Understanding how streamflow and its components, baseflow and quickflow, vary spatially according to climate and landscape characteristics is fundamental for dealing with different water-related issues. Analytical formulations have been proposed to investigate their long-term behavior and additional influencing factors, suggesting that they are mainly controlled by the aridity index ( Φ). Nevertheless, these studies assume the catchment as a closed water balance system, neglecting inter-catchment groundwater flow (IGF). This simplification makes the analysis of the long-term streamflow components and their main control mechanisms challenging, given that many catchments cannot be considered as closed hydraulic entities. Here, we assessed the controls of the mean-annual streamflow components and their behavior under an open water balance assumption, using observed data of 731 Brazilian catchments with diverse hydroclimatic conditions. Our results indicate that indeed streamflow components are primarily controlled by at the mean annual timescale. The consideration of an open water-balance significantly improved the performance of the functional forms to describe streamflow components while also elucidating the assessment of other influencing factors on the streamflow behavior. Land cover, groundwater, climate seasonality and topographic attributes appeared as the main control mechanisms beyond aridity. Overall, our study provides new insights of the main control mechanism of the streamflow behavior at the mean-annual scale, while shedding light on the importance of the open water-balance assumption for model development and water resources management.

Brazil is one of the richest countries in water resources and is currently the second largest global supplier of food and agricultural products. Furthermore, more than 70% of Brazilian energy comes from hydropower. Therefore, to ensure water availability for future generations it is necessary to consider the Nexus thinking (water, food, energy, ecosystem service, and social aspects integrated) in the country. However, few studies have been developed considering the Nexus thinking in Brazil. Understanding the interconnected risks and vulnerability to these sectors under climatic change conditions is crucial for the development of sustainable resources management plans and for mitigating competition among them. Here, we assess the Food-Energy-Water Nexus considering climate and land cover and land use changes (LCLUC) scenarios in the São Francisco river basin. This basin is the third largest basin in Brazil and supplies water to approximately 14 million inhabitants, with the Metropolitan Region of Belo Horizonte being the most populous area. The São Francisco river has been used for water supply, irrigation, agriculture and transportation by waterways, but its preponderant use is for hydroelectric power generation. However, this basin has been suffering from LCLUC and drought that has plagued the region since 2012. In addition, part of the river flow has been diverted due to the transposition of the São Francisco river to supply water to the semi-arid region of Brazil. We will calibrate and evaluate the Soil and Water Assessment Tool (SWAT) using hydrometeorological data from 1972 to 2017. We will also use LCLUC scenarios from the OTIMIZAGRO model and regional climate change models (HadGEM2-ES and MIROC5, RCP 4.5 and 8.5). Then, we will compute the demands of water by different sectors and integrate water availability and demand to reach an optimal water use based on the Nexus thinking. Our results will provide decision-makers with information regarding the risks and trade-offs and will support water resources management decisions in order to allocate scarce water resources toward food or energy.

Direct-runoff and baseflow are the two primary components of total streamflow and their accurate estimation is indispensable for a variety of hydrologic applications. While direct runoff is the quick response stemming from surface and shallow subsurface flow paths, and is often associated with floods, baseflow represents the groundwater contribution to streams and is crucial for environmental flow regulations, groundwater recharge, and water supply, among others. L’vovich (1979) proposed a two-step water balance where precipitation is divided into direct runoff and catchment wetting followed by the disaggregation of the latter into baseflow and evapotranspiration. Although arguably a better approach than the traditional Budyko framework, the physical controls of direct runoff and baseflow are still not fully understood. Here, we investigate the role of the aridity index (ratio between mean annual potential evapotranspiration and precipitation) in controlling the long-term (mean-annual) fluxes of direct runoff and baseflow. We present an analytical solution beginning with similar assumptions as proposed by Budyko (1974), leading to two complementary expressions for the two fluxes. The aridity index explained 83% and 91% of variability in direct runoff and baseflow from 499 catchments within the continental US, and our formulations were able to reproduce the patterns of water balance proposed by L’vovich (1979) at the mean annual timescale. Our approach allows for the prediction of baseflow and direct runoff at ungauged basins and can be used to further understand how climate and landscape controls the terrestrial water balance at mean annual timescales.

Many remote sensing-based evapotranspiration (RSBET) algorithms have been proposed in the past decades and evaluated using flux tower data, mainly over North America and Europe. Model evaluation across South America has been done locally or using only a single algorithm at a time. Here, we provide the first evaluation of multiple RSBET models, at a daily scale, across a wide variety of biomes, climate zones, and land uses in South America. We used meteorological data from 25 flux towers to force four remote sensing based ET models: Priestley & Taylor Jet Propulsion Laboratory (PT-JPL), Global Land Evaporation Amsterdam Model (GLEAM), Penman-Monteith Mu model (PM-MOD), and Penman-Monteith Nagler model (PM-VI). ET was predicted satisfactorily by all four models, with correlations consistently higher (R²>0.6) for GLEAM and PT-JPL, and PM-MOD and PM-VI presenting overall better responses in terms of PBIAS (-10

Climate change affects the global water cycle and has the potential to alter water availability for food-energy-water production and the ecosystems services on regional and local scales. In southeastern Brazil, the Cantareira Water Supply System reached unprecedented low levels in January 2015 compromising the water supply for the Metropolitan Region of São Paulo (MRSP). However, there is still few studies investigating the effects of climate change on water availability in this region. Here, we assess the influence of climate change on water availability in the Jaguari Basin, Southeastern Brazil using a modeling approach. This basin covers and area of about 1200 km2 and it is the main source of the Cantareira Water Supply System, responsible for providing water for about 7 million people in the MRSP. To evaluate climate change scenarios, we use the lumped conceptual HYMOD model on daily time step. This model was calibrated and evaluated using daily observed data of precipitation, evapotranspiration, and discharge for the period of 1990 to 2009. To evaluated climate change scenarios, we used data of an ensemble of 17 General Circulation Models (GCMs), downscaled by MarkSim GCM working off a 30 arc-second climate surface spatial resolution forced by two Representative Concentration Pathways (RCP): RCP 4.5 and RCP 8.5. These data were integrated into the HYMOD to projected scenarios (up to 2095) of water discharge. We find values of Nash-Sutcliffe Efficiency Coefficient (NSE) and Coefficient of Determination (R2) greater than 0.80 for the calibration and evaluation period. We also noticed an increase in the peak of runoff and a decrease and baseflow for both scenarios. Such changes reflect in a higher interannual variability, therefore, increasing the risk of drought and flood. In terms of Environmental Flow Requirement, the probability of exceedance Q90, reveal a clear pattern of decreasing, about 23% from 2010 to 2040, and reaching 28% by the end of the century. Our findings indicate that the water discharge could not be enough for the current and future water demand. Our results expose the fragility of the studied basin, presenting a technical and scientific information focusing on guiding the plans and strategies to deal with situations of water scarcity.

Water scarcity is a key challenge to global development. In Brazil, the Sao Francisco River Basin (SFB) has experienced water scarcity problems because of decreasing streamflow and increasing demands from multiple sectors (e.g., food and energy production and urban supply). However, the drivers of decreased streamflow, particularly the potential role of groundwater withdrawals, have not been yet investigated. Here, we assess long-term trends in baseflow, quickflow, and streamflow of the SFB during 1980–2015 and constrain the most likely drivers of observed decreases through trend analysis of precipitation (P), evapotranspiration (ET), and terrestrial water storage change (TWS). We found that over 82% of the observed decrease in streamflow can be attributed to a significant decreasing baseflow trend (< -20 m3 s-1 y-1) along the SFR with spatial agreement between decreased baseflow, increased ET, and irrigated agricultural land. We also found a decrease in TWS across the SFB with trends exceeding -20 mm y-1. Overall, our findings indicate that decreasing groundwater contributions (i.e., baseflow) are the primary cause of the observed reduction in total SFR flow. A lack of significant climate trends indicates that climate variability likely has not caused the observed baseflow reduction, mainly in the Middle and Sub-middle SFB, and therefore groundwater and surface withdrawals may be the most likely driver of water scarcity over the SFB. We call for increased attention on groundwater’s key role for the SFB and integrated regional management of surface and groundwater.