Abstract
According to local villagers, the main Beiguo landslide on the Heyang Loess Tableland in China occurred several decades ago (the specific time is unknown). After the initial occurrence of the main landslide, there was no major activity for several decades until December 17–22, 2017 (2 months after the rainy season), when the main landslide was completely reactivated. One of the interesting findings of our field survey is that there are two erosion gullies and numerous loess caves (groups) developed at the top of the Beiguo landslide, which are generally considered to be the dominant channels by which rainwater or surface runoff enters the deep underground space. In order to better understand the process and triggering factors of the main landslide’s reactivation, a multidisciplinary investigation including satellite remote sensing, interferometric synthetic aperture radar (InSAR), an unmanned aerial vehicle (UAV) survey, a terrestrial laser scanning (TLS) survey, an electrical resistivity tomography (ERT) survey, laboratory experiments, real-time monitoring using an inclinometer, and field investigations of the Beiguo landslide was conducted. We believe that the reactivation of the main Beiguo landslide may have been occurring for more than 10 years, or perhaps even longer. Since 2011, there have been several signs of local activity in the main landslide body, and the last accelerated stage of the entire deformation process occurred in 2017. The development and existence of loess caves directly destroyed the stability of the slope, and the rainfall and surface runoff quickly entered the interior of the main landslide. The influence of the loess caves on the landslide’s reactivation was long-term and continuous. The results of this study suggest that the several days of abundant and continuous rainfall in the rainy season in 2017 were the last direct trigger of the final reactivation of the main landslide. The secondary landslide significantly changed the original slope, including the aspect of the slope profile’s morphology, the slope gradient, relief, erosion, and accumulation, which accelerated the evolution of the tableland landform. This study enhances our understanding of the reactivation behavior of loess landslides, and also provides important support for the monitoring and prevention of reactivated landslides in the future.
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References
Bayer B, Simoni A, Schmidt D, Bertellob L (2017) Using advanced InSAR techniques to monitor landslide deformations induced by tunneling in the Northern Apennines, Italy. Eng Geol 226:20–32. https://doi.org/10.1016/j.enggeo.2017.03.026
Borgatti L, Corsini A, Barbieri M, Sartini G, Truffelli G, Caputo G, Puglisi C (2006) Large reactivated landslides in weak rock masses: a case study from the Northern Apennines (Italy). Landslides 3(2):115–124. https://doi.org/10.1007/s10346-005-0033-9
Bozzano F, Esposito C, Martini G, Martino S, Prestininzi A, Rinaldis D, Romeo RW, Mugnozza GS (2013) Earthquake-reactivated landslide scenarios in Southern Italy based on spectral-matching input analysis. Bull Earthq Eng 11(6):1927–1948. https://doi.org/10.1007/s10518-013-9477-9
Ciuffi P, Bayer B, Berti M, Franceschini S, Simoni A (2021) Deformation detection in cyclic landslides prior to their reactivation using two-pass satellite interferometry. Appl Sci 11(7):3156. https://doi.org/10.3390/app11073156
Crozier MJ (2010) Landslide geomorphology: an argument for recognition, with examples from New Zealand. Geomorphology 120(1):3–15. https://doi.org/10.1016/j.geomorph.2009.09.010
Dąbski M, Zmarz A, Pabjanek P, Korczak-Abshire M, Karsznia I, Chwedorzewska KJ (2017) UAV-based detection and spatial analyses of periglacial landforms on Demay Point (King George Island, South Shetland Islands, Antarctica). Geomorphology 290:29–38. https://doi.org/10.1016/j.geomorph.2017.03.033
De Rose RC (2013) Slope control on the frequency distribution of shallow landslides and associated soil properties, North Island, New Zealand. Earth Surf Proc Land 38(4): 356–371. https://onlinelibrary.wiley.com/doi/epdf/https://doi.org/10.1002/esp.3283
Derbyshire E (2001) Geological hazards in loess terrain, with particular reference to the loess regions of China. Earth Sci Rev 54(1–3):231–260. https://doi.org/10.1016/S0012-8252(01)00050-2
Dewitte O, Chung CJ, Cornet Y, Daoudi M, Demoulin A (2010) Combining spatial data in landslide reactivation susceptibility mapping: a likelihood ratio-based approach in W Belgium. Geomorphology 122(1):153–166. https://doi.org/10.1016/j.geomorph.2010.06.010
Doglioni A, Fiorillo F, Guadagno FM, Simeone V (2012) Evolutionary polynomial regression to alert rainfall-triggered landslide reactivation. Landslides 9:53–62. https://doi.org/10.1007/s10346-011-0274-8
Duan Z, Cheng WC, Peng JB, Wang QY, Chen W (2019) Investigation into the triggering mechanism of loess landslides in the south Jingyang platform, Shaanxi province. Bull Eng Geol Env 78(7):4919–4930. https://doi.org/10.1007/s10064-018-01432-8
Floris M, Bozzano F (2008) Evaluation of landslide reactivation: A modified rainfall threshold model based on historical records of rainfall and landslides. Geomorphology 94(1):40–57. https://doi.org/10.1016/j.geomorph.2007.04.009
Guzzetti F, Mondini AC, Cardinali M, Fiorucci F, Santangelo M, Chang K (2012) Landslide inventory maps: new tools for an old problem. Earth Sci Rev 112(1–2):42–66. https://doi.org/10.1016/j.earscirev.2012.02.001
Hou XK, Vanapalli SK, Li TL (2018) Water infiltration characteristics in loess associated with irrigation activities and its influence on the slope stability in Heifangtai loess highland, China. Eng Geol 234:27–37. https://doi.org/10.1016/j.enggeo.2017.12.020
Hu S (2019) Spatial pattern of landslide in Loess Plateau and its influence on geomorphologic evolution. Xi’an: Northwest University. (in Chinese)
Hu S, Qiu HJ, Wang NL, Cui YF, Wang JD, Wang XG, Ma SY, Yang DD, Cao MM (2020) The influence of loess cave development upon landslides and geomorphologic evolution: A case study from the northwest Loess Plateau, China. Geomorphology 359:107167. https://doi.org/10.1016/j.geomorph.2020.107167
Hu S, Qiu HJ, Wang XG, Gao Y, Wang NL, Wu J, Yang DD, Cao MM (2018) Acquiring high-resolution topography and performing spatial analysis of loess landslides by using low-cost UAVs. Landslides 15(3):593–612. https://doi.org/10.1007/s10346-017-0922-8
Hu ZB, Pan BT, Wang JP, Cao B, Gao HS (2012) Fluvial terrace formation in the eastern Fenwei Basin, China, during the past 1.2 Ma as a combined archive of tectonics and climate change. J Asian Earth Sci 60:235–245. https://doi.org/10.1016/j.jseaes.2012.09.016
Klimeš J, Vilímek V (2011) A catastrophic landslide near Rampac Grande in the Cordillera Negra, northern Peru. Landslides 8(3):309–320. https://doi.org/10.1007/s10346-010-0249-1
Kohler MA, Linsley RK (1951) Predicting the runoff from storm rainfall. Weather Bureau, US Department of Commerce, Research Paper No.34, Washington.
Leng Y, Peng J, Wang Q, Meng Z, Huang W (2018) A fluidized landslide occurred in the Loess Plateau: a study on loess landslide in South Jingyang tableland. Eng Geol 236:129–136. https://doi.org/10.1016/j.enggeo.2017.05.006
Li F, Zhang MS, Jin Z, Zhang SS, Sun PP, Gu TF, Liu XB, Lin H, An ZS, Peng JB (2021a) The genesis, development, and evolution of original vertical joints in loess. Earth Sci Rev 214:103526. https://doi.org/10.1016/j.earscirev.2021.103526
Li X, Wei Y, Li F (2021b) Optimality of antecedent precipitation index and its application. J Hydrol 595:126027. https://doi.org/10.1016/j.jhydrol.2021.126027
Li XA, Song YX, Ye WJ (2010) Engineering geological research on tunnel-erosion in loess. Tongji University Press, Shanghai ((in Chinese))
Lian B, Peng J, Zhan H, Huang Q, Wang X, Hu S (2020) Formation mechanism analysis of irrigation-induced retrogressive loess landslides. CATENA 195:104441. https://doi.org/10.1016/j.catena.2019.104441
Liu TS (1985) Loess and the Environment. China Ocean Press, Beijing
Liu X, Zhao C, Zhang Q, Peng J, Zhu W, Lu Z (2018) Multi-Temporal Loess Landslide Inventory Mapping with C-, X- and L-Band SAR Datasets-A Case Study of Heifangtai Loess Landslides, China. Remote Sens-Basel 10(11):1756. https://doi.org/10.3390/RS10111756
Liu XJ (2019) Study on the influence of sinkhole on the stability of loess slope-Taking the slope of Quanxingzhai in Heyang County as an example. Xi’an: Chang’an University. (in Chinese)
Luo SL, Jin XG, Huang D (2019) Long-term coupled effects of hydrological factors on kinematic responses of a reactivated landslide in the Three Gorges Reservoir. Eng Geol 261:105271. https://doi.org/10.1016/j.enggeo.2019.105271
Ma PH, Peng JB, Wang QY, Duan Z, Meng ZJ, Zhuang JQ (2019a) Loess landslides on the South Jingyang Platform in Shaanxi Province, China. Q J Eng Geol Hydroge 52(4):547–556. https://doi.org/10.1144/qjegh2018-115
Ma PH, Peng JB, Wang QY, Zhuang JQ, Zhang FY (2019b) The mechanisms of a loess landslide triggered by diversion-based irrigation: a case study of the South Jingyang Platform, China. Bull Eng Geol Env 78(7):4945–4963. https://doi.org/10.1007/s10064-019-01467-5
Ma SY, Qiu HJ, Hu S, Yang DD, Liu ZJ (2021) Characteristics and geomorphology change detection analysis of the Jiangdingya landslide on July 12, 2018, China. Landslides 18:383–396. https://doi.org/10.1007/s10346-020-01530-3
Mao ZL, Hu S, Wang NL, Long YQ (2021) Precision Evaluation and Fusion of Topographic Data Based on UAVs and TLS Surveys of a Loess Landslide. Front Earth Sci 9:801293. https://doi.org/10.3389/feart.2021.801293
Massey CI, Petley DN, McSaveney MJ, Archibald G (2016) Basal sliding and plastic deformation of a slow, reactivated landslide in New Zealand. Eng Geol 208:11–28. https://doi.org/10.1016/j.enggeo.2016.04.016
Medjkane M, Maquaire O, Costa S, Roulland T, Letortu P, Fauchard C, Antoine R, Davidson R (2018) High-resolution monitoring of complex coastal morphology changes: cross-efficiency of SfM and TLS-based survey (Vaches-Noires cliffs, Normandy, France). Landslides 15(6):1097–1108. https://doi.org/10.1007/s10346-017-0942-4
Migoń P, Jancewicz K, Różycka M, Duszyński F, Kasprzak M (2017) Large-scale slope remodelling by landslides-Geomorphic diversity and geological controls, Kamienne Mts., Central Europe. Geomorphology 289:134–151. https://doi.org/10.1016/j.geomorph.2016.09.037
Mishra SK, Singh VP (2003) Soil conservation service curve number (SCS-CN) methodology. Kluwer Academic Publisher, Dordrecht. https://link.springer.com/book/10.1007%2F978-94-017-0147-1
Niethammer U, James MR, Rothmund S, Travelletti J, Joswig M (2012) UAV-based remote sensing of the Super-Sauze landslide: evaluation and results. Eng Geol 128:2–11. https://doi.org/10.1016/j.enggeo.2011.03.012
Notti D, Wrzesniak A, Dematteis N, Lollino P, Fazio NL, Zucca F, Giordan D (2021) A multidisciplinary investigation of deep-seated landslide reactivation triggered by an extreme rainfall event: a case study of the Monesi di Mendatica landslide, Ligurian Alps. Landslides 18:2341–2365. https://doi.org/10.1007/s10346-021-01651-3
Pánek T, Smolková V, Hradecký J, Baroň I, Šilhán K (2013) Holocene reactivations of catastrophic complex flow-like landslides in the Flysch Carpathians (Czech Republic/Slovakia). Quatern Res 80(1):33–46. https://doi.org/10.1007/978-3-319-04999-1_33
Pasternack GB, Wyrick JR (2017) Flood-driven topographic changes in a gravel-cobble river over segment, reach, and morphological unit scales. Earth Surf Proc Land 42(3):487–502. https://doi.org/10.1002/esp.4064
Peng D, Xu Q, Liu F, He Y, Zhang S, Qi X, Zhao K, Zhang X (2017) Distribution and failure modes of the landslides in Heitai terrace, China. Eng Geol 236:97–110. https://doi.org/10.1016/j.enggeo.2017.09.016
Peng JB, Li QC, Chen ZX, Liu YH, Fan W, Li XA (2008) Loess Cave Hazards. Science Press, Beijing ((in Chinese))
Peng JB, Sun P, Igwe O, Li XA (2018) Loess caves, a special kind of geo-hazard on loess plateau, northwestern China. Eng Geol 236:79–88. https://doi.org/10.1016/j.enggeo.2017.08.012
Peng D, Xu Q, Zhang X, Xing H, Zhang S, Kang K, Qi X, Ju Y, Zhao K (2019a) Hydrological response of loess slopes with reference to widespread landslide events in the Heifangtai terrace, NW China. J Asian Earth Sci 171:259–276. https://doi.org/10.1016/j.jseaes.2018.12.003
Peng JB, Wang QY, Men YM, Xu Q, Zhuang JQ (2019b) Landslide disaster in Loess Plateau. Science Press, Beijing ((in Chinese))
Peng JB, Wang SK, Wang QY, Zhuang JQ, Huang WL, Zhu XH, Leng YQ, Ma PH (2019c) Distribution and genetic types of loess landslides in China. J Asian Earth Sci 170:329–350. https://doi.org/10.1016/j.jseaes.2018.11.015
Phillips JD (2006) Evolutionary geomorphology: thresholds and nonlinearity in landform response to environmental change. Hydrol Earth Syst Sci 10(5):731–742. https://doi.org/10.5194/hess-10-731-2006
Qi X, Xu Q, Liu FZ (2017) Analysis of retrogressive loess flowslides in Heifangtai, China. Eng Geol 236:119–128. https://doi.org/10.1016/j.enggeo.2017.08.028
Ronchetti F, Borgatti L, Cervi F, Lucente CC, Veneziano M, Corsini A (2007) The Valoria landslide reactivation in 2005–2006 (Northern Apennines, Italy). Landslides 4:189–195. https://doi.org/10.1007/s10346-006-0073-9
Samia J (2018) Landslide path dependency and its impact on space-time modelling of landslide susceptibility. Wageningen University. https://doi.org/10.18174/459815
Samia J, Temme A, Bregt A, Wallinga J, Guzzetti F, Ardizzone F, Rossi M (2017a) Do landslides follow landslides? Insights in path dependency from a multi-temporal landslide inventory. Landslides 14(2):547–558. https://doi.org/10.1007/s10346-016-0739-x
Samia J, Temme A, Bregt A, Wallinga J, Guzzetti F, Ardizzone F, Rossi M (2017b) Characterization and quantification of path dependency in landslide susceptibility. Geomorphology 292:16–24. https://doi.org/10.1016/j.geomorph.2017.04.039
Selby MJ (1982) Hillslope Materials and Processes. Oxford University Press, Oxford
Shahverdian S, Macfarlane W, Stevens G, Meier M, Wheaton JM (2017) Geomorphic response to Pilot River restoration on the San Rafael River, Utah: a pilot installation of Beaver Dam Analogues. https://doi.org/10.13140/RG.2.2.18963.37928
Shangguan CC (2017) The risk evaluation on the geological hazards in Heyang County. Xi’an: Chang’an University. (in Chinese)
Shen YD, Qiu HJ, Hu S, Liu ZJ, Zhang Y, Yang DD, Cao MM (2019) Hydrogeological structure exploration and failure cause analysis of landslide in loess tableland. Quaternary Sciences 39(6): 1503–1513. http://www.dsjyj.com.cn/CN/https://doi.org/10.11928/j.issn.1001-7410.2019.06.17 (in Chinese)
Smolíková J, Blahut J, Vilímek V (2016) Analysis of rainfall preceding debris flows on the Smědavská hora Mt., Jizerské hory Mts., Czech Republic. Landslides 13(4): 683–696. https://doi.org/10.1007/s10346-015-0601-6
Sun Q, Zhang L, Ding XL, Hu J, Li ZW, Zhu JJ (2015) Slope deformation prior to Zhouqu, China landslide from InSAR time series analysis. Remote Sens Environ 156:45–57. https://doi.org/10.1016/j.rse.2014.09.029
Sun W, Tian Y, Mu X, Zhai J, Gao P, Zhao G (2017) Loess landslide inventory map based on GF-1 satellite imagery. Remote Sens-Basel 9(4):314. https://doi.org/10.3390/rs9040314
Tarolli P (2014) High-resolution topography for understanding Earth surface processes: Opportunities and challenges. Geomorphology 216:295–312. https://doi.org/10.1016/j.geomorph.2014.03.008
Temme AJ, Keiler M, Karssenberg D, Lang A (2015) Complexity and non-linearity in earth surface processes–concepts, methods and applications. Earth Surf Proc Land 40(9):1270–1274. https://doi.org/10.1002/esp.3712
Wang W, Wang Y, Sun Q, Zhang M, Qiang Y, Liu M (2018) Spatial variation of saturated hydraulic conductivity of a loess slope in the South Jingyang Plateau, China. Eng Geol 236:70–78. https://doi.org/10.1016/j.enggeo.2017.08.002
Wang W, Li J, Li X, Wang Y (2020a) Evolution of the hydrogeological structure and disaster-generating mechanisms of landslides in loess slopes of the southern Jingyang Plateau, Shaanxi, China. Hydrogeol J 28:2223–2239. https://doi.org/10.1007/s10040-020-02195-x
Wang XG, Wang JD, Li P, Zhan HB, Qiu HJ, Hu S (2020b) Moisture Content Effect on the Creep Behavior of Loess for the Catastrophic Baqiao Landslide. CATENA 187:104371. https://doi.org/10.1016/j.Catena.2019.104371
Wen BP, He L (2012) Influence of lixiviation by irrigation water on residual shear strength of weathered red mudstone in Northwest China: Implication for its role in landslides’ reactivation. Eng Geol 151:56–63. https://doi.org/10.1016/j.enggeo.2012.08.005
Wheaton JM, Brasington J, Darby SE, Sear DA (2010) Accounting for uncertainty in DEMs from repeat topographic surveys: improved sediment budgets. Earth Surf Proc Land 35(2):136–156. https://doi.org/10.1002/esp.1886
Wu Q, Xu LQ, Li SZ, Liu X, Wang PC, Cao XZ, Suo YH (2013) Active tectonics in the Central North China Block and the cause of the formation of the Fenwei Graben. Earth Science Frontiers 20(4): 104–114. http://www.earthsciencefrontiers.net.cn/CN/Y2013/V20/I4/104 (in Chinese)
Wu WJ, Su X, Meng XM (2014) Characteristics and Origin of Loess Landslides on Loess Terraces at Heifangtai, Gansu Province, China. Appl Mech Mater 694:455–461. https://doi.org/10.4028/www.scientific.net/AMM.694.455
Xie M, Zhao W, Ju N, He C, Huang H, Cui Q (2020) Landslide evolution assessment based on InSAR and real-time monitoring of a large reactivated landslide, Wenchuan, China. Eng Geol 277:105781. https://doi.org/10.1016/j.enggeo.2020.105781
Xu L, Dai FC, Tu XB, Javed I, Woodard MJ, Jin YL, Tham LG (2013) Occurrence of landsliding on slopes where flowsliding had previously occurred: an investigation in a loess platform, North-west China. CATENA 104:195–209. https://doi.org/10.1016/j.catena.2012.11.010
Xu L, Qiao X, Wu C, Iqbal J, Dai F (2012) Causes of landslide recurrence in a loess platform with respect to hydrological processes. Nat Hazards 64(2):1657–1670. https://doi.org/10.1007/s11069-012-0326-y
Xu Q, Peng DL, Qi X, Zhu X, Dong XJ (2020) Genesis mechanism, early identification, monitoring and warning of Heifangtai loess landslides. Science Press, Beijing ((in Chinese))
Yan Y, Cui Y, Guo J, Hu S, Wang Z, Yin S (2020a) Landslide reconstruction using seismic signal characteristics and numerical simulations: case study of the 2017 “6.24” Xinmo landslide. Eng Geol 270: 105582. https://doi.org/10.1016/j.enggeo.2020.105582
Yan Y, Cui Y, Tian X, Hu S, Guo J, Wang Z, Yin S, Liao L (2020b) Seismic signal recognition and interpretation of the 2019 “7.23” Shuicheng landslide by seismogram stations. Landslides 17(5): 1191-1206. https://doi.org/10.1007/s10346-020-01358-x
Yang DD, Qiu HJ, Hu S, Zhu YR, Cui YF, Du C, Liu ZJ, Pei YQ, Cao MM (2021) Spatiotemporal distribution and evolution characteristics of successive landslides on the Heifangtai tableland of the Chinese Loess Plateau. Geomorphology 378:107619. https://doi.org/10.1016/j.geomorph.2021.107619
Yang SL, Ding ZL (2017) Spatial changes in grain size of loess deposits in the Chinese Loess Plateau and implications for palaeoenvironment. Quaternary Sciences 37(5): 934–944. https://doi.org/10.11928/j.issn.1001-7410.2017.05.02 (in Chinese)
Yang W, Qi W, Wang M, Zhang J, Zhang Y (2017) Spatial and temporal analyses of post-seismic landslide changes near the epicentre of the Wenchuan earthquake. Geomorphology 276:8–15. https://doi.org/10.1016/j.geomorph.2016.10.010
Zeng RQ, Meng XM, Zhang FY, Wang SY, Cui ZJ, Zhang MS, Chen G (2016) Characterizing hydrological processes on loess slopes using electrical resistivity tomography-a case study of the Heifangtai Terrace, Northwest China. J Hydrol 541:742–753. https://doi.org/10.1016/j.jhydrol.2016.07.033
Zhang C, Yin Y, Dai Z, Huang B, Zhang Z, Jiang X, Wang L (2021) Reactivation mechanism of a large-scale ancient landslide. Landslides 18:397–407. https://doi.org/10.1007/s10346-020-01538-9
Zhang FY, Wang GH (2018) Effect of irrigation-induced densification on the post-failure behavior of loess flowslides occurring on the Heifangtai area, Gansu, China. Eng Geol 236:111–118. https://doi.org/10.1016/j.enggeo.2017.07.010
Zhang MS, Li TL (2011) Triggering factors and forming mechanism of loess landslides. Journal of Engineering Geology 19(4): 530–410. http://www.gcdz.org/article/id/10051 (in Chinese)
Zhang Y, Guo C, Lan H, Zhou N, Yao X (2015) Reactivation mechanism of ancient giant landslides in the tectonically active zone: a case study in Southwest China. Environ Earth Sci 74(2):1719–1729. https://doi.org/10.1007/s12665-015-4180-6
Zhang Z (2020) Mechanism of the 2019 Yahuokou landslide reactivation in Gansu, China and its causes. Landslides 17:1429–1440. https://doi.org/10.1007/s10346-020-01384-9
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Thanks for the free universal remote monitoring equipment for geological disasters provided by Insentek Co. Ltd (https://www.insentek.com/).
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This research was funded by the National Natural Science Foundation of China (grant nos. 42001006 and 41771539), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (grant no. 2019QZKK0902), the China Postdoctoral Science Foundation (grant no. 2019M663792), the International Science & Technology Cooperation Program of China (grant no. 2018YFE0100100), and the Open Fund of Shaanxi Key Laboratory of Earth Surface System and Environmental Carrying Capacity (grant no. 2019KFJJ01).
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Conceptualization: SH, HQ, NLW; methodology: SH, ZL, DY; formal analysis and investigation: SH, XW, SM, DY, NW, ZL, YS, ZS; writing, first draft: SH; review and editing: SH, HQ, NLW; approval of final draft: SH, HQ, NLW; supervision: HQ, NLW, MC.
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Hu, S., Qiu, H., Wang, N. et al. Movement process, geomorphological changes, and influencing factors of a reactivated loess landslide on the right bank of the middle of the Yellow River, China. Landslides 19, 1265–1295 (2022). https://doi.org/10.1007/s10346-022-01856-0
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DOI: https://doi.org/10.1007/s10346-022-01856-0