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The consequences of climate change in Latin America and the Caribbean, according to Herrán (2012), are reduced availability of water, diminished performance of agriculture in low altitude areas, loss of biodiversity, and increase of flooding and droughts.
Between 1992 and 2001, floods were the most frequent natural disaster (43% of 2257 disasters) and affected more than 1.2 billion people all over the world in a decade. These floods are the result of climate change associated with the impact on ecosystems (Millennium Ecosystem Assessment, 2005). In recent years, climate change has been aggravated by natural or anthropogenic causes, or a combination of these (Moraes et al., 2015).
According to Frank (1995) the region of Vale do Itajaí, in Santa Catarina, and the municipality of Itajaí in particular, have a history of floods and flooding due to environmental characteristics. The most devastating floods in the region occurred in 1983, 1984, 2001, 2008 and 2011 (Santos et al., 2012). The last flood in the city occurred in 2015. Combined with the fact that Itajaí is located on a plain that comprises the mouth of river Itajaí and that it receives the energy and matter inputs of the entire river basin, the forest fragmentation and urbanization of this region increases its susceptibility to floods in rainy periods (Marenzi, 2012). According to Tucci (2008), floods occur in two ways: the riverbanks, because of the temporal and spatial variability of rainfall and runoff in the river basin; and due to urbanization, which occur in urban drainage because of the effect of waterproofing the soil or preventing the flow of water.
Itajaí is located in the Atlantic Forest and it is represented by the Montane Atlantic Forest and Lowland Atlantic Forest (Veloso et al., 1991). The lowland coastal forest was extensively fragmented and mostly occupied by settlements, port structures, and agricultural and pasture systems, especially rice crops in the rural area (Marenzi, 2012).
Several works have been carried out to better understand the impacts of urban planning on the hydrological cycle (Barron et al., 2013; Braud et al., 2013; Davies et al., 2008; Miller et al., 2014; Randhir and Raposa, 2014; Rougé and Cai, 2014; Sillanpää and Koivusalo, 2015; Suriya and Mudgal, 2012; Yang et al., 2014; Dalagnol et al., 2017; Santos et al., 2017) and the effects of irrigated agriculture (Foster et al., 2015; Blainski, et al., 2016). Some works specifically sought to analyse the influence of vegetation on water infiltration (Alves et al., 2005; Costa et al., 1999; Godim et al., 2010; Nunes et al., 2012; Sidiras and Roth, 1987; Touma and Albergel, 1992; Zwirtes et al., 2011; Rossato et al., 2016). However, no works on the subject in the region of Vale do Itajaí, in Santa Catarina were identified.
Based on the history of physical and material damages caused by flood phenomena in Santa Catarina, specifically in the Itajaí Valley, there is a need for research on measures to minimize this trend (Moraes et al., 2015). Therefore, this work sought to verify the influence of vegetation cover on the rate of infiltration and physical characteristics of soil inherent to infiltration. Consequently, the aim of this study is to analyse the physical and hydrological characteristics of soil in pasture and forest areas and understand the functional role of forests to provide information on floods in Itajaí.
MATERIALS AND METHODS
The study area is located in the municipality of Itajaí, in the geographical coordinates 26°54'28"S and 48°39'42"W, at an altitude of 2.0 m. Research was conducted from May 2014 to February 2015.
According to the Köppen classification, the regional climate is humid with a warm summer (Cfa). Annual average rainfall in the region is 1545.3 mm, average temperature is 20.3 °C, and average relative humidity is 83.73%. The heaviest rainfall occurs from January to March and from October to December and the lowest rainfall occurs from April to June and July to September (Araújo et al., 2006).
Google Earth images were used to classify soil use and occupation and select the sample areas and ArcMap 10.0 software was used to georeference a scene from the urban part of the municipality, with information from the database of the Empresa de Pesquisa Agropecuária e Extensão Rural de Santa Catarina (EPAGRI, 2004). For the other scenes, we created a mosaic using Regeemy software 0.2.43 (Fedorov et al., 2003). ArcMap 10.0 software was used for supervised classification.
The experimental areas were selected according to similar environmental characteristics, like soil, climate and relief although one area consisted of secundary atlantic forest (21.44 ha) and the other consisted of artificial pastures (15.63 ha) with semi-extensive livestock farming. Both areas are situated along the right bank of river Itajaí - Mirim and are classified as Histosols (PMI, 2006).
Two sampling campaigns were executed, one in August (dry season) and one in October (rainy season) of 2014. Soil was collected from four random sampling points in each area to determine moisture content, organic matter content, and grain size. The basic infiltration rate was determined at the location.
Each sample contained five kilos of soil at a depth of 50 cm. For the hydraulic conductivity analysis, soil was collected at a depth of 50 cm to obtain an undisturbed sample of 15 cm in height and 10 cm in diameter.
The basic infiltration rate test was run according to the methodology double ring, described by Bernardo et al. (2013), using I=K*T^A, where K and A are constant and depend of the soil. Moisture content was analysed according to EMBRAPA (1997). This methodology was used for grain size analysis of soils rich in organic matter. Fifty grams of soil were weighed in a beaker and portions of oxygen peroxide (H2O2) were successively added to the soil. The samples were heated to accelerate the reaction and stirred using a glass rod to observe the effervescent reaction. The addition of H2O2 was suspended and the samples were covered with a clock glass and left to sit overnight. This process was repeated another two times. To remove the excess of H2O2, water was added to the beaker instead of washing the sample filter paper, as indicated in the methodology. The samples were left to sit overnight, after which the supernatant was withdrawn and the water was restored. Also, this procedure was repeated two more times. The material was put to dry in an oven at 105 oC for 24 h. The material was buffered and 20 g were weighed and sieved (#0.063 mm) with the addition of water. The retained material was collected and dried in an oven at 105 oC for 24 h. The weight difference was used to calculate the percentage of silt/clay and, consequently, of sand. Organic matter content of the soil samples was determined using the methodology described by Wright (2008). Permeability was determined using the variable load methodology described by Das (2006).
For the analysis of variance, the following factors were considered: 1) soil cover (forest and pasture); 2) campaign, both considered orthogonal and fixed. The analysed variables were: basic infiltration rate, hydraulic conductivity, moisture content and organic matter content. Normality was verified using the Kolmogorov-Smirnov test and the homogeneity of variances was verified using the Cochran test; when necessary, transformations to Log10 (X+1) were applied (Underwood, 1997).
RESULTS AND DISCUSSION
The municipality of Itajaí presented fairly fragmented areas, mainly by agricultural activities and urban pressure (Figure 1). We defined the following six classes of soil use and occupation: culture, urban patch, reforestation, exposed soil, forest, and water. Once the study area was selected, the following classes were specifically verified: cultivation, exposed soil and vegetation, with a predominance of the former, mainly consisting of pasture for cattle farming. However, this type of farming is not prevalent in the municipality; irrigated agriculture represents most of the cultures in the municipality, occupying 9283 ha, with a production of 69,000 t of grain (EPAGRI, 2010).
The analysis of soil use and occupation showed that the municipality of Itajaí has small portions of forest in the lowlands. According to Bedin (2013), since the 1970s, the urban fabric of Itajaí has expanded rapidly and lots have replaced cultivated areas. Ribas (2013) stresses that in 1985, the cultivation and reforestation areas occupied most of the municipality. Around 1995, the forest cover increased in sparse fragments, possibly resulting from the abandonment of part of the agricultural areas. The urban area also expanded due to the economic dynamics of the region. This expansion caused the process of forest fragmentation, especially in plains that are most sought after for urban expansion, and intensified the problem of soil sealing and flooded areas in periods of intense rainfall. These facts support the importance of understanding the functional role of forest cover in these flood plains. This considering the natural vulnerability to flooding due Itajai location at the mouth of a large river basin (Santos et al., 2012; Moraes et al., 2015). Natural vulnerability intensifies with the urbanization process (Tucci, 2008). It is also important to consider that climate change intensifies vulnerability, because in many parts of the world there is a significant increase in temperature, resulting in extreme events. (Righi and Robaina, 2010; IPCC, 2014), such as floods. The scientific community has reinforced the conviction that climate variability has intensified in recent decades as a result of human activities (Moraes et al., 2015). All the analysed samples contained more than 99% of clay. Organic matter content ranged from 13.4 to 51.7%, moisture content ranged from 59.8% and 84.5%, the basic infiltration rate ranged from 0.6 and 39 mm h-1, and conductivity ranged from 0.05 and 1.13 cm h-1 (Table 1).
Table 1 Observed values for the variables hydraulic conductivity, moisture content, organic matter content, basic infiltration rate and grain size for two soil covers in Itajaí (SC).
The differences found for the factor soil cover with the variable basic infiltration rate corroborate the findings of Alves et al. (2005) and Zwirtes et al. (2011), considering the most of basic infiltration rate in the soils of the forest. However, the values found in the forest area by these authors are significantly higher (116.5 cm h-1 and 122.48 cm h-1, respectively). This difference can be associated with the type of studied soil. Histosol or organic soil have hydromorphic organic horizons and these horizons remain saturated with water during a period of the year (Zanella et al., 2018). Therefore, with lower infiltration rate if compared another soils. The moisture content of the soil in the two areas was slightly different, with smaller amounts in the forest area. This can be explained by the fact that larger vegetation areas require more water (Raven, 2007).
The analysis of variance indicated significant differences in relation to cover for the infiltration rate (Table 2). The significantly higher values were in forest areas regardless of the time of the year (Figure 2). The significantly higher moisture content was in campaign 1 for both types of cover (Figure 3).
Table 2 Significance value and F value of the variables: hydraulic conductivity, moisture content, organic matter content and basic infiltration rate obtained in the August and October campaigns of 2014 for different soil covers (forest and pasture), in the city of Itajaí (SC).
Figure 2 Basic infiltration rate, average and respective standard error, obtained in the August and October campaigns of 2014 for different soil covers (forest and pasture), in the city of Itajaí (SC).
Figure 3 Moisture Content, average and respective standard error, obtained in the August and October campaigns of 2014 for different soil covers (forest and pasture), in the city of Itajaí (SC).
The analysis of variance proved to be significant for the factor campaign and can be explained by the occurrence of increased rainfall in the period. However, there were no significant differences between the soil covers, which suggests the same moisture condition of the analysed soil samples. According to Araújo and Reis (2014), the accumulated volume in August (118 mm) was higher than the accumulated volume of October (63 mm). Therefore, the result of greater moisture in the first campaign is expected.
The high values of clay in all the analysed samples suggest that any difference between the studied areas can be the result of the forest function on the soil, especially in terms of aeration due to the root system (Gliessman, 2000).
The content of organic matter was numerically smaller in the forest area of the first campaign. This fact cannot be explained since the forest generates the most amount of organic matter, as verified by Cerri et al. (1991) and Leite et al. (2003), even considering recycling. However, the increase in organic matter content may derive from the semi-extensive livestock farming in the area and the faeces residue. In the October campaign, the organic matter content was numerically greater in the forest area, which is closer to the expected result.
The hydraulic conductivity results followed the same trend of the organic matter content, according to which the presence of cattle could have been more frequent in August (first campaign) since the semi-extensive system generates organic waste that may have contributed to the hydraulic conductivity. In the October campaign, the forest cover was compensated in organic matter and therefore provided greater hydraulic conductivity. Significant correlations were observed between the organic matter and the physical attributes of the soil (Beutler et al., 2001) since organic matter contributes to the increase in the porosity of soil (Khaleel et al., 1981; Logan and Harrison, 1995), and therefore facilitates permeability.
These results indicate the possibility of greater contribution of forest compared to pasture to minimize flooding in Itajaí. Thus, conservation actions and educational programs or actions are extremely important, because they can minimize environmental problems, especially those associated with the silting of the river and soil sealing (Moraes et al., 2015).
CONCLUSIONS
The water infiltration was significantly higher in the presence of forest vegetation (P=0.041) indicating its potential role in the infiltration by rainwater absorption in the floodplain of Itajaí/SC, Brazil. In this way, the pressure for occupation urban and agriculture stresses the importance of protecting the last forest remnants.
It is important to consider that sampling in one area limited the study, since other samples could confirm the value of the forest in the infiltration of the water in the soil, minimizing the problems of floods, common in the region and intensified by events due to climate change. In any case, the result of this study indicates the importance of transforming the last forest remnants into Protected Areas.
