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Nanjing University (2018)

Changes in Rainfall Seasonality and Their Impacts on Vegetation Across Tropical Savannas Based on Multi-Source Data

张文敏;

Titre : Changes in Rainfall Seasonality and Their Impacts on Vegetation Across Tropical Savannas Based on Multi-Source Data

Auteur : 张文敏;

Grade : Doctoral Dissertation 2018

Université : Nanjing University

Résumé
Global warming is expected to cause climate change with a higher frequency of extreme rainfall events.More extreme events may affect the water balance and primary productivity,ultimately threatening food security and people’s general wellbeing.Therefore,knowledge on changes in rainfall seasonality and how such changes affect vegetation productivity and ecosystem functioning are important for understanding vegetation phenology and guiding agricultural production.Moreover,it also contributes to the understanding of global warming effect on rainfall regimes and our biosphere.Here,our study applied daily satellite based rainfall estimates(TRMM,CHIRPS,GPCC and MSWEP),monthly rainfall data sets(CRU TS4)and Normalized difference vegetation indices(GIMMS 3g-vl and MODIS NDVI),leaf area index(GIMMS 3g LAI)and vegetation depth optical(VOD)to investigate:1,changing characteristics of rainfall seasonality ;2,impacts of changed rainfall regimes(e.g.onset of wet season,rainfall intensity)on vegetation productivity and season NDVI ;3,increased heavy rainfall frequency impact on ecosystem structure(by altering the herbaceous/woody vegetation composition).The main results of this study were shown as follows :(1)In this study,the combined use of a generalized additive model(GAM)(to fit a model describing changes of cumulative rainfall anomalies on a daily scale)and the Akaike information criterion(AIC)(generally used to assess the goodness of fit)was found to succeed in capturing global wet season domains.Areas of distinct rainfall seasonality are observed to be mainly located in the Amazon rainforest,northern Australia,southern Asia,South Africa and Sahel.Seasonal rainfall amount accounted for 82 ± 10(%)of the annual rainfall over wet season domains.The seasonality of rainfall and the length of wet season showed decreasing tendencies moving from the equator towards higher/lower latitudes,and the onset and cessation of wet season showed opposite patterns in the northern and southern hemispheres.We then found that largely consistent results of inter-annual variability in seasonal rainfall metrics(seasonal rainfall,onset,cessation and length of wet season)can be derived for different rainfall products for different monsoon regions(Brazil,Sahel,South Africa and India).Only exception is for GPCC rainfall of which moderate differences in seasonal rainfall metrics were observed in Sahel and southern Africa when compared to the other three rainfall products(TRMM,CHIRPS and MSWEP).The significant correlation(p<0.05)between the onset,cessation and length of wet season and seasonal rainfall implied that the changed seasonal rainfall metrics can contribute to understanding the change of rainfall.Specifically,the correlation between seasonal rainfall and the length of wet season is the strongest with r =0.58,while the onset of wet season is negatively related to seasonal rainfall with r =-0.45.(2)This study tested the importance of rainfall metrics in the wet season(onset and cessation of the wet season,number of rainy days,rainfall intensity,number of consecutive dry days and heavy rainfall events)on growing season ANPP.We focused on the Sahel and north-Sudanian region(100-800 mm yr-1)and applied daily satellite based rainfall estimates(CHIRPS v2.0)and growing season integrated NDVI(MODIS)as a proxy for ANPP over the study period 2001—2015.Growing season ANPP in the arid zone(100—300 mm yr-1)was found to be rather insensitive to variations in the seasonal rainfall metrics,whereas vegetation in the semi-arid zone(300—700 mm yr-1)was significantly impacted by most metrics,especially by the number of rainy days and timing(onset and cessation)of the wet season.We analyzed critical breakpoints for all metrics to test if vegetation response to changes in a given rainfall metric surpasses a threshold beyond which vegetation functioning is significantly altered.It was shown that growing season ANPP was particularly negatively impacted after>14 consecutive dry days and that a rainfall intensity of~13 mm day-1 was detected for optimum growing season ANPP.(3)This study analyze impacts of seasonal rainfall metrics(onset,cessation,length of wet season and seasonal rainfall)on season NDVI across tropical savannas for areas of rainfall showing a well-defined annual cycle.The study makes use of the daily of rainfall estimates(CHIRPS v2)and the average NDVI(GIMMS 3g-vl)of the growing season covering the period 1982 to 2015.We first used the timing of wet season to determine the growing season and the period between onset and cessation was used to calculate the growing season NDVI at the per-pixel level.Seasonal rainfall amount was found to be significantly positive correlated with NDVI for largest areas across tropical drylands,closely followed by sizable areas characterized by a significant correlation between NDVI and length and onset of wet season

Mots clés : Global climate change; Rainfall seasonality; Aboveground net primary productivity; Rainy season areas; Tropical savannas;

Présentation (CNKI)

Page publiée le 11 avril 2019