Groundwater dynamics, land cover and salinization in the dry Chaco in Paraguay
Introduction
Water is vital to satisfy both human needs, production and to maintain ecosystem functions. Despite this fundamental role, there is a high probability that water resources are among the most limited during this century, due, among other factors, to the continued growing World population and the increase in demand for fresh and potable water (Gottle and Sène, 1997; Heathcote, 2009; Jewitt, 2002; Oki and Kanae, 2006; Ostfeld et al., 2012). Water availability and quality are also threatened around the world, not only because of their scarcity in some regions, but also because of inadequate management, high levels of pollution and degradation of ecosystems (le Polain de Waroux et al., 2018; Ostfeld et al., 2012; Boer and Radersma, 2011; Peters and Meybeck, 2000; Rengasamy, 2006). Water pollution poses significant threat to marginal populations particularly in countries where water balance is negative or where dry conditions prevail (Marchesini et al., 2017; Nelson and Chomitz, 2006; Vorbsmarty et al., 2000).
Water quality not only depends on the management of its distribution and health in the cities, but also on the management measures taken at their sources of origin. In addition to measures which should accompany the development of cities at the urban level, it is necessary to consider that what happens at the rural level and in the catchments as well as the rainfall pattern and water infiltration do have a great influence on the availability and quality of this resource (Laino et al., 2016). From this perspective, the practices of adequate soil management and conservation are closely related to the improvement of the water supply, both in its quality and in its quantity, and justify the approach of the human activities being integrated with ecosystems and abiotic factors, particularly in the areas of water catchment, runoff, infiltration and groundwater recharge (Cotler-Ávalos 2004; Dourojeanni and Nelson, 1987; Heathcote, 2009). The hydrological cycle is the movement of water from the atmosphere to the earth and back again. Most salts are highly soluble, so water is key to the movement of salts in the landscape (Podmore, 2009). Even though soluble salts are inherent in all soils, there are many processes that can contribute to the build-up of salts in a given soil layer (Boer and Radersma, 2011). Soil salinization, defined as the accumulation above a certain level of water-soluble salts within soil layers, can lead to undesirable effects on the ground surface, limiting production and altering ecosystems functionality (Williams, 1999).
Soil salinity is described and characterized in terms of the concentration and composition of soluble salts. Evaporation, agricultural activities, historical depositions, fresh water extraction, soil mineral weathering and pollution can lead to salinization of soil and groundwater (Boer and Radersma, 2011). These factors combined with climatic, hydrological, geological, as well as groundwater depth and salinity, vegetation cover and human influences determine where salinization occurs (Williams, 1999). Salinization commonly causes damages when saline groundwater reaches the soil surface by capillary ascension (Podmore, 2009). Dryland salinity is strongly dependent on saline groundwater dynamics along landscape profiles (Williams and Chartres, 1991), and land use cover has shown to have a direct impact (although not necessarily the predominant one) on the depth of the water table (Peck and Williamso, 1987; Smerdon et al., 2009) thus land use change could eventually modify vulnerability to salinization. All soil types can be affected by salinization (Marchesini et al., 2017; Rengasamy, 2006).
Salinization is a phenomenon that can have an important impact on ecology and productivity (Williams, 1999). Ecohydrological transformation in the dry Chaco and the risk of dryland salinity has been studied and warned of by few studies (Marchesini et al., 2017; Magliano et al., 2016), however, these studies focus on the Argentinean dry Chaco and not the Paraguayan. While much attention has been paid to coastal areas (Cardona et al., 2004; Carol et al., 2009; Giménez and Morell, 1997) and to some of the world's most populated areas prone to salinization such as in some regions of Australia (Gordon et al., 2003; Ruprecht and Schofield, 1991) and China (Wang et al., 2008; Wu et al., 2014), the semi-arid Chaco, especially in its Paraguayan portion still remains understudied or studied but unpublished. Even though some studies have focused on the benefits of land use planning to maintain connectivity (Torrella et al., 2018) or others have analysed bird conservation (Macchi et al., 2016) in agricultural landscapes in the dry Chaco, few have focused on studying salinization as a land use planning tool. Understanding saline groundwater and salinization dynamics and the effects of land use and land cover is crucial to prevent damage and decide on proper agricultural and livestock activities allowing for sustainable production. The challenge to find grazing systems that are sustainable is shown to lie in recognizing that ecosystems must be studied in an integrated way. Not focusing only on short term animal productivity without considering all other ecosystem essential components (Williams and Chartres, 1991). This study aims to provide insights of the relation among water fluxes, soil cover and salinization in areas dedicated to livestock production in the Paraguayan dry Chaco.
Section snippets
Study sites
The study area is located in a 900 mm rainfall region, considered part of the semi-arid or dry Paraguayan Chaco. It is located within a belt of some 500 km length and some 50 to 75 km width extending from northeast to southwest right across the Paraguayan Chaco (from General Diaz to Bahia Negra), which is particularly prone to dryland salinity due to a high saline groundwater table (Vogt, 2016). The Gran Chaco is a semiarid to subhumid mostly plain ecoregion extending from north Argentina, over
Data analysis
Data was collected from July 2002 until November 2004, 75% of the measurements were made in Campo Maria (21 wells), and 25% in Campo Bello (7 wells). The measurements were made under similar conditions in both locations except from the sand fraction in both soils. The higher sand concentration makes infiltration a little easier in Campo Bello. Measurements of precipitation, water table and electric conductivity (as an indirect measure of Total Dissolved Salts) were collected every two weeks.
Results and discussion
The highest groundwater conductivity values were found under forest land cover, followed by pasture, fringe and lagoon in Campo Maria (Fig. 4, left above). In the same way, in Campo Bello the conductivity under forest was higher than under pasture (Fig. 4, left below). According to the classification of Martens and Wichmann (2007) only groundwater under the lagoon is considered brackish, while the rest is considered saline.
In both Campo Maria and Campo Bello, the groundwater conductivity
Conclusions
This study shows considerable interdependencies among topography, land cover, precipitation, groundwater depth and salinity and surface salinization. Groundwater under native forest showed consistently the highest levels of conductivity, followed by pasture, fringe and lagoon. This indicates that forest plays an important role in reducing rainfall infiltration and in keeping the saline water table at a safe distance, thus reducing vulnerability to surface salinization. This study confirms the
Conflict of Interest
Authors declare that there is no conflict of interest in this article.
Ethical Statement
Authors state that the research was conducted according to ethical standards.
Acknowledgments
This research has been possible thanks to the support of AVINA.
Funding body
Financial support for the conduction of this research was provided by AVINA. Article preparation and writing was supported by the SuLu and PaCha projects, supported by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety of Germany and the Ministry of Foreign Affairs of the Netherlands respectively.
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