Abstract: The structure of mangrove vegetation, soil salinity, and topographic relief of the mangrove ecosystem were studied along the south coast of Puerto Rico, Culebra, and Mona Island. All systems, with the exception of the forest at Mona Island, were characterized by having a coastal fringe of live vegetation (usually dominated by the red mangrove), a zone of dead trees, and a hypersaline lagoon or dry salt flat on the landward side. Mean soil salinities were 44 0/00 for the zone of live trees, 72 0/00 for the zone of dead trees, and 87 0/00 for the salt flats. The Mona Island forest grew over a sandy hardpan that prevented mixing of sea water and the fresh water. Soil salinities were low in this forest, and trees reached a height of 15 m. Tree height was inversely proportional to soil salinity (r = 0.72) between 17 and 72 0/00. When soil salinities exceeded 65 0/00, dead tree basal area was higher than live tree basal area. It is suggested that mangrove growth is limited by soil salinities higher than 90 0/00. Mangrove succession in offshore overwash islands and arid coastlines is described. It is proposed that cyclic rainfall patterns and hurricanes act as regulators of speed and direction of succession. Rainy periods are associated with lower soil salinities and expansion of the red mangrove zone. Droughty periods result in high soil salinities, mangrove mortality, and expansion of salt flats. Hurricanes set back succession and reverse successional trends that reduce mangrove areas. A model of mangrove ecosystem function and implications for management are also discussed.

Abstract: The geographic distribution of C4 species in the Dicotyledonae of North America was examined in relation to climatic parameters in an attempt to identify environmental conditions which favor the presence of C4 species. The percentage of C4 dicot species in the spermatophyte flora in a geographic region is best predicted by a combination of summer pan evaporation and dryness ratio. Both of these variables are closely related to plant water balance, supporting previous contentions that the C4 pathway arose in the Dicotyledonae as an adaptation to arid conditions. However, the distribution of the C3 species in the C4 dicot families is also strongly correlated with summer pan evaporation. Thus, there appear to exist adaptive properties of these families, in addition to the type of photosynthetic pathway, which favor their presence in arid regions. The climatic correlations of C4 dicot species differ from those of the C4 species of the Gramineae, in which minimum daily temperatures are most important. It appea...

Abstract: Small mammal communities were studied along a 1000 km latitudinal transect on the northern Chilean coast. Vegetation here is dominated by evergreen and drought-deciduous shrubs in the south, which are progressively replaced by succulents in the more arid northern regions. Nine localities were sampled twice during 1973-74 for small mammals; measurements were taken also of vegetation characteristics including cover, height, volume and foliage density as well as site slope, number of hiding holes and soil hardness. Precipitation was determined from weather station records. Using two measures of mammal species numbers, the total number of species known to be present (TNS) and the number of species actually caught (NSC), a stepwise multiple regression with all environmental variables indicated significant relationships to logarithmically-transformed measures of precipitation and herbaceous cover. Both latter variables demonstrate a northward decrease, and are significantly correlated with each other. Since available ecological information indicates that none of the small mammals are water-independent, and a large component (the caviomorphs) are primarily herbivorous, precipitation may act as an ultimate factor and herbaceous cover a proximate one in limiting the number of small mammals in the northern arid zone. Although some northern localities have insular characteristics, recent glacial history and potential avenues of dispersal down Andes flanks and river valleys suggest that isolation has not accounted for the present northward decrease in species numbers.

Abstract: Summary The arid and extremely arid areas of the Middle East are outlined, and the past and present climatic regimes within these areas considered and related to the existing rock and soil conditions. A system of classification of the ground into four simple zones based on their desert characteristics and the natural processes operating within the zone is described. The general engineering character of each zone is discussed and particular problems highlighted. Zone I is the mountainous areas, zone II the large gravel fans bounding mountains, zone III the alluvial plains beyond the fans, and zone IV the central base level plains between mountains or adjacent to coasts. The great majority of desert soils are granular and their engineering behaviour is directly related to their grading characteristics. Flooding is an important hazard in the fans and alluvial plains, while in the base level plains high water tables are of major importance. Ground conditions of significance in deserts, which do not generally occur elsewhere include aggressive salty ground, wind blown sands and silts, and hard and soft duricrusts. These and certain other ground conditions are described.

Abstract: against Risk in Arid and Semi-Arid Tropical Areas of India N S Jodha Weather-induced instability of farming is the principal source of risk in agriculture in arid and semi-arid tropical areas of India The problem presents itself in its most magnified form during drought years During such occasions, which are quite frequent, farming fails to ensure even the minimum subsistence requirement of humans and livestock

Abstract: The riverine forest formation on the levees along the Orange River in South Africa shows a shift in floristic composition as the river traverses various climatic zones on its course through the temperate area in the eastern parts of the subcontinent, the central semi-desert region, and the desert area near the Atlantic in the west. Leaf size classes and leaf consistency types of the woody species in the riverine forest were determined for each community. Analysis of these data revealed a diversification of leaf sizes as the climate changed from temperate to hot and arid and particularly microphylls became relatively less important and were replaced by smaller leaves in the hot areas. In the same direction malacophylls, which are of the "low-cost, quick-profit" strategy type and are well represented in the temperate, frosty areas, disappear and xeromorphic leaves ("high-cost, slow-profit" strategy type) increase in importance. It is suggested that the cooler conditions with higher wind speeds and higher degrees of average air humidity near the Atlantic coast are reflected by an increase in mesophylls which are sclerophyllous as an adaptation to the frequently and rapidly changing temperature conditions here.

Abstract: The hypothesis that present-day ground-water gradients in arid regional sedimentary basins are a decay feature of very old recharge mounds is examined using a resistance network analogue. A longitudinal section model of a conceptual basin with some specific control of aquifer permeability and ground-water age is used. With reasonable combination of aquifer permeability and specific yield it is shown that an acceptable head decay could have occurred over the past 10,000 years to produce the present-day gradient. In the model adopted, decreasing permeability with depth was found to be required to accord with 14C ground-water age data. Although the model is not considered to be definitive for any particular aquifer the results show that fossil gradient conditions could exist in the regional basins for long periods and should be taken account of in ground-water resources assessments.

Abstract: An analysis of the coarse (>40 µm) and clay fractions of Neogene sediments from Sites 135, 139, 140, 141 (DSDP Leg 14) has been carried out. The Early Miocene sediments of Sites 139 and 140 are eolian sand turbidites consisting of coarse quartz sand and shallow water particles. They have been formed during a period with arid climate and lowered sea-level. Upwelling on the upper slope produced a rather high opal content of the allochthonous sediments. Smectite (70-85%) and kaolinite (15-30%) are the dominant clay minerals. The clay assemblage of the Late Miocene of Site 141 indicates a climate with contrasting seasons on the African continent, the arid season being most important. The Early Pliocene (Sites 135, 141) and part of the Late Pliocene (141) reveal a mainly arid climate on the continent, with only small scale changes. Variations in detrital clay mineral contents, especially chlorite, mixed-layers and smectite, suggest some more humid periods on the continent, which cannot be detected from coarse terrigenous components, because the general trend in climate is more arid than humid and because transport agents were too weak to transport major amounts of terrigenous particles >40 µm. There is a good correlation between coarse and clay sized terrigenous components in the Late Pliocene and the Pleistocene of Site 141. Terrigenous components >40 µm reveal a constantly arid climate and changing trade wind velocities in the latitude of 20°N. Clay minerals, transported by the Canary current from African regions northeast of Site 141, suggest changes in humidity and in soil formation and soil destruction. Strong trade wind velocities in 20°N correlate with a humid climate north of 20°N ("glacial type"), small trade wind velocities with arid climate ("interglacial type"). The correspondence of results is due to rather strong transport energies, which supply clay and >40 µm sized terrigenous material. Calcium carbonate dissolution is strongest (100% fragmentation of planktonic foraminifers) in the Late Miocene of Site 141, decreasing in the Pliocene and showing cycles in the Late Pliocene/Pleistocene which might be correlated to periods of advancing cold arctic/antarctic bottom water. Sorting effects in the Late Pliocene/Pleistocene of Site 141 in those sections with strong CaCO3 dissolution and strong trade winds ("glacial type of climate") are tentatively interpreted by increased velocities of bottom currents, perhaps of arctic or antarctic origin.

Abstract: Rabbit burrows in three different habitats in an arid area showed characteristic variations and differences. Burrows in stony soil were generally cooler and more humid than burrows in sand and gravel soil. The temperature of the burrows in sand and gravel showed similar characteristics but the humidity in the gravel burrows was almost always significantly less than that of the sand burrows. Air and ground temperatures were significantly correlated but air and ground humidities showed a less significant correlation. Seasonal differences in microclimate within and between the three types of burrows were pronounced and are thought to cause differences in the survival of rabbits in arid areas.

Abstract: adapted species and to the unpredictability of natural precipitation. This study was designed to sift the native flora of the benchlands of the Kaiparowits Basin, a potentially important coal field, for native species capable of naturally recolonizing disturbed sites. We also provide some information concerning ecological situations where such species might best be used.

Abstract: Environmental effects of arid land irrigation in developing countries , Environmental effects of arid land irrigation in developing countries , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Abstract: Ecological systems in the semi-arid savanna of southern Africa are subject to an erratic moisture supply. This results in large and unpredictable fluctuations in plant production and changes in plant cover and botanical composition. Notably in the herbaceous layer. With rapidly increasing pressure on the land there is an urgent need to define land use procedures which will give maximum production over the years and which will minimise stress on livestock and the vegetation during droughts. Present knowledge is inadequate for this purpose, and there is a strong case for comprehensive study of the effects of rainfall fluctuations on plant and livestock production, botanical composition and soil cover. The feasibility of developing models to estimate plant production from simple meteorological, soil-water and plant growth parameters is indicated. It is shown how such models could be used in conjunction with empirical relations between stocking pressure and gain per animal, and stocking pressure and intake, to simulate fluctuations in animal and plant production over a long period of years. With such a model a variety of management options could be examined with particular emphasis on the critical seasons in which plant growth is at a minimum. Keywords: animal production; botanical composition; botany; drought; ecological systems; growth parameters; herbaceous layer; intake; land use; livestock; maximum production; model; models; plant cover; plant growth; production; rainfall; rainfall variability; semi-arid savanna; soil water; southern africa; stress; variability; vegetation

Abstract: Irrigation has been defined as the artificial control of soil moisture for agricultural purposes (Rydzewski 1974). Therefore the soil is probably the first consideration in any proposed scheme for irrigation, and geological survey must be a starting point to provide a clear understanding of the origin and distribution of the soils and groundwater conditions underlying them. The study of soils and their suitability for irrigated agriculture is a specialized subject. Its basis is an appreciation of the geology and in particular the geomorphology of the region. Problems in irrigation engineering associated with ground conditions usually arise as a result of changes in moisture content of the soils or of the ground water regime in the underlying material. This paper deals with problems that are universal in irrigated agriculture of arid or semi-arid lands, but the experience on which it is based and the examples quoted are derived from the Middle East. Irrigation drawing its supply from ground water is becoming increasingly important in the region, but this paper does not deal with strictly hydrogeological problems associated with supply. At the present time in the truly arid areas of the Middle East there is only limited experience of large scale irrigation. In some peripheral areas however, soils exist which, if irrigated, would be potentially of agricultural value. Extensive irrigation projects for semi-deserts and dry grassland indicate some of the problems that undoubtedly would be encountered if large scale irrigation schemes were attempted in the truly arid areas of the

Abstract: The distribution of rainfall in Sudan, despite its generally low value, is also highly seasonal in character. The country is therefore dependent on its river system for perennial irrigation for reliable agricultural production. At present about 4.6 million acres (1.9 million ha) are under irrigated agriculture. In all these areas, the system of irrigation is achieved by a gravity-flow method based on hydraulic principles which take maximum advantage of land contours. It consists of hierarchies of supply and distributive canals. The lower stages of these hierarchies, called Minor-canals, produce ponding conditions and are thus very favourable for extensive growth of aquatic weeds (macrophytes).Because future agricultural development will be based on gravity-flow, and for economic reasons, attention is focused on the increasing extent of this man-made aquatic habitat in the arid region of central Sudan.

Abstract: There are numerous instances where, through injudicious use, Man has contributed to desertification of the agricultural environment by increasing the aridity and decreasing the ability of the soil to support vegetation. This may come through erosion, even in areas with very high rainfall (e.g. Amazonia). But in many arid regions dedesertification has occurred under the influence of Man (e.g. Imperial Valley in California, Hungry Desert in Uzbekistan, 90-Miles Desert in Australia).However, under certain conditions, such as those of fog-drip, an irreversible desertification can be caused by removal of the original vegetation (e.g. Central Chile).Dew may be an important source of water for plants even in deserts, where there is a shortage cf liquid water for plant growth.

Abstract: This paper describes a simulation game prepared as an exercise for a course entitled 'Arid and Semi-arid Lands, their Status and Potential' which the authors taught in the Department of Geography of the University of Nairobi during the 1977 78 academic year. The simulation was intended to tie together the socio-economic and the physical aspects of arid and semiarid areas. It is concerned with the problem of land allocation in a situation where there is land-use conflict. The participants are divided ihto groups of farmers and pastoralists and are required to adjudicate the land in a manner acceptable to the government. The problem is complicated by a government proposal to set up a national park in the area. The subject of the simulation is relevant not only to students of arid and sami-arid areas, but also to those interested in resource management in developing countries. Sufficient information is given in this paper to allow the reader to set up the simulation.

Abstract: Summary Embankment stability, inadequate catchment areas, seepage losses and high salinity are the main problems in the successful construction of small water storages in arid and semi-arid regions. It is essential for success that storages be correctly sited, and that the design be suited to the materials available. The investigation for an excavated tank-earth dam system near Broken Hill, Australia, is described as an example.