 million cubic meters, MCM) was up to 10% greater (78.7% non-exceedence probability) than during the instrumental period. The results indicate that the gaged record does not contain the full range of high and low flows or the variability in the probability distributions of flows present in the long-term record. Therefore current and future water management and planning based on the instrumental period may not adequately buffer against the natural variability in the climate and streamflow systems. This valuable paleo-hydrologic information is in the process of being incorporated into water supply planning for the City of Santa Fe (e.g., modeling future water supply scenarios directly from reconstructed periods of streamflow).

A key feature of anticipated 21st century droughts in Southwest North America is the concurrence of elevated temperatures and increased aridity. Instrumental records and paleoclimatic evidence for past prolonged drought in the Southwest that coincide with elevated temperatures can be assessed to provide insights on temperature-drought relations and to develop worst-case scenarios for the future. In particular, during the medieval period, ∼AD 900–1300, the Northern Hemisphere experienced temperatures warmer than all but the most recent decades. Paleoclimatic and model data indicate increased temperatures in western North America of approximately 1 °C over the long-term mean. This was a period of extensive and persistent aridity over western North America. Paleoclimatic evidence suggests drought in the mid-12th century far exceeded the severity, duration, and extent of subsequent droughts. The driest decade of this drought was anomalously warm, though not as warm as the late 20th and early 21st centuries. The convergence of prolonged warming and arid conditions suggests the mid-12th century may serve as a conservative analogue for severe droughts that might occur in the future. The severity, extent, and persistence of the 12th century drought that occurred under natural climate variability, have important implications for water resource management. The causes of past and future drought will not be identical but warm droughts, inferred from paleoclimatic records, demonstrate the plausibility of extensive, severe droughts, provide a long-term perspective on the ongoing drought conditions in the Southwest, and suggest the need for regional sustainability planning for the future.

We examine how the seasonality of precipitation signals embedded within the North American Drought Atlas varies across the continent. Instrumental records of average summer (JJA) Palmer Drought Severity Index (PDSI) are characterized by major regional differences in the relative importance of precipitation during summer and winter (DJF). The Atlas, which is based on a network of drought-sensitive tree-ring records, is able to reproduce the main geographic patterns of these biases, but tree-ring reconstructions exaggerate the influence of seasonal precipitation anomalies in the southwestern United States and northern Mexico (towards a stronger winter signal) and western Canada (towards a stronger summer signal). Drought reconstructions from the Southwest and Tex-Mex regions are tuned mainly to winter precipitation and display strong teleconnections to both El Niño and La Niña. In contrast, winter precipitation signals are either weak or absent in drought reconstructions from northwestern North America, and tree-ring estimates of PDSI show a much less robust association with the El Niño-Southern Oscillation. Geographical differences in the relative strength of seasonal precipitation signals are likely due to (i) local factors that influence tree growth but are not incorporated into the PDSI algorithm and (ii) real differences in regional climatology. These seasonal biases must be taken into account when comparing drought reconstructions across North America, when comparing tree-ring PDSI to drought records developed from other proxies or when attempting to use the Drought Atlas to link past droughts to potential forcing mechanisms.

Changes in precipitation patterns and the frequency and duration of drought are likely to be the feature of anthropogenic climate change that will have the most direct and most immediate consequences for human populations. The latest generation of state-of-the-art climate models project future widespread drying in the subtropics. Here, we reconstruct spatially-complete gridded Palmer drought severity index values back to A.D. 1179 over Morocco, Algeria, and Tunisia. The reconstructions provide long-term context for northwest African hydroclimatology, revealing large-scale regional droughts prior to the sixteenth century, as well as more heterogeneous patterns in sixteenth, eighteenth, and twentieth century. Over the most recent decades a shift toward dry conditions over the region is observed, which is consistent with general circulation model projections of greenhouse gas forced enhanced regional subtropical drought.

Despite the extensive network of moisture-sensitive tree-ring chronologies in western North America, relatively few are long enough to document climatic variability before and during the Medieval Climate Anomaly (MCA) ca. AD 800-1300. We developed a 2300-yr tree-ring chronology extending to 323 BC utilizing live and remnant Douglas-fir (Pseudotsuga menziesii) from the Tavaputs Plateau in northeastern Utah. A resulting regression model accounts for 70% of the variance of precipitation for the AD 1918–2005 calibration period. Extreme wet and dry periods without modern analogues were identified in the reconstruction. The MCA is marked by several prolonged droughts, especially prominent in the mid AD 1100s and late 1200s, and a lack of wet or dry single-year extremes. The frequency of extended droughts is not markedly different, however, than before or after the MCA. A drought in the early AD 500s surpasses in magnitude any other drought during the last 1800 yr. A set of four long high-resolution records suggests this drought decreased in severity toward the south in the western United States. The spatial pattern is consistent with the western dipole of moisture anomaly driven by El Niño and is also similar to the spatial footprint of the AD 1930s "Dust Bowl" drought.

Precipitation over the southwestern United States exhibits distinctive seasonality, and contrasting ocean–atmospheric dynamics are involved in the interannual variability of cool- and warm-season totals. Tree-ring chronologies based on annual-ring widths of conifers in the southwestern United States are well correlated with accumulated precipitation and have previously been used to reconstruct cool-season and annual precipitation totals. However, annual-ring-width chronologies cannot typically be used to derive a specific record of summer monsoon-season precipitation. Some southwestern conifers exhibit a clear anatomical transition from the earlywood and latewood components of the annual ring, and these exactly dated subannual ring components can be measured separately and used as unique proxies of cool- and warm-season precipitation and their associated large-scale ocean–atmospheric dynamics. Two 2139-yr-long reconstructions of cool- (November–May) and early-warm season (July) precipitation have been developed from ancient conifers and relict wood at El Malpais National Monument, New Mexico. Both reconstructions have been verified on independent precipitation data and reproduce the spatial correlation patterns detected in the large-scale SST and 500-mb height fields using instrumental precipitation data from New Mexico. Above-average precipitation in the cool-season reconstruction is related to El Niño conditions and to the positive phase of the Pacific decadal oscillation. Above-average precipitation in July is related to the onset of the North American monsoon over New Mexico and with anomalies in the 500-mb height field favoring moisture advection into the Southwest from the North Pacific, the Gulf of California, and the Gulf of Mexico. Cool- and warm-season precipitation totals are not correlated on an interannual basis in the 74-yr instrumental or 2139-yr reconstructed records, but wet winter–spring extremes tend to be followed by dry conditions in July and very dry winters tend to be followed by wet Julys in the reconstructions. This antiphasing of extremes could arise from the hypothesized cool- to early-warm-season change in the sign of large-scale ocean–atmospheric forcing of southwestern precipitation, from the negative land surface feedback hypothesis in which winter–spring precipitation and snow cover reduce surface warming and delay the onset of the monsoon, or perhaps from an interaction of both large-scale and regional forcing. Episodes of simultaneous interseasonal drought (“perfect” interseasonal drought) persisted for a decade or more during the 1950s drought of the instrumental era and during the eighth- and sixteenth-century droughts, which appear to have been two of the most profound droughts over the Southwest in the past 1400 yr. Simultaneous interseasonal drought is doubly detrimental to dry-land crop yields and is estimated to have occurred during the mid-seventeenth-century famines of colonial New Mexico but was less frequent during the late-thirteenth-century Great Drought among the Anasazi, which was most severe during the cool season.

An October–June precipitation reconstruction was developed from a Pinus halepensis regional tree-ring chronology from four sites in northwestern Tunisia for the period of 1771–2002. The reconstruction is based on a reliable and replicable statistical relationship between climate and tree-ring growth and shows climate variability on both interannual and interdecadal time scales. Thresholds (12th and 88th percentiles) based on the empirical cumulative distribution of observed precipitation for the 1902–2002 calibration period were used to delineate dry years and wet years of the long-term reconstruction. The longest reconstructed drought by this classification in the 232-year reconstruction is 2 years, which occurred in the 19th century. Analysis of 500 mb height data for the period 1948–2002 suggests reconstructed extreme dry and wet events can provide information on past atmospheric circulation anomalies over a broad region including the Mediterranean, Europe and eastern Asia.

A network of 54 ring-width chronologies is used to estimate changes in summer climate within the Winnipeg River basin, Canada, since AD 1783. The basin drains parts of northwestern Ontario, northern Minnesota and southeastern Manitoba, and is a key area for hydroelectric power production. Most chronologies were developed from Pinus resinosa and P. strobus, with a limited number of Thuja occidentalis, Picea glauca and Pinus banksiana. The dominant pattern of regional tree growth can be recovered using only the nine longest chronologies, and is not affected by the method used to remove variability related to age or stand dynamics from individual trees. Tree growth is significantly, but weakly, correlated with both temperature (negatively) and precipitation (positively) during summer. Simulated ring-width chronologies produced by a process model of tree-ring growth exhibit similar relationships with summer climate. High and low growth across the region is associated with cool/wet and warm/dry summers, respectively; this relationship is supported by comparisons with archival records from early 19th century fur-trading posts. The tree-ring record indicates that summer droughts were more persistent in the 19th and late 18th century, but there is no evidence that drought was more extreme prior to the onset of direct monitoring.

The authors develop a model framework that includes a set of hydrologic modules as a water resources management and planning tool for the upper Santa Cruz River near the Mexican border, Southern Arizona. The modules consist of: (1) stochastic generation of hourly precipitation scenarios that maintain the characteristics and variability of a 45-year hourly precipitation record from a nearby rain gauge; (2) conceptual transformation of generated precipitation into daily streamflow using varied infiltration rates and estimates of the basin antecedent moisture conditions; and (3) surface-water to ground-water interaction for four downstream microbasins that accounts for alluvial ground-water recharge, and ET and pumping losses. To maintain the large inter-annual variability of streamflow as prevails in Southern Arizona, the model framework is constructed to produce three types of seasonal winter and summer categories of streamflow (i.e., wet, medium, or dry). Long-term (i.e., 100 years) realizations (ensembles) are generated by the above described model framework that reflects two different regimes of inter annual variability. The first regime is that of the historic streamflow gauge record. The second regime is that of the tree ring reconstructed precipitation, which was derived for the study location. Generated flow ensembles for these two regimes are used to evaluate the risk that the regional four ground-water microbasins decline below a preset storage threshold under different operational water utilization scenarios.

Tree-ring data were analyzed for a long-term perspective on ecological and hydroclimatic variations on the Peace-Athabasca Delta (PAD). The ecosystem of this Ramsar Wetland of International Importance in northeastern Alberta is sensitive to fluctuations in water levels of the numerous lakes, channels and perched basins, and attention has been focused in recent decades on the possibility of deleterious effects from climatic change and regulation of the Peace River by W.A.C. Bennett Dam. A network of eight Picea glauca tree-ring chronologies was developed, time series features of growth variation were summarized, and the chronologies were applied in a regression model to reconstruct an annual time series of early summer (July 11 to 20) water levels of Lake Athabasca for the period A.D. 1801-1999. Though statistically weak (R2 = 0.36), the reconstruction verifies adequately in cross-validation and is consistent with anecdotal written records of a dramatic decline in water levels in 1879-81. Correlation of re...

Growing and changing demands on water supply, along with natural climate variability and possible anthropogenically induced climate change, make water resource management and planning increasingly challenging, particularly in arid regions. Instrumental climate and gaged streamflow records provide just a snapshop of recent natural hydrologic variability. In this paper, we use tree-ring-based annual streamflow reconstructions for the Sacramento River in California and the Blue River in western Colorado to analyze the temporal and spatial variability of widespread drought simultaneously affecting both basins over the past five centuries. Stability of joint-drought episodes and the covariation of reconstructed flows in the two basins are analyzed with sliding correlations, spectral analysis and a hypergeometric test. Year-to-year spatial patterns of moisture anomalies in a singular joint-drought episode in the late-1500s are mapped with a network of tree-ring data. Climatological aspects of joint droughts of the 20th century are investigated with 500-mb geopotential height data and climatic indices. Although flow in the two rivers is only very weakly correlated over the full 538-yr reconstruction period, more years of joint drought occur than would be expected by chance alone. Covariation in reconstructed flows is stronger in the late 1500s and mid-1700s than at any time since 1800. The late 1500s period of drought is not characterized as a decades-long unbroken drought, but as a series of drought impulses broken by wet years, with widespread moisture deficits in joint dry years. Periods of high inter-basin correlation in reconstructed flow are characterized by coherency at frequencies within the ENSO band. However, joint droughts in instrumental gage records do not display any consistent relationship with ENSO or the Pacific Decadal Oscillation (PDO), and so it is difficult to infer either as a causal mechanism for joint droughts in the past.

The western United States is experiencing a severe multiyear drought that is unprecedented in some hydroclimatic records. Using gridded drought reconstructions that cover most of the western United States over the past 1200 years, we show that this drought pales in comparison to an earlier period of elevated aridity and epic drought in AD 900 to 1300, an interval broadly consistent with the Medieval Warm Period. If elevated aridity in the western United States is a natural response to climate warming, then any trend toward warmer temperatures in the future could lead to a serious long-term increase in aridity over western North America.

In the 10,000 km 2 San Pedro River watershed area in south-eastern Arizona, high-resolution spatial patterns of long-term precipitation and temperature were better reproduced by kriging climate data with elevation as external drift (KED) than by multiple linear regression on station location and elevation as judged by the spatial distribution of interpolation error. Mean errors were similar overall, and interpolation accuracy for both methods increased with increasing correlation between climate variables and elevation. Uncertainty in station locations had negligible effect on mean estimation error, although error for individual stations varied as much as 27%. Our future ability to examine spatial aspects of climate change at high spatial resolution will be severely limited by continuing closures of climate stations in this part of the United States.

Two reconstructions of spring (May–June) precipitation have been developed for southwestern Turkey. The first reconstruction (1776–1998) was developed from principal components of nine chronologies of Cedrus libani, Juniperus excelsa, Pinus brutia ,a ndPinus nigra. The second reconstruction (1339–1998) was derived from principal components of three J. excelsa chronologies. Calibration and verification statistics of both reconstructions indicate reasonably accurate reconstruction of spring precipitation for southwestern Turkey, and show clear evidence of multi-year to decadal variations in spring precipitation. The longest period of reconstructed spring drought, defined as consecutive years with less than 80% of normal May–June precipitation, was 4 years (1476–79). Only one drought event of this duration has occurred during the last six centuries. Monte Carlo analysis indicates a less than 33% probability that southwestern Turkey has experienced spring drought longer than 5 years in the past 660 years. Apart from the 1476–79 extended dry period, spring droughts of 3 years in length have only occurred from 1700 to the present. The longest reconstructed wet period, defined as consecutive years with more than 120% of normal May–June precipitation, was 4 years (1532–35). The absence of extended spring drought during the 16th and 17th centuries and the occurrence of extended wet spring periods during these centuries suggest a possible regime shift in climate. Preliminary analysis of links between large-scale climatic variation and these climate reconstructions shows that there is a relationship between extremes in spring precipitation and anomalous atmospheric circulation in the region. Copyright  2003 Royal Meteorological Society.

US National Science Foundation; ATMGEO/ATM-Paleoclimate Program [0758486]; AGS-Paleo Perspectives on Climate Change Program [1103314]

A time series of annual flow of the Sacramento River, California, is reconstructed to A.D. 869 from tree rings for a long-term perspective on hydrologic drought. Reconstructions derived by principal components regression of flow on time-varying subsets of tree-ring chronologies account for 64 to 81 percent of the flow variance in the 1906 to 1977 calibration period. A Monte Carlo analysis of reconstructed n-year running means indicates that the gaged record contains examples of drought extremes for averaging periods of perhaps = 6 to 10 years, but not for longer and shorter averaging periods. For example, the estimated probability approaches 1.0 that the flow in A.D. 1580 was lower than the lowest single-year gaged flow. The tree-ring record also suggests that persistently high or low flows over 50-year periods characterize some parts of the long-term flow history. The results should contribute to sensible water resources planning for the Sacramento Basin and to the methodology of incorporating tree-ring data in the assessment of the probability of hydrologic drought.

The variability of the North American Monsoon System (NAMS) is important to the precipitation climatology of Mexico and the southwestern United States. Tree-ring studies have been widely applied to climatic reconstruction in western North America, but as yet, have not addressed the NAMS. One reason is the need for highly resolved seasonal dendroclimatic information. Latewood-width, the portion of the annual tree ring laid down late in the growing season, can potentially yield such information. This paper describes a pilot study of the regional summer precipitation signal in latewood-width from a network of five Pseudotsuga menziesii chronologies developed in the heart of the region of NAMS influence in Arizona, USA. Exploratory data analysis reveals that the summer precipitation signal in latewood is strong, but not equally so over the full range of summer precipitation. Scatter in the relationship increases toward higher levels of precipitation. Adjustment for removal of inter-correlation with earlywood-width appears to strengthen the summer precipitation signal in latewood-width. To demonstrate a possible application to climatic reconstruction, the latewood precipitation signal is modelled using a nonlinear model—a binary recursive classification tree (CT) that attempts to classify summers as dry or not dry from threshold values of latewood-width. The model identifies narrow latewood-width as an effective predictor of a dry summer. Of 14 summers classified dry by latewood-width for the period 1868–1992, 13 are actually dry by the instrumental precipitation record. The results suggest that geographical expansion of coverage by latewood-width chronologies and further development of statistical methods may lead to successful reconstruction of variability of the NAMS on century time scales. Copyright © 2001 Royal Meteorological Society

The development of a 2° lat × 3° long grid of summer drought reconstructions for the continental United States estimated from a dense network of annual tree-ring chronologies is described. The drought metric used is the Palmer Drought Severity Index (PDSI). The number of grid points is 154 and the reconstructions cover the common period 1700–1978. In producing this grid, an automated gridpoint regression method called “point-by-point regression” was developed and tested. In so doing, a near-optimal global solution was found for its implementation. The reconstructions have been thoroughly tested for validity using PDSI data not used in regression modeling. In general, most of the gridpoint estimates of drought pass the verification tests used. In addition, the spatial features of drought in the United States have been faithfully recorded in the reconstructions even though the method of reconstruction is not explicitly spatial in its design. The drought reconstructions show that the 1930s “Dust Bow...

Water resources are the lifeblood of the Near East region. Careful planning and management of water resources in dry land regions requires information on the likelihood of extreme events, especially prolonged drought. It is essential to understand the variability of climate on time scales of decades to centuries to assign reasonable probabilities to such events. Tree-ring analysis is one way to increase our knowledge of the climate variability beyond the short period covered by the instrumental data. In this paper, we reconstruct October-May precipitation from a Juniperus phoenicia tree-ring chronology in southern Jordan to gain a long-term (A.D. 1600–1995) perspective on runs of dry years and on time series fluctuations in precipitation averaged over several years. The reconstruction equation derived by regression of log-transformed precipitation on tree-ring indices explains 44 percent of the variance of observed precipitation. The longest reconstructed drought, as defined by consecutive years below a threshold of 217.4 mm, was four years, compared with three years for the 1946–95 instrumental data. A Monte Carlo analysis designed to account for uncertainty in the reconstruction indicates a lower than 50 percent chance that the region has experienced drought longer than five years in the past 400 years.

The overall amount of precipitation deposited along the West Coast and western cordillera of North America from 258 to 558N varies from year to year, and superimposed on this domain-average variability are varying north‐south contrasts on timescales from at least interannual to interdecadal. In order to better understand the north‐south precipitation contrasts, their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large-scale climatic patterns. Spatial empirical orthogonal functions (EOFs) and spatial moments (domain average, central latitude, and latitudinal spread) of zonally averaged precipitation anomalies along the westernmost parts of North America are analyzed, and each is correlated with global sea level pressure (SLP) and sea surface temperature series, on interannual (defined here as 3‐7 yr) and decadal (7 yr) timescales. The interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much (%) (%) p ¯

A new drought area index (DAI) for the United States has been developed based on a high-quality network of drought reconstructions from tree rings. This DAI is remarkably similar to one developed earlier based on much less data and shows strong evidence for a persistent bidecadal drought rhythm in the western United States since 1700. This rhythm has in the past been associated with possible forcing by the 22-yr Hale solar magnetic cycle and the 18.6-yr lunar nodal tidal cycle. The authors make a new assessment of these possible forcings on DAI using different methods of analysis. In so doing, they confirm most of the previous findings. In particular, there is a reasonably strong statistical association between the bidecadal drought area rhythm and years of Hale solar cycle minima and 18.6-yr lunar tidal maxima. The authors also show that the putative solar and lunar effects appear to be interacting to modulate the drought area rhythm, especially since 1800. These results do not eliminate the pos...

Tree-ring site chronologies, the predictors for most dendroclimatic reconstructions, are essentially mean-value functions with a time varying sample size (number of trees) and sample composition. Because reconstruction models are calibrated and verified on the most recent, best-replicated part of the chronologies, regression and verification statistics can be misleading as indicators of long-term reconstruction accuracy. A new reconstruction method is described that circumvents the use of site chronologies and instead derives predictor variables from indices of individual trees. Separate regression models are estimated and cross validated for various time segments of the tree-ring record, depending on the trees available at the time. This approach allows the reconstruction to extend to the first year covered by any tree in the network and yields direct evaluation of the change in reconstruction accuracy with tree-ring sample composition. The method includes two regression stages. The first is to ...

Touchan, R.... et al., This report describes an international summer course, >Tree Rings, Climate, Natural Resources, and Human Interaction>, held in Valsaín, Spain, in summer of 2012. The course, with 14 participants from three countries (Spain, Algeria, and Russia), included basic training in dendrochronology skills as well as applied projects in dendroclimatology, dendroecology and dendrogeomorphology. Copyright © 2013 by The Tree-Ring Society., This International Summer School was carried out in collaboration with the Laboratory of Tree-Ring Research, University of Arizona, from the United States, and IGME (Instituto Geológico y Minero de España), IPE-CSIC (Instituto Pirenaico de Ecología - Consejo Superior de Investigaciones Científicas), ARAID foundation, INIA-CIFOR (Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria-Centro de Investigación Forestal) and University of Córdoba, from Spain. The organizers sincerely thank the CENEAM (National Center for Environmental Education), Montes de Valsaín and OAPN (Organismo Autónomo de Parques Nacionales) from the Ministerio de Agricultura, Alimentación, y Medio Ambiente in Spain for the support and facilities to organize this course. We thank Drs. Rafael Herrera, Mar Génova and Guillermo Gea-Izquierdo for their contributions in the course. We thank AEET (Asociación Española de Ecología Terrestre) from Spain for generous grants provided to students. We thank the course sponsors: Tree-Ring Society, National Science Foundation-USA (Grant AGS-Paleo Perspectives on Climate Change Program 1103314), Regents Instruments, Beta Analytic, and Cox Analytical Systems.

Frequent and persistent droughts exacerbate the problems caused by the inherent scarcity of water in the semiarid to arid parts of the southwestern United States. The occurrence of drought is driven by climatic variability, which for years before about the beginning of the 20th century in the Southwest must be inferred from proxy records. As part of a multidisciplinary study of the potential hydrologic impact of severe sustained drought on the Colorado River, the physical basis and limitations of tree rings as indicators of severe sustained drought are reviewed, and tree-ring data are analyzed to delineate a “worst-case” drought scenario for the Upper Colorado River Basin (UCRB). Runs analysis of a 121-site tree-ring network, 1600–1962, identifies a four-year drought in the 1660s as the longest-duration large-scale drought in the Southwest in the recent tree-ring record. Longer tree-ring records suggest a much longer and more severe drought in 1579–1598. The regression estimate of the mean annual Colorado River flow for this period is 10.95 million acre-feet, or 81 percent of the long-term mean. The estimated flows for the 1500s should be used with caution in impact studies because sample size is small and some reconstructed values are extrapolations.

Effective planning for use of water resources requires accurate information on hydrologic variability induced by climatic fluctuations. Tree-ring analysis is one method of extending our knowledge of hydrologic variability beyond the relatively short period covered by gaged streamflow records. In this paper, a network of recently developed tree-ring chronologies is used to reconstruct annual river discharge in the upper Gila River drainage in southeastern Arizona and southwestern Arizona since A.D. 1663. The need for data on hydrologic variability for this semi-arid basin is accentuated because water supply is inadequate to meet current demand. A reconstruction based on multiple linear regression (R2=0.66) indicates that 20th century is unusual for clustering of high-discharge years (early 1900s), severity of multiyear drought (1950s), and amplification of low-frequency discharge variations. Periods of low discharge recur at irregular intervals averaging about 20 years. Comparison with other tree-ring reconstructions shows that these low-flow periods are synchronous from the Gila Basin to the southern part of the Upper Colorado River Basin.

The problem of constructing millennia-long tree-ring chronologies from overlapping segments of cross-dated ring-width series is reviewed, with an emphasis on preserving very low-frequency signals potentially due to climate. In so doing, a fundamental statistical problem coined the 'segment length curse' is introduced. This 'curse' is related to the fact that the maximum wavelength of recoverable climatic information is ordinarily related to the lengths of the individual tree-ring series used to construct the millennia-long chronology. Simple experiments with sine waves are used to illustrate this fact. This is followed by more realistic experiments using a long bristlecone pine series that is randomly cut into a number of 1000-, 500- and 200-year segments and standardized using three very conservative methods. When compared against the original, uncut series, the resulting 'chronologies' show the effects of segment length even when the most conservative and noncommittal method of tree-ring standardiza tion is applied (i.e., a horizontal line through the mean). Alternative schemes of chronology development are described that seek to exorcise the segment length curse. While they show some promise, none is universal in its applicability and this problem still remains largely unsolved.

The authors wish to thank the Forestry Departments in Turkey, Greece, Cyprus, Lebanon, and Syria for their great help and support in making this study possible. We would like to thank the Cyprus Meteorological Service for providing us with climate data. We thank Professor Alexandros P Dimitrakopoulos, of the Laboratory of Forest Protection, School of Forestry and Natural Environment, Aristotle University of Thessaloniki, Greece, for his help and support. We thank Christopher Baisan, Russell Biggs, and Gurudas C Bock for their valuable assistance in the field. We also thank Russell Biggs, Victoria L Frazier, Alicia Stout, Gurudas C Bock, Jessica L Little, and Anthony P Trujillo for their assistance in sample preparation and measurement. We wish to thank the anonymous reviewers for their constructive comments and suggestions on the manuscript. Funding was provided by the US National Science Foundation under Grant Earth System History (Grant No. 0075956), ATM-GEO/ATM-Paleoclimate Program 0758486, and Paleo Perspectives on Climate Change (Award No. 1103314). V. Shishov (software development and data analysis) was supported by the Russian Science Foundation (Grant # 14-14-00219). The tree-ring data will be available at the International Tree-Ring Data Bank (ITRDB) web site (https://www.ncdc.noaa.gov/paleo/study/19016).

https://doi.org/10.15447/sfews.2011v9iss1art4 Blue oak tree-ring chronologies correlate highly with winter–spring precipitation totals over California, with Sacramento and San Joaquin river stream flow, and with seasonal variations in the salinity gradient in San Francisco Bay. The convergence of fresh and saline currents can influence turbidity, sediment accumulation, and biological productivity in the estuary. Three selected blue oak chronologies were used to develop a 625-year-long reconstruction of the seasonal salinity gradient, or low salinity zone (LSZ), which provides a unique perspective on the interannual-to-decadal variability of this important estuarine habitat indicator. The reconstruction was calibrated with instrumental LSZ data for the winter–spring season, and explains 73% of the variance in the February–June position of the LSZ from 1956 to 2003. Because this calibration period post-dates the sweeping changes that have occurred to land cover, channel morphology, and natural streamflow regimes in California, the reconstruction provides an idealized estimate for how the LSZ might have fluctuated under the seasonal precipitation variations of the past 625 years, given the modern geometry and bathymetry of the estuary and land cover across the drainage basin. The February–June season integrates precipitation and runoff variability during the cool season, and does not extend into the late-summer dry season when LSZ extremes can negatively affect Sacramento–San Joaquin Delta (Delta) agriculture and some aquatic organisms. However, there is such strong inter-seasonal persistence in the instrumental LSZ data that precipitation totals during the cool season can strongly pre-condition LSZ position in late summer. The 625-year-long reconstruction indicates strong interannual and decadal variability, the frequent recurrence of consecutive 2-year LSZ maxima and minima, large-scale ocean atmospheric forcing, and an interesting asymmetrical influence of warm El Niño–Southern Oscillation (ENSO) events.

A network of 248 tree-ring chronologies in the conterminous United States is assembled and analyzed by rotated principal components analysis (RPCA) to delineate “regions” of common tree-growth variation during the period 1705–1979. Spatial continuity of the tree-ring data is summarized by variogram analysis, and tree-ring data are gridded before RPCA to reduce effects of site clustering. Principal component drought information is evaluated by comparing PC scores and primary pattern coefficients with Palmer Drought Severity Index (PDSI) data from instrumental records. High PC pattern coefficients group geographically into regions coinciding roughly with nine drought regions delineated by RPCA of PDSI by other researchers. The drought signal as measured by the correlation between tree-ring PC scores and July PDSI, 1929–79, is strongest in the South and the interior West (r>0.7), and weakest in the Northeast and Pacific Northwest (r

Streamflow reconstruction—the statistical augmentation of streamflow time series using tree-ring data—has been increasingly applied as a planning and research tool in water resources studies over the past few decades. Streamflow reconstruction in North America has evolved from a largely qualitative science in the first half of the twentieth century into a highly quantitative science that draws heavily on probabilistic theory. The historical development of streamflow reconstruction from a western United States perspective is reviewed, with an emphasis on developments of the last 30 years. Contributions to the study of water resources are discussed. Temporal extension of gauge flow records is the central contribution of the paleo record, but the statistical summary of those records and their manner of presentation are important factors in determining the value to water resources management. Probabilistic interpretations of flow reconstructions are needed because of uncertainty stemming both from limitations of the basic data and from the reconstruction process itself. Case studies are presented for the Colorado River, at the large spatial scale, and for the water-supply region of a metropolitan area—Denver, Colorado—at the smaller scale. Interaction of the tree-ring scientists with water managers and the public is a hallmark of modern applied reconstruction studies. Aspects receiving increased attention are the extent to which seasonal flows can be resolved, and how water managers and planners concerned with future conditions can overlay impacts of expected climate change on the natural hydroclimatic variability of the past as reflected in the tree-ring record.

[1] Anthropogenic climate change is projected to exacerbate midlatitude aridity. Here, we analyze newly developed multi-century tree-ring records for a long-term perspective on drought in Tunisia and Algeria. We use a new set of 13 Cedrus atlantica and Pinus halepensis chronologies with a strong signal for warm-season drought (May–August) to generate a robust, well-validated reconstruction of the Palmer Drought Severity Index (PDSI) for the period AD 1456–2002. Key features of the reconstruction reveal the magnitude of pre-instrumental droughts from the historic record. Remarkably, the most recent drought (1999–2002) appears to be the worst since at least the middle of the 15th century. This drought is consistent with the early signature of a transition to more arid midlatitude conditions, as projected by general circulation models.

[1] New tree-ring records of ring-width from remnant preserved wood are analyzed to extend the record of reconstructed annual flows of the Colorado River at Lee Ferry into the Medieval Climate Anomaly, when epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western North America. The most extreme low-frequency feature of the new reconstruction, covering A.D. 762-2005, is a hydrologic drought in the mid-1100s. The drought is characterized by a decrease of more than 15% in mean annual flow averaged over 25 years, and by the absence of high annual flows over a longer period of about six decades. The drought is consistent in timing with dry conditions inferred from tree-ring data in the Great Basin and Colorado Plateau, but regional differences in intensity emphasize the importance of basin-specific paleoclimatic data in quantifying likely effects of drought on water supply.

[1] Updated proxy reconstructions of water year (October–September) streamflow for four key gauges in the Upper Colorado River Basin were generated using an expanded tree ring network and longer calibration records than in previous efforts. Reconstructed gauges include the Green River at Green River, Utah; Colorado near Cisco, Utah; San Juan near Bluff, Utah; and Colorado at Lees Ferry, Arizona. The reconstructions explain 72–81% of the variance in the gauge records, and results are robust across several reconstruction approaches. Time series plots as well as results of cross-spectral analysis indicate strong spatial coherence in runoff variations across the subbasins. The Lees Ferry reconstruction suggests a higher long-term mean than previous reconstructions but strongly supports earlier findings that Colorado River allocations were based on one of the wettest periods in the past 5 centuries and that droughts more severe than any 20th to 21st century event occurred in the past.

Since 2000, southwestern North America has experienced widespread drought. Lakes Powell and Mead are now at less than 50% of their reservoir capacity, and drought or fire-related states of emergency were declared this past summer by governors in six western states. As with other prolonged droughts, such as the Dust Bowl during the 1930s, aridity has at times extended from northern Mexico to the southern Canadian prairies. A synthesis of climatological and paleoclimatological studies suggests that a transition to a more arid climate may be occurring due to global warming, with the prospect of sustained droughts being exacerbated by the potential reaction of the Pacific Ocean to warming.

The two most severe, sustained droughts in the continental United States during the 20th century occurred in the 1930s and 1950s. The 1950s drought was most extreme over the southwest and southern Great Plains, where ecological consequences are still evident on the landscape [Swetnam and Betancourt], 1998].The Dust Bowl,vividly recounted in John Steinbeck's The Grapes of Wrath, was the nations most severe, sustained,and widespread drought of the past 300 years, according to tree-ring reconstructions of the Palmer drought severity index (PDSI) across the continental United States [Cook et al., 1999] (http://www.ngdc.noaa.gov/paleo/pdsiyear.html. Droughts during the 1750s, 1820s, and 1850s–1860s estimated from tree rings were similar to the 1950s drought in terms of magnitude, persistence, and spatial coverage, but these earlier episodes do not appear to have surpassed the severity or extent of the Dust Bowl drought. However, longer tree-ring reconstructions of PDSI for the United States and precipitation for northwestern Mexico and western Canada indicate that the “megadrought” of the 16th century far exceeded any drought of the 20th century (Figure 1) [also see Wood-house and Overpeck, 1998], and is considered to be the most severe prolonged drought over much of North America for at least the last 500 years [Meko et al., 1995].