The studied area embraces the Itacaiúnas River watershed in the southeast region of the Pará state. According to the Agência Nacional das Águas (ANA), the Itacaiúnas river watershed (Figure 1) is classified as Order 3, according to the Otto Pfafstetter system (NCWR Resolution No. 30/2002), and has an area of 42,000 km². This basin averages 2200 mm of mean annual rainfall and the mean annual average temperature is 30°C. It has a tropical monsoon climate, according to the Köppen climate classification.

The Itacaiúnas River rises in Seringa mountain, in the municipality of Água Azul do Norte, and is formed primarily by the junction of the D’Água Preta and Azul rivers. Its mouth is on the west bank of the Tocantins River, in the area of Marabá City, and presents a fairly high level of deforestation, being in the region denominated the "arc of deforestation," with approximately 62% of all its area deforested, according to the Monitoramento do Desmatamento na Amazônia Legal por Satélite (PRODES project, 2012). It is among the watersheds whose water flows to the Tucuruí hydroelectric power plant, this area is of the ultmost importance to the economic and social development of Brazil.

Figure 1:
Watershed area of the Itacaiúnas River from the digital elevation model (DEM) of the SWAT model.

Table 1 presents the procedures adopted and considered as a reference base as follows: Arnold et al. (1998), Machado (2003), Winchel et al. (2013), Arnold et al. (2012), Neitsch et al. (2011), Wagener (2011), Nunes (2012) and Schuol (2008). This presents an integration interface in a Geographic Information System (GIS) enabling a dynamic interaction with the use of remote sensing and GIS, and can be applied to hydrology, hydrometeorology, and hydrogeology.

Table 1
Criteria adopted by SWAT.

For calibration of the model, we used the SWAT-CUP 5.0 software, applying the method of calibrating SUFI2. For validation of the results, we used the statistical indexes recommended by the International Precipitation Working Group (IPWG) and described by Liu et al. (2013) and replicated by Adam and Vecchia (2011) and Serrão et al. (2016). Thus, five statistical indexes were used: the Pearson correlation coefficient (r), the coefficient of determination (r²), the Nash-Sutcliffe efficiency coefficient (NS), the concordance index (CI) based on Willmott (1985), Hallak and Pereira Filho (2011), and Santos et al. (2011) and the quality coefficient (U) (Theil, 1966) for qualitative analysis. The following equations were used:

Table 2
Statistical indexes used to validate the SWAT.

In the calibration of SWAT, the SUFI2 method was used in the SWAT-CUP. For the 41 years of hydrological modeling (1969-2010), the calibration was used for five years (2010 to 2015) to calibrate the model. Table 3 shows the parameters that were calibrated in SWAT-CUP. After calibration of the SWAT model, it was again photographed using new parameters and new results for hydrological variables were obtained. The parameters shown in Table 3 represent difficult variable measurement. Moreover, there is limited information available regarding these variables for river basins in Brazil, mainly in the Amazonian Basin.

Table 3
Parameters used in the calibration of the SWAT model with their variations

It is noticeable that the south and the central portion of the basin present highest slopes with a maximum of 66 degrees. These are followed by the flat areas in the south portion of the basin, heading downstream along the Itacaiúnas, with 20 degrees of slope (Figure 3). The catchment area of the Itacaiúnas River has varied slopes resulting from various altitudes in the basin, and this increases the likelihood of erosional processes and siltation of rivers.

As shown in Figure 3, 43% of the basin is composed of native Amazonian forest area (is a conservation unit, of the Carajás National Forest), and 41% of the basin consists of pasture, followed by reforestation (11%), mining (0.35%) and urban areas (0.57%). The basin is quite heterogenous with different types of soil use and occupation, within the catchment area of the Itacaiúnas River. Within the large area of forest are the areas of preservation. Notably, within the areas of mineral extraction there is deforestation, increasing environmental degradation and endangering the fauna, flora, and entire energy and water balance.

Figure 3:
Slope, land use and land cover of the Itacaiúnas River basin

The distribution of the rainfall and consequently the runoff by sub-basin of the Itacaiúnas River is shown in Figure 4. The annual mean rainfall is higher in the central and western portions of the basin (between 1,990 mm and 2,216 mm) and it is lower in the southern portion of the basin. This pattern is related whit the displacement (north - south) of the Inter Tropical Convergence Zone (ITCZ), together whit South Atlantic Convergence Zone (SACZ), and to the regional atmospheric circulation regime, as mesoscale systems. Sub basin 20 presented the lowest level of annual mean precipitation, with 1,500 mm.

The Amazon rainforest, through vegetation, has a strong influence on evapotranspiration processes and, consequently, on the hydrological cycle, besides contributing to the interception of rainfall and reduction of the surface runoff process. However, this dynamic is affected by several meteorological variables and the physical factors of the basin (Alves and Pérez-Cabello, 2017).

The spatial distribution of the surface runoff is similar to rainfall. The sub-basin with the highest level of rainfall resulted in highest levels of runoff, and the sub-basins 13 and 14 had the lowest runoff. This is maybe by the land use and occupation in the basin, as shown in Figure 3, where it is clear that the sub-basin has various types of land use and is dominated by Neossolo flúvico, resulting in a higher infiltration and lower runoff. The largest runoff is in sub-basins 1, 3, 5, and 16 with more than 831 mm and the smallest runoff are in the range of 382 mm to 417 mm in sub-basins 13 and 14, respectively.

Figure 4:
Spatial distribution of precipitation and surface runoff by sub-basin.

It is clear that the surface runoff is directly dependent on the rainfall and physical factors of the basin (slope, land, and land use), and each of these variables influence the surface runoff. In its movement towards the exit of a river basin, the intercepted water flows over the rocks and the lands that form or cover the slopes and drainage network gutters (Tucci, 2005). And these obstacles, and the slope, determine the speed of runoff. (Tucci, 2009).

The surface runoff and the yield of sediments depend on the rain and the physical factors of the basin, as shown in Figure 5. It is clear that the sub-basins 8, 11, 13, 14, and 15 are the sub-basins with less yield of sediments (0.3 to 0.7 t. ha^-1. year^-1), and they are the sub-basins with the lowest rainfall and runoff. Sub-basins 7 and 4 are the ones that carry considerable sediment and show the highest values of rainfall and runoff. They are sub-basins with red-yellow latosol and high dynamics of land use, with reforestation, cities, woods, pasture, and agriculture. When evaluating land cover and land use changes in Rift Valley, Kenya, using the SWAT model, Baker and Miller (2013) showed that land use changes resulted in the corresponding increase in surface runoff, yield sediment and decrease in groundwater recharge.

Figure 5:
Spatial distribution of yield sediment by sub-basin

In this respect, the Sub basin 20 is among the sub-basins with higher sediment yield and is the basin with the least precipitation and runoff, but is among the regions of greater slope in the basin and have high land use dynamics. this respect, Machado and Vettorazzi (2003) argued that the hydrological processes associated with land use and management clearly play a dominant role in sediment production and transport. Within this subject, Martins Filho et al. (2009) asserted that the vegetation plays an important role in controlling erosion and reducing the loss of organic matter and nutrients through sedimentation. the problems caused by sediment are numerous, including siltation of rivers, increasing the magnitude of flooding, and siltation of reservoirs, diminishing their capacity and supply for irrigation, among others (Coe et al., 2011). These problems depend of the quantity and nature of the sediment that are related with the production processes (geologic formation), transportation (slope and rainfall), and deposition (depth of the riverbed).

Figure 6 shows various types of land use in the Itacaiúnas River basin and rainfall variations (P), actual evapotranspiration (ET) and runoff by sub-basin. The sub-basins with areas of forest as predominant in the use of the land have greater stability in the P, ET and runoff variables, with the exception of sub-basin 8, which has areas of deforestation, mining, grazing, and reforestation resulting in a greater runoff. Therefore, the hydrological processes associated with land cover and management clearly play a dominant role in sediment production and transport (Silva et al., 2018).

Analyzing the sub-basins in the east area of the forest, using the land predominantly for pasture, a contrast with the other uses of land is noticeable, particularly, regarding the runoff. In sub-basins 5, 11, and 12, there is an increase in surface runoff at pasture areas, once this type of land usage facilitates greater surface water flow because of forest removal (Figure 7). A similar result was obtained by Golmohammadi et al. (2017) who assessed the capability of SWAT to identify areas contributing to runoff at the Gully Creek Watershed in Ontario, Canada.

Normally low rates of evapotranspiration are associated with the withdrawal the forest and the compacted land (normally because of low slope), and a small leaf area, like pasture. A similar result was obtained by Abiodun et al. (2018) comparing of MODIS and SWAT evapotranspiration with different land cover at different spatial scales. However, the sub-basins with the predominant forest as land use, ET is more significant. Similar results were found by Coe et al. (2011) in which the runoff increased after the simulation of the model that replaced forested areas by pasture and decreased the evapotranspiration.

Figure 6:
Relation between land use and land cover classes and water balance variables per sub-basin, simulated using the SWAT model and 41 years of data.

Figure 7:
Runoff by land use and land cover.

When applying the statistical tests in this series of data, the Mann-Kendall test detected that sub-basins dominated by pasture tended to increase runoff. The series showed no tendency for the sub-basins that predominate the use of land as forest. Silva et al. (2017b) found a similar result when replacing native vegetation caatinga by pasture.

For the sub-basin with multiple uses of land, an anomalous result was obtained, but it was consistent with the history of the basin, as previously described, in which the runoff had a tendency to decay over the years, and it was not a homogenous series over the years, and there is a break in the mean of this variable. Chen-Feng et al. (2014) obtained a similar result for a model of river flow in Taiwan, where for areas of multiple uses of land, the flow will tend to fit the greater use shown in the area. The land cover affects sediment yield, potential evapotranspiration, and surface runoff of the basin and hence influences the water balance. Hydrologic models are essential for studying hydrologic processes and their responses to both natural and anthropogenic factors (Lironga and Jianyuna, 2012).

Figure 8 shows the annual flow (observed and simulated) during the simulation period. I 1996 the simulated flow began to underestimate the flow observed. Again, the issue of high dynamics of land use during these years can influence the downstream flow. It is seen that the land use types inserted into the model are the uses that already exist in the database of SWAT and have similarity to the current usage of the basin, creating even more uncertainties in the simulation. The change in vegetation cover in a watershed can change the water balance in the region.

Deforestation alters the porosity of the soil by pore sealing during precipitation events, changing their hydraulic properties. Deforestation also often results in increased erosion of the Earth's surface and deposition in channels and floodplains because of the increased area of bare soil, changes in infiltration and runoff and poor management practices (Bruijnzeel, 1991). Costa et al. (2003) analyzed the data and climate in a 175,000 km² sub-basin of the basin of the Tocantins River in the eastern Amazonia and concluded that most of the observed increase of 25% in that discharge drainage basin was attributable to deforestation.

Figure 8:
Observed and simulated streamflow series

Validation of the variables simulated by SWAT flow data were used for the period 2010 to 2016 and evapotranspiration was used for weather data from Marabá and applied to the statistical indexes described in table 2. Thus, we obtained the validation of flow and evapotranspiration for the Itacaiúnas River basin. Figure 9 shows the relationship of flow observed and simulated flow, it is apparent that during some years in the simulation using SWAT some outliers occurred, in some years the model did not obtain a good simulation of the streamflow.

Figure 9:
Relationship observed and simulated flow and validation.

Even if the model does not accurately reproduce some flow periods, statistical indices obtained satisfactory performance for the validation period, the correlation coefficient of Pearson was 0.86 and the determination was 0.74 being a good value for modeling uncertainty. For the Nash-Sutcliffe coefficient and index of concordance of Willmott a value of 0.99 was obtained, both considered excellent by the literature. The U coefficient of Theil was 0.03, an excellent value. Finally, when applying statistical tests of trend and homogeneity in the flow and sediment series, it was observed that the two series remained without a trend and were homogeneous, thus, it is assumed that on the whole, the water is in balance, where the different types of use if the soil, slope, and soil types and finally the weather, fit.

E-mail:oliveiraserrao@gmail.comE-mail:madson.geotech@gmail.comE-mail:fsouza2011@gmail.comE-mail:lorenaataide07@gmail.comE-mail:vicente.paulo@ufcg.edu.brE-mail:alinemeiguins@gmail.comE-mail:cleberassis.ufpa@gmail.com