Properties of Solar Radiation over Chinese Arid and Semi-Arid Areas
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Climatic characteristics of broadband solarradiation (Rs) in Chinese arid and semi-arid areas are reported in this study. The annual average daily Rs in thearid and semi-arid areas is 16.3 ± 5.77 and 15.3 ± 5.16 MJm-2 d-1, respectively. The highest value (17.2 ± 5.84 MJm-2 d-1) appears in an arid area at Linze. The lowest valueappears in the semi-arid area of Ansai. Pronounced seasonal variation of Rs is observed with the highest value insummer and the lowest in winter. The clearness indexshowed similar seasonal pattern at most sites, with theminimum observed in the summer and the highest valuesin winter at both arid and semi-arid areas. The seasonalvariation of the ratio of Rs to its extraterrestrial value Kt inthe arid area is more significant than that observed in thesemi-arid region, and it is caused by the different range ofvariation of water vapor between arid and semi-arid areasThe seasonal fluctuations in Rs and Kt are mainly controlled by the water vapor content in these areas. Theaerosol particles have significant influence on Rs and Kt atstations with higher aerosol burden.Keywords:
Seasonality
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Abstract Water resources are essential for survival of both the ecosystem and human society in arid regions. The impact of precipitation on vegetation cover, especially in arid and semiarid areas, has received increasing interest. Although correlation between precipitation and vegetation cover has been reported in arid and semi-arid areas (with annual precipitation from 50 to 200 mm and from 200 to 400 mm, respectively), it is unclear whether the same correlation holds in extremely arid regions (with annual precipitation less than 50 mm). This study used remote sensing data of precipitation and normalized difference vegetation index (NDVI) to investigate their correlation at different temporal and spatial scales in a typical arid mountain-oasis river system located in northwest China. The results showed that precipitation and NDVI are not evenly distributed in space. In the period 1999–2007, precipitation was declining at a rate of 1.1785 mm/year, while during 2008–2015, it has increased at a rate of 2.0516 mm/year. NDVI showed no significant temporal trend in most areas (trend slope = 0.001, Significance > 0.05), except for a slight increase in regions where cropland expanded. The Pearson correlation coefficient between the time series of spatial mean precipitation and NDVI over the whole study area was 0.46 (Significance
Vegetation Cover
Aridity index
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Forcing (mathematics)
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This work focused on studying the relationships between Normalized Difference Vegetation Index (NDVI) and daytime and nighttime Land Surface Temperature (LST) in winter, spring, summer and fall and investigating the effects of land cover on these variables in Jordan, which represents a typical semi-arid to arid environment. Using MODIS-based data for the year 2017, multiple procedures were applied: one-way analysis of variance followed by comparison between means, Pearson correlation coefficient, global Moran’s index, simple linear regression, second-order polynomial regression, recursive-partitioning regression and geographically weighted regression. The results showed that land cover explained fair amount of the variability in NDVI but small amount of the variability in daytime and nighttime LST. In addition, an inverted surface urban heat island pattern was observed in daytime. Finally, applying different regression procedures produced different perspectives about the complex and variable relationships between daytime and nighttime LST and NDVI in different seasons.
Land Cover
Urban Heat Island
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According to the distribution of arid and humid regions in China,the typical arid region (Erjina),the typical semi-arid/semi-humid region (Guanzhong basin/Loess Plateau) and the typical humid region (Poyang Lake basin) were selected as the study areas.Based on NDVI data from 1982 to 2000 and meteorological observing data of three study areas from 1981 to 2000,the interactions between vegetation NDVI and climatic factors (temperature and precipitation) in typical arid and humid regions were discussed in this study.The results showed that in the responses of vegetation to climatic factors,vegetation in the typical arid region (Erjina) was more sensitive to precipitation,while vegetation in the typical semi-arid/semi-humid region (Guanzhong basin/Loess Plateau) was more sensitive to both temperature and precipitation,and vegetation in the typical humid region (Poyang Lake basin) was more sensitive to temperature.As for effects of vegetation on climatic factors,there was a remarkable negative correlation between vegetation NDVI in the past winter and temperature in the present summer,and also a significant positive correlation between vegetation NDVI in the past winter and precipitation in the present summer.However,in the typical semi-arid/semi-humid region (Guanzhong basin/Loess Plateau),there was a significant positive correlation between vegetation NDVI in the present spring and temperature in the present summer.
Loess plateau
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Using data from the intensive observation period (May-June 2000) of the NWC-ALIEX (The Field Experiment on Interaction between Land and Atmosphere in Arid Region of North-west China), the characteristics and mechanisms of some key land-surface process over Gobi in a typical arid region of north-west China are analyzed and several parameters of land-surface processes are calculated. The weighted mean of the surface albedo over a typical arid region of the Dunhuang Gobi is calculated using the relative reflection as a weighting factor, and its value is 0.255 ± 0.021. After removing the influence of precipitation, the mean soil heat capacity over a typical arid region of the Dunhuang Gobi is 1.12 × 10 6 Jm −3 K −1 , which is smaller than that observed in the Heihe (China) River basin Field Experiment (HEIFE). The mean soil heat diffusivity and conductivity are about half of those observed in HEIFE.
Albedo (alchemy)
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Aridity index
Trend analysis
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Vegetation greenness dynamics in arid and semi-arid regions are sensitive to climate change, which is an important phenomenon in global climate change research. However, the driving mechanism, particularly for the longitudinal and latitudinal changes in vegetation greenness related to climate change, has been less studied and remains poorly understood in arid and semi-arid areas. In this study, we investigated changes in vegetation greenness and the vegetation greenness line (the mean growing season normalized difference vegetation index (NDVI) = 0.1 contour line) and its response to climate change based on AVHRR-GIMMS NDVI3g and the fifth and latest global climate reanalysis dataset from 1982 to 2015 in the arid and semi-arid transition zone of the Mongolian Plateau (ASTZMP). The results showed that the mean growing season NDVI increased from the central west to east, northeast, and southeast in ASTZMP. The vegetation greenness line migrated to the desert during 1982–1994, to the grassland during 1994–2005, and then to the desert during 2005–2015. Vegetation greenness was positively correlated with precipitation and negatively correlated with temperature. The latitudinal variation of the vegetation greenness line was mainly affected by the combination of precipitation and temperature, while the longitudinal variation was mainly affected by precipitation. In summary, precipitation was a key climatic factor driving rapid changes in vegetation greenness during the growing season of the transition zone. These results can provide meaningful information for research on vegetation coverage changes in arid and semi-arid regions.
Growing season
Desert climate
Aridity index
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The semi-arid and arid drylands of China, which are located in the inland region of Eurasia, have experienced rapid climate change. Some regions in particular, have shown upward trends in the observational records of precipitation. However, there is more to drying and wetting than just changes in precipitation which still have large uncertainties. Coherent results, however, can be obtained, at the regional scale, with the use of multiple indices as shown in the recent literature. We divided the drylands of China into three sub-regions, i.e., a semi-arid (SA), an eastern-arid (EA) and a western-arid (WA) region. Precipitation from the China Meteorological Administration (CMA) and Climatic Research Unit (CRU), statistical and physical drought indices, including the Standardized Precipitation Evapotranspiration Index (SPEI), the Palmer Drought Severity Index (PDSI), self-calibrating PDSI (sc_PDSI), Root zone soil moisture (Root_sm) and Surface soil moisture (Surf_sm) from Global Land Evaporation Amsterdam Model (GLEAM), and Normalized Difference Vegetation Index (NDVI) were used to identify temporal and spatial patterns in drying and wetting. Data were selected from 1982–2012, in line with the availability of the remotely sensed vegetation data. Results show that the drylands of China exhibits a pattern of wetting in the west and drying in the east. The semi-arid region in the east is becoming drier and the drought area is increasing, with the values of CMA_P, CRU_P, PDSI, sc_PDSI, SPEI-01,SPEI-06, SPEI-12, Root_sm, Surf_sm at −1.064 mm yr−1, −0.834 mm yr−1, −0.050 yr−1 (p < 0.1), −0.174 yr−1 (p < 0.1), −0.014 yr−1, −0.06, −0.021 (p < 0.1), −0.257×10−3 m3 m−3 yr−1, −0.024×10−3 m3 m−3 yr−1, respectively. The arid region generally exhibits a wetting trend, while the area in drought declines only in the western arid region, but not in the eastern arid part. In the semi-arid region, growing season (May to September) NDVI is significantly correlated (p < 0.1) with eight out of nine indicators. We show in this study that the semi-arid region needs more study to protect the vegetation ecosystem and the water resources.
Cru
Aridity index
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Aridity index
Penman–Monteith equation
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Aridity is one of the main factors which distinguish the climate of a region and has significant influence on human activities. This study investigated the spatial distribution of the aridity indices to determine the climate conditions in Iraq over the period (1981-2015), depending on the data of the air temperature and rainfall which obtained from 28 stations distributed through Iraq. The used aridity indices are: Lang, Erinc, Emberger, UNEP, De Martonne and Thornthwaite. The spatial distribution was determined using inverse distance weighting (IDW) interpolated method. The results of aridity indices analysis shows that the hyper-arid, arid, and semi-arid categories are predominant with almost (91%) to (100%) of the country’s area. Dry sub-humid, moist sub-humid and humid categories occupies less than (10%) with most of indices at stations of (Arbil, Sulaymaniyah, and Salahaddin). To evaluate the seasonal spatial distributions, De Martonne was utilized. During winter, the climate types ranged from semi-arid to very-humid, while at spring season from arid to humid. Autumn season dominated by arid at (97%) of study area. The summer season was the driest compared with the other seasons. The change point for aridity indices was detected by using the cumulative sum charts (CUSUMs), it is found for the most stations in (1997). Consequently, the spatial distribution for the aridity indices were analyzed through two periods (1981-1997 and 1998-2015), this analysis showed that the arid and hyper-arid areas were increased in the second period compared with the first period with obvious extension toward the north of Iraq.
Aridity index
Inverse distance weighting
Desert climate
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