Climate change impacts on spatial patterns in drought risk in the Willamette River Basin, Oregon, USA

Climate change is likely to lead more frequent droughts in the Pacific Northwest (PNW) of America. Rising air temperature will reduce winter snowfall and increase earlier snowmelt, subsequently reducing summer flows. Longer crop-growing season caused by higher temperatures will lead to increases in evapotranspiration and irrigation water demand, which could exacerbate drought damage. However, the impacts of climate change on drought risk will vary over space and time. Thus, spatially explicit drought assessment can help water resource managers and planners to better cope with risk. This study seeks to identify possible drought-vulnerable regions in the Willamette River Basin of the PNW. In order to estimate drought risk in a spatially explicit way, relative Standardized Precipitation Index (rSPI) and relative Standardized Runoff Index (rSRI) were employed. Statistically downscaled climate simulations forcing two greenhouse gas emission scenarios, A1B and B1, were used to investigate the possible changes in drought frequency with 3-, 6-, 12-, and 24-month time scales. The results of rSPI and rSRI showed an increase in the short-term frequency of drought due to decreases in summer precipitation and snowmelt. However, long-term drought showed no change or a slight decreasing pattern due to increases in winter precipitation and runoff. According to the local index of spatial autocorrelation analysis, the Willamette Valley region was more vulnerable (hot spot) to drought risk than the mountainous regions of the Western Cascades and the High Cascades (cold spot). Although the hydrology of the Western Cascades and the High Cascades will be affected by climate change, these regions will remain relatively water-rich. This suggests that improving the water transfer system could be a reasonable climate adaptation option. Additionally, these results showed that the spatial patterns of drought risk change were affected by drought indices, such that appropriate drought index selection will be important in future studies of climate impacts on spatial drought risk.     

Temporal and spatial variability of rainfall over Greece

Recent studies have showed that there is a significant decrease in rainfall over Greece during the last half of the pervious century, following an overall decrease of the precipitation at the eastern Mediterranean. However, during the last decade an increase in rainfall was observed in most regions of the country, contrary to the general circulation climate models forecasts. An updated high-resolution dataset of monthly sums and annual daily maxima records derived from 136 stations during the period 1940–2012 allowed us to present some new evidence for the observed change and its statistical significance. The statistical framework used to determine the significance of the slopes in annual rain was not limited to the time independency assumption (Mann-Kendall test), but we also investigated the effect of short- and long-term persistence through Monte Carlo simulation. Our findings show that (a) change occurs in different scales; most regions show a decline since 1950, an increase since 1980 and remain stable during the last 15 years; (b) the significance of the observed decline is highly dependent to the statistical assumptions used; there are indications that the Mann-Kendall test may be the least suitable method; and (c) change in time is strongly linked with the change in space; for scales below 40 years, relatively close regions may develop even opposite trends, while in larger scales change is more uniform.     

Temporal and spatial variability of dryness/wetness in China during the last 530 years

Summary The main characteristics of spatial and temporal variability of dryness and wetness during the last 530 years (1470–1999) are classified over five centuries. They have been investigated by using 100-site dryness/wetness index data that has recorded the historical weather conditions that affect agriculture and living conditions in eastern China. A set of principal modes of spatial variability and time coefficient series describing the dominant temporal variability are extracted by a diagnostic method, the rotated empirical orthogonal function (REOF) analysis. The long-term precipitation around Beijing, north China and the long-term runoffs in the middle Yangtze River are used to confirm the dry/wet variability in north China and the mid-low Yangtze River over the last two centuries.When considering the data from the last 530 years as a whole, the first two modes of dryness/wetness variability are found in the mid to low sections of two major valleys in eastern China, the Yellow and Yangtze River valleys. These valleys experienced the largest dryness/wetness variability in the history of eastern China. The third and fourth modes are located in northwest and northeast China. The fifth and sixth modes are situated in south and southwest China. However, over the last 500 years the strength and location of principal modes have experienced significant changes. During the 20th century, the first mode is found in the lower Yangtze River valley, the second mode in south China while the third mode is located in the mid-low Yellow River valley. During the 19^th century, the first three modes are situated in the mid-low Yellow River, the mid-low Yangtze River and south China, respectively. The first two modes in the 18^th century are located in the mid-low Yellow River and the mid-low Yangtze River valleys. The largest change of all modes occurred in the 17^th century with the first mode in northeast China, the second mode in northwest China, and the third mode in the mid-low Yangtze River valley. During the 16^th century, the first two modes are found in the mid-low Yangtze River and the mid-low Yellow River valleys.In each of the last five centuries, some special dryness/wetness processes are characterized in the mid-low Yangtze River and the mid-low Yellow River (north China). During the 20^th century, continuous and severe wetness is experienced in the mid-low Yangtze River in the last two decades. A two-decade wetness period in north China was followed by a severe dry period in the late 19^th century. Inter-annual variability, decade and two-decade oscillations of dryness/wetness are experienced in the series of different modes from one century to another. Dry/wet variations in north China and the middle Yangtze River are confirmed by series of data on local precipitation and runoff.     

Temporal and spatial variability of the global water balance

An analysis of simulated global water-balance components (precipitation [P], actual evapotranspiration [AET], runoff [R], and potential evapotranspiration [PET]) for the past century indicates that P has been the primary driver of variability in R. Additionally, since about 2000, there have been increases in P, AET, R, and PET for most of the globe. The increases in R during 2000 through 2009 have occurred despite unprecedented increases in PET. The increases in R are the result of substantial increases in P during the cool Northern Hemisphere months (i.e. October through March) when PET increases were relatively small; the largest PET increases occurred during the warm Northern Hemisphere months (April through September). Additionally, for the 2000 through 2009 period, the latitudinal distribution of P departures appears to co-vary with the mean P departures from 16 climate model projections of the latitudinal response of P to warming, except in the high latitudes. Finally, changes in water-balance variables appear large from the perspective of departures from the long-term means. However, when put into the context of the magnitudes of the raw water balance variable values, there appears to have been little change in any of the water-balance variables over the past century on a global or hemispheric scale.     

基于SPEI指数分析华中地区近40a干旱时空分布特征

分析了1961-2009年华中地区降水量、气温及蒸发量变化特征,在此基础上,采用标准化降水蒸散指数(SPEI)确定华中地区的干旱强度,按照SPEI指数的标准界值将干旱强度划分为4个等级并分析了各干旱等级的发生频率和空间分布。不同时间尺度SPEI指数的EOF分析表明:华中地区干旱的主要空间分布具有较好的全区一致性,且春季干旱的强度在四季中是最强的。     

1960—2010年中国南方地区夏季旱涝时空分布特征

基于1960—2010年中国南方地区51 a降水资料,进行Z指数变换,再利用降水距平场作REOF分区,并分别对各区域代表站点Z指数序列作线性趋势分析和M-K分析及Morlet小波分析。结果表明：中国南方5个分区岳阳站、广州站、丽水站、信阳站和巢湖站5个典型站点的年际及年代际波动幅度都比较明显,但均呈由干变湿的趋势。且5个站点分别在1973、1993、1992、2002年和1979年发生突变,突变后UF一直呈上升趋势,总体均由干变湿5,个分区旱涝异常具有相对一致的性质。中国南方地区夏季降水具有多时间尺度振荡的特点,存在2—3 a和5—6 a的周期。     

Spatiotemporal variability of four precipitation-based drought indices in Xinjiang,China

Global increases in duration and prevalence of droughts require detailed drought characterization at various spatial and temporal scales. In this study, drought severity in Xinjiang, China was investigated between 1961 and 2012. Using meteorological data from 55 weather stations, the UNEP (1993) index (I _A), Erinç’s aridity index (I _m), and Sahin’s aridity index (I _sh) were calculated at the monthly and annual timescales and compared to the Penman-Monteith based standard precipitation evapotranspiration index (SPEI_PM). Drought spatiotemporal variability was analyzed for north (NX), south (SX), and entire Xinjiang (EX). I _m could not be calculated at 51 stations in winter as T _max was below 0. At the monthly timescale, I _A, I _m, and I _sh correlated poorly to SPEI_PM because of seasonality and temporal variability, but annual I _A, I _m, and I _sh correlated well with SPEI_PM. Annual I _A, I _m, and I _sh showed strong spatial variability. The 15 extreme droughts denoted by monthly SPEI_PM occurred in NX but out of phase in SX. Annual precipitation, maximum temperature, and relative and specific humidity increased, while air pressure and potential evapotranspiration decreased over 1961–2012. The resulting increases in the four drought indices indicated that drought severity in Xinjiang decreased, because the local climate became warmer and wetter.

     

Temporal and spatial evolution trends of drought in northern Shaanxi of China: 1960–2100

Theoretical and Applied Climatology - Drought prediction and assessment are the basis for addressing climate change and extreme weather. Northern Shaanxi is an important energy base and ecological...     

Spatial and temporal variability of reference evapotranspiration and influenced meteorological factors in the Jialing River Basin,China

Theoretical and Applied Climatology - Reference evapotranspiration (ETr) is one of the important parameters in the hydrological cycle. The spatio-temporal variation of ETr and other meteorological...     

Temporal and spatial variability of SSTs in the tropical Atlantic and Indian Oceans

Summary The objective of this study is to describe spatial and temporal patterns of sea-surface temperature (SST) variability in the Atlantic and Indian Oceans. The analysis domain extends from 40°S to 25°N and 50°W to 80°E, hence the tropical and most of the South Atlantic and central and western Indian Oceans. The investigation, covering the years 1948 to 1979, utilizes the COADS marine data set. Empirical orthogonal functions and spectral analysis are used to analyze SST fields.A major finding of this investigation is that SSTs vary coherently throughout most of the analysis domain. The greatest coherence is evident from 10°N to 30°S in the Atlantic and from 20°N to 35°S in the western Indian Ocean. Spectral analysis of regional time series shows that throughout this region the time scale of 5–6 years is the dominant one in the fluctuations; this is also the case for the Southern Oscillation and for equatorial rainfall. SST variations are roughly in-phase within each ocean and the two oceans are roughly in-phase with each other, i.e., the lags which exist are much smaller than the dominant time scale of the fluctuations. The SST anomalies appear to propagate eastward from NE Brazil; the eastern Atlantic lags the western by two to six months and the Indian Ocean lags the western Atlantic by four to eight months.With 15 Figures     

Temporal variability and spatial distribution of Sea Surface Temperatures in the Aegean Sea

Summary Sea Surface Temperature (SST) is a critical factor in the assessment of the climatic structure of a coastal region affected mainly by air temperature (AT). A regionalization of the Aegean Sea based on SST values measured at twenty-one representative stations is presented using a widely accepted clustering method. With this method, the Aegean Sea is divided into sub-regions with similar characteristics, on a seasonal basis.Principal Component Analysis (PCA) shows that SSTs oscillate simultaneously in the Aegean Sea and that the major part of the variance is concentrated in the first mode. Spectral analysis shows a high concentration of energy for periods of one year, as is expected. When the annual oscillation is filtered, two secondary peaks can be distinguished, one representing periods from two to three years and another representing periods from three to five years. The first peak is caused by the Quasi-Biennial Oscillation (QBO) and the second as a result of the Southern Oscillation (SO).Interannual variability of the SSTs, although present in the overall signal, seems to be low compared with the strong annual period signal.With 7 Figures     

Analysis of drought determinants for the Colorado River Basin

Ongoing drought in the Colorado River Basin, unprecedented urban growth in the watershed, and numerical model simulations showing higher temperatures and lower precipitation totals in the future have all combined to heighten interest in drought in this region. In this investigation, we use principal components analysis (PCA) to independently assess the influence of various teleconnections on Basin-wide and sub-regional winter season Palmer Hydrological Drought Index (PHDI) and precipitation variations in the Basin. We find that the Pacific Decadal Oscillation (PDO) explains more variance in PHDI than El Niño-Southern Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO), and the planetary temperature combined for the Basin as a whole. When rotated PCA is used to separate the Basin into two regions, the lower portion of the Basin is similar to the Basin as a whole while the upper portion, which contains the high-elevation locations important to hydrologic yield for the watershed, demonstrates poorly defined relationships with the teleconnections. The PHDI for the two portions of the Basin are shown to have been out of synch for much of the twentieth century. In general, teleconnection indices account for 19% of the variance in PHDI leaving large uncertainties in drought forecasting.     

贵州夏季最高气温变率的时空分布特征

选取贵州19个代表站点47 a夏季日最高气温月平均及季平均资料,采用EOF分解方法对贵州夏季最高气温的变化进行分析,结果表明:前两个模态的累积方差贡献在82%~89.7%,特征值对应特征向量和时间系数能够较好地反映时空分布特征,第一特征向量代表的空间分布显示:全省变化趋势一致,东北部变化较西南部大,北部、东北部变率变化较南部西南部大;相应地时间系数变化幅度大,都有2~4 a的周期振荡;从第二特征向量代表的空间分布可知:各时段呈现出不一样的变化趋势,时间系数的值也普遍较第一特征向量小,突变不明显。     

2011—2019年中国长江三角洲区域闪电时空分布特征

利用2011—2019年江苏、浙江和安徽27个探测站ADTD闪电监测定位系统获取的资料,对中国长江三角洲三省一市(上海、江苏、浙江、安徽)一体化协同发展区域云地闪的发生频次、强度进行统计,运用EOF方法分析闪电频次的空间分布特征.结果表明:中国长江三角洲区域负闪数均占总闪数的95％以上,正闪平均强度均大于负闪平均强度绝...     

2020年主汛期长江流域短时强降水时空分布特征

长江流域(Yangtze River Basin, YZRB)是中国降水集中地。在气候变暖背景下,短时强降水(Short-Duration Heavy Rainfall, SDHR)有增加趋势。2020年主汛期(6—8月)YZRB出现多轮强降水,发生了新中国成立以来仅次于1954年、1998年的流域性大洪水。本文利用中国气象局国家气象信息中心逐小时降水资料,分析了长江上游(YR-A)、长江中游(YR-B)和长江下游(YR-C)三个区域SDHR时空分布以及不同类型短时强降水事件(Short-Duration Heavy Rainfall Event, SDHRE)的统计特征。得到结论如下：1)受地形影响,YZRB山区降水频次增加、降水强度增强,且地形作用会增加山区SDHR的频次,进而增强山区SDHR的降水量;YZRB降水强度的空间分布依赖于SDHR降水量的空间分布。2)YZRB三个区域SDHR降水量和频次的日变化均表现为双峰型,双峰时间在YZRB区域自西向东有从夜间移向白天的趋势,这与对流活动日变化的区域差异有关;SDHR的降水量和频次具有相似的日变化,说明SDHR的降水量主要源自其降水...     

Analysis of climatic variability and snow cover in the Kaligandaki River Basin,Himalaya, Nepal

Various remote sensing products and observed data sets were used to determine spatial and temporal trends in climatic variables and their relationship with snow cover area in the higher Himalayas, Nepal. The remote sensing techniques can detect spatial as well as temporal patterns in temperature and snow cover across the inaccessible terrain. Non-parametric methods (i.e. the Mann–Kendall method and Sen's slope) were used to identify trends in climatic variables. Increasing trends in temperature, approximately by 0.03 to 0.08 °C year^?1 based on the station data in different season, and mixed trends in seasonal precipitation were found for the studied basin. The accuracy of MOD10A1 snow cover and fractional snow cover in the Kaligandaki Basin was assessed with respect to the Advanced Spaceborne Thermal Emission and Reflection Radiometer-based snow cover area. With increasing trends in winter and spring temperature and decreasing trends in precipitation, a significant negative trend in snow cover area during these seasons was also identified. Results indicate the possible impact of global warming on precipitation and snow cover area in the higher mountainous area. Similar investigations in other regions of Himalayas are warranted to further strengthen the understanding of impact of climate change on hydrology and water resources and extreme hydrologic events.     

Spatial and Temporal Variability of Water Availability in the Yellow River Basin

The changes in hydrological processes in the Yellow River basin were simulated by using the Community Land Model(CLM,version 3.5),driven by historical climate data observed from 1951 to 2008.A comparison of modeled soil moisture and runoff with limited observations in the basin suggests a general drying trend in simulated soil moisture,runoff,and precipitation-evaporation balance(P-E) in most areas of the Yellow River basin during the observation period.Furthermore,annual soil moisture,runoff,and P-E averaged over the entire basin have declined by 3.3%,82.2%,and 32.1%,respectively.Significant drying trends in soil moisture appear in the upper and middle reaches of the basin,whereas a significant trend in declining surface runoff and P-E occurred in the middle reaches and the southeastern part of the upper reaches.The overall decreasing water availability is characterized by large spatial and temporal variability.