Michelle Ho

The future role of dams in the United States of America

Storage and controlled distribution of water have been key elements of a human strategy to overcome the space and time variability of water, which have been marked by catastrophic droughts and floods throughout the course of civilization. In the United States, the peak of dam building occurred in the mid-20th century with knowledge limited to the scientific understanding and hydrologic records of the time. Ecological impacts were considered differently than current legislative and regulatory controls would potentially dictate. Additionally, future costs such as maintenance or removal beyond the economic design life were not fully considered. The converging risks associated with aging water storage infrastructure and uncertainty in climate in addition to the continuing need for water storage, flood protection, and hydropower result in a pressing need to address the state of dam infrastructure across the nation. Decisions regarding the future of dams in the United States may, in turn, influence regional water futures through groundwater outcomes, economic productivity, migration, and urban growth. We advocate for a comprehensive national water assessment and a formal analysis of the role dams play in our water future. We emphasize the urgent need for environmentally and economically sound strategies to integrate surface and groundwater storage infrastructure in local, regional, and national water planning considerations. A research agenda is proposed to assess dam failure impacts and the design, operation, and need for dams considering both paleo and future climate, utilization of groundwater resources, and the changing societal values toward the environment.

A paleoclimate rainfall reconstruction in the Murray‐Darling Basin (MDB), Australia: 2. Assessing hydroclimatic risk using paleoclimate records of wet and dry epochs

Estimates of hydrological risk are crucial to enable adequate planning and preparation for extreme events. However, the accurate estimation of hydrological risk is hampered by relatively short instrumental records in many parts of the world. Information derived from climate-sensitive paleoclimate proxies provide an opportunity to resolve hydroclimatic variability, but many regions, such as Australias Murray-Darling Basin (MDB), currently lack the suitable in situ proxies necessary to do this. Here, new MDB rainfall reconstructions are presented based on a novel method using paleoclimate rainfall proxies in the Australasian region spanning from 749 BCE to 1980 CE. Our results emphasize the need to develop additional reconstructions and, with the companion paper, demonstrate how this information can be used to benefit water resource management. This study shows that prior to the 20th century both dry and wet epochs have persisted for longer periods than observed in the instrumental record – with the probability of both dry and wet periods exceeding a decade at least 10 times more likely prior to 1883 than suggested by the instrumental records. Some reconstructed rainfalls exceeded the instrumental range (i.e. drier dry epochs and wetter wet spells) despite a systematic underestimation of extremes due to a combination of proxy quality and model bias. Importantly, the results demonstrate that the instrumental record does not cover the full range of hydroclimatic variability possible in the MDB. Therefore hydroclimatic risk assessments based on the instrumental record likely underestimate, or at least misinterpret, the frequency, duration and magnitude of wet and dry epochs. This article is protected by copyright. All rights reserved.

Broadening the Spatial Applicability of Paleoclimate Information—A Case Study for the Murray–Darling Basin, Australia

Recent advances in the collection and analysis of paleoclimate data have provided significant insights into preinstrumental environmental events and processes, enabling a greater understanding of long-term environmental change and associated hydroclimatic risks. Unfortunately, it is often the case that there is a dearth of readily available paleoclimate data from regions where such insights and long-term data are most needed. The Murray‐Darling basin (MDB), known as Australia’s ‘‘food bowl,’’ is an example of such a region where currently there are very limited in situ paleoclimate data available. While previous studies have utilized paleoclimate proxy records of large-scale climate mechanisms to infer preinstrumental MDB hydroclimatic variability, there is a lack of studies that utilize Australian terrestrial proxy records to garner similar information. Given the immediate need for improved understanding of MDB hydroclimatic variability, this paper identifies key locations in Australia where existing and as yet unrealized paleoclimate records will be most useful in reconstructing such information. To identify these key locations, rainfall relationships between MDB and non-MDB locations were explored through correlations and principal component analysis. An objective analysisusing optimalinterpolation was thenused to pinpoint the most strategic locations to further developproxyrecordsandgaininsightsintothebenefitsofobtainingthis additionalinformation.Thefindings reveal that there is potential for the future assembly of high-resolution paleoclimate records in Australia capable of informing MDB rainfall variability, in particular southeast Australia and central-northern Australia. This study highlights the need for further investment in the development of these potential proxy sources to subsequently enable improved assessments of long-term hydroclimatic risks.

Can PDSI inform extreme precipitation?: An exploration with a 500 year long paleoclimate reconstruction over the U.S.

This study explores whether it is possible to reconstruct the frequency of extreme precipitation occurrence across the contiguous United States (CONUS) using the Living Blended Drought Atlas (LBDA), a 500 year paleoclimate reconstruction of the summer (June–August) Palmer Drought Severity Index (PDSI). We first identify regions of the country where the LBDA may reflect the occurrence of extremes based on their seasonality and contribution to total annual moisture delivery. Correlation measures are used to assess the relationship between the frequencies of extreme precipitation occurrence and both the instrumental monthly PDSI and the annual LBDA-estimated PDSI. Extreme precipitation is found to account for a large portion of total precipitation west of the Mississippi River and clusters in particular seasons (winter and summer), supporting a strong relationship with the LBDA without much information loss from the instrumental PDSI data. Dimension reduction techniques are used to explore the joint spatiotemporal structure of extreme precipitation occurrence and LBDA across the country. The primary modes of variability of the LBDA and extreme precipitation occurrence relate remarkably well for a region centered over the southwest that exhibits an ENSO-like time-frequency structure. Generalized linear models (GLMs) are used to demonstrate the feasibility of reconstructing the annual extreme precipitation frequency over the 500 year prehistoric record at two sites in the southwest and Southern Plains. GLM-based reconstructions show a high degree of structured variability in the likelihood of extreme precipitation occurrences over the prehistoric record.

Can a paleodrought record be used to reconstruct streamflow?: A case study for the Missouri River Basin

Recent advances in paleoclimatology have revealed dramatic long-term hydroclimatic variations that provide a context for limited historical records. A notable data set derived from a relatively dense network of paleoclimate proxy records in North America is the Living Blended Drought Atlas (LBDA): a gridded tree-ring-based reconstruction of summer Palmer Drought Severity Index. This index has been used to assess North American drought frequency, persistence, and spatial extent over the past two millennia. Here, we explore whether the LBDA can be used to reconstruct annual streamflow. Relative to streamflow reconstructions that use tree rings within the river basin of interest, the use of a gridded proxy poses a novel challenge. The gridded series have high spatial correlation, since they rely on tree rings over a common radius of influence. A novel algorithm for reconstructing streamflow using regularized canonical regression and inputs of local and global covariates is developed and applied over the Missouri River Basin, as a test case. Effectiveness in reconstruction is demonstrated with reconstructions showing periods where streamflow deficits may have been more severe than during recent droughts (e.g., the Civil War, Dust Bowl, and 1950s droughts). The maximum persistence of droughts and floods over the past 500 years far exceeds those observed in the instrumental record and periods of multidecadal variability in the 1500s and 1600s are detected. Challenges for an extension to a national streamflow reconstruction or applications using other gridded paleoclimate data sets such as adequate spatial coverage of streamflow and applicability of annual reconstructions are discussed.

A paleoclimate rainfall reconstruction in the Murray‐Darling Basin (MDB), Australia: 1. Evaluation of different paleoclimate archives, rainfall networks, and reconstruction techniques

From ∼1997 to 2009 the Murray-Darling Basin (MDB), Australias largest water catchment and reputed “food bowl,” experienced a severe drought termed the “Millennium Drought” or “Big Dry” followed by devastating floods in the austral summers of 2010/2011, 2011/2012, and 2012/2013. The magnitude and severity of these extreme events highlight the limitations associated with assessing hydroclimatic risk based on relatively short instrumental records (∼100 years). An option for extending hydroclimatic records is through the use of paleoclimate records. However, there are few in situ proxies of rainfall or streamflow suitable for assessing hydroclimatic risk in Australia and none are available in the MDB. In this paper, available paleoclimate records are reviewed and those of suitable quality for hydroclimatic risk assessments are used to develop preinstrumental information for the MDB. Three different paleoclimate reconstruction techniques are assessed using two instrumental rainfall networks: (1) corresponding to rainfall at locations where rainfall-sensitive Australian paleoclimate archives currently exist and (2) corresponding to rainfall at locations identified as being optimal for explaining MDB rainfall variability. It is shown that the optimized rainfall network results in a more accurate model of MDB rainfall compared to reconstructions based on rainfall at locations where paleoclimate rainfall proxies currently exist. This highlights the importance of first identifying key locations where existing and as yet unrealized paleoclimate records will be most useful in characterizing variability. These results give crucial insight as to where future investment and research into developing paleoclimate proxies for Australia could be most beneficial, with respect to better understanding instrumental, preinstrumental and potential future variability in the MDB.

The paleoclimate context and future trajectory of extreme summer hydroclimate in eastern Australia

Eastern Australia recently experienced an intense drought (Millennium Drought, 2003-2009) and record-breaking rainfall and flooding (austral summer 2010-2011). There is some limited evidence for a climate change contribution to these events, but such analyses are hampered by the paucity of information on long-term natural variability. Analyzing a new reconstruction of summer (December-January-February) Palmer Drought Severity Index (the Australia-New Zealand Drought Atlas; ANZDA, 1500-2012 CE), we find moisture deficits during the Millennium Drought fall within the range of the last 500 years of natural hydroclimate variability. This variability includes periods of multi-decadal drought in the 1500s more persistent than any event in the historical record. However, the severity of the Millennium Drought, which was caused by autumn (March-April-May) precipitation declines, may be underestimated in the ANZDA because the reconstruction is biased towards summer and antecedent spring (September-October-November) precipitation. The pluvial in 2011, however, which was characterized by extreme summer rainfall faithfully captured by the ANZDA, is likely the wettest year in the reconstruction for Coastal Queensland. Climate projections (RCP 8.5 scenario) suggest that eastern Australia will experience long-term drying during the 21st century. While the contribution of anthropogenic forcing to recent extremes remains an open question, these projections indicate an amplified risk of multi-year drought anomalies matching or exceeding the intensity of the Millennium Drought.

America's water: Agricultural water demands and the response of groundwater

Agricultural, industrial, and urban water use in the conterminous United States (CONUS) is highly dependent on groundwater that is largely drawn from nonsurficial wells (>30 m). We use a Demand-Sensitive Drought Index to examine the impacts of agricultural water needs, driven by low precipitation, high agricultural water demand, or a combination of both, on the temporal variability of depth to groundwater across the CONUS. We characterize the relationship between changes in groundwater levels, agricultural water deficits relative to precipitation during the growing season, and winter precipitation. We find that declines in groundwater levels in the High Plains aquifer and around the Mississippi River Valley are driven by groundwater withdrawals used to supplement agricultural water demands. Reductions in agricultural water demands for crops do not, however, lead to immediate recovery of groundwater levels due to the demand for groundwater in other sectors in regions such as Utah, Maryland, and Texas.

Multiscale temporal variability and regional patterns in 555 years of conterminous U.S. streamflow

The development of paleoclimate streamflow reconstructions in the conterminous United States (CONUS) has provided water resource managers with improved insights into multidecadal and centennial scale variability that cannot be reliably detected using shorter instrumental records. Paleoclimate streamflow reconstructions have largely focused on individual catchments limiting the ability to quantify variability across the CONUS. The Living Blended Drought Atlas (LBDA), a spatially and temporally complete 555 year long paleoclimate record of summer drought across the CONUS, provides an opportunity to reconstruct and characterize streamflow variability at a continental scale. We explore the validity of the first paleoreconstructions of streamflow that span the CONUS informed by the LBDA targeting a set of U.S. Geological Survey streamflow sites. The reconstructions are skillful under cross validation across most of the country, but the variance explained is generally low. Spatial and temporal structures of streamflow variability are analyzed using hierarchical clustering, principal component analysis, and wavelet analyses. Nine spatially coherent clusters are identified. The reconstructions show signals of contemporary droughts such as the Dust Bowl (1930s) and 1950s droughts. Decadal-scale variability was detected in the late 1900s in the western U.S., however, similar modes of temporal variability were rarely present prior to the 1950s. The twentieth century featured longer wet spells and shorter dry spells compared with the preceding 450 years. Streamflows in the Pacific Northwest and Northeast are negatively correlated with the central U.S. suggesting the potential to mitigate some drought impacts by balancing economic activities and insurance pools across these regions during major droughts.

How wet and dry spells evolve across the conterminous United States based on 555 years of paleoclimate data

AbstractEvolving patterns of droughts and wet spells in the conterminous United States (CONUS) are examined over 555 years using a tree-ring-based paleoclimate reconstruction of the modified Palmer...