Southwestern Desert Research Articles

Some Floristic Characteristics of the Northern Chihuahuan Desert: A Search for Its Northern Boundary

An index of Chihuahuan floristic affinity was constructed to characterize the floristic variation over a 5° latitudinal gradient in the northern Chihuahuan Desert, with an emphasis on evaluating northern boundary conditions. The index was based on evaluating the ranges of 494 species from 590 vegetation plots from Bouteloua eriopoda and B. gracilis grasslands and Larrea tridentata scrub, distributed among four sites along the gradient. The most northern site, the Sevilleta National Wildlife Refuge (34°N latitude), while maintaining a complement of primarily southern distributed species, had the lowest index values and the fewest Chihuahuan endemics (3%) and Southwestern desert species (19%) in general. At the intermediate sites (Jornada/Tularosa basins and Otero Mesa), one and two degrees further south, index values increased conspicuously along with number of Chihuahuan endemics (7%) and the Southwestern desert species (37%). At 29°N latitude, the Sierra del Carmen Protected Area in northern Mexico was the furthest southern site. It represented typical hot and dry Chihuahuan Desert conditions where Chihuahuan endemics comprised 29% of species complement and Southwestern desert species made up 55%. With respect to floristic boundaries, the Sevilleta was clearly transitional to the southern Shortgrass Prairie and Intermountain (Great Basin-Colorado Plateau) provinces. While it supported several grassland and shrubland associations that maintained their overall Chihuahuan character, there were also several associations with primarily northern affinities, and also a set unique to the Sevilleta, reflecting its ecotonal nature. The results show that sound biosystematics, biogeographical analyses and indices like those presented here can provide an important context for questions in synecology, plant ecology, and conservation biology.

Some floristic characteristics of the northern Chihuahuan Desert: a search for its northern boundary

AbstractAn index of Chihuahuan floristic affinity was constructed to characterize the floristic variation over a 5° latitudinal gradient in the northern Chihuahuan Desert, with an emphasis on evaluating northern boundary conditions. The index was based on evaluating the ranges of 494 species from 590 vegetation plots from Bouteloua eriopoda and B. gracilis grasslands and Larrea tridentata scrub, distributed among four sites along the gradient. The most northern site, the Sevilleta National Wildlife Refuge (34°N latitude), while maintaining a complement of primarily southern distributed species, had the lowest index values and the fewest Chihuahuan endemics (3%) and Southwestern desert species (19%) in general. At the intermediate sites (Jornada/Tularosa basins and Otero Mesa), one and two degrees further south, index values increased conspicuously along with number of Chihuahuan endemics (7%) and the Southwestern desert species (37%). At 29°N latitude, the Sierra del Carmen Protected Area in northern Mexico was the furthest southern site. It represented typical hot and dry Chihuahuan Desert conditions where Chihuahuan endemics comprised 29% of species complement and Southwestern desert species made up 55%. With respect to floristic boundaries, the Sevilleta was clearly transitional to the southern Shortgrass Prairie and Intermountain (Great Basin­Colorado Plateau) provinces. While it supported several grassland and shrubland associations that maintained their overall Chihuahuan character, there were also several associations with primarily northern affinities, and also a set unique to the Sevilleta, reflecting its ecotonal nature. The results show that sound biosystematics, biogeographical analyses and indices like those presented here can provide an important context for questions in synecology, plant ecology, and conservation biology.

Nostalgia por la tierra, nostalgia por el dolar: Guatemalan transnational lives and ideology of return migration

This paper addresses the ideology of return among contemporary Guatemalan migrants living and working in the southwestern desert city of Phoenix in Arizona. In Phoenix’s metropolitan area, Guatemalans (both Ladino and Maya ethnic groups) add to the city’s cultural mosaic, and at the same time, are vital agents for change in Guatemalan society. Little is known about the processes and patterns behind Guatemalan migration despite the fact that over 10 percent of Guatemala’s population currently resides in the United States. Most Guatemalans in Phoenix, regardless of their length of residence in the United States, express longings for return to their homeland. In part, this strong notion of return prevails because the Guatemalan community in the Phoenix metropolitan area is not cohesive.

Environmental pollutant isotope measurements and natural radioactivity assessment for North Tushki area, south western desert, Egypt

Natural radioactive materials under certain conditions can reach hazardous radiological levels. The natural radionuclide ( 238U, 232Th, 40K) contents of rock samples at various locations in the North Tushki area were investigated using gamma-spectrometric analysis. Estimates of the measured radionuclide content have been made for the absorbed dose rate of gamma radiation. The equivalent radium ( R eq) and the external hazard index ( H ex) which resulted from the natural radionuclides in soil are also calculated and tabulated. The studied samples have been collected from various rock exposures in the North Tushki area. The distribution of major oxides, U and Th were studied. It is found that the enrichment and depletion of the major oxides are mainly due to the effect of hydrothermal alteration, which caused mobility of some major oxides, which increases some elements and decreases others. It is important to mention that the study area is far from the development region of the Tushki project and is only a local hazard. Therefore, additional regional studies of the Tushki Project area should be under taken to explore any unexpected environmental hazard due to the high concentration of the radioactive elements, which have been observed at its north boundary.

ALFALFA BASAL CROP COEFFICIENTS FOR FAO–56 PROCEDURES IN THE DESERT REGIONS OF THE SOUTHWESTERN U.S.

The FAO Irrigation and Drainage Paper No. 56 (FAO56) is expected to provide a universally consistentmethodology for obtaining reliable estimates of crop evapotranspiration (ETc) from standard weather data and cropcoefficient (Kc) information. Yet, for many areas, including the southwestern desert region of the U.S., evaluation of theFAO56 crop coefficient methods for local conditions has been limited. In this study, measurements of daily alfalfa ETc wereused to evaluate the dual crop coefficient approach of FAO56, which separates the single Kc into two coefficients, a basalcoefficient, Kcb (primarily transpiration), and a wet soil evaporation coefficient, Ke. A second objective was to calibratealfalfa Kcb baselines for constructing localized, climateadjusted FAO56 Kcb curves for growth cycles under the seasonalclimatic variations encountered in the semiarid, southwestern U.S. Daily ETc was measured for alfalfa in three weighinglysimeters subjected to varying irrigation regimes for eight cutting cycles in 1985 at a field site in Phoenix, Arizona. DailyKc data were determined from the measured ETc and calculations of the FAO56 grassreference evapotranspiration (ETo)using meteorological data obtained at the site. Daily Kc values were partitioned into the dual crop coefficients (Kcb and Ke)using backcalculations of the FAO56 dual procedures. Examples are given to illustrate how the magnitudes of Kcb, Ke, andKc varied with respect to crop development and wet and dry surface soil conditions. The effects of water stress on ETc wereconsidered in the backcalculations using the FAO56 water stress reduction coefficient (Ks). Linearstyle FAO56 Kcbcurves were then constructed separately for each lysimeter for each of the eight cutting cycles based on the backcalculatedKcb data. Daily ETc based on the constructed curves and FAO56 calculation procedures were highly correlated to the dailymeasured lysimeter ETc (r2 = 0.98), and the mean daily difference between calculated and measured ETc (0.03 mm) was notsignificant (p > 0.7). Comparison of measured ETc for waterstressed and wellwatered alfalfa indicated that calculated Ksdescribed ETc reductions due to soil water stress adequately when atmospheric evaporative demand was considered in theKs calculation. Baseline Kcb values for constructing local climateadjusted FAO56 alfalfa Kcb curves were determined as0.30, 1.22, and 1.05 for the initial, midseason, and end of late season, respectively.

Uranium isotopic study of artesian and pluvial contributions to the Nubian Aquifer, Western Desert, Egypt

The groundwater systems of the Nubian Aquifer beneath six major oases (Baris, Kharga, Dakhla, Farafra, Bahariya, and Siwa) in the Western Desert of Egypt have been studied to establish their sources and mixing volumes using uranium isotopes. One hundred six groundwater samples from different depths of the Nubian Aquifer have been analyzed for uranium content and 234U/ 238U activity ratio (AR). The aquifer under the Western Desert is known to have more than one source of water. At Bahariya and Farafra Oases, the Nubian artesian water migrating from the south has been augmented by local recharge during pluvial times. At Siwa Oasis in the northwestern desert, a shallower aquifer component migrating from the north or west is also present. At Dakhla, Kharga and Baris Oases in the southwestern desert, the main source is the Nubian artesian water migrating from southeast Uweinat Upland and northwest Sudan. These water masses have distinctive uranium isotopic signatures. The Nubian Aquifer water is characterized by very low U concentrations (<0.05 ppb) and a relatively high 234U/ 238U AR (>1.5). The shallow northwest aquifer water also has a high AR but a much higher U concentration. The locally recharged pluvial waters have high U concentrations (>0.1 ppb) but low ARs, near unity. A diagnostic derivative parameter is excess 234U content. The deep Nubian Aquifer water is characterized by a relatively low excess 234U (<0.02 ppb equivalent), while the shallow Siwa water has a very high excess 234U [(AR−1)(U conc.)]. The Bahariya and Farafara waters are also high in this component, probably because of pluvial water percolation through phosphate rich strata. At all oases, U isotopic mixing diagrams show that the deep aquifer sources predominate; however, the pluvial contributions are significant, ranging from about 5% at remote Baris Oasis to about 26% at the more northerly Farafra Oasis. The observed lowering of potentiometric surfaces in the Western Desert is caused not only by pumping at a rate greater than inflow from the aquifer systems, but also by the withdrawal of pluvial water which in modern times is not being replaced at all.

Large-Scale BRDF Retrieval over New Mexico with a Multiangular NOAA AVHRR Dataset

In this study a number of linear semiempirical kernel-driven (LiSK) bidirectional reflectance distribution function (BRDF) models are adjusted against an extensive Advanced Very High Resolution Radiometer (AVHRR) dataset collected over a variety of semiarid cover types in the southern part of New Mexico and parts of Chihuahua, Mexico as part of the May 1997 Prototype Validation Exercise (PROVE) campaign, an activity of the NASA Earth Observing System Terra validation program. The aim is to investigate model behavior under conditions of sparse angular sampling such as that provided by the AVHRRs and MODIS over a wide variety of southwestern desert surface types. Linear semiempirical models of the type to be used in the MODIS/MISR BRDF/albedo product (MOD43) are inverted, since these are appropriate for use over large areas. The results of the inversions show that these models are able to describe BRDF for a wide variety of surfaces and provide both a means for correcting for directional phenomena in satellite data and for extracting structural information from multiangular reflectance datasets.

Molt of the Gray Vireo

AbstractUsing museum specimens, I document the molt cycles and molting grounds of the Gray Vireo (Vireo vicinior). During prebasic molts, adult female Gray Vireos replace their primaries in 57 days, whereas adult males take 70 days; all body plumage is replaced during this molt. Prebasic molts occur almost exclusively on the breeding grounds; just 3 of 41 specimens replacing primaries were collected away from breeding grounds. No molting specimens were collected from wintering areas. Prealternate molt occurs on the wintering grounds, and appears limited to the replacement of innermost secondaries and a limited molt of body plumage. By performing prebasic molt on breeding grounds, the Gray Vireo differs from several other western breeding passerine species that use desert regions of the southwestern United States and northwestern Mexico to exploit late summer food resources. The areas of these southwestern desert regions used by other species form a small portion of the breeding grounds, and encompass the entire wintering grounds, of Gray Vireos. I hypothesize that this contrast in molting regions is not due to differences in the general timing of prebasic molts among these species; rather, the contrast may be due to constraints imposed on Gray Vireos by a dietary shift to fruit during winter and the need to defend winter territories.

Geoarchaeology of the Holocene climatic optimum at Nabta Playa, Southwestern Desert, Egypt

Except for the Kharga–Dakhla Depression in the north, Nabta Playa is probably the largest internally drained basin in the Southwestern Desert of Egypt. The center of the basin is filled with clastic materials, deposited from suspension during inundations. These finer-grained clastics interfinger with sandy beach deposits associated with early Holocene phytogenic dunes. The shores contain embedded Early Neolithic occupations ranging in age from ca. 9400 to 7250 14C yr B.P. (all radiocarbon ages reported in this article are uncalibrated). There was a phase of reduced sedimentation in the playa between 8100 and 7300 14C yr B.P. During this period, at least 130 species of plants, including 10 varieties of trees and bushes, were growing in the area. These plants may have formed a dense cover over the uplands surrounding the basin and greatly reduced the sediment load in the seasonal runoff. This was the Holocene climatic optimum at Nabta Playa. It was also the time of maximal development of human settlement in the Nubia–Sheb Pediplain. © 2001 John Wiley & Sons, Inc.

MOLT OF THE GRAY VIREO

Using museum specimens, I document the molt cycles and molting grounds of the Gray Vireo (Vireo vicinior). During prebasic molts, adult female Gray Vireos replace their primaries in 57 days, whereas adult males take 70 days; all body plumage is replaced during this molt. Prebasic molts occur almost exclusively on the breeding grounds; just 3 of 41 specimens replacing primaries were collected away from breeding grounds. No molting specimens were collected from wintering areas. Prealternate molt occurs on the wintering grounds, and appears limited to the replacement of innermost secondaries and a limited molt of body plumage. By performing prebasic molt on breeding grounds, the Gray Vireo differs from several other western breeding passerine species that use desert regions of the southwestern United States and northwestern Mexico to exploit late summer food resources. The areas of these southwestern desert regions used by other species form a small portion of the breeding grounds, and encompass the entire wintering grounds, of Gray Vireos. I hypothesize that this contrast in molting regions is not due to differences in the general timing of prebasic molts among these species; rather, the contrast may be due to constraints imposed on Gray Vireos by a dietary shift to fruit during winter and the need to defend winter territories.

Two subphenotypes of childhood asthma that differ in maternal and paternal influences on asthma risk.

Asthma is a phenotypically heterogeneous disease. Two subgroups are defined here based initially on skin test reactivity to the allergen Alternaria at age 6 from among a large population of children born and raised in the Southwestern desert environment of Tucson, Arizona. When compared with asthma among Alternaria-positive subjects, asthma among Alternaria-negative subjects was associated with lower levels of total serum IgE, no relation to local aeroallergen skin tests, a younger age at diagnosis, greater remittance by age 11, and more frequent wheezing lower respiratory illnesses (LRIs) in the first year of life. Despite the difference in total serum IgE, however, IgE concentrations were significantly higher in each asthma group compared with its respective control group. Asthma in each parent contributed approximately equivalent risk for Alternaria-positive asthma in the child. However, neither parental skin test sensitization nor total serum IgE levels provided risk for asthma in the child. Inheritance patterns for Alternaria-negative asthma revealed a contribution from maternal but not paternal asthma. Thus, dividing asthma in children at age 6 into Alternaria-positive and Alternaria-negative groups identifies subphenotypes that are further distinguished by differences in phenotypic markers and parental influences.

Below Ground Pot-in-Pot Effects on Growth of Two Southwest Landscape Trees was Related to Root Membrane Thermostability

Abstract Two southwestern desert landscape trees, Acacia smallii L. (sweet acacia) and Cercidium floridum Benth. ex A. Gray (blue palo verde), were grown outdoors in full sun during Summer 1997 in 19-liter (#5) containers placed either pot-in-pot (PIP) below ground or unshielded in above-ground containers (AGC). Soil moisture sensors wired to electronic solenoid valves regulated occurrence of six cyclic micro-irrigation pulses per day (0600, 0900, 1200, 1500, 1800, and 2100 HR) such that container substrate moisture tensions were continuously maintained between −0.005 to −0.01 MPa (90% of water holding capacity) in both PIP and AGC. Mean maximum recorded root-zone temperatures in PIP containers were 19C (34F) lower than for AGC. Micro-irrigation volumes were 40% less for trees grown PIP compared with those in AGC. Growth of sweet acacia was enhanced by PIP placement while in containers and one year after transplanting trees into field plots in 1998. Only caliper growth of blue palo verde was increased by PIP placement while in containers, but had no effect on blue palo verde growth one year after transplanting into field plots. The critical killing temperature (TM) for root tissues of sweet acacia and blue palo verde were 45.3 ± 1.8C (113.5 ± 3.2F) and 49.4 ± 0.8C (120.9 ± 1.4F), respectively, indicating differences in root membrane thermostability. Based on our data, we suggest that sweet acacia trees benefitted from PIP placement more than blue palo verde trees because root-zone temperatures in PIP containers were lower than for AGC in central Arizona, and sweet acacia roots were more susceptible to injury by supraoptimal root zone temperatures.

Phylogeography of a Wide-Ranging Desert Lizard, Gambelia wislizenii (Crotaphytidae)

Intraspecific phylogeography of the long-nosed leopard lizard (Gambelia wislizenii) was examined using 419 base pairs of sequence of the mitochondrially encoded COIII gene from 23 individuals sampled from populations representing five regional deserts in western North America. Intraspecific population structure was evaluated under two biogeographic models: (1) gene flow among populations during late Pleistocene to Recent times, possibly in conjunction with range expansions from Pleistocene refugia; or (2) subdivision among southwestern desert regions that occurred in conjunction with late Pliocene/early Pleistocene geological events or as a result of isolation in separate Pleistocene refugia. Phylogenetic analysis indicated subdivision of 18 mitochondrial DNA haplotypes into two major clades separated by approximately 6.1% sequence divergence: Western (Mojave + Great Basin + Colorado Plateau deserts) and Eastern (Chihuahuan desert). A third, more divergent, lineage with unknown geographic distribution was represented by a single haplotype from southwestern Arizona. The Western clade contained two northern Great Basin subclusters (eastern Great Basin and western Great Basin), representing populations separated from one another by approximately 5.4% sequence divergence. These phylogeographic breaks are consistent with the second model of late Pliocene/early Pleistocene vicariance. These data are also consistent with assessments of the biogeographic histories of several other arid-dwelling vertebrates.

Biolistic transformation of the human pathogenic fungus Coccidioides immitis

Coccidioides immitis is a fungal respiratory pathogen of humans. The infectious, saprobic phase of the microbe resides in soil of southwestern desert regions of the United States. The genetics of this pathogen are essentially undeveloped. Progress in studies of the molecular biology of C. immitis have been hampered by the absence of a transformation system. We report here the first stable integration of plasmid DNA into chromosomes of C. immitis by the method of biolistic DNA delivery. A previously-described pAN7-1 plasmid and a newly-constructed C. immitis URA5 gene disruptant (pU5H-1) were used to transform C. immitis strain 735. The 6.8-kilobase (kb) pAN7-1 plasmid and the 10.1-kb pU5H-1 construct both contained the hygromycin phosphotransferase gene ( hph) which encodes a protein that inactivates hygromycin B (HmB) by phosphorylation. Germinated, saprobic phase spores (arthroconidia) and parasitic phase cells (spherules) of C. immitis were subjected to bombardment by gold particles coated with either the linearized pAN7-1 or the circular pU5H-1 plasmid DNA (50 μg DNA per 25 mg gold particles). A single pAN7-1 and two pU5H-1 transformants were initially selected from approximately 10 7 cells on the basis of their ability to grow on glucose–yeast extract agar supplemented with 60–80 μg/ml HmB. The transformants showed resistance to HmB up to a concentration of approximately 320 μg/ml of media, compared to the parental strain which was unable to grow in the presence of 40 μg HmB/ml of media. Southern hybridization analyses of the transformants suggested that both the pAN7-1 and pU5H-1 DNAs were integrated into chromosomal DNA of C. immitis as tandem repeats. After animal passage, all transformants showed mitotic stability of the integrated DNA. This newly-developed transformation system permits genetic manipulations of C. immitis which were not previously possible.

El Niño Drives Spectacular Flower Show

TUCSON, ARIZONA-- This spring9s eruption of wildflowers in the Southwestern desert, one of the most dazzling blooms of the century, came courtesy of the heavy rainfalls of last year9s El NiA±o. The desert display has been a boon to scientists hoping to learn more about how wildflowers survive in harsh climates--and why not every flower blooms in every rainy year. Botanists are finding that not just total rainfall, but its timing and how it interacts with temperature and plants9 own survival strategies, determines when these blooms occur.

Paleozoic Stratigraphy and Petroleum Systems of the Western and Southwestern Deserts of Iraq

ABSTRACT A stratigraphic scheme for the Paleozoic of the Southwestern Desert of Iraq is proposed based upon the review of recently published data from several deep wells in the western part of the country and from outcrops in other regions in Iraq. The main formations are described in terms of facies distribution, probable age, regional thickness, and correlations with the well-reported Paleozoic successions of the adjacent countries (e.g. Jordan and Saudi Arabia), as well as with the Thrust Zone of North Iraq. The Paleozoic depositional and tectonic evolution of the Western and Southwestern Deserts of Iraq, particularly during Cambrian, Ordovician and Silurian, shows marked similarity to those of eastern Jordan and northern Saudi Arabia. However, local lithological variations, which are due to Late Paleozoic Hercynian tectonics, characterize the Upper Paleozoic sequences. The Lower Silurian marine “hot” shale, 65 meters thick in the Akkas-1 well in the Western Desert, is believed to be the main Paleozoic source rock in the Western and Southwestern Deserts. Additional potential source rocks in this region could be the black shales of the Ordovician Khabour Formation, the Upper Devonian to Lower Carboniferous Ora Shale Formation, and the lower shaly beds of the Upper Permian Chia Zairi Formation. The main target reservoirs are of Ordovician, Silurian, Carboniferous and Permian ages. Similar reservoirs have recently been reported for the Western Desert of Iraq, eastern Jordan and northern Saudi Arabia. In addition, two main regional seals (Lower Silurian shales and Permian evaporites) of northeastern Arabia extend over most of the Southwestern Desert, together with several other local seals. These considerations render the unexplored Paleozoic Southwestern Desert of Iraq prospective.

Punctuated aridity in southern Africa during the last glacial cycle: The chronology of linear dune construction in the northeastern Kalahari

Abstract The Mega Kalahari of central southern Africa is one of the most extensive Quaternary desert basins. On a regional scale, present-day aeolian activity is restricted to episodic dune crest reactivation in the most arid southwestern desert core. There is, however, abundant evidence of former periods of both more arid and more humid conditions, many of which have little or no chronological control. We have employed optical dating of quartz sand grains to develop a chronology of arid intervals as recorded by phases of linear dune construction in the northeastern sector of the Mega Kalahari. We identify repeated phases of aeolian deposition during the last interglacial-glacial cycle, at ca. 95–115, 41–46, 20–26 and post-20 ka, which are separated by depositional hiatuses that we infer to correspond to more humid periods. These aeolian depositional events correlate with and are inferred to relate to millennial-scale cold sea surface temperature events in the southeast Atlantic which have been linked to sub-milankovitch climate changes recognised in northern hemisphere oceanic and cryospheric environmental archives covering the same time period. While the present landscape is the product of either post-20 ka (Hwange National Park dune field) or 20–30 ka (Victoria Falls dune field) aeolian activity and subsequent erosion and reworking, much of the vertical expression of the larger dune forms corresponds to the earlier periods of activity. The linear ridges of the area are rich archives of late Quaternary terrestrial climate change.

Detailed gravimetric geoid for the Egyptian south-western desert

Different gravimetric geoid solutions were carried out for the south-western Egyptian desert. A set of 2682 measured gravity stations were available for this investigation. The discrete gravity data are reduced to the geoid through the free-air reduction. Also terrain reduction is performed using a set of fine 30″ × 30″ and coarse 3′ × 3′ Digital Height Models for the test area. Different earth's geopotential models were used for the removing of the reference field. The geoid computations were performed in the space domain using the discrete Stokes integration technique and in the spectral domain using the spherical 2-D FFT technique. For both techniques, the reduced gravity anomalies are gridded on a 3′ × 3′ geographical grid using the least-squares interpolation technique with local covariance functions. A broad comparison among the different geoid solutions is made. The effect of the inner zone densification is in the order of about 2 cm. The effect of the spherical 2-D FFT approximation is mostly less than 1 cm.

Urban waterways: changing historical uses and users in a southwestern desert city

This study traces the changing historical uses of irrigation canals and documents current activities along the canals in a rapidly urbanizing section of the Phoenix metropolitan area in the State of Arizona in the southwestern United States. The Highline and Western Canals, in an area of South Phoenix, provide a case study for understanding the role of the canals as their surroundings change from agricultural land use to an urban context. Reviews of land use changes, archival data, and photographs; interviews with local leaders; and canal user surveys offer a historical picture of the area, describe the characteristics of today's canal users, and provide a context for planning of the canal system. This study serves as a pilot project for further research about the users and historical and current uses of canals that run through urbanizing cities. Survey findings indicate that two-thirds of the canal way users are male. Sixty percent of the respondents are in the 30- to 49-year old age group. The most frequent activity noted on the canal path is bicycling, accounting for approximately four-fifths of all activity, while other activities observed on the canal paths include walking, jogging, and horseback riding. Statistical analysis suggests that the gender of the respondent is directly related to his or her safety concerns, regardless of the activity involvement, with almost four times as many women being concerned about personal safety as men. A key finding of the survey was that almost 60% of the canal users have resided in the same dwelling for more than ten years, suggesting the opportunity to involve these long-term residents in canal planning activities. The historical picture offered in this study, along with data about present canal uses and users, can provide planners with a clearer picture of the urban system that surrounds these irrigation canals. Their potential as an amenity for urban communities can be better realized if planners take into account their history and their present uses.

Multiple episodes of aridity in southern Africa since the last interglacial period

There is generally a dearth of evidence of the nature of Quaternary climate change within desert systems, which has limited previous interpretations of past environmental change at low latitudes. The Last Glacial Maximum has previously been identified as the peak of Late Quaternary aridity, when desert systems expanded to five times their present extent1,2,3, and low-latitude aridity has been described for previous glaciations4. But little evidence has been derived directly for large desert basins, particularly southern Africa. Here we report new chronological (optical dating) evidence of arid episodes recorded in aeolian sediments from the Mega Kalahari sand sea. Episodic aeolian activity is recorded at the northeastern desert margin, whereas more sustained activity is evident from the southwestern desert core. Several significant arid events are apparent since the last interglacial period, with dune-building (arid) phases at ∼95–115, 41–46, 20–26 and 9–16 kyr before present. Existing atmospheric general circulation model simulations and independent palaeoclimate data indicate that the changes in aridity are related to changes in the northeast–southwest summer rainfall gradient, which are in turn related to sea surface temperatures in the southeastern Atlantic Ocean.