Multi-year Drought Research Articles

Groundwater–surface water interaction and the impact of a multi-year drought on lakes conditions in South-East Australia

South-East Australia is currently experiencing a severe multi-year drought (not, vert, similar11 years of below long-term average rainfall between 1997 and 2008). To refine our understanding of the impacts of such climate stresses on lake systems, and to overcome the limitations of insufficient field based measurements, lake water levels and lake salinity values are combined with remote sensing data (Landsat-5TM) to analyse water and solute budgets for two large lakes in the Corangamite catchment. For these two lakes, data during pre-drought (1992–1996) and drought (1997–2006) periods are used to investigate the impact of the drought on lake and groundwater interaction and possible changes in mechanisms controlling the salinity of the lakes. For 26 other lakes in the Corangamite catchment with limited or no on-ground data, water and evaporite mapping using remote sensing data (Landsat-5TM) were used as proxies for drought impacts on lake quantity and salinity. Within the study area, this drought has resulted in a 60% loss of lakes area inundated (220 × 106 m2), and for the two largest lakes the water loss equates to an 80% decrease in volume (714 × 106 m3). Although the lakes are groundwater-fed, the rapid declines in lake water levels have not resulted in systematic increases in groundwater discharge to lakes across the study region. The lakes that are most sensitive to changes in the hydrologic budget within the catchment (10 of the 28 monitored lakes) have changed from groundwater throughflow or discharge lakes to intermittent recharge lakes. The decrease in lake levels is accompanied by an increase in lake salinity, however the dominant controls on increases in salinity varies for different lakes between just evaporation, and evaporation plus episodes of increased net groundwater discharge. The drought has exacerbated pre-existing high lake salinity levels; by 2006 one quarter of the lakes have evaporites where previously (before 1997) these lakes were evaporite free. These observations on changes in lakes quantity, interaction with groundwater, and salinity highlights the spatial variability in the lakes response to regional drought impacts.

Effect of extreme rainfall events on the water resources of the Jordan River

As a response to climate change, shifting rainfall trends including increased multi-year droughts and an escalation in extreme rainfall events are expected in the Middle East. The purpose of this study is to evaluate the potential impact of these shifting trends on stream flow in the Jordan River and its tributaries. We use a non-homogeneous hidden Markov model to generate artificial daily rainfall simulations which capture independently shifting trends of increased droughts and escalated extreme. These simulations are then used as input into a hydrological model calibrated for the upper catchments of the Jordan River to compare the impact on stream flow and water resources between the different rainfall scenarios. We compare the predicted baseflow and surface flow components of the tested watersheds, and find that while an increase in extreme rainfall events increases the intensity and frequency of surface flow, the over all flow to the Jordan River, and the characteristics of the baseflow in the Jordan River system is not largely impacted. In addition, though it has been suggested that in the case of a multi-year drought the karstic nature of the aquifer might lead to more intense, non-linear reductions in stream flow, here we quantify and show the conditions when annual stream flow reduce linearly with rainfall, and when these relations will become non-linear.

Basin‐scale, integrated observations of the early 21st century multiyear drought in southeast Australia

The Murray‐Darling Basin in southeast Australia is experiencing one of the most severe droughts observed recently in the world, driven by several years of rainfall deficits and record high temperatures. This paper provides new basin‐scale observations of the multiyear drought, integrated to a degree rarely achieved on such a large scale, to assess the response of water resources and the severity of the drought. A combination of Gravity Recovery and Climate Experiment (GRACE) data with in situ and modeled hydrological data shows the propagation of the water deficit through the hydrological cycle and the rise of different types of drought. Our observations show the rapid drying of soil moisture and surface water storages, which reached near‐stationary low levels only ∼2 years after the onset of the drought in 2001, with a loss of ∼80 and ∼12 km3 between January 2001 and January 2003, respectively. The multiyear drought has led to the almost complete drying of surface water resources which account for most of the water used for irrigation and domestic purposes. High correlation between observed groundwater variations and GRACE data substantiates the persistent reduction in groundwater storage, with groundwater levels still declining 6 years after the onset of the drought (groundwater loss of ∼104 km3 between 2001 and 2007). The hydrological drought continues even though the region returned to average annual rainfall during 2007.

Climatic Signatures in Yucatecan Wills and Death Records

Dense collections of eighteenth-century wills and death registers from Tekanto and Ixil, two towns in northern Yucatan, represent hitherto unexplored sources for documenting the relationship between natural disasters and mortality patterns among the Yucatecan Maya during colonial times. They provide detailed, sometimes daily, records of the impact of famines caused by multiyear droughts, hurricanes, and plagues of locusts on the agrarian population of the peninsula, which supplement the brief, impressionistic accounts of historians.

Topographic position modulates the mycorrhizal response of oak trees to interannual rainfall variability

California coast live oak (Quercus agrifolia) forms tripartite symbiotic associations with arbuscular (AMF) and ectomycorrhizal (EMF) fungi. We selected oak individuals differing in topographic position and depth to groundwater (mesic valley vs. xeric hill sites) to investigate changes of tree mycorrhizal status in response to interannual rainfall variability. EMF root colonization, as well as hyphal abundance and viability in upper rhizosphere soil (0-30 cm), were negatively affected by severe multi-year drought, although not to the same extent in each topographic location. Oak trees growing in hill sites showed EMF colonization levels <1% in upper roots during drought. By contrast, oaks in valley sites maintained much higher EMF colonization (>19%) in upper roots during drought. EMF root colonization increased sharply at both topographic positions during the ensuing wet year (78% in valley, 49% in hill), which indicates that the mycorrhizal status of roots in upper rhizosphere soil is highly responsive to interannual rainfall variability. Across sites and years, percentage EMF colonization and soil hyphal density and viability were strongly positively correlated with soil moisture potential, but percentage AMF root colonization was not. Interestingly, changes in percentage EMF root colonization and density of viable hyphae between a wet and a dry year were proportionally much greater in xeric hill sites than in mesic valley sites. The mycorrhizal status of oak trees was particularly responsive to changes in soil moisture at the hill sites, where roots in upper rhizosphere soil shifted from almost exclusively AMF during severe drought to predominantly EMF during the ensuing wet year. By contrast, the mycorrhizal status of oaks in the valley sites was less strongly coupled to current meteorological conditions, as roots in upper soil layers remained predominantly EMF during both a dry and a wet year. Canopy shading and hydraulic lift by oaks in valley sites likely contributed to maintain the integrity and viability of EMF roots and extraradical hyphae in upper rhizosphere soil during extended drought. Our results suggest that oak woodlands in water-limited ecosystems may become increasingly reliant on the AMF symbiosis under future climate change scenarios for the U.S. southwest and other world regions.

Droughts and drought mitigation in water resources planning

Many parts of Australia have experienced their worst single- and multi-year droughts on record over the last decade. These recent climate conditions have severely stressed our water supply systems and the communities that depend on them. With the threat of climate change potentially further exacerbating droughts in the years ahead, Engineers Australia firmly believes that: (i) A national review of the adequacy of drought response plans is urgently required. A recent national overview of long-term water resources planning by the Academy of Technological Sciences and Engineering (ATSE, 2007) revealed large inconsistencies and inadequacies in the standard of that planning. Based on the knowledge of the authors of this paper, similar outcomes are expected in the area of drought response planning. The scope of the review should include assessment of the transparency and defensibility of the timing of implementation of drought response actions; (ii) Continuing investment is needed in climate research to better understand the underlying drivers of our climate variability and their interactions at a range of time scales. The majority of research to date has focused on short-term sea surface temperature or pressure differentials across the globe (eg. the El Niño/Southern Oscillation) without understanding the longer-term drivers behind these temperature and pressure responses, which are being investigated through the Indian Ocean Climate Initiative and, more recently, the South East Australia Climate Initiative. This investment should include consolidation for and communication to the water industry of existing knowledge of links between long-term climate variability and key climate drivers including Earth’s orbital cycles, solar activity, atmospheric chemistry, major volcanic eruptions, surface and deeper ocean circulations (including the thermohaline circulation – the “great ocean conveyor belt”), sea ice, and major bushfires; (iii) Greater collaboration is required between climatologists, paleoclimatologists and water resource engineers to take advantage of recent scientific advances that have direct implications for drought response planning. Engineers Australia proposes that the National Water Commission should conduct a national forum to bring these three industry groups together with the aim of providing meaningful, useful advice to water industry practitioners from the latest climate research and to guide research activities towards more practical outcomes. This will enable water resource planners to better understand natural climate variability in the longer term and appropriately take into account greenhouse gas induced climate change, including an understanding of uncertainties surrounding both current and future climate; (iv) The development of an industry standard end-use demand model for urban areas should be accelerated. The absence of an industry standard tool in this area currently inhibits the ability to rapidly assess the financial costs of imposing restrictions on particular sectors of the community versus supply augmentation costs to avoid those restrictions. Such a tool is needed to make informed decisions about preparedness to pay for new infrastructure to avoid or reduce restrictions; and (v) More sophisticated water products and market mechanisms have a strong role to play in better managing scarce resources between similar types of users during times of drought. This has become increasingly important with demand hardening over recent decades, which has reduced the water savings that are achievable from water restrictions. Any expansion of water markets as a drought management tool must, however, occur within a regulatory framework that seeks to achieve equitable social outcomes by acknowledging the inherent social value of water to communities and the food security value of water to the nation. It is acknowledged in preparing this position paper that our understanding of drought and our responses to it are rapidly changing, particularly in light of climate change. This is particularly the case with the current reviews of aspects of the National Drought Policy, which were being undertaken by several government agencies at the time of drafting this paper. It is hoped that this position paper will encourage both greater dialogue and investment in drought forecasting, planning, adaptation and mitigation to enable Australia to be well placed to cope with future climate conditions.

Multi-year streamflow drought in eastern Australia

Streamflow drought characteristics were calculated for 123 coastal and inland catchments in eastern Australia. The characteristics for each catchment were determined by stochastic generation of2000 years of daily rainfalls, which were converted to streamflows using the AWBM rainfall-runoff model. Droughts were calculated from the 2000 years of generated streamflow for durations of 1, 2, 3, 5 and 10 years, and expressed as a percentage of mean annual flow to facilitate comparisons among the catchments. There were six river basins with five or more catchments in the basin - three coastal basins in northern New South Wales and three inland basins in the Murray-Darling drainage division. These showed more variation within each basin than between basins. A long sequence of 100,000 years of streamflow was generated on one catchment, which showed that the severity of droughts increased with average recurrence interval to the limit of the generated data. There was no evidence of any regional influence on or geographical pattern of drought severity. The main result was the large variability of drought severity from catchment to catchment even within river basins.

Climate variability and change in the drylands of Western North America

We argue that it is important to expand the consideration of climate in the context of provision of ecosystem services in drylands. In addition to climate change, it is necessary to include climate variability on timescales relevant to human and ecological considerations, namely interannual to decadal and multidecadal. The period of global instrumental record (about a century and a half long at the very most) is neither an adequate nor an unbiased sample of the range and character of natural climate variability that might be expected with the climate system configured as it is now. We base this on evidence from W. N. America, where there has recently been a major multi-year drought, of a scale and intensity that has occurred several times in the last 2000 years, and on attempts to provide explanations of these phenomena based on physical climatology. Ensembles of runs of forced climate system models suggest the next 50 years will bring much more extensive and intense drought in the continental interior of North America. The trajectory followed by the supply of ecosystem services will be contingent not only on the genotypes available and the antecedent soil, economic and social conditions but also on climate variability and change. The critical features of climate on which patterns of plant growth and water supply depend may vary sharply during and between human generations, resulting in very different experiences and hence, expectations.

Dry and Getting Drier

CLIMATE SCIENCE Global warming is expected to have a substantial impact on the amount and pattern of rainfall worldwide. Although projections indicate that the overall effect should be an increase in precipitation, at a regional scale there will be areas that receive less rainfall; many such areas are already arid and particularly vulnerable to further drying. One of these regions is northwestern Africa, which recently suffered a severe drought from 1999 to 2002. In order to establish a context for understanding drought frequency and severity in the region, Touchan et al. constructed a 547-year summer drought record by measuring and analyzing ring widths of cedar and pine trees across Algeria and Tunisia. They found that the multiyear drought of 1999 to 2002 was the longest in their entire record and that 2002 was the single driest year, a troubling set of statistics if the data do indeed reflect ongoing anthropogenic climate change. Climate models are unable to identify the physical causes of drought in this region, however, so a mechanistic understanding of rainfall dynamics there remains elusive. — HJS Geophys. Res. Lett. 35 , L13705 (2008).

Simulating multiyear drought events in North America with the HadCM3 climate model

WeatherVolume 63, Issue 8 p. 240-243 Research Article Simulating multiyear drought events in North America with the HadCM3 climate model Simon J. Busby, Corresponding Author Simon J. Busby s.busby@uea.ac.uk Climatic Research Unit, University of East Anglia, NorwichClimatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UKSearch for more papers by this author Simon J. Busby, Corresponding Author Simon J. Busby s.busby@uea.ac.uk Climatic Research Unit, University of East Anglia, NorwichClimatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UKSearch for more papers by this author First published: 28 July 2008 https://doi.org/10.1002/wea.310Citations: 4AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume63, Issue8Special Issue: RMetS Student Conference 2007August 2008Pages 240-243 RelatedInformation

Would Advance Knowledge of 1930s SSTs Have Allowed Prediction of the Dust Bowl Drought?*

Abstract Could the Dust Bowl drought of the 1930s have been predicted in advance if the SST anomalies of the 1930s had been foreknown? Ensembles of model simulations forced with historical observed SSTs in the global ocean, and also separately in the tropical Pacific and Atlantic Oceans, are compared with an ensemble begun in January 1929 with modeled atmosphere and land initial conditions and integrated through the 1930s with climatological SSTs. The ensemble with climatological SSTs produces values for the precipitation averaged over 1932–39 that are not statistically different from model climatology. In contrast, the ensembles with global SST forcing produce a drought centered in the central plains and southwestern North America that is clearly separated from the model climatology. Both the tropical Pacific and northern tropical Atlantic SST anomalies produce a statistically significant model drought in this region. The modeled drought has a spatial pattern that is different from the observed drought, which was instead centered in the central and northern plains and also impacted the northern Rocky Mountain states but not northeastern Mexico. The model error in extending the Dust Bowl drought too far south is attributed to an incorrect response of the model to warm subtropical North Atlantic SST anomalies. The model error in the northern states cannot be attributed to an incorrect response to tropical SST anomalies. The model also fails to reproduce the strong surface air warming across most of the continent during the 1930s. In contrast, the modeled patterns of precipitation reduction and surface air temperature warming during the 1950s drought are more realistic. Tree-ring records show that the Dust Bowl pattern of drought has occurred before, suggesting that while the extensive human-induced land surface degradation and dust aerosol loading of the 1930s drought may have played an important role in generating the observed drought pattern, natural processes, possibly including land interactions, are capable of generating droughts centered to the north of the main ENSO teleconnection region. Despite this caveat, advance knowledge of tropical SSTs alone would have allowed a high-confidence prediction of a multiyear and severe drought, but one centered too far south and without strong cross-continental warming.

한강수계 장기 가뭄 수문시나리오 개발

장기지속 가뭄에 대한 많은 연구는 과거 기록 자료에 제한을 받아왔다. 장기지속가뭄과 관련된 연구의 애로점은 분석을 위한 많은 가뭄 사상을 얻는데 필요한 장기간의 과거 수문자료의 기록이 부족하다는데 있다. 이와 같은 어려움을 극복하기 위해 근래의 연구방법중 하나는 합성유량자료 계열을 이용하는 것이다. 과거 관측 자료의 추계학적 특성에 근거하여 수문시나리오를 개발하는 것으로, 방법론은 분석에 필요한 다양한 수문사상이 포함될 수 있도록 희망하는 수문자료계열을 인위적으로 생성하는 것이다. 본 논문에서는 이와 같은 방법을 이용하여 수자원 평가를 위한 장기 가뭄 수문시나리오를 한강수계의 저수지 시스템을 대상으로 개발하였다. 아울러 갈수분포모형과 비교하여 합성 유량계열이 주어진 재현기간내에서 신뢰성 있는 장기가뭄빈도와 이에 해당하는 가뭄시나리오 개발에 이용될 수 있음을 확인하였다. Many studies on sustained droughts have often been limited to the analysis of historic flow series. A major disadvantage in this approach can be described as the lack of long historic flow records needed to obtain a significant number of drought events for the analysis. To overcome this difficulty, one of the present study idea is to use synthetically generated hydrologic series. A methodology is presented to develop flow series based on the probabilistic analysis of the stochastic properties of the observed flows. The method can be utilized to generate a flow series of desired length so as to include many multiyear drought events within the process. In this paper, a concept of creating multiyear drought scenarios is introduced, and its development procedure is illustrated by a case study of the water supply system in Han River Basin. Also, it was found that the generated flow series can be reliably used to predict the long drought duration and sustained drought hydrologic scenarios within a given return period.

Effect of climate fluctuations on long-term vegetation dynamics in Carolina bay wetlands

Carolina bays and similar depression wetlands of the U.S. Southeastern Coastal Plain have hydrologic regimes that are driven primarily by rainfall. Therefore, climate fluctuations such as drought cycles have the potential to shape long-term vegetation dynamics. Models suggest two potential long-term responses to hydrologic fluctuations, either cyclic change maintaining open emergent vegetation, or directional succession toward forest vegetation. In seven Carolina bay wetlands on the Savannah River Site, South Carolina, we assessed hydrologic variation and vegetation response over a 15-year period spanning two drought and reinundation cycles. Changes in pond stage (water depth) were monitored bi-weekly to monthly each year from 1989–2003. Vegetation composition was sampled in three years (1989, 1993, and 2003) and analyzed in relation to changes in hydrologic conditions. Multi-year droughts occurred prior to the 1989 and 2003 sampling years, whereas 1993 coincided with a wet period. Wetland plant species generally maintained dominance after both wet and dry conditions, but the abundances of different plant growth forms and species indicator categories shifted over the 15-year period. Decreased hydroperiods and water depths during droughts led to increased cover of grass, upland, and woody species, particularly at the shallower wetland margins. Conversely, reinundation and longer hydroperiods resulted in expansion of aquatic and emergent species and reduced the cover of flood-intolerant woody and upland species. These semi-permanent Upper Coastal Plain bays generally exhibited cyclic vegetation dynamics in response to climate fluctuation, with wet periods favoring dominance by herbaceous species. Large basin morphology and deep ponding, paired with surrounding upland forest dominated by flood-intolerant pines, were features contributing to persistence of herbaceous vegetation. Drought cycles may promote directional succession to forest in bays that are smaller, shallower, or colonized by flood-tolerant hardwoods.

Drought effects on litterfall, wood production and belowground carbon cycling in an Amazon forest: results of a throughfall reduction experiment

The Amazon Basin experiences severe droughts that may become more common in the future. Little is known of the effects of such droughts on Amazon forest productivity and carbon allocation. We tested the prediction that severe drought decreases litterfall and wood production but potentially has multiple cancelling effects on belowground production within a 7-year partial throughfall exclusion experiment. We simulated an approximately 35–41% reduction in effective rainfall from 2000 through 2004 in a 1 ha plot and compared forest response with a similar control plot. Wood production was the most sensitive component of above-ground net primary productivity (ANPP) to drought, declining by 13% the first year and up to 62% thereafter. Litterfall declined only in the third year of drought, with a maximum difference of 23% below the control plot. Soil CO2 efflux and its 14C signature showed no significant treatment response, suggesting similar amounts and sources of belowground production. ANPP was similar between plots in 2000 and declined to a low of 41% below the control plot during the subsequent treatment years, rebounding to only a 10% difference during the first post-treatment year. Live aboveground carbon declined by 32.5 Mg ha−1 through the effects of drought on ANPP and tree mortality. Results of this unreplicated, long-term, large-scale ecosystem manipulation experiment demonstrate that multi-year severe drought can substantially reduce Amazon forest carbon stocks.

The Turn of the Century North American Drought: Global Context, Dynamics, and Past Analogs*

Abstract The causes and global context of the North American drought between 1998 and 2004 are examined using atmospheric reanalyses and ensembles of atmosphere model simulations variously forced by global SSTs or tropical Pacific SSTs alone. The drought divides into two distinct time intervals. Between 1998 and 2002 it coincided with a persistent La Niña–like state in the tropical Pacific, a cool tropical troposphere, poleward-shifted jet streams, and, in the zonal mean, eddy-driven descent in midlatitudes. During the winters reduced precipitation over North America in the climate models was sustained by anomalous subsidence and reductions of moisture convergence by the stationary flow and transient eddies. During the summers reductions of evaporation and mean flow moisture convergence drove the precipitation reduction, while transient eddies acted diffusively to oppose this. During these years the North American drought fitted into a global pattern of circulation and hydroclimate anomalies with noticeable zonal and hemispheric symmetry. During the later period of the drought, from 2002 to 2004, weak El Niño conditions prevailed and, while the global climate adjusted accordingly, western North America remained, uniquely among midlatitude regions, in drought. The ensemble mean of the climate model simulations did not simulate the continuation of the drought in these years, suggesting that the termination of the drought was largely unpredictable in terms of global ocean conditions. The global context of the most recent, turn of the century, drought is compared to the five prior persistent North American droughts in the instrumental record from the mid-nineteenth century on. A classic La Niña pattern of ocean temperature in the Pacific is common to all. A cold Indian Ocean, also typical of La Niña, is common to all five prior droughts, but not the most recent one. Except in southern South America the global pattern of precipitation anomalies of the turn of the century drought is similar to that during the five prior droughts. These comparisons suggest that the earlier period of this most recent drought is the latest in a series of multiyear droughts forced by persistent changes in tropical Pacific Ocean temperatures. Warm tropical North Atlantic Ocean temperatures may play a secondary role.

Warming may create substantial water supply shortages in the Colorado River basin

The high demand for water, the recent multiyear drought (1999–2007), and projections of global warming have raised questions about the long‐term sustainability of water supply in the southwestern United States. In this study, the potential effects of specific levels of atmospheric warming on water‐year streamflow in the Colorado River basin are evaluated using a water‐balance model, and the results are analyzed within the context of a multi‐century tree‐ring reconstruction (1490–1998) of streamflow for the basin. The results indicate that if future warming occurs in the basin and is not accompanied by increased precipitation, then the basin is likely to experience periods of water supply shortages more severe than those inferred from the long‐term historical tree‐ring reconstruction. Furthermore, the modeling results suggest that future warming would increase the likelihood of failure to meet the water allocation requirements of the Colorado River Compact.

Future projections of winter rainfall in southeast Australia using a statistical downscaling technique

Much of southeast Australia has experienced rainfall substantially below the long-term average since 1997. This protracted drought is particularly noticeable in those parts of South Australia and Victoria which experience a winter (May through October) rainfall peak. For the most part, the recent meteorological drought has affected the first half of the rainfall season May–June–July (MJJ), while rainfall during the second half August–September–October (ASO) has been much closer to the long term average. The recent multi-year drought is without precedent in the instrumental record, and is qualitatively similar to the abrupt decline in rainfall which was observed in the southwest of Western Australia in the 1960 and 1970s. Using a statistical downscaling technique, the rainfall decline is linked to observed changes in large-scale atmospheric fields (mean sea level pressure and precipitable water). This technique is able to reproduce the statistical properties of rainfall in southeast Australia, including the interannual variability and longer time-scale changes. This has revealed that the rainfall recent decline may be explained by a shift to higher pressures and lower atmospheric precipitable water in the region. To explore the likely future evolution of rainfall in southeast Australia under human induced climate change, the same statistical downscaling technique is applied to five climate models forced with increasing greenhouse gas concentrations. This reveals that average rainfall in the region is likely to decline in the future as greenhouse gas concentrations increase, with the greatest decline occurring during the first half of winter. Projected declines vary amongst models but are generally smaller than the recent early winter rainfall deficits. In contrast, the rainfall decline in late winter–spring is larger in future projections than the recent rainfall deficits have been. We illustrate the consequences of the observed and projected rainfall declines on water supply to the major city of Melbourne, using a simple rainfall run-off relationship. This suggests that the water resources may be dramatically affected by future climate change, with percentage reductions approximately twice as large as corresponding changes in rainfall.

Stochastic simulation of droughts. Part 2: regional droughts

AbstractIn Part 1 we demonstrated the applicability of stochastic models to predicting the characteristics of point drought events within any planning period by means of a case study (Mohan S, Sahoo PK (2007) Hydrological Processes 21: this issue). In addition, studies on regional droughts are important in the context of regional level planning and evolving management strategies. The small number of drought events from a particular streamflow or rainfall series, when subjected to statistical analysis in order to predict future occurrences, produces results that are not very reliable. To overcome this difficulty, we propose using a long sequence of synthetically generated annual rainfall series at various rain‐gauge stations of a region, and multiyear regional droughts were derived from both historic and generated series. The key parameters for a successful regional multiyear drought study are the critical area ratio and the critical level, and the area affected by the drought can be ascertained using these parameters. The important regional drought parameters were determined and their suitable probability distributions were arrived at by studying a total of nine possible probability models; these models can be used in predicting the longest regional drought duration and the greatest regional drought severity with a given return period. The effect of change of critical parameters on the regional drought parameters is also studied and reported. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Multidecadal Climate Variability and Drought in the United States

AbstractThe history of multidecadal climate variability and drought in the United States during the past millennium is reviewed, and recent research into the physical mechanisms that produce long‐term aridity is examined. Numerous multiyear droughts that have major social repercussions for contemporary and prehistoric societies can be identified in instrumental and proxy records. The 16th‐century megadrought in the Southwest was the worst long period of aridity in the past 500 years. A prolonged period of aridity in the western United States from 900 to 1300 AD, which coincided with the Medieval Warm Period, is indicative of a mean shift in climate that ended in roughly 1400 AD. While the causes of these and other multiyear droughts are still under investigation, there is strong statistical and modeled evidence that persistent La Niña‐like sea surface temperatures in the tropical Pacific force hemispheric and zonal symmetry in multidecadal droughts. In other words, persistent cooler than normal sea surface temperatures in the tropical Pacific force anomalous atmospheric circulations that produce multiyear droughts not only in the Great Plains and the Southwest, but also in the Mediterranean region of Europe, the Pampas region of South America, the steppes of Central Asia, and the outback of Western Australia. Other possible causes of long period drought include low‐frequency variability in the north Pacific and Atlantic oceans and global warming in the Anthropocene.

Annual and Warm Season Drought Intensity–Duration–Frequency Analysis for Sonora, Mexico

Abstract Located in northwestern Mexico, Sonora is a region affected by the North American monsoon (NAM). The region covers nearly 50% of the North American Sonoran Desert and is characterized by climatic conditions ranging from extremely arid to semiarid. The region has suffered from drought since 1995, and consequently, water supplies are threatened. The objectives of this work are to characterize the spatial and temporal variabilities of precipitation in Sonora and to conduct a meteorological drought intensity–duration–frequency analysis based on annual and warm season precipitation records. Monthly precipitation data are compiled from 76 meteorological stations located in Sonora, along with 19 stations in the neighboring American state of Arizona, for the period 1961–2004. For increased reliability, data are pooled within five plausible climatic regions. Among the results reported herein are summaries of precipitation variability, drought frequency estimates for annual and seasonal durations and return periods of 10–100 yr, and an estimate of the return period of the most recent multiyear drought.