Evaluation of Six Land Surface Evapotranspiration Products over the Tibetan Plateau
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摘要: 鉴于基于卫星遥感和地面观测开发出的不同时空分辨率蒸散发(ET)产品在青藏高原(TP)仍存在不确定性,从而限制了这些产品在水文气象和气候评估方面的应用。本文基于涡动观测的ET对六种ET产品(PML、EB-ET_V2、GLEAM、GLDAS、ERA5_Land和MOD16)进行评估并比较各产品之间的差异,对TP区域ET产品不确定性做了分析。结果表明:(1)观测值与对应像元ET值之间的年平均态和季节循环存在较好的相关性、一致性。GLEAM产品与观测值吻合度较高并拥有适用性;MOD16产品在大部分站点性能较差。(2)在季节性变化方面,春季ERA5_Land产品与观测的变化较为一致;夏季和冬季GLEAM产品与观测的变化更为接近,而EB-ET_V2产品在秋季表现更有优势。(3)在空间上,GLEAM、EB-ET_V2产品和GLDAS产品存在更高的相关性(相关系数R>0.88)和一致性(一致性指数IOA>0.89);各产品季节时空分布有较大的差异,尤其是春季;相对其他产品,MOD16产品在大部分区域夏季低估且冬季高估。(4)除MOD16外的各产品年平均ET大小相差较大,多年平均的高原ET大小排序为ERA5_Land(401.46 mm a−1)>PML(334.37 mm a−1)>GLEAM(298.46 mm a−1)>EB-ET_V2(271.39 mm a−1)>GLDAS(249.67 mm a−1),六套产品估算的青藏高原的总体年蒸发量为330.59 mm a−1。青藏高原不同蒸发产品的比较有助于对高原蒸发的动态变化有更深入的了解,可以为青藏高原水资源评估和区域水管理提供参考。Abstract: Uncertainties in the evapotranspiration (ET) products used in the Tibetan Plateau (TP) region were determined based on the data from satellite remote sensing and observations having different spatial and temporal resolutions, limiting their utility for hydrometeorological and climate assessment. Six ET (PML, EB-ET_V2, GLEAM, GLDAS, ERA5_Land, and MOD16) products were evaluated based on eddy observations, and the differences between the products were compared. Moreover, the uncertainties in ET products in the TP region were analyzed. The results of the analysis are as follows: (1) A good correlation and consistency exist in the mean state and seasonal cycle between the observed and ET values of the corresponding pixel. Moreover, GLEAM product exhibits a high degree of agreement with the observed values and has applicability, and MOD16 product exhibits poor performance at most sites. (2) In terms of seasonal changes, ERA5_Land product values are highly consistent with the observed changes during spring, GLEAM product values are nearly consistent with the observed changes during summer and winter, and EB-ET_V2 product values are highly consistent with observed values during autumn. (3) Spatially, GLEAM product has higher correlation (the correlation coefficient R>0.88) and consistency (index of agreement IOA>0.89) compared to those of EB-ET_V2 product and GLDAS product. Substantial differences exist in the temporal and spatial distribution of various products during different seasons, especially during spring. Compared with other products, MOD16 product is underestimated in summer and overestimated in winter in most regions. (4) The annual average ET for each product except for MOD16 product is considerably different. The annual average ET values of the remaining five products over multiple years arranged in descending order are as follows: ERA5_Land product (401.46 mm a−1)>PML product (334.37 mm a−1)>GLEAM product (298.46 mm a−1)>EB-ET_V2 product (271.39 mm a−1)>GLDAS product (249.67 mm a−1). The total annual evaporation in the TP region is 330.59 mm a−1. The assessment results provide a detailed understanding of the quality and dynamics of ET products in the TP region, which can serve as reference data for regional water management and water resource assessment in the TP region.
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图 2 6种ET产品ET值与草地站(Arou、Maqu、BJ、NAMOR和SETORS)和裸土站(QOMS、MAWORS和NADORS)通量塔观测ET值的时间序列和散点图
Figure 2. Time series and scatter plots for ET values obtained from six ET products (M) and flux tower data (G) over grassland sites (Arou, Maqu, BJ, NAMOR, and SETORS) and bare soil sites (QOMS, MAWORS, and NADORS)
表 1 8个通量站点的地理特征和观测数据的年份范围
Table 1. Geographic feature and year range of observation data of eight flux sites
站点类型 站点名称 站点位置 植被类型 海拔/m 测量高度/m 数据年份 草地 阿柔(Arou) 38.05°N, 100.46°E 草地 2995 2.96 2015~2017 玛曲(Maqu) 33.92°N, 102.16°E 草地 3500 3.50 2012~2014 那曲(BJ) 31.37°N, 91.90°E 高寒草甸 4509 3.10 2016~2018 纳木错(NAMOR) 30.77°N, 90.99°E 高寒草甸 4730 3.06 2008~2010 林芝站(SETORS) 29.76°N, 94.73°E 草地 3326 3.04 2015~2017 裸土 珠峰(QOMS) 28.21°N, 86.56°E 砾石或稀疏短草 4298 3.25 2008~2010 慕士塔格(MAWORS) 38.41°N, 75.05°E 荒或稀疏短草 3668 2.30 2015~2017 阿里(NADORS) 33.39°N, 79.70°E 荒或稀疏短草 4270 2.75 2013~2015 表 2 6个ET产品数据集的时空分辨率、研究年份
Table 2. Spatial–temporal resolution and research year of six ET (evapotranspiration) products datasets
ET产品 时间分辨率 空间分辨率 研究年份 PML 8 d 0.05°×0.05° 2002~2019 EB-ET_v2 逐日 0.05°×0.05° 2000~2017 GLEAM 逐日 0.25°×0.25° 1980~2018 GLDAS 3 h 0.25°×0.25° 2000~2020 ERA5_Land 1 h 0.1°×0.1° 1981~2020 MOD16 8 d 0.05°×0.05° 2000~2018 表 3 6种ET产品数据集的主要强迫数据来源
Table 3. Major forcing data of the six ET products datasets
ET产品 数据来源 降水 辐射 气象数据(风、温度、湿度和气压) 植被指数 模型 年份 PML GLDAS_Noah_2.1 GLDAS_Noah_2.1 GLDAS_Noah_2.1 MODIS PML遥感模型 2002~2019 EB-ET_V2 / ERA-Interim ERA-Interim MODIS 地表能量平衡法 2000~2017 GLEAM MSWEP v1.0 ERA-Interim ERA-Interim MODIS Gash分析模型、Priestley-Taylor方程等模型,融合了卫星和环境观测数据 1980~2018 GLDAS UMFD UMFD UMFD / 陆面模型和数据同化 2000~2020 ERA5_Land / ERA5近地表气象数据集和通量场 ERA5近地表气象数据集和通量场 / ECMWF集成预报系统CY41R2中的4D-Var数据同化 1981~2020 MOD16 / MERRA GMAO MODIS PM遥感模型 2000~2018 注:“/”表示没有相应的驱动数据集。 表 4 两种不同生态系统的通量塔观测ET值与六种ET产品数据集ET值之间的统计指标值
Table 4. Statistical indicator values between ET observations obtained from flux tower in two different ecosystems and six ET products datasets
站点类型 ET数据集 ET/mm (8 d)−1 MB/mm (8 d)−1 RMSE/mm (8 d)−1 IOA R 通量站平均 产品平均 草地站 PML 13.34 8.44 −2.17 5.44 0.91 0.88** EB-ET_V2 11.15 −4.65 8.62 0.74 0.72** GLEAM 12.50 −0.92 5.37 0.90 0.88** GLDAS 10.85 −2.45 7.26 0.85 0.86** ERA5_Land 8.01 −5.34 7.81 0.79 0.85** MOD16 12.31 −1.20 7.56 0.73 0.72** 裸土站 PML 8.66 3.81 0.71 7.34 0.70 0.58* EB-ET_V2 9.24 −4.61 9.21 0.56 0.54* GLEAM 4.28 −4.41 7.47 0.71 0.74** GLDAS 6.63 −2.12 7.58 0.70 0.57* ERA5_Land 3.30 −5.43 8.77 0.66 0.72** MOD16 / / / / / 注:**、*分别表示各ET产品与通量塔观测的相关性通过显著性水平为0.01、0.05的显著性t检验;黑体数字表示不同下垫面下各ET产品的验证统计指标最优值;“/”表示没有相应统计结果。 表 5 6种ET产品数据集ET值与通量塔观测ET值在TP地区春、夏、秋、冬季的相关系数
$R $ 和一致性指数IOATable 5. Correlation coefficients (
$R $ ) and index of agreement (IOA) between ET observations obtained from flux tower and six ET products datasets in TP region in spring, summer, autumn, and winterET数据集 春季 夏季 秋季 冬季 R IOA R IOA R IOA R IOA PML 0.38* 0.65 −0.22 0.27 −0.19 0.23 0.21 0.49 EB-ET_V2 0.23 0.52 0.24 0.52 0.36 0.62 0.15 0.46 GLEAM −0.27 0.18 0.33 0.62 0.14 0.44 0.43* 0.54 GLDAS 0.47** 0.72 0.32 0.58 0.15 0.49 0.03 0.41 ERA5_Land 0.18 0.52 0.09 0.31 0.28 0.54 −0.02 0.37 MOD16 −0.04 0.39 0.32 0.61 0.41* 0.68 0.14 0.51 注:**、*、黑体数字分别表示各ET产品与通量塔观测的相关性通过显著性水平为0.01、0.05、0.1的显著性t检验。 表 6 青藏高原各ET产品不同季节ET值和占比
Table 6. Statistics of the mean ET of seven datasets in the TP region in different seasons
ET数据集 春季 夏季 秋季 冬季 ET量/mm season−1 占比 ET量/mm season−1 占比 ET量/mm season−1 占比 ET量/mm season−1 占比 PML 85.78 25.65% 167.12 49.98% 59.53 17.8% 22.94 6.86% EB-ET_V2 92.96 34.24% 114.28 42.11% 46.29 17.05% 17.90 6.59% GLEAM 58.46 19.58% 167.43 59.09% 53.47 17.92% 19.10 6.38% GLDAS 36.10 14.48% 140.72 56.45% 55.83 22.39% 16.62 6.46% ERA5_Land 75.52 18.78% 219.86 54.69% 83.75 20.84% 22.83 5.68% Han_ET 122.46 25.34% 243.64 50.42% 82.37 17.05% 34.73 7.18% 平均 80.25 23.27% 168.12 48.76% 66.78 19.26% 29.62 8.59% 表 7 青藏高原7种ET产品陆面蒸散发总量(单位:mm a−1)
Table 7. The total ET (units: mm a−1) of seven datasets in the TP region
年份 陆面蒸散发总量/mm a−1 PML EB-ET_V2 GLEAM GLDAS ERA5_Land MOD16 Han_ET 2001 / 307.08 267.98 213.61 404.77 386.48 490.51 2002 / 299.68 280.42 216.85 407.18 375.88 467.39 2003 319.16 272.67 289.65 231.60 393.17 381.92 494.98 2004 306.77 256.38 282.15 220.16 391.47 370.69 467.51 2005 313.76 278.01 294.07 238.85 387.65 392.48 510.35 2006 312.82 266.67 299.28 253.69 409.96 351.14 503.53 2007 312.51 259.89 296.96 256.58 414.62 337.98 498.11 2008 312.25 279.16 302.23 258.25 410.43 363.16 501.36 2009 290.68 261.98 283.71 243.21 384.64 353.22 476.88 2010 332.02 267.65 309.74 270.83 403.15 374.28 500.82 2011 337.21 260.14 298.79 271.71 404.77 377.98 493.35 2012 321.60 280.32 295.89 261.49 407.18 393.76 519.22 2013 338.11 259.24 307.58 271.16 393.17 371.12 450.46 2014 310.83 261.71 294.36 246.15 391.48 372.39 440.94 2015 341.84 261.60 303.38 260.97 387.65 366.77 450.64 2016 410.99 270.14 321.84 265.77 409.96 397.76 491.84 2017 407.31 / 323.61 256.42 414.62 425.31 485.74 2018 382.06 / 320.72 256.69 410.43 393.02 456.83 多年平均 334.37 271.39 298.46 249.67 401.46 376.96 483.36 标准偏差 34.48 14.15 14.5 18.11 9.84 19.45 22.15 总体平均 330.59 -
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