A full analysis of 3-month Standardized Precipitation-Evapotranspiration index (SPEI-3) changes and attribution analyses are of significance for deeply understanding dryness/wetness evolutions and thus formulating specific measures to sustain regional development. In this study, we analyze monthly and annual SPEI-3 changes over Southwest China (SWC; including Sichuan (SC), Chongqing (CQ), Guizhou (GZ), Yunnan (YN), and west Guangxi (wGX)) during 1961–2012, using the SPEI model and routine meteorological measurements at 269 weather sites. For SWC and each subregion (excluding wGX), annual SPEI-3 during 1961–2012 tends to decrease, and drying is at most of months in January and September–December, but wetting is in February–August (excluding March for wGX). Additionally, more than 50% of sites show declined and increased SPEI-3 in January, April, June, and August–December and the remaining months, respectively. Except for wGX with dominant of ET0, annual SPEI-3 changes in SWC and other four subregions have dominant of precipitation. Spatially, annual SPEI-3 changes at 59% of sites are because of precipitation, generally located in southeast SC, south YN, CQ, GZ, and south and northeast wGX. Nevertheless, dominants at regional and site scales vary among months, e.g., SWC, SC, CQ, and GZ, having dominant of precipitation (ET0) during September–December (most of months during January–August), YN always with dominant of precipitation, and wGX with dominant of precipitation (ET0) in February–April and July–December (January, May, and June). Importantly, this study provides a reference for quantitatively evaluating spatiotemporal dryness/wetness variations with climate change, especially for regions with significant drying/wetting.
Drought is usually regarded as one of the most damaging natural hazards [
Because of drought complexity and much limited observations, to directly conduct drought analyses is difficult. Therefore, a number of drought indices have been developed to characterize drought evolutions [
Reviewing the previous studies, there is a general agreement that SWC is becoming drier with more frequent and higher intense droughts during the past five decades [
To fill these gaps, we have quantitatively assessed precipitation and ET0 impacts on annual and monthly dryness/wetness (reflected by 3-month SPEI (SPEI-3)) changes at regional and site scales during 1961–2012 over SWC. Therefore, this study aims to (1) analyze precipitation, FAO (Food and Agriculture Organization)–56 Penman–Monteith ET0 and SPEI-3 variations on different spatiotemporal scales and (2) then identify dominants of SPEI-3 changes based on individual contributions of precipitation and ET0, which are separated by a method from Sun et al. [
Here, we specify SWC between 21–34°N and 97–110°E (Figure
Location of 269 weather sites over SWC. The shading indicates elevation, which is represented with DEM (Digital Elevation Model;
For running the FAO–56 Penman–Monteith equation and the SPEI model, a monthly meteorological dataset during 1960–2012, including precipitation (mm), mean, maximum and minimum air temperatures (°C), wind speed at 10 meter (m/s), sunshine duration (h/mon) and relative humidity (%), was collected from 334 weather sites of the China Metrological Administration (CMA). Before using, two data quality issues, i.e., inhomogeneity caused by nonclimatic factors (e.g., changes in instruments, station locations, and environment around observation field) and missing values, should be solved. For simplicity, we will not show the detailed procedures to process these two issues, but more information can be found in Sun et al. [
Trend of each hydroclimatic variable can be obtained using
Considering our major objectives to quantify climate change impacts on SPEI-3, the selected ET0 equation should involve a comprehensive physical concept of evapotranspiration processes. Therefore, the FAO–56 Penman–Monteith equation [
Since the SPEI is proposed by Vicente-Serrano et al. [
The SPEI is a function of precipitation and driving factors of ET0. Then, interactions among these factors potentially bias contributions of each factor to SPEI changes and finally influence conclusions [
Detailed information about numerical experiments.
Experiments | Inputs |
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SPEI_CTR | Monthly precipitation, Rn, Tave, Wnd, and Vpd during 1960–2012 |
SPEI_P | Monthly precipitation fixed at 52-year (1961–2012) mean; monthly Rn, Tave, Wnd, and Vpd during 1960–2012 |
SPEI_Rn | Monthly Rn fixed at 52-year monthly mean; monthly precipitation, Tave, Wnd, and Vpd during 1960–2012 |
SPEI_Tave | Monthly Tave fixed at 52-year monthly mean; monthly Rn, precipitation, Wnd, and Vpd during 1960–2012 |
SPEI_Wnd | Monthly Wnd fixed at 52-year monthly mean; monthly Rn, Tave, precipitation, and Vpd during 1960–2012 |
SPEI_Vpd | Monthly Vpd fixed at 52-year monthly mean; monthly Rn, Tave, Wnd, and precipitation during 1960–2012 |
This approach will be conducted at each site and each year. Note that ET0 contributions to annual and monthly SPEI-3 trends are sum of respective contributions of Rn, Tave, Vpd, and Wnd.
During the past 52 years, annual precipitation for SWC and all subregions consistently decreases with a range from −2.16 mm/yr to −0.82 mm/yr (the right panel of Figure
Monthly and annual trends of regional mean precipitation, ET0, and SPEI-3 during 1961–2012. The asterisk denotes that trend is significant.
Seen from Table
Site percentage (%) with increased and decreased precipitation/ET0/SPEI-3.
Annual | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | ||
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Precipitation | SI | 1.1 | 25.0 | 1.9 | 9.3 | 5.6 | 6.3 | 9.0 | 4.5 | 0 | 1.1 | 1.9 | 0.4 | 0 |
II | 16.4 | 64.6 | 55.2 | 76.1 | 33.6 | 51.5 | 43.7 | 61.2 | 14.2 | 26.5 | 16.4 | 14.6 | 17.5 | |
SD | 23.5 | 0 | 0 | 0.4 | 26.5 | 3.4 | 6.7 | 3.4 | 28.7 | 18.3 | 16.4 | 12.7 | 5.6 | |
ID | 59.0 | 10.4 | 42.9 | 14.2 | 34.3 | 38.8 | 40.7 | 31.0 | 57.1 | 54.1 | 65.3 | 72.4 | 76.9 | |
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ET0 | SI | 11.6 | 9.0 | 13.8 | 1.9 | 7.5 | 1.9 | 6.0 | 1.1 | 6.7 | 6.3 | 7.5 | 25.7 | 10.8 |
II | 18.7 | 15.7 | 53.0 | 15.7 | 36.9 | 41.0 | 22.8 | 12.3 | 31.3 | 42.2 | 44.8 | 41.4 | 28.0 | |
SD | 42.9 | 46.6 | 3.4 | 22.8 | 11.9 | 14.9 | 31.0 | 46.6 | 19.8 | 11.6 | 10.1 | 8.2 | 23.1 | |
ID | 26.9 | 28.7 | 29.9 | 59.7 | 43.7 | 42.2 | 40.3 | 39.9 | 42.2 | 39.9 | 37.7 | 24.6 | 38.1 | |
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SPEI-3 | SI | 4.9 | 5.2 | 8.2 | 25.7 | 10.4 | 9.7 | 4.9 | 10.1 | 5.2 | 0 | 0 | 0 | 2.2 |
II | 30.2 | 38.1 | 59.7 | 62.7 | 38.4 | 45.5 | 42.9 | 54.5 | 38.4 | 23.1 | 6.3 | 12.7 | 8.2 | |
SD | 11.9 | 4.1 | 2.2 | 0.4 | 7.8 | 7.5 | 2.2 | 1.5 | 10.1 | 8.6 | 28.7 | 34.3 | 29.5 | |
ID | 53.0 | 52.6 | 29.9 | 11.2 | 43.3 | 37.3 | 50.0 | 34.0 | 46.3 | 68.3 | 64.9 | 53.0 | 60.1 |
Note: SI (SD) suggests that increasing (decreasing) trend is significant, while II (ID) represents that increasing (decreasing) trend is insignificant.
Annual spatial distributions of precipitation, ET0, and SPEI-3 changes (a1–3) and precipitation and ET0 alone contributions to SPEI-3 trends (b1–2) with dominants (b3) during 1961–2012. The plus sign denotes that trend is significant.
For SWC, annual ET0 significantly decreases by 0.55 mm/yr during 1961–2012 (the right panel of Figure
Overall, sites with significant changes in annual ET0 are widespread across SWC, accounting for 54.5% of sites (Table
Taking SWC as a whole (right panel of Figure
Table
Spatial distributions of monthly ET0 changes during 1961–2012. The plus sign denotes that trend is significant.
With climate change (reflected by precipitation and ET0), dry/wet conditions at different temporal (i.e., annual and monthly) and spatial (i.e., regional and site) scales have changed over SWC during 1961–2012 (see Section
Monthly and annual regional mean contributions of precipitation and ET0 alone to SPEI-3 changes (a1–6) and those of the driving factors alone to ET0 trends (b1–6) during 1961–2012. For each panel, left and right
Comparing Figures
Site percentage (%) with dominants of changed SPEI-3/ET0.
Dominants | Annual | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
SPEI-3 | Precipitation | 59.0 | 54.9 | 40.7 | 64.6 | 69.4 | 69.4 | 74.6 | 76.9 | 79.1 | 82.8 | 92.9 | 90.0 | 90.7 |
ET0 | 41.0 | 45.1 | 59.3 | 35.4 | 30.6 | 30.6 | 25.4 | 23.1 | 20.9 | 17.2 | 7.1 | 10.0 | 9.3 | |
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ET0 | Tave | 0 | 0 | 11.2 | 1.1 | 0 | 1.5 | 0.4 | 0 | 0.4 | 0.4 | 2.2 | 3.4 | 1.9 |
Rn | 48.5 | 27.2 | 8.6 | 31.3 | 26.9 | 38.8 | 70.5 | 83.6 | 66.0 | 53.0 | 34.7 | 16.4 | 17.9 | |
Vpd | 24.6 | 37.7 | 51.5 | 25.0 | 37.3 | 31.7 | 19.8 | 11.9 | 30.2 | 37.3 | 41.4 | 49.6 | 36.6 | |
Wnd | 26.9 | 35.1 | 28.7 | 42.5 | 35.8 | 28.0 | 9.3 | 4.5 | 3.4 | 9.3 | 21.6 | 30.6 | 43.7 |
Based on respective contributions of precipitation (Figures
Spatial patterns of dominants of monthly SPEI-3 changes during 1961–2012.
Numerous researchers generally agreed that with global climate change, regional mean annual precipitation consistently decreased for SWC and its subregions (e.g., SC and YN [
Also, external forcings (e.g., greenhouse gases, various aerosols, and solar activities) can influence regional precipitation by a series of complex physical processes and mechanisms [
At first, we used the FAO–56 Penman–Monteith equation to perform five experiments (details in Table
Notably, considering major aims of this study and extensive investigations of ET0 driving factors’ changes over SWC [
In this study, a new separation method was applied to obtain precipitation and ET0 alone contributions to SPEI-3 dryness/wetness changes over SWC during 1961–2012, and then attribution analyses are performed; however, there still exist potential uncertainties, which may influence the confidence level of our conclusions. We hereby show some possible sources of uncertainties from several perspectives. The first is Rn estimation. Because of limited sites with solar radiation measurements, Rn is computed using the semi-physical formulas of Allen et al. [
In addition, several limitations of this study should be kept in mind. One widely used drought index at a 3-month scale (i.e., SPEI-3) coupled with only the FAO–56 Penman–Monteith equation is employed in this study, and this treatment may potentially influence the universality of our findings, e.g., whether the roles of precipitation and ET0 in dryness/wetness changes are consistent among different drought indices, and for a specific drought index with different potential evapotranspiration formulations. Considering the complex associations of drought with precipitation and ET0, the separated contributions by linear equations (i.e., equation (
In this study, we firstly investigate precipitation, ET0, and SPEI-3 changes over SWC during 1961–2010 on different spatiotemporal scales. Then, attribution analyses of changed SPEI-3 are conducted based on separated respective contributions from precipitation and ET0. Major results can be summarized below: Regionally, annual precipitation (ET0) changes between −2.16 mm/yr (−1.09 mm/yr) and −0.82 mm/yr (−0.02 mm/yr) over SWC and its five subregions, which also show declined precipitation (ET0) at most of months. Over all regions (except for wGX), annual SPEI-3 slightly decreases, implying that these regions become drying during 1961–2010. Among 12 months, drying and wetting trends for each region generally occur in January and September–December, and the remaining months, respectively. Precipitation reduction is major contributor of annual SWC drying during 1961–2012. However, annual dominants of changed SPEI-3 exhibit evident regional differences; e.g., positive (negative) trends in wGX (other four subregions) are due to decreased ET0 (precipitation). There exist 59% of sites with dominant of precipitation in southeast SC, south YN, CQ, GZ, and south and northeast wGX, while 41% of sites with dominant of ET0 are located in the remaining regions. Major contributors of regional mean SPEI-3 trends indicate obvious intra-annual differences, i.e., SWC, SC, CQ, and GZ with dominant of precipitation (ET0) in September–December (most of January–August), YN always with the major contributor of precipitation, and wGX with dominant of precipitation (ET0) in February–April and July–December (January, May, and June). Moreover, intra-annual differences in dominants also exist at each site.
Based on a full investigation of SPEI-3 changes on different spatiotemporal scales, we confirm that dryness/wetness has changed across SWC due to climate change during 1961–2012, and major contributors (i.e., precipitation and ET0) exhibit evident spatiotemporal differences. These detailed analyses is of significance for deeply understanding spatial-temporal evolutions of dry/wet conditions (e.g., drying) and even droughts (e.g., ongoing and intensifying droughts) over SWC and formulating specific measures to sustain regional development (e.g., water resources, ecosystem, and agriculture). Findings, e.g., higher or comparable impact of ET0 on SPEI-3 than that of precipitation and spatiotemporal differences in dominants of dryness/wetness changes, suggest that role of ET0 and spatiotemporal differences in dominants should be involved in drought monitoring and forecasting system. Also, this study provides a reference framework for quantitatively evaluating spatiotemporal dryness/wetness variations with climate change in other regions of the globe, especially for those with significant drying/wetting.
All the original meteorological observations supporting this article are not available to the public, but they are obtained and used through cooperation with the China Meteorological Administration (CMA). If the readers would like to use this dataset, they can contact National Meteorological Information Center of CMA (
(i) A full analysis of dryness/wetness changes over Southwest China (SWC) is performed. (ii) To attribute dryness/wetness changes, climate change contributions are quantified. (iii) Dominants of dryness/wetness changes show evident spatiotemporal differences. (iv) This study provides a reference framework to attribute dryness/wetness variation.
The authors declare that they have no conflicts of interest.
This work was jointly supported by the National Key Research and Development Program of China (Grant nos. 2018YFC1507101 and 2017YFA0603701), the National Natural Science Foundation of China (Grant nos. 41605042 and 41875094), the Natural Science Foundation of Jiangsu Province, China (Grant no. BK20151525), the Qinglan Project of Jiangsu Province of China, and the Science Technology Department of Zhejiang Province, China (Grant no. LGN18D050001).
Text S1: Rn estimation. Text S2: spatial distributions of monthly precipitation and ET0 changes. Text S3: changes in major driving factors of ET0. Text S4: spatial distributions of dominants of monthly ET0. Figure S1: spatial distributions of monthly precipitation change (a1–a12) and its contribution to SPEI-3 (b1–b12) during 1961–2012. The plus sign in a1–a12 denotes that this trend is significant with