Journal of Geo-information Science >
Application of Random Forest Algorithm in Global Drought Assessment
Received date: 2020-08-19
Request revised date: 2020-12-23
Online published: 2021-08-25
Supported by
National Key Research and Development Program of China(2016YFA0601500)
National Natural Science Foundation of China(42071040)
Fundamental Research Funds for the Central Universities(2019B04714)
Copyright
Drought is one of the most frequent and widespread climate extremes, causing devasting social, economic and ecological damages. It is of key importance to evaluate drought reliably and effectively. In this study, in order to assess global drought grade, the Random Forest (RF) algorithm was used to establish the drought grade assessment models for the 11 climate zones in the world. We chose monthly mean precipitation, mean temperature, maximum temperature, minimum temperature, soil moisture, evapotranspiration (ET), Normalized Difference Vegetation Index (NDVI), and Sun/Solar-induced Chlorophyll Fluorescence (SIF) as explanatory variables and drought grades based on Standardized Precipitation Index (SPI) as target variable. The SPI on different timescales of 1 month, 3 months, 6 months and 12 months were labeled as SPI1, SPI3, SPI6 and SPI12, respectively. The data from 2007 to 2012 were used as training data of the assessment models while those from 2013 to 2014 were used as prediction data. The results showed that: (1) The temporal scale of SPI influenced the model accuracy. Among the models with drought grade based on SPI1、SPI3、SPI6 and SPI12, the one with drought grade based on SPI1 had the highest accuracy (60%~75%) and prediction performance. The model with drought grade based on SPI1 was able to capture 90.91% of the drought records in the global emergency events database (EM-DAT). It could capture 78.47% of the drought duration month in the EM-DAT. The agreements with records and drought duration month in the EM-DAT indicated the good performance of the drought grade assessment model based on 1-month SPI and RF algorithm. (2) The drought grading criterion had little impact on the model performance. Users could select criterion I (drought/not drought) or criterion II (severe/not severe) depending on the real needs. (3) The relative importance of each explanatory variable depended on both the temporal scale of SPI and climatic differences. Precipitation was the most important factor for the drought grade based on SPI1. The importance of precipitation decreased and the ones of other explanatory variables such as temperature, soil moisture, NDVI, and ET increased as the timescale of SPI increased. The importance of variables except precipitation showed differences in different climate zones. Among the tropical, subfrigid, and tundra climate zones, temperature or ET is relatively important for drought. Soil moisture is relatively important in dry climate zone and precipitation is the most important in mild temperate climate zone, while vegetation is relatively important in the humid continental climate zone.
Key words: drought evaluation; drought grade; SPI; random forest; climate zone; precipitation; temperature; soil moisture
FANG Xiuqin , GUO Xiaomeng , YUAN Ling , YANG Lulu , REN Liliang , ZHU Qiuan . Application of Random Forest Algorithm in Global Drought Assessment[J]. Journal of Geo-information Science, 2021 , 23(6) : 1040 -1049 . DOI: 10.12082/dqxxkx.2021.200474
表1 气候区划分条件及文中缩写[11]Tab. 1 Climatic condition and acronym for each climate zone |
气候区 | 归类条件 | 文中缩写 |
---|---|---|
热带雨林气候 | Tmin≥18 °C,Pmin≥60 mm | Ay |
热带季风气候 | Tmin≥18 °C,Pann≥(100-Pmin)×25 | Aj |
热带稀树草原气候 | Tmin≥18 °C,Pann<(100-Pmin)×25 | Ax |
半干旱气候 | 5 Pth<Pann<10 Pth | Bb |
沙漠气候 | Pann≤5 Pth | Bs |
地中海气候 | -3°C<Tmin<18 °C,Psmin<Pwmin,Pwmax>3Psmin,Psmin<40 mm | Cd |
湿润亚热带气候 | -3°C<Tmin<18 °C,Tmax≥22 °C | Cs |
海洋性气候 | -3°C<Tmin<18 °C,Tmax<22 °C | Ch |
湿润大陆性气候 | Tmin≤-3 °C,至少有4个月的Tmon≥10 °C | Ds |
亚寒带气候 | Tmin≤-3 °C,Tmon≥10 °C少于4个月 | Dy |
冰原气候 | Tmax<0 °C | Eb |
苔原气候 | 0 °C≤Tmax<10 °C | Et |
表2 研究数据基本信息Tab. 2 Information of the data |
数据 | 来源 | 时间分辨率 | 空间分辨率 | 时间序列 |
---|---|---|---|---|
SPI | http://amir.eng.uci.edu/ | 月 | 0.625° | 1980.06—2016.12 |
SM | http://cci.esa.int/ | 月 | 0.25° | 1978.11—2016.12 |
ET | http://www.ntsg.umt.edu/ | 月 | 0.05° | 2000.01—2014.12 |
PRE | https://crudata.uea.ac.uk/ | 月 | 0.5° | 1901.01—2017.12 |
TMN | https://crudata.uea.ac.uk/ | 月 | 0.5° | 1901.01—2017.12 |
TMP | https://crudata.uea.ac.uk/ | 月 | 0.5° | 1901.01—2017.12 |
TMX | https://crudata.uea.ac.uk/ | 月 | 0.5° | 1901.01—2017.12 |
NDVI | https://modis.gsfc.nasa.gov/ | 月 | 1 km | 2000.02至今 |
SIF | https://avdc.gsfc.nasa.gov/ | 月 | 0.5° | 2007.01—2018.10 |
表3 不同时间尺度SPI反映的现象及其相关应用Tab. 3 Phenomena reflected by SPI on different timescales and their applications |
SPI时间尺度 | 反映现象 | 应用 |
---|---|---|
1个月 | 短期湿度条件 | 短期土壤水分和作物胁迫(特别是在生长季节) |
3个月 | 短期和中期湿度条件 | 对降水量的季节估计 |
6个月 | 中期降水趋势 | 表征不同季节降水的潜力 |
12个月 | 长期降水模式 | 与水流、水库水位和地下水水位有关 |
表4 标准化降水指数干旱等级划分表Tab. 4 Classification of the drought grades indicated by SPI |
分类标准 | 等级 | SPI |
---|---|---|
标准Ⅰ | 非干旱 | SPI >-1.0 |
干旱 | SPI≤-1.0 | |
标准Ⅱ | 非重旱 | SPI >-2.0 |
重旱 | SPI≤-2.0 |
表5 各气候区8个解释变量对基于SPI1、SPI3、SPI6、SPI12的干旱等级的相对重要性排序Tab. 5 Rank of the relative importance of the 8 explanatory variables for drought grades based on different SPI (SPI1, SPI3, SPI6, SPI12) in each climate zone |
![]() | 热带雨林气候 | 热带季风气候 | 热带稀树草原气候 | 半干旱气候 | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
SPI1 | SPI3 | SPI6 | SPI12 | SPI1 | SPI3 | SPI6 | SPI12 | SPI1 | SPI3 | SPI6 | SPI12 | SPI1 | SPI3 | SPI6 | SPI12 | |
1 | PRE | PRE | TMX | TMX | PRE | TMX | TMX | TMX | PRE | ET | ET | ET | PRE | PRE | SM | SM |
2 | TMX | TMP | SM | SM | TMP | TMP | TMP | TMP | ET | PRE | PRE | TMX | SM | SM | PRE | PRE |
3 | TMP | TMX | TMP | TMN | TMN | PRE | SM | SM | TMN | TMX | TMN | PRE | ET | ET | TMX | TMN |
4 | SM | SM | PRE | ET | TMX | SM | TMN | TMN | TMX | TMN | SM | TMN | TMX | TMN | ET | TMX |
5 | TMN | ET | TMN | TMP | SM | TMN | PRE | NDVI | TMP | TMP | TMX | SM | TMP | TMX | TMN | NDVI |
6 | ET | TMN | ET | PRE | ET | ET | NDVI | PRE | SM | SM | TMP | TMP | NDVI | TMP | NDVI | ET |
7 | SIF | SIF | NDVI | NDVI | SIF | NDVI | ET | ET | NDVI | NDVI | NDVI | NDVI | TMN | NDVI | TMP | TMP |
8 | NDVI | NDVI | SIF | SIF | NDVI | SIF | SIF | SIF | SIF | SIF | SIF | SIF | SIF | SIF | SIF | SIF |
重要性排序 | 沙漠气候 | 地中海气候 | 湿润亚热带气候 | 海洋性气候 | ||||||||||||
SPI1 | SPI3 | SPI6 | SPI12 | SPI1 | SPI3 | SPI6 | SPI12 | SPI1 | SPI3 | SPI6 | SPI12 | SPI1 | SPI3 | SPI6 | SPI12 | |
1 | PRE | PRE | PRE | TMN | PRE | PRE | PRE | PRE | PRE | PRE | PRE | PRE | PRE | PRE | PRE | PRE |
2 | SM | SM | SM | SM | TMX | SM | TMX | TMN | TMX | TMX | TMX | TMX | TMX | ET | ET | TMX |
3 | ET | ET | ET | NDVI | TMN | TMN | SM | SM | SM | ET | ET | TMN | ET | TMX | TMX | ET |
4 | TMN | TMN | TMX | PRE | TMP | TMP | TMN | ET | NDVI | TMN | TMN | ET | NDVI | TMN | TMN | TMN |
5 | TMX | TMX | TMN | TMX | ET | ET | TMP | TMX | TMP | SM | SM | TMP | SIF | SM | SM | SM |
6 | TMP | TMP | TMP | ET | NDVI | TMX | ET | TMP | TMN | SIF | TMP | SM | TMN | TMP | NDVI | NDVI |
7 | NDVI | NDVI | NDVI | TMP | SM | NDVI | NDVI | NDVI | ET | NDVI | SIF | NDVI | TMP | SIF | TMP | TMP |
8 | SIF | SIF | SIF | SIF | SIF | SIF | SIF | SIF | SIF | TMP | NDVI | SIF | SM | NDVI | SIF | SIF |
重要性排序 | 湿润大陆性气候 | 亚寒带气候 | 苔原气候 | |||||||||||||
SPI1 | SPI3 | SPI6 | SPI12 | SPI1 | SPI3 | SPI6 | SPI12 | SPI1 | SPI3 | SPI6 | SPI12 | |||||
1 | PRE | PRE | PRE | SM | PRE | PRE | PRE | PRE | PRE | PRE | PRE | PRE | ||||
2 | NDVI | SM | TMN | PRE | ET | ET | ET | ET | TMX | TMN | TMN | TMN | ||||
3 | SIF | TMX | SM | TMN | TMX | TMX | TMN | TMN | ET | TMX | TMX | TMX | ||||
4 | TMX | TMN | ET | ET | NDVI | TMN | TMX | TMX | TMN | ET | ET | ET | ||||
5 | ET | NDVI | TMX | TMX | TMN | SM | SM | SM | TMP | TMP | TMP | TMP | ||||
6 | TMP | ET | NDVI | NDVI | SM | NDVI | TMP | NDVI | SM | SM | SM | NDVI | ||||
7 | TMN | TMP | TMP | SIF | TMP | TMP | NDVI | TMP | NDVI | NDVI | NDVI | SM |
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