1. Introduction

Population growth worldwide will bring with it an increasing number of problems regarding drinking water demand and its relationship with availability. According to the water statistics for Mexico in 2019, published by the Comisión Nacional del Agua (National Water Commission, ^{Conagua [2019]}), Mexico receives approximately 1 449 471 million cubic meters of water annually through precipitation. Of this total, approximately 72.1% returns to the atmosphere through evapotranspiration, 21.4% flows through rivers and streams, and the remaining 6.4% infiltrates into the subsoil, naturally recharging aquifers. When accounting for cross-border outflows and inflows, Mexico has a 451 584.7 Hm³ annual renewable freshwater availability (^{Conagua, 2019}).

Surface water availability in Mexico is a critical issue influenced by climatic variability, population growth, and unsustainable water management practices. Studies such as those by ^{Mekonnen and Hoekstra (2016)} highlight the global and regional water scarcity challenges, with Mexico facing significant stress due to uneven distribution and overexploitation of surface water resources. ^{López-Morales (2017)} provides a comprehensive overview of Mexico’s water resources, highlighting that surface water availability is highly seasonal and spatially uneven, with most resources concentrated in the southern regions, while the north of the country faces severe water scarcity. Climate change further complicates this scenario, as highlighted by ^{Magaña et al. (1997)}, who projected reduced precipitation and increased evaporation rates, thereby threatening the long-term sustainability of surface water. Additionally, ^{Hernández-Espriú et al. (2014)} discussed the interplay between surface and groundwater systems, noting that over-reliance on groundwater in regions like Mexico City indirectly impacts surface water availability. Collectively, these studies underscore the urgent need for integrated water management strategies to address Mexico’s surface water challenges in the face of the growing demand and climate variability.

Despite these substantial resources, Mexico faces a complex water challenge, including regional imbalances in water availability, basin overexploitation, and increasing demand. The 2030 water agenda (^{Conagua, 2012}) emphasizes these disparities, particularly in critical basins. While regions such as Grijalva-Usumacinta (HR 30) use only a small fraction of their available water, others, like the Lerma and Bravo basins, exceed 100% of their natural surface water availability. This overuse threatens ecosystems and undermines sustainable economic development. Currently, Mexico’s national water demand stands at 78 400 Hm³ annually, of which 11 500 Hm³ are unsustainably sourced. Projections suggest that, without intervention, this gap could double within the next 20 years. Addressing these challenges requires an integrated water management strategy that not only tackles water scarcity but also considers issues of water quality, governance, and sustainability. These efforts must integrate the interconnected social, economic, and environmental dimensions of water resources. Collaborative action among government agencies, stakeholders, and communities is essential to ensure equitable access to clean water, promote environmental sustainability, and build resilience to future water-related challenges.

This paper examines surface water availability in Mexico across 757 basins, organized into 37 hydrological regions, using the methodology outlined in the Official Mexican Standard NOM-011-CONAGUA-2015 (^{Semarnat, 2015}). It is essential to note that while numerous manuals are available for estimating water availability in a basin, these primarily focus on natural availability or natural runoff. This is distinct from water availability for administrative or legal purposes, as outlined in ^{Silva-Hidalgo et al. (2013)}. For instance, ^{López-García et al. (2017)} conducted a water balance to determine natural availability under climate change scenarios for the Galeana Valley aquifer in the state of Nuevo León, Mexico. Similarly, ^{Loor (2017)} performed a watershed balance in Ecuador, and ^{Ordóñez-Gálvez (2011)} presented a methodology to estimate the natural surface water balance in Peru. The United Nations Educational, Scientific and Cultural Organization (UNESCO) incorporated the consumptive use variable (Uc) into the surface water balance of the Valley of Mexico basin, and calculated its water balance variability and uncertainty components (^{Aparicio-Mijares et al., 2006}),

Although most publications focus on the natural water balance, countries such as Chile, Spain, Mexico, and the USA have normative documents for water resource management. These documents are based on the fundamental equation (dV)(dt)^-1 = E - S, which states that the volume change (V) is equal to the inputs (E) minus the outputs (S) of water over a specific period (t) (^{Aparicio-Mijares et al., 2006}). The primary difference lies in the methodology used to estimate the data in the balance equation. Unlike the natural water balance, in the administrative or regulatory balance, consumptive use (Uc) is considered an output (S).

This paper uses a robust methodology to assess average annual surface water availability, quantifying the delta between runoff volume and existing downstream water commitments. Crucially, it extends beyond mere quantification to pinpoint basins exhibiting optimal potential for hydroelectric development by 2034. This is achieved through a meticulous evaluation of topographic characteristics, interwoven with a comprehensive consideration of ecological, social, and infrastructural constraints. By dissecting these multifaceted dynamics, this paper aims to make a significant contribution to a sustainable and resilient water management strategy for Mexico. It seeks to ensure responsible and equitable stewardship of natural resources, while simultaneously illuminating viable pathways for sustainable energy production. Figure 1 illustrates Mexico’s division into 37 hydrological regions, representing the country’s primary watersheds or basins. These regions define water flow dynamics, availability, and catchment boundaries, serving as a critical framework for water resource management, conservation, and infrastructure planning.

Fig. 1 Mexican hydrological regions. 

2. Methods

As outlined in the Official Mexican Standard NOM-011-CONAGUA-2015 (^{Semarnat, 2015}), which provides specifications and methodology for determining the average annual availability of national waters in Mexico, average surface water availability is estimated using the following equation:

where: D is the average annual surface water availability in Hm³, A _b is the average annual runoff volume downstream in Hm³, and R _xy is the average annual volume committed downstream in Hm³.

The average runoff volume downstream (A _b ) is further calculated using Eq. (2):

where A _r is the average annual runoff volume from upstream basin; C _p is the average annual natural runoff volume; R _e is the annual returns volume; I _m is the annual import volume; E _x is the annual export volume; U _c is the annual surface water volume extraction (U _ca : annual surface water volume extraction through titles currently registered/assigned in the Water Rights Public Registry [REPDA, for its Spanish acronym], U _cb : annual surface water volume extraction from titles in the registration process at REPDA, and U _cc : annual volume corresponding to reserves and regulated areas); E _v is the average annual evaporation volume in reservoirs and water bodies; and A _v is the average annual storage volume variation in reservoirs (all variables are reported in Hm³).

Eq. (2) defines A _b as water balance inflows and outflows within a basin. Positive variables represent the water volume entering the basin, while negative variables indicate the water volume leaving it. To calculate water availability, both natural and anthropogenic factors that influence these variables are considered. The behavior of these variables over time was analyzed, and projected values were compared to historical records. The percentage change for each variable was then applied to the latest values published in a 2020 availability study (^{SINA, 2020}).

Hydrological regions in Mexico are composed of multiple hydrological basins, and water availability was calculated both by basin and by hydrological region using Eq. (1). The D value in Eq. (1) was calculated for each hydrological basin. However, the methodology established in NOM-011-CONAGUA-2015 specifies that this calculation must be applied to the entire hydrological region, as it is performed from the downstream basin to the upstream basin. It is important to note that NOM-011-CONAGUA-2015 was designed for its application to a single hydrological basin, which must have a single outlet. In contrast, a hydrological region comprises multiple hydrological basins, and not all hydrological regions in Mexico drain into a single outlet. Many hydrological regions in Mexico have multiple outlets draining into the sea, which does not align with the requirements of NOM-011-CONAGUA-2015.

In this analysis, natural variables include precipitation and temperature, and anthropogenic variables include water usage or consumptive uses (U _c ). These inputs are essential for calculating the water balance within a basin (Cp). The methodology to estimate consumptive water uses and the climatological variables required for assessing water availability in Mexico for 2020 and 2034 are detailed below.

3. Results and discussion

3.1 Water use

The agricultural sector is the primary water consumer in Mexico. To ensure sustainable water use, it is crucial to improve water efficiency and adopt sustainable agricultural practices. Water use patterns vary across hydrological regions, requiring region-specific water management strategies. Figure 2 illustrates the significant increases in agricultural water consumption projected for the Balsas, Bravo, Sinaloa, and Nazas-Aguanaval regions. Industrial water use will remain concentrated along the central axis, with increasing demand in northwestern states such as Chihuahua, Sonora, Sinaloa, and Durango. Urban public water use will continue to be highest in basins with large cities. By 2034, the Bajo Atoyac River basin in the Balsas region and the Río Blanco in the Papaloapan region will have the highest extraction volumes, while the Valley of Mexico basin will remain the primary basin for urban public water use. Given the large number of hydrological basins, the results are presented by hydrological regions, which group multiple basins into broader, more manageable categories for analysis.

Fig. 2 Participation percentage by consumptive use. 

3.2 Climate change impacts

Climate change is expected to have a significant impact on Mexico’s water resources. Studies by ^{Boulanger et al. (2005)} and ^{Koshida et al. (2015)} highlighted the potential consequences of shifting climate patterns on water availability. While long-term climate forecasts inherently carry uncertainty, it is crucial to recognize the potential impacts of climate change on Mexico’s water resources. Factors such as altered rainfall patterns, rising temperatures, and increased frequency of extreme weather events could exacerbate water scarcity and disrupt hydrological cycles. Our climatic variable findings are detailed in the following sections.

3.2.1 Precipitation

Precipitation patterns across Mexico exhibit significant regional variability, with the Baja California Peninsula, along with the northwest and north-central regions, experiencing the lowest rainfall, while the southeast region, including Veracruz, Oaxaca, Tabasco, Chiapas, and Campeche, receives the highest precipitation. The projected precipitation for 2034 shows significant regional variability, with some areas projected to experience substantial increases (e.g., Rh 30: +41%) and others facing decreases (e.g., Rh 1, Rh 4, Rh 7: -48%). These changes underscore the need for region-specific water management strategies to address the challenges of water scarcity and flooding. These changes underscore the increasing variability in precipitation patterns, with some regions facing heightened water stress while others experience higher rainfall. Figure 5 illustrates the percentage change in precipitation between the 1976-2018 average and the projected values for 2034, emphasizing the need for region-specific water management strategies to address these shifts. Additionally, the linear regression model’s performance across 757 basins reveals significant disparities, with basins like 1804 (Río Nexapa) showing relatively strong predictive accuracy (R² = 0.33 and RMSE = 78.55), while others, such as 2302 (Tepanatepec), 2321 (Coatán), and 3030 (Paredón), exhibit poor performance (R² < 0.05, RMSE > 500). These values reinforce what is described in the methodology; the precipitation and temperature values projected for 2034 have a high level of uncertainty, so they are only considered as a scenario (Figs. 3 and 4).

Fig. 3 R² values for precipitation linear regression. 

Fig. 4 RMSE values for precipitation linear regression. 

Fig. 5 Change percentage in precipitation between 1976-2018 and 2034. 

3.2.2 Temperature

• In 101 basins, maximum temperatures are projected to decrease slightly by 1% (0.5 ºC).

• In 656 basins, maximum temperatures are projected to increase by 3% (almost 1 ºC).

• Decreases in maximum temperature are concentrated in northern Mexico, Michoacán, coastal Oaxaca, and Campeche, and the southern border with Guatemala.

• Historical minimum temperatures are concentrated in the areas between the Sierra Madre Occidental and the Sierra Madre Oriental, together with Baja California Norte.

• In 299 basins, minimum temperatures are projected to decrease by 3% (-0.4 ºC), while in 458 basins, they are projected to increase by 3% (0.4 ºC).

• Decreases in minimum temperature are observed in northwestern Mexico, particularly in the mountains of Sinaloa, Chihuahua, and Durango.

• Increases in minimum temperature are observed in the Valley of Mexico basin and parts of Campeche.

Similarly, Figures 6 and 7 show the percentage changes in maximum and minimum temperatures, respectively, over the same period.

Fig. 6 Change percentage in maximum temperature between 1976-2015 and 2034. 

Fig. 7 Change percentage in minimum temperature between 1976-2015 and 2034. 

3.3 Water volume per local basin (Cp )

Turc’s and Rc methods were employed to estimate Cp under both historical and projected climate conditions, as well as to calculate the corresponding percentage changes. Turc’s method typically results in larger changes in Cp compared to Rc, which accounts solely for variations in precipitation. Figure 8 illustrates the water volume per local basin (Cp), as reported in the study published by ^{Semarnat (2020)}.

Fig. 8 Water volume local basin (^{Semarnat, 2020}). 

Figure 8 also shows that in 2020, most basins generated a runoff lower than 500 Hm³ per year, except for the Tarahumara mountains, the Balsas River, and the eastern slopes of the Sierra Madre. The change percentage predicts a decrease in Cp in Baja California Peninsula and northwestern territories, and an increase in the south-central region (Fig. 9). The Rc method shows a less significant change, but also indicates a decrease in Cp in the northwest (Fig. 10).

Fig. 9 Water volume per local basin by 2034, estimated with Turc’s method. 

Fig. 10 Water volume per local basin by 2034, estimated with the RC method. 

3.4 Water availability

For 2020, basins without water availability were primarily concentrated in the following hydrological regions: 8 Sonora Norte, 9 Sonora Sur, 24 Río Bravo Conchos, 12 Lerma, and 18 Balsas, as well as basin 3405 Río Santa María 1 together with several basins in the southwestern area of hydrological region 25 San Fernando Soto la Marina. Figure 11 shows surface water availability for 2020, as published by ^{Semarnat (2020)}.

Fig. 11 Surface water availability in 2020 (^{Semarnat, 2020}). 

In the 2034 scenario, using Turc’s method, projections show 154 basins (25%) without availability, 266 basins (36%) with availability (but less than 100 Hm³), and 337 basins (38%) with more than 100 Hm³. For the 2034 projection, using the percentage change calculated by Turc’s method (Fig. 12), basins without availability are expected to be concentrated in the northwest of the country and Rh 29 Coatzacoalcos, with additional scattered basins without availability distributed throughout the national territory. Figure 12 displays the surface water availability projected for 2034, calculated using Turc’s method.

Fig. 12 Surface water availability by 2034, estimated with Turc’s method. 

Figure 13 shows the surface water availability for 2034, calculated using the Rc method. Additionally, Figure 14 illustrates the comparative behavior of water availability between 2020 and the projected scenarios for 2034. These figures collectively provide a comprehensive comparison of the results obtained in this study with the official water availability data published by the Mexican government, as well as the projected scenarios based on the applied methodologies. Similarly, projections with the runoff coefficient method (Fig. 13) show that basins without availability will also be concentrated in the northwest of the country, though to a lesser extent. Additional affected areas include Rh 30 Grijalva-Usumacinta, 18 Balsas, and 35 Mapimí. With the Rc method, in the 2034 scenario, there are 103 basins without availability (23%), 334 basins (42%) with availability (but less than 100 Hm³) and 320 basins (35%) with more than 100 Hm³.

Fig. 13 Surface water availability by 2034, estimated with the Rc method. 

Figure 14 graphically contrasts the calculated results with the 2020 water availability data published by ^{Semarnat (2020)}, providing a clearer understanding of trends and variations. The basins without availability, as reported in the 2020 study, increase slightly when the estimated percentage changes derived from the Turc’s and Rc methods are applied. In both scenarios, the number of basins with availability of less than 100 Hm³ decreases, while the number of basins with availability exceeding 100 Hm³ increases. Nationwide, the total number of basins without availability rose significantly from 91 in 2020 to 154 when applying the percentage change from Turc’s method and to 103 when using the Rc method.

Fig. 14 Percentage of basins with and without availability of surface water for each scenario analyzed. 

3.6 Hydropower potential

To identify potential sites for hydropower development, basins with surface water availability (D) exceeding 2 m³ s^-1 and slopes greater than 2% were considered. Additionally, downstream flow (AB), as defined in NOM-011-CONAGUA-2015, was utilized to pinpoint basins suitable for further analysis. This approach enables the identification of sites with hydroelectric potential, which can accommodate either large dams or smaller infrastructure, such as pipelines running parallel to the riverbeds. Figures 15 and 16 present the results of the calculations, employing each previously mentioned method, based on the scenery’s water availability for 2034.

Fig. 15 Map of basins with available flow greater than 2 m³ s^-1 by 2034, estimated with Turc’s method. 

Fig. 16 Map of basins with available flow greater than 2 m³ s^-1 by 2034, estimated with the Rc method. 

Figure 17 offers a comprehensive visual representation of the intricate relationships between hydrological regions, projected water availability for 2034, and ecologically critical areas. To assess the potential impacts of water availability on sensitive ecosystems, this study incorporated existing designations of ecologically critical areas, including Protected National Areas (ANPs, for their acronym in Spanish), Ramsar wetlands, and Important Bird Conservation Areas (AICAs, for their acronym in Spanish). The AB 2034 analysis, utilizing Turc’s method (as depicted in Fig. 12), identified basins with sufficient water flow to meet future demands, while also revealing the highest number of basins projected to experience water scarcity by 2034. These results were then overlaid with a map of protected areas, encompassing NPAs, Ramsar wetlands, and AICAs, emphasizing the imperative need for sustainable infrastructure planning. This integrated approach, facilitated by the calculations and development shown in Figure 17, provides a crucial tool for understanding the intersection of water resources and ecological preservation.

Fig. 17 Relationship between hydrological regions, projected water availability (using Turc’s method) by 2034, and critical ecological areas. 

4. Conclusions

In this work, the results of the analysis of historical precipitation and temperatures conducted by Ramírez-Villa et al. (2020) and their projection to 2034, based on a linear behavior, were utilized. The growth rates of water volume in agricultural, industrial, and urban public consumptive uses were analyzed. Using two equations (Turc’s and Rc), the volume per local basin (Cp) was estimated and applied in Eqs. 1 and 2, to calculate the available volume of surface water by 2034. Subsequently, the basins with the possibility of developing hydroelectric projects were identified.

R² and RMSE values obtained from the linear regression applied to historical precipitation and temperature data provide results with a low probability of occurrence through 2034; however, they can be considered a scenario or an average value. On the other hand, if these trends in rainfall and temperature behavior, as well as water use behavior, continue, surface water availability scenarios can be obtained through 2034.

The results of this work enable a first analysis of the surface water resources available in 2020 and their scenarios by 2034. For example, according to the ^{Semarnat (2020)} study, 321 953.36 Hm³ of water are generated in Mexico by natural runoff (Cp), and 192 022.94 Hm³ are extracted for consumptive uses (60%). There is sufficient water in the country, but the problem arises when the analysis is carried out by hydrological region. In 2020, the natural runoff in the north was 81 095.08 Hm³, with a Uc of 78 830.86 Hm³ (97.2%). In the rest of the regions (south), the natural runoff was 240 858.28 Hm³ and the Uc of 113 192.08 Hm³ (47%). That is, in the north, almost 100% of surface water is consumed for different purposes, resulting in severe water stress.

The northern regions experience severe water stress, characterized by high local basin (Cp) values and consumptive use (Uc). The substantial exploitation of available surface water across various sectors intensifies pressure on already scarce resources.

The southern regions, which face significant barriers to infrastructure development, exhibit lower water utilization rates. These challenges include irregular precipitation patterns, limited investment, environmental constraints, social resistance, and the recognition of indigenous rights. Furthermore, limited hydropower potential necessitates the exploration of alternative energy sources. Developing hydropower infrastructure presents a significant challenge, demanding a careful equilibrium between energy generation and ecological and social considerations. Southeastern basins, like the Usumacinta and Grijalva, exhibit substantial hydropower potential, yet they also face considerable hurdles.

Legal requirements for ecological flows, as demonstrated by the environmental flow decrees for the Usumacinta River, frequently clash with hydropower operational demands. Furthermore, indigenous rights and local opposition create additional complexities for the implementation of infrastructure. Achieving sustainable hydropower development requires a commitment to prioritizing biodiversity conservation, respecting indigenous rights, and ensuring equitable distribution of resources. This approach aims to harmonize energy needs with the preservation of vital ecosystems and the well-being of local communities. This paper underscores the urgent need for stronger regulatory interventions to combat illegal surface water extraction and restore ecological balance. Strict enforcement of ecological flow requirements in perennial channels is crucial for protecting ecosystems. The remaining flows should be allocated for consumptive uses, prioritizing human consumption and essential agriculture.

In conclusion, Mexico faces complex challenges in managing its water resources and meeting its energy demands. However, proactive and integrated measures that prioritize sustainability, equity, and resilience can overcome these obstacles. By embracing innovative solutions and fostering the responsible use of natural resources, Mexico can secure long-term environmental health and equitable access to resources for all its regions.