The Impact of Agricultural Irrigation on Landslide Triggering: a Review from Chinese, English, and Spanish Literature

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Review The Impact of Agricultural Irrigation on Landslide Triggering: A Review from Chinese, English, and Spanish Literature

Pablo Garcia-Chevesich 1,2,*, Xiaolu Wei 1, Juana Ticona 3, Gisella Martínez 3 , Julia Zea 3 , Vilma García 3 , Francisco Alejo 3 , Yao Zhang 4 , Hanna Flamme 1, Andrew Graber 1, Paul Santi 1, John McCray 1 , Edgard Gonzáles 3 and Richard Krahenbuhl 1

1 Center for Mining Sustainability, Colorado School of Mines, Golden, CO 80401, USA; [email protected] (X.W.); hﬂ[email protected] (H.F.); [email protected] (A.G.); [email protected] (P.S.); [email protected] (J.M.); [email protected] (R.K.) 2 International Hydrological Programme, UNESCO, 11200 Montevideo, Uruguay 3 Centro de Minería Sostenible, Universidad Nacional de San Agustín de Arequipa. Calle Santa Catalina 117, Cercado, Arequipa 04000, Peru; [email protected] (J.T.); [email protected] (G.M.); [email protected] (J.Z.); [email protected] (V.G.); [email protected] (F.A.); [email protected] (E.G.) 4 Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-520-270-9555

Abstract: Landslides are considered a natural process, with hundreds of events occurring every year in many regions of the world. However, human activities can signiﬁcantly affect how stable a slope or cliff is, increasing the chances of slope collapses. Moreover, agricultural irrigation has potential to saturate subsurface materials well below ground level and is known to be an important factor that can trigger landslides in many countries. A macroregional literature review of irrigation-induced landslides was developed in this investigation, considering what has been published in Chinese, English, and Spanish. A total of 115 peer-reviewed papers, books and book chapters, graduate and Citation: Garcia-Chevesich, P.; Wei, undergraduate theses, and government reports were found, including 82 case studies (23 in Chinese, X.; Ticona, J.; Martínez, G.; Zea, J.; 26 in English, and 33 in Spanish). Results from this analysis indicate that studies focusing on this García, V.; Alejo, F.; Zhang, Y.; Flamme, important topic have increased exponentially since the 1960s, with most irrigation-induced landslides H.; Graber, A.; Santi, P.; et al. The occurring in dry climates (precipitation less than 600 mm/year), with rainfall concentrated during Impact of Agricultural Irrigation on summer months. The majority of studies have been done in the loess region of China (Asian region), Landslide Triggering: A Review from followed by Peru (Latin American region), though cases were found from other macroregions (African, Chinese, English, and Spanish Litera- Indian, Russian, Angloamerican, and Indonesian). Based on this global review, new agricultural irrigation ture. Water 2021, 13, 10. projects located in collapsible areas must include a landslide risk analysis. Cultivated areas can follow https://dx.doi.org/10.3390/w13010010 a series of measures to minimize the chances of triggering a landslide, which would put human lives,

Received: 10 November 2020 ecosystems, food production, and infrastructure at risk. Accepted: 18 December 2020 Published: 23 December 2020 Keywords: landslides; irrigation; agriculture; slope stability; review

Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institutional 1. Introduction affiliations. Landslides occur around the world in a variety of geologic settings, representing a significant hazard to human lives [1]. Either as soil mass movement [2], debris flow [3], rock- fall [4], or combinations of these, landslides naturally occur because of a number of factors,

Copyright: © 2020 by the authors. Li- which can act alone or in combination, including saturation by rain water inﬁltration [5–11] censee MDPI, Basel, Switzerland. This and snow melting [12], both leading to groundwater changes and seepage erosion [13–15]; article is an open access article distributed increase in hydrostatic pressure in cracks and fractures [16]; topography [17,18]; ground under the terms and conditions of the shaking caused by earthquakes [19–24]; and even physical and chemical weathering [25]. Creative Commons Attribution (CC BY) Similarly, rock and soil properties strongly affect the likelihood of slope collapse [26,27], license (https://creativecommons.org/ with chances increasing with certain soil textures, e.g., [28,29], and geological origin, licenses/by/4.0/). e.g., [30], among other factors. The factors above can determine, for example, how far a

Water 2021, 13, 10. https://dx.doi.org/10.3390/w13010010 https://www.mdpi.com/journal/water Water 2021, 13, 10 2 of 17

landslide will travel from its origin [31], influencing how likely a populated area is to be affected by such a phenomenon. Landslides can also be triggered by human activities [32], with numerous cases around the world documented in the literature, including mass movements initiated by excavations [33], piping of soil [34], terracing [35,36], deforestation [37,38], and urbanization [39], among others. Moreover, agricultural irrigation can also lead to slope instability, a topic that has been studied for decades (e.g., [40]), simply because of its effects on percolation and aquifer recharge [41], decreasing effective soil cohesion in certain portions of the slope profile due to saturation [42]. Irrigation effects are even stronger when combined with other previously mentioned factors. While climate change continues to increase the likelihood of landslides occurring due to the presence of storms with higher intensities and rainfall depths [43–45], new agricultural projects are established every year, e.g., [46], affecting the water cycle not only by shortening the availability of the resource [47], but also by increasing the amount of water that percolates through the soil profile, increasing water table levels [48] and exacerbating the other adverse effects on slope stability. Considering all of the above and following the global macroregional classification defined by Andˇelet al. [49], herein we develop a worldwide review focused on the effects of agricultural irrigation on landslide triggering. The literature from this review was primarily from studies reported in Chinese, English, and Spanish languages (due to a lack of translation expertise, other languages such as Arabic, Japanese, Russian, etc., were not included). This extensive search was based on case studies and reviews from peer- reviewed scientific literature (SciELO, Latindex, Scopus, Web of Science, etc.), books and book chapters, graduate/undergraduate theses, and reports from government agencies, all under the following keywords: Landslides; Mass movement; Irrigation; Agriculture; Crops; Landslide-induced; and Landslide triggering. The above work was developed using different methods such as universities’ inter-library systems from China, Peru, and the United States, government agencies’ publication databases, and Internet search engines. Only cases where agricultural irrigation, irrigation canals, and/or storage ponds were the main landslide-inducing factor were considered in this analysis (not necessarily excluding other contributing factors such as rainfall). The overall purpose was to obtain a better understanding of the impacts of agricultural irrigation on slope mass movement mechanisms and identifying the best management practices intended to prevent future episodes.

2. A Worldwide Review on Landslides Triggered by Agricultural Irrigation Many irrigation-induced landslides have been reported in the Asian region, specifically in the Loess Plateau, which covers 635 thousand km2 in China. As a result of dam construction for hydroelectric power in parts of the Yellow River (West Gansu Province, North-West China), thousands of farmers were re-settled in the Heifangtai loess platform located 42 km southwest of Lanzhou city in Yongjing County (Gansu Province), where they started irrigating intensively around 1968, resulting in geological hazards such as soil salinization, among others [50], but also increasing perched water table levels and satu- rating subsurface soil layers, e.g., [51]. This resulted in thousands of soil mass movement events in this area [40,52–56]. These events have been widely studied by Chinese scientists to describe the process of slope collapse, e.g., [13,57–68], and to understand why many of them transition to dangerous fluidized forms [69–74]. Furthermore, Derbyshire [75] developed a complete analysis of the risk that landslides represent in China’s loess region, concluding that agricultural activities represent a clear threat. Irrigation-induced landslides in the region are often triggered by infiltration through surface cracks and sinkholes [76–80]. In fact, irrigation not only triggers landslides in this area [81], but the occurrence of a landslide itself also increases the chances for new events to happen [82], as observed in the Jiaojiayatou region [83,84]. Studies suggest a noticeable relationship between groundwater table level changes (as a result of agricultural irrigation) and landslide history [85–87], with authors such as Zhang [88] reporting water Water 2021, 13, 10 3 of 17

table increases of more than 20 m over four decades in some areas of the Heifangtai loess plateau, and Xu et al. [89], who estimated an annual increase of 0.18 m in the same region. Moreover, Xu et al. [90] simulated the hydrological effects of irrigation on the stability of a loess cliff edge in Heifangtai, concluding that the main triggering cause of the ongoing landslides within this geological formation was the establishment of large agricultural fields. Also in the Heifangtai Platform, Gu et al. [91] and Wu et al. [92] investigated the effects of irrigation on the stability of the loess landform, with both studies concluding that the excess water from agricultural flooding irrigation systems was primarily responsible for recent landslides in the area, agreeing with the results obtained by Hou et al. [93] on the instability of the Heifangtai platform, and Duan et al. [94] on the instability of the Jingyang loess platform located in the Shaanxi Province (northern China). In that same region, Li and Jin [95] analyzed the initiation of deep landslides, concluding that the triggering cause was an increase in water tables created by intense irrigation activities, similar to what Gu et al. [96], Gao et al. [97], and Qui et al. [98] concluded in their analyses of landslides in the Gansu, Sichuan, and Wuhai provinces, respectively. Early studies such as Lei [99] and Meng and Derbyshire [55] describe how seepage water from earthen irrigation canals on the upper portions of loess formations can trigger landslide events. Later on, Wen and He [100] evaluated the effects of percolated water from irrigation projects on the reactivation of landslides located in red mudstone plateaus of northern China, suggesting that agriculture represents a concern for future mass movements in the Lanzhou area, one of the largest cities in northern China. Ma et al. [101] used field investigations, geological exploration, numerical simulation, isotropically con- solidated undrained triaxial tests, and ring shear tests on four landslides that occurred simultaneously in the Shaanxi Province to identify their initiation and movement mechanisms, concluding that the main cause for such events was the presence of diversion-based irrigation canals, which constantly percolate water downward. This finding agrees with Xi et al. [102], who concluded that leakage from irrigation canals was the main triggering factor in the Gaoloucun landslide (Shaanxi Province). Wang [103] was a pioneer of irrigation-induced landslide prediction in China, an- ticipating the occurrence of the Huangci landslide in Yongjing county (Gansu Province). Furthermore, landslide modeling in China’s loess region has also been done by authors such as Wang et al. [104], Cui et al., [105], Lian et al. [106], Xing [107], and Zhang and Wang [108], including centrifuge experiments [109,110], Visual MODFLOW [87], and FEFLOW [84] to simulate the relationship between the evolution of groundwater dynamics and landslide disasters. These studies all identified intensive irrigation as the main factor triggering mass movements. Similarly, experimental rainfall and irrigation simulations made by Cao and Yin [111] and Ding [112], as well as slope stability analyses made by Duan et al. [113], also concluded that artificially induced water percolation from agricultural activities was the main triggering factor in landslides occurring in dry climatic regions of the country. According to Zhuang et al. [114], most landslides in the Chinese loess region are shallow, with volumes of less than 100,000 m3, with concave south-east-facing slope profiles (i.e., less vegetation), slope angles of 20–35◦, and occurring as long run-out events that generally transform into mudflows. In this sense, antecedent soil moisture (from rainfall or irrigation) plays an important role in triggering loess landslides. Four types of landslides have been identified for the loess region of China [67]: ir- regularly moving, fast moving, slow moving, and mudflows. Lei [115] notes that human activities can cause all of these, and one initiating from plateau surfaces is often related to irrigation activities. Most irrigation-induced landslides in this region occur during the thaw period (March) and the rainy season (July) [116], and they have been carefully monitored using high-resolution satellite data [117] and mitigated using a variety of measures [50]. Irrigation-induced landslides have been also documented in the Latin American region, with most of the events located in Peru, a country that started evaluating this type of mass movement in the 1960s. Landslides have occurred primarily in the Aczo (Ancash) [118] and Yuncanpata regions (Cerro de Pasco) [119]. One of the most recent studies was developed Water 2021, 13, 10 4 of 17

by Lacroix et al. [120], who used 40 years of satellite data to document the long-term effects of irrigation on landslides in the Vitor and Siguas valleys (southern Peru, an extremely dry region), concluding that irrigation initiated very large slow-moving events, affecting one-third of the valleys within their study area. They indicate that this phenomenon started about 20 years after irrigation began, being concentrated in areas near surface water ponds. Also in southern Peru, Araujo-Huamán et al. [121] characterized a large landslide that occurred in 2005 in the Siguas region, assuming (though not scientifically proving) that the mass movement occurred as a result of an irrigation project established decades ago on the desert plateau above. Similarly, Valderrama et al. [122] documented landslides in the nearby Lari region since 1960 (the beginning of intensive agricultural irrigation projects in the area), with dangerous events that happened in 1983, 1987, and 2009. In a series of landslides that occurred in Matarani (Tacna), Soncco and Manrique [123] concluded that water seeping from irrigation canals initiated the soil mass movements. Similarly, other landslides have been triggered by water leaking from irrigation canals in San Pedro (Ayacucho) [124], Challa (Tacna) [125], Llocche (Lima) [126], Cerro Pucutura (Lima) [127], Rodeopampa (Cajamarca) [128] , Horno Huayoc (Huancavelica) [129] , Santiago de Anchucaya (Lima) [130], Astobamba (Cajamarca) [131], San Mateo, Lima [132], Aurahuá (Huancavelica) [133], Carampa (Huancavelica) [134], Huamancharpa (Cusco) [135], Ccochalla (Ayacucho) [136], Huellap (Ancash) [137], La Lampa (Cajamarca) [138], Higos- bamba, Hichabamba, Huayllabamba, and Churucana (Cajamarca) [139], Vilcabamba [140], and Santa María de Otopongo (Lima) [141]. Water leaking from irrigation ponds had initiated landslides in Pilchaca (Huancavelica) [142], and Llapay (Lima) [143]. In Argentina, Jurio et al. [144] used over 20 years of monitoring data to characterize and analyze the gravitational movements affecting the edge of the plateau in Vista Alegre (Neuquén Province). They attributed the landslides to an irrigated area of only 12 hectares located above the collapsed volume in this arid region of the country. Similarly, Villaseñor-Reyes et al. [145] evaluated two landslide events on volcanic- derived soils through an integrated study, including detailed lithology, morpho-structural inventories, analysis of land use, and pluviometric regimes in eastern Michoacán (Mexico), attributing excessive agricultural irrigation as the main triggering factor. Also in Mexico, García [146] concluded that excessive irrigation was one of the main contributing factors leading to landslides in Metztitlán. In Ecuador, Yadún et al. [147] documented several irrigation-triggered landslides in the Escudillas watershed, developing a risk map using GIS techniques and field visits, emphasizing the need to be aware of how irrigation canals affect soil mass movement. Similarly, Samaniego [148] evaluated past irrigation-triggered landslides in Pueblo Viejo (Ecuador), an old mining town that became farmland later on. The author developed a risk analysis for future episodes so that local authorities can make proper prevention decisions. Rosales and Centeno [149] evaluated the susceptibility to slope failure in La Conquista (Nicaragua), attributing inefficient irrigation as the main reason why several landslide episodes have occurred (and might continue to occur) in the area, similar to what Reyes et al. [150] and Carbajal [151] concluded in their studies at the Talgua (Honduras) and Tabarcia (Costa Rica) watersheds, respectively. As for the Indonesian region, the 2018 Palu valley landslides in Indonesia killed thousands of people and were initially attributed to a strong earthquake. However, a recent study by Bradley et al. [152] concluded that the main triggering factor in such devastating landslide was actually rice irrigation fields located uphill, a statement corroborated later on by Watkinson and Hall [153] and Cummins [154]. Evidence of irrigation-induced landslides have been also documented in the An- gloamerican region. Knott [155] investigated the causes triggering two long-dormant landslides in Ventura County (California), attributing irrigation of avocado fields as the main factor. Similarly, Clague and Evans [156] evaluated a series of landslides next to the Thomp- son River valley (British Columbia, Canada) that have been occurring since 1880 over a 10 km long section of the riverside, concluding that most ancient events were charac- Water 2021, 13, 10 5 of 17

terized as slow moving, until uphill agriculture began, triggering rapid and dangerous mass movements. After a landslide killed 300 people in Bududa (eastern Uganda, African region), Gorokhovich et al. [157] concluded that even though the event was triggered by heavy rains, a variety of factors contributed to initiation of the slope failure, including the absence of drainage systems, the local topography, as well as land use changes (Arabica coffee plantations). The abundance of rainfall in the area, however, means that irrigation is not required, except perhaps during the dry season (January), so land use and cover changes may be more important than the addition of water. Through geophysics, Domej et al. [158] evaluated the contributions of irrigation canals on slope stability in Tajikistan (Russian region), where landslides occurred and were initially thought to be caused by either glacial retreat and/or strong earthquakes. This area of the country is known for its extreme aridity and for the presence of a permeable surface layer that covers a clay-rich horizon, making it susceptible to landslides since agricultural ﬁelds were established, according to the authors. Similarly, though triggered by an earthquake, Ishihara et al. [159] demonstrated that agricultural irrigation was a critical component for the initiation of a landslide in that same country. Finally, in Pakistan (Indian region), Ali et al. [160] challenged the attribution of climate change as the primary factor triggering landslides, identifying the political dimension of water management that also contributed to mass movement in the Yourjogh’s mountainous region. The authors concluded that while geological factors conducive to landslides are present in the area, the main cause of the events is the deterioration of the water governance system, leading to negligent overuse of the resource, which is readily available. No irrigation-induced landslides were found for the Islamic, European, Sino-Japanese, and Australian-Oceanic regions within the Chinese, English, or Spanish scientiﬁc literature, or were not reviewed because of translation issues.

3. Discussion and Conclusions A total of 115 irrigation-induced landslide investigations around the world, including 82 case studies in Chinese (23), English (26), and Spanish (33), were found, considering scientiﬁc publications, university theses, books and book chapters, and reports from government agencies. It is interesting to note that the number of worldwide studies focusing on the effects of irrigation on landslide triggering has increased exponentially during the last decades (Figure1), with four additional studies (including this one) already been published in 2020 (Lacroix et al. [120] and Ingemmet [141] in Peru, and Yadún et al. [147] in Ecuador). This is expected, considering that increasing global agricultural demands are anticipated to double by 2050 [161]. As a consequence, new irrigation projects are established every year in areas that are susceptible irrigation-induced landslides, which are often mistakenly attributed to earthquakes, e.g., [152]. Results from this literature review indicate that irrigation-induced landslides represent a growing risk in areas where agriculture and certain climatic and geologic conditions are present. Moreover, documented landslides from around the world suggest that the majority of irrigation-induced landslides occur in dry climates (Table1), with 80% of cases happening on areas with less than 600 mm/year of rainfall. Similarly, 95% of cases have occurred in climates where rainfall is concentrated during summer months (see Table1) , possibly because of the lower kinematic viscosity of rainwater (i.e., higher inﬁltration capacity) at warmer temperatures, e.g., [162], resulting in higher effective permeability of soil and higher saturation of soil masses, leading to collapse, e.g., [112,113]. In other words, wherever underground geologic layers are dry and stable under ambient conditions, but highly unstable when saturated due to percolating water from irrigation projects (and many times combined with summer rainfall), landslides are more likely to occur. Such collapses are triggered mainly due to an increase in water table levels (e.g., [99]) or even from small seepage impediments [51]. Finally, 49% of the reported irrigation-induced landslides have Water 2021, 13, 10 6 of 17

been documented on loess terrains, followed by sedimentary (26%), alluvial (13%), and Water 2020, 12, x FOR PEER REVIEW 6 of 17 volcanic (12%) geologic materials.

2010-2019

2000-2009

1990-1999 Time period

1989 or earlier

0 1020304050607080 Number of publications

Figure 1. Evolution of worldwide studies (in Chinese, English, and Spanish) focusing on irrigation- inducedFigure landslides, 1. Evolution by decade of worldwide (not including studies investigations (in Chinese, English, published and in Spanish) 2020). focusing on irrigation-induced landslides, by decade (not including investigations published in 2020).

Table 1. Documented cases of irrigation-inducedResults from landslides this literature within the review Chinese indicate (*), English that (**), irrigation-induc and Spanish (***)ed literature. landslides represent aAnnual growing Precipitation risk in areas where agriculture and certain climatic and geologic conditions Country Region Rainy Season Geological Material Source are present. Moreover,(mm) documented landslides from around the world suggest that the majority of irrigation-induced landslides occur in dry climates (Table 1), with 80% of cases Argentina Neuquén 200 Alluvial [144] *** happening on areas with less than 600 mm/year of rainfall. Similarly, 95% of cases have Canada Spences Bridgeoccurred in climates 233 where rainfall is concentrated during Sedimentary summer months [156(see] Table ** 1), possibly because of the lower kinematic viscosity of rainwater (i.e., higher infiltration[105] ** capacity) at warmer temperatures, e.g., [162], resulting in higher effective permeability of [85]* soil and higher saturation of soil masses, leading to collapse, e.g., [112,113]. In other words, wherever underground geologic layers are dry and stable under ambient[86]* conditions, but highly unstable when saturated due to percolating water from irrigation[96]* projects (and many times combined with summer rainfall), landslides are more likely to oc- [91] ** cur. Such collapses are triggered mainly due to an increase in water table levels (e.g., [99]) or even from small seepage impediments [51]. Finally, 49% of the reported irrigation-in-[83]* duced landslides have been documented on loess terrains, followed by[ 106sedimentary] ** (26%), alluvial (13%), and volcanic (12%) geologic materials. [55] ** China Gansu 316Summer Loess Table 1. Documented cases of irrigation-induced landslides within the Chinese (*), English[103]* (**), and Spanish (***) literature. [80]* Annual Precipitation Rainy [116]* Country Region Geological Material Source (mm) Season [68]* Argentina Neuquén 200 Alluvial [144] *** [100] ** Spences Canada 233 Sedimentary [156] ** Bridge [92] ** [56]*[105] ** [107]*[85] * [90] **[86] * Summer [96] * [63] ** [91] ** China Gansu 316 Loess [88]*[83] * [77]*[106] ** [55] ** [103] * [80] *

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Table 1. Cont.

Annual Precipitation Country Region Rainy Season Geological Material Source (mm) 350 [87]* 358 [93]* 549 [78]* 504 [79]* [72] ** [95] ** [76]*

548 [58] ** [113] ** [94] ** Shaanxi [101] ** [73]* [70] ** [71]* 585 [69] ** [102]* 648 [99]* 1000 [97]* Wuhai 159 [98]* Costa Rica Mora 844 Summer, fall Volcanic [151] *** Bolívar [147] *** Ecuador 1626 Summer Pueblo Viejo Sedimentary [148] *** Honduras Talgua 1337 Summer, fall [150] *** [152] ** Spring and Indonesia Palu valley 1432 Alluvial [154] ** summer [153] ** Metztitlán 437 Summer Sedimentary [146] *** Mexico Michoacán 553 Volcanic [145] *** Summer, fall Nicaragua La Conquista 1300 Sedimentary [149] *** Pakistan Yourjogh 451 Winter Volcanic [160] ** Aczo 350 Summer [118] *** Ancash 1000 Year round [137] *** Peru Sedimentary [123] *** Arequipa 350 Summer [122] *** Water 2021, 13, 10 8 of 17

Table 1. Cont.

Annual Precipitation Country Region Rainy Season Geological Material Source (mm) Ayacucho 350 Volcanic [136] *** Barranca 5Summer Alluvial [141] *** 32 Sedimentary [138] *** Cajamarca 400 Fall Alluvial [128] *** 700 [139] *** Sedimentary Cerro Pucutura 525 [127] *** Challa 12 [125] *** Volcanic Summer [140] *** Cusco 700 Sedimentary [135] *** 72 Alluvial [133] *** Huancavelica 350 Sedimentary [142] *** 500 Spring Alluvial [129] *** 702 Summer Volcanic [134] *** 350 Spring and Alluvial [130] *** Lima 550summer Volcanic [131] *** Llapay 500 Alluvial [143] *** Llocche 350 Sedimentary [126] *** San Mateo 342 [132] *** Summer Volcanic San Pedro 29 [124] *** Siguas 16 [121] *** Sedimentary Vítor-Siguas 16 [120] ** Spring and Yuncanpata 100 Alluvial [119] *** summer Fall, winter, Dushanbe 568 Loess [159] ** Tajikistan and spring Tusion 170 Spring [158] ** Uganda Bududa 188 Year roundSedimentary [157] ** United States Oak Ridge, CA 432 Winter [155] **

In terms of global geographical distribution, most irrigation-induced landslides have been reported in the Asian Region, with the majority of cases documented in China, followed by Peru in the Latin American Region (see Figures2 and3, and Table1). Moreover, the most widely studied geological formation on the planet, when it comes to irrigation-induced landslides, is the Loess Plateau of China. In fact, just like volcanic, alluvial, and sedimentary materials in general, dry loess usually has high stability, e.g., [75], so disturbances such as earthquakes and excavations are normally the main factors triggering landslides in these regions, e.g., [22]. However, loess may collapse rapidly with increasing water content from irrigation [65,163], which is not only one of the most common triggers of loess landslides in China (as discussed earlier), but it also contributes to collapsed masses travelling longer distances downwards, thereby putting local communities at risk [69]. The most direct external manifestations of the collapsibility of loess are sinkholes and surface cracks, with the former formed on the edge of the plateau, while the latter formed mostly through the Water 2021, 13, 10 9 of 17

upper and lower layers of hillslopes, reducing the integrity of the soil mass and affecting Water 2020, 12, x FOR PEER REVIEW the stability of the plateau, e.g., [59,71]. Both phenomena represent important ways9 of for17 surface storm runoff to percolate into lower layers, in addition to the constant percolation Water 2020, 12, x FOR PEERthat REVIEW occurs below irrigation. 9 of 17 Asian

Latin American Asian

IndonesianLatin American

AngloamericanIndonesian Russian Macroregion Angloamerican

Indian Russian Macroregion

African Indian

0African 1020304050607080 Number of publications 0 1020304050607080

Number of publications Figure 2. Geographical distribution of publications focusing on irrigation-induced landslides, fol-

lowingFigure the 2. Geographical global macroregional distribution classification of publications defined focusing by Ande on irrigation-inducedľ et al. [49] and the landslides, scientific litera- follow- tureing thepublished globalFigure macroregionalin 2.Chinese, Geographical English, classification distribution and Spanish. defined of publications by Andel˘ fo etcusing al. [49 ]on and irrigation-induced the scientific literature landslides, following the global macroregional classification defined by Andeľ et al. [49] and the scientific litera- published in Chinese, English, and Spanish. ture published in Chinese, English, and Spanish.

Figure 3. Number of irrigation-inducedFigure 3. Number landslides of irrigation-induced documented within landslides the Chinese, do English,cumented and within Spanish the literature, Chinese, by English, country. and Spanish literature, by country. DespiteFigure the 3. above, Number studies of irrigation-induced have shown that landslides irrigation-induced documented within events the can Ch beinese, avoided, English, and e.g.,Despite [164Spanish]. A the simple above,literature, geological studies by country. have analysis shown of that the irrigation-induced material located below events current can be oravoided, future agricultural projects could lead to the prevention of future episodes, speciﬁcally looking e.g., [164]. A simple geological analysis of the material located below current or future for the presence of geological components that can be stable when dry, but very collapsible agricultural projectsDespite could the above, lead to studies the prevention have shown of thatfuture irrigation-induced episodes, specifically events looking can be avoided, when saturated such as alluvial, loess, sedimentary, or volcanic units (as shown in this for the presencee.g., [164]. of geological A simple componentsgeological analysis that can of be the stable material when located dry, but below very collapsi-current or future review), e.g., [27,165–167]. Therefore, human activities (in this case, agricultural irrigation) ble when agriculturalsaturated such projects as alluvial, could loess,lead to sedimentary, the prevention or volcanic of future units episodes, (as shown specifically in this looking should include a landslide-triggering risk analysis, especially for projects located in dry review), e.g.,for the [27,165–167]. presence of Theref geologicalore, human components activities that (in can this be case, stable agricultural when dry, irrigation) but very collapsi- climates with rainfall concentrated in summer months [115]. Irrigation programs should should includeble when a landslide-triggering saturated such as alluvial, risk analysis, loess, sedimentary, especially for or projects volcanic located units (as in showndry in this avoid the establishment of new crops above hillslopes and cliffs, or on edges of plateaus, climates withreview), rainfall e.g., concentrated[27,165–167]. inTheref summerore, human months activities [115]. Irrigation (in this case, programs agricultural should irrigation) e.g., [120]. Moreover, landslide modeling, e.g., [168], as well as monitoring mass movement avoid theshould establishment include ofa landslide-triggeringnew crops above hill riskslopes analysis, and cliffs, especially or on edges for projects of plateaus, located in dry through high-resolution remote sensing [169] and other methods, e.g., [170–173], are key e.g., [120].climates Moreover, with landslide rainfall concentrated modeling, e. g.,in summer[168], as monthswell as [115].monitoring Irrigation mass programs move- should tools for predicting future irrigation-induced landslides. ment throughavoid high-resolution the establishment remote of new sensing crops [169] above and hill otherslopes methods, and cliffs, e.g., or [170–173], on edges are of plateaus, key tools e.g.,for predicting [120]. Moreover, future irrigation-inducedlandslide modeling, landslides. e.g., [168], as well as monitoring mass move- However,ment throughin most high-resolutioncases the damage remote has already sensing been [169] done, and other and agricultural methods, e.g., (irriga- [170–173], are tion) fieldskey have tools been for predictingestablished future for decades irrigation-induced in collapsible landslides. areas. In these cases, useful mitigation optionsHowever, include in mostminimizing cases the irrigation damage water has already input andbeen controlling done, and agriculturalthe rise of (irriga- water tables,tion) with fields the have purpose been ofestablished avoiding satufor decadesration of in slopes collapsible [88,99]. areas. This Incan these be done cases, useful mitigation options include minimizing irrigation water input and controlling the rise of water tables, with the purpose of avoiding saturation of slopes [88,99]. This can be done

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However, in most cases the damage has already been done, and agricultural (irrigation) fields have been established for decades in collapsible areas. In these cases, useful mitigation options include minimizing irrigation water input and controlling the rise of water tables, with the purpose of avoiding saturation of slopes [88,99]. This can be done through significantly decreasing the size of irrigation projects, using pumping wells to prevent groundwater from flowing towards potentially unstable slopes, setting up efficient groundwater and surface drainage systems on slopes, and returning farmland to forest and grass in potentially unstable areas [174]. Other approaches include perforating imperme- able layers to allow seepage water from irrigation to pass through, e.g., [175], or stabilizing landslides by constructing retaining walls or emplacing concrete and steel piles within the slope, for example [55,57]. Replacing flood-irrigation and earth irrigation canals (which result in more seepage water, soil mass saturation, and increases in water table levels) with more efficient irrigation practices can significantly reduce the probability of future landslide events. Moreover, drip irrigation systems have the advantage of minimizing the amount of water needed per unit area, simply because the technique provides moisture to the plant in a localized manner within the soil, normally directly to the root system. If applied during daylight (when plants are active), roots can immediately absorb the provided water, thus significantly reducing seepage volumes, e.g., [176]. Equally important is the application of water-retaining polymers (ideally, potassium- based to avoid environmental impacts), which can save up to 90% of water volumes needed to irrigate a certain area and can rapidly absorb the applied water during each irrigation period, retaining the water for use by plants over time. Additionally, polymers can decrease the number of times that a certain crop needs to be irrigated (e.g., from daily to biweekly in extreme cases, depending on site conditions and species). Therefore, the application of water retention polymers allows a farmer to cultivate more hectares with the same amount of available water, but more importantly it limits seepage processes, reducing the probability of landslides initiation, e.g., [177]. Following the same approach, reducing seepage from current irrigation structures such as canals and storage ponds can minimize the risk of landslides as well. This can be accomplished using liners of low-cost geotextiles instead of expensive concrete [178]. Finally, slopes can also be stabilized by afforestation, since the roots of trees help to reinforce soil and resist sliding [179]. It should be noted that revegetation programs typically should not include trees that require irrigation to become established (i.e., native or low water demanding species, ideally with water retaining polymers), since adding more water to the slope would defeat the purpose of stabilizing the slope by revegetation. While climate change continues to increase the risk of landslides [180,181], findings from this review suggest that agricultural irrigation can drastically change local hydrologic cycles in many regions of the world, e.g., [182], leading to the collapse of slopes that can endanger human lives, ecosystems, infrastructure, and food production, and possibly result in lawsuits, e.g., [183]. Finally, as Hermanns et al. [184] suggest, there is an urgent need to improve international collaboration for the mapping of landslides, as this review indicates that each country acts on its own.

Author Contributions: Conceptualization: P.G.-C., X.W., J.M.; methodology: P.G.-C., X.W., Y.Z., J.T., G.M., J.Z., V.G., F.A.; validation: P.G.-C., X.W., Y.Z., J.T., G.M., J.Z., V.G., F.A.; writing—original draft preparation: P.G.-C., X.W., Y.Z.; writing—review and editing: A.G., H.F., E.G., R.K., P.S., J.M.; project administration: E.G., R.K.; funding acquisition: E.G., R.K. All authors have read and agreed to the published version of the manuscript. Funding: Funding for this project was provided by the Center for Mining Sustainability, a joint venture between the Universidad Nacional San Agustin (Arequipa, Peru) and Colorado School of Mines (USA). Water 2021, 13, 10 11 of 17

Acknowledgments: Besides the valuable contributions from the Center for Mining Sustainabil- ity, Arequipa, Peru, the authors also thank the contributions from the Natural Resource Ecology Laboratory at Colorado State University. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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