Short Wet Periods Research Articles

Assessing current and future available resources to supply urban water demands using a high-resolution SWAT model coupled with recurrent neural networks and validated through the SIMPA model in karstic Mediterranean environments

Hydrological simulation in karstic areas is a hard task due to the intrinsic intricacy of these environments and the common lack of data related to their geometry. Hydrological dynamics of karstic sites in Mediterranean semiarid regions are difficult to be modelled mathematically owing to the existence of short wet episodes and long dry periods. In this paper, the suitability of an open-source SWAT method was checked to estimate the comportment of a karstic catchment in a Mediterranean semiarid domain (southeast of Spain), which wet and dry periods were evaluated using box-whisker plots and self-developed wavelet test. A novel expression of the Nash–Sutcliffe index for arid areas (ANSE) was considered through the calibration and validation of SWAT. Both steps were completed with 20- and 10-year discharge records of stream (1996–2015 to calibrate the model as this period depicts minimum gaps and 1985–1995 to validate it). Further, SWAT assessments were made with records of groundwater discharge and relating SWAT outputs with the SIMPA method, the Spain’s national hydrological tool. These methods, along with recurrent neural network algorithms, were utilised to examine current and predicted water resources available to supply urban demands considering also groundwater abstractions from aquifers and the related exploitation index. According to the results, SWAT achieved a “very good” statistical performance (with ANSE of 0.96 and 0.78 in calibration and validation). Spatial distributions of the main hydrological processes, as surface runoff, evapotranspiration and aquifer recharge, were studied with SWAT and SIMPA obtaining similar results over the period with registers (1980–2016). During this period, the decreasing trend of rainfalls, characterised by short wet periods and long dry periods, has generated a progressive reduction of groundwater recharge. According to algorithms prediction (until 2050), this declining trend will continue reducing groundwater available to meet urban demands and increasing the exploitation index of aquifers. These results offer valuable information to authorities for assessing water accessibility and to provide water demands in karstic areas.

Rain process models and convergence to point processes

Abstract. A variety of stochastic models have been used to describe time series of precipitation or rainfall. Since many of these stochastic models are simplistic, it is desirable to develop connections between the stochastic models and the underlying physics of rain. Here, convergence results are presented for such a connection between two stochastic models: (i) a stochastic moisture process as a physics-based description of atmospheric moisture evolution and (ii) a point process for rainfall time series as spike trains. The moisture process has dynamics that switch after the moisture hits a threshold, which represents the onset of rainfall and thereby gives rise to an associated rainfall process. This rainfall process is characterized by its random holding times for dry and wet periods. On average, the holding times for the wet periods are much shorter than the dry ones, and, in the limit of short wet periods, the rainfall process converges to a point process that is a spike train. Also, in the limit, the underlying moisture process becomes a threshold model with a teleporting boundary condition. To establish these limits and connections, formal asymptotic convergence is shown using the Fokker–Planck equation, which provides some intuitive understanding. Also, rigorous convergence is proved in mean square with respect to continuous functions of the moisture process and convergence in mean square with respect to generalized functions of the rain process.

Effects of Temperature and Wetness on Components of the Infection Cycle of Valdensia heterodoxa in Lowbush Blueberry

Valdensia leaf spot, caused by Valdensia heterodoxa, is a serious disease of lowbush blueberry. The disease may develop rapidly, resulting in extensive defoliation of fields. The purpose of this study was to examine the effects of temperature and wetness duration on various components of the infection cycle to gain a better understanding of epidemic development that might lead to improved management practices. Lesions on leaves appeared 6 h after inoculation at 20°C and were larger on young 3-week-old leaves compared with 8-week-old leaves. Incidence of infection on 3-week-old leaves was lowest at 5°C, highest at 15 and 20°C, and failed to occur at 30°C. Defoliation began 48 h after inoculation at 20 and 25°C but was slower at higher and lower temperatures. Conidia production and release from colonized leaves began 48 h after inoculation at 15 and 19°C. Total conidia production was lowest at 7°C, highest at 15°C, and progressively declined at 19 and 23°C. Production of conidia lasted 2 to 3 days. Sclerotia formed mainly along the midveins and were similar in size at 5 to 15°C, largest at 20°C, and smallest at 25°C. Conidia formed directly on sclerotia that were overwintered outdoors and then incubated on moist filter paper. Conidia production began after 48 h at 10, 15, and 20°C. Total production was lowest at 5°C, highest at 20°C, failed to occur at 25°C, and ceased after 10 days at all temperatures. These data show that at optimal temperatures, relatively short wet periods are required for conidia production on overwintered sclerotia, infection of leaves, and subsequent conidia production on diseased leaves that may account for the sudden and rapid spread of disease in fields. The data will be useful for helping growers identify weather conditions favorable for disease development.

Long-term spatiotemporal variability in occurrences of wet and dry days across South Mongolian Plateau

Precipitation has been widely considered as the key point toward the apprehension of climate change, to which the arid and semi-arid southern Mongolian plateau is highly responsive. Using the daily data from 1960 to 2017 at 25 weather stations across the plateau, this study aimed to assess the spatial patterns and temporal trends in precipitation in terms of occurrences of wet and dry days. In this regard, four surrogate indices, namely frequencies of the occurrence of wet day following a dry day (F(d-w)), dry day following a wet day (F(w-d)), wet day following a wet day (F(w-w)), and dry day following a dry day (F(d-d)), were derived. The modified Mann-Kendall test, Sen's slope estimator, and sequential Mann-Kendall method were applied to examine trend significance, rate, and shifting year. The results indicated that the climate across the plateau was characterized by fairly long dry phases, short wet periods, and low frequent alternation of wet and dry spells, as well as an obvious northeast-southwest aridity increasing gradient. This study identified an overall decreasing trend for F(d-w), F(w-d) and F(w-w) and an increasing trend for F(d-d). The significant decreases in F(d-w) and F(w-d) concurrently occurred in the central part of the plateau, whereas the decrease in F(w-w) and the increase in F(d-d) were significant in the middle and northeastern parts. The tendency was most prominent in summer: this season's average annual decreasing rates of F(d-w), F(w-d), and F(w-w) were 0.072, 0.064, and 0.166%, respectively, while this season's average annual increasing rate of F(d-d) was 0.244%. A trend change point around the 1990s was found for half of the significant trends. Regionally mixed (increasing/decreasing) trends were detected for all the indices at temporal scales of dry season, spring, fall, and winter, only a few of which were statistically significant. It can be implied that over the southern Mongolian plateau, both duration and frequency of the precipitation events tended to decrease, along with longer-lasting dry spells and less frequent wetting-drying alternations in the past five decades. The overall drying trend was prominent across the central and northeastern parts, which was especially true in recent three decades. The advanced understanding of climate change provides a scientific base for sustaining such arid and semiarid hydro-ecosystems.

Thresholds of key disaster-inducing factors and drought simulation in the Xilinguole Grassland

The frequency of extreme events such as droughts is increasing due to climate change. Grassland ecosystems are extremely sensitive to drought. This study aims to analyse how droughts respond to meteorological factors, establish a method to quantitatively evaluate the threshold values of key meteorological factors that can induce drought in different time periods, and explore a drought prediction model applicable to the Xilinguole Grassland. The Xilinguole Grassland has experienced an obvious aridification trend, with notable fluctuations in short dry and wet periods and the probability of drought (generally ranked in the order of moderate drought > light drought > severe drought > extreme drought). The Xilinguole Grassland is expected to face more intense drought conditions in the next 5 years. Among the meteorological elements, temperature, precipitation, water vapor pressure and solar radiation are the key factors influencing drought. Both short-term and long-term changes in precipitation and temperature have a strong influence on drought. Short-term changes in water vapor pressure have a significant impact on drought. Drought is sensitive to extreme climate events and is most sensitive to the influence of meteorological factors in May–September, particularly in July. In July, drought can be induced by any of the following: monthly precipitation below 50 mm, an average temperature above 23 °C, sunshine percentage above 69%, a monthly sunshine duration above 319 h, or an average water vapor pressure below 12 hPa. In addition, Markov chain modelling is a feasible approach for predicting drought in the Xilinguole Grassland. This study provides a scientific basis for drought prevention and control.

Coastal wetlands as markers of transgression in proximal extensional systems (Berriasian, W Cameros Basin, Spain)

The early stages of intraplate extensional systems commonly are recorded by deposition of continental sediments. In this context, given appropriate tectonics and eustasy, transgressions can be well recorded in the areas of the basins located close to the sea, but they may be difficult to recognize in the innermost landwards areas of the system. This situation occurs in the innermost Upper Jurassic-Early Cretaceous Cameros Basin, part of the Iberian Extensional System (N. Spain), where a Berriasian transgression is recorded. The Berriasian succession in this area consists of siliciclastic deposits (sandstone and mudstone) of the Salcedal Formation and of carbonate and mixed carbonate-fine siliciclastic deposits (limestone and marl) of the San Marcos Formation. The sedimentological analysis of this depositional succession indicates that a Berriasian carbonate coastal wetland system occupied that sector of the Cameros Basin during deposition of the San Marcos Formation. This carbonate coastal wetland system consisted of shallow and quiet water bodies including some with marine influence others with no to very little marine influence, and palustrine areas. A semiarid climate characterized by the seasonal alternation of short wet and long dry periods caused water bodies of the system to undergo episodic desiccation and subaerial exposure. Moreover, this complex mosaic of sub-environments was connected laterally with a distal zone of a distributive fluvial system that was rimmed by siliciclastic tidal flats during phases of greater marine influence. The paleogeographic arrangement of this coastal wetland depositional system indicates that the marine influence came from the Basque-Cantabrian Basin to the north. During the period of Berriasian maximum marine influence, accommodation linked to the eustatic rise added to accommodation generated by tectonic subsidence from the extensional reactivation of late Variscan strike-slip faults. All these factors favored marine incursion into the west Cameros Basin from the Basque-Cantabrian Basin to the north. The example of the Berriasian transgression recorded in the W Cameros Basin by establishment of coastal wetland systems matches the interpretations of previous studies in neighboring areas. In those areas, complex coastal systems record transgressions in the innermost parts of the intraplate extensional basins of the Iberian Plate. This observation suggests that this paleogeographic and sedimentological arrangement may be common in the innermost parts of intraplate extensional basins during transgressive episodes throughout the geological record.

Climatic Water Balance in AL-Amaid Area/ Muthana Governorate/ Southwest Iraq

Ground water is a vital source for agricultural sector and rural communities. The global climate change is expected to change the hydrometeorological processes parameters. The climate considered as part of the southern Iraqi desert general climate with long, extremely hot, and dry summer and short wet period with little rain. So it is vital to investigate the groundwater quality for irrigation purposes. The meteorological data of Samawa meteorological station for the period 1980-2015 was used to evaluate the climatic conditions for Muthana Governorate. It was found that the averages of annual rainfall was 105.7mm and the everages of evaporation is 3182 mm, while the mean monthly relative humidity % , mean temperature, wind speed and sunshine were 40.9 %, 25.3 Co, 5.2m/sec and 9 h/day respectively. The results show that the mean evapotranspiration is 1717 mm, while the correct evapotranspiration is 1520 mm and the climate of the study area is classified as dry. And water deficit equals to (72%) of the total rainfall amount, and water surplus formed (28%) of the total rainfall amount.

Hydrotime model analysis of Trachyspermum ammi (L.) Sprague seed germination

Trachyspermum ammi (L.) Sprague (Apiaceae) is an annual herb that is known medicinal plant. Information about germination of T. ammi is scarce. The aims of this study were (1) to develop a general hydrotime model to predict seed germination of T. ammi and (2) to evaluate seed germination of T. ammi using hydrotime parameters. Seeds of 20T. ammi ecotypes were germinated at five water potentials: 0, −0.1, −0.2, −0.3, and −0.4MPa. Quantification of seed germination was conducted in relation to water potential using the hydrotime model for each of the 20 ecotypes. The ecotypes differed for estimated values of hydrotime (θH), base water potential (ψb(50)) and σψb. The average θH was 13.6MPa-h. The ψb(50) varied from −0.32 to −0.12MPa, with an average of −0.21MPa. Average σψb was 0.18 and it varied from 0.13 to 0.31. A general model was fitted to all germination data for the 20 ecotypes and 5 levels of water potentials based on an average of θH, ψb(50), and σψb. The general model predictions agreed closely with observed germination. High ψb(50) and low θH values were associated with a rapid germination rate under wet conditions but inhibited germinating at reduced water potentials. Low θH values are contributed by rapid germination and high ψb(50) is showing that germination in dry condition would not be done. These lead to a good establishment of T. ammi by rapid germination after rainfall and resulted to take advantage of the short wet periods in arid and semiarid environments.

Estimation of consistent wet period characteristics for further improvement of water resources studies

AbstractThis study presents an analysis of consistent wet periods for further improvement of water resources studies. While previous studies have focused on characterizing dry periods, this study uses tree‐ring reconstructed precipitation to detect the occurrence of wet periods of magnitude larger than what can be achieved from the analysis of the historical precipitation records. Tree‐ring reconstructed precipitation for Madaba region in Jordan that began in 1743 was used. The analysis of the historical precipitation indicated the occurrence of short wet periods (1–4 years). The analysis of the 239 years of the reconstructed precipitation indicated the occurrence of long wet periods (7 years), and wet period magnitude of 551 mm, that is 20% larger than the observed from the analysis of historical precipitation. Furthermore, this study presents theoretical formulation that can be used to estimate the expected number of wet events of desired length that could emerge during the operation period of surface resources.

Refugia and dispersal promote population persistence under variable arid conditions: a spatio‐temporal simulation model

In arid environments, population dynamics of many organisms follow resource pulses in time and space. This heterogeneity in resource accessibility is due to irregular and local rainfalls. In drought periods, the population density of small mammals such as rodents falls so that animals seem absent across the landscape. How can they avoid extinction and persist in time and space at low densities during droughts? We hypothesize that a fraction of the population may survive in refugia—less arid patches—and recolonize the landscape after drought‐breaking rains. When precipitation and resources abound again, rodents become abundant and disperse over large areas. Our spatio‐temporal simulation tests the hypotheses that refugia and dispersal promote population persistence over large temporal and spatial scales. We programmed a virtual desert (100 × 100 matrix) in which a virtual population changes over the course of 100 time steps representing 100 years. In our simulations, when rainfall is scarce, refugia and dispersal are insufficient to produce population persistence, and when rainfall is heavy or widespread, these factors are not necessary. At moderate rainfall frequency, refugia and dispersal are essential for persistence, and long‐distance dispersers need fewer refugia than short‐distance ones. With cyclic rainfall patterns mimicking La Niña's influence on desert precipitation, long drought periods and short wet periods, persistence requires both abundant refugia and long distance dispersal.

Climate drying amplifies the effects of land-use change and interspecific interactions on birds

Climate change may amplify the effects of land-use change, including induced changes in interspecific interactions. To investigate whether an avifauna changed over a period of severe drought, and if changes in avifaunas were related to changes in vegetation characteristics and the irruption of a despotic native species, the noisy miner Manorina melanocephala. In the box–ironbark forests of south-eastern Australia, we resurveyed the avifaunas and remeasured vegetation characteristics in 120 forest transects in 2010–2011 that had previously been measured in 1995–1997. The avifauna changed markedly over the prolonged drought, and changes were more marked in smaller fragments of remnant vegetation in which more pronounced vegetation change had occurred. The noisy miner increased differentially in smaller remnants adding to the declines, especially for small-bodied birds. Long droughts interspersed with short wet periods are projected for the region, so the imposition of climate effects on an already much-modified region has profound implications for the avifauna. The noisy miner has (and continues) to benefit from both land-use and climate change, so future sequences of drought interspersed with short wet periods are likely to lead to further changes in the avifauna as the miner extends its occupancy. Differential reductions in small nectarivores and insectivores will affect ecosystem processes, including the control of defoliating insects, seed dispersal and pollination.

Changing structure of precipitation evolution during 1957–2013 in Yangtze River Delta, China

Based on the daily precipitation of 16 rain gauges in Yangtze River Delta (YRD), the spatial and temporal changing properties of precipitation structure during 1957–2013 are investigated in the view of consecutive rainy duration. The results show that: (1) increasing precipitation amount can be detected in YRD while rainy days exhibit decreasing tendency over the basin. So precipitation intensity has been increasing in the past 57 years. (2) During the past 57 years, the mean wet periods become shorter over most of YRD. Wet periods with short duration show upward occurrence and fractional contribution to annual precipitation. Being shorter, wet periods in Yangtze River Delta are characterized by intensified precipitation, which may result in high risk of flash flood. (3) Extreme precipitation events during 1957–2013 are more evidently associated with short wet periods and peak for 2-day duration both in occurrence and fractional contribution. (4) An combination of climate change and urbanization intensifies the change in the distribution of temporal evolution of precipitation structure towards the precipitation events with shorter duration and higher intensity. Therefore, these changing properties are of significance to understanding the impact of climate change and human interference on hydrological processes and the flood prevention in the future in Yantze River Delta.

APPLICATIONS OF INNOVATIVE MATERIALS FOR PERFORMANCE IMPROVEMENT OF FLEXIBLE PAVEMENT OVER EXPANSIVE SUBGRADE

Expansive soils are one of the most problematic materials that are widely encountered in significant land areas in several parts of the world; like Africa, Australia, India, United States and Canada. The South Gujarat region in India have majority of top soil as black cotton soil. The black cotton soil has characteristics of shrinking on drying and heaving on wetting. This soil being expansive creates several types of damages to pavement structures, and in some cases the pavement may even become unserviceable. The normal climate condition of study area shows short wet and long dry period which aggravate the problem of swelling and shrinkage. The IRC: 37 - 2001, Annexure - 4 suggest 0.6 to 1.0 m thick non-cohesive soil cushion on the expansive soil for road construction which led to higher cost for road construction. Also for new urban areas it is difficult to raise the embankment or to excavate the subgrade upto such a depth due to existing structures and under laying service lines. To provide economical solution along with feasible application two innovative materials were used namely, CONSOLID and Geotextile for flexible pavement. The CONSOLID application shows the great improvement of CBR values helping the overall stability of the pavement. The Geotextile provided below the pavement components to act against the heaving of the swelling soil at the same time it helps as drainage layer also. Field study is undertaken to observe the effect of Geotextile in flexible pavement performance and 2 specific boundary conditions are created for observations. The Observations shows about 50 % reduction in shrinkage effect for paved road reinforced with Geotextile subjected to drying and wetting cycles. Both the materials are having its unique advantage in the performance improvement of flexible pavement over expansive subgrade.

Changing structure of wet periods across southwest China during 1961-2012

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 61:123-131 (2014) - DOI: https://doi.org/10.3354/cr01247 Changing structure of wet periods across southwest China during 1961-2012 Yang Chen1,2, Panmao Zhai1,2,* 1Department of Atmospheric Science, Nanjing University of Information Science & Technology, Nanjing 210044, China 2State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, CMA, Beijing 100081, China *Corresponding author: pmzhai@cma.gov.cn ABSTRACT: Using homogenized daily precipitation observations during 1961 to 2012, changes in duration-based structure of precipitation over southwest China under drought conditions were investigated. The results reveal a distinct regrouping of wet periods of different length. Wet days and precipitation related to long wet periods (>3 d) decreased significantly. Such decreases resulted in significantly fewer fractional contributions of long wet periods to total wet days and annual precipitation totals. In contrast, contributions from short wet periods (<3 d) showed significant ascending trends. Substantial decreases in precipitation of long wet periods after 2000 accounted for most reductions in annual precipitation totals and resultant drought tendency. A more specific analysis on seasonal timescales highlights the significance of changes in autumn precipitation structure in causing the drying tendency across southwest China. KEY WORDS: Changing precipitation structure · Duration · Wet periods Full text in pdf format PreviousNextCite this article as: Chen Y, Zhai P (2014) Changing structure of wet periods across southwest China during 1961-2012. Clim Res 61:123-131. https://doi.org/10.3354/cr01247 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 61, No. 2. Online publication date: September 23, 2014 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2014 Inter-Research.

Performance Improvement of Flexible Pavement on Swelling Subgrade Using Geotextile

Expansive soils are one of the most problematic materials that are widely encountered in significant land areas in several parts of the world; like Africa, Australia, India, United States and Canada. The South Gujarat region in India have majority of top soil as black cotton soil. The black cotton soil has characteristics of shrinking on drying and heaving on wetting. This soil being expansive creates several types of damages to pavement structures, and in some cases the pavement may even become unserviceable. The normal climate condition of study area shows short wet and long dry period which aggravate the problem of swelling and shrinkage. The IRC: 37 – 2001, Annexure – 4 suggest 0.6 to 1.0 m thick non-cohesive soil cushion on the expansive soil for road construction which led to higher cost for road construction. Also for new urban areas it is difficult to raise the embankment or to excavate the subgrade upto such a depth due to existing structures and under laying service lines. To provide economical solution along with feasible application Geotextile used as reinforcement material for flexible pavement. It is provided below the pavement components to act against the heaving of the swelling soil at the same time it helps as drainage layer also. Field study is undertaken to observe the effect of Geotextile in flexible pavement performance and 2 specific boundary conditions are created for observations. Observations shows about 50 % reduction in shrinkage effect for paved road reinforced with Geotextile subjected to drying and wetting cycles. 

Towards soil property retrieval from space: A one-dimensional twin-experiment

Summary Soil moisture is among the key environmental variables controlling evaporation, infiltration and runoff. However, the temporal evolution of soil moisture is not easy to measure or monitor at large scales due to its spatial variability, which is largely driven by local variation in soil properties and vegetation cover. Consequently, soil moisture estimates using land surface models are typically made using effective physical parameterisations based on low-resolution and/or erroneous soil property information. Thus, land surface models have an urgent need for more accurate and detailed soil parameter data sets than are currently available, in order to undertake regional or global simulation studies at high spatial resolution and with the required accuracy. To overcome this limitation, the possibility of estimating the soil hydraulic properties through model calibration to remotely sensed near-surface soil moisture observation is explored. The study presents a methodology that demonstrates this potential using a synthetic twin experiment framework, thus avoiding the need to deal with possible model-observation biases. Moreover, it explores a range of scenarios, with the objective to determine the best meteorologic conditions for soil property retrieval and hence the most efficient use of computational resources when applying the methodology at large scales. These scenarios include: (a) short dry-down period, (b) short dry period, (c) short wet-up period, (d) short wet period and (e) full 12-months with multiple wetting and drying periods. The methodology was also tested for four different soil types including a homogeneous column of sand, a homogeneous column of clay, a duplex column of clay over sand, and a duplex column of silty sand over clay. The study showed that soil hydraulic parameters were best retrieved when using the full 12-month period, with the sequential retrieval of three parameters at a time being the most suitable approach when retrieving the six parameters, with the most sensitive parameters retrieved first.

Precipitation variations of Longxi, northeast margin of Tibetan Plateau since AD 960 and their relationship with solar activity

Abstract. The precipitation variations of Longxi area, northeast margin of the Tibetan Plateau since AD 960 are reconstructed from Chinese historical documentary records. These records show that since AD 960, the precipitation of Longxi decreased and reached the lowest level at the end of the 17th and the 18th centuries. After this period, the precipitation gradually increased. The three short wet periods of Longxi in the last millennium were: from the end of the 10th century to the early years of the 11th century, from the end of the 12th century to the early years of the 13th century and during the first half of the 20th century. The precipitation variations coincide well with variations of the Northern Hemisphere temperature and the atmospheric 14C concentration, as well as the averaged 10Be concentration and the reconstructed solar modulation record which show that solar activity may be an important driving force of the precipitation variations of Longxi on multi-decadal to centennial scales during the last millennium. Solar activity controls the motion of the north edge of the Asian summer monsoon by affecting the Asia summer monsoon intensity, the East Asian winter monsoon intensity and the locations of westerlies, thus further dominating precipitation variations of Longxi. Synchronous variations of Longxi precipitation and Northern Hemisphere temperature may also be ascribed to the same control of solar activity.

The records of shallow well in Lake Jiezechaka shoreline of Tibet and the lake evolution

藏西北结则茶卡湖面到高位湖岸线间湖泊记录发育,沿岸湖积物中3个U系年龄分别为14.2±1.2kaBP、38.0±3.5 kaBP和41.6±3.2 kaBP,6个浅井沉积物中CaO、MgO、NaCl、LiCl、B₂O₃的含量与沉积物形成时代和环境有一定的关联性.在中更新末期,该湖可能处于高湖位期,随着晚更新世初期以来气候变干,湖面开始下降,湖泊逐渐萎缩,湖水盐度逐渐增加,Ca和Mg含量变高,但在30kaBP左右,气候有过短暂湿润期,湖水出现返淡趋势,之后湖水进一步浓缩,湖水中Na、B、Li含量显著增高,盐湖大约在15kaBP左右形成.;The lacustrine sediments deposited between the present water level and the high perched lake shoreline in Lake Jiezechakalocated in the northwestern Tibet provided good records of the lake evolution history. The three U series ages of the lacustrine sedimentsalong the bank are 14.2±1.2kaBP, 38.0±3.5kaBP and 41.6±3.2 kaBP, respectively. The contents of CaO, MgO, NaCl, LiCl and B₂O₃ in sixshallow wells are related to the age and environment when the sediments were deposited. In the end of middle Pleistocene, the lakeshoreline lied in the high perched position probably. From the early period of the Late Pleistocene, the levels of the lake began fallingbecause of the dry climate. The lake shrank gradually and the salinity was increased. Ca and Mg also increased correspondingly. However,around 30kaBP, there was a short wet period when the lacustrine water became less salty. Then, the lake water was further condensed withthe increases in the content of Na, B, and Li significantly. The present salt lake was formed at about 15 kaBP.

Halloysite as a kinetically controlled end product of arid-zone basalt weathering

Mineralogical and isotope results paired with field observations suggest that halloysite is the favored, albeit metastable, aluminosilicate end product of arid-zone basalt weathering on Kohala, Hawai'i, and that the formation of smectite has been inhibited by kinetic factors. Soils sampled on constructional lava flows having ages ranging from 4 to 350 ka provide a chronosequence that has had minimal physical disturbance, thus allowing us to interpret chemical and mineralogical changes as the result of a time-dependent process. The halloysite content of the soil increases with increasing soil age; its growth is at the expense of allophane, which, in turn, forms at the expense of primary minerals. These mineral relationships suggest that halloysite has been forming continuously throughout the lifetime of the soil. Smectite, the thermodynamically stable phyllosilicate phase predicted by soil solution composition, is not found in these arid soils. We determined that our soil system is controlled by kinetic factors, and that, therefore, thermodynamic predictions do not reflect reality. The main factors favoring kinetic control of halloysite formation are intense, but short wet periods followed by prolonged extremely dry seasons, and microenvironmental conditions leading to immediate uptake of released Al by the halloysite-precursor mineral allophane. The δ 18O relationship between present-day soil water and halloysite, formed over the last 170 ka, was used as a tracer of past soil conditions. Results from a reconstruction of paleo-climate and -soil conditions, combined with δ 18O data, observed mineral relationships along the 350 ka chronosequence, and field evidence for long-term aridity rule out the possibility that the arid side of Kohala was affected by prolonged periods of higher rainfall that could have produced more dilute soil waters and, therefore, altered mineralogical stabilities. We conclude that pedogenic halloysite has been continually forming from the early stage of soil formation, and that it has consistently formed under isotopic equilibrium conditions, indicating that halloysite δ 18O compositions imply paleoclimatic conditions over the time of its formation that are similar to today's.

Influence of spore dose and interrupted wet periods on the development of pear scab caused by Venturia pirina on pear (Pyrus communis) seedlings

The influence of spore dose and interrupted wet periods on pear scab infection was studied under controlled conditions by assessing disease development on pear seedlings. Under optimal conditions (20°C and continuous wetness), the severity of leaf scab (lesions/cm2) increased with conidial concentrations ranging from 102 to 105 conidia/mL. Pear seedlings inoculated with conidia were also exposed to an initial short wet period (3 and 5 h) and a final 24 h wet period which was interrupted with 1, 6, 8, 12, 24, 48 or 90 h of dryness under low ( 90%) relative humidity. The length of the dry period reduced disease severity. During interrupted wet periods under high relative humidity (>90%) at 18" and 20°C, the number of lesions per cm2 of leaf tissue decreased from 2.7 to 0.24 and 2.7 to 0.15, respectively as the dry period ranged from 1 to 90 h. In the same two experiments with similar wet/dry/wet regimes under low relative humidity (<70%), disease severity was also reduced with no scab lesions occurring if leaves were dry for more than 12 h.