Drought Length Research Articles

Atlantic Forcing of Persistent Drought in West Africa

Although persistent drought in West Africa is well documented from the instrumental record and has been primarily attributed to changing Atlantic sea surface temperatures, little is known about the length, severity, and origin of drought before the 20th century. We combined geomorphic, isotopic, and geochemical evidence from the sediments of Lake Bosumtwi, Ghana, to reconstruct natural variability in the African monsoon over the past three millennia. We find that intervals of severe drought lasting for periods ranging from decades to centuries are characteristic of the monsoon and are linked to natural variations in Atlantic temperatures. Thus the severe drought of recent decades is not anomalous in the context of the past three millennia, indicating that the monsoon is capable of longer and more severe future droughts.

Population differences in juvenile survival under increasing drought are mediated by seed size in cork oak ( Quercus suber L.)

Many studies have focused on the physiological and morphological responses of Quercus suber to high temperatures and summer drought. Nevertheless, our understanding of the potential of this species to cope with climate change is incomplete. An increase in severity, length and frequency of summer droughts is expected in the Iberian Peninsula over the 21st century. We investigated the potential of cork oak to adapt to climate change in a 4-year study comparing seedling survival from thirteen Spanish populations in a common garden. Acorn size was evaluated as a possible adaptive trait enhancing stress resistance during establishment. Populations originating from sites with the driest summers exhibited the highest survival rates under dry conditions. These populations were characterized by bigger acorns, suggesting selection for this trait across sites experiencing drought. Our findings reveal that northern populations are not well adapted to cope with increasing drought but suggest that they might cope well with the moderate drought increase expected for these zones. However, continental populations are intermediately adapted to dry conditions, while drastic and fast increases in summer droughts are expected to occur in these regions. This extensive and quick change will provide scant chances to adapt making the populations of cork oak in these regions particularly vulnerable to the future climatic conditions.

Probabilistic characterization of drought properties through copulas

This paper shows an application of copulas to the probabilistic analysis of drought characteristics. Drought occurrences are analyzed by the Standardized Precipitation Index (SPI) computed on the mean areal precipitation, aggregated at 6months, observed in Sicily between 1921 and 2003. Assuming a drought period as a consecutive number of intervals where SPI values are less than −1, several characteristics are determined, namely: drought length, mean and minimum SPI values, and drought mean areal extent.Results of a preliminary analysis based on Kendall’s correlation and upper tail dependence coefficient, computed on observed and resampled data, show significant dependence properties between almost all the considered pairs. The four-dimensional joint distribution required to correctly model the stochastic structure of variables is determined by resorting to copula approach. This allows flexibility in choosing suitable marginals and dependence structure, and in simplifying the inference procedure as well. Drought return periods are then computed as mean interarrival time, taking into account two drought characteristics at a time by means of the corresponding bivariate marginals of the fitted four-dimensional distribution.Application of the proposed methodology to Sicilian precipitation series shows a good correspondence between empirical and theoretical joint return periods, thus indicating that copulas are adequate to jointly model drought characteristics and to compute exceedance probabilities of drought events.

The role of phenology, growth and nutrient retention during leaf fall in the competitive potential of two species of mediterranean shrubs in the context of global climate changes

Phenological patterns, growth and internal nutrient cycling of the Mediterranean shrubs Cistus salviifolius and Cistus ladanifer were analysed during 2 years of contrasted precipitation to compare their life strategies and their competitive potential to cope with future climate changes, leading to increased drought. The phenophases of both study species occurred mostly in spring, the most favourable period of the year, but while C. salviifolius produced greater number of shoot and leaves, C. ladanifer showed greater leaf area production, greater specific leaf mass and longer leaf life span. The latter species also showed more efficient N and P leaf resorption and recycling. Growth of both species was highly dependent on rainfall distribution pattern, but the two species exhibited different strategies to face the limitations imposed by the summer drought. C. salviifolius exhibited high seasonal dimorphism in plant structure, with great leaf shedding before summer drought, while the structure and biomass of C. ladanifer showed lower relative change throughout the year. The increase in length and intensity of drought also caused greater variation on growth rates and leaf duration and shedding in C. salviifolius than in C. ladanifer. Results suggest that C. ladanifer has greater stress-tolerance ability and hence higher competitive potential in response to varying environmental conditions, including increasing drought stress, which is expected to occur with the ongoing global climatic changes.

Flow characteristics of rivers in northern Australia: Implications for development

Summary Annual, monthly and daily streamflows from 99 unregulated rivers across northern Australia were analysed to assess the general surface water resources of the region and their implications for development. The potential for carry-over storages was assessed using the Gould-Dincer Gamma method, which utilises the mean, standard deviation, skewness and lag-one serial correlation coefficient of annual flows. Runs Analysis was used to describe the characteristics of drought in northern Australia and the potential for ‘active’ water harvesting was evaluated by Base Flow Separation, Flow Duration Curves and Spells Analysis. These parameters for northern Australia were compared with data from southern Australia and data for similar Koppen class from around the world. Notably, the variability and seasonality of annual streamflow across northern Australia were observed to be high compared with that of similar Koppen classes from the rest of the world (RoW). The high inter-annual variability of runoff means that carry-over storages in northern Australia will need to be considerably larger than for rivers from the RoW (assuming similar mean annual runoff, yield and reliability). For example, in the three major Koppen zones across the North, it was possible (theoretically) to only exploit approximately 33% (Koppen Aw; n = 6), 25% (Koppen BSh; n = 12) and 13% (Koppen BWh; n = 11) of mean annual streamflow (assuming a hypothetical storage size equal to the mean annual flow). Over 90% of north Australian rivers had a Base Flow Index of less than 0.4, 72% had negative annual lag-one autocorrelation values and in half the rivers sampled greater than 80% of the total flow occurred during the 3-month peak period. These data confirm that flow in the rivers of northern Australia is largely event driven and that the north Australian environment has limited natural storage capacity. Hence, there is relatively little opportunity in many northern rivers to actively harvest water for on-farm storage, particularly under environmental flow rules that stipulate that water can only be extracted during the falling limb of a hydrograph. Streamflow drought severity, the product of drought length and magnitude, was found to be greater in northern Australia than in similar climatic regions of the RoW, due to higher inter-annual variability increasing the drought magnitude over the course of normal drought lengths. The high likelihood of severe drought means that agriculturalists seeking to irrigate from rivers in northern Australia should have especially well developed drought contingency plans.

Soil water content at the catchment level and plant water status relationships in a Mediterranean Quercus ilex forest

This paper presents an analysis of the forest hydrology and plant water status interaction, focusing on the relationship between the hydrological water balance at the catchment level and the predawn leaf water potential of the species Quercus ilex (holm oak). The catchment water balance approach was applied to a Mediterranean watershed forested with holm oak to evaluate the daily soil water reserve at the catchment level. After this, evapotranspiration and soil water content were combined to estimate the potential soil water reserve and evaluate plant water status at the catchment level. A close relationship was detected between leaf water potential and the soil water reserve, and was fitted to a negative exponential curve to estimate predawn leaf water potential from a hydrological database. The proposed equation can help to predict the frequency, intensity, and length of droughts potentially capable of causing structural damage to the forest, from the hydrological time series records.

Regional variations of the Caribbean mid-summer drought

Pentad satellite-based precipitation estimates were input into a wavelet analysis to quantify the length, timing, and strength of the mid-summer drought (MSD) for the Caribbean Sea and surrounding regions. For most of the Caribbean the time between the first and second summer precipitation maxima is 98 to 117 days (∼3 to 4 months). The MSD appears in early-June over Puerto Rico and Hispaniola, and develops progressively later in the summer season towards the west, finally occurring in early-October over the Gulf of Mexico. The MSD is most intense in the eastern Pacific, strong and significant in the western Caribbean, and almost nonexistent in the eastern Caribbean. Forcing mechanisms are examined to help explain the regional variability in the Caribbean. A July increase in surface pressure and surface divergence, caused by the changing wind field, appears to contribute to a strong concurrent MSD over the waters bounded by Jamaica, Cuba, and the Yucatan peninsula. Finally, the island of Jamaica itself appears to block the flow of the tradewinds as they migrate northward and intensify into mid-summer, thus enhancing the divergence, and in turn MSD, immediately to the west.

ENVIRONMENTAL HETEROGENEITY, BIRD-MEDIATED DIRECTED DISPERSAL, AND OAK WOODLAND DYNAMICS IN MEDITERRANEAN SPAIN

Vegetation dynamics in complex landscapes depend on interactions among environmental heterogeneity, disturbance, habitat fragmentation, and seed dispersal processes. We explore how these features combine to affect the regional abundances and distributions of three Quercus (oak) species in central Spain: Q. faginea (deciduous tree), Q. ilex (evergreen tree), and Q. coccifera (evergreen shrub). We develop and parameterize a stochastic patch occupancy model (SPOM) that, unlike previous SPOMs, includes environmentally driven variation in disturbance and establishment. Dispersal in the model is directed toward local (nearby) suitable habitat patches, following the observed seed-caching behavior of the European Jay. Model parameters were estimated using Bayesian methods and survey data from 12 047 plots. Model simulations were conducted to explore the importance of different dispersal modes (local directed, global directed, local random, global random). The SPOM with local directed dispersal gave a much better fit to the data and reproduced observed regional abundance, abundance-environment correlations, and spatial autocorrelation in abundance for all three species. Model simulations suggest that jay-mediated directed dispersal increases regional abundance and alters species-environment correlations. Local dispersal is estimated to reduce regional abundances, amplify species-environment correlations, and amplify spatial autocorrelation. Parameter estimates and model simulations reveal important species-specific differences in sensitivity to environmental perturbations and dispersal mode. The dominant species Q. ilex is estimated to be highly fecund, but on the edge of its climatic tolerance. Therefore Q. ilex gains little from directed dispersal, suffers little from local dispersal, and is relatively insensitive to changes in habitat cover or disturbance rate; but Q. ilex is highly sensitive to altered drought length. In contrast, the rarest species, Q. coccifera, is well adapted to the climate and soils but has low fecundity; thus, it is highly sensitive to changes in dispersal, habitat cover, and disturbance but insensitive to altered drought length. Finally, Q. faginea is estimated to be both at the edge of its climatic tolerance and to have low fecundity, making it sensitive to all perturbations. Apparently, co-occurring species can exhibit very different interactions among dispersal, environmental characteristics, and physiological tolerances, calling for increased attention to species-specific dynamics in determining regional vegetation responses to anthropogenic perturbations.

Non-linear regional relationships between climate extremes and annual mean temperatures in model projections for 1961–2099 over Europe

A simple method is tested for scaling climate-extreme results from high resolution regional climate models (RCMs) to time-periods and emission scenarios for which the RCMs have not been run. The 30 yr mean relationships between indicators of extremes (IoEs) and annual mean daily maximum temperature (T-xa) are investigated. Such relationships from the UK Met Office Hadley Centre RCM HadRM3P, along with temperatures from the global climate model HadCM3, are used to scale IoEs to other time periods and scenarios. This is tested for selected indicators of heat-wave and drought over Europe for the period 1961-2099. Curvature is demonstrated in the relationships between these quantities and T-xa. Such non-linearities are shown to have a large potential effect on how these climate extremes are likely to evolve during the century, as well as their sensitivity to emissions. A broad picture of possible changes in European heat-wave and drought severity is presented. For drought over the Mediterranean and western Europe, a very clear positive curvature in the relationship between drought length and annually averaged temperature is found. (This feature is also common in a brief study of 6 other RCMs.) It suggests a rapid increase in drought length towards the end of the century, and a strong sensitivity to the emission scenario. Extended summer dry spells are projected to become a much more regular feature of western European climates. For European heatwaves, we find a slightly earlier onset of increases in heat-wave severity and a reduced sensitivity to emission scenarios than might be expected from a more straightforward interpretation of the Hadley Centre model results. This is linked with extreme dryness occurring at high summer in all years by the end of the century, but was not evident in the 6 other RCMs studied. Based on these results, suggestions are made for choices of future RCM experiments. (Less)

On the dynamics of soil moisture, vegetation, and erosion: Implications of climate variability and change

We couple a shear‐stress‐dependent fluvial erosion and sediment transport rule with stochastic models of ecohydrological soil moisture and vegetation dynamics. Rainfall is simulated by the Poisson rectangular pulses rainfall model with three parameters: mean rainfall intensity, duration, and interstorm period. These parameters are related to mean annual precipitation on the basis of published data. The model is used to investigate the sensitivity of grass cover and erosion potential to drought length, changes in storm frequency under fixed mean seasonal rainfall, and variations in mean annual precipitation. Three fundamental factors, amount of precipitation, storm frequency, and soil type, are predicted to control the system response. Variation in storm frequency is predicted to have a significant influence on sediment transport capacity because of its influence on vegetation dynamics. Our results predict soil loss potential to be more sensitive to a reduction in storm frequency (under fixed mean annual precipitation) in humid ecosystems than in arid and semiarid regions. The well‐known dependence between mean annual sediment yields and precipitation (e.g., Langbein and Schumm, 1958) is reproduced by the model. Numerical experiments using different soil types underscore the importance of soil texture in controlling the magnitude and shape of such dependence. Coupling abiotic and biotic Earth surface processes under random climatic forcing is the salient aspect of our approach, opening new avenues for research in the emerging field of complex climate‐soil‐vegetation‐landscape dynamics.

Plio-Pleistocene climatic change in the Turkana Basin (East Africa): Evidence from large mammal faunas

We investigated palaeoclimatic change in the Turkana Basin during the Pliocene climatic shift toward increased aridity in Africa. We analyzed the palaeoecology of this area using mammal faunas as environmental indicators. Twenty Plio-Pleistocene fossil assemblages and a comparative dataset of 16 modern localities covering a wide range of climatic and ecological conditions across Africa were analyzed. We constructed community profiles using taxonomic variables which reflect ecological information. Principal component analysis and bivariate correlation were used to study changes in the community structure of these mammalian faunas and to draw palaeoenvironmental inferences. Subsequently, least-squares regressions yielded climatic estimates (annual rainfall and drought length) for the studied period. An additional set of 8 modern faunas was used to validate these regression models. The climatic estimates showed a drying trend throughout the sequence. The biomes in the Turkana Basin changed from semi-evergreen rain forest to deciduous woodland and savanna during the middle-late Pliocene. This was the most important climatic shift detected in our study. Evidence suggests a continuous presence of savannas from 2.5 million years ago onwards. This pattern of climatic change is consistent with isotopic evidence on global climate, and with independently derived regional palaeoenvironmental evidence (i.e., micromammals, palaeovegetation, soil carbonates and palaeosols).

Multi-year drought frequency analysis at multiple sites by operational hydrology – A comparison of methods

This paper compares two generators of yearly water availabilities from sources located at multiple sites with regard to their ability to reproduce the characteristics of historical critical periods and to provide reliable results in terms of the return period of critical sequences of different length. The two models are a novel multi-site Markov mixture model explicitly accounting for drought occurrences and a multivariate ARMA. In the case of the multisite Markov mixture model parameter estimation is limited to a search in the parameter space guided by the value of parameter λ to show the sensitivity of the model to this parameter. Application to two of the longest time series of streamflows available in Sicily (Italy) shows that the models can provide quite different results in terms of estimated return periods of historic droughts, although they seem to perform more uniformly when it comes to simulate drought-related statistics such as drought length, severity and intensity. The role of parameter selection for the multisite Markov mixture model and of the marginal probability of generated flows in providing results consistent with the characteristics of the observed series is discussed. Both models are applied to the system of sources supplying the city of Palermo (Sicily) and its environs showing the applicability of the newly developed multisite Markov mixture model to medium-to-large scale water resources systems.

Validation of precipitation events in a regional climate model simulation using methods from complex systems theory

We show that daily precipitation is characterized by a behaviour regulated by power laws. Both for the frequency distribution of both event intensity and drought duration. In this respect, precipitation appears to follow self-organized criticality laws, much as other geophysical phenomena such as avalanches and earthquakes. We use this feature to validate the simulation of daily precipitation events in a multi-decadal regional climate model experiment for the European region. Our focus is on the Italian peninsula and we show that both the model results and the station observations for daily precipitation intensity and drought length follow power laws with comparable values of the relevant parameters. We suggest that complex systems theory can provide useful tools for the validation of precipitation statistics in climate models.

CLIMATE AND LIFE-HISTORY EVOLUTION IN EVENING PRIMROSES (OENOTHERA, ONAGRACEAE): A PHYLOGENETIC COMPARATIVE ANALYSIS

Evolutionary ecologists have long sought to understand the conditions under which perennial (iteroparous) versus annual (semelparous) plant life histories are favored. We evaluated the idea that aridity and variation in the length of droughts should favor the evolution of an annual life history, both by decreasing adult survival and by increasing the potential for high seedling survival via reduced plant cover. We calculated phylogenetically independent contrasts of climate with respect to life history in a clade of winter-establishing evening primroses (sections Anogra and Kleinia; Oenothera; Onagraceae), which includes seven annuals, 12 perennials, and two variable taxa. Climate variables were quantified from long-term records at weather stations near collection localities. To explicitly account for phylogenetic uncertainty, contrasts were calculated on a random sample of phylogenetic trees from the posterior distribution of a Bayesian analysis of DNA sequence data. Statements of association are based on comparing the per-tree mean contrast, which has a null expectation of zero, to a set of per-tree mean contrasts calculated on the same trees, after randomizing the climate data. As predicted, increased annual aridity, increased annual potential evapotranspiration, and decreased annual precipitation were associated with transitions to the annual habit, but these trends were not significantly different from the null pattern. Transitions to the annual habit were not significantly associated with increases in one measure of aridity in summer nor with increased summer drought, but they were associated with significantly increased maximum summer temperatures. In winter, increased aridity and decreased precipitation were significantly associated with transitions to the annual habit. Changes in life history were not significantly associated with changes in the coefficient of variation of precipitation, either on an annual or seasonal (summer vs. winter) basis. Though we cannot attribute causality on the basis of a correlational, historical study, our results are consistent with the idea that increased heat and drought at certain times of the year favor the evolution of the annual habit. Increased heat in summer may cause adult survival to decline, while increased aridity and decreased precipitation in the season of seedling recruitment (winter) may favor a drought-avoiding, short-lived annual strategy. Not all of the predicted patterns were observed: the capability for drought-induced dormancy may preclude change in habit in response to summer drought in our study group.

CLIMATE AND LIFE-HISTORY EVOLUTION IN EVENING PRIMROSES (OENOTHERA, ONAGRACEAE): A PHYLOGENETIC COMPARATIVE ANALYSIS

Evolutionary ecologists have long sought to understand the conditions under which perennial (iteroparous) versus annual (semelparous) plant life histories are favored. We evaluated the idea that aridity and variation in the length of droughts should favor the evolution of an annual life history, both by decreasing adult survival and by increasing the potential for high seedling survival via reduced plant cover. We calculated phylogenetically independent contrasts of climate with respect to life history in a clade of winter-establishing evening primroses (sections Anogra and Kleinia; Oenothera; Onagraceae), which includes seven annuals, 12 perennials, and two variable taxa. Climate variables were quantified from long-term records at weather stations near collection localities. To explicitly account for phylogenetic uncertainty, contrasts were calculated on a random sample of phylogenetic trees from the posterior distribution of a Bayesian analysis of DNA sequence data. Statements of association are based on comparing the per-tree mean contrast, which has a null expectation of zero, to a set of per-tree mean contrasts calculated on the same trees, after randomizing the climate data. As predicted, increased annual aridity, increased annual potential evapotranspiration, and decreased annual precipitation were associated with transitions to the annual habit, but these trends were not significantly different from the null pattern. Transitions to the annual habit were not significantly associated with increases in one measure of aridity in summer nor with increased summer drought, but they were associated with significantly increased maximum summer temperatures. In winter, increased aridity and decreased precipitation were significantly associated with transitions to the annual habit. Changes in life history were not significantly associated with changes in the coefficient of variation of precipitation, either on an annual or seasonal (summer vs. winter) basis. Though we cannot attribute causality on the basis of a correlational, historical study, our results are consistent with the idea that increased heat and drought at certain times of the year favor the evolution of the annual habit. Increased heat in summer may cause adult survival to decline, while increased aridity and decreased precipitation in the season of seedling recruitment (winter) may favor a drought-avoiding, short-lived annual strategy. Not all of the predicted patterns were observed: the capability for drought-induced dormancy may preclude change in habit in response to summer drought in our study group.

Drought length properties for periodic‐stochastic hydrologic data

Extreme droughts may be characterized by their duration, severity (magnitude or intensity), spatial extent, and frequency or return period. Comparing the time series of water supply and water demand and analyzing droughts based on the theory of runs may determine these characteristics. This study is focused on drought analysis where the underlying water supply process is periodic stochastic, such as for monthly streamflows. The probability mass function (pmf) of drought length and associated low‐order moments are derived assuming a periodic simple Markov chain. The derived pmf allows estimating the occurrence probability of droughts of a given length and its return period. The applicability of the drought formulations has been illustrated using a variety of water supply series such as monthly and weekly precipitation, monthly streamflows, the Palmer hydrologic drought index, and the standardized precipitation index. The results obtained confirm the validity of the analytical derivations for drought lengths and associated return periods. The overall conclusion of the study is that simple definitions of droughts enables one characterizing droughts using stochastic approaches and analytical derivations. They are particularly useful for drought analysis because the limited hydrologic records that are generally available do not allow observing many drought events of a particular duration and, in fact, extremely long droughts may not even be observable from the historical sample. This hinders the applicability of an inferential approach for finding the probability distributions of drought lengths and their associated return periods because it is either impractical or not feasible.

Phenotypic plasticity and acclimation to water deficits in velvet-grass: a long-term greenhouse experiment. Changes in leaf morphology, photosynthesis and stress-induced metabolites

Wild velvet-grass (Hocus lanatus) collected from a natural population along a drought gradient, previously demonstrated as genetically homogeneous, was studied for phenotypic plasticity and acclimation mechanisms to water deficits. A three months greenhouse experiment was designed with plants submitted to nine levels of water availability, three months being the usual length of summer drought in field conditions. Aboveground biomass, total soluble proteins, free proline, free polyamines, leaf morphology, stomatal and hair characteristics, and net photosynthetic rates were analysed, and their significance discussed. Moderate drought did not damage the velvet-grass, and the plants grew better than without water limitation. Water stress-typical responses were shown as time and severity dependent in all the measured parameters. Lowest water availability treatments induced significative increase in free proline and soluble protein contents, as well as reductions in leaf size and aboveground biomass. Spermidine contents rose significatively in the most severe water stress. Furthermore, velvet-grass acclimated progressively to long-term water limitation.

Spatial and temporal characterization of annual extreme droughts in Catalonia (northeast Spain)

Several characteristics of annual extreme drought episodes for Catalonia (northeast Spain) are investigated from different points of view. Firstly, possible temporal trends on the length of these episodes are analysed, by means of Student's t test and a Monte Carlo procedure, for each complete pluviometric series. Secondly, the spatial distribution of the seasonal predominance of the annual extreme droughts is analysed, showing the influence of the orographic factors. Thirdly, most of the annual extreme episodes are modelled correctly by the Gumbel distribution of extremes, and the expected lengths for return periods of 2, 5, 10, 25 and 50 years are computed. Nevertheless, some discordances were detected between this theoretical distribution and the empirical extremes. Thus, the return periods for some gauges were computed by means of the more general Jenkinson model. Finally, the set of pluviometric stations are characterized by the extreme drought lengths of the different return periods, and they are classified according to the average linkage algorithm into clusters of homogeneous gauges. In short, the results obtained suggest that most of the country is characterized by a typical Mediterranean behaviour. However, some differences can be established between littoral areas, some parts of the Central Basin and the pre-Pyrenean and Pyrenean ranges. © 1998 Royal Meteorological Society.

Spatial and temporal characterization of annual extreme droughts in Catalonia (northeast Spain)

Several characteristics of annual extreme drought episodes for Catalonia (northeast Spain) are investigated from different points of view. Firstly, possible temporal trends on the length of these episodes are analysed, by means of Student's t test and a Monte Carlo procedure, for each complete pluviometric series. Secondly, the spatial distribution of the seasonal predominance of the annual extreme droughts is analysed, showing the influence of the orographic factors. Thirdly, most of the annual extreme episodes are modelled correctly by the Gumbel distribution of extremes, and the expected lengths for return periods of 2, 5, 10, 25 and 50 years are computed. Nevertheless, some discordances were detected between this theoretical distribution and the empirical extremes. Thus, the return periods for some gauges were computed by means of the more general Jenkinson model. Finally, the set of pluviometric stations are characterized by the extreme drought lengths of the different return periods, and they are classified according to the average linkage algorithm into clusters of homogeneous gauges. In short, the results obtained suggest that most of the country is characterized by a typical Mediterranean behaviour. However, some differences can be established between littoral areas, some parts of the Central Basin and the pre-Pyrenean and Pyrenean ranges. © 1998 Royal Meteorological Society.

Predictions of drought length extreme order statistics using run theory

A new model has been developed here, incorporating prospective run theory to predict events based on order statistics. The difference equations for such a system are presented and the closed form solution is obtained in terms of parameters to which the practising hydrologist has access. Predictions are provided for duration of maximum drought lengths as a function of the definition of drought, the number of years over which the prediction is made, and the probability of inadequate water resources.