Semi-arid Climate Zones Research Articles

Outline of the Yellow River basin, China

The paper aims to summarize the features of the Yellow River basin from viewpoints of physiographic conditions, climate, hydrography, economic development and water resources demand. The Yellow River basin is higher in the west and lower in the east, descending in three huge topographic steps. Most of the area belongs to the arid and semiarid continental monsoon climate zones and relatively dry. The main stream of the basin is 5,464 km long, and divided into upper, middle and lower reaches. The catchment area for upper reach is 38.6×10 km, and that for middle reach is the same, but lower reach very small (2.24×10 km). Both the upper and middle reaches are sections of rich water resources, but the middle reaches are also the main source for silt load. Economic development is presently unbalanced between upper-middle and middle-lower reaches. Based on these datasets, water demand for 2010 and 2020 is predicted.

Sensitivity of Normalized Difference Vegetation Index (NDVI) to Seasonal and Interannual Climate Conditions in the Lhasa Area, Tibetan Plateau, China

ABSTRACT The Normalized Difference Vegetation Index (NDVI) derived from NOAA AVHRR Global Vegetation Index (GVI) for Lhasa area in the central Tibetan Plateau from 1985 to 1999 and its relationship with climate conditions (precipitation and temperature) are established in this study. Climate data from the Lhasa meteorological station provided by the Tibet Climate Data Center are used for this analysis. The NDVI-derived vegetation growth patterns show very strong seasonal cycles and interannual variations. The growing season length varies between years. The correlation between NDVI and precipitation (r = 0.75, P < 0.01) in Lhasa area is higher than the correlation between NDVI and temperature (r = 0.63, P < 0.01), suggesting that NDVI is more sensitive to precipitation than temperature in this semiarid climate zone. Furthermore, the time series of NDVI demonstrate a positive trend from 1985 to 1999, which means that the vegetation biomass present on land surface is increasing. This trend is strongly correlated to increased rainfall and temperature from mid-1980s to 1990s in the Lhasa area of the Tibetan Plateau.

Age-related change in canopy traits shifts conspecific facilitation to interference in a semi-arid shrubland

Shifts between facilitation and interference and their importance in shaping plant population and community dynamics have received wide recognition. Nevertheless, the causes and spatio-temporal scales of these shifts are poorly understood, yet strongly debated. This study tested the hypothesis that age-related changes in canopy structure shift the effect of a on their protege from facilitation to interference, using as a model system the interaction between the dwarf Sarcopoterium spinosum and conspecific new recruits, in the shrubland of the transition area between the Mediterranean and the semi-arid climatic zones of Israel. Foliation level (i.e. the percentage of canopy surface area covered with leaves), a measure of canopy structure, increased with age. Shading level was significantly and positively related to foliation level. Densities of new recruits in the shrubland showed a unimodal response to canopy structure and cover: the highest densities were associated with canopies presenting low and medium foliation levels (providing 71 and 82% shade, respectively), while high foliation levels (93% shade) and open spaces among canopies were characterized by very low densities. A related field experiment using shading nets revealed that seedling survival rates followed a similar unimodal pattern, with the highest survival (ca 60%) detected in moderate shade (70%), twice as much as in full sun, and the lowest survival (ca 10%) observed in extreme shade (90%). These results support the study hypothesis on age-dependent interactions. Thus, in a semi-arid shrubland ecosystem, the transition of the nurse shrub from young to old stage can shift facilitation to interference. Hence, the age structure of established populations determines a) the availability of suitable sites for seedling recruitment and b) the balance between facilitation versus interference effects on seedling establishment.

Salinity in the southeastern Anatolia project (GAP), Turkey: issues and options

The GAP area faces salinity problems caused by irrigation. The region is situated in the arid and semi-arid climatic zone of Turkey. After the irrigation systems became operational, salinity and sodicity problems along with drainage problems increased. Salinity and also drainage problems relating to GAP are at present more a priority than irrigation. The research trend is to develop an appropriate approach to tackle the salinity problem in the region. In this study, salinity in GAP, particularly salinity problems related to the Sanliurfa-Harran Plain, was considered, and options for solution of the problem were discussed. Copyright © 2005 John Wiley & Sons, Ltd.

Paleoclimate reconstruction based on the timing of speleothem growth and oxygen and carbon isotope composition in a cave located in the rain shadow in Israel

High-resolution 230Th/ 234U ages and δ 18O and δ 13C compositions of speleothems in Ma’ale Efrayim Cave located to the east of the central mountain ridge of Israel enable us to examine the nature of the rain shadow aridity during glacial and interglacial intervals. Speleothem growth occurred during marine glacial isotopic periods, with no growth during the two last marine isotope interglacial intervals and during the peak of the Last Glacial Maximum. This contrasts with speleothem growth in caves located on the western flank of the central mountain ridge, in the Eastern Mediterranean semiarid climatic zone, which continued throughout the last 240,000 yr. Thus, during glacial periods water reached both sides of the central mountain ridge. A comparison of the present-day rain and cave water isotopic compositions and amounts at the Ma’ale Efrayim Cave site with those on the western flank shows that evaporation and higher temperatures on the eastern flank are major influences on isotopic composition and the lack of rainfall. The δ 18O and δ 13C profiles of the speleothems deposited between 67,000 and 25,000 yr B.P. match the general trends of the isotopic profiles of Soreq Cave speleothems, suggesting a similar source (eastern Mediterranean Sea) and similar climatic conditions. Thus, during glacial periods the desert boundary effectively migrated further south or east from its present-day location on the eastern flank, whereas interglacial periods appear to have been similar to the present, with the desert boundary at the same position. The decrease in overall temperature and a consequent reduction in the evaporation to precipitation ratios on the eastern flank are viewed as the major factors controlling the decay of the rain shadow effect during glacial periods.

Reactive glass and vegetation patterns.

The formation of vegetation patterns in the arid and the semiarid climatic zones is studied. Threshold for the biomass of the perennial flora is shown to be a relevant factor, leading to a frozen disordered pattern in the arid zone. In this "glassy" state, vegetation appears as singular plant spots separated by irregular distances, and an indirect repulsive interaction among shrubs is induced by the competition for water. At higher precipitation rates, the diminishing of hydrological losses in the presence of flora becomes important and yields spatial attraction and clustering of biomass. Turing patterns with characteristic length scale emerge from the disordered structure due to this positive-feedback instability.

Rainfall-recharge relationships within a karstic terrain in the Eastern Mediterranean semi-arid region, Israel: δ 18O and δD characteristics

Annual rainfall variations and processes in the upper vadose zone exert a profound influence on the chemical and isotopic compositions of waters of carbonate aquifers in semi-arid climatic zones. In order to define these processes we have studied the surface temperatures during rainfall events, the isotopic composition of rain, infiltrating and groundwaters. This study was carried out within a karstic terrain (Soreq Cave), Israel, located in Cenomanian dolomitic rocks, approximately 40 km inland of the Mediterranean Sea, and 400 m above sea level. The climate is typical of the Eastern Mediterranean semi-arid conditions, with a rainy winter and dry summer and the average annual rainfall in the area is ∼500 mm. Close monitoring indicates that the δD and δ 18O values of individual rainstorm events decrease with increasing rainfall. Annual average isotopic values of years with rainfall of 500–600 mm do not vary systematically. Years with extreme rainfall values define a negative covariation between the δ 18O and rainfall. The δD-δ 18O relationship of all rain events of more than 20 mm fall on the Mediterranean Meteoric Water Line (MMWL) with a slope of ∼8 and d-excesses of 20–30%. These rain events occur when mid-winter surface temperatures are 5 to 10°C. Rainfall events of less than 20 mm, mainly occurring at above 10°C, have slopes of less than 8 and smaller d-excess as a consequence of evaporation processes beneath the clouds. Two main water-types infiltrating into the cave are recognized: slow- and fast-drip. Slow-drip occurs from the tips of stalactites and takes place throughout the year; these waters represent seepage water that remains in the upper vadose zone for up to several decades. Fast-drip emanates from fissures in the cave roof during the winter seasons; these waters represent vadose flow with a short residence time of less than 1 year. The infiltration of the fast-drip water into the cave depends on the fracture system of the rock cover and on the intensity of the rainstorms. In the inner parts of the cave, where the rock cover is thick, fast-drip starts only after several massive (>20 mm) rainstorms, whereas below a thin roof the response time of fast-drip is a few hours to a few days after intensive rainstorms. Both fast- and slow-drip water fall on the MMWL, indicating that they are derived mainly from the relatively intensive rainstorms that compose about 2 3 of the annual rainfall. Although fast-dri water represents massive rainstorms, their δ 18O and δD values are nonetheless higher by ∼1% and 10% respectively, relative to the average rainwater compisition. This indicates that the fast-drip water consists predominantly of water derived from intensive rainstorms admixed with a minor component of isotopically 18 O- and D-enriched water, which is represented by the slow-drip waters. The fast-drip waters are derived mainly from massive rain events at surface temperatures of 5 to 10°C, which thermally equilibrate with the host rock and upon reaching the cave they have already acquired the mean temperature (∼20°C) of the cave.

Carbon and oxygen isotope study of the active water-carbonate system in a karstic Mediterranean cave: Implications for paleoclimate research in semiarid regions

In a semiarid climatic zone, such as the Eastern Mediterranean region, annual rainfall variations and fractionation processes in the epikarst zone exert a profound influence on the isotopic compositions of waters seeping into a cave. Consequently, the isotopic compositions of speleothems depositing from cave waters may show complex variations that need to be understood if they are to be exploited for paleoclimate studies. This is confirmed by a four-year study of the active carbonate-water system in the Soreq cave (Israel). The δ 18O (SMOW) values of cave waters range from −6.3 to −3.5%.. The highest δ 18O values occur at the end of the dry season in waters dripping from stalactites, and reflect evaporation processes in the epikarst zone, whereas the lowest values occur in rapidly dripping (fast-drip) waters at the peak of the rainy seasons. However, even fast-drip waters are about 1.5%. heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual evaporated water in the epikarst zone. δ 13C (PDB) values of dissolved inorganic carbon (DIC) vary from −15.6 to −5.4%., with fast-drip waters having lower δ 13C values (mostly −15.6 to −12%.) and higher DIC concentrations relative to pool and stalactite-drip water. The low δ 13C values of fast-drip waters and their supersaturation with respect to calcium carbonate indicates that the seepage waters have dissolved both soil-CO 2 derived from overlying C 3-type vegetation and marine dolomite host rock. The δ 18O (PDB) values of various types of present-day low-magnesium calcite (LMC) speleothems range from −6.5 to −4.3%. and δ 13C values from −13 to −5.5%. and are not correlated with speleothem type. An analysis of δ 18O values of present-day calcite rafts and pool waters shows that they form in oxygen isotope equilibrium. Similarly, the measured ranges of δ 13C and δ 18O values for all types of present-day speleothems are consistent with equilibrium deposition at cave temperatures. The δ 13C– δ 18O range of contemporary LMC thus reflects the variations in temperatures and isotopic compositions of the presentday cave waters. The 10%. variation in the δ 13C values in waters can be modeled by a simple Rayleigh calculation of the carbon isotope fractionation accompanying CO 2-degassing and carbonate precipitation. These variations may obscure the differences in the carbon isotopic composition of speleothems that could arise when vegetation cover changes from C 3 to C 4-type plants. This consideration emphasizes that it is necessary to characterize the full range of δ 13C values associated with contemporaneous speleothems in order to clarify the effects of degassing from those due to differing vegetation types. Isotopic studies of a number of different types of fossil LMC speleothems show many of them to exhibit isotopic trends that are similar to those of present-day LMC, but others show both higher and lower δ 18O ranges. In particular, the higher δ 18O range has been shown by independent age-measurements to be associated with a period of drier conditions. The results of the study thus indicate that it is necessary to work on a well calibrated cave system in semiarid climates and that the fossil speleothem record should be obtained from different types of contemporaneous deposit in order to fully characterize the δ 18O–δ 13C range representative of any given climatic period.

Lineament identification for groundwater drilling in a hard-rock terrain of Sirohi district, western Rajasthan

In hard-rock terrain under semi-arid climatic zone of western Rajasthan, prospective groundwater zones are poorly defined. The area of Sirohi district studied is regarded as a critical zone for tubewell siting. The present study involves delineation of lineaments on spaceborne and airborne data and their identification in field. Lineaments are identified with surface and subsurface geological features for selection of drilling sites. The study has resulted in 100 percent high-yielding exploratory wells in the area. Identical results obtained in their respective observation wells reaffirm utility of the approach adopted.

Three-Fold Remedial Measures for Groundwater Protection: Case Study of the Khathkari Stream, Pune District, India

A vast tract in Maharashtra State of India, constituted of horizontally disposed basaltic flows of Cretaceous-Eocene age, falls in the semi-arid climatic zone characterised by low precipitation and high temperature and evaporation. Such areas are marked by the variety of calcretes including massive, honeycomb, nodular, and powdery types. These calcretes which owe their origin to both pedogenic and nonpedogenic processes, are responsible for imparting the salinity to the groundwater. An integrated approach, involving geological, hydrogeological and hydrochemical studies, has been employed for delineating the boundary of saline and fresh waters in the groundwater system and evaluating the variation in salinity in the area around the Khathkari stream (18 13’ to 18 18’ N latitudes, and 74 13’ to 74 22’E longitudes) in Pune district. The paper briefly presents the data collected during the course of the work and incorporates the three-fold remedial measures such as i) protecting fresh water that occurs at deeper depth, ii) improving the present quality of water and iii) suggesting an appropriate crop-pattern that can stand for present and improved quality of water.

Comparative growth performance of some multipurpose trees and shrubs grown at Machakos, Kenya

Growth rates of 29 multipurpose trees grown in an agroforestry arboretum for six years at a sub-humid to semi-arid climatic zone are presented. Exotic species such as Grevillea robusta, Sesbania grandiflora, Leucaena leucocephala, Cassia siamea and Sesbania sesban, some of which were outside their traditional climatic zones, had higher diameters, heights and bole volumes/tree (upto 130% more in certain cases) than of the indigenous species. However, poor performance of several species (both exotic and indigenous) would limit their agroforestry potentials at the evaluation site or other similar areas.

Some engineering properties of stabilized salina soil

A large portion of the land surface in the Middle East arid and semi-arid climate zone has a high salt concentration. The presence of salt in the soil profile influences its geotechnical properties. This paper discusses local practices and techniques used to improve some of the engineering properties of salina soil similar to that found in Saudi Arabia. The effectiveness of using portland cement to improve the behaviour of salina sand was investigated. The best lime percent to improve the behaviour of fine desert salina was determined. The effectiveness of vibro-replacement, dynamic consolidation techniques in improving the properties of salina beds was evaluated. It is suggested that both portland cement and lime in quantities as little as 5% by weight improves salina behaviour significantly. While dynamic consolidation with sand pillar seems to give satisfactory results in treating muddy deep salina beds.

Practical generation of synthetic rainfall event time series in a semi-arid climatic zone

An event-based approach adapted to semi-arid climatic conditions and applicable in those cases when only unreliable observation data are available is developed. Emphasis is placed on ease of modeling and computation. The case study of a 20,000 km 2 area in Central Tanzania where rainfall is mostly from convective-type storms is used to illustrate the approach. A phenomenological description of both convective-storm and frontal system precipitation is provided by a regional study of point rainfall events defined as an uninterrupted succession of days with more than 5 mm of precipitation, and dry events defined as a succession of dry days. The rainfall events are defined by depth and duration, which are found to be correlated. An analysis of the depth per event is thus performed, conditioned on the event duration. The log Pearson type III distribution appears to provide a good overall fit for the depth per event, especially for events lasting more than one day, while the one-day long events fit the negative binomial distribution. The event duration is found to fit a geometric distribution. The dry events during the rainy season seem to fit a negative binomial distribution. The climatic cycle length is fitted to a log Pearson type III distribution. A step-by-step procedure provided herein to simulate regional rainfall can easily be implemented on a microcomputer or even a programmable hand calculator.

Carbon and oxygen isotopic composition of a recent megapolygon-spelean limestone, fisheman by, South Australia

In the supratidal zone of broad peritidal flats at Fisherman Bay, in the semi-arid climatic zone of South Australia, a diagenetic limestone containing megapolygons, tepee structures and speleothems is forming by cementation of nearsurface marine carbonates. Cementation occurs as aragonite precipitates from upwardly-flowing saline springs formed by groundwater of meteoric origin which has flowed seawards across the coastal plain. The 180 content of the emerging spring waters δ 18O, −2.45%osmow) is higher than that of local rainfall ( δ 18O ∼-3.5%o) but has not reached the positive values found in other highly evaporative environments, such as the sabkhas of the Persian Gulf. The 13C content of the dissolved carbonate ( δ 13C, −9.1‰ PDB) is slightly higher than that of altered Tertiary marine carbonate sediments ( δ 13C, −8.7 to −6.3%o) which form that part of the coastal aquifers in which the dissolved carbonate is generated. Aragonite precipitating in cavities within the limestone has an isotopic composition ( 13C; −4.4%o; δ 18 O +26.7% o) which reflects that of the emergingwaters. Elsewhere, higher rates of evaporation and evolution of C0 2 produce a range of isotopically heavier cements. The isotopic composition of the host limestone is marginally below that of the local uncemented intertidal marine carbonates ( δ 13C, +0.8 to +4.5%o; δ 18O, +33.0 to +34.3%o) but it is within the general range for Holocene and Pleistocene marine carbonates. Despite the highely evaporative climate, the isotopic characteristics of the spring waters remain distinctively nonmarine. Evaporative increases in 18O content are limited by the partly confined nature of the coastal aquifer system and, in the near-surface environment, by the overlying limestone. The 3C content of the dissolved carbonate in the aquifers increases considerably as a result of water-sediment interactions but only towards a limit set by the isotopically light composition of the aquifer carbonates. The limited influence of the groundwaters on the bulk isotopic composition of the limestone is a consequence of their seawater-like chemical composition, which minimizes dissolution and alteration of the original marine carbonates. Ancient examples of cementation of peritidal carbonates by aragonite-precipitating upward-flowing spring waters should be recognizable as thin lenses located at sub-aerial exposure surfaces. Such lenses would contain juxtaposed isotopically-different non-marine cavity cements and unaltered marine sediments.

Geomorphic features of a part of Cuddapah and Nellore districts, Andhra Pradesh — Using remote sensing techniques

An area of about 1000 square kilometres in Cuddapah and Nellore districts of Andhra Pradesh is studied to bring out the different landforms and other geomorphic features, their material content and distribution, the processes that have acted and are acting in the evolution of landscape and ultimately to get an idea of the geomorphic history of the area. The data obtained during the present study is mainly through remote sensing-techniques (Landsat image and airphoto interpretation) with field checks. The study area forms a part of southeastern Cuddapah Basin and consists of structurally disturbed and deformed sedimentary sequence of varied lithology. The impor-tant rock types in the area are granites, gneisses, quartzites, shales, phyllites, limestones and dolomites. The area presently comes under semi-arid climatic zone. The different landforms recognised and mapped in the area are hog-back ridges, piedmont fans, valley fills, river built plain, abandoned channels, point bars, ’ V‘ shaped valleys, piedmont zone, colluvial plain, residual hills, besides lineaments.

The effects of vegetation on the swelling and shrinking of soils in Australia

Soil movement is of great importance in Australia because about 20% of the total area consists of expansive soils, most of which occur in the semi-arid climatic zone. The first measurements of soil movement were made in Adelaide under natural grassland. From this limited information, it was deduced that there was little soil movement below 1·6m. Recently, movements have been measured in clay layers at much greater depths. These movements have been attributed to shrinkage due to desiccation by deep rooted vegetation, and swelling due to ponding of water. Several case histories are discussed where the effects of vegetation have caused movements of buildings of up to 150mm settlement and 100mm heave. To calculate potential soil movement caused by vegetation, the extremes of total soil suction and a quantitative value of the volume change properties of the soil are required. The instability index, Ip has been suggested as a measure of this soil property and was measured on two of the most expansive and widespread soil types in Adelaide. Values for the black earths were 0·06 and for the underlying dispersive clay, 0·10, compared with 0·02 for the red-brown earths down to 5 m. Changes in total soil suction were measured in these two soils both near and away from several species of trees. Near eucalyptus trees, the total suction of red-brown earths attained a maximum value of 2MPa to depths exceeding 5m. Under natural grassland, the profile was uniformly wetted to a suction of 0·3 MPa. In the saline dispersing clay underlying the black earths, the corresponding suctions were of the order of 3 MPa and 1 MPa respectively. However, in this latter case, the effect of Ip and the depth of the clay, causes much greater movements in spite of the lower suction changes. At present, soil movements due to the effects of vegetation and its associated watering regime, calculated from measurements of Ip are only a guide. Past experience of the movements of buildings on similar soil profiles in similar circumstances is still essential. Le mouvement du sol est très important en Australie, parce que 20% environ du continent entier comporte des sols expansibles, dont la plupart se trouvent dans la zone climatique demi-aride. Les premières mesures du mouvement des sols furent prises à Adelaide sous des herbages naturels. Sur la base de ces données limitées on a déduit qu'il n'y avait que peu de mouvement du sol à une profondeur supérieure à 1·6m. Récemment des mouvements ont été mesurés dans des couches d'argile à des profondeurs beaucoup plus grandes. Ces mouvements ont été attribués au retrait causé par la dessiccation due à la végétation profondément enracinée, tandis qu'une autre cause possible est le gonflement produit par la formation de mares. L'article discute plusieurs exemples où les effets de la végétation ont causé des tassements jusqu'à 150mm et des soulèvements jusqu'à 100mm de bâtiments. Pour calculer le mouvement potentiel du sol produit par la végétation, il faut connaître les valeurs extrêmes de la succion totale du sol et une valeur quantitative de ses propriétés de changement de volume. L'indice d'instabilité Ip a été proposé comme mesure de cette propriété des sols, et cette valeur a été déterminée sur deux des sols les plus expansibles et les plus usuels à Adelaide. Les valeurs pour les terres noires ont été de 0·06, la valeur pour l'argile sousjacente dispersive étant de 0·10, à comparer avec 0·02 dans le cas des terres brunes-rougeâtres jusqu'à une profondeur de 5m. Des changements dans la succion totale du sol ont été mesurés dans ces deux sols dans le voisinage d'arbres de plusieurs essences, ainsi qu'à quelque distance. Dans le voisinage des eucalyptus on a trouvé une valeur maximale de 4 MPa pour la succion totale des terres brunes-rougeâtres, jusqu'à des profondeurs dépassant 7 m. Sous des herbages naturels le profil a été humidié de façon uniforme à une succion de 0·3 MPa. Dans l'argile dispersive saline sous les terres noires les succions correspondantes étaient repectivement de l'ordre de 1 MPa et 3 MPa. Cependant, dans le dernier cas l'effet de Ip et la profondeur de l'argile causent des mouvements beaucoup plus marqués, malgré les changements réduits dans la suction. A present les mouvements du sol dus aux effets de la végétation et au régime d'eau associé calculé à partir des mesures du Ip ne donnent qu'une indication générale. On a encore essentiellement besoin de données concernant les mouvements des bâtiments dans le passé sur des prols de sol similaire dans des circonstances analogues.

The Nature of Rainfall Fluctuations in Subtropical West Africa

Abstract The nature of rainfall fluctuations in the West African subtropics is described on the basis of three analyses of rainfall departures. Areally averaged. normalized rainfall departure series for the years 1901–75 are derived for four semi-arid sub-Saharan climatic zones: the Sahelo-Saharan, the Sahel, the Soudan and the Soudano-Guincan. Similar series are derived for four or five regions within each zone and the regional series are intercorrelated. Finally, correlation between annual rainfall departure maps is used to determine frequently occurring rainfall anomaly types. Several characteristics of West African climate variation emerge from these investigations. A strong coherence of variation occurs throughout the region from∼20–10°N, a latitude which frequently marks a climatic discontinuity with large departures of opposite sign prevailing to the north and south. This coherence is very pronounced in both the north-south and east-west directions, although certain longitudinal sectors occasionall...