Frequency Analysis Of Drought And Its Impact On Terrestrial Ecosystem Water Use Efficiency

Drought is one of the most complicated natural hazards that affect the function,structure,and composition of terrestrial ecosystems.As an intermittent disturbance of the water cycle,drought has adverse effects on the carbon cycle of terrestrial ecosystems such as limiting vegetation growth,causing vegetation mortality,and triggering wildfire.Drought has been projected to be more frequent and severe under the background of global climate warming,which may increase the vulnerability of ecosystems.Therefore,a deep understanding of how drought influences the carbon and water balances of terrestrial ecosystems is essential to ecosystem management for mitigating drought risk.On the basis of reanalysis and remote sensing datasets,this research focus on the terrestrial ecosystem vegetation,the global drought frequency analysis was conducted,the spatiotemporal variation of ecosystem water use efficiency was performed,and the impact of drought on ecosystem water use efficiency was quantified.Firstly,drought is described by the standardized soil moisture index(SSMI),and drought variables(i.e.,duration,severity,and peak)are extracted using the run theory,the multivariate joint distributions of drought variables are established by the nonparametric Kernel Density Estimation(KDE)method,further the joint probabilities and return periods of drought variables are calculated.Secondly,from the perspective of climate regions and land cover types,the annual and monthly variations of Gross Primary Production(GPP),Evapotranspiration(ET),and Water Use Efficiency(WUE)are analyzed,the spatial distribution of GPP,ET,and WUE are explored,the trend of mean annual GPP,ET,and WUE are estimated by the Sen’s slope,the significance of the trend is tested through the Modified Mann-Kendall(MMK)method.Finally,the lagged effect of drought on WUE are quantified by which lagged months of SSMI that has the maximum significant correlation with WUE,the relative change of mean value of WUE between drought and non-drought periods is computed to quantify the impact of drought on WUE,the sensitivity of GPP and ET to drought are examined by simple linear regression.The main conclusions are as follows:(1)Drought risk was higher in West North America,North-East Brazil,Southeastern South America,Central Asia,and Tibetan Plateau.At the global scale,North Australia experienced fewer drought events(33 times),whereas drought events were relatively higher in Alaska/N.W.Canada,North Europe,and North Asia(70 times).Drought duration,severity,and peak across all of the climate regions were in the range of 1.94-5.18(months),0.92-2.81,and 0.49-0.72,respectively.The dependence between drought variables was strong,with an average correlation coefficient all over 0.73,and therefore appropriate to construct joint distribution function.KDE can establish reliable joint distributions of drought variables after passing Kolmogorov-Smirnov(K-S)and AndersonDarling(A-D)tests at the 5% significance level,with root-mean-square error being in the range of 0.002-0.217.Besides,it could be noted that the univariate return period was longer than the multivariate return period for “or” case,but was shorter than that for “and” case.In addition,the joint return period of “or” case was always less than that of “and” case,namely drought risk will be overestimated if just the joint return period of “or” case is used.On the other hand,drought risk will be underestimated if only the joint return period of the “and” case is considered.Drought risk was lower in North Europe and North Asia,while West North America,North-East Brazil,Southeastern South America,Central Asia,and Tibetan Plateau experienced higher drought risks.(2)The GPP,ET,and WUE of terrestrial ecosystem mainly showed upward trend during1982-2018.On average,the annual GPP,ET,and WUE of terrestrial ecosystem were 1027.74 g C m-2,705.11 mm,and 1.05 g C kg-1 H2 O,respectively.Monthly GPP,ET,and WUE presented nshaped curve in the middle and high latitudes such as East North America,North Europe,and Central Europe,while they showed w-shaped curve in the low latitudes such as Central America/Mexico,Amazon,and Southeast Asia;GPP,ET,and WUE usually peak between July and August.Spatially,GPP,ET,and WUE decreased poleward from the equator.Among the climate regions,GPP and ET reached maximum in Amazon,with the values of 2765.38±636.88 g C m-2 and 1603.04±272.16 mm,respectively,whereas Southeast Asia had the largest annual WUE of 1.67 ± 0.29 g C kg-1 H2 O.Among all land cover types,forest ecosystems had the highest levels of annual GPP and WUE,reaching 1405.42±916.08 g C m-2 and 1.21±0.47 g C kg-1 H2 O,respectively,while ET achieved maximum value of 836.31±416.03 mm in mixed vegetation.The MMK trend test results showed that,46.74%,56.32%,and 35.95% of the vegetated areas showed significant increasing trend of GPP,ET,and WUE,while 6.67%,5.20%,and 6.85% of the vegetated areas showed significant decreasing trend of GPP,ET,and WUE.Compared with other climate regions,GPP increased most obviously in North Europe,which accounted for 95.22% of the vegetated areas;while ET and WUE increased most clearly in South Europe/Mediterranean,which covered 91.40% and 72.88% of the vegetated areas.(3)When drought occurred,WUE decreased in 67.30% of the terrestrial vegetated areas,while it increased across 29.59% of the terrestrial vegetated areas.Drought had lagged effect on WUE in 70.87% of the global vegetated areas,and the mean value of lagged months was 4.12.Regionally,the lagged effect of drought on WUE was 0-4months for North-East Brazil,Southern Africa,and North Australia,and 5-12 months for East North America,Central Europe,and West Asia.At the vegetation level,drought usually had no and short-term lagged effect on WUE of shrubland and sparse vegetation,while the lagged effect of drought on forest WUE was mainly at middle and long periods.67.30% of the terrestrial vegetated areas showed a positive relationship between SSMI and WUE,while29.59% of the terrestrial vegetated areas showed a negative relationship between SSMI and WUE,suggesting that drought mainly had an adverse impact on WUE.Drought reduced WUE by 36.95% in the Tibetan Plateau and 24.93% in West Africa,whereas WUE in North Europe,Alaska/N.W.Canada,and West Asia increased by 14.64%,8.83%,and 8.53% respectively during drought periods.The sensitivity of GPP to drought was higher in humid climate regions,conversely,ET in semi-humid and semi-arid climate regions was more sensitive to drought.