3.2 Hydrological responses of the São João river watershed
As expected by the water balance, in long-term average values, evaporation reduction (Fig. 4B) suggests an increase in streamflow generation (Fig. 4A). The hydrological responses simulated with the LU/LC scenarios wherein only the hilltop protected areas are preserved per the Forest Code (3CF) or the Inea Resolution (3CI) show similar variation to the extreme scenario of the completely deforested watershed. In contrast, given that most areas defined as protected areas are currently forested, the hydrological response scenarios for Control, 2CF and 2RI are close to the response found when the watershed is fully forested. In these scenarios, despite the lower evapotranspiration at the dry season and the beginning of the rainy season, streamflow is still lower than in the Forest scenario because of the lower soil hydraulic conductivity capacity inputted in the model.
An analysis of long-term monthly mean amounts reveals a rise in the streamflow regime and a reduction in evapotranspiration processes in the seasonal variation, which shows the effects on maintaining forest cover only on the hilltop protected areas (Fig. 4A and Fig. 4B, 3CF and 3RI). In all other scenarios, among which the Forest scenario, mean hydro-climatic variables show behavior closer to that of the watershed’s present state (Fig. 4A and Fig. 4B, Control).
The FDC descriptors, when applied to the numerical modelling results considering the Pasture and Forest scenarios, show two extreme hydrological response patterns (Fig. 5). Dry and rainy season streamflow rates are higher in the Pasture scenario (Fig. 5A and 5C), with a greater slope in the middle section of the FDC (Fig, 5B), suggesting loss of regularity with rapid responses to rainfall events. Seasonal variation streamflow rates are also higher in the Pasture scenario (Fig. 5D).
The Control scenario shows a pattern close to that of the Forest scenario. The MWL index, associated with dry season streamflow rates, is close to that of the Forest scenario (Fig. 5A), while the MWH index, associated with the rainy season, is higher (Fig. 5C). The slope of the middle section of the FDC, shown by the MS index, is greater in the Control scenario than in the Forest scenario (Fig. 5B), revealing a quicker, less regular hydrological response; also, seasonal variation is greater in the Control scenario (Fig. 5D).
The addition of forest to current LU/LC in the hilltop protected areas shown in the 2CF and 2RI scenarios resulted in a decline in dry and rainy season streamflow rates (Fig. 5A and 5C), along with changes in soil hydraulic properties and better hydrological response in the watershed (Fig. 5B and 5D). On the other hand, when the model assumes that only the hilltop protected areas have native forest (scenarios 3CF and 3RI), the hydrological response of the watershed changes to a pattern similar to that of the Pasture scenario. In these scenarios, dry and rainy season volumes rise (Fig. 5A and 5C), while hydrological response is more rapid and less regulated (Fig. 5B and 5D).
Streamflow rate rises in the watershed under the native vegetation loss scenarios are associated with a direct response increase (Fig. 6A). As such, the São João river watershed shows direct response increases under scenarios 3CF and 3RI (Fig. 6A). Conversely, native vegetation loss is associated with a decline in response rates for baseflow, which is the phenomenon that feeds channel runoff at dry season (Fig. 6B).
This variation in flow paths is associated with variations in saturated zones, where direct runoff into the watershed occurs (Fig. 6C). The replacement of Atlantic Rainforest with pasture results in a decline in evapotranspiration (Fig. 4B), which, associated with a decline in soil water retention capacity, increases saturated zones. In the pasture scenarios (Pasture, 3CF, 3RI), the watershed keeps a higher saturated zone rate than in the Forest, Control, 2CF and 2RI scenarios.
Saturated zones had seasonal variation in size, covering a larger area of the watershed during the rainy season (January, February and March) than in the dry season (July, August and September) (Fig. 7). Under the Control scenario (Fig. 7A), saturated zones are clustered around the main channel where the floodplain is located.
In the Pasture scenario, saturated zones are the largest at both seasons, including regions with high elevation in the rainy season (Fig. 7B). On the other extreme, i.e., the Forest scenario, saturated zones are smaller and show fewer variations between seasons (Fig. 7E). The biggest differences between the Forest and Control scenarios (Fig. 7A) are in the size of the floodplain occupied by pastures (Fig. 2A).
In a LU/LC scenario wherein the Atlantic Rainforest is restricted only to the hilltop protected areas (Fig. 7D and Fig. 7G), saturated zone size is close that of the Pasture scenario in the watershed (Fig. 7B).
In the scenarios that keep LU/LC as currently existing in most of the watershed (2CF, 2RI) (Fig. 7C and Fig. 7F), saturated zones are closer in size to those of the Forest scenario (Fig. 7B). This is because of the small area of hilltop classified as protected area under current legislation, which protects a smaller rate of natural vegetation than what is found in the current scenario (Magdalena et al, 2018). Nevertheless, we noted that the presence of forests on hilltops reduces saturated zones in the watershed (Fig. 7C, Fig. 7F, Fig. 7G).
These responses suggest that, although liquid discharges increase in volume, reducing native vegetation coverage to the minimum prescribed by law would affects water supply in the watershed, which can cause problems in water resource management in the Juturnaíba reservoir and intensify conflicts over water use. Besides, the greater streamflow rates in the rainy season suggest a rise in energy flows for sediment displacement, which may cause soil loss, silting of the main channel and deposit of sediments in the Juturnaíba reservoir (Chen et al., 2007; Cunha, 1995; Fohrer et al., 2001).
3.3 Responses to sediment deposit controls in the São João River watershed
Per our analysis of the spatial distribution of average annual soil loss in the watershed under the Control scenario, 76% of the catchment area shows values lower than 5 t ha− 1 year− 1 (Fig. 8A). Nevertheless, 14% of the catchment area has an average loss of 5–30 t ha− 1 year− 1 (slight and moderate). This soil loss is in lowland areas, a region currently covered with pasture areas because of the removal of floodplain vegetation and the riparian forest for the channeling of the São João river, as well as due to agricultural activity (Cunha, 1995, 1991). Areas showing severe and very severe soil loss amount to only 2% and 3% of the catchment area (Fig. 8A).
As expected, under the Pasture scenario, we see a decline in the share of areas with soil loss at less than 5 t ha− 1 year− 1 and an increase in areas with slight (5–15 t ha− 1 year− 1) and moderate (15–30 t ha− 1 year− 1) soil loss, which represent 34% and 21% of the catchment area. The Pasture scenario also shows 13% of the catchment area with soil loss greater than 50 t ha− 1 year− 1 (very severe), concentrated in the water divisor regions (Fig. 8B), while, under the Forest scenario, this soil loss is less than 5 t ha− 1 year− 1 (Fig. 8E).
For the scenarios that consider only hilltop protected areas will retain vegetation coverage (3CF and 3RI), soil loss is close to that of the Pasture scenario; when compared to the Control scenario, forest decline results in intensified soil loss, especially in the watershed water divisors (Fig. 8D and Fig. 8G). Greater soil loss can intensify drainage system processes over time – an issue further compounded by increased surface runoff (Fig. 6A) in high-slope areas – and reduce useful storage volume in the Juturnaíba reservoir.
In the 2CF and 2RI scenarios, which are characterized by minimum vegetable coverage reduction or a small increase thereof when compared to the Control scenario, the spatial distribution of soil loss is close to that of the Control scenario (Fig. 8A, Fig. 8C and Fig. 8F). The differences between the scenarios that take into account Federal and State legislation (3CF and 3RI) are expressed in the number of hilltop protected area fragments when considering the Inea Resolution criteria, which considers the foot of the hill or the nearest body of water as the criteria for delimitation.
3.4 Discussion
Changes in hydrological responses caused by forest cover loss intensify sediment transport in the watershed. Restricting protected area status to hilltops alone does not meet the goal of the Forest Code and the Inea Resolution, which explicitly state that hilltop protected areas have the environmental role of safeguarding water resources and controlling soil erosion.
Maintaining only hilltops as protected areas does not prevent abrupt changes in the hydrological behavior of the São João river watershed. This is because protected areas in the watershed are not very sizeable if calculated according to new federal and state legislation (Magdalena et al., 2018).
Besides the liquid discharge increases associated with the decline in evapotranspiration, LU/LC changes lead to significant shifts in streamflow patterns because of changes in flow paths. Atlantic Rainforest areas have smaller saturated zones, which leads to decreased surface runoff generation and larger retention of groundwater storage, resulting in better flow regulation in the dry season (Bonell, 2010; Khaledian et al., 2017; Salemi et al., 2013). Thus, the streamflow increases along the main channel in the reduced forest cover scenarios result in rapid responses to rainfall events and a decline in baseflow.
Surface runoff increases have a positive feedback effect on soil erosion processes over time (Bruijnzeel, 2004; Chen et al., 2007; Guo et al., 2019; Sthiannopkao et al., 2007). Associated with erosion processes energy sources (Fohrer et al., 2001; García Rodríguez and Giménez Suárez, 2010; Goss et al., 2014), these can also affect the drainage system and stability of the Juturnaíba reservoir, namely because of sediment deposition causing silting. This process can overload the reservoir with sediment and reduce the regularity of tributary streamflow rates (Chen et al., 2007; Sthiannopkao et al., 2007).
As such, preserving Atlantic Rainforest vegetation only in the areas delimited by legislation has a direct impact on the economic development of the watershed, a result of soil loss and the overload of sediments in the drainage system and the reservoir, subsequently affecting the water supply system. The results obtained with the Control scenario, wherein forested area remains larger than that required by law in association with agricultural uses in the plains, suggest that preserving a greater forested area in the watershed will lead to better regulated water flow in the channel and reduce soil loss, with consequent declines in silting and sediment deposition, thus facilitating better water resource retention for sustainable management in the watershed.