Showing papers in "Journal of Arid Land in 2017"

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

Abstract: 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 AgMERRA 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 AgMERRA 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 (R2=0.99). Furthermore, the higher correlations (R2=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 AgMERRA precipitation data and from the station-observed precipitation data, we suggest that the AgMERRA 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 AgMERRA precipitation data may be used for the data-lacking areas.

Mechanisms of bush encroachment and its inter-connection with rangeland degradation in semi-arid African ecosystems: a review

Abstract: Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local woody species and/or incursion of woody species introduced from other ecosystems into semi-arid savannas and grassland ecosystems. Rangeland degradation due to bush encroachment causes several challenges, affecting the production of livestock and pastoral people livelihoods in most parts of Africa. Scientists have long been attempting to develop schematic and mathematical theories to explain the observed phenomenon of bush encroachment, and several theories were proposed and developed. The well-regarded theories include: (1) Walter’s two-layer model, (2) Moir’s one-layer model, (3) state-and-transition theory, (4) equilibrium theory, (5) disequilibrium theory, and (6) non-equilibrium theory. Within those theories, the most frequently-indicated driving factors that explain bush encroachment include over-grazing, availability of soil nutrient and moisture, elevated CO2 levels, frequency and intensity of fire, spread of seeds of woody species by livestock and wild animals. It should be stressed that couplings and interactions among diverse driving factors are more often at work in determining the condition of bush encroachment. To summarize, the effort in managing semi-arid ecosystems needs critical knowledge to understand the cause-effect relationships of underlying factors through integrated approach. Therefore, future research on encroachment of woody plants should be multi-discipline oriented and multi-partnership involved.

Assessing the impacts of human activities and climate variations on grassland productivity by partial least squares structural equation modeling (PLS-SEM)

Abstract: The cause-effect associations between geographical phenomena are an important focus in ecological research. Recent studies in structural equation modeling (SEM) demonstrated the potential for analyzing such associations. We applied the variance-based partial least squares SEM (PLS-SEM) and geographically-weighted regression (GWR) modeling to assess the human-climate impact on grassland productivity represented by above-ground biomass (AGB). The human and climate factors and their interaction were taken to explain the AGB variance by a PLS-SEM developed for the grassland ecosystem in Inner Mongolia, China. Results indicated that 65.5% of the AGB variance could be explained by the human and climate factors and their interaction. The case study showed that the human and climate factors imposed a significant and negative impact on the AGB and that their interaction alleviated to some extent the threat from the intensified human-climate pressure. The alleviation may be attributable to vegetation adaptation to high human-climate stresses, to human adaptation to climate conditions or/and to recent vegetation restoration programs in the highly degraded areas. Furthermore, the AGB response to the human and climate factors modeled by GWR exhibited significant spatial variations. This study demonstrated that the combination of PLS-SEM and GWR model is feasible to investigate the cause-effect relation in socio-ecological systems.

Endophytic bacteria associated with endangered plant Ferula sinkiangensis K. M. Shen in an arid land: diversity and plant growth-promoting traits

Abstract: Ferula spp. are traditional medicinal plants found in arid land. Large-scale excavation for extracting bioactive compounds from the plants in arid regions of Xinjiang over the last few years has, however, significantly decreased their distributions. Due to the urgent need for preservation of these plant resources, along with the need of searching for alternative source of the useful metabolites, it is important to screen the endophytic microbial resources associated with the plant Ferula sinkiangensis K. M. Shen. In the study, a total of 125 endophytic bacteria belonging to 3 phyla, 13 orders, 23 families, and 29 genera were isolated based on 16S rRNA gene sequencing data. Among the different isolates, three strains isolated from roots were potential novel species of the genera Porphyrobacter, Paracoccus and Amycolatopsis. In this study, 79.4% and 57.1% of the total isolates were capable of producing indole-3-acetic acid (IAA) and siderophore, respectively. And, 40.6% of the strains inhibit the growth of fungal pathogen Alternaria alternata, 17.2% and 20.2% strains were positive for antagonism against Verticillium dahlia 991 and V. dahlia 7, respectively. These results demonstrated that F. sinkiangensis is a rich reservoir of endophytic bacterial resources with potential for production of biologically important functions such as plant growth-promoting factors.

Meteorological impacts on evapotranspiration in different climatic zones of Pakistan

Abstract: Arid regions are highly vulnerable and sensitive to drought. The crops cultivated in arid zones are at high risk due to the high evapotranspiration and water demands. This study analyzed the changes in seasonal and annual evapotranspiration (ET) during 1951–2016 at 50 meteorological stations located in the extremely arid, arid, and semi-arid zones of Pakistan using the Penman Monteith (PM) method. The results show that ET is highly sensitive and positively correlated to temperature, solar radiation, and wind speed whereas vapor pressure is negatively correlated to ET. The study also identifies the relationship of ET with the meteorological parameters in different climatic zones of Pakistan. The significant trend analysis of precipitation and temperature (maximum and minimum) are conducted at 95% confidence level to determine the behaviors of these parameters in the extremely arid, arid, and semi-arid zones. The mean annual precipitation and annual mean maximum temperature significantly increased by 0.828 mm/a and 0.014°C/a in the arid and extremely arid zones, respectively. The annual mean minimum temperature increased by 0.017°C/a in the extremely arid zone and 0.019°C/a in the arid zone, whereas a significant decrease of 0.007°C/a was observed in the semi-arid zone. This study provides probabilistic future scenarios that would be helpful for policy-makers, agriculturists to plan effective irrigation measures towards the sustainable development in Pakistan.

A remote sensing-based agricultural drought indicator and its implementation over a semi-arid region, Jordan

Abstract: The objective of the study was to develop a remote sensing (i.e., Landsat-8 and MODIS)-based agricultural drought indicator (ADI) at 30-m spatial resolution and 8-day temporal resolution and also to evaluate its performance over a heterogeneous agriculture dominant semi-arid region in Jordan. Firstly, we used principal component analysis (PCA) to evaluate the correlations among six commonly used remote sensing-derived agricultural drought related variables. The variables included normalized difference water index (NDWI), normalized difference vegetation index (NDVI), visible and shortwave drought index (VSDI), normalized multiband drought index (NMDI), moisture stress index (MSI), and land surface temperature (LST). Secondly, we integrated the relatively less correlated variables (that were found to be NDWI, VSDI, and LST) to generate four agricultural drought categories/conditions (i.e., wet, mild drought, moderate drought, and severe drought). Finally, we evaluated the ADI maps against a set of 8-day ground-based standardized precipitation index values (i.e., SPI-1, SPI-2, …, SPI-8) by use of confusion matrices and observed the best results for SPI-4 (i.e., overall accuracy and Kappa-values were 83% and 76%, respectively) and SPI-5 (i.e., overall accuracy and Kappa-values were 85% and 78%, respectively). The results demonstrated that the method would be valuable for monitoring agricultural drought conditions in semi-arid regions at both a reasonably high spatial resolution (i.e., 30-m) and a short time period (i.e., 8-day).

Spatial and temporal variations of vegetation cover and the relationships with climate factors in Inner Mongolia based on GIMMS NDVI3g data

Abstract: Variation in vegetation cover in Inner Mongolia has been previously studied by the remote sensing data spanning only one decade However, spatial and temporal variations in vegetation cover based on the newly released GIMMS NDVI3g data spanning nearly thirty years have yet to be analyzed In this study, we applied the methods of the maximum value composite (MVC) and Pearson’s correlation coefficient to analyze the variations of vegetation cover in Inner Mongolia based on GIMMS NDVI3g data spanning from 1982 to 2013 Our results indicate that the normalized difference vegetation index (NDVI) increased at a rate of 00003/a during the growing seasons despite of the drier and hotter climate in Inner Mongolia during the past three decades We also found that vegetation cover in the southern agro-pastoral zone significantly increased, while it significantly decreased in the central Alxa The variations in vegetation cover were not significant in the eastern and central regions NDVI is positively correlated with precipitation (r=0617, P=0000) and also with air temperature (r=0425, P=0015), but the precipitation had a greater effect than the air temperature on the vegetation variations in Inner Mongolia

Responses of water productivity to irrigation and N supply for hybrid maize seed production in an arid region of Northwest China

Abstract: Water and nitrogen (N) are generally two of the most important factors in determining the crop productivity. Proper water and N managements are prerequisites for agriculture sustainable development in arid areas. Field experiments were conducted to study the responses of water productivity for crop yield (WP Y-ET ) and final biomass (WP B-ET ) of film-mulched hybrid maize seed production to different irrigation and N treatments in the Hexi Corridor, Northwest China during April to September in 2013 and also during April to September in 2014. Three irrigation levels (70%–65%, 60%–55%, and 50%–45% of the field capacity) combined with three N rates (500, 400, and 300 kg N/hm2) were tested in 2013. The N treatments were adjusted to 500, 300, and 100 kg N/hm2 in 2014. Results showed that the responses of WP Y-ET and WP B-ET to different irrigation amounts were different. WP Y-ET was significantly reduced by lowering irrigation amounts while WP B-ET stayed relatively insensitive to irrigation amounts. However, WP Y-ET and WP B-ET behaved consistently when subjected to different N treatments. There was a slight effect of reducing N input from 500 to 300 kg/hm2 on the WP Y-ET and WP B-ET , however, when reducing N input to 100 kg/hm2, the values of WP Y-ET and WP B-ET were significantly reduced. Water is the primary factor and N is the secondary factor in determining both yield (Y) and final biomass (B). Partial factor productivity from applied N (PFPN) was the maximum under the higher irrigation level and in lower N rate (100–300 kg N/hm2) in both years (2013 and 2014). Lowering the irrigation amount significantly reduced evapotranspiration (ET), but ET did not vary with different N rates (100–500 kg N/hm2). Both Y and B had robust linear relationships with ET, but the correlation between B and ET (R 2=0.8588) was much better than that between Y and ET (R 2=0.6062). When ET increased, WP Y-ET linearly increased and WP B-ET decreased. Taking the indices of Y, B, WP Y-ET , WP B-ET and PFP N into account, a higher irrigation level (70%–65% of the field capacity) and a lower N rate (100–300 kg N/hm2) are recommended to be a proper irrigation and N application strategy for plastic film-mulched hybrid maize seed production in arid Northwest China.

A proposed surface resistance model for the Penman-Monteith formula to estimate evapotranspiration in a solar greenhouse

Abstract: Greenhousing is a technique to bridge season gap in vegetable production and has been widely used worldwide Calculation of water requirement of crops grown in greenhouse and determination of their irrigation schedules in arid and semi-arid regions are essential for greenhouse maintenance and have thus attracted increased attention over the past decades The most common method used in the literature to estimate crop evapotranspiration (ET) is the Penman-Monteith (PM) formula When applied to greenhouse, however, it often uses canopy resistance instead of surface resistance It is understood that the surface resistance in greenhouse is the result of a combined effect of canopy restriction and soil-surface restriction to water vapor flow, and the relative dominance of one restriction over another depends on crop canopy In this paper, we developed a surface resistance model in a way similar to two parallel resistances in an electrical circuit to account for both restrictions Also, considering that wind speed in greenhouse is normally rather small, we compared three methods available in the literature to calculate the aerodynamic resistance, which are the r a 1 method proposed by Perrier (1975a, b), the r a 2 method proposed by Thom and Oliver (1977), and the r a 3 method proposed by Zhang and Lemeu (1992) We validated the model against ET of tomatoes in a greenhouse measured from sap flow system combined with micro-lysimeter in 2015 and with weighing lysimeter in 2016 The results showed that the proposed surface resistance model improved the accuracy of the PM model, especially when the leaf area index was low and the greenhouse was being irrigated We also found that the aerodynamic resistance calculated from the r a 1 and r a 3 methods is applicable to the greenhouse although the latter is slightly more accurate than the former The proposed surface resistance model, together with the r a 3 method for aerodynamic resistance, offers an improved approach to estimate ET in greenhouse using the PM formula

Organochlorine pesticides and polycyclic aromatic hydrocarbons in water and sediment of the Bosten Lake, Northwest China

Abstract: We evaluated organic pollution in Bosten Lake, Xinjiang, China, by measuring the concentrations and distributions of organochlorine pesticides (OCPs) and polycyclic aromatic hydrocarbons (PAHs). Water and sediment samples were collected from 19 sites (B1–B19) in the lake for analysis. Our analytical results show that the concentrations of total OCPs in water ranges from 30.3 to 91.6 ng/L and the concentrations of PAHs ranges from undetectable (ND) to 368.7 ng/L. The concentrations of total OCPs in surface (i.e., lake bottom) sediment ranges from 6.9 to 16.7 ng/g and the concentrations of PAHs ranges from 25.2 to 491.0 ng/g. Hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs) account for large proportions of the OCPs. Low α- to γ-HCH ratios in both water and sediment samples indicate possible contributions from both industrial products and lindane. DDTs in water are probably from historical input, whereas DDTs in sediments are from both historical and recent inputs. Moreover, DDT products in both water and sediments were from multiple sources in the northwestern part of the lake (B11, B12, B13, and B14). Fugacity ratios for DDT isomers (p,p′-DDE and p,p′-DDT) at these sites were generally higher than equilibrium values. These results suggest that the input from the Kaidu River and diffusion of DDTs from the sediment to the water are responsible for DDT pollution in the water. Lower-molecular-weight PAHs, which originate primarily from wood and coal combustion and petroleum sources, represent the major fraction of the PAHs in both water and sediment samples. Our findings indicate that OCPs and PAHs in Bosten Lake can be attributed primarily to human activities. A risk assessment of OCPs and PAHs in water and sediment from Bosten Lake, however, suggests that concentrations are not yet high enough to cause adverse biological effects on the aquatic ecosystem.

Two energy balance closure approaches: applications and comparisons over an oasis-desert ecotone

Abstract: Studies of energy balance that rely on eddy covariance (EC) are always challenged by energy balance closure, which is mainly caused by the underestimations of latent heat flux (LE) and sensible heat flux (Hs) The Bowen ratio (BR) and energy balance residual (ER) approaches are two widely-used methods to correct the LE A comprehensive comparison of those two approaches in different land-use types is essential to accurately correcting the LE and thus improving the EC experiments In this study, two energy balance approaches (ie, BR and ER) were compared to correct the LE measured at six EC sites (ie, three vegetated, one mixed and two non-vegetated sites) in an oasis-desert ecotone of the Heihe River Basin, China The influences of meteorological factors on those two approaches were also quantitatively assessed Our results demonstrated that the average energy closure ratio ((LE+Hs)/(Rn–Gs); where Rn is the surface net radiation and Gs is the surface soil heat flux) was approximately close to 10 at wetland, maize and village sites, but far from 10 at orchard, Gobi and desert sites, indicating a significant energy imbalance at those three latter sites After the corrections of BR and ER approaches that took into account of soil heat storage, the corrected LE was considerably larger than the EC-measured LE at five of six EC sites with an exception at Gobi site The BR and ER approaches yielded approximately similar corrected LE at vegetated and mixed sites, but they generated dissimilar results at non-vegetated sites, especially at non-vegetated sites with low relative humidity, strong wind, and large surface-air temperature difference Our findings provide insight into the applicability of BR and ER approaches to correcting EC-based LE measurements in different land-use types We recommend that the BR-corrected and ER-corrected LE could be seriously reconsidered as validation references in dry and windy areas

Degree of coupling and coordination of eco-economic system and the influencing factors: a case study in Yanchi County, Ningxia Hui Autonomous Region, China

Abstract: Based on the statistical data, we analyzed and evaluated the degree of coupling and coordination of the eco-economic system in Yanchi County for the period spanning from 1983 to 2014 The eco-economic system can be divided into socioeconomic and ecological sub-systems and their relationship can reveal the interaction state between the two sub-systems and help the local government to establish a coordinated development mode An index system was constructed to assess the development of the two sub-systems before the evaluation of the degree of coupling and coordination The principal component regression analysis was adopted to quantitatively assess the influences of natural, economic and social factors on the degree of coupling and coordination of the eco-economic system Results showed that, from 1983 to 2014, the development trends of both sub-systems were increasing with the ecological sub-system having more fluctuations The degree of coupling and coordination of the eco-economic system in the study area increased gradually from 1983 to 2014, but experienced five different development stages from the verge of disorder to favorable coordination The development of the local social and economic conditions was the most important factor influencing the degree of coupling and coordination The second most important factor was the financial support from the local government In addition, the environment protection policies also played undeniable roles Due to the diversity of the influence factors, the government should take comprehensive measures to promote the sustainable development of the eco-economic system

Effects of freeze-thaw on soil erosion processes and sediment selectivity under simulated rainfall

Abstract: The freeze-thaw (FT) processes affect an area of 46.3% in China. It is essential for soil and water conservation and ecological construction to elucidate the mechanisms of the FT processes and its associated soil erosion processes. In this research, we designed the control simulation experiments to promote the understanding of FT-water combined erosion processes. The results showed that the runoff of freeze-thaw slope (FTS) decreased by 8% compared to the control slope (CS), and the total sediment yield of the FTS was 1.10 times that of the CS. The sediment yield rate from the FTS was significantly greater than that from the CS after 9 min of runoff (P 0.98, P<0.01). Significant differences in the mean weight diameter (MWD) values of particles were observed for washed particles and splashed particles between the CS and the FTS treatments in the erosion process (P<0.05). The mean MWD values under CS were smaller than those under FTS for both washed and splashed particles. The ratio of the absolute value of a regression coefficient between the CS and the FTS was 1.15, being roughly correspondent with the ratio of K between the two treatments. Therefore, the parameter a of the power function between cumulative runoff and sediment yield could be an acceptable indicator for expressing the soil erodibility. In conclusion, the FTS exhibited an increase in soil erosion compared to the CS.

Influences of drip and flood irrigation on soil carbon dioxide emission and soil carbon sequestration of maize cropland in the North China Plain

Abstract: The need is pressing to investigate soil CO2 (carbon dioxide) emissions and soil organic carbon dynamics under water-saving irrigation practices in agricultural systems for exploring the potentials of soil carbon sequestration. A field experiment was conducted to compare the influences of drip irrigation (DI) and flood irrigation (FI) on soil organic carbon dynamics and the spatial and temporal variations in CO2 emissions during the summer maize growing season in the North China Plain using the static closed chamber method. The mean CO2 efflux over the growing season was larger under DI than that under FI. The cumulative CO2 emissions at the field scale were 1959.10 and 1759.12 g/m2 under DI and FI, respectively. The cumulative CO2 emission on plant rows (OR) was larger than that between plant rows (BR) under FI, and the cumulative CO2 emission on the irrigation pipes (OP) was larger than that between irrigation pipes (BP) under DI. The cumulative CO2 emissions of OP, BP and bare area (BA) under DI were larger than those of OR, BR and BA under FI, respectively. Additionally, DI promoted root respiration more effectively than FI did. The average proportion of root respiration contributing to the soil CO2 emissions of OP under DI was larger than that of OR under FI. A general conclusion drawn from this study is that soil CO2 emission was significantly influenced by the soil water content, soil temperature and air temperature under both DI and FI. Larger concentrations of dissolved organic carbon (DOC), microbial biomass carbon (MBC) and total organic carbon (TOC) were observed under FI than those under DI. The observed high concentrations (DOC, MBC, and TOC) under FI might be resulted from the irrigation-associated soil saturation that in turn inhibited microbial activity and lowered decomposition rate of soil organic matter. However, DI increased the soil organic matter quality (the ratio of MBC to TOC) at the depth of 10–20 cm compared with FI. Our results suggest that the transformation from conventional FI to integrated DI can increase the CO2 emissions and DI needs to be combined with other management practices to reduce the CO2 emissions from summer maize fields in the North China Plain.

Effects of vegetation types on soil water dynamics during vegetation restoration in the Mu Us Sandy Land, northwestern China

Abstract: The arid and semi-arid northwestern China has been undergoing ecological degradation and the efforts to reverse the ecological degradation have been undertaken for many years. Some shifting dunes have been fixed and the vegetation has been partially recovered in certain areas and the Mu Us Sandy Land in the Ordos Plateau is an example of the success. The present study attempts to reveal the relationships between the vegetation restoration and ecohydrology in the Mu Us Sandy Land. We continuously measured soil water content at 10-min intervals under three vegetation types (i.e., shifting dune, shrub-dominated community, and herb-dominated community) in the Mu Us Sandy Land from April 2012 to October 2013. The results show the infiltration coefficient increased with increased rainfall amount and eventually reached a stable value. Infiltration coefficients were 0.91, 0.64, and 0.74 in the shifting dune, in the shrub-dominated community, and in the herb-dominated community, respectively. Cumulative infiltration and soil texture are two vital factors affecting the depths of rainfall penetration. Only rainfall events larger than 35.0 mm could recharge soil water at the 60–80 cm layer in the herb-dominated community. Our results imply that the expected forward succession of restored vegetation may be destined to deterioration after reaching the climax simply because of following two facts: (1) soil water is mainly retained at shallower layer and (2) plant fine roots mainly distribute in deeper layer in the herb-dominated community.

Wind tunnel test on the effect of metal net fences on sand flux in a Gobi Desert, China

Abstract: The Lanzhou-Xinjiang High-speed Railway runs through an expansive windy area in a Gobi Desert, and sand-blocking fences were built to protect the railway from destruction by wind-blown sand. However, the shielding effect of the sand-blocking fence is below the expectation. In this study, effects of metal net fences with porosities of 0.5 and 0.7 were tested in a wind tunnel to determine the effectiveness of the employed two kinds of fences in reducing wind velocity and restraining wind-blown sand. Specifically, the horizontal wind velocities and sediment flux densities above the gravel surface were measured under different free-stream wind velocities for the following conditions: no fence at all, single fence with a porosity of 0.5, single fence with a porosity of 0.7, double fences with a porosity of 0.5, and double fences with a porosity of 0.7. Experimental results showed that the horizontal wind velocity was more significantly decreased by the fence with a porosity of 0.5, especially for the double fences. The horizontal wind velocity decreased approximately 65% at a distance of 3.25 m (i.e., 13H, where H denotes the fence height) downwind the double fences, and no reverse flow or vortex was observed on the leeward side. The sediment flux density decreased exponentially with height above the gravel surface downwind in all tested fences. The reduction percentage of total sediment flux density was higher for the fence with a porosity of 0.5 than for the fence with a porosity of 0.7, especially for the double fences. Furthermore, the decreasing percentage of total sediment flux density decreased with increasing free-stream wind velocity. The results suggest that compared with metal net fence with a porosity of 0.7, the metal net fence with a porosity of 0.5 is more effective for controlling wind-blown sand in the expansive windy area where the Lanzhou-Xinjiang High-speed Railway runs through.

Livestock induces strong spatial heterogeneity of soil CO 2 , N 2 O and CH 4 emissions within a semi-arid sylvo-pastoral landscape in West Africa

Abstract: Greenhouse gas (GHG) emissions from the surface soils and surface water receiving animal excreta may be important components of the GHG balance of terrestrial ecosystems, but the associated processes are poorly documented in tropical environments, especially in tropical arid and semi-arid areas. A typical sylvo-pastoral landscape in the semi-arid zone of Senegal, West Africa, was investigated in this study. The study area (706 km² of managed pastoral land) was a circular zone with a radius of 15 km centered on a borehole used to water livestock. The landscape supports a stocking rate ranging from 0.11 to 0.39 tropical livestock units per hectare depending on the seasonal movements of the livestock. Six landscape units were investigated (land in the vicinity of the borehole, natural ponds, natural rangelands, forest plantations, settlements, and enclosed plots). Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes were measured with static chambers set up at 13 sites covering the six landscape units, and the 13 sites are assumed to be representative of the spatial heterogeneity of the emissions. A total of 216 fluxes were measured during the one-year study period (May 2014 to April 2015). At the landscape level, soils and surface water emitted an average 19.8 t C-CO2 eq/(hm²•a) (CO2: 82%, N2O: 15%, and CH4: 3%), but detailed results revealed notable spatial heterogeneity of GHG emissions. CO2 fluxes ranged from 1148.2 (±91.6) mg/(m²•d) in rangelands to 97,980.2 (±14,861.7) mg/(m²•d) in surface water in the vicinity of the borehole. N2O fluxes ranged from 0.6 (±0.1) mg/(m²•d) in forest plantations to 22.6 (±10.8) mg/(m²•d) in the vicinity of the borehole. CH4 fluxes ranged from–3.2 (±0.3) mg/(m²•d) in forest plantations to 8788.5 (±2295.9) mg/(m²•d) from surface water in the vicinity of the borehole. This study identified GHG emission “hot spots” in the landscape. Emissions from the surface soils were significantly higher in the landscape units most frequently used by the animals, i.e., in the vicinity of the borehole and settlements; and emissions measured from surface water in the vicinity of the borehole and from natural ponds were on average about 10 times higher than soil emissions.

An experimental study of salt expansion in sodium saline soils under transient conditions

Abstract: Salt expansion in sulfate saline soils that are widely distributed in northwestern China causes serious infrastructural damages under low-temperature conditions. However, the mechanism of salt expansion under low temperatures is not clear. In this study, we conducted a series of cooling experiments combined with salt crystallization to study this mechanism, and employed an ionic model to calculate the supersaturation ratio of the solution. During the experiments, the strength and the process of salt expansion were examined under different cooling rates and various crystal morphologies. The relationship between temperature and supersaturation ratio under transient conditions was also considered. Results indicate that the initial supersaturation ratio of a sodium sulfate solution is closely related to environmental conditions, and that this ratio decreases with slowing the cooling rates and stabilizing the crystal forms. Higher initial supersaturation ratios lead to an increased non-steady-state zone, resulting in less salt expansion. On the other hand, chloride ion content has a distinct influence on the crystallization supersaturation ratio of the sodium sulfate solution, and higher chloride ion content can inhibit salt expansion in sodium saline soils. These findings help explain salt expansion mechanisms in complex conditions such as seasonally frozen soils, and thus help search for improved methods of preventing salt expansion in sulfate saline soils.

Relative importance of climate factors and human activities in impacting vegetation dynamics during 2000–2015 in the Otindag Sandy Land, northern China

Abstract: In recent years, there has been increasing research interests in differentiating the relative importance of climate factors and human activities in impacting vegetation dynamics. In this study, based on residual trend method, we used MOD13A3 (MODIS vegetation index product), MCD12Q1 (MODIS land cover product) and meteorological datasets to differentiate the relative importance of climate factors and human activities in impacting vegetation dynamics during 2000–2015 in the Otindag Sandy Land, northern China. Results show that during the study period (2000–2015), the overall vegetation condition had improved in the Otindag Sandy Land. The driving forces of vegetation dynamics differed spatially in the whole study area over the study period. The area with vegetation degradation solely resulted from human activities accounted for 8.23% of the study area, while the area with vegetation degradation resulted from others (including climate factors and combination of climate factors and human activities) occupied 1.53%. The area with vegetation recovery benefitted from human activities occurred over 26.02% of the study area; the area benefitted from climate factors accounted for 23.69%; and the area benefitted from both climate factors and human activities occupied 37.74%. All in all, impacts of climate factors and human activities on vegetation dynamics varied at the county/city/banner scales and locality-specific measures should be adopted to protect the environments.

Atmospheric deposition of inorganic nitrogen in a semi-arid grassland of Inner Mongolia, China

Abstract: Due to increasing global demand for crop production and energy use, more and more reactive nitrogen (Nr) has been generated and emitted to the environment. As a result, global atmospheric nitrogen (N) deposition has tripled since the industrial revolution and the ecological environment and human health have been harmed. In this study, we measured dry and wet/bulk N deposition from July 2013 to December 2015 in a semi-arid grassland of Duolun County, Inner Mongolia, China. The samples of dry and wet/bulk N deposition were collected monthly with a DELTA (DEnuder for Long Term Atmospheric sampling) system and with Gradko passive samplers and a precipitation gauge. The measured results show that the annual mean concentrations of NH3, NO2, HNO3, particulate NH4+ (pNH4+) and particulate NO3− (pNO3−) in atmosphere were 2.33, 1.90, 0.18, 1.42 and 0.42 μg N/m3, respectively, and that the annual mean volume-weighted concentrations ofNH4+-N and NO3−-N in precipitation were 2.71 and 1.99 mg N/L, respectively. The concentrations of Nr components (including NH3, NO2, HNO3, pNH4+, pNO3−,NH4+-N and NO3−-N) exhibited different seasonal variations. Specifically, NO2 and HNO3 exhibited higher concentrations in autumn than in summer, while the other Nr components (NH3, pNH4+, pNO3−,NH4+-N and NO3−-N) showed the highest values in summer. Based on measured concentrations of Nr components and their deposition velocities estimated using the GEOS-Chem global atmospheric chemical transport model, the calculated annual mean dry deposition fluxes were 3.17, 1.13, 0.63, 0.91 and 0.36 kg N/(hm2•a) for NH3, NO2, HNO3, pNH4+ and pNO3−, respectively, and the calculated annual mean wet/bulk deposition fluxes were 5.37 and 3.15 kg N/(hm2•a) forNH4+-N and NO3−-N, respectively. The estimated annual N deposition (including dry N deposition and wet/bulk N deposition) reached 14.7 kg N/(hm2•a) in grassland of Duolun County, approaching to the upper limit of the N critical load (10–15 kg N/(hm2•a)). Dry and wet/bulk deposition fluxes of all Nr components (with an exception of HNO3) showed similar seasonal variations with the maximum deposition flux in summer and the minimum in winter. Reduced Nr components (e.g., gaseous NH3 and pNH4+ in atmosphere andNH4+-N in precipitation) dominated the total N deposition at the sampling site (accounted for 64% of the total N deposition), suggesting that the deposited atmospheric Nr mainly originated from agricultural activities. Considering the projected future increases in crop and livestock production in Inner Mongolia, the ecological and human risks to the negative effects of increased N deposition could be increased if no mitigation measures are taken.

Physiological responses of artificial moss biocrusts to dehydration-rehydration process and heat stress on the Loess Plateau, China

Abstract: Ex-situ cultivation of biological soil crusts (biocrusts) is a promising technology to produce materials that can induce the recovery of biocrusts in the field for the purposes of preventing soil erosion and improving hydrological function in degraded ecosystems. However, the ability of artificially cultivated biocrusts to survive under adverse field conditions, including drought and heat stresses, is still relatively unknown. Mosses can bolster biocrust resistance to the stresses (e.g., drought and heat) and the resistance may be introduced prior to field cultivation. In this study, we subjected the well-developed artificial moss biocrusts (dominant species of Didymodon vinealis (Brid.) Zand.) that we cultivated in the phytotron to a dehydration-rehydration experiment and also a heat stress experiment and measured the activities of protective enzymes (including peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT)) and the contents of osmoregulatory substances (including soluble proteins and soluble sugars) and malondialdehyde (MDA, an indicator of oxidative stress) in the stem and leaf fragments of mosses. The results showed that, during the dehydration process, the activities of protective enzymes and the contents of osmoregulatory substances and MDA gradually increased with increasing duration of drought stress (over 13 days). During the rehydration process, values of these parameters decreased rapidly after 1 d of rehydration. The values then showed a gradual decrease for 5 days, approaching to the control levels. Under heat stress (45°C), the activities of protective enzymes and the content of soluble proteins increased rapidly within 2 h of heat exposure and then decreased gradually with increasing duration of heat exposure. In contrast, the contents of soluble sugars and MDA always increased gradually with increasing duration of heat exposure. This study indicates that artificial moss biocrusts possess a strong drought resistance and this resistance can be enhanced after a gradual dehydration treatment. This study also indicates that artificial moss biocrusts can only resist short-term heat stress (not long-term heat stress). These findings suggest that short-term heat stress or prolonged drought stress could be used to elevate the resistance of artificial moss biocrusts to adverse conditions prior to field reintroduction.

Monitoring recent changes in snow cover in Central Asia using improved MODIS snow-cover products

Abstract: Snow cover plays an important role in the fields of climatology and cryospheric science. Remotely-sensed data have been proven to be effective in monitoring snow covers. Improved methods to process the 8-day snow-cover products derived from MODIS Terra/Aqua data can dramatically increase the data quality and reduce noise. A five-step algorithm for removing cloud effects was designed to improve the quality of MODIS snow products, and the overall accuracy of the MODIS snow data without cloud (defined as cloud-free snow-cover dataset) was enhanced by more than 90% based on direct and indirect validation methods. The snow-cover frequency (SCF) and snow-cover rate (SCR) of Central Asia were analyzed from 2000 to 2015 using trend analysis and empirical orthogonal functions (EOFs). Over the plain regions, the SCF displayed a significant north-south declining trend with a rate of 0.03 per degree of latitude, and the SCR showed a similar north-south gradient. In the mountainous areas, the SCF significantly increased with altitude by 0.12 per kilometer. Within the study area, the SCF in 65% of the study area experienced an increasing trend, but only 4.3% of the SCF-increasing pixels passed a significance test. The remaining 35% of the area underwent a decreasing trend of SCF, but only 5.2% of the SCF-decreasing pixels passed a significance test. For the entire Central Asia, the inter-annual variations of snow-cover presented a slight and insignificant increase trend from 2000 to 2015. However, the change trends of snow cover are different between the plain and mountainous regions. That is, the annual mean SCR in the plain areas displayed an increasing trend, but a decreasing trend was found in the mountainous areas.

Streamflow responses to climate change and LUCC in a semi-arid watershed of Chinese Loess Plateau

Abstract: Climate change and Land Use/Cover Change (LUCC) have been identified as two primary factors affecting watershed hydrological regime. This study analyzed the trends of streamflow, precipitation, air temperature and potential evapotranspiration (PET) from 1962 to 2008 in the Jihe watershed in northwestern Loess Plateau of China using the Mann-Kendall test. The streamflow responses to climate change and LUCC were quantified independently by the elasticity method. The results show that the streamflow presented a dramatic decline with a turning point occurred in 1971, while the precipitation and PET did not change significantly. The results also show that the temperature rose markedly especially since 1990s with an approximate increase of 1.74°C over the entire research period (1962–2008). Using land use transition matrix, we found that slope cropland was significantly converted to terrace between 1970s and 1990s and that forest cover increased relatively significantly because of the Grain for Green Project after 2000. The streamflow reduction was predominantly caused by LUCC and its contribution reached up to 90.2%, while the contribution of climate change to streamflow decline was only 9.8%. Although the analytical results between the elasticity method and linear regression model were not satisfactorily consistent, they both indicated that LUCC (human activity) was the major factor causing streamflow decline in the Jihe watershed from 1962 to 2008.

Characterizing urban expansion of Korla City and its spatial-temporal patterns using remote sensing and GIS methods

Abstract: Cities provide spatial contexts for populations and economic activities Determining the spatial-temporal patterns of urban expansion is of particular significance for regional sustainable development To achieve a better understanding of the spatial-temporal patterns of urban expansion of Korla City, we explore the urban expansion characteristics of Korla City over the period 1995–2015 by employing Landsat TM/ETM+ images of 1995, 2000, 2005, 2010, and 2015 Urban land use types were classified using the supervised classification method in ENVI 45 Urban expansion indices, such as expansion area, expansion proportion, expansion speed, expansion intensity, compactness, and fractal dimension, were calculated The spatial-temporal patterns and evolution process of the urban expansion (eg, urban gravity center and its direction of movement) were then quantitatively analyzed The results indicated that, over the past 25 years, the area and proportion of urban land increased substantially with an average annual growth rate of 1518% Farmland and unused land were lost greatly due to the urban expansion This result might be attributable to the rapid population growth and the dramatic economic development in this area The city extended to the southeast, and the urban gravity center shifted to the southeast as well by about 2118 m The degree of urban compactness tended to decrease and the fractal dimension index tended to increase, indicating that the spatial pattern of Korla City was becoming loose, complex, and unstable This study could provide a scientific reference for the studies on urban expansion of oasis cities in arid land

Drought monitoring and reliability evaluation of the latest TMPA precipitation data in the Weihe River Basin, Northwest China

Abstract: The high resolution satellite precipitation products bear great potential for large-scale drought monitoring, especially for those regions with sparsely or even without gauge coverage. This study focuses on utilizing the latest Version-7 TRMM Multi-satellite Precipitation Analysis (TMPA 3B42V7) data for drought condition monitoring in the Weihe River Basin (0.135×106 km2). The accuracy of the monthly TMPA 3B42V7 satellite precipitation data was firstly evaluated against the ground rain gauge observations. The statistical characteristics between a short period data series (1998–2013) and a long period data series (1961–2013) were then compared. The TMPA 3B42V7-based SPI (Standardized Precipitation Index) sequences were finally validated and analyzed at various temporal scales for assessing the drought conditions. The results indicate that the monthly TMPA 3B42V7 precipitation is in a high agreement with the rain gauge observations and can accurately capture the temporal and spatial characteristics of rainfall within the Weihe River Basin. The short period data can present the characteristics of long period record, and it is thus acceptable to use the short period data series to estimate the cumulative probability function in the SPI calculation. The TMPA 3B42V7-based SPI matches well with that based on the rain gauge observations at multiple time scales (i.e., 1-, 3-, 6-, 9-, and 12-month) and can give an acceptable temporal distribution of drought conditions. It suggests that the TMPA 3B42V7 precipitation data can be used for monitoring the occurrence of drought in the Weihe River Basin.

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

Abstract: 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 1×104 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.

Effects of different plantation types on soil properties after vegetation restoration in an alpine sandy land on the Tibetan Plateau, China

Abstract: Large areas of Artemisia ordosica Krasch., Caragana korshinskii Kom., and Caragana intermedia Kuang and H. C. Fu plantations were established on moving sand dunes in the Gonghe Basin (northeastern Tibetan Plateau) for vegetation restoration. Elevating our understanding of the changes in soil characteristics after the establishment of different plantation types can be useful in the context of combating desertification. To assess the effects of these plantation types on the restoration of sandy land, we measured soil physical-chemical properties at four depths (0–5, 5–10, 10–20, and 20–50 cm) in each of the three plantation types and also in non-vegetated moving sand dunes (as control sites). Generally, the establishment of A. ordosica, C. korshinskii and C. intermedia plantations on sand dunes has greatly ameliorated soil quality in the Gonghe Basin. Specifically, relative to the moving sand dunes, shrub plantation has increased the silt and clay contents, total porosity and water holding capacity, soil organic matter, total nitrogen, total phosphorus and total potassium contents. The calculated soil quality index suggested that in the Gonghe Basin, C. intermedia is the best choice for soil amelioration. In all the three plantation types, soil amelioration mainly occurred in the shallow depths.

Diurnal dynamics of soil respiration and the influencing factors for three land-cover types in the hinterland of the Taklimakan Desert, China

Abstract: Knowledge of soil respiration and the influencing factors in desert ecosystems is essential to understanding carbon dynamics and responses of biotic and abiotic processes in soils to climate change. In this study, soil respiration rate (R s ) for three land-cover types (shifting sandy land, sandy land with straw checkerboard barriers, and shelter forest land) in the hinterland of the Taklimakan Desert was measured in May 2015 using an automated soil CO2 flux system. The effects of soil temperature (T s ) and soil water content (W s ) on R s were also analyzed. The results showed that R s values in shifting sandy land, sandy land with straw checkerboard barriers, and shelter forest land were all low and exhibited obvious diurnal fluctuations. The establishment of straw checkerboard barriers in sandy land had no significant effect on R s , while the establishment of shelterbelts significantly increased R s . Shifting sandy land and sandy land with straw checkerboard barriers were carbon sinks at night and early morning and were carbon sources in the daytime, while shelter forest land always acted as a carbon source during the whole day. The synergistic effect of T s and W s could better explain the diurnal dynamics in R s than single factor. In shifting sandy land and sandy land with straw checkerboard barriers, W s was identified as a limiting factor influencing the diurnal dynamics of R s . Furthermore, a relatively strong hysteresis loop existed between R s and T s . In contrast, in shelter forest land, R s was significantly influenced by T s , and a relatively weaker hysteresis loop existed between R s and W s .

Vegetation composition differentiation and species-environment relationships in the northern part of Isfahan Province, Iran

Abstract: A considerable proportion of Iran’s territory is covered with arid and semi-arid rangelands and mismanagement and overexploitation of those rangelands have resulted in serious ecological degradation. Thus, the need is pressing to examine the present species composition and the relationships with environmental factors for providing the needed scientific references to species conservation and ecological rehabilitation efforts. The aims of this study were to examine the species composition and to delineate the most important factors influencing the distributions of plant species and groups in the northern rangelands of Isfahan Province (Iran) using two-way indicator species analysis (TWINSPAN), detrended correspondence analysis (DCA), principal component analysis (PCA), and canonical correspondence analysis (CCA). Field investigations were conducted in the growing season of 2014 using stratified random method in 22 homogeneous sampling units. In total, 75 plant species belonging to 52 genera and 19 families were identified. The most important families were Asteraceae and Papilionaceae, the most important genera were Astragalus, Cousinia, and Acanthophyllum, and the most important species were Artemisia aucheri and Artemisia sieberi. Plant species were classified into 10 groups using TWINSPAN. DCA was used to estimate the magnitude of changes in species composition along the first two ordination axes to provide gradient length estimations for PCA and CCA ordinations. The first three PCA axes and the first three CCA axes demonstrated similar cumulative percentage of variance, indicating that the environmental factors (selected by PCA) used in CCA ordination were acceptable for explaining the species composition and the distributions. CCA ordination showed that the first axis was closely related to elevation, slope, surface bare soil cover, surface litter cover, gravel proportion, organic matter, total nitrogen, CaCO3 content, and grazing intensity and that the second axis was closely related to sand proportion, silt proportion, clay proportion, and saturation percentage. Among these factors, elevation was the most effective factor to separate the plant groups and grazing was the major cause of rangeland degradation.

Impacts of land disturbance and restoration on runoff production and sediment yield in the Chinese Loess Plateau

Abstract: Land disturbance and land restoration are important factors influencing runoff production and sediment yield in the semi-arid loess regions of China. This study compared the runoff production and sediment yield during the early stage after land disturbance (ESLD) with those during restoring stage after land disturbance (RSLD). Grey relational analysis was used to analyse the importance of each one of the influencing factors (vegetation, rainfall, soil and topography) in affecting the runoff production and sediment yield. Our results showed that during ESLD, topography was the most critical factor controlling the runoff production, while soil was the most important factor controlling the sediment yield. During RSLD, vegetation was more important in affecting runoff production, while rainfall was more important in affecting sediment yield. In additional, this study demonstrated that both the runoff production and the sediment yield can be effectively reduced by restoring vegetation on severely-disturbed lands, thus providing an important theoretical basis for better implementations of the Grain for Green Program. Our results revealed that the vegetation types of Hippophae rhamnoides+Pinus tabulaeformis and H. rhamnoides are better plant selections for land restoration in this area, especially for relatively gentle slopes (i.e., less than 20 degrees).