2.1. Study Area
The Pandeiros River basin (PRB) is located in the northern state of Minas Gerais, Brazil, and has an area of 396,028 ha, which encompasses part of Januária, Bonito de Minas, and Cônego Marinho municipalities (
Figure 1). The climate of the region is predominantly dry with mean annual temperature of 24.6 °C, which occasionally reaches a maximum of 33 °C in October, and a minimum of 14 °C in July [
44]. Rainfall is concentrated in April to September, since the climate is semiarid, with mean annual rainfall of approximately 1,050 mm year [
45,
46,
47]. The longest and largest tributaries of the Pandeiros River are the Catolé, Suçuarana, Borrachudo and Macaúbas streams.
According to the Brazilian Institute for Geography and Statistics (2018), the municipalities comprising the PRB have a total population of 86,311 inhabitants, most of them living in Januária. The largest part of this population live in rural areas, especially in the municipalities of Bonito de Minas and Cônego Marinho (
Table 1) [
48].
The PRB is in an area with transitional vegetation, presenting phytophysionomies of Cerrado and Caatinga biomes [
49]. This ecotone is characterized by swamp regions that contain the springs of the São Francisco River. These springs are responsible for the reproduction of most fishes that live between the Três Marias (MG) and Sobradinho (BA) dams [
50].
The relief is predominantly flat. The plain was formed by the filling of the São Francisco Depression with sediments that were sourced from the erosion of rocks from the São Francisco Plateau [
51]. This process is also responsible for a small proportion of steep slope areas in the basin, as shown in
Figure 2 and quantified in
Table 2 [
52].
As regards the local geology, there is the presence of alluvial deposits over the main drains in the area and in wetland regions (Veredas), which are the results of natural and anthropogenic erosion processes with consequent transport and deposition of sediments.
Figure 3 and
Table 3 show the spatial distribution and proportion of lithotypes in the studied basin, respectively.
The Soil Map of Minas Gerais [
53] presents an area predominantly composed of red–yellow latosols, which cover more than 87% of the hydrographic basin, followed by fluvic neosols (5.48%). The occurrence of quartzarenic neosols, litolic neosols, cambisols and melanic gleysols are related to much smaller areas (
Figure 4 and
Table 4).
The spatial distribution of the land use and cover classes in the basin (
Figure 5) shows the predominance of a typical Cerrado vegetation (savanna) of low to medium size [
54], which is usually associated with the occurrence of latosols and sandstone regions. These are found mainly in the central part, towards the northern and northeast of the Pandeiros River basin. This phytophysiognomy covers 183,719.88 ha in the basin, representing 46.3% of its area (
Table 5). The second most significant land use and cover class in the study area is the dense Cerrado (21.5%), followed by the sparse Cerrado (10.42%).
2.3. Methodology
The spatially explicit groundwater recharge potential in the Pandeiros River basin was evaluated using the methodology proposed by Costa et al. (2019) [
10], with the adjustments described below. The workflow was divided into 5 main steps: (i) acquisition of a land use and cover map, digital elevation model, soil type map, and climate data (rainfall and evapotranspiration); (ii) calculation of surface runoff using the slope length and steepness factor, and runoff coefficients for the land use and cover types; (iii) calculation of water percolation based on the PUC method [
43] (replacing the effective porosity of Costa’s approach and representing the proposed methodological improvement) and adapted hydraulic conductivity values with fuzzy logic [
55,
56]; (iv) calculation of groundwater recharge in different points of the basin and a mean value for the whole basin, using a geographical information system; (v) validation of results by comparing the previously calculated mean groundwater recharge with the value estimated by the hydrograph recession curve analysis (
Figure 6).
In the first step, the mean annual rainfall and evapotranspiration of municipalities near the study area were estimated using data from the Brazilian Institute for Meteorology (INMET) relative to the 2009–2018 period, which were obtained from meteorological stations in Arinos (MG), Januária (MG), Montes Claros (MG), Salinas (MG), Cariranha (BA), Espinosa (MG), Formoso (MG), Posses (GO), and Brasília (DF). Longer temporal series would be more adequate for estimating groundwater recharge. However, these records were not available. The flaws in the temporal series were resolved by using the regional weighting method, and the information was interpolated through inverse distance weighting (IDW) raised to the power of two [
58]. This method was used because it raises the importance of closer stations in the interpolation. The data were spatialized and trimmed up to the study area limits.
In the second step, the land use and cover map and the topographical information from the digital elevation model (slope length and steepness factor—LS factor) were used to calculate the surface runoff factor, based on the method proposed by Böhner and Selige (2002) [
59]. The runoff factor was calculated according to Equation (1), reproduced from Costa et al. 2019 [
10].
where
RF is the runoff factor (dimensionless);
C is the runoff coefficient (dimensionless values adopted from
Table 7);
LSFUZZY is the slope length and steepness factor estimated using the method of Desmet and Govers (1996) [
57], but recast to the 0–1 range using a fuzzy logic algorithm (the steeper the slope the closer to 1).
In the third step, the water percolation was calculated considering the soil classes in the basin [
53]. A systematic literature review was performed to set up hydraulic conductivity values for each soil class, at the second category level of Brazilian soil classification system (SiBCS) [
61]. The hydraulic conductivity values were established according to Freire et al. (2003), Costa et al. (2019), Pedron (2011) and Amaral (2017) [
10,
55,
56,
62].
Different from the method proposed by Costa et al. (2019) [
10], the water percolation factor in this model was calculated using the groundwater recharge category of the PUC method, developed by Costa et al. (2017) [
43]. The PUC is a method that allows for mapping areas of a basin based on their limitations and potentialities for conservationist land use, through the combined assessment and weighting of several environmental variables (soils, geology, and geomorphology) [
43].
The PUC assigns values from 1 to 5 to the different classes of lithology, slope and soil in the watersheds from the state of Minas Gerais, Brazil. The analyses are focused on groundwater recharge, agricultural use potential and soil resistance to erosion in the catchments [
43]. For the identification of lithologies, slopes and soil classes existing in Minas Gerais, the available official databases are used.
The attribution of grades for the different types of lithologies took into consideration their potential to provide nutrients (greater weight for rocks with higher absolute content of essential macro elements to plants) and their susceptibility to weathering processes (considering the main mineral constituents and scored according to their resistance to weathering, based on Goldish’s stability [
63]). Regarding the slope parameter, the same weights were attributed for groundwater recharge potential for agricultural use and resistance to erosion. For groundwater recharge, it was considered that the slope has a direct relationship with the flow velocity and the opportunity time for water infiltration. The higher the slope, the higher the water velocity and the shorter the time for water infiltration. In this context, the mountainous relief received a weight 1 and the flat relief a weight 5.
For the attribution of grades to different soils, the variables texture, drainage, effective depth and fertility were considered. For groundwater recharge, the attribute fertility was disregarded and the classes of soils characterized as favorable to water infiltration and percolation received greater weight. The recharging potential for each soil type was obtained by the simple average of the values of texture, drainage and effective depth, normalized so that the final scale was in the range of 1 to 5.
In this work, only the soil parameter of groundwater recharge was used, which assigns the basin´s soil classes a score from 1 to 5. This parameter takes into account the effective depth, texture and drainage of each soil class at the first category level of SiBCS [
61]. Thus, to the soil that presents considerable effective depth, satisfactory drainage and texture favorable to infiltration, a higher score regarding groundwater recharge is attributed.
However, in order to adapt the PUC method to the recharge model, a rescaling process was implemented, whereby the ratings from 1 to 5 were recast to the range 0 to 1. The values used in the model are represented in
Table 8. The percolation factor was evaluated using Equation (2), reproduced from Costa et al. 2019 [
10]:
where
PF is the water percolation factor (dimensionless);
KsFUZZY (dimensionless) is the soil’s hydraulic conductivity fitted to the 0 to 1 range by the fuzzy logic algorithm (the higher the soil’s hydraulic conductivity in the class the closer to 1);
PUC are the scores of groundwater recharge set up by the PUC method but fitted to the 0 to 1 range.
In the fourth step, the groundwater recharge potential was calculated for each point in the basin, using Equation (3), according to Costa et al. 2019 [
10]:
where
RPot is the groundwater recharge potential (m
3 ha
−1 year
−1);
P is the mean annual rainfall depth (mm year
−1);
ETR is the mean evapotranspiration (mm year
−1);
RF is the surface runoff factor; and
PF is the water percolation factor.
The results were validated in the fifth step through comparison of the calculated mean groundwater recharge with a homologous median value estimated by the hydrograph recession method based on the Maillet equation [
64,
65]. The hydrograph for the Pandeiros River was drawn from daily average stream flow data measured at the hydrometric station nº 4425000 located in the Pandeiros River mouth (Pandeiros River Dam), compiled from the Hidroweb portal [
66]. Streamflow data from 2013 to 2018 were the most recent and more continuous in the historical series, showing no flaws or absent data. Thus, these data were used to calculate the Maillet equation (Equation (4)). To separate and analyze the recession curve and the recession days, the methodology proposed by Barnes (1939), Dewandel et al. (2003) and Kovacs et al. (2005) was used [
64,
65,
67,
68,
69]:
where
QT is the flow at time
t (m
3 s
−1);
Q0 is the flow at the beginning of a recession (m
3 s
−1);
α is the coefficient of recession;
t is the time (days); and
e is the basis of Neperian logarithm (2.71828).
Thus, the coefficient of recession can be determined numerically, based on the logarithmic form of Equation (4), represented and rearranged in Equation (5):
Subsequently, the groundwater recharge volume was calculated using Equation (6):
where
V is the recharge volume (m
3);
Q0 is the flow at the beginning of recession (m
3 s
−1);
t’ is the converter of the
t unit (days into seconds; 86,400);
α is the coefficient of recession (dimensionless).
The constant of recession (α; Equation (5)) is dependent on the aquifer characteristics and therefore should not vary significantly from year to year. On a Q versus t plot (hydrograph), where the Q values are represented in logarithmic scale and the values are in linear scale, the baseflow within a hydrologic year (from the recharge period to the end of recession) should define as a straight line, the slope to which is related in terms of α. If tcycle is the time of a log cycle for discharge, meaning the time for discharge to change from 1 to 10 m3/s, from 10 to 100 m3/s, and so forth, then and α = 1/(0.4343tcycle). This simplified representation of Equation (5) is frequently used in the calculation of α and will be adopted in the present study. Conversely, the values of Q0 can vary in response to the annual variations of precipitation. In this case, a value of Q0 should be calculated for each hydrologic year, while mean ± standard deviation values are derived therefrom.