Using aridity indices to describe some climate and soil features in Eastern Europe: a Romanian case study

Summary As a result of climatic change associated with global warming, aridity is an increasing problem in many parts of the world, including south-eastern and southern regions of Romania. This paper clarifies the concept of aridity, and discusses related concepts including indices of aridity, and their influence on some landscape and soil features including climatic water deficit (WD) and the depth to soil carbonates (DC). As used here, WD is calculated as the difference between precipitation sum (P) and the Penman-Monteith reference evapotranspiration sum (ET_o-PM) over certain periods. Another three well-known aridity indices are also considered: De Martonne’s index (I_ar-DM), Thornthwaite’s index (I_ar-TH), the UNESCO (1979) P/ET_o-PM ratio index (I_ar-P/ETo-PM). WD is as high as −450 mm during the growing season in the most arid, south-eastern and southern regions of Romania, especially in the Dobrogea and Baragan areas. In other regions of Romania, including most of the plains and plateaus where agriculture is an important branch of the economy, WD reaches −100 to −300 mm during the growing season. The above aridity indices were spatially interpolated for specific periods by kriging, to generate relatively homogeneous areas. WD can also be seen as an aridity index which has the advantage of a more accurate quantification of the water supply needed for a reference crop, e.g. grass under standardised conditions, for various geographical regions. WD is significantly correlated with the other aridity indexes and with DC. This paper also examines the risk of aridity spreading, and suggests improvements to the water management system for agriculture in Romania.     

Spatiotemporal variations of potential evapotranspiration and aridity index in relation to influencing factors over Southwest China during 1960–2013

This study investigated the spatial–temporal patterns and trends of potential evapotranspiration (ET₀) and aridity index (AI) over Southwest China during 1960–2013 based on daily temperature, precipitation, wind speed, sunshine duration, total solar radiation, and relative humidity data from 108 meteorological stations. The Penman–Monteith model, Mann–Kendall (M–K) test, moving t test, and Morlet wavelet method were used. The results indicated that ET₀ and AI across the region displayed decreasing trends, but the former was significant. After 2000, regionally average trends in ET₀ and AI increased rapidly, indicating that droughts increased over Southwest China in recent years. Spatially, the changes of ET₀ and AI were dissimilar and not clustered, either. Temporally, both ET₀ and AI displayed obvious abrupt change points over different timescales and that of AI was during the winter monsoon period. Significant periodic variations with periods of 27, 13, and 5 years were found in ET₀, but only of 13 and 5 years existed in AI. Correlation analysis revealed that the sunshine duration and wind speed were the dominant factors affecting ET₀ and that AI showed strong negative correlation with precipitation. The findings of this study enhance the understanding of the relationship between climate change and drought in Southwest China, while the mechanism controlling the variation in drought requires further study.     

标准化降水蒸发指数在中国区域的应用

利用中国气象局160个站1951～2010年月降水和月平均气温资料,分析了最近定义的一种干旱指数——标准化降水蒸发指数(SPEI)在我国不同等级降水区域的适用性,并与标准化降水指数(SPI)和湿润指数H进行了对比分析。结果表明:1)在我国年均降水量大于200 mm的地区,各种时间尺度的SPEI分析均适用;在干旱区(年均降水量小于200 mm),只有12个月以上的大尺度SPEI分析适用性较好;其中12个月尺度的SPEI分析在各区适用性最好。2)由于干旱区冬季的潜在蒸发量和降水量0值均较多,导致1、3、6个月的小尺度SPEI分析在该区不适用。3)与SPI和H指数相比,SPEI既能充分反映1997年气温跃变以后增温效应对干旱程度的影响,又可作为监测指数识别干旱是否发生和结束,能较准确地表征干旱状况。     

Dry-wet variations and cause analysis in Northeast China at multi-time scales

Global warming has caused unevenly distributed changes in precipitation and evapotranspiration, which has and will certainly impact on the wet-dry variations. Based on daily meteorological data collected at 91 weather stations in Northeast China (NEC), the spatiotemporal characteristics of dry and wet climatic variables (precipitation, crop reference evapotranspiration (ET₀), and humid index (HI)) are analyzed, and the probable reasons causing the changes in these variables are discussed during the period of 1961–2014. Precipitation showed non-significant trend over the period of 1961–2014, while ET₀ showed a significant decreasing trend, which led to climate wetting in NEC. The period of 2001–2012 exhibited smaller semiarid area and larger humid area compared to the period of 1961–1980, indicating NEC has experienced wetting process at decadal scale. ET₀ was most sensitive to relative humidity, and wind speed was the second most sensitive variable. Sunshine hours and temperature were found to be less influential to ET₀ in the study area. The changes in wind speed in the recent 54 years have caused the greatest influence on ET₀, followed by temperature. For each month, wind speed was the most significant variable causing ET₀ reduction in all months except July. Temperature, as a dominant factor, made a positive contribution to ET₀ in February and March, as well as sunshine hours in June and July, and relative humidity in August and September. In summary, NEC has experienced noticeable climate wetting due to the significantly decreasing ET₀, and the decrease in wind speed was the biggest contributor for the ET₀ reduction. Although agricultural drought crisis is expected to be partly alleviated, regional water resources management and planning in Northeast China should consider the potential water shortage and water conflict in the future because of spatiotemporal dry-wet variations in NEC.     

近40年河北省地表干燥度的时空变化

利用河北省1970-2007年48个气象台站逐日资料, 采用Penman-Monteith模型计算潜在蒸散量, 由潜在蒸散量和降水量之比构建干燥度指数, 并采用Kriging插值法进行空间插值以分析其区域特征。结果表明:1970-1985年, 由于降水量减少和潜在蒸散量减少, 蒸散量的减少速率大于降水量的减少速率, 地表干燥度指数呈下降趋势, 潜在蒸散量的显著减少是地表干燥度下降的主要原因, 而风速和日照时数的显著降低决定了潜在蒸散量的显著下降; 1986-2007年, 由于年平均气温的显著升高, 潜在蒸散量增加, 使得地表干燥度略呈上升趋势。河北省地表干燥度高值区分布在张家口地区的桑洋盆地和坝西高原, 而低值区主要在燕山南麓低山丘陵地区的承德西南部、唐山的北部和秦皇岛中北部大部分地区。干燥度减少区域主要集中在河北省东北部至河北省西部的带状区域。     

Probabilistic assessment of projected climatological drought characteristics over the Southeast USA

The study makes a probabilistic assessment of drought risks due to climate change over the southeast USA based on 15 Global Circulation Model (GCM) simulations and two emission scenarios. The effects of climate change on drought characteristics such as drought intensity, frequency, areal extent, and duration are investigated using the seasonal and continuous standard precipitation index (SPI) and the standard evapotranspiration index (SPEI). The GCM data are divided into four time periods namely Historical (1961–1990), Near (2010–2039), Mid (2040–2069), and Late (2070–2099), and significant differences between historical and future time periods are quantified using the mapping model agreement technique. Further, the kernel density estimation approach is used to derive a novel probability-based severity-area-frequency (PBS) curve for the study domain. Analysis suggests that future increases in temperature and evapotranspiration will outstrip increases in precipitation and significantly affect future droughts over the study domain. Seasonal drought analysis suggest that the summer season will be impacted the most based on SPI and SPEI. Projections based on SPI follow precipitation patterns and fewer GCMs agree on SPI and the direction of change compared to the SPEI. Long-term and extreme drought events are projected to be affected more than short-term and moderate ones. Based on an analysis of PBS curves, especially based on SPEI, droughts are projected to become more severe in the future. The development of PBS curves is a novel feature in this study and will provide policymakers with important tools for analyzing future drought risks, vulnerabilities and help build drought resilience. The PBS curves can be replicated for studies around the world for drought assessment under climate change.     

Comparison of the streamflow sensitivity to aridity index between the Danjiangkou Reservoir basin and Miyun Reservoir basin, China

The central route of China’s South-to-North Water Diversion Project would divert water from the Danjiangkou Reservoir basin (DRB) to Beijing beginning in the year 2014. The current main surface water source for Beijing is the Miyun Reservoir basin (MRB). The observed streamflows into the DRB and the MRB decreased significantly due to climatic variation and human activities from 1960 to 2005. The climate elasticity method is widely used to quantitatively separate the impacts of climatic variation and human activities on streamflow. One of the uncertainties of the method is that the impacts of changes in precipitation and potential evapotranspiration on streamflow are separated with the assumption that they are independent. However, precipitation and potential evapotranspiration are not totally independent. Aridity index, as the ratio between potential evapotranspiration and precipitation, could be considered as the representative indicator of climatic variation. In this study, the sensitivity of streamflow to aridity index is evaluated to assess the impact of climatic variation on streamflow in the DRB and the MRB. The result shows that streamflow in the MRB is more sensitive to climatic variation than that in the DRB. However, the effective impact of aridity index on streamflow is the product of the sensitivity and the change rate of aridity index. The attribution results show that change in aridity index contributed 68.8 % of the decrease in streamflow in the DRB while it contributed 31.5 % of the decrease in streamflow in the MRB. This indicated that the impact of climatic variation was the main reason of decrease in streamflow in the DRB while human activities such as increasing water consumption and land use change were the main reasons of decreasing streamflow in the MRB.     

The potential of vegetation feedback to alleviate climate aridity over the United States associated with a 2×CO2 climate condition

This study examines the potential impact of vegetation feedback on changes in summer climate aridity over the contiguous United States (US) due to the doubling of atmospheric CO₂ concentration using a set of 100-year-long climate simulations made by a global climate model interactively coupled with a dynamic vegetation model. The Thornthwaite moisture index (I _m ), which quantifies climate aridity on the basis of atmospheric water supply (i.e., precipitation) and atmospheric water demand (i.e., potential evapotranspiration, PET), is used to measure climate aridity. Warmer atmosphere and drier surface resulting from increased CO₂ concentration increase climate aridity over most of the contiguous US. This phenomenon is due to larger increments in PET than in precipitation, regardless of the presence or absence of vegetation feedback. Compared to simulations without active dynamic vegetation feedback, the presence of vegetation feedback significantly alleviates the increase in aridity. This vegetation-feedback effect is most noticeable in the subhumid regions such as southern, midwestern and northwestern US, primarily by the increasing vegetation greenness. In these regions, the greening in response to warmer temperatures enhances moisture transfer from soil to atmosphere by evapotranspiration (ET). The increased ET and subsequent moistening over land areas result in weaker surface warming (1–2?K) and PET (3–10?mm?month^?1), and greater precipitation (4–10?mm?month^?1). Collectively, they result in moderate increases in I _m . Our results suggest that moistening by enhanced vegetation feedback may prevent aridification under climatic warming especially in areas vulnerable to climate change, with consequent implications for mitigation strategies.     

滕州市近50年气候干湿变化

利用滕州市1956~2005年降水量、平均气温资料,用Holdridge干燥度指数来分析近50年气候干湿变化趋势和特征。滕州市近50年来在年生物温度、年可能蒸散量极显著上升背景下,年降水量不显著的减小趋势,造成年水分盈亏量显著亏损及年干燥度指数显著增大,总体呈现暖干化趋势。年干燥度指数变化有明显的阶段性,干湿期交替变化,大体经历了3个湿期和2个干期。1976年年干燥度指数发生由偏湿向偏干的突变,突变后气候类型分布发生显著变化。通过对近50年年干燥度指数滑动平均值和标准差分析发现:随着干燥度指数平均值的增大,异常湿事件明显减少,而异常干事件明显增多,同时,随着标准差的增大,异常干湿事件频率明显增大。     

A New method for identifying possible causal relationships between CO2, total solar irradiance and global temperature change

The study compares two formulas for calculating the daily evapotranspiration ET₀ for a reference crop. The first formula was proposed by Allen et al. (AL), while the second one was proposed by Katerji and Perrier with the addition of the carbon dioxide (CO₂) effect on evapotranspiration (KP). The study analyses the impact of the calculation by the two formulas on the irrigation requirement (IR). Both formulas are based on the Penman-Monteith equation but adopt different approaches for parameterising the canopy resistance r _c . In the AL formula, r _c is assumed constant and not sensitive to climate change, whereas in the KP formula, r _c is first parameterised as a function of climatic variables, then ET₀ is corrected for the air CO₂ concentration. The two formulas were compared in two periods. The first period involves data from two sites in the Mediterranean region within a measured climate change period (1981–2006) when all the input climatic variables were measured. The second period (2070–2100) involves data from a future climate change period at one site when the input climatic variables were forecasted for two future climate scenarios (A2 and B2). The annual cumulated values of ET₀ calculated by the AL formula are systematically lower than those determined by the KP formula. The differences between the ET₀ estimation with the AL and KP formulas have a strong impact on the determination of the IR for the reference crop. In fact, for the two periods, the annual values of IR when ET₀ is calculated by the AL formula are systematically lower than those calculated by the KP formula. For the actual measured climate change period, this reduction varied from 26 to 28 %, while for the future climate change period, it varied based on the scenario from 16 % (A2) to 20 % (B2).     

Improved predictability of droughts over southern Africa using the standardized precipitation evapotranspiration index and ENSO

The provision of timely and reliable climate information on which to base management decisions remains a critical component in drought planning for southern Africa. In this observational study, we have not only proposed a forecasting scheme which caters for timeliness and reliability but improved relevance of the climate information by using a novel drought index called the standardised precipitation evapotranspiration index (SPEI), instead of the traditional precipitation only based index, the standardised precipitation index (SPI). The SPEI which includes temperature and other climatic factors in its construction has a more robust connection to ENSO than the SPI. Consequently, the developed ENSO-SPEI prediction scheme can provide quantitative information about the spatial extent and severity of predicted drought conditions in a way that reflects more closely the level of risk in the global warming context of the sub region. However, it is established that the ENSO significant regional impact is restricted only to the period December–March, implying a revisit to the traditional ENSO-based forecast scheme which essentially divides the rainfall season into the two periods, October to December and January to March. Although the prediction of ENSO events has increased with the refinement of numerical models, this work has demonstrated that the prediction of drought impacts related to ENSO is also a reality based only on observations. A large temporal lag is observed between the development of ENSO phenomena (typically in May of the previous year) and the identification of regional SPEI defined drought conditions. It has been shown that using the Southern Africa Regional Climate Outlook Forum’s (SARCOF) traditional 3-month averaged Nino 3.4 SST index (June to August) as a predictor does not have an added advantage over using only the May SST index values. In this regard, the extended lead time and improved skill demonstrated in this study could immensely benefit regional decision makers.     

联合干旱指数在干旱监测中的应用——以广东韶关地区为例

气候变化影响了水循环与地区的水量平衡过程,一定程度上改变了干旱的形成与演变条件。以标准化降水量与蒸散发量差值表征水分偏离正常程度的标准化降水蒸散发指数(SPEI)为基础,从多时间尺度联合的视角建立联合干旱指数(JDI),并以广东韶关为例分析修正的新指数JDI在干旱监测中的准确性和有效性。结果表明,综合了不同时间尺度干旱特征信息的JDI能够较全面地反映干旱的形成与演进过程。通过对干旱监测的评价以及与实际旱情的对比分析,验证了联合干旱指数JDI在实际干旱监测中的准确性和有效性,其可作为未来干旱监测的新理想指标。     

利用条件植被指数评价西藏植被对气象干旱的响应

基于2000—2014年4—10月西藏气象站遥感干旱指数 (条件植被指数,VCI) 和气象干旱指数 (标准降水指数,SPI) 之间的相关性,评估植被对气象干旱的响应特征,通过分析气候环境要素对响应特征的影响并归纳相应规则,获取西藏地区植被对气象干旱有明显响应的区域分布。结果显示:VCI与12周时间尺度的SPI具有较强相关性,说明西藏地区植被生长对降水的响应大约滞后12周;植被对气象干旱响应不敏感的原因主要包括气候极度干燥或极度湿润、土地覆盖类型为森林、年平均归一化植被指数 (NDVI) 值过小、多年NDVI变化标准差过小、有降水之外的其他水源补给等;基于对区域气候环境要素特征的分析,可以得出西藏中部偏南地区植被对气象干旱有明显响应,主要包括拉萨地区、山南地区北部、日喀则地区东部、那曲地区中部和西南部、阿里地区的东南部。     

MLP-based drought forecasting in different climatic regions

Water resources management is a complex task and is further compounded by droughts. This study applies a multilayer perceptron network optimized using Levenberg–Marquardt (MLP) training algorithm with a tangent sigmoid activation function to forecast quantitative values of standardized precipitation index (SPI) of drought at five synoptic stations in Iran. The study stations are located in different climatic regions based on De Martonne aridity index. In this study, running series of total precipitation corresponding to 3, 6, 9, 12, and 24?months were used and the corresponding SPIs were calculated: SPI3, SPI6, SPI9, SPI12, and SPI24. The multilayer perceptrons (MLPs) for SPIs with the 1-month lead time forecasting, were tested and validated. Four different input vectors were considered during network development. In the first model, MLP constructed by importing antecedent SPI with 1-, 2-, 3-, and 4-month time lags and antecedent precipitation with 1- and 2-month time lags (MLP1). Addition of antecedent North Atlantic Oscillation or antecedent Southern Oscillation Index with 1-month time lag or both of them to MLP1 led to MLP2, MLP3, and MLP4, respectively. The MLP models were evaluated using the root mean square error (RMSE) and the coefficient of determination (R ²). The results showed that MLP4 had a higher prediction efficiency than the other MLPs. The more satisfactory results of RMSE and R ² values of MLP4 for 1-month lead time for validation phase were equal to 0.35 and 0.92, respectively. Also, results indicated that MLPs can forecast SPI24 and SPI12 more accurately than the other SPIs.     

Application of relative drought indices in assessing climate-change impacts on drought conditions in Czechia

The common versions (referred to as self-calibrated here) of the Standardized Precipitation Index (SPI) and the Palmer Drought Severity Index (PDSI) are calibrated and then applied to the same weather series. Therefore, the distribution of the index values is about the same for any weather series. We introduce here the relative SPI and PDSI, abbreviated as rSPI and rPDSI. These are calibrated using a reference weather series as a first step, which is then applied to the tested series. The reference series may result from either a different station to allow for the inter-station comparison or from a different period to allow for climate-change impact assessments. The PDSI and 1–24 month aggregations of the SPI are used here. In the first part, the relationships between the self-calibrated and relative indices are studied. The relative drought indices are then used to assess drought conditions for 45 Czech stations under present (1961–2000) and future (2060–2099) climates. In the present climate experiment, the drought indices are calibrated by using the reference station weather series. Of all drought indices, the PDSI exhibits the widest spectrum of drought conditions across Czechia, in part because it depends not only on precipitation (as does the SPI) but also on temperature. In our climate-change impact experiments, the future climate is represented by modifying the observed series according to scenarios based on five Global Climate Models (GCMs). Changes in the SPI-based drought risk closely follow the modeled changes in precipitation, which is predicted to decrease in summer and increase in both winter and spring. Changes in the PDSI indicate an increased drought risk at all stations under all climate-change scenarios, which relates to temperature increases predicted by all of the GCMs throughout the whole year. As drought depends on both precipitation and temperature, we conclude that the PDSI is more appropriate (when compared to the SPI) for use in assessing the potential impact of climate change on future droughts.     

Performance evaluation of modified versions of Hargreaves equation across a wide range of Iranian climates

Reference evapotranspiration (ET_o) is significant for water resources planning and environmental studies. Many equations have been developed for ET_o estimation in various geographic and climatic conditions, of which, the Penman–Monteith FAO 56 (PMF-56) equation was accepted as reference method. A major complication in estimating ET_o by the PMF-56 model is the requirement for meteorological data that may not be readily available from typical weather stations in many areas of the globe. This restriction necessitates use of simpler models which require less input data. In this study, the original and five modified versions of the Hargreaves equation that require only temperature and rainfall were evaluated in humid, semi-humid, semi-arid and arid climates in Iran. The results showed that the original and modified versions of the Hargreaves equation had the poorest performance in semi-humid climate and the best performance in windy humid environment. Further, the ET_o estimations with the Hargreaves equations having rainfall parameter were poor as compared to those from the PMF-56 method under majority of the climatic situations studied.     

Assessment of an Evapotranspiration Deficit Drought Index in Relation to Impacts on Ecosystems

Ecosystems have increasingly been subject to the challenge of heavy drought under global warming. To quantitatively evaluate the impacts of drought on ecosystems, it is necessary to develop a drought index that can sensitively depict the response of vegetation to drought evolution at a biological time scale. For the ability of direct connection between climate and ecosystem by deficit of evapotranspiration, in the present study, a drought index was defined based on standardized evapotranspiration deficit (SEDI), according to the difference between actual and potential evapotranspiration, to meet the need for highlighting drought impacts on ecological processes. Comparisons with traditional indices show that SEDI can reasonably detect droughts and climatic dry and wet transitions, especially at a monthly time scale, and can also regenerate long-term trends. Moreover, SEDI can more sensitively capture the biological changes of ecosystems in response to the dynamics of drought intensity, compared with the indices of precipitation and temperature. SEDI is more practical than the precipitation and temperature indices to highlight signals of biological effects in climate droughts. Hence, it has potential for use in assessments of climate change and its impact on ecosystems.     

Differential regional responses in drought length, intensity and timing to recent climate changes in a Mediterranean forested ecosystem

The Mediterranean area is one of the regions of the world where GCMs agree the most on precipitation changes due to climate change. In this study we aim to assess the impact of recent climate change on drought features of Mediterranean ecosystems in Southern France. Regional climatic trends for the 1971–2006 period are compared to drought trends based on a water balance model accounting for soil properties, vegetation structure and functioning. Drought, defined here as periods when soil water potentials drop below ??0.5 MPa, is described in terms of intensity, duration and timing, which are integrative of both climate variability and site conditions. Temporal trends in precipitation, temperature and solar radiation lead altogether to drier and warmer conditions over the region but with a high spatial heterogeneity; for similar climatic trends, a significant increase in drought intensity was detected in the wettest areas of the region, whereas drought intensity in the driest areas did not change. Indeed, in the wettest areas, we observed an earlier onset of drought by about 1 month, but a constant end of drought. In the driest areas of the region, we observed the same earlier onset of drought but combined with an earlier end of drought, thus leading to a shift of the dry season without increasing its duration. The definition of drought features both in terms of intensity but also of seasonal timing appears relevant to capture historical or forecasted changes in ecosystem functioning. Studies concerning climate change impacts on forested ecosystems should be interpreted with caution when using climate proxies alone.     

20世纪60~90年代辽东地区气候年代际变化特征分析

利用1961～2000年辽东地区13个观测台站的逐月降水和气温资料,分析了辽东地区降水和气温的年代际变化。并利用高桥公式计算出辽东地区的蒸发量,得出了该地区蒸发量和降水蒸发差的年代际变化特征;同时利用干燥度指标研究了辽东地区气候干旱对气候变暖的响应。结果表明:近40 a来辽东地区气候变化呈暖干变化趋势,即气温升高、降水减少,尤其以20世纪90年代变化最为明显。     

Sensitivity analysis of climate on streamflow in north China

The economics and crowded cities of north China play important roles in China’s overall economic development. Streamflow is a hot issue in ecohydrological studies, and research into changes in streamflow in north China is of great significance. In this study, the sensitivities of streamflow to the aridity index, precipitation, and potential evapotranspiration are evaluated to assess the impact of climatic variation in streamflow in north China. The results show that the average coefficient of sensitivity of streamflow to aridity index is ?2.24, and streamflow would decrease by 22.4 % with a 10 % increase in the aridity index. The average coefficients of sensitivity of streamflow to precipitation and potential evapotranspiration are 3.21 and ?2.21, respectively. A 10 % increase in precipitation or potential evapotranspiration would induce a 32.1 % increase or a 22.1 % decrease of streamflow, respectively. Basins with low streamflows would be more sensitive to climatic variation than basins with high streamflows.