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The frequency of extreme drought events is projected to increase under global climate change, causing damage to plants and crop yield despite potential acclimation. We investigated whether grasses remain acclimated to drought even after a harvest and remember early summer drought exposure over a whole vegetation period. For this, we compared the response of Arrhenatherum elatius plants under a second, late, drought (they were pre-exposed to an early drought before), to plants exposed to a single, only late, extreme drought. Surprisingly, the percentage of living biomass after a late drought increased for plants that were exposed to drought earlier in the growing season compared to single-stressed plants, even after...
One of the challenges of range management is to compare one system of management with another with regard to their response to a different natural or management-induced factor. Drought and grasshoppers were dominant influences on rangelands of the southwest in the late 1980s. Drought has long been recognized as a good test of a grazing system as drought tends to magnify any weakness of management practice. The drought and subsequent grasshopper invasion of 1988 through 1990 provided a good opportunity to stress test two different grazing systems of the Arizona Strip. The two allotments examined in this paper are adjacent to each other, have similar range sites, and similar precipitation. Two major range sites occur...
Historical stream flow records and the forecast for 2004 make the current (1999?2004) drought in the southwestern United States the worst one in the past 80 years for portions of the Upper Colorado River Basin (UCRB). For the Colorado River (near Cisco, Utah), the cumulative stream flow deficit (departure from long-term mean) for the current drought is almost 11 km3, or approximately 2 years of average stream flow.Although the current drought is the most significant,based on historical stream flow records, is it the worst ever? Published in Eos, Transactions, American Geophysical Union, volume 85, issue 32, on pages 301 - 308, in 2004.
The definition of desertification accepted in the ad hoc conference held by UNEP in Nairobi in 1977 and confirmed at the Earth Summit on Environment and Development held in Rio de Janeiro in 1992 is: ?arid, semi-arid and dry-subhumid land degradation?. There is no global long-term trend in any rainfall change over the period of instrumental record (c. 150 years), but there has been an increase of 0�5�C in global temperature over the past 100 years. This increase seems partly due to urbanization, as there is no evidence of it resulting from atmospheric pollution by CO2and other warming gases (SO2, NO2, CH4, CFH etc.). On the other hand, the thermal increase is uneven, increasing with latitudes above 40� N and S....
The impacts of the United States drought of 2007 to both society and ecosystems were substantive and included multi-billion dollar agricultural losses and the second worst wildfire season on record. The purpose of this paper is to place the 2007 drought in historical perspective relative to the climate record from 1895?2007 to increase our understanding of this hazard and contribute to improvements of drought mitigation plans. We compared the 2007 drought historically against the climatic record (1895?2007) using the Palmer Drought Severity Index (PDSI). We then examined the temporal progression of the 2007 drought and placed the peak month of drought severity (November) in historical perspective using rankings...
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...
Severe droughts have been associated with regional-scale forest mortality worldwide. Climate change is expected to exacerbate regional mortality events; however, prediction remains difficult because the physiological mechanisms underlying drought survival and mortality are poorly understood. We developed a hydraulically based theory considering carbon balance and insect resistance that allowed development and examination of hypotheses regarding survival and mortality. Multiple mechanisms may cause mortality during drought. A common mechanism for plants with isohydric regulation of water status results from avoidance of drought-induced hydraulic failure via stomatal closure, resulting in carbon starvation and a cascade...
In response to recent severe drought conditions throughout the state, Arizona recently developed its first drought plan. The Governor's Drought Task Force focused on limiting the economic and social impacts of future droughts through enhanced adaptation and mitigation efforts. The plan was designed to maximize the use of new, scientific breakthroughs in climate monitoring and prediction and in vulnerability assessment. The long term objective of the monitoring system is to allow for evaluation of conditions in multiple sectors and at multiple scales. Stakeholder engagement and decision support are key objectives in reducing Arizona's vulnerability in light of the potential for severe, sustained drought. The drivers...
Southern California relies heavily upon imported water from the Sacramento and Colorado river systems to augment local supplies and to mitigate the impacts of drought. In this paper a ‘perfect drought’ is defined as a prolonged drought that affects southern California, the Sacramento River basin and the upper Colorado River basin simultaneously. Examination of instrumental climate and discharge records shows that over the past century such perfect droughts do occur, but generally persist for less than five years. Perfect droughts that extend across all three areas are associated with anomalous upper-level high pressure off west coast and over western North America which is in turn associated with anomalously...
New tree-ring records of ring-width from remnant preserved wood are analyzed to extend the record of reconstructed annual flows of the Colorado River at Lee Ferry into the Medieval Climate Anomaly, when epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western North America. The most extreme low-frequency feature of the new reconstruction, covering A. D. 762-2005, is a hydrologic drought in the mid-1100s. The drought is characterized by a decrease of more than 15% in mean annual flow averaged over 25 years, and by the absence of high annual flows over a longer period of about six decades. The drought is consistent in timing with dry conditions inferred from tree-ring...
Ongoing drought in the Colorado River Basin, unprecedented urban growth in the watershed, and numerical model simulations showing higher temperatures and lower precipitation totals in the future have all combined to heighten interest in drought in this region. In this investigation, we use principal components analysis (PCA) to independently assess the influence of various teleconnections on Basin-wide and sub-regional winter season Palmer Hydrological Drought Index (PHDI) and precipitation variations in the Basin. We find that the Pacific Decadal Oscillation (PDO) explains more variance in PHDI than El Nio-Southern Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO), and the planetary temperature combined...
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...
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....
Artemisia tridentata Nutt, is a drought-tolerant shrub that expands inflorescences during summer and autumn, even though xylem pressure potentials may be less than -5.0 MPa. Supplemental watering increased total inflorescence biomass and the biomass of floral heads over 450 and 550%, respectively. Inflorescences displayed positive net photosynthesis until xylem pressure potentials decreased below -5.0 MPa. Growth of inflorescences was not dependent on carbon imported from vegetative leaves; removal of all leaves on vegetative branches did not change inflorescence biomass, and 14CO2 fixed by adjacent vegetative branches was not transferred into inflorescences. Expanding inflorescences during summer may enhance competitive...
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...
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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...
Drought and freezing are both known to limit desert plant distributions, but the interaction of these stressors is poorly understood. Drought may increase freezing tolerance in leaves while decreasing it in the xylem, potentially creating a mismatch between water supply and demand. To test this hypothesis, we subjected Larrea tridentata juveniles grown in a greenhouse under well-watered or drought conditions to minimum temperatures ranging from -8 to -24 �C. We measured survival, leaf retention, gas exchange, cell death, freezing point depression and leaf-specific xylem hydraulic conductance (k?). Drought-exposed plants exhibited smaller decreases in gas exchange after exposure to -8 �C compared to well-watered...
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. Published in Journal of the...
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To better understand drought occurrence in the Colorado River Basin (CRB) of the southwestern United States we used a hydroclimatic index to create a historical record of drought coverage and analysed the linear trend and relationships with key climate teleconnections. The past century was characterized by an increase in drought coverage during the warm portion of the year almost exclusively as a result of climatic warming. In recent decades, a significant increase in the drought coverage occurred earlier in the year, during the spring season, primarily as a function of warming, but in combination with a decline in precipitation for a significant portion of the basin. The El Ni o (La Ni a) phase of the El Ni o-Southern...
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 streamfiows for the Colorado River basin with a 38-year streamfiow trace extracted from the recent historic record. The impacts of the severe drought on streamfiows, water allocation, storage, hydropower generation, and salinity were assessed. Estimated deliveries to consumptive uses in the Upper Basin states of Colorado,...
map background search result map search result map The source hydrology of severe sustained drought in the southwestern United States Do plants remember drought? Hints towards a drought-memory in grasses A hydroclimatic index for examining patterns of drought in the Colorado River Basin A hydroclimatic index for examining patterns of drought in the Colorado River Basin Do plants remember drought? Hints towards a drought-memory in grasses The source hydrology of severe sustained drought in the southwestern United States