Interannual Rainfall Research Articles

Rainfall variability drives interannual variation in N2O emissions from a humid, subtropical pasture

Variations in interannual rainfall totals can lead to large uncertainties in annual N2O emission budget estimates from short term field studies. The interannual variation in nitrous oxide (N2O) emissions from a subtropical pasture in Queensland, Australia, was examined using continuous measurements of automated chambers over 2 consecutive years. Nitrous oxide emissions were highest during the summer months and were highly episodic, related more to the size and distribution of rain events than soil water content. Over 48% of the total N2O emitted was lost in just 16% of measurement days.Interannual variation in annual N2O estimates was high, with cumulative emissions increasing with decreasing rainfall. Cumulative emissions averaged 1826.7±199.9gN2O-Nha−1yr−1 over the two year period, though emissions from 2008 (2148±273gN2O-Nha−1yr−1) were 42% higher than 2007 (1504±126gN2O-Nha−1yr−1). This increase in annual emissions coincided with almost half of the summer precipitation from 2007 to 2008. Emissions dynamics were chiefly driven by the distribution and size of rain events which varied on a seasonal and annual basis.Sampling frequency effects on cumulative N2O flux estimation were assessed using a jackknife technique to inform future manual sampling campaigns. Test subsets of the daily measured data were generated for the pasture and two adjacent land-uses (rainforest and lychee orchard) by selecting measured flux values at regular time intervals ranging from 1 to 30days. Errors associated with weekly sampling were up to 34% of the sub-daily mean and were highly biased towards overestimation if strategically sampled following rain events. Sampling time of day also played a critical role. Morning sampling best represented the 24hour mean in the pasture, whereas sampling at noon proved the most accurate in the shaded rainforest and lychee orchard.

Evaluation of two GCMs in simulating rainfall inter-annual variability over Southern Africa

We evaluate the performance of two global circulation models (GCMs) over Southern Africa, as part of the efforts to improve the skill of seasonal forecast from a multi-model ensemble system over the region. The two GCMs evaluated in the study are the Community Atmosphere Model version 3 (CAM3) and the Hadley Centre Atmospheric Model version 3 (HadAM3). The study analyzed 30-year climate simulations from the models and compared the results with those from Climate Research Unit (CRU) and National Center for Environmental Prediction (NCEP) reanalysis dataset. The evaluation focused on how well the models simulate circulation features, seasonal variation of temperature and rainfall, and the inter-annual rainfall and circulations during El Nino Southern Oscillation (ENSO) years. The study also investigated the relationship between the regional rainfall from the models and global sea surface temperature (SST) during the El Nino and La Nina years. The results show that both GCMs simulate the circulation features and the seasonal cycles of rainfall and temperature fairly well. The location and magnitude of maxima and minima in surface temperature, sea level pressure (SLP), and rainfall fields are well reproduced. The maximum error in the simulated temperature fields is about 2 °, 4 mb in SLP and 8 mm/day in rainfall. However, CAM3 shows a major bias in simulating the summer rainfall; it simulates the maximum rainfall along the western part of Southern Africa, instead of the eastern part. The phase of the seasonal cycles is well reproduced, but the amplitude is underestimated over the Western Cape. Both CAM3 and HadAM3 give reasonable simulations of significant relationship between the regional rainfall and SST over the Nino 3.4 region and show that ENSO strongly drives the climate of Southern Africa. Hence, the model simulations could contribute to understanding the climate of the region and improve seasonal forecasts over Southern Africa.

Climatic Variables Mapping Using Geostatistics in Semi Arid Region: Case Study of Tunisia

Climatic Variables Mapping Using Geostatistics in Semi Arid Region: Case Study of Tunisia In this paper, two geostatistical algorithms are compared: universal kriging (UK) and kriging with external drift (KED). These algorithms are used to interpolate interannual rainfall observations and interannual potential evapotranspiration (Et0) measured at 22 weather stations in Tunisia covering an area of 164 150 km2. Experimental sample semi variograms are constructed and fitted to estimate levels of interannual rainfall and interannual Et0. Contour maps and corresponding uncertainty maps are then created. Maps of rainfall and Et0 obtained using UK showed a slightly smoother pattern than those obtained using KED. The largest kriging variance values are located in the west and south parts of the study area for both methods with a larger maximum variance for Et0 than for rainfall. Cross validation showed that the RMSE obtained for KED gave better results for rainfall interpolation while the UK gave better result for Et0 interpolation.

氮素添加和刈割对内蒙古弃耕草地土壤氮矿化的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 氮素添加和刈割对内蒙古弃耕草地土壤氮矿化的影响 DOI: 10.5846/stxb201403040364 作者: 作者单位: 山西农业大学;中国科学院植物研究所,中国科学院植物研究所,中国科学院植物研究所,山西农业大学动物科技学院 山西 太谷 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目(31170455,41371111);中国科学院方向性项目 Effect of nitrogen addition and mowing on soil nitrogen mineralization in abandoned grasslands in Inner Mongolia Author: Affiliation: Shanxi Agriculture University;State Key Laboratory of Vegetation and Environmental Change,Institute of Botany,the Chinese Academy of Sciences,State Key Laboratory of Vegetation and Environmental Change,Institute of Botany,the Chinese Academy of Sciences,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:以内蒙古多伦县恢复生态学试验示范研究站弃耕10余年的草地为研究对象,于2006年起分别设置对照、氮素添加、刈割和氮素添加+刈割4种处理,每种处理6次重复,研究弃耕草地氮素添加和刈割对土壤氮矿化的影响,结合土壤理化性质和植被地上生产力的动态变化,分析弃耕草地土壤氮矿化对植被恢复的响应,为当地草地恢复与重建提供理论依据和数据支持。实验结果表明:① 氮素添加显著增加了植物地上净初级生产力(ANPP)和土壤无机氮库,与对照相比分别提高115% 和196%,同时显著提高了土壤总硝化速率;但是氮素添加对总氨化速率、土壤微生物生物量碳(MBC)、微生物生物量氮(MBN)、微生物生物量碳氮比(MBC/MBN)、微生物呼吸(MR)以及呼吸熵(qCO2)均无显著影响;② 总氨化速率和硝化速率对刈割处理的响应均不显著,但是刈割处理显著降低了土壤MR(P < 0.05);③ 氮素添加+刈割处理5-7a后,土壤总氨化和硝化速率均无显著变化;但是氮素添加+刈割处理显著增加了ANPP、土壤无机氮库和qCO2,同时显著降低了MBC和MBC/MBN。这说明在弃耕草地适应性管理中,氮素添加可以显著提高草地生产力,但是长期的氮添加对土壤微生物氮的转化是否有利还值得我们进一步研究。 Abstract:Agricultural restructuring and conservation priorities have led to the abandonment of farmlands in China and elsewhere. Restoration of croplands to grassland has become an important management practice during the past ten years in the grassland of Inner Mongolia in northern China. In our experimental region, grassland (steppe) represents the typical arid and semiarid regional vegetation type, which is also typical across a vast area of the Eurasian continent. Mowing of grassland is an important management practice in this area. N availability and microbial N transformation processes are key parameters for evaluation of this management practice, because these parameters greatly affect steppe productivity. However, few studies have evaluated their relative importance and interaction in old-fields in China. We investigated effects of N addition and mowing on soil N mineralization in abandoned croplands, and examined the relationships between soil N mineralization and vegetation recovery in combination with dynamic variation in soil physical-chemical properties and aboveground productivity, to provide a scientific foundation for restoration and reconstruction practices. In this study, we examined how two management practices, namely mowing and N addition, affect gross microbial N mineralization (ammonification and nitrification), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and microbial respiration (MR) in a grass-dominated grassland in an old-field community in northern China. We began the long-term N addition and mowing experiment in 2006 in grassland that had been abandoned for ten years, near the Duolun Restoration Ecology Experimentation and Demonstration Station in Inner Mongolia, China. Four treatments were conducted, each with six replicates: control (no N addition or mowing), N addition, mowing, and N addition + mowing. N addition increased gross nitrification rate, while it had no effect on gross ammonification rate, soil MBC, MBN, microbial biomass carbon/nitrogen ratio (MBC/MBN), MR, or microbial metabolic quotient (qCO2). However, N addition significantly enhanced aboveground net primary productivity (ANPP) and soil inorganic nitrogen, which increased by 115% and 196% respectively compared to controls. Mowing had no significant effect on gross nitrification and ammonification rates, while it significantly reduced soil MR. After five to seven years, N addition + mowing had no significant effect on soil gross ammonification and nitrification rates. However, this treatment had significant positive effects on ANPP, NH4+-N + NO3--N, and qCO2, while it had significant negative effects on MBC and MBC/MBN. Therefore, N addition is an effective means of increasing productivity in abandoned grasslands, but we need think carefully whether long-term N addition promotes soil microbial N transformation in abandoned grasslands. Our results highlight the need for further study of the importance of belowground C supply as a control of microbial N cycling processes. Furthermore, during the restoration of degenerated croplands, N cycling rates are stimulated, but the magnitude of this stimulation is modulated by plant community composition. Considering the significant effect of inter-annual rainfall variability on the key microbial parameters we investigated in our semi-arid study region, we emphasize that further long-term studies are needed to examine the responses of N transformation, microbial biomass, and MR to anthropogenic perturbations such as mowing and N addition. 参考文献 相似文献 引证文献

Seasonal variations in nutrients and secondary metabolites in semi-arid savannas depend on year and species

Semi-arid savannas are characterised by alternating wet and dry seasons and large inter-annual rainfall fluctuations that affect plant growth. Carbon-based secondary metabolites (CBSMs) vary inversely with growth and nutrients because of the physiological trade-off between cellular growth and differentiation. We predicted that (1) nutrient concentrations decrease during the wet season, (2) CBSM concentrations increase during the wet season and (3) nutrient concentrations are lowest and CBSM concentrations are highest in the dry season. We measured nitrogen, phosphorus, total polyphenols and condensed tannins in six woody species (including one evergreen) seasonally at the Nkuhlu exclosure, Kruger National Park, South Africa, for three consecutive years, including one ‘wet’ year (above-average rainfall) and two ‘dry’ years (below-average rainfall). Neither N nor P consistently decreased during wet seasons, while CBSMs did not consistently increase. Neither N nor P in the evergreen species was consistently lowest in dry seasons, while CBSMs were not consistently highest in dry seasons. We discuss the inconsistent responses in the context of species-specificity and high inter-annual rainfall variation. We conclude that seasonal variations in N, P and CBSMs in semi-arid savannas cannot be easily generalised because they depend on species and year.

Peak-summer East Asian rainfall predictability and prediction part I: Southeast Asia

The interannual variation of East Asia summer monsoon (EASM) rainfall exhibits considerable differences between early summer [May–June (MJ)] and peak summer [July–August (JA)]. The present study focuses on peak summer. During JA, the mean ridge line of the western Pacific subtropical High (WPSH) divides EASM domain into two sub-domains: the tropical EA (5°N–26.5°N) and subtropical-extratropical EA (26.5°N–50°N). Since the major variability patterns in the two sub-domains and their origins are substantially different, the Part I of this study concentrates on the tropical EA or Southeast Asia (SEA). We apply the predictable mode analysis approach to explore the predictability and prediction of the SEA peak summer rainfall. Four principal modes of interannual rainfall variability during 1979–2013 are identified by EOF analysis: (1) the WPSH-dipole sea surface temperature (SST) feedback mode in the Northern Indo-western Pacific warm pool associated with the decay of eastern Pacific El Nino/Southern Oscillation (ENSO), (2) the central Pacific-ENSO mode, (3) the Maritime continent SST-Australian High coupled mode, which is sustained by a positive feedback between anomalous Australian high and sea surface temperature anomalies (SSTA) over Indian Ocean, and (4) the ENSO developing mode. Based on understanding of the sources of the predictability for each mode, a set of physics-based empirical (P-E) models is established for prediction of the first four leading principal components (PCs). All predictors are selected from either persistent atmospheric lower boundary anomalies from March to June or the tendency from spring to early summer. We show that these four modes can be predicted reasonably well by the P-E models, thus they are identified as the predictable modes. Using the predicted PCs and the corresponding observed spatial patterns, we have made a 35-year cross-validated hindcast, setting up a bench mark for dynamic models’ predictions. The P-E hindcast prediction skill represented by domain-averaged temporal correlation coefficient is 0.44, which is twice higher than the skill of the current dynamical hindcast, suggesting that the dynamical models have large rooms to improve. The maximum potential attainable prediction skills for the peak summer SEA rainfall is also estimated and discussed by using the PMA. High predictability regions are found over several climatological rainfall centers like Indo-China peninsula, southern coast of China, southeastern SCS, and Philippine Sea.

Climate-smart tank irrigation: A multi-year analysis of improved conjunctive water use under high rainfall variability

Although water harvesting is receiving renewed attention as a strategy to cope with increasing seasonal and inter-annual rainfall variability, many centuries-old local water-harvesting reservoirs (tanks) in India are rapidly deteriorating. Easy access to groundwater is seen as one of the major threats to their maintenance and functioning. Potentially, however, conjunctive use of water from rain, tanks and groundwater reserves, supported by proper monitoring, could improve the resilience and productivity of traditional tank irrigation systems. To date, few quantitative multi-annual analyses of such climate-smart systems have been published. To redress this, we assess the sustainability of a rehabilitated tank irrigation system, by monitoring all inputs and outputs over a period of six years (12 cropping seasons). Our results show that during the period considered, improved conjunctive use resulted in a more stable cropping intensity, increased economic water productivity and higher net agricultural income. Groundwater tables were not negatively affected. We argue that improved conjunctive use can considerably reduce the vulnerability of tank irrigation to rainfall variability and thus is a valuable strategy in light of future climate change.

Rainfall and climate variability: long-term trends in the Metropolitan Area of São Paulo in the 20th century

We investigated spatialtemporal variability and long-term trends of rainfall over the Metropolitan Area of Sao Paulo (MASP). Due to a lack of continuous and consistent rainfall obser- vations in the MASP, a database for the spatial analysis was created from a composite of 94 sta- tions, with daily rainfall data for 1973�1997, plus data from the University of Sao Paulo's Institute of Geophysics and Astronomy (IAG/USP) station for 1933�2011, as the base of the long-term vari- ability analysis. It is shown that the seasonal and interannual rainfall variability result from under- lying local influences and remote large-scale atmospheric dynamics. These effects appear to be more complex during the austral spring and fall seasons, and seem to determine the duration of the rainy period over the MASP. The intraseasonal (30�60 d) oscillation of rainfall does not exhibit any long-term modulation; interannual rainfall variability for 1975�1990 appears modulated by the El NinoSouthern Oscillation (ENSO) scale (2�8 yr). Nonetheless, there exists statistical evi- dence that the rainfall climate over the MASP had an abrupt change at the end of the 1950s, but the global climatic change that occurred in the middle of the 1970s is not evident. The progressive increase in the frequency of rainy days and total daily throughout the period of analysis are promi- nent factors in the trend of annual and seasonal rainfall. Furthermore, rainfall variability over the MASP may be modulated by the Southern Annular Mode (SAM), Pacific Decadal Oscillation (PDO) and ENSO at seasonal scales, altering the normal progression of the seasonal rainfall cycle, while at interannual scales, each one of these patterns drives the rainfall variability, mainly when each pattern reaches its extreme value.

Rainfall conditions and rainwater harvesting potential in the urban area of Khartoum

Runoff water management is among the inherent challenges which face the sustainability of the development of arid urban centers. These areas are particularly at risk from flooding due to rainfall concentration in few heavy showers. On the other hand, they are susceptible to drought. The capital of Sudan (Khartoum) stands as exemplary for these issues. Hence, this research study aims at investigating the potential of applying rainwater harvesting (RWH) in Khartoum City Center as a potential urban runoff management tool. Rapid urbanization coupled with the extension of impervious surfaces has intensified the heat island in Khartoum. Consequently, increased frequency of heat waves and dust storms during the dry summer and streets flooding during the rainy season have led to environmental, economical, and health problems. The study starts with exposing the rainfall behavior in Khartoum by investigating rainfall variability, number of raindays, distribution of rain over the season, probability of daily rainfall, maximum daily rainfall and deficit/surplus of rain through time. The daily rainfall data show that very strong falls of >30mm occur almost once every wet season. Decreased intra- and inter-annual rainfall surpluses as well as increased rainfall concentration in the month of August have been taking place. The 30-year rainfall variability is calculated at decade interval since 1941. Increasing variability is revealed with 1981–2010 having coefficients of variation of 66.6% for the annual values and 108.8–118.0% for the wettest months (July–September). Under the aforementioned rainfall conditions, this paper then explores the potential of RWH in Khartoum City Center as an option for storm water management since the drainage system covers only 40% of the study area. The potential runoff from the 6.5km2 center area is computed using the United States Natural Resources Conservation Services method (US-NRCS), where a weighted Curve Number (CN) of 94% is found, confirming dominant imperviousness. Rainfall threshold for runoff generation is found to be 3.3mm. A 24,000m3 runoff generated from a 13.1mm rainfall (with 80% probability and one year return period) equals the drainage system capacity. An extreme rainfall of 30mm produces a runoff equivalent to fourfold the drainage capacity. It is suggested that the former and latter volumes mentioned above could be harvested by applying the rational method from 18% and 80% rooftops of the commercial and business district area, respectively. Based on the above results, six potential sites can be chosen for RWH with a total roof catchment area of 39,558m2 and potential rooftop RWH per unit area of 0.033m3. These results reflect the RWH potential for effective urban runoff management and better water resources utilization. RWH would provide an alternative source of water to tackle the drought phenomenon.

Climatic variations in Central America: further exploration

ABSTRACTResuming earlier work and based on recently available data sources, this study explores interannual rainfall variability in Central America and underlying circulation mechanisms during 1958–2012. With the changes of circulation in the annual cycle, precipitation increases into May–June, become scarcer in July–August (JA), yet most abundant in September–October; the edge of the Atlantic subtropical high extending westward most markedly and the Pacific Intertropical Convergence Zone (ITCZ) retreating southward in JA. Interannual variations of rainfall are accompanied with circulation changes involving vertical motion over Central America. Overall, with weaker Atlantic trade winds southerly wind component over Pacific tends to be stronger and Central America rains more abundant. Regarding long‐term variations from protracted dry regime 1971–1978 to protracted wet regime 2005–2012, on the Atlantic side ocean surface warmed, pressure dropped, trade winds weakened, whereas over the Pacific northward pressure gradient and winds increased. The study builds on work half a century ago and extends analysis into the most recent decades, using recent and updated circulation and raingauge data sets.

Impacts of climate change and variability on cattle production in southern Ethiopia: Perceptions and empirical evidence

Climate change and variability can severely constrain the productivity of pastoral herds by reducing water availability, forage production and quality, and hence the carrying capacity of rangelands. In particular, the risk of heavy livestock losses suffered during recurrent severe droughts associated with climate change and variability presents one of the most serious threats to pastoral livestock keepers. To generate insights into how climate change and variability adversely affect cattle production in the Borana of southern Ethiopia, we analyzed perceptions of herders and long-term changes in cattle numbers and climate data. A total of 242 households were surveyed to generate data on perceived trends in climate, rangeland condition and livestock production. Socio-demographic characteristics of households and cattle mortality due to the 2010/2011 drought were also recorded. Using a local time calendar, cattle herd history was reconstructed for a period spanning five major droughts to portray the linkage between changes in cattle numbers and changes in rainfall and temperature. Most of the herders perceived that rainfall has become more unpredictable, less in amount and shorter in duration, while drought recurrence and temperature have increased. Similarly, the majority perceived a decreasing trend in cattle herd sizes and their production performances. The 2010/2011 drought was associated with a substantial decline in cattle herd sizes due to increased mortality (26%) and forced off-take (19%). Death occurrences and mortality rates varied significantly by district, herd size and feed supplementation. Spectral density analysis revealed a quasi-periodic pattern in the annual rainfall with an approximate cycle period of 8.4years, suggesting that droughts recur approximately every 8.4years. A downward trend in cattle population mirrored a similar underlying trend in the interannual rainfall variation. Accordingly, changes in cattle number were significantly linked with changes in rainfall. In conclusion, perceptions corroborated by empirical evidences showed that climate change and variability were associated with declining cattle numbers, portending a precarious future to the sustainability of cattle pastoralism in southern Ethiopia and other pastoral systems.

China's water sustainability in the 21st century: a climate-informed water risk assessment covering multi-sector water demands

Abstract. China is facing a water resources crisis with growing concerns as to the reliable supply of water for agricultural, industrial and domestic needs. High inter-annual rainfall variability and increasing consumptive use across the country exacerbates the situation further and is a constraint on future development. For water sustainability, it is necessary to examine the differences in water demand and supply and their spatio-temporal distribution in order to quantify the dimensions of the water risk. Here, a detailed quantitative assessment of water risk as measured by the spatial distribution of cumulated deficits for China is presented. Considering daily precipitation and temperature variability over fifty years and the current water demands, risk measures are developed to inform county level water deficits that account for both within-year and across-year variations in climate. We choose political rather than watershed boundaries since economic activity and water use are organized by county and the political process is best informed through that unit. As expected, the risk measures highlight North China Plain counties as highly water stressed. Regions with high water stress have high inter-annual variability in rainfall and now have depleted groundwater aquifers. The stress components due to agricultural, industrial and domestic water demands are illustrated separately to assess the vulnerability of particular sectors within the country to provide a basis for targeted policy analysis for reducing water stress.

Seasonal and interannual variation in vegetation composition: Implications for survey design and data interpretation

AbstractUnderstanding how vegetation composition varies with season and interannual climate variability is important for any ecological research that uses vegetation data derived from surveys for the basis of inference. Misunderstanding this variation can influence land management and planning decisions, leading to poor implementation of biodiversity offsetting mechanisms, for example. We monitored plots (400 m2) grazed by livestock paired with adjacent ungrazed plots in derived native pastures four times a year over 2.5 years on the North‐West Slopes of New South Wales. Species density in plots varied greatly with season and interannual rainfall. Highest species density was recorded in spring, though species density in summer was not significantly lower, nor was a spring–summer peak in species density evident in the 2009 drought. Surveys in spring 2008 had the highest species density, and recorded only 60–72% of the total species recorded at each site over 2.5 years. Variation in the proportion of total site diversity represented in combinations of two or three surveys was large, though the best combinations comprised surveys from spring and summer in years of above‐average rainfall, either from the same spring‐summer, or from different years. Compositional differences among sites were much greater than within sites, showing that differences among sites related to broad environmental gradients were not overwhelmed by seasonal and interannual variability in site composition. When grazing was excluded, there was no evidence of competitive exclusion by the dominant grasses, and no directional shift in composition. The implications of these findings for ecological research depend on the question being addressed: if capturing a large proportion of site diversity is important, then surveys must be carefully timed, or repeat surveys must be conducted. Single surveys did not effectively capture site diversity for use in biodiversity offsetting, and the timing of repeat surveys was critical.

Questioning the impact and sustainability of percolation tanks as aquifer recharge structures in semi-arid crystalline context

Since the beginning of the green revolution in the 1970s, large areas of India have been strongly dependent on groundwater for irrigation which has led to aquifer overexploitation in various parts of the country. To tackle this problem, Indian authorities rely on managed aquifer recharge (MAR). Perennial water resources in semi-arid regions of South India consist mostly of crystalline aquifers. Despite limited and only local storage capacity and high potential evaporation loss, MAR is promoted through percolation tanks. To assess the efficiency of MAR, a monitoring program and a water balance were developed for a percolation tank in Andhra Pradesh, India. The developed water balance shows that the tank has a limited local impact and only a few events contribute to tank replenishment. Enhanced infiltration (10,100 m3) is of the same order of magnitude as evaporation (8,000 m3). Potential benefits of the tank system depend on the observation scale and rainfall conditions. Generally, tanks have a local positive impact in their immediate surroundings. However, in the case of low rainfall, if there is no stream water flowing out of the watershed, the tank merely redistributes the water within the watershed. Inversely, during years with high precipitation, there may be a net benefit in terms of underground storage. Regarding the hydrological impact assessment of percolation tanks, it is necessary to take into account (1) the net “nonmanaged” water balance at watershed scale compared to “managed” water balance, (2) the inter-annual rainfall variations and (3) the impact for downstream users.

Spatial variability and mechanisms underlying El Niño-induced droughts in Mexico

The El Nino Southern Oscillation plays a key role in modulating interannual rainfall variability in Mexico. While El Nino events are linked to drought in Mexico, uncertainty exists about the spatial pattern and causal mechanisms behind El Nino-induced drought. We use lead/lag correlation analysis of rainfall station data to identify the spatial pattern of drought associated with the summer before, and the spring following, the peak of warm SST anomalies in the eastern equatorial Pacific. We also use atmospheric fields from the North American Regional Reanalysis to calculate the anomalous moisture budget and diagnose the mechanisms associated with El Nino-induced drought in Mexico. We find that reduced rainfall occurs in Mexico in both the summer before and the spring after a peak El Nino event, especially in regions of climatologically strong convection. The teleconnection in the developing phase of El Nino is primarily driven by changes in subsidence resulting from anomalous convection in the equatorial Pacific. The causes of drought during the decaying phase of El Nino events are varied: in some years, descent anomalies dominate other moisture budget terms, while in other years, drying of the boundary layer on the Mexican plateau is important. We suggest that the latter may result from the interaction of weakened southeasterly winds in the Intra-Americas Sea with high topography along the Atlantic coast of Mexico. Weakened winds are likely driven by a reduced sea level pressure gradient between the Atlantic and the Pacific. Changes in easterly wave activity may contribute to drought in the developing phase of El Nino, but may be less important in the decaying phase of El Nino.

Caractérisation de la Variabilité Climatique : Cas du Bassin Versant de La Medjerda ( Nord-Est Algérien ) = Characterization of Climate Variability : Case of Watershed Medjerda ( North East of Algeria )

جفاف | تغير المناخ | تجزئة هطول الأمطار | حوض مجردة | شرق الجزائر |Sécheresse | Variabilité Climatique | Fraction Pluviométrique | Bassin Versant Medjerda | Est de L'Algérie | Drought | Climate Variability | Rainfall Fraction | Watershed Medjerda | East of Algeria

Sensitivity of the agroecosystem in the Ganges basin to inter-annual rainfall variability and associated changes in land use

The rate of growth in agricultural production has been decreasing in several regions of the world in recent years. The availability of water, which is one of the main inputs, is becoming limiting and more variable. In this article, we study the sensitivity of the agroecosystem to rainfall variability in order to identify vulnerable areas. We applied a longitudinal assessment of remote sensing time-series data, using the correlation between inter-annual rainfall anomalies and anomalies in Normalized Difference Vegetation Index (NDVI), a proxy for crop production. With a novel approach, we then identified whether the sensitivity results from a variation in crop growth or from a deliberate adjustment in the cropping pattern, reflecting a coping strategy. In our case study area, the Ganges basin, 25% of the basin area showed a significant correlation (p < 0.10) between rainfall and NDVI anomalies during the summer monsoon-dominated cropping season, both positive and negative. During the consecutive dry season, 18 % of the basin area showed a significant correlation, mostly positive. This variation in sensitivity shows the added value of spatially explicit information from remote sensing over lumped crop statistics. Primarily in the drier western part of the basin, a coping strategy of increasing fallow land in years with below-average rainfall was detected. Distinguishing a coping strategy from a crop yield reduction is important from both an economic and a hydrologic perspective.

The Impact of El Niño/Southern Oscillation on Hydrology and Rice Productivity in the Cauvery Basin, India: Application of the Soil and Water Assessment Tool

This study was performed to further understanding of the variations in hydrology and rice crop productivity during different El Niño/Southern Oscillation (ENSO) events in the Cauvery River Basin of Tamil Nadu, India using the Soil and Water Assessment Tool (SWAT). The entire Cauvery Basin was divided into 301 sub-basins and further subdivided into 3,601 Hydrological Response Units (HRU). Based on the National Oceanic and Atmospheric Administration (NOAA) official website, information on El Niño (1972, 1982, 1987, 1991, 1997, 2002 and 2004) and La Niña (1970, 1971, 1973, 1974, 1975, 1988, 1998, 1999 and 2000) years were obtained. The SWAT model was continuously run from 1970 to 2008, and a composite for El Niño, La Niña and normal years was constructed to understand their influence on hydrology and rice crop productivity in the study area. From the analysis, it was clear that an El Niño episode is correlated with rainfall, hydrology and rice productivity in the Cauvery river basin. The validation of the SWAT model indicated the capability of SWAT to accurately predict stream flow and rice productivity. It was evident from the investigation that the quantum of rainfall was more during El Niño years with high inter-annual rainfall variability (809.3mm to 2,366mm) compared with La Niña and normal years. As a result, the soil water recharge, including percolation and soil water availability in the surface layers, was increased in the El Niño years. Simulated rice productivity over 39 years in the Cauvery Basin ranged between 1,137 and 7,865kgha−1 with a mean productivity of 3,955kgha−1. The coefficient of variation in rice productivity was higher during La Niña (21.4%) years compared with El Niño (14.7%) and normal years (14.6%). The mean rice productivity was increased in both El Niño and normal years, indicating the possibility of higher yields than those in La Niña years. An analysis of the hydrological data and rice productivity showed that the risk of failure was low during El Niño years compared with normal or La Niña years. This behavior could be utilized for forecasting rice crop productivity under different ENSO conditions and can provide information for policy makers when deciding on water allocation and import / export policies.

The effects of short-term rainfall variability on leaf isotopic traits of desert plants in sand-binding ecosystems

Sand-binding vegetation is effective in stabilizing sand dunes and reducing soil erosion, thus helps minimize the detrimental effects of desertification. The aim of this study is to better understand the relationships between water and nutrient usage of sand-binding species, and the effects of succession and rainfall variability on plants’ water–nutrient interactions. We examined the effects of long-term succession (50 years), inter-annual rainfall variability (from 65% of the mean annual precipitation in 2004 to 42% in 2005) and seasonality on water–nutrient interactions of three major sand-binding species (Artemisia ordosica, Hedysarum scoparium and Caragana korshinskii) by measuring foliar δ13C, δ15N and [N]. Long-term succession in general did not significantly alter δ13C, δ15N and [N] of the three species. Short-term rainfall variability, however, significantly increased foliar δ13C levels of all three species by 1.0–1.8‰ during the severely dry year. No significant seasonal patterns were found in foliar δ13C and δ15N values of the three species, whereas foliar [N] varied by season. For the two leguminous shrubs, the correlations between δ13C and δ15N were positive in both sampling years, and the positive correlation between [N] and δ13C was only found in the severely dry year. The results indicate that these sand-binding plants have developed into a relatively stable stage and they are able to regulate their nitrogen and water use in responding to environmental conditions, which reinforces the effectiveness of plantation of native shrubs without irrigation in degraded areas. However, the results also indicate that short-term climate variability could have severe impact on the vegetation functions.

Ecosystem Consequences of Tree Monodominance for Nitrogen Cycling in Lowland Tropical Forest

Understanding how plant functional traits shape nutrient limitation and cycling on land is a major challenge in ecology. This is especially true for lowland forest ecosystems of the tropics which can be taxonomically and functionally diverse and rich in bioavailable nitrogen (N). In many tropical regions, however, diverse forests occur side-by-side with monodominant forest (one species >60% of canopy); the long-term biogeochemical consequences of tree monodominance are unclear. Particularly uncertain is whether the monodominant plant-soil system modifies nutrient balance at the ecosystem level. Here, we use chemical and stable isotope techniques to examine N cycling in old-growth Mora excelsa and diverse watershed rainforests on the island of Trinidad. Across 26 small watershed forests and 4 years, we show that Mora monodominance reduces bioavailable nitrate in the plant-soil system to exceedingly low levels which, in turn, results in small hydrologic and gaseous N losses at the watershed-level relative to adjacent N-rich diverse forests. Bioavailable N in soils and streams remained low and remarkably stable through time in Mora forests; N levels in diverse forests, on the other hand, showed high sensitivity to seasonal and inter-annual rainfall variation. Total mineral N losses from diverse forests exceeded inputs from atmospheric deposition, consistent with N saturation, while losses from Mora forests did not, suggesting N limitation. Our measures suggest that this difference cannot be explained by environmental factors but instead by low internal production and efficient retention of bioavailable N in the Mora plant-soil system. These results demonstrate ecosystem-level consequences of a tree species on the N cycle opposite to cases where trees enhance ecosystem N supply via N2 fixation and suggest that, over time, Mora monodominance may generate progressive N draw-down in the plant-soil system.