基于遥感和Penman-Monteith模型的内陆河流域不同生态系统蒸散发估算

遥感数据具有很好的时空连续性,它是区域蒸散发通量估算的有效方法。引入了一个简单的具有生物物理基础的Penman-Monteith(P-M)模型,分别利用黑河流域高寒草地阿柔站和干旱区农田盈科站2008—2009年的气象数据和MODIS(Moderate Resolution Imaging Spectroradiometer)叶面积指数(LAI),实现了2008—2009年日蒸散发的估算,并同时实现了对植被蒸腾和土壤蒸发的分别估算。结果表明,利用P-M公式模拟的蒸散发与实测的蒸散发具有较好的一致性,日蒸散发模拟的决定系数(R2)超过0.8。估算的高寒草甸和干旱区农田玉米全年平均的蒸腾分别为0.78 mm/d和1.20 mm/d,分别占总蒸散发的60%和61%,土壤蒸发分别为0.53和0.77 mm/d,占总蒸发的40%和39%。可见两种生态系统的作物蒸腾均强于土壤蒸发,同时农田玉米蒸腾强于高寒草甸蒸腾。研究结果证明了基于遥感的P-M公式可以很好地实现对高寒草地和干旱区农田生态系统蒸散发的估算。通过考虑土壤水分变化对气孔导度的影响,可以提高模型对农田蒸散发的模拟精度。     

三江源区退化高寒草甸蒸散特征及冻融变化对其的影响

蒸散(ET)是陆地生态系统水分收支的重要分量。为探究三江源区退化高寒草甸的蒸散特征,基于2016和2017年涡度相关和微气象系统的观测数据,定量研究了其生态系统的蒸散变化及其环境和生物因子的影响。为深入探讨不同时段的蒸散变化,根据土壤冻融状态和植被生长状况进一步将年蒸散划分为3个时段:冻结期、冻融交替期和消融期,其中在消融期中又划分出植物生长季(5—9月),并探讨了土壤冻融对年蒸散量的影响。结果表明:研究区2016和2017年的降水量分别为451.8 mm和442.3 mm,但2017年ET为485.6 mm,明显高于2016年的428.6 mm,两年ET的季节变化趋势相同,ET的最高值出现在生长旺季的7—8月,最低值出现在12月或1月,生长季ET分别占全年ET的73%和72%。2017年的冻结期和冻融交替期比2016年分别减少了8 d,2017年消融期的蒸散量比2016年增加了63.1 mm,其中生长季的蒸散量多36.3 mm。2016和2017年消融期的日蒸散速率分别为1.81 mm/d和1.97 mm/d,其中生长季为2.05 mm/d和2.29 mm/d,冻融交替期分别为0.97 mm/d和0.73 mm/d,而冻结期最低,分别为0.27 mm/d和0.33 mm/d。逐步回归分析结果表明:2016年净辐射(R_n)对ET的影响最大,其次是气温(T_a)和土壤含水量(SWC_5);2017年ET主要受R_n和T_a的影响。生长季和消融期的冠层导度(g_c)和解耦系数(Ω)明显高于其他两个时段,且2017年g_c和Ω值均高于2016年同期。本研究说明,由于辐射、温度等引起的冻融时间变化和植被的年际间差异,导致三江源区退化草甸各时段及年蒸散量出现明显的变化,该研究结果为全面探讨三江源区蒸散特征提供了参考。     

基于改进的双源模型模拟荒漠河岸胡杨林蒸散发

蒸散发是水循环和能量平衡过程中的重要组成部分,其准确量化对于深刻揭示干旱半干旱地区的生态水文过程具有重要意义。以黑河下游荒漠河岸胡杨林为研究对象,在2014和2015年胡杨主要生长季内,基于涡度相关技术实测数据,分析了蒸散发日及各物候期变化规律,结合改进的双源Penman-Monteith-Priestley-Taylor(PM-PT)模型,模拟了黑河下游荒漠河岸胡杨林蒸散发,并分析了模型的参数敏感性,得到的主要结论如下:(1)胡杨林蒸散发日变化大致呈先升高后降低的趋势。上午随着太阳辐射的逐渐增强,气温逐渐升高,蒸散速率逐渐增大,在中午12:00左右达到峰值。随后,太阳辐射减弱,气温逐渐降低,空气中相对湿度增加,胡杨叶片内外水汽压差减小,蒸散速率随之降低。(2)胡杨生长季内蒸散发整体上呈先升高后降低的趋势。2014和2015年生长季蒸散发总量分别为612 mm和658 mm,果期和种子散播期累积蒸散发为生长季内蒸散发总量的主体部分,果期内累积蒸散发分别为316 mm和348 mm,分别占各年生长季蒸散发总量的51.65%和52.87%;种子散播期内平均蒸散发略低于果期,2014和2015年胡杨林种子散播期内累积蒸散发分别为261 mm和271 mm,分别占各年生长季蒸散发总量的42.71%和41.12%,展叶期和叶变色期内平均蒸散发最低,原因在于展叶期胡杨叶片尚未完全成形,而叶变色期叶片活性逐渐降低。(3)改进的双源PM-PT模型与传统的双源Shuttleworth-Wallace(SW)模型相比,在模型结构与参数数量方面均得到了优化,其模拟精度也更高。(4)改进的双源PM-PT模型对净辐射最为敏感。     

基于茎流-蒸渗仪法的荒漠草原带人工灌丛群落蒸散特征

中国西北地区通过大量种植中间锦鸡儿(Caragana liouana)进行生态治理, 在荒漠草原带上形成人工灌丛景观, 改变了生态系统的结构和功能, 影响到地-气水汽循环过程, 研究该人工灌丛群落的蒸散特征, 对揭示其生态水文效应和指导地方生态治理实践具有重要意义。该文以宁夏盐池荒漠草原带上的人工灌丛群落为例, 利用茎流-蒸渗仪法测定了2018年5-8月的灌木蒸腾和丛下蒸散, 并分析了环境因子对人工灌丛群落蒸散的影响。结果表明: (1)茎流-蒸渗仪法所测的群落蒸散与水量平衡法、涡度相关法得到的群落蒸散有较好的一致性, 茎流-蒸渗仪法能适用于荒漠草原带人工灌丛群落蒸散及其组分结构的测定; (2)观测期内晴天的灌木蒸腾速率和丛下蒸散速率日变化趋势相近, 均为单峰曲线, 群落蒸散主要发生在日间, 但灌丛最大蒸腾速率的出现时间比丛下蒸散最大速率的出现时间晚1 h; (3) 5-8月间灌木累积蒸腾为83.6 mm, 日平均蒸腾量为0.7 mm·d^-1, 季节变化呈抛物线状; 同期丛下累积蒸散为182.5 mm, 日平均蒸散量为1.5 mm·d^-1; 丛下蒸散明显大于灌木蒸腾; (4)观测期间人工灌丛群落累积蒸散266.1 mm, 而同期的降水量为222.6 mm, 陆面水分收支处于亏缺状态; (5)净辐射是影响蒸散最主要、最直接的驱动因素, 且能够影响其他因子进而对人工灌丛群落蒸散产生作用。综上, 人工灌丛引发荒漠草原地带陆面水分收支亏缺的现象, 在生态恢复与重建中须引起注意。     

Characteristics of evapotranspiration in planted shrub communities in desert steppe zone based on sap flow and lysimeter methods

中国西北地区通过大量种植中间锦鸡儿(Caragana liouana)进行生态治理, 在荒漠草原带上形成人工灌丛景观, 改变了生态系统的结构和功能, 影响到地-气水汽循环过程, 研究该人工灌丛群落的蒸散特征, 对揭示其生态水文效应和指导地方生态治理实践具有重要意义。该文以宁夏盐池荒漠草原带上的人工灌丛群落为例, 利用茎流-蒸渗仪法测定了2018年5-8月的灌木蒸腾和丛下蒸散, 并分析了环境因子对人工灌丛群落蒸散的影响。结果表明: (1)茎流-蒸渗仪法所测的群落蒸散与水量平衡法、涡度相关法得到的群落蒸散有较好的一致性, 茎流-蒸渗仪法能适用于荒漠草原带人工灌丛群落蒸散及其组分结构的测定; (2)观测期内晴天的灌木蒸腾速率和丛下蒸散速率日变化趋势相近, 均为单峰曲线, 群落蒸散主要发生在日间, 但灌丛最大蒸腾速率的出现时间比丛下蒸散最大速率的出现时间晚1 h; (3) 5-8月间灌木累积蒸腾为83.6 mm, 日平均蒸腾量为0.7 mm·d^-1, 季节变化呈抛物线状; 同期丛下累积蒸散为182.5 mm, 日平均蒸散量为1.5 mm·d^-1; 丛下蒸散明显大于灌木蒸腾; (4)观测期间人工灌丛群落累积蒸散266.1 mm, 而同期的降水量为222.6 mm, 陆面水分收支处于亏缺状态; (5)净辐射是影响蒸散最主要、最直接的驱动因素, 且能够影响其他因子进而对人工灌丛群落蒸散产生作用。综上, 人工灌丛引发荒漠草原地带陆面水分收支亏缺的现象, 在生态恢复与重建中须引起注意。     

灌丛化的蒸散耗水效应数值模拟研究——以内蒙古灌丛化草原为例

灌丛化是干旱半干旱草原一种常见的全球性变化现象,由于野外土壤、灌丛和草本的蒸散耗水难于拆分的限制,关于灌丛化蒸散耗水效应的研究较少。该文将已有的二源模型应用于我国内蒙古灌丛化草原估算其蒸散发,并用波文比系统观测结果对模型进行了率定。研究结果表明改进的模型可以较好地重建灌丛化草地的蒸散发特征;敏感性分析结果表明模型输入变量及参数对蒸散发组分拆分结果产生的误差较小。在此基础上进行了灌丛化的情景模拟,研究其耗水效应。结果表明:灌丛化对蒸散发总量影响较小,而对蒸散发组分影响较大。灌丛化初期盖度5%、中期盖度15%及后期盖度为30%的情境下,对应的生长季内蒸散发(ET)平均值分别为182.97、180.38和176.72 W·m~(–2);土壤蒸发(E)占蒸散发比率(E/ET)平均值分别为52.9%、53.9%和55.5%。灌丛化从初期到中期、中期至后期,蒸散发降幅平均值分别为0.34%和0.44%,E/ET升幅分别达2.04%及3.25%。该研究结果表明在内蒙古太仆寺旗站点灌丛化导致的土壤水分差异并不明显,但随着灌丛化加剧,灌丛逐渐替代草本,改变了原有的生态系统结构,植被叶面积指数变小,导致冠层导度降低。研究结果强调我国半干旱草原区灌丛化加剧对生态系统总蒸散耗水量影响不大,但其土壤蒸发无效损耗快速增加会导致系统水分利用效率降低。     

蒸渗仪法和涡度相关法测定蒸散的比较

为准确揭示三江源区退化高寒草甸生态系统的水分收支状况,本文利用涡度相关系统(EC)和蒸渗仪对退化高寒草甸的蒸散进行了连续观测,结果表明:两种方法测定的蒸散量年变化趋势一致,涡度相关法测定的年蒸散量为481 mm(约占降水量的86%),低于蒸渗仪法的558 mm。涡度相关法的湍流能量与有效能的闭合度(EBR)年均值约为0.78,生长季的EBR要高于非生长季;当摩擦风速(u*)0.25 m·s~(-1)时,EBR随u*的增大而增加,当u*0.25 m·s~(-1)时,EBR随u*的增大无明显变化。此外,土壤热通量相对于太阳净辐射的滞后可能也是导致能量不闭合的原因之一。随着净辐射、温度和风速的升高,涡度相关法测定的蒸散逐渐接近于蒸渗仪法的测定值,而土壤含水量对两种方法观测蒸散的影响不明显。本研究结果说明,涡度相关和蒸渗仪法测定的蒸散具有较好的相关性,但涡度相关法可能会低估该生态系统的蒸散量。     

基于Shuttleworth-Wallace模型的科尔沁沙地流动半流动沙丘蒸散发模拟

陆面蒸散发在气候调节和维持区域水量平衡中起关键作用.量化蒸散发及其各组分项,对深刻揭示干旱半干旱地区的生态水文过程具有重要意义.本研究基于科尔沁沙地流动半流动沙丘2017年生长季气象监测系统的原位监测数据,利用Shuttleworth-Wallace(S-W)模型对沙丘蒸散发进行模拟,在此基础上,对蒸散各组分进行拆分,并利用涡度相关对模拟蒸散发值进行验证.结果表明: 整个生长季模型模拟蒸散发值为308 mm,涡度相关实测值为296 mm,偏差较小,证明S-W模型适用于该地区的蒸散发模拟.蒸散发整体呈生长旺盛期>生长后期>生长初期,分别为192、71和45 mm,分别占总量的62.3%、23.1%和14.6%.日尺度上模型模拟值与实测蒸散发值一致性较高,模型模拟精度大体表现为: 晴天>阴天>雨天,且阴雨天模型模拟值较涡度相关实测值偏低.经拆分,土壤蒸发和植被蒸腾分别为176和132 mm,分别占总量的57.1%和42.9%,表明沙地水分利用效率较低.持续干旱和降水后,蒸散发规律明显不同,且土壤蒸发对降水的敏感性强于植被蒸腾.     

西北黑河中游荒漠绿洲农田作物蒸腾与土壤蒸发区分及作物耗水规律

利用中国生态系统研究网络临泽内陆河流域研究站绿洲农田2009年小气候、湍流交换、土壤蒸发和叶片气孔导度等综合观测试验数据,应用Shuttleworth-Wallace(S-W)双源模型以半小时为步长估算了绿洲农田玉米生长季实际蒸散量,并利用涡动相关与微型蒸渗仪实测数据对田间蒸散发量和棵间土壤蒸发量计算结果进行了检验。结果表明:S-W模型较好地估算研究区的蒸散量,并能有效区分农田作物蒸腾和土壤蒸发;全生育期玉米共耗水640 mm,其中作物蒸腾累积量为467 mm,土壤蒸发累积量为173 mm,分别占总量的72.9%和27.1%;日时间尺度上,作物蒸腾和土壤蒸发分别在0—6.3 mm/d和0—4.3 mm/d之间变化,其日平均分别为2.9和1.0 mm/d;田间供水充足,作物蒸腾与土壤蒸发比值明显受作物生长过程影响,播种—出苗期、出苗—拔节期、拔节—抽雄期、抽雄—灌浆期、灌浆—成熟期,其比值分别为0.04、0.8、7.0、5.2和1.4,不同阶段的比值差异主要受叶面积指数影响。     

黑河下游柽柳根系水力提升对林分蒸散的贡献

准确量化植物根系水力提升(HL)及其生态-水文效应对于陆地生态系统水分循环和全球变化研究具有重要意义。基于2011—2012年黑河下游柽柳林地土壤含水量和涡度协方差观测资料,通过将土壤体积含水量分割为HL和水分损失量(WD),结合涡度协方差测定的潜热通量计算的蒸散量(ET),首次定量黑河下游柽柳根系HL及其对ET的贡献。据估算,柽柳根系HL主要发生在20—60 cm深度,生长季HL大小在0—1.4 mm/d之间变化,平均为0.22 mm/d,WD在0—0.76 mm/d之间变化,平均为0.23 mm/d,HL与WD的年内变化存在同步性,且HL与WD处于正平衡状态,表明HL通过将深层吸收的土壤水或地下水释放在根系吸收层以供植物蒸腾消耗外,还有剩余水分留存在该层内。生长季ET在0.31—5.38 mm/d之间变化,平均为2.82mm/d,但值得注意的是,HL与ET的年内变化存在时间滞后性,HL在5月最高,但ET在7月最大。HL对ET的贡献率在0.06%—108.25%之间变化,平均为19.25%,比例高于100%的时段主要在生长初期,也就是说在蒸散最大的夏季,HL是相对较小的,其原因可能有2个:一是HL受到夏季深层土壤干化的抑制,二是HL受到夏季夜间蒸腾的抑制,究竟是何种原因还有待进一步研究。     

川西亚高山流域水碳平衡研究

森林生态系统的产水量与固碳效益之间存在着一种可交易的平衡关系。基于WaSSI-C水碳耦合模型和趋势分析, 研究了1982-2006年川西杂古脑河上游22个子流域内不同植被类型空间分布对水碳平衡的影响并分析了其水碳耦合关系, 发现: 1)针叶林主导的流域在生长季增加土壤水分入渗的功能明显高于其他植被类型, 但不足以补偿其高蒸散带来的水分消耗, 因而其年平均土壤含水量明显低于高山草甸和混交林类型; 且森林土壤含水量随着森林覆盖率的升高而降低。2) 25年的土壤水分蓄变量的平均值, 高山草甸流域为-44 mm, 混交林为-18 mm, 针叶林为-5 mm, 说明川西亚高山植被的整体维持稳定产水量及其潜力在下降, 其中高山草甸流域下降趋势尤为显著。3)流域产流量和净生态系统生产力具有显著负相关性, 且不同植被组成对固碳和产水效益的转化具有重要影响: 高山草甸主导的子流域具有较高的产水量和较低的固碳能力, 常绿针叶林主导的子流域具有较高固碳能力和较低产水量, 且森林覆盖率越高, 产水量越低。三种植被类型的净生态系统生产力在研究期间均呈现上升趋势, 且高山草甸的上升趋势显著。     

Trade-offs between water yield and carbon sequestration for sub-alpine catchments in western Sichuan,China

Aims There is increasing concern on the trade-off between carbon sequestration and water yield of forest ecosystems. Our objective was to explore the effects of vegetation composition on water and carbon trade-off in the sub-alpine watersheds of western Sichuan during 1982-2006.Methods The WaSSI-C, which is an eco-hydrological model with coupled water and carbon cycles, was employed to calculate the key components in water balance and carbon sequestration for the 22 sub-catchments in the upper reaches of Zagunao River. Spearman’s Rho trend analysis was used to examine the trends in runoff and net ecosystem productivity. Important findings Compared with either subalpine meadow or mixed forest dominated catchments, the conifer-dominated catchments had much higher water loss due to high evapotranspiration, and the loss was not offset by its higher soil water infiltration during the growing season. The change in soil water storage for subalpine meadow, mixed forest and coniferous forest are -44 mm, -18 mm and -5 mm, respectively, which indicated significant decline in soil water storage and thus water yield particularly in alpine meadow catchments. Significant negative relationship was found between runoff and net ecosystem productivity, the alpine meadow as the dominant vegetation type showed high water yield and low carbon sequestration, and the conifer-dominant and mixed forest vegetation showed low water yield and high carbon sequestration, moreover, the higher the forest coverage, the lower the water yield. Upward trends in net ecosystem productivity were observed in the three vegetation types during the study period and the alpine meadow type was significant.     

Trade-offs between water yield and carbon sequestration for sub-alpine catchments in western Sichuan,China北大核心CSCD

Aims: There is increasing concern on the trade-off between carbon sequestration and water yield of forest ecosystems. Our objective was to explore the effects of vegetation composition on water and carbon trade-off in the sub-alpine watersheds of western Sichuan during 1982-2006. Methods: The WaSSI-C, which is an eco-hydrological model with coupled water and carbon cycles, was employed to calculate the key components in water balance and carbon sequestration for the 22 sub-catchments in the upper reaches of Zagunao River. Spearman's Rho trend analysis was used to examine the trends in runoff and net ecosystem productivity. Important findings: Compared with either subalpine meadow or mixed forest dominated catchments, the conifer-dominated catchments had much higher water loss due to high evapotranspiration, and the loss was not offset by its higher soil water infiltration during the growing season. The change in soil water storage for subalpine meadow, mixed forest and coniferous forest are -44 mm, -18 mm and -5 mm, respectively, which indicated significant decline in soil water storage and thus water yield particularly in alpine meadow catchments. Significant negative relationship was found between runoff and net ecosystem productivity, the alpine meadow as the dominant vegetation type showed high water yield and low carbon sequestration, and the conifer-dominant and mixed forest vegetation showed low water yield and high carbon sequestration, moreover, the higher the forest coverage, the lower the water yield. Upward trends in net ecosystem productivity were observed in the three vegetation types during the study period and the alpine meadow type was significant.     

Ecohydrology and the Partitioning AET Between Transpiration and Evaporation in a Semiarid Steppe

Water availability defines and is the most frequent control on processes in arid and semiarid ecosystems. Despite widespread recognition of the importance of water in dry areas, knowledge about key processes in the water balance is surprisingly limited. How water is partitioned between evaporation and transpiration is an area about which ecosystem ecologists have almost no information. We used a daily time step soil water model and 39 years of data to describe the ecohydrology of a shortgrass steppe and investigate how manipulation of soil and vegetation variables influenced the partitioning of water loss between evaporation and transpiration. Our results emphasize the overwhelming importance of two environmental factors in influencing water balance processes in the semiarid shortgrass steppe; high and relatively constant evaporative demand of the atmosphere and a low and highly variable precipitation regime. These factors explain the temporal dominance of dry soil. Annually and during the growing season 60–80% of the days have soil water potentials less than or equal to −1.5 MPa. In the 0–15 cm layer, evaporation accounts for half of total water loss and at 15–30 cm it accounts for one third of the loss. Annual transpiration/actual evapotranspiration (T/AET) ranged from 0.4–0.75 with a mean of 0.51. The key controls on both T/AET and evaporation/actual evapotranspiration in order of their importance were aboveground biomass, seasonality of biomass, soil texture, and precipitation. High amounts of biomass and late timing of the peak resulted in the highest values of T/AET.     

玉米农田蒸散过程及其对气候变化的响应模拟

应用基于生理生态学过程的EALCO模型,对玉米农田生态系统的蒸散(ET)过程进行了模拟,在模型检验基础上,使用该模型模拟了玉米农田生态系统ET过程对未来气候变化的响应。结果表明,EALCO模型中能量与水过程的动态耦合机制使模型能够较好地模拟农田蒸散过程,基于涡度相关法的观测值与模型模拟值在小时、日尺度上均吻合较好,模型可以解释67%的日蒸散的变化特征。对土壤蒸发与冠层蒸腾的分别模拟显示,生长季土壤蒸发约占ET的36%。温度的升高会引起ET与冠层蒸腾的增加,同时土壤蒸发减少;ET对降水减少的响应较为敏感,主要表现在土壤蒸发的下降。大气CO2浓度升高对冠层蒸腾影响显著,该情景下冠层蒸腾下降幅度最大。研究所假设的2100年气候情景下,该农田生态系统生长季蒸散将减少,然而相对于降水的减少而言,蒸散的减少量较小,即水分支出项相对增加,因此,发生土壤水分匮乏的可能性加大,这可能会加剧该地区的暖干化趋势,给作物产量及生态环境带来威胁。     

Alpine grassland water use efficiency based on annual precipitation,growing season precipitation and growing season evapotranspiration

Water use efficiency (WUE) is an important parameter to understand the coupling between the water, and carbon cycles of terrestrial ecosystems. Previous studies on the grassland ecosystem WUE on the Qinghai-Xizang Plateau mainly based on annual precipitation (AP). However, vegetation water use mainly occurs in growing season. Therefore, we aimed to explore the differences of ecosystem WUE between alpine meadow and alpine steppe, and the relationships between ecosystem WUE and environmental factors from 2000 to 2010, using annual precipitation use efficiency (PUE_a), growing season precipitation use efficiency (PUE_gs), growing season water use efficiency (WUE_gs) based on AP, growing season precipitation (GSP) and growing season evapotranspiration (ET_gs ) respectively. Combining satellite-derived above-ground net primary productivity (ANPP), satellite-derived evapotranspiration and meteorological data from 2000 to 2010, we calculated PUE_a (ANPP / AP), PUE_gs (ANPP / GSP) and WUE_gs (ANPP / ET_gs) to find the differences of PUE_a, PUE_gs and WUE_gs between alpine meadow and alpine steppe. Moreover, we explored the relationships between PUE_a, PUE_gs or WUE_gs and precipitation (or evapotranspiration) or air temperature. We found that (1) the PUE_a and PUE_gs of alpine meadow were higher than that of alpine steppe, but there were no significant difference between WUE_gs of the two grassland types, indicating that there may be similar intrinsic water use efficiencies of the two grassland types. (2) The inter-annual variation of PUE_a and PUE_gs were similar while WUE_gs showed a larger fluctuation, implying that ET-based WUE_gs was more sensitive than precipitation-based PUE_a and PUE_gs, therefore WUE_gs is a better indicator of ecosystem water use efficiency than PUE_a or PUE_gs. (3) The PUE_a, PUE_gs and WUE_gs were negatively correlated with AP, GSP and ET_gs respectively, reflecting a consistency of the three water use efficiency measurements. In the alpine steppe, only WUE_gs was observed positively correlated with air temperature among the three measurements, but in the alpine meadow, no significant relationships between water use efficiency and air temperature was detected, suggesting that the WUE_gs of alpine steppe was more sensitive to air temperature than that of alpine meadow.     

基于年降水、生长季降水和生长季蒸散的高寒草地水分利用效率

水分利用效率是深入理解生态系统碳、水循环间耦合关系的重要指标。以前研究青藏高原的水分利用效率多基于年降水量(AP)来分析, 但植物对水分的利用主要在生长季。该研究采用以AP、生长季降水量(GSP)和生长季蒸散量(ET_gs)分别计算的年降水利用效率(PUE_a)、生长季降水利用效率(PUE_gs)和生长季水分利用效率(WUE_gs), 分析了2000-2010年间青藏高原两种主要植被类型高寒草甸和高寒草原PUE_a、PUE_gs和WUE_gs的差异及其与降水量、蒸散量和气温的关系。结果表明: (1)高寒草甸的PUE_a、PUE_gs均大于高寒草原, 但两种草地类型的WUE_gs无显著差别, 这说明两种草地类型可能存在相似的内在的水分利用效率。(2)从年际动态来看, PUE_a和PUE_gs的波动范围相似, 而WUE_gs的波动范围更大, 说明以蒸散为依据的WUE_gs可能比PUE_a和PUE_gs更敏感, 因而可能更好地反映生态系统的水分利用能力。(3)高寒草甸和高寒草原的PUE_a、PUE_gs和WUE_gs分别与AP、GSP和ET_gs呈单调递减趋势, 表明3种水分利用效率均随降水量或蒸散量的增加而降低。高寒草原的3种水分利用效率中仅WUE_gs随着气温的增加而增加, 而高寒草甸的3种水分利用效率均与气温无显著关系, 这说明相比高寒草甸, 高寒草原的水分利用效率对气温更加敏感。     

Intraseasonal Variation in Water and Carbon Dioxide Flux Components in a Semiarid Riparian Woodland

Abstract We investigated how the distribution of precipitation over a growing season influences the coupling of carbon and water cycle components in a semiarid floodplain woodland dominated by the deep-rooted velvet mesquite (Prosopis velutina). Gross ecosystem production (GEP) and ecosystem respiration (R _eco) were frequently uncoupled because of their different sensitivities to growing season rainfall. Soon after the first monsoon rains, R _eco was high and was not proportional to slight increases in GEP. During the wettest month of the growing season (July), the system experienced a net carbon loss equivalent to 46% of the carbon accumulated over the 6-month study period (114 g C m⁻²; May–October). It appears that a large CO₂ efflux and a rapid water loss following precipitation early in the growing season and a later CO₂ gain is a defining characteristic of seasonally dry ecosystems. The relative contribution of plant transpiration (T) to total evapotranspiration (ET) (T/ET) was 0.90 for the entire growing season, with T/ET reaching a value of 1 during dry conditions and dropping to as low as 0.65 when the soil surface was wet. The evaporation fraction (E) was equivalent to 31% of the precipitation received during the study period (253 mm) whereas trees and understory vegetation transpired 38 and 31%, respectively, of this water source. The water-use efficiency of the vegetation (GEP/T) was higher later in the growing season when the C4 grassy understory was fully developed. The influence of rain on net ecosystem production (NEP) can be interpreted as the proportion of precipitation that is transpired by the plant community; the water-use efficiency of the vegetation and the precipitation fraction that is lost by evaporation.     

Interannual Invariability of Forest Evapotranspiration and Its Consequence to Water Flow Downstream

Although drought in temperate deciduous forests decreases transpiration rates of many species, stand-level transpiration and total evapotranspiration is often reported to exhibit only minor interannual variability with precipitation. This apparent contradiction was investigated using four years of transpiration estimates from sap flux, interception–evaporation estimates from precipitation and throughfall gauges, modeled soil evaporation and drainage estimates, and eddy covariance data in a mature oak-hickory forest in North Carolina, USA. The study period included one severe drought year and one year of well above-average precipitation. Normalized for atmospheric conditions, transpiration rates of some species were lower in drought than in wet periods whereas others did not respond to drought. However, atmospheric conditions during drought periods are unlike conditions during typical growing season periods. The rainy days that are required to maintain drought-free periods are characterized by low atmospheric vapor pressure deficit, leading to very low transpiration. In contrast, days with low air vapor pressure deficit were practically absent during drought and moderate levels of transpiration were maintained throughout despite the drying soil. Thus, integrated over the growing season, canopy transpiration was not reduced by drought. In addition, high vapor pressure deficit during drought periods sustained appreciable soil evaporation rates. As a result, despite the large interannual variation in precipitation (ranging from 934 to 1346 mm), annual evapotranspiration varied little (610–668 mm), increasing only slightly with precipitation, due to increased canopy rainfall interception. Because forest evapotranspiration shows only modest changes with annual precipitation, lower precipitation translates to decreased replenishment of groundwater and outflow, and thus the supply of water to downstream ecosystems and water bodies.