Changes In Aridity Index Research Articles

Aridity Index Variations and Dust Events in Iran from 1990 to 2018

This study was carried out to determine whether the changes in dust concentration (DC) in Iran were attributed to changes in aridity index (AI) from 1990 to 2018. Long-term precipitation and potential evapotranspiration were used to compute the United Nations Environment Program AI and horizontal visibility data were used to calculate the DC in the study area. The DC spatial distribution maps were prepared to better identify areas affected by sand-dust events. Pearson’s correlation analysis was performed to explore the association between DC and AI in Iran and study stations during the monitoring period. Spatiotemporal variations in DC showed that more than 60 percent of Iran’s area experienced severe wind erosion events in 2001, 2008, and 1990 when AI was less than 0.2. The results also showed that approximately 45 percent of Iran’s area has suffered from the average DC over the study period (565 µg/m3). The incremental changes in DC were observed in almost half of the entire area of Iran during the second period (2000–2008) and one third of this area during the third period (2009–2018). Areas affected by decreasing changes in AI during the second and third periods were 90 percent and 73 percent of the area affected during the previous period, respectively. Overall, there was a significant negative correlation between AI and DC in Iran over the monitoring period (r = −0.51, p value < 0.01). Insignificant positive correlations were observed in humid regions, whereas negative correlations were mostly detected in arid and semiarid regions of Iran.

Future changes in Aridity Index at two and four degrees of global warming above preindustrial levels

AbstractIn this study, we project the global terrestrial changes in Aridity Index (AI), as defined by the ratio of annual precipitation to potential evapotranspiration (PET), at 2 and 4°C levels of global warming above the preindustrial conditions based on the outputs from 21 Coupled Model Intercomparison Project Phase 5 models under the Representative Concentration Pathways 8.5 (RCP8.5) scenario. Compared to the preindustrial period, the global terrestrial annual precipitation will increase by an average of 0.02% (3%), the annual PET will increase by 6% (15%) and AI will decrease by 0.1 (0.2) at the 2°C (4°C) warming level. More remarkable decreases in the annual AI are seen in the northern high latitudes, where will also experience greater increases in PET and precipitation. In general, the seasonal changes in AI display a similar spatial pattern to the annual changes and are characterized by the greatest decrease in December–January–February and the smallest in June–July–August. The largest expansion of drylands occurs in semi‐arid regions, followed by the expansion of arid regions, with area changes of 4% (7%) and 3% (4%) under the 2°C (4°C) scenario, respectively. Further analyses indicate that the AI changes are determined by PET in the northern high latitudes, and the contribution of PET to AI increases from the 2 to 4°C warming levels. Furthermore, changes in thermodynamic factors contribute most to the PET, and at a higher level of global warming, the contribution of thermodynamic factors increases and that of dynamic factors decreases.

Influences of the Internal Mixing of Anthropogenic Aerosols on Global Aridity Change

Influences of the mixing treatments of anthropogenic aerosols on their effective radiative forcing (ERF) and global aridity are evaluated by using the BCC_AGCM2.0_CUACE/Aero, an aerosol–climate online coupled model. Simulations show that the negative ERF due to external mixing (EM, a scheme in which all aerosol particles are treated as independent spheres formed by single substance) aerosols is largely reduced by the partial internal mixing (PIM, a scheme in which some of the aerosol particles are formed by one absorptive and one scattering substance) method. Compared to EM, PIM aerosols have much stronger absorptive ability and generally weaker hygroscopicity, which would lead to changes in radiative forcing, hence to climate. For the global mean values, the ERFs due to anthropogenic aerosols since the pre-industrial are–1.02 and–1.68 W m–2 for PIM and EM schemes, respectively. The variables related to aridity such as global mean temperature, net radiation flux at the surface, and the potential evaporation capacity are all decreased by 2.18/1.61 K, 5.06/3.90 W m–2, and 0.21/0.14 mm day–1 since 1850 for EM and PIM schemes, respectively. According to the changes in aridity index, the anthropogenic aerosols have caused general humidification over central Asia, South America, Africa, and Australia, but great aridification over eastern China and the Tibetan Plateau since the pre-industrial in both mixing schemes. However, the aridification is considerably alleviated in China, but intensified in the Arabian Peninsula and East Africa in the PIM scheme.

Spatiotemporal changes in aridity index and reference evapotranspiration over semi-arid and humid regions of Iran: trend, cause, and sensitivity analyses

The existence of significant trend and change point in the aridity index (AI), precipitation, and reference evapotranspiration (ET0) was evaluated using the Mann-Kendall and Pettitt tests, respectively, over 22 semi-arid, humid, and sub-humid sites of Iran during 1966–2012. Detrending method was also employed to quantify each factor’s contribution to the AI and ET0 trends. The ET0 and AI sensitivity to their input perturbations was analyzed using a derivative sensitivity coefficient. The results indicated a downward trend in precipitation and AI and an upward trend in ET0 on both seasonal and annual scales for majority of sites. Compared with AI and precipitation, more significant change points were detected, mainly during the 1990s, in the ET0 series. Except for wintertime, solar radiation (SR) was found as the most sensitive factor on the AI and ET0 dynamics in the semi-arid regions. However, with an exception for summertime, AI and ET0 exhibited greater sensitivity to the RH changes followed by the SR changes in the humid/sub-humid areas. For more than 70% of semi-arid stations and Gorgan (a sub-humid location), wind speed (U) was the most important variable contributing to seasonal and annual ET0 trends. However, the ET0 trend was primarily caused by the mean temperature (Tmean) changes for the humid environments. Further, the precipitation changes made the largest contribution to seasonal and annual AI trends for most cases followed by the U changes. An integrated water resource management is required to be implemented to reduce negative impacts of decreased AI and ET0 increment over Iran.

Changes in aridity index and reference evapotranspiration over the central and eastern Tibetan Plateau in China during 1960–2012

Based on climate data from 68 meteorological stations over the Tibetan Plateau (TP) observed by the China Meteorological Administration in 1960–2012, temporal and spatial variations in aridity index (AI) and reference evapotranspiration (ET0) were comprehensively investigated. The abrupt change and the period in AI and ET0 were characterized using a comprehensive time series analysis conducted with Mann–Kendall test and Morlet wavelet. The results indicated that the regionally averaged value of AI significantly decreased by 0.04/decade during 1960–2012 period, with the maximum observed in 1972. Similarly, the regional trend for ET0 was at the rate of −9.6 mm/decade with statistically significant at the 0.01 level. Most of these stations with positive value for AI were primarily distributed at the northern southwestern TP. Moreover, a majority of stations with low values of ET0 were substantially distributed at the central and eastern TP, and amounts of the stations with high values of ET0 were mainly located at a lower elevation. Abrupt changes of both AI and ET0 primarily happened in 1980s. The major cycles of AI and ET0 were 15 y and 17 y scale over the study period with apparent periodic oscillation characteristics, respectively, and together with other different scale cycles co-existing. The significant correlations between AI and East Asian Summer Monsoon Index (EASMI) indicated that AI over the TP was related to the EASMI.

Spatial and temporal changes in aridity index in northwest China: 1960 to 2010

Northwest China is the driest region in China and the regional climate fluctuated dramatically during the last century. Aridity index, as the ratio between potential evapotranspiration and precipitation, is a good indicator to represent regional climate character. In this study, the change and attribution of the aridity index was investigated in northwest China using the observed climate data from 80 national meteorological stations during 1960–2010. The spatial and temporal variabilities of the aridity index shows that the annual aridity index decreased significantly (P < 0.05) by 0.048 year−1, indicating that northwest China became wetter from 1960 to 2010. A differentiation equation method was used to attribute the change in aridity index to climate variables. The results indicate that the aridity index was most sensitive to the change in precipitation, followed by vapor pressure, solar radiation, wind speed, and air temperature. Increase in air temperature should have led to an increase in aridity index, but this effect had been offset by the increase in precipitation and vapor pressure and the decrease in wind speed. Increasing precipitation, which contributed 91.7 % of the decrease in the aridity index, was the dominant factor to the decrease in the aridity index in northwest China from 1960 to 2010.