Streamflow Reconstruction Research Articles

Suitability of ANN-Based Daily Streamflow Extension Models: a Case Study of Gaoping River Basin, Taiwan

It is well known that sufficiently long and continuous streamflow data are required for accurate estimations and informed decisions in water-resources planning, design, and management. Although streamflow data are measured and available at most river basins, streamflow records often suffer from insufficient length or missing data. In this work, artificial neural networks (ANNs) are applied to extend daily streamflow records at Lilin station located in Gaoping River basin, southern Taiwan. Two ANNs, including feed forward back propagation (FFBP) and radial basis function (RBF) networks, associated with various time-lagged streamflow and rainfall inputs of nearby long-record stations are employed to extend short daily streamflow records. Performances of ANNs are evaluated by root-mean-square error (RMSE), coefficient of efficiency (CE), and histogram-matching dissimilarity (HMD). Inconsistency among these evaluation measures is solved by the technique for order performance by similarity to ideal solution (TOPSIS), a widely used multi-criteria decision-making approach, to find an optimal model. The results indicate that RBF-E1 (entire-year data training with Q t and Q t−1 inputs) has the minimum RMSE of 104.4 m3/s, second highest CE of 0.956, and third lowest HMD of 0.0096, which outperforms other ANNs and provide the most accurate reconstruction of daily streamflow records at Lilin station.

Beyond annual streamflow reconstructions for the Upper Colorado River Basin: A paleo‐water‐balance approach

AbstractIn this paper, we present a methodology to use annual tree‐ring chronologies and a monthly water balance model to generate annual reconstructions of water balance variables (e.g., potential evapotranspiration (PET), actual evapotranspiration (AET), snow water equivalent (SWE), soil moisture storage (SMS), and runoff (R)). The method involves resampling monthly temperature and precipitation from the instrumental record directed by variability indicated by the paleoclimate record. The generated time series of monthly temperature and precipitation are subsequently used as inputs to a monthly water balance model. The methodology is applied to the Upper Colorado River Basin, and results indicate that the methodology reliably simulates water‐year runoff, maximum snow water equivalent, and seasonal soil moisture storage for the instrumental period. As a final application, the methodology is used to produce time series of PET, AET, SWE, SMS, and R for the 1404–1905 period for the Upper Colorado River Basin.

Palaeohydrology in climatological context: Developing the case for use of remote predictors in Australian streamflow reconstructions

The dry continent of Australia experiences frequent periods of devastating regional drought, making high quality palaeohydrological reconstructions essential for water resource management and planning. In other parts of the world tree-rings form a core component of such reconstructions. Yet for much of Australia, annually resolved palaeohydrological reconstructions derived from tree-rings have proven elusive. The island state of Tasmania in the far south is an important exception, with over 50 tree-ring chronologies available. Coupled ocean-atmosphere processes that drive precipitation across mainland Australia also influence Tasmanian precipitation. Here, we provide a basic analysis of how geographic relationships between important drivers of seasonal Tasmanian streamflow and potential streamflow predictors such as tree-ring chronologies may extend the spatial applicability of Tasmania's tree-ring predictors beyond the local context. We find clear geographically and seasonally defined patterns in Tasmanian streamflow that are reflected in the relationships between individual Tasmanian tree-ring chronologies and their relationships with streamflows. We find strong evidence that quality Tasmanian summer (DJF) streamflow reconstructions based on tree-rings are possible. Evidence also suggests that streamflow reconstructions for other seasons are also likely to be possible, especially as additional chronologies based on wood properties such as tracheid radial diameter, microfibril angle, density and cell wall thickness become available. Based on the relationships between streamflows and tree-ring chronologies, and between streamflows and major precipitation drivers across Australia, we conclude that Tasmanian tree-ring chronologies are likely to prove useful as predictors for seasonal streamflow reconstructions, particularly in eastern Australia. Successful streamflow reconstructions for a much broader portion of Australia than currently available will be invaluable for future water resource management and planning.

Optimal ranking regime analysis of TreeFlow dendrohydrological reconstructions

Abstract. The optimal ranking regime (ORR) method was used to identify 6–100-year time windows containing significant ranking sequences in 55 western US streamflow reconstructions, and reconstructions of the level of the Great Salt Lake and San Francisco Bay salinity during 1500–2007. The method's ability to identify optimally significant and non-overlapping runs of low- and high-rankings allows it to re-express a reconstruction time series as a simplified sequence of regime segments marking intra- to multi-decadal (IMD) periods of low or high streamflow, lake level, and salinity. Those ORR sequences, referred to here as Z-lines, can be plotted to identify consistent regime patterns in the analysis of numerous reconstructions. The Z-lines for the 57 reconstructions evaluated here show a common pattern of IMD cycles of drought and pluvial periods during the late 16th and 17th centuries, a relatively dormant period during the 18th century, and the reappearance of alternating dry and wet IMD periods during the 19th and early 20th centuries. Although this pattern suggests the possibility of similarly active and inactive oceanic modes in the North Pacific and North Atlantic, such centennial-scale patterns are not evident in the ORR analyses of reconstructed Pacific Decadal Oscillation (PDO), El Niño–Southern Oscillation, and North Atlantic sea-surface temperature variation. However, given the inconsistency in the analyses of four PDO reconstructions, the possible role of centennial-scale oceanic mechanisms is uncertain. In future research the ORR method might be applied to climate reconstructions around the Pacific Basin to try to resolve this uncertainty. Given its ability to compare regime patterns in climate reconstructions derived using different methods and proxies, the method may also be used in future research to evaluate long-term regional temperature reconstructions.

Preliminary December–January inflow and streamflow reconstructions from tree rings for western Tasmania, southeastern Australia

AbstractProjected decreases and changes in the seasonal distribution of precipitation will have profound impacts on southeastern Australia, including its ability to generate renewable hydroelectricity. Recent decreases in precipitation over the region may be significant in the context of instrumental records, but the question of whether these decreases are within long‐term natural variability remains. To help address this issue, we present December–January streamflow and dam inflow reconstructions for southeastern Australia. These reconstructions for the Tasmanian west coast are based solely on local tree ring chronologies and span up to 1600 years. Nonparametric estimates, however, indicate good model skill for the last 458 years (streamflow) and 478 years (dam inflow). The reconstructions indicate that twentieth century conditions were well within the range of historical variability, and were in fact relatively wet. The period from approximately 1600 to 1750 CE was one of the enhanced variability and a high proportion of low and high flow events occurred in the seventeenth century. There are significant relationships between streamflow and inflow reconstructions and large‐scale ocean‐atmosphere processes such as ENSO and the Southern Annular Mode. Critically, our two reconstructions rely heavily on new tree ring chronologies based on properties such as tracheid radial diameter, cell wall thickness, and density, underscoring the importance of these different types of chronologies in reconstructions.

Alternative standardization approaches to improving streamflow reconstructions with ring-width indices of riparian trees

Old, multi-aged populations of riparian trees provide an opportunity to improve reconstructions of streamflow. Here, ring widths of 394 plains cottonwood ( Populus deltoides, ssp. monilifera) trees in the North Unit of Theodore Roosevelt National Park, North Dakota, are used to reconstruct streamflow along the Little Missouri River (LMR), North Dakota, US. Different versions of the cottonwood chronology are developed by (1) age-curve standardization (ACS), using age-stratified samples and a single estimated curve of ring width against estimated ring age, and (2) time-curve standardization (TCS), using a subset of longer ring-width series individually detrended with cubic smoothing splines of width against year. The cottonwood chronologies are combined with the first principal component of four upland conifer chronologies developed by conventional methods to investigate the possible value of riparian tree-ring chronologies for streamflow reconstruction of the LMR. Regression modeling indicates that the statistical signal for flow is stronger in the riparian cottonwood than in the upland chronologies. The flow signal from cottonwood complements rather than repeats the signal from upland conifers and is especially strong in young trees (e.g. 5–35 years). Reconstructions using a combination of cottonwoods and upland conifers are found to explain more than 50% of the variance of LMR flow over a 1935–1990 calibration period and to yield reconstruction of flow to 1658. The low-frequency component of reconstructed flow is sensitive to the choice of standardization method for the cottonwood. In contrast to the TCS version, the ACS reconstruction features persistent low flows in the 19th century. Results demonstrate the value to streamflow reconstruction of riparian cottonwood and suggest that more studies are needed to exploit the low-frequency streamflow signal in densely sampled age-stratified stands of riparian trees.

Tree-ring based reconstruction of Río Bermejo streamflow in subtropical South America

Summary Precipitation on the subtropical Andes in northwestern Argentina is the main source of freshwater to rivers, which provide water for consumption, hydroelectric generation and irrigation of agricultural fields. Regional streamflow records for the past 60 years indicate a period of enhanced inter-annual and inter-decadal variability during recent decades. Here we present a long-term perspective of streamflow variations for Rio Bermejo, a major river in the subtropics of Argentina–Bolivia. This streamflow reconstruction covers the period 1680–2001 and was performed using principal component regression models based on precipitation sensitive tree-ring width series. Composite tree-ring chronologies from Juglans australis , Cedrela lilloi and Schinopsis lorentzii were used as predictors to reconstruct the May–October (dry season) streamflow variations. Monthly instrumental streamflow records were used to assess the temporal relationship between climatic data and tree-ring records. The regression model explains 52% of the variance of May–October Bermejo discharge ( R 2 adj = 0.499) over the period 1941–1992. Analyses of the frequency, intensity and duration of pluvials and droughts indicates a sustained increase in streamflow since the 1960s, which is exceptional since 1680. Pearson’s correlation coefficients between Rio Bermejo records against gridded interpolated precipitation and outgoing longwave radiation indicate that the South American monsoon is the main source of regional precipitation and the consequent discharge of subtropical rivers in northern Argentina. Long-term streamflow reconstructions in subtropical South America contribute to our understanding of past and present climate variations and the related large-scale atmospheric features that drive these variations.

A millennium-length reconstruction of Bear River stream flow, Utah

Summary The Bear River contributes more water to the eastern Great Basin than any other river system. It is also the most significant source of water for the burgeoning Wasatch Front metropolitan area in northern Utah. Despite its importance for water resources for the region’s agricultural, urban, and wildlife needs, our understanding of the variability of Bear River’s stream flow derives entirely from the short instrumental record (1943–2010). Here we present a 1200-year calibrated and verified tree-ring reconstruction of stream flow for the Bear River that explains 67% of the variance of the instrumental record over the period from 1943 to 2010. Furthermore, we developed this reconstruction from a species that is not typically used for dendroclimatology, Utah juniper (Juniperus osteosperma). We identify highly significant periodicity in our reconstruction at quasi-decadal (7–8 year), multi-decadal (30 year), and centennial (>50 years) scales. The latter half of the 20th century was found to be the 2nd wettest (∼40-year) period of the past 1200 years, while the first half of the 20th century marked the 4th driest period. The most severe period of reduced stream flow occurred during the Medieval Warm Period (ca. mid-1200s CE) and persisted for ∼70 years. Upper-level circulation anomalies suggest that atmospheric teleconnections originating in the western tropical Pacific are responsible for the delivery of precipitation to the Bear River watershed during the October–December (OND) season of the previous year. The Bear River flow was compared to recent reconstructions of the other tributaries to the Great Salt Lake (GSL) and the GSL level. Implications for water management could be drawn from the observation that the latter half of the 20th century was the 2nd wettest in 1200 years, and that management for future water supply should take into account the stream flow variability over the past millennium.

An Annual Streamflow Reconstruction of the Red River, Kentucky Using a White Pine (Pinus Strobus) Chronology

Using tree ring data (dendrochronology) a reconstruction of the historical annual stream flow of the Red River Basin of Kentucky was performed. Tree ring widths provide a surrogate record of the past climate and hence stream flow of a particular area, with thicker rings corresponding to wetter periods and thinner rings corresponding to drier conditions. This type of research is common in river basins across the western USA which are prone to serious droughts and flooding on annual and decadal scales. However, little research has been conducted in this manner in the eastern USA, Kentucky in particular, despite the potential for intermittent dry and wet periods over time. The use of the Daniel Boone National Forest is the focus of this study due to the well documented past flooding and drought events of the Red River during the 20th Century. The technique provided a reconstructed record from 1713 up to the end of the observed period in 1999. The results show that there has been a decrease in the number and frequency of prolonged wet and dry periods, although there has been an increase in the extreme wet and dry conditions during the later observed record which suggests a shift in the hydroclimatological regime of the river basin.

A water balance approach for reconstructing streamflow using tree-ring proxy records

• Seasonal water balance models performed well in reproducing instrumental streamflow. • Seasonal PRISM and proxy precipitation and temperature inputs were used with models. • Annual reconstructions did not reproduce observations as well as seasonal ones. • The method also enabled reconstruction of streamflows, SWE and evapotranspiration. • A water balance method is useful for assessing uncertainty in proxy reconstructions. Tree-ring data have been used to augment limited instrumental records of climate and provide a longer view of past variability, thus improving assessments of future scenarios. For streamflow reconstructions, traditional regression-based approaches cannot examine factors that may alter streamflow independently of climate, such as changes in land use or land cover. In this study, seasonal water balance models were used as a mechanistic approach to reconstruct streamflow with proxy inputs of precipitation and air temperature. We examined a Thornthwaite water balance model modified to have seasonal components and a simple water balance model with a snow component. These two models were calibrated with a shuffled complex evolution approach using PRISM and proxy seasonal temperature and precipitation to reconstruct streamflow for the upper reaches of the West Walker River basin at Coleville, CA. Overall, the modified Thornthwaite model performed best during calibration, with R 2 values of 0.96 and 0.80 using PRISM and proxy inputs, respectively. The modified Thornthwaite model was then used to reconstruct streamflow during AD 1500–1980 for the West Walker River basin. The reconstruction included similar wet and dry episodes as other regression-based records for the Great Basin, and provided estimates of actual evapotranspiration and of April 1 snow water equivalence. Given its limited input requirements, this approach is suitable in areas where sparse instrumental data are available to improve proxy-based streamflow reconstructions and to explore non-climatic reasons for streamflow variability during the reconstruction period.

RECONSTRUCCIÓN DEL ESCURRIMIENTO HISTÓRICO EN LA CUENCA ALTA DEL RÍO NAZAS, DURANGO

The drought of the past few years in north-central Mexico underscores the need for extensive hydroclimatic data that can reveal fluctuations and trends over time, in order to establish water resource planning and mitigation measures. Using dendrochronological techniques, an 800-year streamflow reconstruction was developed for the upper Nazas River watershed in Durango. The reconstruction showed high inter-annual and multiannual hydroclimatic variability and decadal droughts were detected in the 1250s, 1320-1340s, 1370-1380s, 1440-1450s, 1460-1470s, 1650-1660s, 1710-1730s, 1790-1810s, 1920-1940s, 1970-1980s, and 1990s. Some of the droughts were longer and of higher intensity than ones observed in climate records. Extreme flow events were detected in the years 1212, 1229, 1242, 1245, 1271, 1313, 1404, 1427, 1451, 1504, 1520, 1563, 1566, 1620,1671, 1681, 1735, 1816, 1833, 1869, 1920, 1956, 1959, and 1992, and they were linked with the El Nino–Southern Oscillation (ENSO) warm phase. Water decision-makers in the Comarca Lagunera region could use this information to establish probabilities of droughts and occurrence of extreme runoff events for management purposes.

A 576‐Year Weber River Streamflow Reconstruction from Tree Rings for Water Resource Risk Assessment in the Wasatch Front, Utah

AbstractWe present a 576‐year tree‐ring‐based reconstruction of streamflow for northern Utah's Weber River that exhibits considerable interannual and decadal‐scale variability. While the 20th Century instrumental period includes several extreme individual dry years, it was the century with the fewest such years of the entire reconstruction. Extended droughts were more severe in duration, magnitude, and intensity prior to the instrumental record, including the most protracted drought of the record, which spanned 16 years from 1703 to 1718. Extreme wet years and periods are also a regular feature of the reconstruction. A strong early 17th Century pluvial exceeds the early 20th Century pluvial in magnitude, duration, and intensity, and dwarfs the 1980s wet period that caused significant flooding along the Wasatch Front. The long‐term hydroclimatology of northern Utah is marked by considerable uncertainty; hence, our reconstruction provides water managers with a more complete record of water resource variability for assessment of the risk of droughts and floods for one of the largest and most rapidly growing population centers in the Intermountain West.

Filyos River Streamflow Reconstruction from Tree‐Ring Chronologies with Nonparametric Approaches

AbstractThe article presents nonparametric methods based on K nearest neighbors (KNNs), modified KNNs, and local polynomial techniques to reconstruct streamflow ensembles from tree‐ring data in Filyos River region (Turkey). Three methods were tested using cross‐validation for the overlap period, 1963‐1997 for which the tree‐ring and streamflow data are available. It was found that for the study where the length of the overlap period was limited, a nonparametric method based on a local polynomial technique provides simulations that have a slightly better solution than the other methods. After verification using standard statistical techniques, these methods were utilized to develop streamflow reconstructions from tree‐ring data for the paleo‐hydrologic period (1657‐1963). These reconstructions of seasonal low and high flows were discussed with the obtained flood duration curve. They were also compared with the historical archives and other tree‐ring reconstructions data available in the same river. Overall, the utility and limitations of these methods and the resulting streamflow simulations were discussed to assess the long‐term discharge behavior of Filyos River and to evaluate water supply reliability.

Streamflow reconstruction of Eastern Himalaya River, Lachen ‘Chhu’, North Sikkim, based on tree-ring data of Larix griffithiana from Zemu Glacier basin

The relationship of streamflow records of the Lachen River with tree-ring parameters of total tree-ring width (TRW), earlywood width (EWW) and latewood width (LWW) chronologies of Larix griffithiana from Lachen, North Sikkim, Eastern Himalaya was generated. These chronologies correlate significantly with the observed discharge of the Lachen River where the EWW chronology explains 61.2% of the streamflow variance. Based on this result, Lachen River discharge for the period of previous year March to current year February was reconstructed using EWW chronology, which extends back to AD 1790. In the smoothed reconstructed data the period of extreme low streamflows were during AD 1791–1805, 1813–1822 and 1914–1925 and the extreme highs were during AD 1823–1835, 1879–1890, 1926–1946 and 1980–1989. The streamflow is also found to be lower than average during the monsoon failure (or East India Drought) of AD 1792–1796 and past great droughts of AD 1876–1878. The lower tree growth during AD 1816–1822 is consistent with that of the Tambora volcanic eruption of Indonesia in AD 1815. High spectral power at 4–8 years in the reconstructed streamflow is similar to that of ENSO range.

A tree-ring based reconstruction of Logan River streamflow, northern Utah

[1] We created six new tree-ring chronologies in northern Utah, which were used with preexisting chronologies from Utah and western Wyoming to reconstruct mean annual flow for the Logan River, the largest tributary of the regionally important Bear River. Two reconstruction models were developed, a “Local” model that incorporated two Rocky Mountain juniper chronologies located within the basin, and a “Regional” model that also included limber pine and pinyon pine chronologies from a larger area. The Local model explained 48.2% of the variability in the instrumental record and the juniper chronologies better captured streamflow variability than Douglas-fir collected within the Logan basin. Incorporating chronologies from the northern and southern margins of the transition zone of the western precipitation dipole increased the skill of the Regional model (r2 = 0.581). We suggest the increased Regional model skill indicates that both nodes of the western precipitation dipole influence northern Utah climate. The importance of Rocky Mountain juniper in both reconstructions of streamflow for this region suggests that future work should target these trees where more traditionally desirable species are not present. The reconstructions provide the first extended record of streamflow in northern Utah. Preinstrumental streamflows (1605–1921) exhibited considerable variability when compared to the instrumental period (1922–2005). Our findings confirm that the inherent uncertainty in contemporary water management and planning in the region is due to hydroclimatic variability that has persisted for at least the last four centuries.

A tree-ring-based reconstruction of the Yimin River annual runoff in the Hulun Buir region, Inner Mongolia, for the past 135 years

Based on the relationships between the regional tree-ring chronology (RC) of moisture-sensitive Pinus sylvestris var. mongolica and the monthly mean maximum temperature, annual precipitation and annual runoff, a reconstruction of the runoff of the Yimin River was performed for the period 1868–2002. The model was stable and could explain 52.2% of the variance for the calibration period of 1956–2002. During the past 135 years, 21 extremely dry years and 19 extremely wet years occurred. These years represented 15.6% and 14.1% of the total study period, respectively. Six severe drought events lasting two years or more occurred in 1950–1951, 1986–1987, 1905–1909, 1926–1928, 1968–1969 and 1919–1920. Four wetter events occurred during 1954–1959, 1932–1934, 1939–1940 and 1990–1991. Comparisons with other tree-ring-based streamflow reconstructions or chronologies for surrounding areas supplied a high degree of confidence in our reconstruction. Power spectrum and wavelet analyses suggested that the reconstructed annual runoff variation in the Hulun Buir region and surrounding area could be associated with large-scale atmospheric-oceanic variability, such as the Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation (ENSO), and sunspot activity.

Case Study of Drought Frequency and Risk Analysis in the Upper Green River Basin, Wyoming

AbstractThe limited length of instrumental streamflow data impacts the true magnitude of natural interdecadal variability of water delivered from the Upper Green River Basin (UGRB). This limited period of instrumental record can be expanded by utilizing proxy records (reconstructed streamflow) derived from tree rings. Recent research has resulted in the development of nine streamflow reconstructions spatially located throughout the UGRB. This paper utilizes four of those nine reconstructed streamflow records and instrumental records to compare and analyze differences between the two streamflow records—human and natural. Three approaches were used for comparison and analysis: (1) Weibull distribution, (2) compound renewal, and (3) drought risk using bivariate probability distribution functions. This analysis has resulted in magnitude-duration-frequency curves for UGRB drought. Such probability curves and stochastic analysis can then be utilized in light of compact agreements and system storage to answer qu...

Multi-century tree-ring based reconstruction of the Neuquén River streamflow, northern Patagonia, Argentina

Abstract. In most cases, gauged river flow records in southern South America extend for only a few decades, hampering the detection of long-term, decadal to centennial-scale cycles and trends. Long streamflow series can be reconstructed from tree-ring records, offering the opportunity of extending the limited hydrological instrumental data to several centuries. In northern Patagonia, Argentina, the Neuquén River has great importance for local and national socio-economic activities such as hydroelectric power generation, agriculture and tourism. In this study, new and updated tree-ring chronologies from Araucaria araucana and Austrocedrus chilensis are used to reconstruct the October–June mean streamflow for the Neuquén River and place the period of gauged flows (1903–2009), in a long-term, multi-century context. The reconstruction covers the period 1346–2000 AD and was developed from a network of 43 tree-ring chronologies, grouped in composite series, using a nested principal component regression approach. Analyses of the frequency, intensity, and duration of droughts and pluvial events indicate that the 20th century contains some of the driest and wettest annual to decadal-scale events in the last 654 yr, but longer and more severe events were recorded in previous centuries. Blackman-Tukey and singular spectral analyses identified quasiperiodic oscillations from 3.5 to 17.5 yr. A dominant 6.8-yr cycle explains ca. 23.6% of the total variance in the Neuquén River streamflow reconstruction. Correlation analyses showed that discharges of the Neuquén River are related to variations in the Southern Annular Mode (SAM), a measure of air mass exchanges between middle and high latitudes in the Southern Hemisphere. This association is consistent with previous studies that indicate a strong correlation between rainfall in northern Patagonia and SAM variations.

Paleoreconstruction of cool season precipitation and warm season streamflow in the Pacific Northwest with applications to climate change assessments

Long‐term streamflow reconstructions help characterize climate variability and extreme climatic events, such as droughts, which play a crucial role in water resource planning. Dendrohydrological reconstructions generally build on indirect associations between streamflow and radial tree growth, both of which depend on cool season precipitation in the Pacific Northwest. We develop a new approach that integrates a tree ring reconstruction of cool season precipitation with historical meteorological data and a physically based hydrologic model to reconstruct warm season streamflow and streamflow uncertainty. The Upper Yakima Basin in Washington state is used as a test case. We applied objective screening, principal components analysis, and multiple linear regression to reconstruct 366 years of basin‐average cool season precipitation. The reconstruction was integrated with five temporally and spatially distributed cool season precipitation patterns spanning the historical range of natural variability. These distributed meteorological reconstructions were used as inputs to the Variable Infiltration Capacity (VIC) hydrologic model over the Yakima basin to produce an ensemble of warm season streamflows for each reconstructed year. The resultant streamflow reconstruction retains dendroclimatic information and quantifies the inherent uncertainty in warm season streamflow associated with historical meteorology. Finally, the meteorological reconstructions were systematically perturbed and used to drive the VIC to examine the potential impacts of climate change on warm season streamflow over the 366‐year record. Despite projected wetter conditions in the future, less precipitation is stored as snowpack (due to warmer winter temperatures) and consequently warm season streamflow will be systematically reduced. The combination of long records of variability and systematic changes related to climate change provides useful information about the combined effects of natural variability and projected systematic changes in climate to support 21st‐century water planning.

Tree ring-based annual streamflow reconstruction for the Heihe River in arid northwestern China from ad 575 and its implications for water resource management

We developed two tree ring-width chronologies (Qilian juniper, Sabina przewalskii Kom.) for the inland Heihe River Basin in arid northwest China using a large number of tree-ring samples (217 samples/92 trees) with accurate information about pith offsets based on Regional Curve Standardization (RCS) and standard dendrochronological (STD) methodologies. Two 1422-year reconstructions of annual (August–July) streamflow for the upstream region of the Heihe River are presented. The STD and RCS reconstructions account for 53.4% and 57.2% of the actual streamflow variance during the period 1958–2006, respectively. Both reconstructions display considerable low frequency (multidecadal to multicentury) fluctuations, although the RCS based reconstruction is superior to the STD based reconstruction for retention of low-frequency trends. Low-flow years in ad 818–852, 1112–1196, 1453–1495 and 1680–1710, and high-flow periods in ad 868–1000, 1056–1094, 1228–1271, 1327–1440, 1510–1583 and 1877–2006 are detected in both reconstructions. Both the STD and the RCS reconstructions testify to the fact that the 20th century witnessed intensified pluvial conditions in the upstream region of the Heihe River in the context of the last 1500 years. The streamflow reconstructions are anticipated to be useful to water resource planning and management for the Heihe River Basin in arid northwestern China.