Effects of land use and land cover on water quality of low-order streams in Southeastern Brazil: Watershed versus riparian zone

Efeitos do uso e cobertura do solo na qualidade da água de riachos de baixa ordem no sudeste do Brasil: bacia hidrográfica versus zona ribeirinha

ABSTRACT

Land-use/land-cover (LULC) pattern influences water quality, however, this relation may be different for various spatial scales. We evaluated the LULC effects on water quality of tropical low-order streams, comparing watershed and riparian zone models. Water quality parameters were analyzed separately and together using linear mixed and multivariate models. The results indicate that the forest cover plays a significant role in keeping water clean, while agriculture and urban areas lead to water quality degradation. Pasture land had mixed effects, but in general was not correlated with poor water quality. Dissolved oxygen, phosphorus, sediment, and fecal coliforms were influenced by LULC pattern at the watershed scale, while nitrogen and organic matter were more affected by the riparian zone composition. The water quality also varies with seasonal changes in streamflow and temperature. The overall water quality variation is explained better by the LULC composition within the watershed than in the riparian zone.

Keywords: Land use pattern, Riparian buffer, Agricultural watershed, Forest cover, Surface water quality, Water resources management

RESUMO

O padrão de uso/cobertura da terra (LULC) influencia a qualidade da água, no entanto, essa relação pode ser diferente para várias escalas espaciais. Avaliamos os efeitos do LULC na qualidade da água de riachos tropicais de baixa ordem, comparando modelos de bacias hidrográficas e zonas ripárias . Os parâmetros de qualidade da água foram analisados ​​separadamente e em conjunto usando modelos lineares mistos e multivariados. Os resultados indicam que a cobertura florestal desempenha um papel significativo na manutenção da água limpa, enquanto a agricultura e as áreas urbanas levam à degradação da qualidade da água. As pastagens tiveram efeitos mistos, mas em geral não foram correlacionadas com a má qualidade da água. Oxigênio dissolvido, fósforo, sedimento e coliformes fecais foram influenciados pelo padrão LULC na escala da bacia hidrográfica, enquanto nitrogênio e matéria orgânica foram mais afetados pela composição da zona ripária . A qualidade da água também varia com as mudanças sazonais no fluxo do rio e na temperatura. A variação geral da qualidade da água é melhor explicada pela composição LULC dentro da bacia hidrográfica do que na zona ripária.

Introduction

Conversion of natural habitats into anthropogenic landscapes to cater to the increasing human demand for resources is one of the main factors behind the degradation of water quality (Giri and Qiu, 2016; Su et al., 2016). Increases in agricultural and urban lands have been described as one of the greatest contributors to the increase of nutrients and sediments in freshwater ecosystems worldwide (Uriarte et al., 2011; Huang et al., 2016). However, non-point sources pollution is difficult to assess due to the complex and diffuse nature of interactions between hydrologic and landscape patterns (Chiwa et al., 2012). Also, the relationship between land use/land cover (LULC) and water quality can occur at different spatial scales, from local to regional effects (Wang et al., 2013; Tanaka et al., 2016).

Low-order streams (1st to 3rd orders) dominate a riverine landscape, and they contribute to the function, health, and biodiversity of the entire river networks (Vannote et al., 1980; Wipfli et al., 2007). Terrestrial inputs strongly influence low-order streams (Vannote et al., 1980), which make them fragile ecosystems that can suffer dramatic impacts of land-use changes. The relationship between LULC and water quality in low-order streams, despite its importance for the watershed, is not well documented (Ding et al., 2016). It is crucial to understand those interactions in low-order streams as they are responsible for water flows, organic matter, sediments, and nutrients transportation downstream (Gomi et al., 2002).

Riparian zones play a significant role in maintaining water quality, and represent an important aquatic-terrestrial ecotone. Riparian zones exert important influence on the waterways by mediating the bi-directional flow of matter and energy between the water body and the surrounding hinterland (Hanser et al., 2010). For example, the riparian forest reduces nitrates, phosphorus, and sediment loading into the stream (Krutz et al., 2005; Oliveira et al., 2010; Gonzales-Inca et al., 2015; Ou et al., 2016). It also influences the energy balance in water bodies (Tanaka et al., 2016). Replacing riparian forest with other land-cover types leads to a decrease in water quality due to bank erosion, and consequently increasing nutrient and sediment loads into the stream (Ding et al., 2013; Ou et al., 2016; Yang et al., 2016).

Some studies have shown that the LULC composition in a riparian zone is a better predictor of water quality than in the whole watershed (Tran et al., 2010; Shen et al., 2015; Shi et al., 2017). Other studies have found that LULC patterns at the watershed scale can better account for the variability in river water quality (Zhou et al., 2012; Ding et al., 2016). Uriarte et al. (2011) and Tanaka et al. (2016) also observe that the water quality indicators have different responses to LULC patterns when evaluated at different spatial scales. Consequently, authors have highlighted the importance of a multiscale analysis, especially, aiming at the understanding of the impacts of LULC on water quality (Uriarte et al., 2011; Zhou et al., 2012; Tanaka et al., 2016).

In this context, this study aims to examine the land-use/land-cover (LULC) effects on water quality of low-order streams, comparing the watershed and the riparian zone influences. The specific objectives are: (1) to identify the key factors that affect the variability in water quality; (2) to model the relationship between LULC patterns and water quality within the whole watershed and in the riparian zone; and (3) to identify which LULC pattern has the strongest influence on water quality in low-order streams.

Section snippets

Study area

The study area is the Sarapuí River basin located in the São Paulo State, southern Brazil (Fig. 1). The Sarapuí River is a tributary of Tiete River, and it supplies four cities in the State, providing water for domestic, agricultural and other purposes. Most of the soil types in the Sarapuí River basin are red or yellow tropical soils, mainly Latosols, dominated by low-activity clay (Oliveira, 1999; Coelho et al., 2003). The watershed was originally covered by Atlantic Forest, with a dense

LULC composition

The proportion of forest cover at the watershed scale was greater than other land uses in watersheds S1, S2, and S5, whereas S5 showed a high forest cover (75%), followed by S2 (57%), and S1 (55%) (Fig. 3 and Table 1). Conversely, S4, S6, and S3 had 35%, 29%, and 25% of forest cover in the watershed, respectively. Likewise, S4 had the largest agricultural area (54%), followed by S6 (33%), S3 (29%), S1 (27%) and S2 (23%), while the S5 had only 16%. Watersheds S6 and S3 had the highest values of

Discussion

The LULC maps indicate that some watersheds are predominantly covered by agriculture and pasture lands due to the agricultural expansion that occurred in this region (Schneider and Costa, 2013). The S4 is an example where more than 50% of the watershed is covered by agriculture. Vettorazzi and Valente (2016) observed that watersheds which had an increase in agricultural areas also had a drastic reduction in forest cover, thereby impairing water quality and leading to increase in sediment and

Conclusion

We conclude, based on our results, that forest cover is the most important LULC type to maintain the water quality of low-order streams, and agriculture and urban areas are responsible for water quality degradation. The sewage from residential areas, sediments, and nutrients loading from the short-cycle crops lead to the nonpoint source pollution into the small rivers. Grassland has mixed impact on water quality, and in general does not result in water quality degradation, if they have

Acknowledgements

This study was supported by the São Paulo Research Foundation (FAPESP, process number 2013-03586-6). We also thank the University of São Paulo and the University of São Carlos for the structural support; Dr. Adriana Cristina Poli Miwa (University of São Paulo) and Monica Almeida (University of São Carlos) for helping in the samples processing; and Dr. John T. Finn (University of Massachusetts) for helping with the statistical analysis.

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