Five Hundred Years of Hydrological Drought in the Upper Colorado River Basin1

Abstract: This article evaluates drought scenarios of the Upper Colorado River basin (UCRB) considering multiple drought variables for the past 500 years and positions the current drought in terms of the magnitude and frequency. Drought characteristics were developed considering water‐year data of UCRB’s streamflow, and basin‐wide averages of the Palmer Hydrological Drought Index (PHDI) and the Palmer Z Index. Streamflow and drought indices were reconstructed for the last 500 years using a principal component regression model based on tree‐ring data. The reconstructed streamflow showed higher variability as compared with reconstructed PHDI and reconstructed Palmer Z Index. The magnitude and severity of all droughts were obtained for the last 500 years for historical and reconstructed drought variables and ranked accordingly. The frequency of the current drought was obtained by considering two different drought frequency statistical approaches and three different methods of determining the beginning and end of the drought period (annual, 5‐year moving, and ten year moving average). It was concluded that the current drought is the worst in the observed record period (1923‐2004), but 6th to 14th largest in terms of magnitude and 1st to 12th considering severity in the past 500 years. Similarly, the current drought has a return period ranging from 37 to 103 years based on how the drought period was determined. It was concluded that if the 10‐year moving average is used for defining the drought period, the current drought appears less severe in terms of magnitude and severity in the last 500 years compared with the results using 1‐ and 5‐year averages.     

Climate extremes and adaptive management on the Colorado River: lessons from the 1997-1998 ENSO event.

The Colorado River system exhibits the characteristics of a heavily over-allocated or 'closing water system'. In such systems, development of mechanisms to allow resource users to acknowledge interdependence and to engage in negotiations and agreements becomes necessary. Recently, after a decade of deliberations and environmental assessments, the Glen Canyon Dam Adaptive Management Program (GCDAMP) was established to monitor and analyze the effects of dam operations on the Grand Canyon ecosystem and recommend adjustments intended to preserve and enhance downstream physical, cultural and environmental values. The Glen Canyon Dam effectively separates the Colorado into its lower and upper basins. Dam operations and adaptive management decisions are strongly influenced by variations in regional climate. This paper focuses on the management of extreme climatic events within the Glen and Grand Canyon Region of the Colorado River. It illustrates how past events (both societal and physical) condition management flexibility and receptivity to new information. The types of climatic information and their appropriate entry points in the annual cycle of information gathering and decision-making (the 'hydro-climatic decision calendar') for dam operations and the adaptive management program are identified. The study then describes how the recently implemented program, lessons from past events, and new climate information on the Colorado River Basin, facilitated responses during the major El Ni?o-Southern Oscillation (ENSO) event of 1997-1998. Recommendations are made for engaging researchers and practitioners in the effective use of climatic information in similar settings where the decision stakes are complex and the system uncertainty is large.     

长三角区域一体化的环境治理效应评估——基于DID模型的实证研究

随着长三角一体化的范围不断扩大,融入区域一体化的城市不断增加,对区域城市环境协同治理提出了更高要求。本文采用2003—2019年中国225个地级以上城市面板数据,利用双重差分法评估长三角区域一体化对地区环境治理的影响和作用机制。研究发现长三角区域一体化能显著促进整体城市和原位城市的污染排放强度下降,但对新进城市的环境治理影响并不显著。长三角区域一体化对不同规模城市的环境治理效应没有显著差异;对非资源型城市和高行政等级城市的环境改善作用则优于资源型城市和低行政等级城市。进一步机制分析表明,长三角区域一体化带来污染排放强度整体下降,这一结果主要来自经济集聚效应和技术进步效应,来自产业结构升级的环境治理效应并不显著;同时,长三角区域一体化对环境治理具有显著的反向空间溢出效应,即在降低本地污染排放强度的同时,加大了周边城市污染排放强度。     

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

We 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.     

Land Use and Climate Variability Amplify Carbon,Nutrient, and Contaminant Pulses: A Review with Management Implications

Nonpoint source pollution from agriculture and urbanization is increasing globally at the same time climate extremes have increased in frequency and intensity. We review >200 studies of hydrologic and gaseous fluxes and show how the interaction between land use and climate variability alters magnitude and frequency of carbon, nutrient, and greenhouse gas pulses in watersheds. Agricultural and urban watersheds respond similarly to climate variability due to headwater alteration and loss of ecosystem services to buffer runoff and temperature changes. Organic carbon concentrations/exports increase and organic carbon quality changes with runoff. Nitrogen and phosphorus exports increase during floods (sometimes by an order of magnitude) and decrease during droughts. Relationships between annual runoff and nitrogen and phosphorus exports differ across land use. CH₄ and N₂O pulses in riparian zones/floodplains predominantly increase with: flooding, warming, low oxygen, nutrient enrichment, and organic carbon. CH₄, N₂O, and CO₂ pulses in streams/rivers increase due to similar factors but effects of floods are less known compared to base flow/droughts. Emerging questions include: (1) What factors influence lag times of contaminant pulses in response to extreme events? (2) What drives resistance/resilience to hydrologic and gaseous pulses? We conclude with eight recommendations for managing watershed pulses in response to interactive effects of land use and climate change.     

Climate Change Impacts and Uncertainties on Spring Flooding of Lake Champlain and the Richelieu River

The source of the Richelieu River is Lake Champlain, located between the states of New York, Vermont, and Québec. In 2011, the lake and the Richelieu River reached historical flood levels, raising questions about the influence of climate change on the watershed. The objectives of this work are to model the hydrology of the watershed, construct a reservoir model for the lake and to analyze flooding trends using climate simulations. The basin was modeled using the HSAMI lumped conceptual model from Hydro‐Québec with a semi‐distributed approach in order to estimate the inflows into Lake Champlain. The discharge at the Richelieu River was computed by using a mass balance equation between the inputs and outputs of Lake Champlain. Future trends were estimated over the 2041‐2070 and 2071‐2100 periods using a large number of outputs from general circulation models and regional climate models downscaled with constant scaling and daily translation methods. While there is a certain amount of uncertainty as to future trends, there is a decreasing tendency in the magnitude of the mean spring flood. A flood frequency analysis showed most climate projections indicate the severity of most extreme spring floods may be reduced over the two future periods although results are subject to a much larger uncertainty than for the mean spring flood. On the other hand, results indicate summer‐fall extreme events such as caused by hurricane Irene in August 2011 may become more frequent in the future.     

长三角区域环境联合执法机制完善研究

开展跨区域联合执法是解决长三角跨界环境问题的重要途径。长三角区域环境联合执法的实质在于打破传统属地管辖模式,在一定程度上实现行政管辖权的共享。目前,长三角区域在建立区域合作机构、开展环境合作活动、统一环境执法依据等方面已经进行了积极探索,但依然存在执法主体不规范、执法权限不充分、执法依据不统一等问题。长三角区域环境联合执法所采取的横向府际协同模式以及统一执法与联合执法相结合的模式具有一定优势,需要在此基础上加以改进。未来需要规范联合执法队伍的组建,通过区域合作和中央批准的方式建立联合执法队伍并对执法主体进行明确授权,充分发挥现有机构的监督协调作用,以及通过分区域、分行业、分时段的方式逐渐统一执法依据。     

Climate Change Impacts on Local Flood Risks in the U.S. Northeast: A Case Study on the Connecticut and Merrimack River Basins

This study investigates the potential impacts of climate change on future flows in the main stem of the Connecticut and Merrimack rivers within Massachusetts. The study applies two common climate projections based on (Representative Concentration Pathways), RCP 4.5 and RCP 8.5 and downscaled gridded climate projections from 14 global climate models (GCMs) to estimate the 100‐year, 24‐h extreme precipitation events for two future time‐periods: near‐term (2021–2060) and far‐term (2060–2099). 100‐year 24‐h precipitation events at near‐ and far‐term are compared to GCM‐driven historical extreme precipitation events during a base period (1960–1999) and results for RCP 8.5 scenario show average increases between 25%–50% during the near‐term compared to the base period and increases of over 50% during the far‐term. Streamflow conditions are generated with a distributed hydrological model where downscaled climate projections are used as inputs. For the near‐term, the medians of the GCMs using the RCP 4.5 and RCP 8.5 suggest 2.9%–8.1% increases in the 100‐year, 24‐h flow event in the Connecticut and an increase of 9.9%–13.7% in the Merrimack River. For the far‐term, the medians of the GCMs using the RCP 4.5 and RCP 8.5 suggest a 9.0%–14.1% increase in the Connecticut and 15.8%–20.6% for the Merrimack River. Ultimately, the results presented here can be used as a guidance for the long‐term management of infrastructures on the Connecticut and Merrimack River floodplains.     

SUWANNEE RIVER LONG RANGE STREAMFLOW FORECASTS BASED ON SEASONAL CLIMATE PREDICTORS1

ABSTRACT: A study of the influence of climate variability on streamflow in the southeastern United States is presented. Using a methodology previously applied to watersheds in Australia and the United States, a long range streamflow forecast (0 to 9 months in advance) is developed. Persistence (i.e., the previous season's streamflow) and climate predictors of the previous season are used to forecast the following season's (winter and spring) streamflow of the Suwannee River located in northern Florida. The winter and spring streamflow is historically the most likely to have severe flood events due to large scale cyclonic (frontal) storms. Results of the analysis indicated that a strong El Nino‐Southern Oscillation (ENSO) signal exists at various lead times to the winter and spring streamflow of the Suwannee River. These results are based on the high correlation values of two commonly used measurements of ENSO strength, the Multivariate ENSO Index (MEI) and Sea Surface Temperature Range 1. Using the relationships developed between climate and streamflow, a continuous exceedance probability forecast was developed for two Suwannee River stations. The forecast system provided an improved forecast for ENSO years. The ability to predict above normal (flood) or below normal (drought) years can provide communities the necessary lead time to protect life, property, sensitive wetlands, and endangered and threatened species.     

REGIONAL FREQUENCY ANALYSIS OF HYDROLOGIC MULTIYEAR DROUGHTS1

This paper is concerned with regional frequency analysis of hydrologic multiyear droughts. A drought event is defined by three parameters: severity, duration, and magnitude. A method is proposed here to standardize drought severities with a duration adjustment to enable comparison among drought events. For purposes of a regional study, the index drought method is selected and applied to standardized droughts to give a regional frequency curve. However, the recurrence intervals of the drought events obtained from index drought method are limited to the historic period of record. Therefore, by taking advantage of random variations of droughts in both time and space, a multivariate simulation model is used to estimate exceedence probabilities associated with regional drought maxima. This method, named the regional extreme drought method, is capable of generating a series of drought events which, although they have not occurred historically, are more severe than historic events. By combining the results of the index drought method and regional extreme drought analysis, a regional drought probability graph is constructed which ranges from severe droughts to more frequent droughts. This procedure is applied to the mean annual flow records of streams located in the San Joaquin Valley of California, and drought-severity-frequency plots are prepared for 1-year, 2-year, and 3-year durations.     

IMPLICATIONS OF CLIMATIC VARIABILITY FOR REGULATORY LOW FLOWS IN THE SOUTH PLATTE RIVER BASIN,COLORADO1

ABSTRACT: The maximum concentration of a regulated substance that is allowed in a wastewater effluent usually is determined from the amount of dilution provided by the receiving water. Dilution flow is estimated from historical data by application of statistical criteria that define low flow conditions for regulatory purposes. Such use of historical data implies that the past is a good indicator of future conditions, at least for the duration of a discharge permit. Short records, however, introduce great uncertainty in the estimation of low flows because they are unlikely to capture events with recurrence frequencies of multiple years (e.g., ENSO events or droughts). We conducted an analysis of daily flows at several gages with long records in the South Platte River basin of Colorado. Low flows were calculated for successive time blocks of data (3‐, 5‐, 10‐, and 20‐years), and these were compared with low flows calculated for the entire period of record (> 70 years). In unregulated streams, time blocks of three or five years produce estimates of low flows that are highly variable and consistently greater than estimates derived from a longer period of record. Estimates of low flow from 10‐year blocks, although more stable, differ from the long term estimates by as much as a factor of two because of climate variation. In addition, the hydrographs of most streams in Colorado have been influenced by dams, diversions, or water transfers. These alterations to the natural flow regime shorten the record that is useful for analysis, but also tend to increase the calculated low flows. The presence of an upward trend in low flows caused by water use represents an unanticipated risk because it fails to incorporate societal response to severe drought conditions. Thus, climate variability poses a significant risk for water quality both directly, because it may not be represented adequately in the short periods of the hydrologic record that are typically used in permits, and indirectly, through its potential to cause altered use of water during time of scarcity.     

区域环境司法协作的理论溯源与制度回应——以长三角地区为例

区域环境司法协作是区域环境资源审判一体化发展的重要面向。而区域环境司法协作的运行机理在于对生态环境整体主义的遵循、对司法理性所蕴含的价值目标的践行以及对协同理论的运用。目前,长三角地区在区域环境司法协作方面已经进行了积极探索,但仍面临着法治实践异化、制度生态化不足、保障机制缺乏等问题。本文建议区域环境司法协作应在把握司法权属性基础上恰当处理政策与法律的互动关系,加强协作制度体系对生态环境利益的考量和保护,并通过信息、人员与考核等保障机制充分激发协作主体的内生动力和活力。     

中国城市空气污染对人口流动的推拉效应研究

传统城市发展理论认为,空气污染会导致人口流出。然而,本文以城市雾霾数据为例,分类别构建固定效应模型后发现,空气污染会促进百强城市的人口净流入(形成拉力效应),非百强城市的人口净流出(形成推力效应)。拉力效应与传统的城市发展理论不符。本文认为,当城市间收入差距过大时,流入居民会形成高水平的收入预期,促使其愿意承受更严重的空气污染,该预期可量化为空气污染的拉力效应。据此进一步研究收入水平对推拉效应的调节作用发现,随着收入水平上升,其能放大百强城市的拉力效应和非百强城市的推力效应,即进一步促进百强城市的人口净流入和非百强城市的人口净流出。在此基础上,本文对百强城市和非百强城市分别提出政策建议,以期能为我国区域协调发展、城镇化科学转型提供理论支持。     

基于区域一体化背景下的长三角环境经济政策优化研究

基于长三角区域社会、经济、环保一体化高质量发展要求,全面调研"三省一市"(上海、江苏、浙江、安徽)2011—2017年环境经济政策出台情况,并开展评估分析。结果显示:长三角区域环境经济政策整体呈现良好发展态势,尤其在财政奖补方面占绝对优势,但各省市差异较为明显,且区域层面联动政策相对欠缺。在此基础上,结合长三角区域环境质量改善目标及污染防治协作深化需求,建议在环保基金、生态补偿、排污权交易、环境信用体系等领域率先开展区域联动试点,并从机制、法律、技术等层面给予保障,探索形成区域环境经济政策共商、共享、共赢模式。     

HISTORICAL EVOLUTION OF FLOODING DAMAGE ON A USA/QUEBEC RIVER BASIN1

ABSTRACT: There is a general belief in the public eye that extreme events such as floods are becoming more and more common. This paper explores this hypothesis by examining the historical evolution of annual expected flooding damage on the Chateauguay River Basin, located at the border between the United States and the province of Quebec, Canada. A database of basin land use was constructed for the years 1930 and 1995 to assess anthropogenic changes and their impact on the basin's hydrology. The progressive modification of the likelihood of a flooding event over the same period was then investigated using homogeneity and statistical tests on available hydrometric data. The evolution of the annual expected flooding damage was then evaluated using a coupled hydrologic/hydraulic simulator linked to a damage analysis model. The simulator and model were used to estimate flooding damage over a wide range of flooding return periods, for conditions prevailing in 1963 and 1995. Results of the analysis reveal the absence of any increasing or decreasing trend in the historical occurrence of flooding events. However, a general increase in the annual expected flooding damage was observed for all studied river sections. This increase is linked to an historical increase in damages for a given flooding event, and is the result of unbridled construction and development within the flood zone. To assess for future trends, this study also examined the potential impacts linked to the anticipated global warming. Results indicate that a significant increase in seasonal flooding events and annual expected flooding damage is possible over the next century. In fact, what is now considered a 100‐year flooding event for the summer/fall season could become a ten‐year event by the end of this century. This shows that potential future impacts linked to climate change should be considered now by engineers, land planners, and decision makers. This is especially critical if a design return period is part of the decision making process.     

VARIATION IN THE RELATIONSHIP BETWEEN SNOWMELT RUNOFF IN OREGON AND ENSO AND PDO1

ABSTRACT: The value of using climate indices such as ENSO or PDO in water resources predictions is dependent on understanding the local relationship between these indices and streamflow over time. This study identifies long term seasonal and spatial variations in the strength of El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) correlations with timing and magnitude of discharge in snowmelt streams in Oregon. ENSO is best correlated with variability in annual discharge, and PDO is best correlated with spring snowmelt timing and magnitude and timing of annual floods. Streams in the Cascades and Wallowa mountains show the strongest correlations, while the southernmost stream is not correlated with ENSO or PDO. ENSO correlations are weaker from 1920 to 1950 and vary significantly depending on whether Southern Oscillation Index (SOI) or Niño 3.4 is used. PDO correlations are strong from 1920 to 1950 and weak or insignificant other years. Although there are not consistent increasing or decreasing trends in annual discharge or spring snowmelt timing, there are significant increases in fractional winter runoff that are independent of precipitation, PDO, or ENSO and may indicate monotonic winter warming.     

COLUMBIA RIVER FLOW AND DROUGHT SINCE 17501

ABSTRACT: A network of 32 drought sensitive tree‐ring chronologies is used to reconstruct mean water year flow on the Columbia River at The Dalles, Oregon, since 1750. The reconstruction explains 30 percent of the variability in mean water year (October to September) flow, with a large portion of unexplained variance caused by underestimates of the most severe low flow events. Residual statistics from the tree‐ring reconstruction, as well as an identically specified instrumental reconstruction, exhibit positive trends over time. This finding suggests that the relationship between drought and streamflow has changed over time, supporting results from hydrologic models, which suggest that changes in land cover over the 20th Century have had measurable impacts on runoff production. Low pass filtering the flow record suggests that persistent low flows during the 1840s were probably the most severe of the past 250 years, but that flows during the 1930s were nearly as extreme. The period from 1950 to 1987 is anomalous in the context of this record for having no notable multiyear drought events. A comparison of the flow reconstruction to paleorecords of the Pacific Decadal Oscillation (PDO) and El Nino/Southern Oscillation (ENSO) support a strong 20th Century link between large scale circulation and streamflow, but suggests that this link is very weak prior to 1900.     

A Long View of Southern California Water Supply: Perfect Droughts Revisited

The impact of drought on water resources in arid and semiarid regions can be buffered by water supplies from different source regions. Simultaneous drought in all major source regions — or perfect drought — poses the most serious challenge to water management. We examine perfect droughts relevant to Southern California (SoCal) water resources with instrumental records and tree‐ring reconstructions for the Sacramento and Colorado Rivers, and SoCal. Perfect droughts have occurred five times since 1906, lasting two to three years, except for the most recent event, 2012–2015. This number and duration of perfect droughts is not unusual in the context of the past six centuries. The modern period stands out for the relatively even distribution of perfect droughts and lacks the clusters of perfect drought documented in prior centuries. In comparison, perfect droughts of the 12th Century were both longer (up to nine years) and more widespread. Perfect droughts of the 20th and 21st Centuries have occurred under different oceanic/atmospheric patterns, zonal and meridional flow, and ENSO or non‐ENSO conditions. Multidecadal coherence across the three regions exists, but it has varied over the past six centuries, resulting in irregular intervals of perfect drought. Although the causes of perfect droughts are not clear, given the long‐term natural variability along with projected changes in climate, it is reasonable to expect more frequent and longer perfect droughts in the future.     

Colorado River Floods, Droughts, and Shrimp Fishing in the Upper Gulf of California, Mexico

Accurate procedures that measure hydrologic variability would have great value for evaluating ecosystem impacts of upstream water use in the Colorado River Basin. Many local extractive income-based stakeholders rely directly or indirectly on ecosystem health and are adversely affected when the river does not flow. This study focuses on the impact of little or no Colorado River flow on the Mexican shrimp industry. Although there have been complaints that U.S. diversions of Colorado River flow have greatly impaired the shrimp fishery, this research demonstrates that freshwater rarely reaches the Gulf even during times of flooding, and that other factors such as overfishing may influence the instability of shrimp populations. Advanced very-high-resolution radiometer (AVHRR) satellite imagery was used to assess water volumes diverted away from the channel of the Colorado River and ultimately the Gulf of California during flooding periods. Analysis of data demonstrated that little freshwater actually reaches the Gulf even during floods because of its diversion into a large dry lake bed basin known as Laguna Salada. Fuller use of the Colorado River throughout its entire course to the sea is possible and could benefit a large cohort of users without catastrophic habitat destruction in delta ecosystems. Reconstruction of a natural earthen berm, as proposed by Ducks Unlimited, would maximize the use of floodwaters for ecosystem benefits. These findings have profound implications for local economic activities dependent on hydrologic resources in the Colorado River Delta and Upper Gulf.     

Gauging the Vulnerability of Local Water Systems to Extreme Events

Water utilities that rely on surface water may be vulnerable to future droughts and floods, a vulnerability that may be magnified by climate perturbations as well as shorter-term and, in some cases, ongoing changes in the political and regulatory environment in which utilities operate. Unfortunately, day-to-day responsibilities currently occupy most utility operators, leaving little time to plan for inherently uncertain effects. The record of actual responses to past droughts and floods can be illuminating, however, particularly when placed in the context of plausible hydrologic disruption and pressures such as population growth, floodplain development and new regulatory demands. This paper draws on interviews with water utility operators in the northwestern USA to highlight opportunities and constraints that water utilities may face vis-à-vis such disruptions. Key considerations affecting vulnerabilities include water rights, institutional barriers to efficient utility operations, hazard management policy and the fiscal status of utilities.