Spatial variability of rainfall trends in Iran

The main objective of this paper is to analyze the spatial variability of rainfall trends using the spatial variability methods of rainfall trend patterns in Iran. The study represents a method on the effectiveness of spatial variability for predicting rainfall trend patterns variations. In rainfall trend analysis and spatial variability methods, seven techniques were used: Mann–Kendall test, Sen’s slope method, geostatistical tools as a global polynomial interpolation and the spatial autocorrelation (Global Moran’s I), high/low clustering (Getis-Ord General G), precipitation concentration index, generate spatial weights matrix tool, and activation functions of semiliner, sigmoid, bipolar sigmoid, and hyperbolic tangent in the artificial neural network technique .For the spatial variability of monthly rainfall trends, trend tests were used in 140 stations of spatial variability of rainfall trends in the 1975–2014 period. We analyzed the long and short scale spatial variability of rainfall series in Iran. Spatial variability distribution of rainfall series was depicted using geostatistical methods (ordinary kriging). Relative nugget effect (RNE) predicted from variograms which showed weak, moderate, and strong spatial variability for seasonal and annual rainfall series. Moreover, the rainfall trends at each station were examined using the trend tests at a significance level of 0.05. The results show that temporal and spatial trend patterns are different in Iran and the monthly rainfall had a downward (decreasing) trend in most stations, and the trend was statistically significant for most of the series (73.5% of the stations demonstrated a decreasing trend with 0.5 significance level). Rainfall downward trends are generally temporal-spatial patterns in Iran. The monthly variations of rainfall decreased significantly throughout eastern and central Iran, but they increased in the west and north of Iran during the studied interval. The variability patterns of monthly rainfall were statistically significant and spatially random. Activation functions in the artificial neural network models, in annual time scale, had spatially dispersed distribution with other clustering patterns. The results of this study confirm that variability of rainfall revealing diverse patterns over Iran should be controlled mainly by trend patterns in the west and north parts and by random and dispersed patterns in the central, southern, and eastern parts.     

Investigation of spatial variability and pattern analysis of soil properties in the northwest of Iran

Analysis of the spatial variability of soil properties is important to explain the site-specific ecosystems. Spatial patterns of some soil properties such as soil texture, exchangeable sodium percentage (ESP), electrical conductivity (ECe), soil pH and cation exchange capacity (CEC) were analyzed in salt and sodic affected soils in the south of the Ardabil province, in the northwest of Iran, to identify their spatial distribution for performance of a site-specific management. Soil samples were collected from 0 to 30, 30 to 60, 60 to 90, 90 to 120 and 120 to 150 cm soil depths at sampling sites. Data were investigated both statistically and geostatistically on the basis of the semivariogram. The spatial distribution model and spatial dependence level varied in the study area. Among the considered parameters, maximum and minimum spatial variability were observed in EC and pH parameters, respectively. Soil properties showed moderate to strong spatial dependence, except for a few. ECe was strongly spatially dependent in the total soil depth and clay was strongly spatially dependent at the first depth. Sand and pH were moderately spatially dependent for three of the five depths. ESP was strongly spatially dependent and silt was moderate in the total soil depths, except at 90–120 cm depth. Furthermore, CEC had strong spatial dependence for three of the five depths. All geostatistical range values were >1,389 m in this study. It was concluded that the strong spatial dependency of soil properties would lead to extrinsic factors such as bedrock, agricultural pollution, drainage and ground water level.     

Spatial contribution of one-day precipitations variability to rainy days and rainfall amounts in Iran

Rainfalls with short persistency are the tangible characteristics of arid and semiarid regions such as Iran. Iran is an arid and semiarid region with dramatic tempo-spatial changes of rainfall. In this regard, the short persistency of rainfall is approximately observed from 1 to 7 days in whole parts, while the greater ones are only separated in eastern parts of Iran. According to the results, the rainfall persistency is ranged from 1 to 45 days, but the maximum amount and rainy days are generated by rainfalls with short persistency. So, the rainfall events with long persistency are considered as an extreme event with extreme variability. One-day precipitations generate the maximum rainy days and rainfall amounts, especially in eastern parts of Iran. Decrease in the one-day precipitations contribution to eastern parts may indicate to decrease in regional precipitation. However, decrease in contribution in western parts may indicate to increased amounts of rainfall at other persistency rates. Our results revealed that the contribution of the one-day precipitation to general rainfall has reductive trends in almost 17.5 % of the whole Iran. The most integrated and significant reductive trend of one-day precipitation contribution to rainfall spreads northeastern and eastern parts of Iran. However, in the western parts of Iran, decreasing one-day precipitation contribution to rainy days affects to increase in the diurnal rainfall. The mentioned variability can be considered as the climate change signals in respect of one-day precipitation.     

晚第四纪印度洋-太平洋交汇区降水时空演化

印度洋-太平洋交汇区(印太交汇区)作为全球的水汽和热量中心,在多种时间尺度的气候变化中都扮演着重要的角色。然而,迄今该区域晚第四纪古降水时空演化仍不清楚,主要体现在不同区域或不同指标的降水重建结果出现明显不一致、甚至相反的现象。本研究基于印太交汇区已有重建记录,总结了该区域轨道和千年时间尺度上古降水演化特征及机制。在轨道时间尺度上,尽管该区域大多数降水记录都主要受到岁差主控的日射量变化影响,但其响应的相位关系却截然不同,推测是多种气候因子共同作用的结果。在千年时间尺度上,降水变化主要受控于北大西洋千年时间尺度气候突变事件导致的热带辐合带(ITCZ)迁移;其次,厄尔尼诺与南方涛动(ENSO)以及印度洋偶极子(IOD)的活动也对该地区局部降水有所贡献。另外,苏拉威西等印太交汇区中部区域的降水受到海平面变化的显著影响。总体来看,印太交汇区古降水记录仍存在重建指标单一、部分区域缺乏高分辨率记录等问题,降水空间分布及其变化机制也尚不明朗,需更多的高分辨率古降水重建以及模拟工作的开展去全方面阐明印太交汇区降水的时空演化特征及机制。

     

中国大陆降水时空变异规律——I.气候学特征

为系统了解大尺度降水气候特征,利用2 300多个国家级气象站逐日观测资料,分析了中国大陆1956—2013年多年平均降水的空间分布和季节性变化规律。主要新认识有:① 暴雨量、暴雨日数和暴雨强度最高的站点在华南沿海,而小雨量、小雨日数最多的站点主要在江南内陆山区、丘陵;东部季风区山地、丘陵多出现低强度降水,平原和沿海易出现高强度降水;② 四季降水量均由西北内陆向东南沿海递增,南方秋季降水量明显小于春季,但华西和江南沿海秋季降水量较多,冬季降水在东南丘陵出现高值中心;③ 珠江和东南诸河流域降水量年内存在2个峰值,其中珠江流域有6月主峰值和8月次峰值,东南诸河流域主峰在6月中下旬,次峰在8月末,长江流域总体表现为单峰型,出现在6月下旬和7月初,西南诸河流域和北方所有流域降水均表现为夏季单峰型;④ 南方各大河流域从2月末到6月中下旬陆续进入雨季,北方各大河流域进入雨季时间集中在6月末、7月初;南、北方雨季结束时间比雨季开始时间集中,从南到北进入雨季时间持续120 d以上,而从北到南退出雨季时间则仅持续不到45 d;⑤ 丰雨期的持续时间,珠江流域从5月初到9月上旬后期,东南诸河从5月上旬到7月上旬,8月末到9月初再度短暂出现,长江流域从6月中下旬到7月中旬,西南诸河从7月中旬到 8月下旬,淮河流域从7月上旬至7月底、8月初,辽河流域在8月初出现极短丰雨期;⑥ 降水年际变异性最高的站点在青藏高原西南、塔里木盆地、阿拉善高原、华北平原北部和汾河谷地,海河流域年降水具有最大的变异系数。     

Distribution of fluoride in groundwater of Maku area, northwest of Iran

High fluoride groundwater occurs in Maku area, in the north of West Azarbaijan province, northwest of Iran. Groundwater is the main source of drinking water for the area residents. Groundwater samples were collected from 72 selected points including 40 basaltic and 32 nonbasaltic springs and wells, in two stages, during June and August 2006. The areas with high fluoride concentrations have been identified, and the possible causes for its variation have been investigated. Regional hydrogeochemical investigation indicates that water-rock interaction is probably the main reason for the high concentration of ions in groundwater. The concentration of F⁻ in groundwater is positively correlated with that of HCO₃ ⁻ and Na⁺, indicating that groundwater with high HCO₃ ⁻ and Na⁺ concentrations help to dissolve some fluoride-rich minerals. All of the water samples, collected from the basaltic areas do not meet the water quality standards for fluoride concentration and some other parameters. Hence, it is not suitable for consumption without any prior treatment. Inhabitants of the area that obtain their drinking water supplies from basaltic springs and wells are suffering from dental fluorosis. The population of the study area is at a high risk due to excessive fluoride intake especially when they are unaware of the amount of fluoride being ingested due to lack of awareness.     

Spatial and temporal monthly precipitation forecasting using wavelet transform and neural networks,Qara-Qum catchment,Iran

This paper aims to provide a spatial and temporal analysis to prediction of monthly precipitation data which are measured at irregularly spaced synoptic stations at discrete time points. In the present study, the rainfall data were used which were observed at four stations over the Qara-Qum catchment, located in the northeast of Iran. Several models can be used to spatially and temporally predict the precipitation data. For temporal analysis, the wavelet transform with artificial neural network (WTANN) framework combines with the wavelet transform, and an artificial neural network (ANN) is used to analyze the nonstationary precipitation time-series. The time series of dew point, temperature, and wind speed are also considered as ancillary variables in temporal prediction. Furthermore, an artificial neural network model was used for comparing the results of the WTANN model. Therefore, four models were developed, including WTANN and ANN with and without ancillary data. Several statistical methods were used for comparing the results of the temporal analysis. It was evident that at three of the four stations, the WTANN models were more effective than the ANN models, and only at one station, the ANN model with ancillary data had better performance than the WTANN model without ancillary data. The values of correlation coefficient and RMSE for WTANN model with ancillary data for the validation period at Mashhad station which showed the best results were equal to 0.787 and 13.525 mm, respectively. Finally, an artificial neural network model was used as an alternative interpolating technique for spatial analysis.     

Analysis of pervasive precipitation in similar gradient areas of Iran

Precipitation is a major climatic element with high spatial variations. Temporal and spatial variations may differ in large and small scales. It is, therefore, of utmost importance to study areas with similar gradients in terms of precipitation patterns in order to shed light on the complexities of precipitation variations. In the present study, attempts were made to identify areas with similar gradients experiencing the same precipitation pattern over a 50-year period (1964–2013). To this end, data were collected from synoptic stations in Iran in two phases (i.e., 1434 stations in the first phase and 673 stations in the second one). Alexanderson’s technique was adopted to examine sudden changes in precipitation patterns. The results showed that five regions with similar gradients could be identified in terms of precipitation patterns: negative and high variations, negative and moderate variations, positive and high variations, positive and moderate variations, and little or no variations. The distribution of such regions indicated that the regions with positive trends experienced more annual variations and had further spatial distribution. Furthermore, the findings revealed that the regions with negative precipitation patterns experienced more sudden changes in comparison with those with positive precipitation patterns. Additionally, more variations were observed in the precipitation patterns in recent years.     

Spatial and temporal variability of soil moisture based on multifractal analysis

Based on the experimental data collected from 2002 to 2010 in Jilin province, we performed multifractal analysis to investigate the spatial variability of soil moisture during 9 years from 2002 to 2010 and about 60 days from April 21 to June 21 in 2010, and analyzed the relationship between soil moisture and precipitation. The results showed that the soil moisture was multifractal. It was intermediate variability in the soil sample. From 2002 to 2010, the spatial variation of soil moisture was charactered by long distance in 2004, 2006, and 2009 and short distance in 2002, 2003, and 2008. The spatial variation of soil moisture was charactered by long distance from April 21 to May 11 and late June. It was charactered by short distance from May 11 to June 21. From April 11 to July 1, 2010, the spatial variation of soil moisture was mainly affected by the spatial variation of precipitation. From 2002 to 2010, the spatial variation of soil moisture was mainly affected by precipitation in 2006, 2009, and 2010. The spatial variation of precipitation had little effect on soil moisture during 2002 to 2005 and 2007 to 2008. This study analyzed the multifractal characteristic of soil moisture from the perspective of water resources divisions, which can provide references for soil water resources evaluation and water resources allocation.     

Proto-Tethyan remnants in northwest Iran: Geochemistry of the gneisses and metapelitic rocks

The Iran continental crust was metamorphosed, intruded by granitoid magmas, folded and faulted during the Late Precambrian by the Pan-African Orogeny. The basement complex in the Takab Complex (northwest Iran) consists of gneisses, amphibolites, pelitic schists, meta-ultramafic and calc-silicate rocks. Geochemically, the protoliths of the Takab gneisses are slightly peraluminous and medium to high-potassic with calc-alkaline affinity. These gneisses may have been emplaced in volcanic arc tectonic setting. Furthermore, the metapelitic protolith is shale deposited in an active continental margin setting. All these characteristics, and presence of paleo-suture zone and ophiolitic rocks (i.e. serpentines, meta-mafic and meta-ultramafic rocks) around the high grade metamorphic rocks suggest that a continental-margin magmatic arc (Andean-type) formed the Takab Precambrian basement. The basement complexes are extensively overprinted by the Pan-African Orogeny and younger igneous events; this supports the inference that Early Cambrian orogenesis in the Takab Complex region of northwest Iran marks one of the fundamental lithospheric boundaries within Gondwana which belonged to a greater Late Neoproterozoic–Early Paleozoic orogenic system that was active along the Proto-Tethyan margin of the Gondwana supercontinent, extending at least from its Arabian margin to the Himalayan margin of the Indian subcontinent.     

Identifying the spatial and temporal distribution characteristics of precipitation in Iran

Precipitation has been regarded as one of the most important meteorological parameters affecting human activities. The findings of research studies confirm that the variability and fluctuation in precipitation has direct impacts on climate of a given region. The purpose of the present study was to investigate long-term patterns in precipitation variation in Iran. To this end, the available data related to rainfall in Iran over the past half century (1957–2007) were retrieved from APHRODITE database in order to analyze the spatial-temporal variations of precipitation. The statistical test performed on the collected data included spatial auto-correlation, global Moran’s index, local Moran’s index, and hotspots. The results obtained showed that the precipitation variation in Iran followed a high clustering pattern. More specifically, the results of the local Moran’s index and hotspot analysis performed on the data revealed that the precipitation along the Caspian Sea coast and western and southwestern parts of the country had a positive spatial auto-correlation while the precipitation variation in some parts of the central Iran and along the coastline of the country showed a negative spatial auto-correlation. Moreover, the findings of the present study showed that the climate change continued during the decades under study, with clustering patterns of precipitation moving from the southern parts of the country upward towards the coastal regions of the Caspian Sea and the regions in the outskirt of Zagros Mountains.     

Spatial variability of climatic hazards in Bangladesh

Natural Hazards - Climate change is evident with the extreme climatic indices changing all over the world. Bangladesh is one of the most vulnerable countries to climate change. The patterns of...     

Trend analysis and change point detection of annual and seasonal precipitation and temperature series over southwest Iran

This paper presents results of trend analysis and change point detection of annual and seasonal precipitation, and mean temperature (TM), maximum temperature (TMAX) and minimum temperature (TMIN) time series of the period 1950–2007. Investigations were carried out for 50 precipitation stations and 39 temperature stations located in southwest Iran. Three statistical tests including Pettitt’s test, Sequential Mann–Kendall test (SQ-MK test) and Mann–Kendall rank test (MK-test) were used for the analysis. The results obtained for precipitation series indicated that most stations showed insignificant trends in annual and seasonal series. Out of the stations which showed significant trends, highest numbers were observed during winter season while no significant trends were detected in summer precipitation. Moreover, no decreasing significant trends were detected by statistical tests in annual and seasonal precipitation series. The analysis of temperature trends revealed a significant increase during summer and spring seasons. TMAX was more stable than TMIN and TM, and winter was stable compared to summer, spring and autumn seasons. The results of change point detection indicated that most of the positive significant mutation points in TM, TMAX and TMIN began in the 1990s.     

树轮记录的西北季风边缘区干旱变化

祁连山东部位于亚洲季风西北边缘区,对气候变化十分敏感。由于该区位于半干旱区,降水的变化直接影响人类社会经济的发展,所以研究该区干旱变化情况及其机制非常重要。 利用采自祁连山东部西北季风边缘、地处半干旱区的吐鲁沟地区的油松树轮样芯,建立了树轮宽度年表,时间跨度为1866-2003AD。树轮宽度年表与气象因子相关分析结果显示树木生长主要响应生长季内的降水和干旱变化。该年表与附近区域利用树轮宽度重建的帕尔默干旱指数（PDSI）进行对比,结果显示严重的干旱时间发生在1920s-1930s、1966年、1974年、1982年和1990s和21世纪初。多窗谱分析研究表明,该区年表序列存在显著的2~4 a周期（P<0.05）,该周期与ENSO变率吻合,与在贺兰山、祁连山其他地区、兴隆山及贵清山研究结果一致,表明这些区域可能受共同气候因子的影响。     

地下水污染场地水质空间相关性分析

地下水污染场地的监控需要依据水文地质条件、地下水流场和溶质运移特征,利用水化学的示踪作用开展空间分析,以识别地下水和溶质运移特征。该方法具有简易和经济的优点。以台湾苗栗县某化工污染场地为例,通过对地下水中多种离子浓度的空间等值线分析及对比,判断污染物的来源和分布范围。采用离子浓度的统计特征值P95、P75、P50、P25构成的等值线,作为判断离子空间分布特征的依据,并形成离子之间对比的统一标准。所有离子都采用浓度P95等值线包围的区域作为其污染源,其它三种等值线表示可能出现的污染物扩散范围。在此基础上通过对比发现10种离子的污染源集中出现在场地一个范围内,并形成一个污染带,表明它们的来源具有密切的联系。10种离子P75等值线划出的污染物分布范围同样比较集中,但几乎都分布在污染带的南部,显示离子的迁移方向和迁移距离是一致的。根据离子空间分布的相似性将其分为三组,空间分布相似性高的离子组同时出现在一个区域的机会更多。通过多种水化学成分识别地下水流场和溶质运移特征,提高了结论的可信度,为污染物监控提供参考。