Influence of land use and climate on the load of suspended solids in catchments of Andean rivers

Influence of land use and climate on the load of suspended solids in catchments of Andean rivers

Understanding the interaction between an- thropogenic land use and the rainfall pattern can be crucial to predict changes in total suspended solids (TSS) in streams and rivers. We assessed the effects of land use and annual rainfall on the TSS load of 19 southern Chilean catchments. The results indicated that the concentration of TSS increased in catchments with a rainy regime and greater annual precipitation. TSS load also increased as the surface of open areas increased at the catchment scale and decreased with increasing cover of glaciers and perennial snow. However, we did not find support for models with interaction terms between cli- mate and land use. Results suggest that a regional de- crease in annual rainfall accompanied by an increase in the altitude of the zero isotherms, as predicted by climate models, should have multiple effects on TSS. In partic- ular, increased TSS load can be expected from a con- traction of glaciers and perennial snow areas as well as the intensification of new crops and urban expansion.

Total suspended solids (TSS) are basic abiotic compo- nents of aquatic ecosystems, influencing food webs and nutrient cycles (Wood and Armitage 1997; Bartley et al. 2012; Odiyo et al. 2012). An increased TSS concentra- tion in freshwater may have detrimental consequences on water quality and lead to increased costs in drink- ing water treatment (Hutton et al. 2007; Mahmoudi et al. 2010; Ozyonar and Karagozoglu 2012; Chow et al. 2012). TSS in rivers can act as carriers of heavy metals (Horowitz 1995) and persistent organic pollutants (Thomas and Meybeck 1992), whereas high TSS loads can cause strong impacts on the biogeochemical condi- tions of coastal areas and sea waters (Xu 2002; Hunter and Walton 2008; Hsua and Lin 2010). Therefore, a crucial step in freshwater management is to determine natural and anthropogenic processes that increase TSS concentration.
Intensive anthropogenic activities at the basin scale can increase the sediment load of rivers and lakes through increasing wind erosion (Neff et al. 2008, see also Fox et al. 2012) and runoff erosion (Zhang et al. 2011; Kavian et al. 2011). Vegetation removal, forest fires, intensive agricultural practices, and other anthro- pogenic activities at the basin scale result in the runoff of soil materials into adjacent streams and rivers (Amiri and Nakane 2006; Zucca et al. 2010; Solaimani et al. 2009; Basso et al. 2012).

This is of special importance in Andean rivers where natural lithogenic characteristics and mining activities have accelerated natural leaching and weathering. The decline of water quality in Chilean rivers and lakes is a growing concern for environmental policy and land managers (Pizarro et al. 2010a, b). In particular, the replacement of native forest by high water-demanding forest plantations and the intensification of agricultural practices could be attributed to an increase in sediment load in streams and shallow lakes of Chile, as suggested by some previous studies (Cisternas et al. 1999; Pepin et al. 2010). Although intensive land use can increase TSS load in Chilean rivers, natural dynamic processes such as rainfall amounts may also contribute to increase the runoff erosion in river basins (Bathurst et al. 2011; Bonilla and Vidal 2011; Mahmoudi et al. 2010; Kavian et al. 2011). These effects are important as the rainfall regime of central and southern Chilean responds to long-term trends in climate conditions (Quintana and Aceituno 2012). In this study, we used a 23-year data- base of TSS load in southern Chilean river catchments (Table 1; Fig. 1) to determine if catchments with inten- sive anthropogenic land use and high amounts of annual rainfall have larger TSS loads. On the basis of these results, and considering future climate scenarios for the region, we discuss possible changes in the sediment load of southern Chilean catchments.

Materials and methods
Study basins
We studied 19 different river catchments of southern Chile (from 33o53′ to 39o50′ S lat., Fig. 2). River catch- ments were delimited using topographic and hydrologi- cal geographic information system (GIS) layers provided by the Ministry of Public Works of Chile. Each basin was associated with its corresponding monitoring sam- pling station (see below). Soil characteristics of these catchments were homogeneous, as they are of volcanic origin (i.e., Andisols). The studied catchments were lo- cated in the Andean mountains (Table 1; Fig. 2). Since rain and snow precipitation can have different effects on sediment runoff (Iida et al. 2012), catchments were classified according to their dominant climate re- gime by distinguishing between rain and mixed rain– snow basins. Mixed rain–snow basins are character- ized by having winter and summer floods, since snow
melting occurs during the warm season. Monthly data series of flow were used to characterize the regime of each river.