Climate Change and Water Resources in Mexico

Abstract

Achieving water security has become the strategic objective of water management, from the point of view of international organizations, governments, and experts. However, at the global, national, and regional levels, water security is a growing concern. In this context, climate change poses enormous additional challenges that need to be addressed. There are expectations of lower precipitation and therefore availability in large regions of the planet, particularly those located in midlatitudes, as well as an increase in the occurrence and severity of droughts and extreme storms. Climate-change research on aspects of vulnerability and impacts should be a guide to decision-making that increases resilience. Mexico, due to its geographical location and climatic conditions, will be one of the most affected countries both concerning its water resources and extreme weather events. In this chapter, the general perspective of the observed climate-change trends and expected effects, especially that of Mexico, is analyzed within the context of current knowledge, including the effects on water availability and extreme phenomena such as droughts, floods, and severe storms, as well as heat waves and their impacts on human health.

9.1 Overview

In the recent global risk report of the World Economic Forum (2017), water crisis is ranked third as a global risk with the greatest impact and is among those crises with the highest probability of occurrence. The water crisis, moreover, is associated with two other major global risks: the occurrence of extreme climatic events and the failure to mitigate and adapt to climate change. These risks, all of them of great impact and probability of occurrence, are in a feedback loop with each other, so that the presence of any of them increases the probability of the occurrence of the rest. Already in 2011, the water initiative of the World Economic Forum stated, “We simply cannot manage water in the future as we have in the past, or else the economic web will collapse (World Economic Forum 2011).”

In many countries, water security has not been achieved and, in fact, it is increasingly threatened. Population growth , economic development, urbanization, and natural climatic variability, as well as the effects of global climate change , continue to increase the pressure on water resources , in such a way that conditions of scarcity , permanent or recurrent, have already been recorded in some regions. The number and impact of natural water-related disasters continue to grow.

Without considering the effects of climate change, the Water Resources Group (2009) analyzed the global water demand by 2030 and found that, if measures are not adopted to increase efficiency in the use of water and to reduce consumption, the demand will be 40% greater than the supply of water. However, these global figures hide the huge differences in regional scarcities .

There are various water scarcity indexes (i.e., Florke and Alcamo 2007), among which it is important to use one that considers environmental needs. In the Human Development Report 2006 that was dedicated to water (UNDP 2006), the Water Stress Index (WSI) proposed by Smakhtin et al. (2004) was used and is as follows:

$$ \mathrm{WSI}=\frac{\mathrm{Withdrawals}\ }{\mathrm{Average}\ \mathrm{Water}\ \mathrm{Available}-\mathrm{Environmental}\ \mathrm{needs}\ } $$

(9.1)

From the results of Smakhtin et al. (2004), Rekacewicsz (2006) produced the map in Fig. 9.1. As can be seen, there are many large basins in which the water resource is overexploited, particularly in Europe, North Africa, the Middle East, India, China, and North America. In the case of Mexico , practically all the basins in the center and north of the country, in addition to the basin of the Lerma River and the Valley of Mexico , have levels of high exploitation or overexploitation.

Fig. 9.1

Water Stress Index (Rekacewicsz 2006)

Increases in water demand, mainly in agriculture, caused by population growth , urbanization, and economic development will be exacerbated by global warming , while in many regions, natural water availability will be depleted and the number of water-related natural disasters will grow. This scenario poses a daunting challenge for water management .

9.2 Climate-Change Scenarios

The Representative Concentration Pathways (RCPs) describe four twenty-first century scenarios that involve increases in the solar radiative force, going from 2.6 W/m² up to 8.5 W/m², which may be easily reached if the actual trends continue. The difference in the resulting temperature, precipitation , and sea-level increase, between both extremes, is illustrated in Fig. 9.2 showing the expected changes by the 2081–2100 period, in comparison with average of the 1986–2005 period.

Fig. 9.2

Coupled Model Intercomparison for 2081–2100 under RCP2.6 (left) and RCP8.5 scenarios (IPCC 2014)

Temperature increases will not be uniform but concentrate more in the northern hemisphere and in some specific regions. As can be seen in Fig. 9.2, the expected temperature increase in Mexico is among the highest on the continent and will affect areas that already suffer from high temperatures, further increasing heat-wave dangers.

From the point of view of water resources , the temperature increase will mean a greater demand for water to meet the most critical uses: the environment, agriculture, and cities. This increase in water demand will be aggravated by the foreseen changes in precipitation because forecasts indicate that there will be a decrease in the midlatitudes, where Mexico is located. The result will be an expansion in demand along with a decrease in the supply of water, which will lead to greater water stress and, in some areas, extreme scarcity .

Because the rainfall –runoff relationship is not linear, it is to be expected that changes in precipitation , by percentage, will lead to even greater changes in runoff. This decrease will have important effects on the availability of water for environmental and human uses. In an extensive study, using an ensemble of 11 global hydrological models, Schewe et al. (2014) developed the map shown in Fig. 9.3, describing the expected variations in annual runoff with a temperature increase of 2 °C. The intensity of each color indicates the level of agreement between the different models used. In North America, with a high level of agreement between the models, reductions of 10–30% in runoff are to be expected. If the temperature increase were 3 °C, the decrease in runoff could reach up to 50% in some places.

Fig. 9.3

Decrease in runoff with an increase in temperature of 2 °C in scenario RCP8.5, compared to the current situation (Schewe et al. 2014)

9.3 Water Resources in Mexico

In a global analysis, Mexico ’s water balance is already negative: more water is used than naturally available on a sustainable basis. In an analysis conducted by the National Water Commission (CONAGUA 2010), it was found that the 2010 gap between availability and demand was already 11,500 million m³ and, if the current trend continues, the gap will rise to 23,000 million m³ in 2030 (Fig. 9.4). These figures are generated without considering the climate change.

Fig. 9.4

Global water balance in Mexico between 2010 and 2030 (CONAGUA 2010)

For water-management purposes, the Mexican territory has been divided into 13 hydrological–administrative regions that group various basins. Table 9.1 shows the main characteristics of these hydrological regions, which can be identified in Fig. 9.5, including the per capita available water in 2014 and the one for 2030.

Table 9.1 Per capita removable water in Mexican hydrological regions , 2014 and 2030 (with data from CONAGUA 2015)

Fig. 9.5

Renewable per capita water in 2030 (CONAGUA 2015)

The water situation in Mexico has already reached critical levels in some regions. Without considering the effects of climate change, but only the demographic ones and according to the Falkenmark water stress criterion, by the year 2030 most of the Mexican territory will be in conditions of water stress , scarcity , or absolute scarcity , as can be clearly seen in Fig. 9.5. The situation in the Valle de Mexico region, home to more than 23 million inhabitants, and with a per capita availability of less than 150 m³/inhabitant/year, is particularly worrisome. This hydrological region already imports large volumes of water from other nearby basins and overexploits its aquifers . This situation worsens over time, and no appropriate solutions have been implemented.

With respect to groundwater , 38% of the allocated water for consumptive uses in Mexico comes from this source (CONAGUA 2015) and, more importantly, 75% of the water used in cities comes from aquifers .

In this regard, of the 653 aquifers registered in Mexico , 195 (almost 30%) have not more availability, that is, natural recharge is completely allocated. Likewise, 106 aquifers are under conditions of overexploitation. Of special concern are those aquifers located in large urban centers and agricultural production in the Mexican highlands, such as the Valley of Mexico , the Bajío Region, the La Laguna Region, and northern Chihuahua. Additionally, there are 31 aquifers with brackish water and 15 coastal aquifers with saline intrusion (CONAGUA 2015). Figure 9.6 shows the aquifers without water availability or those that are already overexploited.

Fig. 9.6

Aquifers without water availability or overexploited, in red (CONAGUA 2015)

9.4 Effects of Climate Change on Mexico

The hydrological cycle is closely linked to the temperature of the planet, both atmospheric and oceanic, so that global warming will have important effects on precipitation , runoff, aquifer recharge , water quality, and hydro-meteorological extremes. In general, in midlatitudes and subtropical zones, significant decreases in precipitation and runoff are expected, which will cause scarcity and greater pressure on water resources in these regions.

In the case of Mexico , both extremes are to be expected, in various parts of its territory. Thus, increases in the severity and frequency of droughts are expected, particularly in its north-central zone, as well as an increase in the intensity of storms and floods in the center-south region and in the coastal areas.

9.5 Temperature

The results of global circulation models are not enough to estimate the effects of climate change on local scales. The Intergovernmental Panel on Climate Change (IPCC) estimated the performance of global models, making a comparison between their results and the climate observed during the period 1980–1999. Regarding temperature, when multimodel results are analyzed (the average of 23 general circulation models), the estimation error (i.e., the difference between the observations and the model) is rarely greater than 2 °C, although individual models can show errors close to 3 °C (Randall and Word 2007). Consequently, the IPCC notes that the characteristics at large scales are simulated with greater accuracy than the regional scales. The local analysis of impact and vulnerability to climate change, therefore, should be strongly based on observational evidence, and it is crucial to study the observed regional trends.

There are already reliable indicators that allow us to affirm that we have begun to observe the effects of climate change in Mexico . According to the 2016 weather report of the American Meteorological Society (Blunden and Arndt 2017), there is already a clear trend of temperature increase in the last decades, as shown in Fig. 9.7, in which one can notice that, compared to the 1981–2010 base period, an anomaly of almost two degrees was recorded between the years 1970 and 2016. This increase in temperature has serious implications for the climate of Mexico , although with differentiated effects on the regional level.

Fig. 9.7

Annual mean temperature anomalies (°C, blue line, 1981–2010 base period) for Mexico . Red line represents the linear trend over this period (Mekonnen et al. 2017)

Future temperature projections are no less worrisome. According to the most recent calculations for Mexico , made by the National Institute of Ecology and Climate Change (INECC), in the RCP6.0 scenario at the end of this century, the temperature in the country would increase 2.5–3 °C in the center-south region of the country and up to 5 °C in the center-north region (Fig. 9.8). In scenario RCP8.5, the most unfavorable, the increase in temperature, practically in the whole territory, would be 5 °C and even more in the central-northern region (INECC 2016).

Fig. 9.8

Expected anomaly in the average annual temperature in Mexico at the end of the twenty-first century (2075–2099) in CPR6.5 Scenario (INECC 2016)

The effects of these temperature changes on water management will be very large. They will be observed in an increase in demand for water for food production, because crops—in such varieties that can withstand these new temperatures—will require greater water consumption. Ojeda Bustamante et al. (2015) conducted a vulnerability analysis of irrigated agriculture in Mexico , finding that the regions where the largest irrigation districts are located will have high or very high levels of vulnerability.

The increase in average temperature is accompanied by a significant increase in maximum temperatures, which produce heat waves that have proven to be extremely dangerous to health.

A heat wave can be defined qualitatively as a period, usually lasting several days, of temperatures significantly higher than average. Their importance lies mainly in their effects on human health, producing disorders that cause minor alterations in or even the collapse of the body’s capacity to regulate its temperature by means of changes in blood circulation or sweating. People with preexisting illnesses, such as respiratory or cardiovascular diseases, can experience negative health consequences during extreme heat episodes. In extreme cases, these health effects can lead to death. The elderly and small children are particularly vulnerable to heat waves.

In the last few decades, there have been records of particularly dangerous heat waves that have caused at least several deaths, even in developed countries with good public-health services. It is worth mentioning here the heat wave that hit Chicago in 1995, causing 514 heat-related deaths (Whitman et al. 1997), the heat wave of 2003 in Europe, which affected mainly France and caused almost 15,000 deaths (Hémon and Jougla 2003, cited by Le Tertre et al. 2006), and the one that occurred in the Russian Federation in 2010, which caused 55,736 deaths (CRED-UNISDR 2016).

The change in maximum temperature is clearly observable in the northeastern region of Mexico and southeastern US, which are in the same climatic region (BWh in the Köppen–Geiger classification), in the Sonora–Mojave Desert area. In a study of six major cities of the region, it was reported that during the warmer months, there occurred an increase of more than two degrees in the maximum temperatures in the period 1960–2011 (Fig. 9.9) and an increase in the number of days per month in which the temperature exceeds the threshold of heat waves. This corresponds to the 90th percentile of the maximum temperatures observed (Fig. 9.10) and had extended from five to 15 days on average. This behavior of the temperature influences the increase in mortality in the hottest periods (Martinez-Austria and Bandala 2017).

Fig. 9.9

Maximum monthly temperature variations and linear trend lines for August, in six cities of the Sonora–Mojave Desert region in the period 1960–2011 (Martinez-Austria and Bandala 2017)

Fig. 9.10

Number of days exceeding the 90th percentile of the maximum temperature during the month of August in the period 1960–2011 in six cities of the Sonora–Mojave Desert region (Martinez-Austria and Bandala 2017)

In the city of Mexicali, which has the highest mortality rates due to heat stroke in Mexico , a correlation can be identified between maximum temperatures and the general mortality rate. At and above a maximum temperature of 47 °C, the mortality rate per 10,000 inhabitants increases drastically, as can be seen in Fig. 9.11.

Fig. 9.11

Total mortality rate (per 10,000 inhabitants) versus maximum temperature during August (1990–2010) in Mexicali, Mexico (Martinez-Austria and Bandala 2017)

9.6 Water Availability

While changes in temperature will increase the demand for water, mainly for the environment and food production, the expected changes in precipitation and runoff point to a decrease in water availability . Figure 9.12 shows the expected changes in rainfall in the RCP6.0 scenario, for spring–summer, which is the period of highest rainfall in the country, calculated by Salinas Prieto et al. (2015), with respect to the base period 1971–2000. Decreases in precipitation are expected throughout Mexico , with reductions of between −9% and −15% in the north and northwest of Mexico , the region where the main irrigation systems are located, and which includes watersheds where water stress and scarcity are already expected, caused by growing demand, which will be aggravated by this decrease in precipitation .

Fig. 9.12

Changes in spring–summer rainfall for the period 2075–2099 in Mexico , for several climate-change scenarios, compared to the period 1971–2000 (Salinas Prieto et al. 2015)

Due to factors such as higher evapotranspiration of natural vegetation, drier soils, and higher evaporation , the expected reduction in runoff will be greater than that estimated in precipitation . As mentioned before (see Fig. 9.3), in Mexico , significant runoff reductions between 10% and 30% are expected. However, these results come from models of global circulation and have great uncertainty at local scales. Determination of its regional magnitudes requires analysis by basin, through the application of rain–runoff models that, although the simplest ones are used, require a significant quantity of local information of the basin under study. Probably for this reason, there are few studies on runoff vulnerability in Mexico .

Rivas Acosta et al. (2010) conducted studies on changes in runoff for some of the major basins in Mexico . In the Conchos River basin, for example, using the balance method of the Mexican official standard for water-balance calculations, and applying the estimated decrease in scenario A2 precipitation , they determined reductions of 25% in the runoff for 2100. These results, in a basin that already has extractions that keep new water allocations practically unavailable, represent a very important challenge for the future management of water and make expensive adaptive measures necessary in the agricultural sector.

9.7 Extreme Events

Most of the research works on the effects of climate change have focused mainly on the averages of climatic variables, that is, for example, the change in medium temperature or average annual precipitation . However, in an atmosphere and in an ocean with greater energy, more intense extreme phenomena are expected, mainly, more frequent heat waves with higher maximums and extreme rainfall or droughts with greater recurrence and intensity, as well as larger storms and floods. These phenomena are no longer a matter only for the future, since in fact they are recorded year after year with greater severity in various parts of the world. As stated: “The signal of climate change is emerging from the “noise”—the huge amount of natural variability in weather” (Carey 2012).

The number and cost of natural disasters related to water have registered a continuous rise in recent decades (UNESCO 2009), as shown in Fig. 9.13. As can be seen, especially since 1990, the number of disasters, such as floods and damaging winds, has continuously increased. Glokany (2009) has conducted a study of trends in the number of climate-related disasters over the last century. The result is not very encouraging, as shown in Fig. 9.14, which makes evident the exponential growth.

Fig. 9.13

Water-related disasters, 1980–2006 (UNESCO 2009)

Fig. 9.14

Water-related disasters, recorded in the period 1980–2006. Average by decade. (With data from Glokany 2009)

Extreme rainfall , with its harmful effects such as floods and landslides, causes loss of human life and damage to infrastructure and the productive sector and often reverses years of progress. Consequently, flood protection is among the greatest challenges to water security in Mexico . In 2010 alone, the cost of damage caused by extreme hydro-meteorological phenomena amounted to US$6600 million. In Nuevo León, the damage from Hurricane Alex in 2010 represented 2.45% of the state’s Gross Domestic Product (GDP), and in Veracruz, in 2016, the floods caused by the storms Karl and Matthew inflicted damage equivalent to 4.8% of the state GDP. In 2010, 739 municipalities in the country received a declaration of natural disaster due to hydro-meteorological events (CENAPRED 2012). In 2007, most of the state of Tabasco suffered floods, with huge economic and social costs. Unfortunately, as can be seen in Fig. 9.15, the number of deaths and the damage costs in Mexico have been increasing continuously in recent years.

Fig. 9.15

Damage costs and deaths in Mexico due to hydro-meteorological disasters. (Prepared with data from CENAPRED’s reports)

On the other hand, droughts are the hydro-meteorological events that cause the most extensive economic and social damage. Drought is associated with phenomena, such as climate migration and damage to the environment, including permanent ones such as desertification. Its frequency and intensity, presumably because of climate change, have also increased since 1970 to date, and are the cause of the greatest number of deaths among extreme climatic events, as is evident from the list of the 10 largest hydro-climatological disasters recorded in the period 1970–2012 (World Meteorological Organization 2014) and shown in Table 9.2.

Table 9.2 Disasters ranked according to reported deaths, globally in the period 1970–2012 (World Meteorological Organization 2014)

In Mexico , recurrent droughts of great intensity occur that cover almost the entire national territory. Figure 9.16 shows the percentage of Mexican territory suffering drought of varying magnitude, from 2003 to 2017, according to the national drought monitor. The drought of 2012 covered more than 90% of the territory.

Fig. 9.16

Area of Mexico , by percentage, under various conditions of drought in the period 2004–2017. (With data from National Meteorological Service, SMN 2017)

On the other hand, there is already a statistically significant trend of decreased precipitation in the northern region of the country, as will be subsequently shown.

One of the most accepted methods for estimating droughts is the Standardized Precipitation Index (SPI) (Hayes et al. 2011), which represents the number of standard deviations in which the transformed value of precipitation deviates from the historical average, which therefore represents the value zero. The SPI is used in most countries due to the availability of data, for its easiness of interpretation, as well as for its ability to be calculated for short or very long periods of time. Another advantage of SPI is that it enables observing not only the abnormally dry periods, but also the extremely humid ones, and the variability between them. Table 9.3 shows the humidity ranges of the SPI and their significance.

Table 9.3 Standard Precipitation Index (SPI) and weather conditions

As an example of precipitation trends in northern Mexico , consider the Conchos Basin, which is the main tributary of the Rio Grande, and is mainly in the state of Chihuahua. This basin is of great importance in Mexico because it supplies large irrigation districts, the capital of the state of Chihuahua, and other cities. In addition, it is of international importance because a proportion of its runoff must be delivered to the US, according to a treaty signed between both countries in 1944.

In Fig. 9.17, the calculated variation of the SPI at the La Mesa weather station, located in the state of Chihuahua, is shown. The SPI values for 6, 9, and 12 months are shown, as well as the linear adjustment for this last case. As can be seen, there is a clearly identifiable tendency to lower amounts of precipitation , especially since the 1970s. While in previous decades the precipitation was characterized by alternating wet and dry periods, there have been no wet periods since the beginning of the 1990s. This situation has caused the area cultivated in the irrigation districts of the region to rarely receive the needed water in all sectors, in addition to difficulties complying with the commitments of Mexico to the US, as established in the Water Distribution Treaty between both countries.

Fig. 9.17

Variation of the Standardized Precipitation Index in the La Mesa climatological station, in the Conchos River basin (Martinez-Austria and Irula-Luztow 2016)

9.8 Concluding Remarks

Climate change will have great repercussions on Mexico ’s water resources . The combination of higher temperature and the current demographic processes will cause increasing growth in demand. Simultaneously, decreases in precipitation are expected in most of the continental territory, which will cause a decrease in the naturally available water. Thus, on the one hand, the demand for water will increase, and on the other, its natural supply will decrease. Together, both trends will cause huge pressure on Mexico ’s water resources .

The extreme weather phenomena, such as storms, floods, and droughts , have been increasing, in such a way that the cost of the damages and the loss of human lives have swelled. These extreme phenomena, under climate change, will continue to grow in intensity and frequency. Another extreme phenomenon of great importance for the people’s health will be the maximum temperatures and their effects on human health, especially during heat waves.

These events are not only in the distant future. Many of the effects are already evident in obvious trends observed during recent decades. It is very important to be aware of this situation and not to postpone further the design and application of adaptive actions to increase resilience and reduce the vulnerability of cities, communities, agriculture, and the environment.