Severe Sustained Drought Research Articles

Featured Collection introduction: Severe sustained drought revisited managing the Colorado River system in times of water shortage 25 years later—Part II

Featured Collection introduction: Severe sustained drought revisited managing the Colorado River system in times of water shortage 25 years later—Part II

Featured Collection Introduction: Severe Sustained Drought Revisited: Managing the Colorado River System in Times of Water Shortage 25 Years Later — Part I

Featured Collection Introduction: Severe Sustained Drought Revisited: Managing the Colorado River System in Times of Water Shortage 25 Years Later — Part I

An Assessment of Potential Severe Droughts in the Colorado River Basin

ABSTRACTMuch has been learned about Colorado River hydrology since the severe sustained drought study in 1995. We summarize our updated understanding of plausible future drought conditions by considering historical flows, tree‐ring reconstructions, and climate change. We focus on natural streamflow at Lees Ferry, the primary metric used to quantify the runoff in the Colorado River Basin. We identify drought periods using historical records and tree‐ring reconstructed streamflow at Lees Ferry, which we then use to characterize potential future droughts. Resampling from past drought periods generates plausible future conditions to consider during planning. We produced three drought scenarios, each comprising 100 streamflow sequences to be used as input to systems operation and management models. We used analysis of the duration‐severity and cumulative deficit relative to the mean natural flow to evaluate droughts and drought simulations and show that the current millennium drought that started in 2000 has an average flow far less than the historical record. However, the flows reconstructed from tree rings or future flows projected from climate models indicate that even more severe droughts are possible. When used as input to the Colorado River Simulation System the drought scenarios developed indicate considerable periods when Lake Powell falls below its hydropower penstocks, indicating a need to rethink management and operation of these reservoirs during these critical conditions.

Water Storage Decisions and Consumptive Use May Constrain Ecosystem Management under Severe Sustained Drought

AbstractDrought has impacted the Colorado River basin for the past 20 years and is predicted to continue. In response, decisions about how much water should be stored in large reservoirs and how much water can be consumptively used will be necessary. These decisions have the potential to limit riverine ecosystem management options through the effect water‐supply decisions have on reservoir elevations. We used projected hydrology and river temperatures to compare the outcome of combinations of water storage scenarios and consumptive use limits on metrics associated with ecosystem management of the Colorado River in Grand Canyon. Ecosystem management metrics included the ability to implement designer flows, temperature suitability for fishes, and fragmentation. We compared current water management operations to prioritizing storage in either Lake Mead or Lake Powell combined with three levels of consumptive use. Projected reservoir levels limited environmental flow delivery and increased fragmentation regardless of where water was stored if consumptive use was not limited. Warmer river temperatures associated with low reservoir levels are likely, creating suitable conditions for non‐native species of concern, such as smallmouth bass. Water storage decisions provided variability and management flexibility, but water storage was less important when less water was available, highlighting the importance of keeping water in the system to provide flexibility for achieving ecosystem goals.

Tree‐Ring Perspectives on the Colorado River: Looking Back and Moving Forward

AbstractTree rings have been central to the understanding of variability of flow of the Colorado River. Spurred by steadily declining flows after the 1920s, early tree‐ring research drew attention to the importance of climate variability to water supply by identifying episodes in the past that were even drier. Application of modern statistical methods to tree‐ring data later yielded a reconstruction of annual flows at Lees Ferry back to the early 1500s that highlighted the unprecedented wetness of the base period for the 1922 Colorado River Compact. That reconstruction served as the framework for a collection of papers in a 1995 special issue of Water Resources Bulletin on coping with severe sustained drought on the Colorado River. This retrospective paper reviews historical aspects of the dendrohydrology of the Colorado River, and the updates since 1995. A constantly expanding tree‐ring network has been subjected to an array of new statistical approaches to reconstruction. Climate change and increasing demand for water have meanwhile driven increased interest in the processing and presentation of reconstructions for optimal use in water resources planning and management. While highlighting the robustness of main findings of earlier studies, recent research yields improved estimates of magnitudes of flow anomalies, extends annual flows to more than 1200 years, and underscores unmatched drought duration in the medieval period.

The Mexican Drought Atlas: Tree-ring reconstructions of the soil moisture balance during the late pre-Hispanic, colonial, and modern eras

Abstract Mexico has suffered a long history and prehistory of severe sustained drought. Drought over Mexico is modulated by ocean-atmospheric variability in the Atlantic and Pacific, raising the possibility for long-range seasonal climate forecasting, which could help mediate the economic and social impacts of future dry spells. The instrumental record of Mexican climate is very limited before 1920, but tree-ring chronologies developed from old-growth forests in Mexico can provide an excellent proxy representation of the spatial pattern and intensity of past moisture regimes useful for the analysis of climate dynamics and climate impacts. The Mexican Drought Atlas (MXDA) has been developed from an extensive network of 252 climate sensitive tree-ring chronologies in and near Mexico. The MXDA reconstructions extend from 1400 CE–2012 and were calibrated with the instrumental summer (JJA) self-calibrating Palmer Drought Severity Index (scPDSI) on a 0.5° latitude/longitude grid extending over land areas from 14 to 34°N and 75–120°W using Ensemble Point-by-Point Regression (EPPR) for the 1944–1984 period. The grid point reconstructions were validated for the period 1920–1943 against instrumental gridded scPDSI values based on the fewer weather station observations available during that interval. The MXDA provides a new spatial perspective on the historical impacts of moisture extremes over Mexico during the past 600-years, including the Aztec Drought of One Rabbit in 1454, the drought of El Ano de Hambre in 1785–1786, and the drought that preceded the Mexican Revolution of 1909–1910. The El Nino/Southern Oscillation (ENSO) is the most important ocean-atmospheric forcing of moisture variability detected with the MXDA. In fact, the reconstructions suggest that the strongest central equatorial Pacific sea surface temperature (SST) teleconnection to the soil moisture balance over North America may reside in northern Mexico. This ENSO signal has stronger and more time-stable correlations than computed for either the Atlantic Multidecadal Oscillation or Pacific Decadal Oscillation. The extended Multivariate ENSO Index is most highly correlated with reconstructed scPDSI over northern Mexico, where warm events favor moist conditions during the winter, spring, and early summer. This ENSO teleconnection to northern Mexico has been strong over the past 150 years, but it has been comparatively weak and non-stationary in the MXDA over central and southern Mexico where eastern tropical Pacific and Caribbean/tropical Atlantic SSTs seem to be more important. The ENSO teleconnection to northern Mexico is weaker in the available instrumental PDSI, but analyses based on the millennium climate simulations with the Community Earth System Model suggest that the moisture balance during the winter, spring, and early summer over northern Mexico may indeed be particularly sensitive to ENSO forcing. Nationwide drought is predicted to become more common with anthropogenic climate change, but the MXDA reconstructions indicate that intense “All Mexico” droughts have been rare over the past 600 years and their frequency does not appear to have increased substantially in recent decades.

Farmers' pro-environmental behavior under drought: Application of protection motivation theory

Recurrent drought in arid and semi-arid regions poses serious challenges for populations whose livelihoods depend principally on natural resources. In order to mitigate the negative impacts of drought, adoption of pro-environmental measures is imperative. While there is an increasing pressure on farmers to support environmental conservation practices, factors influencing their pro-environmental behavior remain poorly understood. To fill this gap, protection motivation theory (PMT) was used as a basis for identification of the main determinants of the farmers' pro-environmental behavior under drought. A survey of 274 farmers in the Fars Province, selected through a multistage stratified random sampling method, revealed that some farmers postponed the protection measures when facing severe sustained drought. In addition, a low tendency of adopting strategies that are appropriate for reducing the environmental impacts of drought was observed. Analysis of the Bayesian network and partial least squares (PLS) path model illustrated that response efficacy, perceived severity, response costs, perceived vulnerability, self-efficacy, income, and social environment significantly influenced the farmers' pro-environmental behavior under drought. Recommendations and implications for the promotion of pro-environmental behaviors are offered to reduce the negative environmental impacts of drought.

Tree-ring reconstructed rainfall variability in Zimbabwe

We present the first tree-ring reconstruction of rainfall in tropical Africa using a 200-year regional chronology based on samples of Pterocarpus angolensis from Zimbabwe. The regional chronology is significantly correlated with summer rainfall (November–February) from 1901 to 1948, and the derived reconstruction explains 46% of the instrumental rainfall variance during this period. The reconstruction is well correlated with indices of the El Nino-southern oscillation (ENSO), and national maize yields. An aridity trend in instrumental rainfall beginning in about 1960 is partially reproduced in the reconstruction, and similar trends are evident in the nineteenth century. A decadal-scale drought reconstructed from 1882 to 1896 matches the most severe sustained drought during the instrumental period (1989–1995), and is confirmed in part by documentary evidence. An even more severe drought is indicated from 1859 to 1868 in both the tree-ring and documentary data, but its true magnitude is uncertain. A 6-year wet period at the turn of the nineteenth century (1897–1902) exceeds any wet episode during the instrumental era. The reconstruction exhibits spectral power at ENSO, decadal and multi-decadal frequencies. Composite analysis of global sea surface temperature during unusually wet and dry years also suggests a linkage between reconstructed rainfall and ENSO.

SOCIAL IMPLICATIONS OF SEVERE SUSTAINED DROUGHT CASE STUDIES IN CALIFORNIA AND COLORADO1

ABSTRACT: Survey data collected in the San Joaquin Valley of southern California and the Grand Valley of western Colorado reveal that residents of both areas believe that a severe sustained drought is likely to occur within the next 20–25 years and that their communities would be seriously impacted by such an event. Although a severe sustained drought affecting the Colorado River Basin would cause major economic and social disruptions in these and other communities, residents express little support for water management alternatives that would require significant shifts in economic development activities or in water use and allocation patterns. In particular, residents of these areas express little support for strategies such as construction and growth moratoriums, mandatory water conservation programs, water transfers from low‐to high‐population areas, water marketing, or reallocations of water from agricultural to municipal/industrial uses. This rejection of water management strategies that would require a departure from “business as usual” with respect to water use and allocations severely restricts the capacity of these and similar communities to respond effectively should a severe sustained drought occur.

COPING WITH A SEVERE SUSTAINED DROUGHT ON THE COLORADO RWER: INTRODUCTION AND OVERVIEW1

ABSTRACT: In arid regions of rapid economic and population growth, adverse effects of droughts are likely to be increasingly serious. This article presents an introduction and overview of the papers collected in this special issue of the Water Resources Bulletin. The papers report on the second phase of a study of the impacts of and responses to a potential severe sustained drought in the Colorado River Basin in the southwestern U.S. The analyses were performed by a consortium of researchers from universities and the private sector located throughout the Basin. Tree ring studies suggest that droughts of duration and magnitude much more serious than any found in the modern records probably occurred in the Basin during earlier centuries. Taking the present‐day configuration of the storage and diversion structures and the economic conditions in the Basin as the base‐point, the general objectives of the study are three: first, to define a representative Severe Sustained Drought (SSD) and assess its hydrologic impacts; second, to forecast the economic, social and environmental impacts on the southwestern U.S.; and finally, to assess alternative institutional arrangements for coping with an SSD. The evaluation of impacts and policies was conducted with two distinct modeling approaches. One involved hydrologic‐economic optimization modeling where water allocation institutions are decision variables. The second was a simulation‐gaming approach which allowed “players” representing each basin state to interact in a real‐time decision making mode in response to the unfolding drought.

MITIGATING IMPACTS OF A SEVERE SUSTAINED DROUGHT ON COLORADO RWER WATER RESOURCES1

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

HYDROLOGIC AND ECONOMIC IMPACTS OF DROUGHT UNDER ALTERNATWE POLICY RESPONSES1

ABSTRACT: A severe sustained drought in the Colorado River Basin would cause economic damages throughout the Basin. An integrated hydrologic‐economic‐institutional model introduced here shows that consumptive water users in headwaters states are particularly vulnerable to very large shortfalls and hence large damages because their rights are effectively junior to downstream users. Chronic shortfalls to consumptive users relying on diversions in excess of rights under the Colorado River Compact are also possible. Nonconsumptive water uses (for hydropower and recreation) are severely affected during the worst drought years as instream flows are reduced and reservoirs are depleted. Damages to these uses exceeds those to consumptive uses, with the value of lost hydropower production the single largest economic impact of a severe sustained drought. Modeling of alternative policy responses to drought suggests three general policy approaches with particular promise for reducing damages. Consumptive use damages can be reduced by over 90 percent through reallocation from low to high valued uses and through reservoir storage strategies which minimize evaporation losses. Reservoir management to preserve minimum power pool levels for hydropower production (and to maintain reservoir recreation) may reduce damages to these nonconsumptive uses by over 30 percent, but it may increase consumptive use shortfalls.

INSTITUTIONAL OPTIONS FOR THE COLORADO RIVER1

ABSTRACT: In many interstate river basins, the institutional arrangements for the governance and management of the shared water resource are not adequately designed to effectively address the many political, legal, social, and economic issues that arise when the demands on the resource exceed the available supplies. Even under normal hydrologic conditions, this problem is frequently seen in the Colorado River Basin. During severe sustained drought, it is likely that the deficiencies of the existing arrangements would present a formidable barrier to an effective drought response, interfering with efforts to quickly and efficiently conserve and reallocate available supplies to support a variety of critical needs. In the United States, several types of regional arrangements are seen for the administration of interstate water resources. These arrangements include compact commissions, interstate councils, basin interagency committees, interagency‐interstate commissions, federal‐interstate compact commissions, federal regional agencies, and the single federal administrator. Of these options, the federal‐interstate compact commission is the most appropriate arrangement for correcting the current deficiencies of the Colorado River institution, under all hydrologic conditions.

THE LAW OF THE COLORADO RIVER: COPING WITH SEVERE SUSTAINED DROUGHT1

ABSTRACT: The waters of the Colorado River are divided among seven states according to a complex ‘Law of the River’ drawn from interstate compacts, international treaties, statutes, and regulations. The Law of the River creates certain priorities among the states and the Republic of Mexico, and in the event of a severe sustained drought, the Law of the River dictates the distribution of water and operation of the elaborate reservoir system. Earlier work indicated that there is remarkable resilience in the system for established uses of water in the Lower Basin of the Colorado River. This work shows, based on an application of the Law of the River using computer modeling of operations of facilities on the Colorado River, that there may be serious environmental consequences and related legal restraints on how the water is used in times of shortage and that the existing legal and institutional framework governing the Colorado River does not adequately address all the issues that would be raised in a severe sustained drought. Several possible legal options for dealing with drought in the context of the Law of the River are identified.

IMPACTS OF A SEVERE SUSTAINED DROUGHT ON COLORADO RWER WATER RESOURCES1

ABSTRACT: The impacts of a severe sustained drought on Colorado River system water resources were investigated by simulating the physical and institutional constraints within the Colorado River Basin and testing the response of the system to different hydrologic scenarios. Simulations using Hydrosphere's Colorado River Model compared a 38‐year severe sustained drought derived from 500 years of reconstructed streamflows for the Colorado River basin with a 38‐year streamflow trace extracted from the recent historic record. The impacts of the severe drought on streamflows, water allocation, storage, hydropower generation, and salinity were assessed. Estimated deliveries to consumptive uses in the Upper Basin states of Colorado, Utah, Wyoming, New Mexico, and northern Arizona were heavily affected by the severe drought, while the Lower Basin states of California, Nevada, and Arizona suffered only slight shortages. Upper Basin reservoirs and streamflows were also more heavily affected than those in the Lower Basin by the severe drought. System‐wide, total hydropower generation was 84 percent less in the drought scenario than in the historical stream‐flow scenario. Annual, flow‐weighted salinity below Lake Mead exceeded 1200 ppm for six years during the deepest portion of the severe drought. The salinity levels in the historical hydrology scenario never exceeded 1100 ppm.

HYDROLOGIC SCENARIOS FOR SEVERE SUSTAINED DROUGHT IN THE SOUTHWESTERN UNITED STATES1

ABSTRACT: This paper considers the risk of drought and develops drought scenarios for use in the study of severe sustained drought in the Southwestern United States. The focus is on the Colorado River Basin and regions to which Colorado River water is exported, especially southern California, which depends on water from the Colorado River. Drought scenarios are developed using estimates of unimpaired historic streamflow as well as reconstructions of streamflow based on tree ring widths. Drought scenarios in the Colorado River Basin are defined on the basis of annual flow at Lees Ferry. The risk, in terms of return period, of the drought scenarios developed, is assessed using stochastic models.

ASSESSING ENVIRONMENTAL EFFECTS OF SEVERE SUSTAINED DROUGHT1

ABSTRACT: Evaluation criteria for reservoir and stream resources were developed to provide decision makers with feedback on environmental consequences of water allocation decisions under conditions of severe sustained drought within the Colorado River Basin by using the AZCOL gaming simulation model. Seven categories of flow dependent resources were identified which highlight resource states associated with reservoirs or river reaches within the AZCOL model. AZCOL directly simulates impact of water management decisions on five resource categories: threatened, endangered or sensitive fish; native nonlisted fish; wetland and riparian elements; national or state wildlife refuges; and hatcheries or other flow dependent facilities. Two additional categories ‐ cold and warm water sport fish ‐ are not modeled explicitly but are incorporated in the evaluation of monetary benefits from recreation on Colorado River waters. Each resource category was characterized at each time step in the simulation according to one of four environmental states: stable, threatened, endangered, or extirpated. Changes in resource states were modeled by time and flow‐dependent decision criteria tied to either reservoir level or stream flows within the AZCOL model structure. Gaming results using the AZCOL model indicate environmental impacts would be substantial and that water allocation decisions directly impacted environmental resource states.

THE TREE‐RING RECORD OF SEVERE SUSTAINED DROUGHT1

ABSTRACT: 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.

The source hydrology of severe sustained drought in the southwestern United States

This paper considers the risk of drought and develops drought scenarios for use in the study of severe sustained drought in the southwestern United States. The focus is on the Colorado River basin and regions to which Colorado River water is exported, especially southern California, which depends on water from the Colorado River as well as the four major rivers in northern California. Drought scenarios are developed using estimates of unimpaired historic streamflow as well as reconstructions of streamflow based on tree ring widths. Drought scenarios in the Colorado River are defined on the basis of annual flow at Lees Ferry. Possible spatial manifestations of the Colorado River drought scenarios for input into a Colorado River system simulation model are developed by disaggregating the Lees Ferry flow to monthly flows at 29 source locations required by the model. The risk, in terms of return period, of the drought scenarios developed, is assessed using stochastic models applied to both the Colorado River basin and the combined flow in four major California rivers. The risk of severe sustained drought occurring concurrently in the Colorado River basin and California is also assessed.

A 1000-year record of winter precipitation from northwestern New Mexico, USA: a reconstruction from tree-rings and its relation to El Niño and the Southern Oscillation

A 1000-year reconstruction of winter (cool season) November-May precipitation for northern New Mexico is developed for AD 985-1970 based on six millenium-long tree-ring records from moisture-sensitive coniferous sites in New Mexico and southern Colorado. Two of the most prolonged and severe droughts occurred during the last 100 years: in the AD 1890s-1904 and the AD 1950s-early 1960s. These were exceeded in length and severity only once in the 1000-year reconstructed record, in the late 1500s, and almost equaled in the early 1200s. The five decadal-scale intervals of greatest drought were, in descending order, AD 1577-1598, AD 1955-64, AD 1895-1904, AD 1217-1226, and AD 1778- 1787. The wettest periods were AD 1835-49, AD 1905-28, AD 1429-1440, AD 1609-23, and AD 1487- 98. The most abrupt shift from severe sustained drought to extremely high precipitation occurred in the 1890s to early 1900s. The intensity of this shift is unprecedented over the 1000-year period of the reconstruction. Relatively dry conditions are indicated for part of the 1400-1600s period around the onset of the Little Ice Age, and the reconstruction indicates average conditions during the Medieval Warm Period. The southwestern US is an area of known teleconnection with the El Nine-Southern Oscillation (ENSO), coinciding with a tendency for wetter conditions to occur during ENSO years (warmer events) and drier conditions during colder events. The chronologies and reconstruction show correspondence with indices of ENSO, including Wright's Southern Oscillation Index (SOI). The years AD 1720, 1484, 1816 and 1941 (interpreted as ENSO-related events in instrumental or historical data) are the four wettest individual years in the reconstruction. Variance spectral analysis of the chronologies and reconstruction reveals significant peaks at approximately 2-7 year, 22 year and 80-100 year. There is potential for modelling the amplitude and frequency of the ENSO signature in the southwestern US over the past 1000 years or more, particularly by integrating tree-ring data from this region with other sources of high-resolution palaeoclimatic information.