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Soil deterioration in the southern Chihuahuan Desert caused by agricultural practices and meteorological events
Journal of Arid Environments 176 (2020) 104097
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Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
Soil deterioration in the southern Chihuahuan Desert caused by agricultural practices and meteorological events
T
María de Jesús Guevara Macíasa, Noel Carbajala, José Tuxpan Vargasb,∗ a b
Instituto Potosino de Investigación Científica y Tecnológica A.C. Camino a la Presa San José 2055, CP, 78216, San Luis Potosí S.L.P, Mexico Cátedras-CONACyT-Instituto Potosino de Investigación Científica y Tecnológica A.C. Camino a la Presa San José 2055, CP, 78216, San Luis Potosí S.L.P, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Remote sensing Land use impact Desertification Arid zones
In arid and semi-arid regions, unregulated land use changes as a result of poor planning, and the expansion of agricultural and livestock activities increase the risk of desertification and other potentially severe environmental impacts. Several consequences of improper land management practices are soil deterioration and erosion, which may be intensified by meteorological events. This paper presents a historical evaluation of the temporal and spatial evolution of soil deterioration in the southern Chihuahuan Desert in Mexico. A multi-temporal analysis of the study area during the 1995–2016 period was carried out using satellite data (Landsat series). Two seasons were analyzed to determine the influence of external events such as rainfall and wind: dry and rainy. The identification of soil conditions was based on the Brightness Index (BI) considering the complete satellite data set. The soil conditions were classified into five categories according to their reflectance values: highly deteriorated, deteriorated, in the process of being deteriorated, in good condition and other (clouds, water, non-soil). The change detection maps clearly show a growing trend wherein areas of deteriorated and eroded soil increase over time. Agriculture and strong winds are the two main factors involved in the soil deterioration process of the study region.
1. Introduction Land use change is generally a consequence of the exponential growth of the world's population and the expansion of urban areas. As the population increases, more land is needed to produce food and raw materials. Primary economic activities, including intensive agriculture and livestock activities, have often expanded into new areas without proper planning, modifying large tracts of land and leading to deforestation. Crop expansion has affected different ecosystems, mainly grassland and forest regions. In tropical regions, croplands have expanded mostly at the expense of forest cover over the past 40–50 years (Gibbs et al., 2010). Several environmental problems resulting from massive land use changes, such as desertification, have been well documented. During the Dust Bowl era in the United States, the cultivation of large crop areas combined with drought created severe dust storms. Inadequate agricultural practices such as the implementation of monocultures and the removal of vegetation cover were the principal causes of this phenomenon (Schoijet, 2005; Ravi et al., 2009). Currently, China is another example of a country experiencing accelerated environmental degradation caused by deforestation, overgrazing, agricultural
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overexploitation and the exhaustion and contamination of aquifers. Since 1978, China has initiated economic reform and an open-door policy, leading to rapid land use and land cover changes across most of its territory (Weng, 2002). As a result, the rate of desertification in China has increased. For example, the areas surrounding the Gobi Desert have undergone desertification because of overgrazing (Schoijet, 2005; Ravi, 2009). The impacts of agriculture in north-eastern China have caused the erosion of black soil to a depth of 0.4 m, resulting in a nearly 10% reduction in soybean production (Gao et al., 2014). In Mongolia, wind erosion and heavy grazing are the leading causes of sandy desertification and have triggered further impacts on the vegetation cover in addition to hastening erosion processes, leading to the loss of soil structure and soil deterioration (Zhao et al., 2005). In Argentina, about 80% of the territory is dedicated to agricultural or forestry activities; however, these activities have similarly increased rates of soil erosion and degradation. Hence, the expansion of the agricultural frontier and unplanned deforestation appear to have severely degraded natural resources (Maris, 2000). In Mexico, more than 80% of agricultural land shows some degree of degradation as a result of monocultures, deforestation or livestock activities. Estimates obtained indicate that around 97% of the land
Corresponding author. E-mail addresses: [email protected] (M. de Jesús Guevara Macías), [email protected] (N. Carbajal), [email protected] (J.T. Vargas).
https://doi.org/10.1016/j.jaridenv.2019.104097 Received 19 December 2018; Received in revised form 10 August 2019; Accepted 30 December 2019 0140-1963/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Arid Environments 176 (2020) 104097
M. de Jesús Guevara Macías, et al.
Fig. 1. Location of the study area. In a) the Chihuahua desert in Mexico is shown in gray tones (Morrone et al., 2017), in b) the state of Zacatecas is delimited with a red line, and the location of the study area is shown, and in c) a true color composite image acquired by the Landsat 8 platform is presented. The red circles indicate the location of meteorological stations within the study area: 1) Agronomía, 2) COBAEZ, 3) Arcinas 4) Mesa de Fuentes. The urban areas of Fresnillo and Zacatecas are identified by the acronyms UA1 and UA2 respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
area have considerably contributed towards land deterioration. Recent research confirmed that land use changes in this region had intensified soil degradation (Céspedes and Moreno, 2010; Velázquez Valle et al., 2014). In general, the mechanism for soil loss is a combination of the effects of wind and water erosion. In this area, the main factors are changes in land use, erosion caused by strong winds and intense dust storms associated with the passage of cold fronts. Bare soil areas contribute to this problem, as they act as sources of dust. Each year, the region experiences more than 50 cold fronts during the winter season according to the records of the meteorological stations near the study area. The National Institute of Forestry, Agriculture and Livestock Research (INIFAP, for its initials in Spanish) estimated a soil loss of about 3500–4000 tonnes km-2 in 2005. In the study region, average rainfall has been decreasing, leading to longer dry seasons, is the most significant impact of droughts during the winter due to increased wind erosion causing soil deterioration. For instance, soil loss during 12 h over a source area of 4320 km2 was previously determined to be approximately 9162 tonnes (Pineda et al., 2011). Concern over the effects of land use changes on the planet has led to the development of different methodologies for evidencing the impact of land use changes on natural resources. Remote sensing combined with image processing enables areas with modified land uses or covers or with a high level of soil deterioration to be identified. Based on satellite images, it is possible to quantify and classify the state of a vegetation cover (e.g., very deteriorated, deteriorated, in the process of being deteriorated or in good condition). Hence, a series of satellite images can be evaluated to determine the state of a particular
dedicated to agriculture in Mexico are very vulnerable to desertification processes. Studies conducted by Oropeza (2004) determined the Mexican territory as 48% highly susceptible to desertification and 49% as moderately susceptible. The change of land use for agricultural and livestock applications was transformed from areas with forest cover (82%) and grasslands (18%) according to Meneses (2008). The effect of land degradation due to land use change mainly affects arid and semiarid zones. Intensive deforestation, overgrazing, corrupt agricultural practices and soil management are elements that increase the risk of desertification. Two studies cases evidenced the process of desertification from the Chihuahua Desert. In 2006 (E. Huber-Sannwald et al.) realized an investigation in a rural area focusing on the interrelationship human-environment system that causes desertification — in the southern part from Chihuahua Desert in a temporal evolution study using indicators from desertification: Normalized Difference Vegetation Index (NDVI), Normalized Water Index (NDWI), Iron Oxides Index (OI) and Surface temperature (ST) (Noyola-Medrano et al., 2017). The results show that the most affected area in San Luis Potosí Plateau is the west portion. The southern part of the Chihuahua Desert in Zacatecas state is a semi-arid region with extensive grassland and scrub cover as well as a temperate forest in the high mountain areas, is not exempt from the environmental problems associated with land use changes in favor of agricultural and livestock activities. In this region, extensive areas of scrub cover have mostly been converted into new croplands and pasturelands. Additionally, the population growth of cities such as Zacatecas and Fresnillo and the corresponding expansion of the urban 2
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Fig. 2. Land use in the study area. Source: Vector data on land use and vegetation (scale 1:250000, series V from INEGI (2015).
Fig. 3. Schematic representation of the desertification process.
Fig. 4. Wind speeds recorded in the study area during the 2015 winter season.
deforestation, increase in cropland area and other changes in vegetation cover over time. This paper focuses on the evolution of soil deterioration in the southern Chihuahua Desert during the period 1995–2016. The study was conducted using Landsat satellite data and the implementation and evaluation of various indexes and classification
phenomenon over time (Singh et al., 1989; Jovanovic, 2011). Moreover, different techniques can be applied in remote sensing, including the calculation of the Normalized Difference Vegetation Index (NDVI), Bare Soil Index (BSI) and Normalized Difference Soil Index (NDSI), which can also be used, for example, to quantify the rate of 3
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dominant soil classes in the study area are xerosoles, yermosols, feozem, Eurasian, and kastanozem, with mostly a medium texture (World Reference Base, 2006) and reddish color. These soils are mainly used as rainfed croplands. The following soil texture classifications are present: loamy clay-sandy, clayey, clayey and sandy (INEGI, 2007). Fig. 2 shows the main land uses in the study area. Seasonal (rainfed) agriculture is dominant across the entire region, but significant irrigation zones agriculture are present. Different types of scrub can be observed, including the microphyllous scrub that covers the entire northeastern to the central portion of the study area. Several areas of rosette desert scrub are present in the southern part. Also, different associations of secondary vegetation can be observed across a large portion of the study area. Finally, a small area of secondary Quercus forest is found in the south-western region of the study area. Desertification in the study area is highly related to soil deterioration. Fig. 3 shows the desertification process in a schematic way, considering as a trigger the change of land use towards seasonal farming. First, a change in land use occurs wherein grasslands or forests are converted into croplands. Crops are mostly irrigated from May to October. However, during the winter dry season, the soils are devoid of vegetation, as shown in Fig. 3b. A series of cold fronts then propagates through the study area, frequently causing strong winds that interact with bare soils and increase the erosion process. The combined action of these natural and anthropogenic phenomena result in increased erosion and soil deterioration. In Fig. 3c, an image of a deteriorated soil is shown; the main character is the sandiness of the soil. It was also observed that irrigated agriculture does not represent a significant component for the erosion process in the study area. Therefore, the analysis focused on the predominant type of agriculture, i.e., rainfed agriculture. It should be noted that in the winter season the maximum wind speeds are obtained (see Fig. 4), promoting the transport and dragging of material and in turn contributing to soil degradation.
2. Methodology
Fig. 5. Methodological process for determining the level of soil deterioration.
Satellite data set were evaluated and acquired from U.S Geological Services on the following dates: Landsat 5 TM for 1995, 1996 and Landsat 8 OLI-TIRS for 2015 and 2016. Images were obtained for both the summer and winter seasons. In particular, the images corresponded with October 1995, January and October 1996, October 2015 and February 2016. Radiometric and atmospheric corrections were performed for each image according to the methods of Chander et al. (2009) and de Keukelaere (2018). For soil characterization and multitemporal analysis, several procedures were used, including the implementation of indices (NDVI, BSI, NDSI, BI), as well as supervised classification algorithms based on minimum distance (MD) and Maximum Likelihood (ML). Of the tested methods, the most consistent result in the multi-temporal analysis was the brightness index (BI), so it was chosen to determine the degree of soil deterioration. The highest values correspond to the land surface with the highest reflectivity, i.e., bare soil or soil with a low density of vegetation cover. This index is based on the reflectance of two visible bands and the near infrared band (NIR), which determine the surface brightness (Escadafal and Bacha, 1996) as follows:
algorithms and being the brightness index (BI) the one that presented better performance and the one used in this work. Changes in soil quality are identified as a result of agricultural and livestock practices, highlighting areas that have been highly deteriorated.
1.1. Study area The study area is located in the central-northern part of Mexico in a semi-arid region of the Chihuahua Desert (Fig. 1). It is situated in the State of Zacatecas on the central plateau of Mexico between the Western Sierra Madre and the Eastern Sierra Madre, encompassing an area of 7336 km2 (Fig. 1, the area enclosed by the black line). At the local level, the study area is located within two physiographic sub-provinces: the Zacatecas mountain ranges, valleys and plains, and the PotosinaZacatecana mountains ranges. The dominant vegetation is grassland and scrubland. Near urban areas, the primary land use is agriculture. The study region includes different types of coniferous forests in the high mountain ranges in addition to mesquite, desert scrub and grassland in the lower regions (Comisión Técnico Consultiva para la Determinación Regional de los Coeficientes de Agostadero 1980). The dominant climate is dry with warm summer, classified as BS1kw. The annual average temperature is 15–18 °C, and the average annual rainfall is 355 mm. The earliest outcrops are formed by low-grade metamorphic rocks (slates, phyllites, and schists) from the Triassic period that rise towards the North and West. Rocks containing clastic marine deposits (sandstones and shales) from the Cretaceous period and volcanic rocks and small intrusive bodies from the Tertiary period are also present. However, Quaternary alluvial deposits predominate (INEGI, 1981). The
BI= (G2+R2+NIR2)1/2
(1)
where G = the green band, R = the red band and NIR = the nearinfrared band. The resulting values were considered as proxies of the level of soil deterioration and were classified into five categories: highly deteriorated, deteriorated, in the process of being deteriorated, in good condition and other. It should be clarified that the spectral thresholds of each category were obtained in-site through observations using the percentage of land cover as a criterion. The BI index and classification were calculated and compared for all periods for which satellite data were available. The methodological process is summarised in Fig. 5. 4
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Fig. 6. Classification of deteriorated areas based on the Brightness Index (BI) during the rainy season of a) October 1995, b) October 1996 and c) October 2015.
3. Results
it is essential to note that highly deteriorated soils did not show significant differences between the analyzed dates. Records from the weather stations located within the study area are shown in Fig. 7 to corroborate the influence of rainfall. The black arrows indicate the days when the satellite images were obtained. In October 1995 and 1996, no rain fell in the days before the images were taken. However, in October 2015, a relatively big rainfall event occurred. Hence, an analysis of Figs. 6 and 7 reveals that the BI index is sensitive to the presence of humidity. The BI index must then be preferably applied during the dry season. It is important to note that the amount of rainfall in the study area is quite similar among the analyzed years, i.e., the amount of precipitation is not very variable.
The analysis of different patterns in the satellite imagery and BI over time allowed the evolution of soils without vegetation cover to be determined. An increase in rainfed croplands was observed. This process is relevant because these croplands are then exposed to the action of winds associated with the passage of cold fronts during winter, which can initiate or increase soil deterioration. In Fig. 6, the BI index classified into five categories of soil quality is displayed for October in 1995, 1996 and 2015, which can be considered as the end of the rainy season. During this month, the areas classified as highly deteriorated (in black) are practically the same in both 1995 and 2015. However, the areas that were deteriorated or in the process of being deteriorated significantly varied depending on the yearly rainfall intensity. By October 22, 2015, it appears that land cover in good condition increased, but this is also due to the amount of rain that fell on days close to the day the image was taken. The rainfall level influences soil humidity and grass growth and thereby impacts the values of the BI index. However,
4. Discussion Over a more extended period, noticeable changes in soil quality are evident. In Fig. 8, the soil quality determined from satellite images captured 20 years apart can be compared. Significant changes are 5
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Fig. 7. Rainfall for several months during a) 1995, b) 1996 and c) 2015 in the study area. The arrows indicate the dates (October 31, 1995; October 17, 1996; October 22, 2015) when satellite images were captured.
Fig. 8. Satellite images of soil deterioration in a) February 1996 and b) February 2016.
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Fig. 9. Rainfall in January and February 2016 in the study area. The arrow indicates the date when the satellite image (February 11, 2016) was captured.
Fig. 10. Detailed comparison of a) the zoomed-in area in b) February 1996 and c) February 2016.
were identified. It represents a significant change in only 20 years after the date that the land tenure law was changed. It is also important to mention that, in the southern part of the study area, highly deteriorated soils were not present in 1996 but underwent a transition from deteriorated to a highly deteriorated by 2016. Microphyllous scrub or its secondary vegetation covers the areas that conserve a proportion of soils in good condition over the analyzed 20-year period. The significant shift towards highly deteriorated soils is worrisome and reflects the result of net erosion and the formation of desert soils. In the images shown in Fig. 3b and c, sandy soils with dunes are found in regions that previously had some vegetation cover. In fact, on our exploratory routes in the field, a large number of sandy soils were found,
observed in the levels or categories of soil deterioration. In Fig. 8a, areas with deteriorated soils or that are in the process of being deteriorated in February 1996 can be observed. Comparatively, in Fig. 8b (20 years later), a significant amount of soils in these latter categories became highly deteriorated. These changes indicate sharp transitions in the extent of soil deterioration. Many deteriorated soils became highly deteriorated, and many soils in the process of being deteriorated also became deteriorated. Most areas corresponding with highly deteriorated and deteriorated soils are located in the western portion of the study area where a substantial increase in these two categories is observed over time. Notably, in the north-western region, many highly deteriorated soils 7
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Fig. 11. Soil area corresponding with the categories of highly deteriorated, deteriorated and in process of being deteriorated during the winter dry season of 1996 and 2016.
Fig. 12. Soil area corresponding with the categories of high deteriorated, deteriorated and in process of being deteriorated during the rainy summer season of 1996 and 2015.
Fig. 13. Soil conditions during the winter in the study area (a). The erosion process is observed in the formation of small dunes (b).
during these months is low. The rainfall level in addition to the low temperatures explains the lack of vegetation cover in rainfed croplands. Maximum rainfall of about 6.5 mm occurred on January 27 and was recorded at four stations. However, near the date of the satellite image, a rainfall of only 2 mm was captured. This rainfall behavior in the winter months, as shown in Fig. 9, is quite representative for this region. A comparison between the rainy season and the dry season reveals a significant difference in the amount of rain received during these seasons. In winter, levels of around 7 mm are recorded, whereas, during summer, rainfall can reach up to 80 mm daily. Additionally, the results revealed that areas of severe soil deterioration are more concentrated in the north-western part of the study
as observed in Fig. 13b, which is characteristic of highly deteriorated soils. Additionally, in several rainfed croplands that were highly deteriorated, soil was brought from the forested areas of the mountain ranges surrounding the study area and used as fertilizer. This process created two fundamental environmental problems: 1) to make a change of land use towards rainfed land with the erosion mentioned above process due to winds associated with cold fronts and 2) the devastation of soils in wooded areas because of the transport of fertile soil to deteriorated areas. Notably, the deterioration process is less intense in areas with irrigation systems, as seen in Fig. 8 (green areas). Fig. 9 shows the rainfall recorded in January and February 2016. In contrast to the rainy season, the rainfall recorded at most stations 8
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5. Conclusion
area. For this reason, it is necessary to examine these areas in greater detail. Fig. 10 shows the distribution of the BI index during February 1996 and February 2016 for a zoomed-in region showing substantial changes in the extent of soil deterioration. A large portion of the surface area classified as in the process of being deteriorated in 1996 was classified 20 years later as highly deteriorated, being the area of rainfed crops as the most vulnerable and affected. Also, the amount of soil in good condition in 1996 (Fig. 10b) is markedly reduced by 2016 (Fig. 10c). This part of the study area shows the magnitude of the environmental problem generated by the massive shift in land use towards rainfed cultivation. In particular, in this region, the percentage of soil in the highly deteriorated category increased from 5.8% in 1996 to 49.2% in 2016. This change is extraordinary and, without a doubt, is detrimental to the environment. The percentage of soil in good condition decreased from 34.3% in 1996 to 17.4% in 2016. These changes are generally the result of inappropriate land management practices in the region, and poor planning for not considering the climatic conditions that exist during the planning of rain-fed croplands. One consequence of poor land planning and management is the formation of desert zones. Anthropogenic activities have changed vegetation cover and allowed the soil to erode gradually, generating a potential risk of soil deterioration for the future. Finally, the areas that changed in terms of soil quality can also be quantified to further highlight processes of soil deterioration over time. Fig. 11 shows the changes during the winter dry season. Highly deteriorated soils were found across 6.4 km2 in 1996 yet expanded to 422 km2 by 2016. Deteriorated soils were found across 60.82 km2 in 1996 yet increased to 1718 km2 by 2016. Finally, roughly 2477.49 km2 of soil was in the process of being deteriorated in 1996–3019 km2 in 2016. Notably, the first two categories show an increasing tendency over time, indicating that the study area is experiencing a process of soil deterioration. However, the calculated values for the BI index differed during the rainy season. The presence of moisture in the soil and growing grass influences the BI values. Therefore, its application must be done with great care. It means that the percentage of soil area corresponding to the categories of highly deteriorated, deteriorated, in the process of being deteriorated and in good condition varies with the dry season. Fig. 12 shows the areas determined to be highly deteriorated during the rainy season: 27.7 km2 in 1996 and 24 km2 in 2015. In the areas corresponding with the deteriorated category the values are of a higher order of magnitude, but the distribution is similar to that of highly deteriorated areas. Likewise, the areas corresponding to the process of being deteriorated show a very similar behavior during the analysis period, which allows us to think that during the rainy season the “apparent” state of stability may be due to the occurrence of small disturbances in the area and a more significant presence of vegetation cover. Based on the present results, unless there is a change in the soil protection policy in both valleys and forested areas, this semi-arid region of the Chihuahua Desert will likely become a desertic area in the future as a result of the combined effect of the expansion of rainfed croplands and meteorological conditions during the winter dry season. In Fig. 13, deteriorating soils are described in the neighborhood of the coordinates 22.977 N and 102.681 W. In (a), an example of soil exposed in winter to the action of strong winds is presented. In (b), an erosion process is observed in the formation of small dunes. Since in the winter season there are on average about 50 cold fronts (according to the portal of the Mexican meteorological service (SMN, 1996, https://smn. cna.gob.mx/es/) and approximately 20% correspond to strong winds, the erosion process is considerable. This region already experiences severe dust storms that impact other areas of Mexico as well as the United States.
The calculation of the Brightness Index (BI) using satellite data and imagery allowed us to perform a multi-temporal analysis of changes in the level of soil deterioration over time in an area of the southern Chihuahuan Desert. In 1993, the pressure on soil resources was lower than in recent years. However, the BI is susceptible to the presence of moisture in the soil. In other words, the characteristics of the index are enhanced during the winter due to the properties of the soil such as the texture and its composition. Its application during the dry season likely provides a more accurate picture of soil degradation than during the rainy season and represents a practical way of determining areas of bare soil exposed to the action of strong winds that cause deterioration and erosion. Since 1993, processes of soil deterioration in the study area have intensified. By 1996, soil deterioration showed a gradual increment. By 2016, large soil areas had been transformed; many soils that were in the process of being deteriorated became highly deteriorated. These transformations may be related to poor soil and land management in the study region. The process of increasing soil deterioration in the study region is concretely related to two factors: changes in land use favoring agricultural activity and the occurrence of strong winds associated with cold fronts. References Céspedes-Flores, S.L., Moreno-Sánchez, 2010. Estimación del valor de la pérdida de recurso forestal y su relación con la reforestación en las entidades federativas de México. Instituto Nacional de Ecología y Cambio Climático (INECC). Investigación Ambiental 2, 5–13. Chander, G., Markham, B.L., Helder, D.L., 2009. Summary of current radiometric calibration coefficients for Landsat MSS, TM, ETM +, and EO-1 ALI sensors. Remote Sens. Environ. 893–903. 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Zonas Áridas XIII. Weng, Q., 2002. Land use change analysis in the Zhujiang Delta of China using satellite remote sensing, GIS and stochastic modelling. J. Environ. Manag. 64, 273–284. World Reference Base (WRB), 2006. World Reference Base for Soil Resources. In: second ed. World Soil Resources Reports No 103 International Union of Soil Sciences (IUSS) Working Group, Food and Agriculture Organization (FAO), Rome report. Zhao, H.L., Zhao, X.Y., Zhou, R.L., Zhang, R.H., Drake, S., 2005. Desertification processes due to heavy grazing in sandy rangeland, Inner Mongolia. J. Arid Environ. 62, 309–319.
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Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
Soil deterioration in the southern Chihuahuan Desert caused by agricultural practices and meteorological events
T
María de Jesús Guevara Macíasa, Noel Carbajala, José Tuxpan Vargasb,∗ a b
Instituto Potosino de Investigación Científica y Tecnológica A.C. Camino a la Presa San José 2055, CP, 78216, San Luis Potosí S.L.P, Mexico Cátedras-CONACyT-Instituto Potosino de Investigación Científica y Tecnológica A.C. Camino a la Presa San José 2055, CP, 78216, San Luis Potosí S.L.P, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Remote sensing Land use impact Desertification Arid zones
In arid and semi-arid regions, unregulated land use changes as a result of poor planning, and the expansion of agricultural and livestock activities increase the risk of desertification and other potentially severe environmental impacts. Several consequences of improper land management practices are soil deterioration and erosion, which may be intensified by meteorological events. This paper presents a historical evaluation of the temporal and spatial evolution of soil deterioration in the southern Chihuahuan Desert in Mexico. A multi-temporal analysis of the study area during the 1995–2016 period was carried out using satellite data (Landsat series). Two seasons were analyzed to determine the influence of external events such as rainfall and wind: dry and rainy. The identification of soil conditions was based on the Brightness Index (BI) considering the complete satellite data set. The soil conditions were classified into five categories according to their reflectance values: highly deteriorated, deteriorated, in the process of being deteriorated, in good condition and other (clouds, water, non-soil). The change detection maps clearly show a growing trend wherein areas of deteriorated and eroded soil increase over time. Agriculture and strong winds are the two main factors involved in the soil deterioration process of the study region.
1. Introduction Land use change is generally a consequence of the exponential growth of the world's population and the expansion of urban areas. As the population increases, more land is needed to produce food and raw materials. Primary economic activities, including intensive agriculture and livestock activities, have often expanded into new areas without proper planning, modifying large tracts of land and leading to deforestation. Crop expansion has affected different ecosystems, mainly grassland and forest regions. In tropical regions, croplands have expanded mostly at the expense of forest cover over the past 40–50 years (Gibbs et al., 2010). Several environmental problems resulting from massive land use changes, such as desertification, have been well documented. During the Dust Bowl era in the United States, the cultivation of large crop areas combined with drought created severe dust storms. Inadequate agricultural practices such as the implementation of monocultures and the removal of vegetation cover were the principal causes of this phenomenon (Schoijet, 2005; Ravi et al., 2009). Currently, China is another example of a country experiencing accelerated environmental degradation caused by deforestation, overgrazing, agricultural
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overexploitation and the exhaustion and contamination of aquifers. Since 1978, China has initiated economic reform and an open-door policy, leading to rapid land use and land cover changes across most of its territory (Weng, 2002). As a result, the rate of desertification in China has increased. For example, the areas surrounding the Gobi Desert have undergone desertification because of overgrazing (Schoijet, 2005; Ravi, 2009). The impacts of agriculture in north-eastern China have caused the erosion of black soil to a depth of 0.4 m, resulting in a nearly 10% reduction in soybean production (Gao et al., 2014). In Mongolia, wind erosion and heavy grazing are the leading causes of sandy desertification and have triggered further impacts on the vegetation cover in addition to hastening erosion processes, leading to the loss of soil structure and soil deterioration (Zhao et al., 2005). In Argentina, about 80% of the territory is dedicated to agricultural or forestry activities; however, these activities have similarly increased rates of soil erosion and degradation. Hence, the expansion of the agricultural frontier and unplanned deforestation appear to have severely degraded natural resources (Maris, 2000). In Mexico, more than 80% of agricultural land shows some degree of degradation as a result of monocultures, deforestation or livestock activities. Estimates obtained indicate that around 97% of the land
Corresponding author. E-mail addresses: [email protected] (M. de Jesús Guevara Macías), [email protected] (N. Carbajal), [email protected] (J.T. Vargas).
https://doi.org/10.1016/j.jaridenv.2019.104097 Received 19 December 2018; Received in revised form 10 August 2019; Accepted 30 December 2019 0140-1963/ © 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. Location of the study area. In a) the Chihuahua desert in Mexico is shown in gray tones (Morrone et al., 2017), in b) the state of Zacatecas is delimited with a red line, and the location of the study area is shown, and in c) a true color composite image acquired by the Landsat 8 platform is presented. The red circles indicate the location of meteorological stations within the study area: 1) Agronomía, 2) COBAEZ, 3) Arcinas 4) Mesa de Fuentes. The urban areas of Fresnillo and Zacatecas are identified by the acronyms UA1 and UA2 respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
area have considerably contributed towards land deterioration. Recent research confirmed that land use changes in this region had intensified soil degradation (Céspedes and Moreno, 2010; Velázquez Valle et al., 2014). In general, the mechanism for soil loss is a combination of the effects of wind and water erosion. In this area, the main factors are changes in land use, erosion caused by strong winds and intense dust storms associated with the passage of cold fronts. Bare soil areas contribute to this problem, as they act as sources of dust. Each year, the region experiences more than 50 cold fronts during the winter season according to the records of the meteorological stations near the study area. The National Institute of Forestry, Agriculture and Livestock Research (INIFAP, for its initials in Spanish) estimated a soil loss of about 3500–4000 tonnes km-2 in 2005. In the study region, average rainfall has been decreasing, leading to longer dry seasons, is the most significant impact of droughts during the winter due to increased wind erosion causing soil deterioration. For instance, soil loss during 12 h over a source area of 4320 km2 was previously determined to be approximately 9162 tonnes (Pineda et al., 2011). Concern over the effects of land use changes on the planet has led to the development of different methodologies for evidencing the impact of land use changes on natural resources. Remote sensing combined with image processing enables areas with modified land uses or covers or with a high level of soil deterioration to be identified. Based on satellite images, it is possible to quantify and classify the state of a vegetation cover (e.g., very deteriorated, deteriorated, in the process of being deteriorated or in good condition). Hence, a series of satellite images can be evaluated to determine the state of a particular
dedicated to agriculture in Mexico are very vulnerable to desertification processes. Studies conducted by Oropeza (2004) determined the Mexican territory as 48% highly susceptible to desertification and 49% as moderately susceptible. The change of land use for agricultural and livestock applications was transformed from areas with forest cover (82%) and grasslands (18%) according to Meneses (2008). The effect of land degradation due to land use change mainly affects arid and semiarid zones. Intensive deforestation, overgrazing, corrupt agricultural practices and soil management are elements that increase the risk of desertification. Two studies cases evidenced the process of desertification from the Chihuahua Desert. In 2006 (E. Huber-Sannwald et al.) realized an investigation in a rural area focusing on the interrelationship human-environment system that causes desertification — in the southern part from Chihuahua Desert in a temporal evolution study using indicators from desertification: Normalized Difference Vegetation Index (NDVI), Normalized Water Index (NDWI), Iron Oxides Index (OI) and Surface temperature (ST) (Noyola-Medrano et al., 2017). The results show that the most affected area in San Luis Potosí Plateau is the west portion. The southern part of the Chihuahua Desert in Zacatecas state is a semi-arid region with extensive grassland and scrub cover as well as a temperate forest in the high mountain areas, is not exempt from the environmental problems associated with land use changes in favor of agricultural and livestock activities. In this region, extensive areas of scrub cover have mostly been converted into new croplands and pasturelands. Additionally, the population growth of cities such as Zacatecas and Fresnillo and the corresponding expansion of the urban 2
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Fig. 2. Land use in the study area. Source: Vector data on land use and vegetation (scale 1:250000, series V from INEGI (2015).
Fig. 3. Schematic representation of the desertification process.
Fig. 4. Wind speeds recorded in the study area during the 2015 winter season.
deforestation, increase in cropland area and other changes in vegetation cover over time. This paper focuses on the evolution of soil deterioration in the southern Chihuahua Desert during the period 1995–2016. The study was conducted using Landsat satellite data and the implementation and evaluation of various indexes and classification
phenomenon over time (Singh et al., 1989; Jovanovic, 2011). Moreover, different techniques can be applied in remote sensing, including the calculation of the Normalized Difference Vegetation Index (NDVI), Bare Soil Index (BSI) and Normalized Difference Soil Index (NDSI), which can also be used, for example, to quantify the rate of 3
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dominant soil classes in the study area are xerosoles, yermosols, feozem, Eurasian, and kastanozem, with mostly a medium texture (World Reference Base, 2006) and reddish color. These soils are mainly used as rainfed croplands. The following soil texture classifications are present: loamy clay-sandy, clayey, clayey and sandy (INEGI, 2007). Fig. 2 shows the main land uses in the study area. Seasonal (rainfed) agriculture is dominant across the entire region, but significant irrigation zones agriculture are present. Different types of scrub can be observed, including the microphyllous scrub that covers the entire northeastern to the central portion of the study area. Several areas of rosette desert scrub are present in the southern part. Also, different associations of secondary vegetation can be observed across a large portion of the study area. Finally, a small area of secondary Quercus forest is found in the south-western region of the study area. Desertification in the study area is highly related to soil deterioration. Fig. 3 shows the desertification process in a schematic way, considering as a trigger the change of land use towards seasonal farming. First, a change in land use occurs wherein grasslands or forests are converted into croplands. Crops are mostly irrigated from May to October. However, during the winter dry season, the soils are devoid of vegetation, as shown in Fig. 3b. A series of cold fronts then propagates through the study area, frequently causing strong winds that interact with bare soils and increase the erosion process. The combined action of these natural and anthropogenic phenomena result in increased erosion and soil deterioration. In Fig. 3c, an image of a deteriorated soil is shown; the main character is the sandiness of the soil. It was also observed that irrigated agriculture does not represent a significant component for the erosion process in the study area. Therefore, the analysis focused on the predominant type of agriculture, i.e., rainfed agriculture. It should be noted that in the winter season the maximum wind speeds are obtained (see Fig. 4), promoting the transport and dragging of material and in turn contributing to soil degradation.
2. Methodology
Fig. 5. Methodological process for determining the level of soil deterioration.
Satellite data set were evaluated and acquired from U.S Geological Services on the following dates: Landsat 5 TM for 1995, 1996 and Landsat 8 OLI-TIRS for 2015 and 2016. Images were obtained for both the summer and winter seasons. In particular, the images corresponded with October 1995, January and October 1996, October 2015 and February 2016. Radiometric and atmospheric corrections were performed for each image according to the methods of Chander et al. (2009) and de Keukelaere (2018). For soil characterization and multitemporal analysis, several procedures were used, including the implementation of indices (NDVI, BSI, NDSI, BI), as well as supervised classification algorithms based on minimum distance (MD) and Maximum Likelihood (ML). Of the tested methods, the most consistent result in the multi-temporal analysis was the brightness index (BI), so it was chosen to determine the degree of soil deterioration. The highest values correspond to the land surface with the highest reflectivity, i.e., bare soil or soil with a low density of vegetation cover. This index is based on the reflectance of two visible bands and the near infrared band (NIR), which determine the surface brightness (Escadafal and Bacha, 1996) as follows:
algorithms and being the brightness index (BI) the one that presented better performance and the one used in this work. Changes in soil quality are identified as a result of agricultural and livestock practices, highlighting areas that have been highly deteriorated.
1.1. Study area The study area is located in the central-northern part of Mexico in a semi-arid region of the Chihuahua Desert (Fig. 1). It is situated in the State of Zacatecas on the central plateau of Mexico between the Western Sierra Madre and the Eastern Sierra Madre, encompassing an area of 7336 km2 (Fig. 1, the area enclosed by the black line). At the local level, the study area is located within two physiographic sub-provinces: the Zacatecas mountain ranges, valleys and plains, and the PotosinaZacatecana mountains ranges. The dominant vegetation is grassland and scrubland. Near urban areas, the primary land use is agriculture. The study region includes different types of coniferous forests in the high mountain ranges in addition to mesquite, desert scrub and grassland in the lower regions (Comisión Técnico Consultiva para la Determinación Regional de los Coeficientes de Agostadero 1980). The dominant climate is dry with warm summer, classified as BS1kw. The annual average temperature is 15–18 °C, and the average annual rainfall is 355 mm. The earliest outcrops are formed by low-grade metamorphic rocks (slates, phyllites, and schists) from the Triassic period that rise towards the North and West. Rocks containing clastic marine deposits (sandstones and shales) from the Cretaceous period and volcanic rocks and small intrusive bodies from the Tertiary period are also present. However, Quaternary alluvial deposits predominate (INEGI, 1981). The
BI= (G2+R2+NIR2)1/2
(1)
where G = the green band, R = the red band and NIR = the nearinfrared band. The resulting values were considered as proxies of the level of soil deterioration and were classified into five categories: highly deteriorated, deteriorated, in the process of being deteriorated, in good condition and other. It should be clarified that the spectral thresholds of each category were obtained in-site through observations using the percentage of land cover as a criterion. The BI index and classification were calculated and compared for all periods for which satellite data were available. The methodological process is summarised in Fig. 5. 4
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Fig. 6. Classification of deteriorated areas based on the Brightness Index (BI) during the rainy season of a) October 1995, b) October 1996 and c) October 2015.
3. Results
it is essential to note that highly deteriorated soils did not show significant differences between the analyzed dates. Records from the weather stations located within the study area are shown in Fig. 7 to corroborate the influence of rainfall. The black arrows indicate the days when the satellite images were obtained. In October 1995 and 1996, no rain fell in the days before the images were taken. However, in October 2015, a relatively big rainfall event occurred. Hence, an analysis of Figs. 6 and 7 reveals that the BI index is sensitive to the presence of humidity. The BI index must then be preferably applied during the dry season. It is important to note that the amount of rainfall in the study area is quite similar among the analyzed years, i.e., the amount of precipitation is not very variable.
The analysis of different patterns in the satellite imagery and BI over time allowed the evolution of soils without vegetation cover to be determined. An increase in rainfed croplands was observed. This process is relevant because these croplands are then exposed to the action of winds associated with the passage of cold fronts during winter, which can initiate or increase soil deterioration. In Fig. 6, the BI index classified into five categories of soil quality is displayed for October in 1995, 1996 and 2015, which can be considered as the end of the rainy season. During this month, the areas classified as highly deteriorated (in black) are practically the same in both 1995 and 2015. However, the areas that were deteriorated or in the process of being deteriorated significantly varied depending on the yearly rainfall intensity. By October 22, 2015, it appears that land cover in good condition increased, but this is also due to the amount of rain that fell on days close to the day the image was taken. The rainfall level influences soil humidity and grass growth and thereby impacts the values of the BI index. However,
4. Discussion Over a more extended period, noticeable changes in soil quality are evident. In Fig. 8, the soil quality determined from satellite images captured 20 years apart can be compared. Significant changes are 5
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Fig. 7. Rainfall for several months during a) 1995, b) 1996 and c) 2015 in the study area. The arrows indicate the dates (October 31, 1995; October 17, 1996; October 22, 2015) when satellite images were captured.
Fig. 8. Satellite images of soil deterioration in a) February 1996 and b) February 2016.
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Fig. 9. Rainfall in January and February 2016 in the study area. The arrow indicates the date when the satellite image (February 11, 2016) was captured.
Fig. 10. Detailed comparison of a) the zoomed-in area in b) February 1996 and c) February 2016.
were identified. It represents a significant change in only 20 years after the date that the land tenure law was changed. It is also important to mention that, in the southern part of the study area, highly deteriorated soils were not present in 1996 but underwent a transition from deteriorated to a highly deteriorated by 2016. Microphyllous scrub or its secondary vegetation covers the areas that conserve a proportion of soils in good condition over the analyzed 20-year period. The significant shift towards highly deteriorated soils is worrisome and reflects the result of net erosion and the formation of desert soils. In the images shown in Fig. 3b and c, sandy soils with dunes are found in regions that previously had some vegetation cover. In fact, on our exploratory routes in the field, a large number of sandy soils were found,
observed in the levels or categories of soil deterioration. In Fig. 8a, areas with deteriorated soils or that are in the process of being deteriorated in February 1996 can be observed. Comparatively, in Fig. 8b (20 years later), a significant amount of soils in these latter categories became highly deteriorated. These changes indicate sharp transitions in the extent of soil deterioration. Many deteriorated soils became highly deteriorated, and many soils in the process of being deteriorated also became deteriorated. Most areas corresponding with highly deteriorated and deteriorated soils are located in the western portion of the study area where a substantial increase in these two categories is observed over time. Notably, in the north-western region, many highly deteriorated soils 7
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Fig. 11. Soil area corresponding with the categories of highly deteriorated, deteriorated and in process of being deteriorated during the winter dry season of 1996 and 2016.
Fig. 12. Soil area corresponding with the categories of high deteriorated, deteriorated and in process of being deteriorated during the rainy summer season of 1996 and 2015.
Fig. 13. Soil conditions during the winter in the study area (a). The erosion process is observed in the formation of small dunes (b).
during these months is low. The rainfall level in addition to the low temperatures explains the lack of vegetation cover in rainfed croplands. Maximum rainfall of about 6.5 mm occurred on January 27 and was recorded at four stations. However, near the date of the satellite image, a rainfall of only 2 mm was captured. This rainfall behavior in the winter months, as shown in Fig. 9, is quite representative for this region. A comparison between the rainy season and the dry season reveals a significant difference in the amount of rain received during these seasons. In winter, levels of around 7 mm are recorded, whereas, during summer, rainfall can reach up to 80 mm daily. Additionally, the results revealed that areas of severe soil deterioration are more concentrated in the north-western part of the study
as observed in Fig. 13b, which is characteristic of highly deteriorated soils. Additionally, in several rainfed croplands that were highly deteriorated, soil was brought from the forested areas of the mountain ranges surrounding the study area and used as fertilizer. This process created two fundamental environmental problems: 1) to make a change of land use towards rainfed land with the erosion mentioned above process due to winds associated with cold fronts and 2) the devastation of soils in wooded areas because of the transport of fertile soil to deteriorated areas. Notably, the deterioration process is less intense in areas with irrigation systems, as seen in Fig. 8 (green areas). Fig. 9 shows the rainfall recorded in January and February 2016. In contrast to the rainy season, the rainfall recorded at most stations 8
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5. Conclusion
area. For this reason, it is necessary to examine these areas in greater detail. Fig. 10 shows the distribution of the BI index during February 1996 and February 2016 for a zoomed-in region showing substantial changes in the extent of soil deterioration. A large portion of the surface area classified as in the process of being deteriorated in 1996 was classified 20 years later as highly deteriorated, being the area of rainfed crops as the most vulnerable and affected. Also, the amount of soil in good condition in 1996 (Fig. 10b) is markedly reduced by 2016 (Fig. 10c). This part of the study area shows the magnitude of the environmental problem generated by the massive shift in land use towards rainfed cultivation. In particular, in this region, the percentage of soil in the highly deteriorated category increased from 5.8% in 1996 to 49.2% in 2016. This change is extraordinary and, without a doubt, is detrimental to the environment. The percentage of soil in good condition decreased from 34.3% in 1996 to 17.4% in 2016. These changes are generally the result of inappropriate land management practices in the region, and poor planning for not considering the climatic conditions that exist during the planning of rain-fed croplands. One consequence of poor land planning and management is the formation of desert zones. Anthropogenic activities have changed vegetation cover and allowed the soil to erode gradually, generating a potential risk of soil deterioration for the future. Finally, the areas that changed in terms of soil quality can also be quantified to further highlight processes of soil deterioration over time. Fig. 11 shows the changes during the winter dry season. Highly deteriorated soils were found across 6.4 km2 in 1996 yet expanded to 422 km2 by 2016. Deteriorated soils were found across 60.82 km2 in 1996 yet increased to 1718 km2 by 2016. Finally, roughly 2477.49 km2 of soil was in the process of being deteriorated in 1996–3019 km2 in 2016. Notably, the first two categories show an increasing tendency over time, indicating that the study area is experiencing a process of soil deterioration. However, the calculated values for the BI index differed during the rainy season. The presence of moisture in the soil and growing grass influences the BI values. Therefore, its application must be done with great care. It means that the percentage of soil area corresponding to the categories of highly deteriorated, deteriorated, in the process of being deteriorated and in good condition varies with the dry season. Fig. 12 shows the areas determined to be highly deteriorated during the rainy season: 27.7 km2 in 1996 and 24 km2 in 2015. In the areas corresponding with the deteriorated category the values are of a higher order of magnitude, but the distribution is similar to that of highly deteriorated areas. Likewise, the areas corresponding to the process of being deteriorated show a very similar behavior during the analysis period, which allows us to think that during the rainy season the “apparent” state of stability may be due to the occurrence of small disturbances in the area and a more significant presence of vegetation cover. Based on the present results, unless there is a change in the soil protection policy in both valleys and forested areas, this semi-arid region of the Chihuahua Desert will likely become a desertic area in the future as a result of the combined effect of the expansion of rainfed croplands and meteorological conditions during the winter dry season. In Fig. 13, deteriorating soils are described in the neighborhood of the coordinates 22.977 N and 102.681 W. In (a), an example of soil exposed in winter to the action of strong winds is presented. In (b), an erosion process is observed in the formation of small dunes. Since in the winter season there are on average about 50 cold fronts (according to the portal of the Mexican meteorological service (SMN, 1996, https://smn. cna.gob.mx/es/) and approximately 20% correspond to strong winds, the erosion process is considerable. This region already experiences severe dust storms that impact other areas of Mexico as well as the United States.
The calculation of the Brightness Index (BI) using satellite data and imagery allowed us to perform a multi-temporal analysis of changes in the level of soil deterioration over time in an area of the southern Chihuahuan Desert. In 1993, the pressure on soil resources was lower than in recent years. However, the BI is susceptible to the presence of moisture in the soil. In other words, the characteristics of the index are enhanced during the winter due to the properties of the soil such as the texture and its composition. Its application during the dry season likely provides a more accurate picture of soil degradation than during the rainy season and represents a practical way of determining areas of bare soil exposed to the action of strong winds that cause deterioration and erosion. Since 1993, processes of soil deterioration in the study area have intensified. By 1996, soil deterioration showed a gradual increment. By 2016, large soil areas had been transformed; many soils that were in the process of being deteriorated became highly deteriorated. These transformations may be related to poor soil and land management in the study region. The process of increasing soil deterioration in the study region is concretely related to two factors: changes in land use favoring agricultural activity and the occurrence of strong winds associated with cold fronts. References Céspedes-Flores, S.L., Moreno-Sánchez, 2010. Estimación del valor de la pérdida de recurso forestal y su relación con la reforestación en las entidades federativas de México. Instituto Nacional de Ecología y Cambio Climático (INECC). Investigación Ambiental 2, 5–13. Chander, G., Markham, B.L., Helder, D.L., 2009. Summary of current radiometric calibration coefficients for Landsat MSS, TM, ETM +, and EO-1 ALI sensors. Remote Sens. Environ. 893–903. 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