Estimation of sediments produced in a subbasin using the Normalized Difference Vegetation Index

Among the parameters considered by the Revised Universal Soil Loss Equation (RUSLE), the soil cover and management factor (C) is the main human influenced factor affecting the estimation of water erosion, and one of the most sensitive to spatiotemporal variations. Consequently, this study aims to compare the efficiency of C factor estimates obtained from the literature for each land-use class (C lit ) and by calculation based on the Normalized Difference Vegetation Index (C NDVI ). We test the hypothesis that soil loss estimates based on C NDVI approach are more accurate than those based on C lit . Water erosion was estimated based on soil morphological, physical, and chemical properties in addition to climate, relief, management practices, and land use and cover. The modeling steps were realized with the help of the Geographic Information System. The results were validated using the data of total sediment transported with water discharge and daily runoff. RUSLE underestimated soil losses by 0.64 Mg ha -1 year -1 using C lit and 0.45 Mg ha -1 year -1 with C NDVI , which corresponds to errors of 21.05% and 14.80%, respectively. Therefore, the C NDVI factor results are more accurate. Both methodologies identified areas with high erosion rates where the adoption of mitigation measures should be prioritized.


INTRODUCTION
Uncontrolled water erosion is the main reason for soil degradation in tropical regions, with the potential to make large areas economically unproductive. The erosion process not only causes soil losses but also leads to many secondary environmental problems such as flooding, siltation, and water body pollution (Beskow et al., 2009;Prasannakumar et al., 2012;Sun et al., 2014).
A quantitative assessment of erosion is required to understand the range and magnitude of the process to determine effective mitigation strategies. However, measuring erosion rates is a complex task, particularly in rural areas of developing countries, due to the high cost of analyses and the long period required to detect trends (Prasannakumar et al., 2012;Anh et al., 2014).
Moreover, soil loss quantification methods based on experimental plots have many limitations in terms of the representativeness and reliability of the results. Such methodologies cannot provide the spatial distribution of soil loss, and their application is often possible only in small areas (Chen et al., 2011). The use of water erosion modeling overcomes the limitations of direct measurement methods and allows the estimation of soil losses with a satisfactory level of accuracy. Moreover, these models are useful tools to increase our understanding of environmental processes and assist in decision-making (Panagos;Katsoyiannis, 2019).
The Revised Universal Soil Loss Equation (RUSLE) is the most widely used in the world model and is relatively simple to apply with low data requirements (Prasannakumar et al., 2012;Ganasri;Ramesh, 2016). The association of RUSLE with Geographic Information System (GIS) and remote sensing allows the assessment of spatial distribution of soil losses and the identification of areas with the most intense erosion rates (Cunha;Bacani;Panachuki, 2017;Imamoglu;Dengiz, 2017;Haidara et al., 2019).
RUSLE estimates annual average soil loss as a function of rainfall erosivity (R), soil erodibility (K), topographic factor (LS), soil cover and management factor (C), and conservation practices factor (P) (Renard et al., 1997). The C factor represents the protective effect of vegetation against the impact of rainfall on soil and is the main factor controlling anthropic erosion (Ouyang et al., 2010;Devátý et al., 2019). In addition, the C factor is one of the most sensitive parameters to spatiotemporal variations when it is influenced by vegetation growth and rainfall dynamics (Nearing et al., 2005).
Traditionally, the C factor is determined from a constant value found in the literature, which was obtained from experimental plots developed for different regions of a study area. This methodology cannot represent the spatial heterogeneity of soil vegetation cover (Almagro et al., 2019). To improve soil loss estimation, Durigon et al. (2014) developed an equation using the Normalized Differences Vegetation Index (NDVI) to determine the C factor.
In southeastern Brazil, several authors estimated soil losses using RUSLE based on C factor values obtained in the literature (Beskow et al., 2009;Oliveira et al., 2014;Mendes Júnior et al., 2018;Tavares et al., 2019), and there were a few NDVI-based approaches (Durigon et al., 2014;Silva et al., 2017). This study aims to compare the efficiency of the C factor estimation based on values obtained from the literature for each land-use class (C lit ) and the C factor calculation based on NDVI (C NDVI ). We test the hypothesis that the results of the soil loss estimates based on the C NDVI approach are more accurate than those based on the C lit .

Study area
The research was carried out in the Coroado Stream subbasin, which belongs to the Rio Grande River basin. The area is in the municipality of Alfenas, Minas Gerais State, southeastern Brazil. According to Köppen, the climate is classified as mesothermal tropical (Cwb) with a mean annual rainfall of 1500 mm and a mean temperature of 22 °C (Alvares et al., 2013;Instituto Nacional de Meteorologia -INMET, 2019).

Revised Universal Soil Loss Equation (RUSLE)
The RUSLE model is expressed according to Equation 1 (Renard et al., 1997) as follows: where A is the average annual soil loss in Mg ha -1 year -1 , R is the rainfall erosivity factor in MJ mm ha -1 h -1 year -1 , K is the soil erodibility factor in Mg h MJ -1 mm -1 , LS is the dimensionless topographic factor (given by the relationship between the length (L) and inclination of the relief (S)), C is the dimensionless cover and management factor, and P is the dimensionless conservation practices factor.
The R factor reflects the effect of rainfall intensity on soil erosion, and its value is determined as a function of continuous rainfall data (Wischmeier;Smith, 1978). Due to the lack of precipitation data, the R factor was determined according to the multivariate geographic model where, X 14 is the code of the hue of the moist soil according to Munsell (dimensionless), X 16 is the structure degree code (dimensionless), X 17 is the structure size code (dimensionless), X 18 is the structure shape code (dimensionless), X 19 is the soil plasticity code (dimensionless), X 22 is the fine sand content dispersed in 0.1 mol L -1 NaOH (g kg -1 ), X 23 is the very fine sand content dispersed in 0.1 mol L -1 NaOH (g kg -1 ), X 24 is the silt content dispersed in 0.1 mol L -1 NaOH (g kg -1 ), X 25 is the clay content dispersed in 0.1 mol L -1 NaOH (g kg -1 ), X 26 is the very coarse sand content dispersed in water (g kg -1 ), X 27 is the coarse sand content dispersed in water (g kg -1 ), X 31 is the silt content dispersed in water (g kg -1 ), X 32 is the clay content dispersed in water (g kg -1 ), and X 34 is the flocculation index (dimensionless).
The K factor parameters were determined using soil samples collected in January 2019 at 18 points distributed in the subbasin area ( Figure 1B). Disturbed and undisturbed soil samples were collected from the surface (0-20 cm) and subsurface (20-40 cm) layers using a probe and a cylinder sampler (92.53 cm³), respectively. The soil particle size distribution was determined by the pipette method, with and without 0.1 mol L -1 NaOH (Gee;Bauder, 1986) and the flocculation index according to Empresa Brasileira de Pesquisa Agropecuária -Embrapa (2017).
The LS factor was calculated based on the DEM ( Figure 1A) using Equation 4, proposed by Moore and Burch (1986) as follows: where A is the altitude in meters, LA is the latitude, and LO is the longitude. Both LA and LO are in negative decimal degrees.
The K factor represents the susceptibility of the soil to erosion (Renard et al., 1997) and was determined from the physical and chemical attributes of the soil according to the indirect method of Silva et al. (1999) (ESRI, 2015) was used in the processing and modeling steps and to convert the parameters to a raster data format. Each pixel of the soil loss maps based on C lit and C NDVI was converted into 46.000 points using the ArcGIS 10.3 Raster to Point tool. Data of the C NDVI factor (X) were plotted against the C lit factor (Y), and a linear relationship was fitted to assess the deviation from a 1:1 slope.

Validation
Soil loss estimated by RUSLE includes both the soil fraction retained along the area and the fraction that reaches the water bodies (net erosion). Consequently, the integration of the model with the sediment delivery ratio (SDR) is necessary to determine the net erosion. The methodology proposed by Gavrilovic (1962) (Equation 7) was used to calculate the SDR because of the satisfactory result obtained in our previous study (Lense et al., 2019) in the same subbasin.
where FA is the flow accumulation expressed as the DEM grid cell number, S is the declivity of the subbasin in degrees, and the spatial resolution of the DEM is 10 m.
The C factor comprises the effects of vegetation cover on soil loss and ranges from 0 (high vegetation cover) to 1 (bare soil) (Oliveira et al., 2014). We calculated the C factor using two approaches: based on the values presented in the literature for each subbasin land-use class (C lit ) and based on NDVI (C NDVI ) according to the methodology proposed by Durigon et al. (2014) as shown in Equation 5.
where C NDVI is the dimensionless soil cover factor. NDVI is a widely used indicator of vegetation health and ranges from -1 to +1, with higher values attributed to areas of higher plant density. The index was obtained using Equation 6 (Tucker, 1979): where NIR and RED are the near and red infrared spectral bands, respectively. The NDVI was calculated from the Landsat-8 OLI images used in the subbasin land use mapping, as previously described. To improve the representability of NDVI in the subbasin, we used the C NDVI factor average value obtained between June 2018 and July 2019.
The P factor value expresses the impact of soil conservation practices on erosion rates and ranges from 0 to 1, where values close to 0 indicate comprehensive soil conservation practices. We assign the P factor values based on the management practices adopted in the subbasin and on conducted field surveys (Bertoni;Lombardi Neto, 2012). The P factor attributed to coffee was 0.5 due to the planting along the contour lines and the maintenance of spontaneous vegetation between the coffee lines. In the areas with cultivation of maize under conventional systems, uneven eucalyptus planting, and access roads with exposed soil, we attributed a P factor of 1, while a P factor of 0.01 was attributed to forests.
where O is the subbasin perimeter (9.28 km), D is the mean elevation difference (0.06 km), obtained by the difference between the mean altitude (861 m) and the minimum altitude (795 m), and L is the length of the subbasin measured from the watercourses (3.32 km).
The model validation was realized by combining RUSLE with the SDR to calculate the net erosion in the subbasin area. Thus, the results were compared to the annual sediment transported according to Beskow et al. (2009). We used total solids data monitored between 2001 and 2018 by a hydrosedimentological station operated by the Minas Gerais Institute of Water Resources Management (IGAM), located at coordinates 45° 53′ 35″ W and 21° 39′ 55″ S).
A curve relating the total sediment transported in the subbasin and the water discharge (Figure 2) was plotted to determine the annual sediment transport, in relation to the flow versus sediment curve and the daily runoff data from 2018 obtained from the National Water Agency (Agência Nacional de Águas -ANA, 2019).

RESULTS AND DISCUSSION
The subbasin soils exhibited the following characteristics: granular structure with a moderate degree and medium size, slight plastic consistency, and a basic hue of 2.5 YR. The clay content ranged from 41% to 59% (clayey texture). The K factor parameters are presented in Table 1. These characteristics provide a soil erodibility of 0.020 Mg h MJ -1 mm -1 for the subbasin Latosols, which were close to those found by Mendes Júnior et al. (2018) (0.040 to 0.026 Mg h MJ -1 mm -1 ) and Silva et al. (1999) (0.002 to 0.034 Mg h MJ -1 mm -1 ). rainfall intensity in the area (1,500 mm). These results are close to those found by Aquino et al. (2012) for the same region, which found a range of 5,145 to 7,776 MJ mm ha -1 h -1 year -1 , indicating a good accuracy for the erosivity factor calculated by Mello et al. (2013) in the Coroado Stream subbasin.
The LS factor presents an average of 4.3, and only 11% of the subbasin showed values higher than 10, which indicates that these areas are more vulnerable to soil erosion (Beskow et al., 2009;Oliveira et al., 2014). Vulnerable areas were mainly concentrated in high-slope places with a higher runoff velocity process (Beskow et al., 2009;Rodrigues et al., 2017). The highest values of the LS factor are spatially distributed throughout the subbasin ( Figure 3B), reinforcing the need for extensive management to reduce erosion. Similar results were observed by Oliveira et al. (2014) and Steinmetz et al. (2018), who analyzed the water erosion in southeastern and southern Brazil, respectively.
The C NDVI factor reflected the effect of vegetal cover density on the soil surface, indicating more comprehensive soil protection. Thus, we found lower C NDVI factor values in areas with higher soil protection, such as eucalyptus, native, and advanced stages of forests ( Figure 3C). Areas with exposed soil such as access roads, early stages of forests, and maize cultivated under the conventional system present higher C factor values and, consequently, higher erosion rates. The C lit factor values are presented in Table 2.  Fine sand content dispersed in NaOH 0.1 mol L -1 (g kg -1 ) 96.00

X 27
Coarse sand content dispersed in water (g kg -1 ) 79.00
We observed a high R factor according to the Foster et al. (1981) classification, ranging from 6,730 to 7,769 MJ mm ha -1 h -1 year -1 ( Figure 3A), which can be explained by the high RUSLE estimated the total soil loss at 11,235.54 and 11,670.00 Mg year -1 based on the C lit and C NDVI values, respectively. The SDR in the subbasin was 0.118, indicating that 11.8% of the eroded soil volume reaches water bodies contributing to siltation and water quality depreciation. This soil fraction corresponds to a net erosion of 1,280.85 and 1,377.02 Mg year -1 with averages of 2.40 and 2.59 Mg ha -1 year -1 based on C lit and C NDVI , respectively.
low-intensity erosion (< 2.5 Mg ha -1 year -1 ) ( Figure 4A), according to the classification by Beskow et al. (2009). However, using the C NDVI factor, this ratio dropped to 66% ( Figure 4B). The sediment generation calculated based on the IGAM hydrosedimentological station was 3.04 Mg ha -1 year -1 . Comparing this value with the results, RUSLE underestimated soil losses by 0.64 Mg ha -1 year -1 using C lit and 0.45 Mg ha -1 year -1 with C NDVI , which corresponds to errors of 21.05% and 14.80%, respectively. According to Pandey, Chowdary and Mal (2007), errors smaller than 20% are considered tolerable. Therefore, only C NDVI based estimates could be validated for the Coroado Stream subbasin. Almagro et al. (2019) obtained a similar result with errors of 13% using C NDVI and 20% using C lit , demonstrating the higher efficiency of the C NDVI factor compared to the traditional method.
In addition to the more accurate results, another advantage of the approach is the use of remote sensing data, which allows the vegetation cover to be estimated anywhere with satellite coverage. There is a lack of C factor values in the literature. In contrast, satellite images with an adequate spatiotemporal resolution for the erosion model are available free throughout the Brazilian territory through the Imaging Division (INPE, 2019), making it possible to estimate the C factor and soil loss at different scales (Almagro et al., 2019).
Considering the land use and occupation classes, we found different erosion rates by the different C factor calculation approaches (Table 3).
The C lit factor considered the static value of 1 for the access roads, representing the absence of vegetation in these areas, which increases the soil loss rates. However, the access roads in coffee often have low agricultural machinery traffic and are occupied by spontaneous vegetation or grass growth, which was observed in the Coroado Stream subbasin ( Figure 6). Consequently, the C NDVI factor results are closer to the actual subbasin vegetation cover. In the case of access roads, the vegetation present in the area can mitigate the soil erosion process resulting in low soil loss estimates. Moreover, the C NDVI factor can provide a better vegetation cover estimation in forested areas in different stages of regeneration, which provide distinct levels of soil protection.  Notes: C lit = C factor based on literature data; C NDVI = C factor based on NDVI data.
The C NDVI factor is calculated cell by cell in a GIS, which enables a more representative result of the heterogeneity of vegetation cover in the area. Based on the deviation from a 1:1 relationship, we found that, in general, the C lit factor overestimates the soil loss estimates compared to those using the C NDVI factor ( Figure 5). The clearest example of this overestimation is that the lowest soil loss value calculated for the access roads by the C NDVI factor is 5.20 Mg ha -1 year -1 while the corresponding C lit factor is 18.67 Mg ha -1 year -1 .  Regardless of the methodology used to determine the C factor, the RUSLE results indicated high soil losses in some subbasin areas (Figure 3). Some alternatives to reduce water erosion would be the introduction of practices that improve soil cover in the areas of maize and sugarcane, such as the adoption of no-till systems and the management of plant residues. Terracing in the eucalyptus areas under uneven planting and the construction of containment basins around the access roads located on steep reliefs could also help to control the erosive process (Bertoni;Lombardi Neto, 2012;Mendes Júnior et al., 2018).
Additionally, to reduce soil losses to a minimum rate along the subbasin area, the conservation practices already in place should be maintained and intensified to ensure the long-term sustainability of agricultural production.
The subbasin presented high erosivity, constant erodibility, and steep slopes distributed throughout the area. Consequently, vegetation cover and soil management (C and P factors) are the main factors responsible for the variations in water erosion, especially in the places where LS factor indicated high vulnerability to the erosive process.

CONCLUSIONS
Soil loss estimates generated by RUSLE based on the determination of the C factor from NDVI were more accurate than the results based on the C factor obtained from the literature data, with errors of 14% and 21%, respectively. However, both methodologies indicated that the Coroado Stream subbasin represents areas with high erosion rates, where the adoption of mitigation measures for water erosion should be prioritized.