Rapel Basin, Central Chile

Natural Environment

The Rapel Basin is located in Central Chile between 34°S-71°W. It covers almost the entire VI Region, shortly known as O'Higgins Region (Cornwell et al., 2020; Rojas et al., 2020). The Rapel river originates in the Cordillera de Los Andes, flows east to west, and discharges after 310 km into the Pacific Ocean (Varas & Varas, 2021).

Topography

Two mountains characterize the region's topography: The Andes, with an elevation of around 5,000 m, and the parallel running Coastal Cordillera, reaching around 2,200 m (Cornwell et al., 2020). The valley between these two mountain ranges is called the Central Depression, also known as Chile's Central Valley (Armesto et al., 2007). Both mountain ranges have a crucial impact on the relief, natural vegetation, soil development, and flows of water resources (Gironás et al., 2021).

Figure 2: Rapel basin topography (SRTM, 2013).

Geology

The geological units in the Rapel Basins are part of the Central Depression and date back to the Quaternary Period and Paleozoic Era. Mainly metamorphic rocks, marine sedimentary deposits, and volcanic formations are exposed within the basin. Pliocene-Quaternary volcanic rocks can be found in the central cordillera, and their very presence illustrates the ongoing magmatic processes in the region. Additionally, volcanic ashes and lava lead to the formation of humid and fertile soils (Suárez et al., 2021).

Soil

Soils in the Rapel Basin originate from sedimentary and volcanic materials. As a result, the soils of the Central Depression are very fertile and hold ideal conditions for agriculture. The map below shows the different soils in the basin (Armesto et al., 2007; Suárez et al., 2021).

Figure 3: Rapel basin topography (SRTM, 2013).

Landscape and vegetation

The basin's landscape is strongly influenced by the winter rainfalls and dry summers, forming a Mediterranean ecosystem with heterogeneous vegetation. This area's common vegetation types are dry xerophytic thorn scrubs, deciduous shrubs, and succulents, while evergreen sclerophyllous trees dominate the coastal area and foothills (Armesto et al., 2007).

Furthermore, anthropogenic influences have profoundly shaped the landscape and impacted the regional vegetation. Humans transformed the forest ecosystems into agricultural lands or used them for wood production. The mining industry emerged in the 19th century and further impacted forest ecosystems and land use (Pizarro-Tapiaet al., 2021).

According to Chile's Agricultural and Forestry Censusfrom from 2007, the O'Higgins region covers 8.2% of the national area devoted to forestry and livestock farming. Nevertheless, forest plantations in the O'Higgins region still stretched across 141,023 ha in 2007 (ODEPA, 2018). The map below shows the tree cover in the watershed and highlights areas with forest loss and gain.

Land Cover and Land Use

The Mediterranean ecosystem dominates the basin and offers favorable conditions for agriculture. As a result, humans and their activities continuously re-shaped the landscape for agricultural purposes and settlements (Armesto et al., 2007). The table below summarizes the distribution of different land uses and land covers in the Rapel basin

*Referred to the following uses: Scrubland, scrubland-grassland, crop-grassland rotation, unrecognized areas, water bodies, snow- glaciers and wetlands

Table 1: Land cover/use distribution in the Rapel basin (DGA 2004)

Figure 4: Land cover and land use in the Rapel Basin in 2013.

References

Armesto, J.J.; Arroyo, M.T.K.; Hinojosa, L.F. (2007): The Mediterranean Environment of Central Chile. In: Veblen, T.T.; Young, K.R.; Orme, A.R. (Ed.): The Physical Geography of South America. Oxford, University Press, pp. 184-199.

Cornwell, E.; Sposito, V.; Faggian, R. (2020): Land suitability projections for traditional sub-alpine cropping in the Australian Alps and Chilean Dry Andes. A combined biophysical and irrigation potential perspective. In: Applied Geography, Vol. 121(102248), pp. 3-18, doi: 10.1016/j.apgeog.2020.102248

DGA(2004): Diagnostico y clasificacion de los cursos y cuerpos de agua segun objetivos de calidad: Cuenca del Rio Rapel’.

Gironás,J.; Fernández.; Salidías, J. (2021): Country Profile. In: Fernández, B.; Gironás, J. (eds.): Water Resources of Chile, Vol. 8, pp. 1-6.

ODEPA (2018): Región del Libertador Bernardo O’Higgins Informe Regional. Available at: https://www.odepa.gob.cl/wp-content/uploads/2018/03/OHiggins.pdf [Accessed 26.07.2021].

Pizarro-Tapiaet, , R.; Ibáñez-Córdova, A.; García-Chevesich, P.; Vallejos-Carrera, C.; Sangüesa-Pool, C.; Mendoza-Mendoza, R.(2021): The Chilean Forest Sector and its Relationship with Water Resources. In: Fernández, B.; Gironás, J. (eds.): Water Resources of Chile, Vol. 8, pp. 301-315.

Rojas, R.; Bennison, G.; Gálvez, V.; Claro, E.; Castelblanco, G. (2020): Advancing Collaborative Water Governance: Unravelling Stakeholders’ Relationships and Influences in Contentious River Basins. In: Water, Vol.12(2020), pp. 1-25, doi:10.3390/w12123316

NASA SHUTTLE RADAR TOPOGRAPHY MISSION (SRTM) (2013): Shuttle Radar Topography Mission (SRTM) Global. Distributed by OpenTopography. https://doi.org/10.5069/G9445JDF [Accessed 26.07.2021].

Suárez, F.; Leray, S.; Sanzana, P. (2021): Groundwater Resources. In: Fernández, B.; Gironás, J. (eds.): Water Resources of Chile, Vol. 8, pp. 347-364.