1. Introduction
Water is among the most indispensable resources on the planet. Current global water scarcity affects two-thirds of the world’s population for at least 1 month a year, and half a billion people throughout the year [
1], placing water resources under intense pressure with unprecedented consequences [
2]. In an annual report on the global risks from the World Economic Forum in 2020, the water crisis was among the 5 greatest risks in terms of impact and among the 10 greatest risks in terms of probability of occurrence, in addition to being listed as the greatest threat to society [
3]. According to projections, water scarcity will continue to worsen in the future, intensified by population and economic growth, changes in consumption patterns, and climate change [
4,
5].
For tackling climate change impacts, two approaches are mainly used: reducing the sources of climate change and preventing further aggravation (i.e., mitigation); making adjustments to cope with their local impacts (i.e., adaptation) [
6]. As one of the main effects of climate change corresponds to the greater frequency and intensity of extreme events [
7], adaptation strategies seek to reduce vulnerabilities to such events and/or increase resilience in response to them [
8]. From the climate standpoint, resilience can be defined as “the outcomes of evolutionary processes of managing change in order to reduce disruptions and enhance opportunities” [
9].
The challenge of climatic adaptation is just one of the various adversities that societies face and which can be addressed by the application of nature-based solutions (NbS) [
10]. According to the International Union for Conservation of Nature’s (IUCN) definition, NbS are: “Actions to protect, sustainably manage and restore natural or modified ecosystems that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits” [
11].
One of the most widespread NbS applied in Brazil for water conservation consists of the recovery of riparian forest areas around water bodies. Such a solution is the main strategy applied by most payments for environmental services (PES) programs developed in Brazil and is based on the assumption that the increase in forest areas, in general, helps to improve hydrological conditions in hydrographic basins [
12]. In addition, the restoration of riverine areas has the advantage of being a legally instituted and mandatory action, present in the Brazilian Forest Act, which facilitates the observance of those measures by rural landowners. Nonetheless, riparian forests are known to have sediment retention as their primordial ecological function [
13], not having a significant influence on other more relevant factors from the standpoint of water supply such as infiltration and recharge of aquifers.
An interesting alternative solution would be focusing conservation efforts on areas of the basin that have greater importance for water recharge such as implementing less extreme land use changes by improving the management of production systems instead of carrying out a drastic conversion of agricultural land use in native forest areas. Such key areas can be easily identified by the application of the conservative use potential (PUC), a Brazilian method developed for mapping areas within a watershed according to their potentialities and limitations for sustainable uses, defined in terms of their water recharge potential, resistance to erosion, and potential for agricultural use [
14].
Therefore, this study aimed to compare the effects on the water supply of adopting different nature-based solutions for water and soil conservation as well as to evaluate the potential of those solutions to promote climate adaptation and increase water availability resilience in the Atibainha River basin.
4. Discussion
Nature-based solutions in implementing best management practices (BMPs) in key areas for water recharge is predicted to reduce the temporal variation in water supply when compared to the NbS of increasing the basin’s forest cover in riparian areas. The focal conservation and combined solution land use scenarios were able to maintain greater water supply stability throughout the analyzed years and between months and, therefore, contributed to an increase in resilience in the face of temperature and precipitation changes over time.
Even though restoring hydric permanent preservation areas (APPs) contributed less to water supply stabilization than BMP implementation in target areas, restoration is extremely important for decreasing sediment influx into water bodies, since trapping sediment is the main ecological function of riparian vegetation [
13]. APP’s efficiency in providing that service was shown by Monteiro and colleagues [
52], who also evaluated this land use scenario using the SWAT model. Our results regarding total sediment transported into the streams must be interpreted with caution, since our model was not calibrated for that variable. Nonetheless, any error that might be present in the simulated values is equally present in all LULC and climate change scenarios, which enables the comparison between them and validates further discussions from a sediment production standpoint.
Simultaneous application of both NbS compared in this study resulted in a synergistic effect that generated greater environmental condition changes than those achieved by the implementation of each scenario separately. This indicates that the increase or reduction effects on a given variable promoted by the adoption of the focal conservation or riparian restoration scenarios were repeatedly larger when these scenarios were used in combination.
The aforementioned environmental condition changes promoted by the alternative land use scenarios enhanced the basin’s environmental quality. Those changes were related to the increase in the basin’s water production resilience, and both factors were promoted by modifications to the contribution of different components of the hydrological cycle to streamflow. In SWAT, water production is the net amount of water that leaves the subbasin and contributes to streamflow in the reach, corresponding to the sum of the contributions of different water cycle compartments (i.e., surface runoff, groundwater. and lateral flow) to streamflow, discounting the transmission losses and pond abstractions [
53]. The increase in water recharge promoted by focal conservation and combined solution scenarios occurred at the expense of reducing water volume contributed by other compartments, explaining the reduction in surface runoff verified in those land use scenarios for the observed historical climate condition. The reduction in surface runoff that occurred in all LULC scenarios for the projected climate conditions, on the other hand, occurred primarily due to the increase in evapotranspiration volumes (approximately 25%) favored by the mean temperature increase. Nonetheless, even in future climate conditions, the effects of LULC can be observed, with the focal conservation and combined solution scenarios producing the greatest surface runoff decrease and groundwater and lateral flow increases.
The supply of water through groundwater recharge and lateral flow helps to prevent abrupt oscillations in water fluxes characterized by floods and drought events, reducing the challenges of water management in the public and private sectors. Moreover, the reduction in surface runoff also favors sediment yield decrease, which improves the quality of the water produced in the basin and, therefore, lowers the costs of its treatment and prevents water bodies silting and its associated issues such as reducing the reservoir’s storage capacity. However, despite all the positive effects aforementioned, favoring sub-superficial water fluxes also enhanced water retention in soil layers, so that a greater portion of precipitation was no longer available as blue water (main interest from the point of view of water supply) and became available as green water. This, in turn, lowers water production and, consequently, the streamflow that contributes to the reservoir, as shown by the alternative land use scenarios’ implementation.
Siqueira and colleagues [
29], modeling LULC and climate changes with SWAT in another Brazilian watershed, also observed that the implementation of water and soil conservation measures caused a reduction in the blue water available in their study area. Furthermore, according to the interpretation of the data by Siqueira [
29], the reduction in streamflow produced by alternative land uses was caused by the conversion of other uses into forest. For the present work, the flow reduction was higher in scenarios that maintained the same land uses as the current scenario, but improved management conditions and increased water infiltration in areas of the basin which were prone for having naturally greater recharge potential. Thus, for this work, the increase in water recharge contributed more to reduce the volume of water produced than the increase in evapotranspiration. However, it is worth mentioning that according to our results, the reduction in annual mean water yield did not present significant differences among the various land use scenarios, while other changes linked to the improvements in environmental quality (such as increased recharge and reductions in sediments loads and surface runoff) were significantly different.
Despite all the projected benefits produced by the combined adoption of the NbS evaluated, their implementation, in itself, is not enough to mitigate the effects of even the most moderate scenarios of climate change and, thus, to ensure the water supply of the Metropolitan Region of São Paulo. Even though mean water yield and streamflow increased in both climate change projections assessed, in the case of the RCP 8.5 scenario, such an increase was not consistent throughout the projected period. In the century’s first half, an abrupt increase in streamflow of 73.5% generated considerable mean discharge values of 5.2 m3/s; nonetheless, those values were not sustained in the second half of the century in which the progressive streamflow declined from a mean of 6.4 m3/s (in 2054) to a mean of 0.6 m3/s (in 2095), which would make it impossible to supply the São Paulo Metropolitan Region while at the same time maintaining the minimum streamflow required for the supply of other cities downstream of the Atibainha reservoir. For the RCP 4.5 scenario, despite precipitation being distributed more evenly throughout the simulated period and the mean streamflow remaining more stable at approximately 3.5 m3/s, river discharge mean values were still close to the mean values observed for the Atibainha basin (3.0 m3/s) over the last ten years, which were marked by a severe drought that occurred in 2014 and 2015 and by the slow recovery of the water supply system following this event. Both the average values of streamflow recorded over the last 10 years and those projected by RCP 4.5 were below the historical average for the region, corresponding to almost half of the mean discharge inflow to the reservoir taken into account in the establishment of the granting rules of the Canteira system of 6 m3/s.
The occurrence of the extreme events projected by RCP 8.5 could have even more drastic consequences in the other sub-basins of the system, since in contrast with Atibainha watershed, they present a more pronounced state of degradation [
30]. This highlights the importance of expanding this modeling exercise for the entire Cantareira to estimate the potential of NbS to improve water supplies for the system as whole and to identify key areas for conservation on a broader scale. Furthermore, the poor environmental conditions of other Cantareira watersheds raises the need for landscape improvement actions to be associated with rapid and severe measures to reduce GHG emissions so that there is a chance of securing a future water supply for the MRSP.
Although the evaluated NbS are listed in the IPCC report as mitigation strategies and have significant potential as carbon sinks, other actions seem more effective and present more immediate results towards the objective of keeping climate change below 2 °C relative to pre-industrial levels [
54]. Such actions involve a drastic cut in anthropogenic GHG emissions by mid-century through large-scale changes in energy systems and, potentially, land uses [
54]. This is also related to the fact that energy production is the economic activity with the greatest contribution to greenhouse gases emissions [
54].
The present study was the first application of the conservative use potential (PUC) method to build and simulate a land use scenario in the SWAT model. Our results not only confirm other works’ findings regarding the effects of riparian restoration on the water cycle but also expand the understanding of their role by verifying a synergistic relationship with other land use scenarios. Moreover, the results suggest that the sequential adoption of the evaluated NbS, starting with the application of best management practices in key areas and then moving on to the revegetation of riparian areas, allows for achieving more effectively the desired impacts of payments for environmental services (PES) schemes.
5. Conclusions
The adoption of best management practices in target areas is more efficient for reducing temporal oscillations in water availability than simply restoring riparian vegetation areas in the basin. Nevertheless, the increment in riparian forest cover has a synergistic effect with best management practices: both NbS produced better results when applied together than separately. Therefore, the application of best management practices in key areas, followed by the revegetation of riparian areas, is a more effective strategy to produce the desired impacts of water payments for environmental services (PES) schemes.
In addition to being more effective, this perspective is more likely to consolidate new programs, since it starts with the application of practices that are more economically viable, easier to adopt, and with more chances to be adhered to by the local population, before moving on to more expensive actions, which involve more abrupt changes in land use and the implementation of which could benefit from greater structure and maturity of the program.
Nevertheless, only implementing these adaptation strategies without taking severe mitigation measures is insufficient to maintain water production at satisfactory levels to meet the São Paulo’s Metropolitan Region’s future water demand, even under more optimistic climate change projections.
Water quality and quantity are intricately connected to the regulation of atmospheric carbon concentration. Actions like riparian restoration, for instance, applied to water quality improvement, can also contribute to sequestering carbon. Finding the balance to invest in these different actions, combining climate adaptation and mitigation, is one of the greatest challenges of this century in the search for ensuring water security today and in the future for Latin America’s larger metropolitan regions.