Spatio-temporal variation of hydrological drought under climate change during the period 1960–2013 in the Hexi Corridor,China

In recent years, climate change has been aggravated in many regions of the world. The Hexi Corridor is located in the semiarid climate zone of Northwest China, which is particularly affected by climate change. Climate change has led to the spatial and temporal variations of temperature and precipitation, which may result in hydrological drought and water shortage. Thus, it is necessary to explore and assess the drought characteristics of river systems in this area. The patterns of hydrological drought in the Hexi Corridor were identified using the streamflow drought index(SDI) and standardized precipitation index at 12-month timescale(SPI12) from 1960 to 2013. The evolution of drought was obtained by the Mann–Kendall test and wavelet transform method. The results showed that both the mean annual SDI and SPI12 series in the Hexi Corridor exhibited an increasing trend during the study period. According to the results of wavelet analysis, we divided the study period into two segments, i.e. before and after 1990. Before 1990, the occurrence of droughts showing decreased SDI and SPI12 was concentrated in the northern part of the corridor and shifted to the eastern part of the corridor after 1990. The probability of drought after 1990 in Shule River basin decreased while increased in Shiyang River basin. The wavelet analysis results showed that Shiyang River basin will be the first area to go through the next drought period. Additionally, the relationships between drought pattern and climate indices were analyzed. The enhanced westerly winds and increased precipitation and glacier runoff were the main reasons of wet trend in the Hexi Corridor. However, the uneven spatial variations of precipitation, temperature and glacier runoff led to the difference of hydrological drought variations between the Shule, Heihe and Shiyang River basins.     

Meteorological drought features in northern and northwestern parts of Mexico under different climate change scenarios

Meteorological drought has been an inevitable natural disaster throughout Mexican history and the northern and northwestern parts of Mexico(i.e., the studied area), where the mean annual precipitation(MAP) is less than 500 mm, have suffered even more from droughts in the past. The aim of this study was to conduct a meteorological drought analysis of the available MAP data(1950–2013) from 649 meteorological stations selected from the studied area and to predict the drought features under the different IPCC-prescribed climate change scenarios. To determine the long-term drought features, we collected 1×10~4 synthetic samples using the periodic autoregressive moving average(PARMA) model for each rainfall series. The simulations first consider the present prevailing precipitation conditions(i.e., the average from 1950 to 2013) and then the precipitation anomalies under IPCC-prescribed RCP 4.5 scenario and RCP 8.5 scenario. The results indicated that the climate changes under the prescribed scenarios would significantly increase the duration and intensity of droughts. The most severe impacts may occur in the central plateau and in the Baja California Peninsula. Thus, it will be necessary to establish adequate protective measures for the sustainable management of water resources in these regions.     

Response of hydrological drought to meteorological drought in the eastern Mediterranean Basin of Turkey

The hydrographic eastern Mediterranean Basin of Turkey is a drought sensitive area. The basin is an important agricultural area and it is necessary to determine the extent of extreme regional climatic changes as they occur in this basin. Pearson's correlation coefficient was used to show the correlation between standardized precipitation index(SPI) and standardized streamflow index(SSI) values on different time scales. Data from five meteorological stations and seven stream gauging stations in four sub-basins of the eastern Mediterranean Basin were analyzed over the period from 1967 to 2017. The correlation between SSI and SPI indicated that in response to meteorological drought, hydrological drought experiences a one-year delay then occurs in the following year. This is more evident at all stations from the mid-1990 s. The main factor causing hydrological drought is prolonged low precipitation or the presence of a particularly dry year. Results showed that over a long period(12 months), hydrological drought is longer and more severe in the upper part than the lower part of the sub-basins. According to SPI-12 values, an uninterrupted drought period is observed from 2002–2003 to 2008–2009. Results indicated that among the drought events, moderate drought is the most common on all timescales in all sub-basins during the past 51 years. Long-term dry periods with moderate and severe droughts are observed for up to 10 years or more since the late 1990 s, especially in the upper part of the sub-basins. As precipitation increases in late autumn and early winter, the stream flow also increases and thus the highest and most positive correlation values(0.26–0.54) are found in January. Correlation values(ranging between –0.11 and –0.01) are weaker and negative in summer and autumn due to low rainfall. This is more evident at all stations in September. The relation between hydrological and meteorological droughts is more evident, with the correlation values above 0.50 on longer timescales(12-and 24-months). The results presented in this study allow an understanding of the characteristics of drought events and are instructive for overcoming drought. This will facilitate the development of strategies for the appropriate management of water resources in the eastern Mediterranean Basin, which has a high agricultural potential.     

Spatial and temporal change patterns of net primary productivity and its response to climate change in the Qinghai–Tibet Plateau of China from 2000 to 2015

The vegetation ecosystem of the Qinghai–Tibet Plateau in China,considered to be the′′natural laboratory′′of climate change in the world,has undergone profound changes under the stress of global change.Herein,we analyzed and discussed the spatial-temporal change patterns and the driving mechanisms of net primary productivity(NPP)in the Qinghai–Tibet Plateau from 2000 to 2015 based on the gravity center and correlation coefficient models.Subsequently,we quantitatively distinguished the relative effects of climate change(such as precipitation,temperature and evapotranspiration)and human activities(such as grazing and ecological construction)on the NPP changes using scenario analysis and Miami model based on the MOD17A3 and meteorological data.The average annual NPP in the Qinghai–Tibet Plateau showed a decreasing trend from the southeast to the northwest during 2000–2015.With respect to the inter-annual changes,the average annual NPP exhibited a fluctuating upward trend from 2000 to 2015,with a steep increase observed in 2005 and a high fluctuation observed from 2005 to 2015.In the Qinghai–Tibet Plateau,the regions with the increase in NPP(change rate higher than 10%)were mainly concentrated in the Three-River Source Region,the northern Hengduan Mountains,the middle and lower reaches of the Yarlung Zangbo River,and the eastern parts of the North Tibet Plateau,whereas the regions with the decrease in NPP(change rate lower than–10%)were mainly concentrated in the upper reaches of the Yarlung Zangbo River and the Ali Plateau.The gravity center of NPP in the Qinghai–Tibet Plateau has moved southwestward during 2000–2015,indicating that the increment and growth rate of NPP in the southwestern part is greater than those of NPP in the northeastern part.Further,a significant correlation was observed between NPP and climate factors in the Qinghai–Tibet Plateau.The regions exhibiting a significant correlation between NPP and precipitation were mainly located in the central and eastern Qinghai–Tibet Plateau,and the regions exhibiting a significant correlation between NPP and temperature were mainly located in the southern and eastern Qinghai–Tibet Plateau.Furthermore,the relative effects of climate change and human activities on the NPP changes in the Qinghai–Tibet Plateau exhibited significant spatial differences in three types of zones,i.e.,the climate change-dominant zone,the human activity-dominant zone,and the climate change and human activity interaction zone.These research results can provide theoretical and methodological supports to reveal the driving mechanisms of the regional ecosystems to the global change in the Qinghai–Tibet Plateau.     

Response of vegetation to temperature and precipitation in Xinjiang during the period of 1998-2009

In this paper,10-day spatio-temporal response of vegetation to the change of temperature and precipitation in spring,summer,autumn and whole year during the period of 1998―2009 was analyzed based on the data of SPOT VEGETATION-NDVI and 10-day average temperature or precipitation from 54 meteorological stations in Xinjiang.The results show that the response of 10-day NDVI to temperature was more significant than that to precipitation,and the maximal response of vegetation to temperature and precipitation lagged for two 10-day periods.Seasonally,the effect of temperature and precipitation on vegetation NDVI was most marked in autumn,then in spring,and it was not significant in summer.The response of vegetation to 10-day change of meteorological factors was positive with a long affecting duration in spring,and it had a relatively short affecting duration in autumn and summer.Spatially,the 10-day maximal response of NDVI to temperature in northern Xinjiang was higher than that in southern Xinjiang.The correlation between the 10-day NDVI in whole year and the temperature in the 0-8th 10-day period was significantly higher than that between the annual NDVI and the annual temperature at all meteorological stations;the interannual change of NDVI was accordant well with the change of annual precipitation.However,the effect of precipitation within a year on NDVI was not strong.The results indicated that interannual change of temperature was not the dominant factor affecting the change of vegetation NDVI in Xinjiang,but the decrease of annual precipitation was the main factor resulting in the fluctuation of vegetation coverage.Ten-day average temperature was an important factor to promote vegetation growth in Xinjiang within a year,but the effect of precipitation on vegetation growth within a year was not strong.     

Responses of gross primary productivity to different sizes of precipitation events in a temperate grassland ecosystem in Inner Mongolia,China

Changes in the sizes of precipitation events in the context of global climate change may have profound impacts on ecosystem productivity in arid and semiarid grasslands. However, we still have little knowledge about to what extent grassland productivity will respond to an individual precipitation event. In this study, we quantified the duration, the maximum, and the time-integrated amount of the response of daily gross primary productivity(GPP) to an individual precipitation event and their variations with different sizes of precipitation events in a typical temperate steppe in Inner Mongolia, China. Results showed that the duration of GPP-response(τR) and the maximum absolute GPP-response(GPPmax) increased linearly with the sizes of precipitation events(Pes), driving a corresponding increase in time-integrated amount of the GPP-response(GPPtotal) because variations of GPPtotal were largely explained by τR and GPPmax. The relative contributions of these two parameters to GPPtotal were strongly Pes-dependent. The GPPmax contributed more to the variations of GPPtotal when Pes was relatively small(20 mm), whereas τR was the main driver to the variations of GPPtotal when Pes was relatively large. In addition, a threshold size of at least 5 mm of precipitation was required to induce a GPP-response for the temperate steppe in this study. Our work has important implications for the modeling community to obtain an advanced understanding of productivity-response of grassland ecosystems to altered precipitation regimes.     

Response of soil N_2O emissions to precipitation pulses under different nitrogen availabilities in a semiarid temperate steppe of Inner Mongolia,China

Short-term nitrous oxide（N2O） pulse emissions caused by precipitation account for a considerable portion of the annual N2O emissions and are greatly influenced by soil nitrogen（N） dynamics. However, in Chinese semiarid temperate steppes, the response of N2O emissions to the coupling changes of precipitation and soil N availability is not yet fully understood. In this study, we conducted two 7-day field experiments in a semiarid temperate typical steppe of Inner Mongolia, China, to investigate the N2O emission pulses resulting from artificial precipitation events（approximately equivalent to 10.0 mm rainfall） under four N addition levels（0, 5, 10 and 20 g N/（m2·a）） using the static opaque chamber technique. The results show that the simulated rainfall during the dry period in 2010 caused greater short-term emission bursts than that during the relatively rainy observation period in 2011（P〈0.05）. No significant increase was observed for either the N2O peak effluxes or the weekly cumulative emissions（P〉0.05） with single water addition. The peak values of N2O efflux increased with the increasing N input. Only the treatments with water and medium（WN10） or high N addition（WN20） significantly increased the cumulative N2O emissions（P〈0.01） in both experimental periods. Under drought condition, the variations in soil N2O effluxes were positively correlated with the soil NH4-N concentrations in the three N input treatments（WN5, WN10, and WN20）. Besides, the soil moisture and temperature also greatly influenced the N2O pulse emissions, particularly the N2O pulse under the relatively rainy soil condition or in the treatments without N addition（ZN and ZWN）. The responses of the plant metabolism to the varying precipitation distribution and the length of drought period prior to rainfall could greatly affect the soil N dynamics and N2O emission pulses in semiarid grasslands.     

Long-term oscillation of drought conditions in the western China:an analysis of PDSI on a decadal scale

Water resource availability is one of the primary limiting factors with regard to ecosystems in the western China.Having a clear understanding of multi-scale drought patterns in this region is a key step for adaption and mitigation to climate change.The Palmer drought severity index(PDSI) is a widely applied index to assess drought conditions.In this study,long-term monthly self-calibrated PDSI data from 1951 to 2012 were examined for drought spatiotemporal variations in the western China.The results clearly indicated that apparent spatial heterogeneities were evidenced between two sub-regions(arid land with annual precipitation less than 200 mm and semiarid land with annual precipitation between 200 to 500 mm) as well as in the entire region of the western China.Ensemble empirical mode decomposition(EEMD) analyses on monthly PDSI and other atmospheric variable time-series obtained from the Department of Civil and Environmental Engineering,Princeton University revealed that all monthly time-series of variables could be completely decomposed into eight intrinsic mode functions(IMFs) and a trend(residual).This indicates that the monthly PDSI and atmospheric variables of the semiarid area in the western China contain eight quasi-period oscillations on various timescale spanning,seasonal to decadal cycles and a trend of a larger timescale from 1951–2012.The multi-scale drought patterns identified in this research could be powerful supports for decision-making regarding coping with droughts in this region.     

How precipitation and grazing influence the ecological functions of drought-prone grasslands on the northern slopes of the Tianshan Mountains,China?

Drought-prone grasslands provide a critical resource for the millions of people who are dependent on livestock for food security.However,this ecosystem is potentially vulnerable to climate change(e.g.,precipitation)and human activity(e.g.,grazing).Despite this,the influences of precipitation and grazing on ecological functions of drought-prone grasslands in the Tianshan Mountains remain relatively unexplored.Therefore,we conducted a systematic field investigation and a clipping experiment(simulating different intensities of grazing)in a drought-prone grassland on the northern slopes of the Tianshan Mountains in China to examine the influences of precipitation and grazing on aboveground biomass(AGB),soil volumetric water content(SVWC),and precipitation use efficiency(PUE)during the period of 2014–2017.We obtained the meteorological and SVWC data using an HL20 Bowen ratio system and a PR2 soil profile hydrometer,respectively.We found that AGB was clearly affected by both the amount and seasonal pattern of precipitation,and that PUE may be relatively low in years with either low or excessive precipitation.The PUE values were generally higher in the rapid growing season(April–July)than in the entire growing season(April–October).Overall,moderate grazing can promote plant growth under water stress conditions.The SVWC value was higher in the clipped plots than in the unclipped plots in the rapid growing season(April–July),but it was lower in the clipped plots than in the unclipped plots in the slow growing season(August–October).Our findings can enhance the understanding of the ecological effects of precipitation and grazing in drought-prone grasslands and provide data that will support the effective local grassland management.     

Estimation of meteorological drought indices based on AgMERRA precipitation data and station-observed precipitation data

Meteorological drought is a natural hazard that can occur under all climatic regimes. Monitoring the drought is a vital and important part of predicting and analyzing drought impacts. Because no single index can represent all facets of meteorological drought, we took a multi-index approach for drought monitoring in this study. We assessed the ability of eight precipitation-based drought indices(SPI(Standardized Precipitation Index), PNI(Percent of Normal Index), DI(Deciles index), EDI(Effective drought index), CZI(China-Z index), MCZI(Modified CZI), RAI(Rainfall Anomaly Index), and ZSI(Z-score Index)) calculated from the station-observed precipitation data and the Ag MERRA gridded precipitation data to assess historical drought events during the period 1987–2010 for the Kashafrood Basin of Iran. We also presented the Degree of Dryness Index(DDI) for comparing the intensities of different drought categories in each year of the study period(1987–2010). In general, the correlations among drought indices calculated from the Ag MERRA precipitation data were higher than those derived from the station-observed precipitation data. All indices indicated the most severe droughts for the study period occurred in 2001 and 2008. Regardless of data input source, SPI, PNI, and DI were highly inter-correlated(R~2=0.99). Furthermore, the higher correlations(R~2=0.99) were also found between CZI and MCZI, and between ZSI and RAI. All indices were able to track drought intensity, but EDI and RAI showed higher DDI values compared with the other indices. Based on the strong correlation among drought indices derived from the Ag MERRA precipitation data and from the station-observed precipitation data, we suggest that the Ag MERRA precipitation data can be accepted to fill the gaps existed in the station-observed precipitation data in future studies in Iran. In addition, if tested by station-observed precipitation data, the Ag MERRA precipitation data may be used for the data-lacking areas.     

Rainfall patterns of Algerian steppes and the impacts on natural vegetation in the 20~(th) century

Since 1960, the steppe regions of North Africa have been subject to an increasing desertification, including the degradation of traditional pastures. The initially dominant species(Artemisia herba-alba, Lygeum spartum and Stipa tenacissima) declined and were progressively replaced by other species(Atractylis serratuloides and Salsola vermiculata) that are more tolerant to the new conditions. It is not clear whether these changes are due to anthropogenic reasons or climatic determinism. We have carried out a statistical analysis of the climate to detect putative rainfall changes during the 20~(th) century in the Algerian steppes based on data from 9 meteorological stations, including 2 Saharan stations(El Oued and Touggourt), 3 pre-Saharan stations(Biskra, Laghouat and Ain Sefra) and 4 steppe stations(Djelfa, Saida, Méchéria and El-Bayadh) located in the arid high plains, which represent the bioclimate diversities of the region. Previous studies suggested that significant rainfall changes for the 20~(th) century only had records in the south of the Oran region. Most of the studies, however, looked at restricted territories over limited periods, and did not integrate the rainiest period 2004–2014. Our work is designed to integrate all the longest time series of meteorological data available for the steppe regions of Algeria. Our results confirm the spatial rainfall distribution(significant rainfall changes only recorded in the southwestern region) evidenced by previous studies, and reveal a decreasing rainfall gradient from northeastern to southwestern Algeria. Moreover, the results reveal a trend of significant decrease of rainfall in the southern Oran region, marked by two drought periods in 1980–1985 and 1999–2003. However, with the exception of the southwestern region, rainfall overall has not declined since the beginning of the 20~(th) century. While less marked in other regions, the drought appear to have affected all territories of the Algerian steppe. Consequently, our study implies that the climate was not a leading influence in the on-going degradation of the vegetation cover of steppe landscapes. Such a vegetation evolution thus appears to be have been determined more by human activities than by climate forcing.     

Drought fluctuations based on dendrochronology since 1786 for the Lenglongling Mountains at the northwestern fringe of the East Asian summer monsoon region

The Lenglongling Mountains(LLM)located in northeastern part of the Tibet Plateau,belong to a marginal area of the East Asian summer monsoon(EASM)and are sensitive to monsoon dynamics.Two tree-ring width chronologies developed from six sites of Picea crassifolia in the LLM were employed to study the regional drought variability.Correlation and temporal correlation analyses showed that relationships between the two chronologies and self-calibrated Palmer Drought Severity Index(sc_PDSI)were significant and stable across time,demonstrating the strength of sc_PDSI in modeling drought conditions in this region.Based on the relationships,the mean sc_PDSI was reconstructed for the period from 1786 to 2013.Dry conditions prevailed during 1817–1819,1829–1831,1928–1931 and 1999–2001.Relatively wet periods were identified for 1792–1795 and 1954–1956.Spatial correlations with other fourteen precipitation/drought reconstructed series in previous studies revealed that in arid regions of Northwest China,long-term variability of moisture conditions was synchronous before the 1950 s at a decadal scale(1791–1954).In northwestern margin of the EASM,most of all selected reconstructions had better consistency in low-frequency variation,especially during dry periods,indicating similar regional moisture variations and analogous modes of climate forcing on tree growth in the region.     

Drought analysis in Antakya-Kahramanmara? Graben,Turkey

The most parts of the Earth experience precipitation variability as a part of their normal climates over both short- and long-time periods. These variations of precipitation will have unpredictable and perhaps unexpectedly extreme consequences(such as drought and flood) with respect to frequency and intensity for many regions of the Earth. Because of high precipitation fluctuations, the Mediterranean region is also the areas of the world sensitive to precipitation changes which often involve frequent drought conditions in Turkey. In this study, drought conditions at annual, seasonal and monthly time scales over the period of 1975–2010 were examined for Antakya-Kahramanmara? Graben which is located in the eastern part of the Mediterranean region of Turkey. Application of appropriate measures to analyze and monitor droughts is recognized as a major challenge to scientists involved in atmospheric studies. Standardized Precipitation Index(SPI) and cumulative deviation curve techniques were used to determine drought conditions. Results indicated that the study area presented a cyclic pattern of variations with alternating drier and wetter years. From analyses of annual, seasonal and monthly drought series it can be seen that precipitation characteristic of the area is changing. By the results, apparent wet and dry periods can be distinguished. This study also indicated that precipitation totals of winter, spring and summer seasons were slightly decreased during the study period. Drought frequency was increased especially for the northern part of the area in the last ten years. Drought periods were divided into 1982–1985, 1999–2002 and 2004–2008, respectively. According to our analyses, the time scale of 1999–2002 was the driest period in the most of the graben area. The study area, which covers agriculturally important fertile alluvial plains, will experience increasing pressure on its water resources because of its growing population and industry, ever-larger demands for intensive agricultural activities, and frequent drought events.     

Impact of climate change on the streamflow in the glacierized Chu River Basin,Central Asia

Catchments dominated by meltwater runoff are sensitive to climate change as changes in precipitation and temperature inevitably affect the characteristics of glaciermelt/snowmelt, hydrologic circle and water resources. This study simulated the impact of climate change on the runoff generation and streamflow of Chu River Basin(CRB), a glacierized basin in Central Asia using the enhanced Soil and Water Assessment Tool(SWAT). The model was calibrated and validated using the measured monthly streamflow data from three discharge gauge stations in CRB for the period 1961–1985 and was subsequently driven by downscaled future climate projections of five Global Circulation Models(GCMs) in Coupled Model Inter-comparison Project Phase 5(CMIP5) under three radiative forcing scenarios(RCP2.6, RCP4.5 and RCP8.5). In this study, the period 1966–1995 was used as the baseline period, while 2016–2045 and 2066–2095 as the near-future and far-future period, respectively. As projected, the climate would become warmer and drier under all scenarios in the future, and the future climate would be characterized by larger seasonal and annual variations under higher RCP. A general decreasing trend was identified in the average annual runoff in glacier(–26.6% to –1.0%), snow(–21.4% to +1.1%) and streamflow(–27.7% to –6.6%) for most of the future scenario periods. The projected maximum streamflow in each of the two future scenarios occurred one month earlier than that in the baseline period because of the reduced streamflow in summer months. Results of this study are expected to arouse the serious concern about water resource availability in the headwater region of CRB under the continuously warming climate. Changes in simulated hydrologic outputs underscored the significance of lowering the uncertainties in temperature and precipitation projection.     

Spatial distribution of δ~2H and δ~18O values in the hydrologic cycle of the Nile Basin

Existing δ2H and δ18O values for precipitation and surface water in the Nile Basin were used to analyze precipitation inputs and the influence of evaporation on the isotopic signal of the Nile River and its tributaries. The goal of the data analysis was to better understand basin processes that influence seasonal streamflow for the source waters of the Nile River, because climate and hydrologic models have continued to produce high uncertainty in the prediction of precipitation and streamflow in the Nile Basin. An evaluation of differences in precipitation δ2H and δ18O values through linear regression and distribution analysis indicate variation by region and season in the isotopic signal of precipitation across the Nile Basin. The White Nile Basin receives precipitation with a more depleted isotopic signal compared to the Blue Nile Basin. The hot temperatures of the Sahelian spring produce a greater evaporation signal in the precipitation isotope distribution compared to precipitation in the Sahara/Mediterranean region, which can be influenced by storms moving in from the Mediterranean Sea. The larger evaporative effect is reversed for the two regions in summer because of the cooling of the Sahel from inflow of Indian Ocean monsoon moisture that predominantly influences the climate of the Blue Nile Basin. The regional precipitation isotopic signals convey to each region's streamflow, which is further modified by additional evaporation according to the local climate. Isotope ratios for White Nile streamflow are significantly altered by evaporation in the Sudd, but this isotopic signal is minimized for streamflow in the Nile River during the winter, spring and summer seasons because of the flow dominance of the Blue Nile. During fall, the contribution from the White Nile may exceed that of the Blue Nile, and the heavier isotopic signal of the White Nile becomes apparent. The variation in climatic conditions of the Nile River Basin provides a means of identifying mechanistic processes through changes in isotope ratios of hydrogen and oxygen, which have utility for separating precipitation origin and the effect of evaporation during seasonal periods. The existing isotope record for precipitation and streamflow in the Nile Basin can be used to evaluate predicted streamflow in the Nile River from a changing climate that is expected to induce further changes in precipitation patterns across the Nile Basin.     

Effects of climate change and land use/cover change on the volume of the Qinghai Lake in China

Qinghai Lake is the largest saline lake in China.The change in the lake volume is an indicator of the variation in water resources and their response to climate change on the Qinghai-Tibetan Plateau(QTP)in China.The present study quantitatively evaluated the effects of climate change and land use/cover change(LUCC)on the lake volume of the Qinghai Lake in China from 1958 to 2018,which is crucial for water resources management in the Qinghai Lake Basin.To explore the effects of climate change and LUCC on the Qinghai Lake volume,we analyzed the lake level observation data and multi-period land use/land cover(LULC)data by using an improved lake volume estimation method and Integrated Valuation of Ecosystem Services and Trade-offs(InVEST)model.Our results showed that the lake level decreased at the rate of 0.08 m/a from 1958 to 2004 and increased at the rate of 0.16 m/a from 2004 to 2018.The lake volume decreased by 105.40×10⁸ m³ from 1958 to 2004,with the rate of 2.24×10⁸ m³/a,whereas it increased by 74.02×10⁸ m³ from 2004 to 2018,with the rate of 4.66×10⁸ m³/a.Further,the climate of the Qinghai Lake Basin changed from warm-dry to warm-humid.From 1958 to 2018,the increase in precipitation and the decrease in evaporation controlled the change of the lake volume,which were the main climatic factors affecting the lake volume change.From 1977 to 2018,the measured water yield showed an"increase-decrease-increase"fluctuation in the Qinghai Lake Basin.The effects of climate change and LUCC on the measured water yield were obviously different.From 1977 to 2018,the contribution rate of LUCC was -0.76% and that of climate change was 100.76%;the corresponding rates were 8.57% and 91.43% from 1977 to 2004,respectively,and -4.25% and 104.25% from 2004 to 2018,respectively.Quantitative analysis of the effects and contribution rates of climate change and LUCC on the Qinghai Lake volume revealed the scientific significance of climate change and LUCC,as well as their individual and combined effects in the Qinghai Lake Basin and on the QTP.This study can contribute to the water resources management and regional sustainable development of the Qinghai Lake Basin.     

Responses of leaf water potential and gas exchange to the precipitation manipulation in two shrubs on the Chinese Loess Plateau

Regulation of leaf gas exchange plays an important role in the survival of trees and shrubs under future climate change. However, the responses of leaf water potential and gas exchange of shrubs in semi-arid areas to the precipitation alteration are not clear. Here, we conducted a manipulated experiment with three levels of precipitation, i.e., a control with ambient precipitation, 50% above ambient precipitation(irrigation treatment), and 50% below ambient precipitation(drought treatment), with two common shrubs, Salix psammophila C. Wang & C. Y. Yang(isohydric plant, maintaining a constant leaf water potential by stomatal regulation) and Caragana korshinskii Kom.(anisohydric plant, having more variable leaf water potential), on the Chinese Loess Plateau in 2014 and 2015. We measured the seasonal variations of predawn and midday leaf water potential(Ψpd and Ψmd), two parameters of gas exchange, i.e., light-saturated assimilation(An) and stomatal conductance(gs), and other foliar and canopy traits. The isohydric S. psammophila had a similar An and a higher gs than the anisohydric C. korshinskii under drought treatment in 2015, inconsistent with the view that photosynthetic capacity of anisohydric plants is higher than isohydric plants under severe drought. The two shrubs differently responded to precipitation manipulation. Ψpd, An and gs were higher under irrigation treatment than control for S. psammophila, and these three variables and Ψmd were significantly higher under irrigation treatment and lower under drought treatment than control for C. korshinskii. Leaf water potential and gas exchange responded to manipulated precipitation more strongly for C. korshinskii than for S. psammophila. However, precipitation manipulation did not alter the sensitivity of leaf gas exchange to vapor-pressure deficit and soil moisture in these two shrubs. Acclimation to long-term changes in soil moisture in these two shrubs was primarily attributed to the changes in leaf or canopy structure rather than leaf gas exchange. These findings will be useful for modeling canopy water-carbon exchange and elucidating the adaptive strategies of these two shrubs to future changes in precipitation.     

Phenology of desert plants from an arid gravel plain in eastern United Arab Emirates

Phenological events for desert plants were recorded and rainfall and temperature data were gathered over a three-year time scale at a gravel plain in the eastern region of the United Arab Emirates. Variations of phenological periods were analyzed and correlations between phenological periods and climate factors were discussed. The study showed that the growth and flowering of therophytes were significantly correlated with air temperature. The timing and abundance of rainfall came to be another factor significantly correlated with the onset and duration of chamaephyte flowering as well as the duration of therophyte growth and flowering. The variation in rainfall did not affect the onset of flowering in phanerophytes. Peak growing seasons were from November to March and flowering from November to December(also February to March depending on the availability of rainfall). Flowering in phanerophytes and chamaephytes occurs 4–6 weeks and in therophytes 4–8 weeks after rain. The results showed that variations in growth and phenology between species were correlated with environmental factors, such as temperature and rainfall or, maybe, differences in life form and photosynthetic pathways, each being adapted for utilizing a particular phase of the seasonally and yearly variable rainfall. The sequence of flowering for the species under study was more or less constant despite the difference in the amount of rainfall. The fruiting patterns of most of the species were also found parallel to their flowering patterns. Our results emphasized the need to study multiple species at many sites for the understanding and forecast of regional changes in phenology.     

Precipitation trends and variability from 1950 to 2000 in arid lands of Central Asia

Climate warming will cause differences in precipitation distribution and changes in hydrological cycle both at regional and global scales. Arid lands of Central Asia(ALCA), one of the largest arid regions at the middle latitudes in the world, is likely to be strongly influenced by climate warming. Understanding the precipitation variations in the past is an important prerequisite for predicting future precipitation trends and thus managing regional water resources in such an arid region. In this study, we used run theory, displacement, extreme deviation theory, precipitation concentration index(PCI), Mann-Kendall rank correlation and climatic trend coefficient methods to analyze the precipitation in wet and dry years, changes in precipitation over multiple-time scales, variability of precipitation and its rate of change based on the monthly precipitation data during 1950–2000 from 344 meteorological stations in the ALCA. The occurrence probability of a single year with abundant precipitation was higher than that of a single year with less precipitation. The average duration of extreme drought in the entire area was 5 years, with an average annual water deficit of 34.6 mm(accounting for 11.2% of the average annual precipitation over the duration). The occurrence probability of a single wet year was slightly higher than that of a single dry year. The occurrence probability of more than 5 consecutive wet years was 5.8%, while the occurrence probability of more than 5 consecutive dry years was 6.2%. In the center of the study area, the distribution of precipitation was stable at an intra-annual timescale, with small changes at an inter-annual timescale. In the western part of the study area, the monthly variation of precipitation was high at an inter-annual timescale. There were clear seasonal changes in precipitation(PCI=12–36) in the ALCA. Precipitation in spring and winter accounted for 37.7% and 24.4% of the annual precipitation, respectively. There was a significant inter-annual change in precipitation in the arid Northwest China(PCI=24–34). Annual precipitation increased significantly(P=0.05) in 17.4% of all the meteorological stations over the study period. The probability of an increase in annual precipitation was 75.6%, with this increase being significant(P=0.05) at 34.0% of all the meteorological stations. The average increasing rate in annual precipitation was 3.9 mm/10a(P=0.01) in the ALCA. There were significant increasing trends(P=0.01) in precipitation in Kazakhstan, Kyrgyzstan and Tajikistan, with rates of 2.6, 3.1 and 3.7 mm/10 a, respectively.     

Performance of different drought indices for agriculture drought in the North China Plain

The Palmer drought severity index(PDSI), standardized precipitation index(SPI), and standardized precipitation evapotranspiration index(SPEI) are used worldwide for drought assessment and monitoring. However, substantial differences exist in the performance for agricultural drought among these indices and among regions. Here, we performed statistical assessments to compare the strengths of different drought indices for agricultural drought in the North China Plain. Small differences were detected in the comparative performances of SPI and SPEI that were smaller at the long-term scale than those at the short-term scale. The correlation between SPI/SPEI and PDSI considerably increased from 1-to 12-month lags, and a slight decreasing trend was exhibited during 12-and 24-month lags, indicating a 12-month scale in the PDSI, whereas the SPI was strongly correlated with the SPEI at 1-to 24-month lags. Interestingly, the correlation between the trend of temperature and the mean absolute error and its correlation coefficient both suggested stronger relationships between SPI and the SPEI in areas of rapid climate warming. In addition, the yield–drought correlations tended to be higher for the SPI and SPEI than that for the PDSI at the station scale, whereas small differences were detected between the SPI and SPEI in the performance on agricultural systems. However, large differences in the influence of drought conditions on the yields of winter wheat and summer maize were evident among various indices during the crop-growing season. Our findings suggested that multi-indices in drought monitoring are needed in order to acquire robust conclusions.