**1. Introduction**

Climate change impacts on the hydrological cycle [1] with particular examples in France [2] and Central Europe [3], both fast and slow [4] where in the case of abrupt changes impacts on the ecosystem [5] and in long-term changes disrupt a pattern of inland moisture advection and convergence zone, increasing cloud base heights and reducing the total column liquid water content over high elevations [6]. Also, this impact has a strong response to global warming [7, 8], influences its extremes [9], and in turn influences via this cycle water resources [10, 11] while, conversely, the hydrological cycle influences climate [5, 12] in general and may, in case of enhancement, moderate transient climate change [13]. Climate changes impact rivers through the hydrological cycle as seen in [14, 15] and directly on river ecosystems as seen in the Danube [16], in the United Kingdom [17, 18], the Narew river [19], and globally [20]. In terms of hydrological cycle "sojourn" river water turnover takes place in 16 days [21]. As a result, river flow is impacted as seen in Europe [22], in the United Kingdom [23], in the Balkans [24], in Ethiopia [25], in India [26], and in West Africa [27]. Precipitation and temperature scenarios of climate change based on atmospheric circulation play an important role [28] and so do diagnostic statistics of daily rainfall variability in an evolving climate [29].

Under local conditions, environmental flows (e-flows) are defined in the 2007 Brisbane conference as "*the quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and wellbeing that depend on these ecosystems.*" [30] and resultant policies showed some moderate success [31] while the general trend of the state of aquatic ecosystems continued to deteriorate [32] due to increased dam building, particularly in ecologically sensitive areas [33]. Subsequently it was refined to "*Environmental flows describe the quantity, timing, and quality of freshwater flows and levels necessary to sustain aquatic ecosystems which, in turn, support human cultures, economies, sustainable livelihoods, and well-being*" in the 20th International Riversymposium and Environmental Flows Conference, held in Brisbane in September 2017, to lend increased support to groundwater-dependent ecosystems (GDEs) [34]. The influence of river flow on environmental flow is seen in [35], that of flow regime type (general regime classification in [36], under a changing climate is seen in [37], in hydroecology context in [38]) on the e-flows releases, and hydropower production is seen in [39], the impact of extreme flow variability on environmental flows is seen in [40], in terms of river basin management in [41, 42], and for natural, hybrid, and novel riverine ecosystems in [43]. A review [44] determined that regarding rivers, at a global level in six world regions encompassing 44 countries, there are applied 207 different environmental flow methodologies focusing onto hydrological (e.g., the 32-parameter range of variability approach (RVA) [45]), hydraulic rating [46], habitat simulation [46], holistic [47], or combinatory approaches [48]. The future is bleak, as a good scenario solution for the year 2050 [49] leads to an 10–20% increase of global virtual water trade so as to retain a semblance of survival in country-level environmental flows.

The adaptation approach is defined as "*Adapting to climate change means taking action to prepare for and adjust to both the current effects of climate change and the predicted impacts in the future*" [50]. The success of an adaptation policy is measured by monitoring, reporting, and evaluation (MRE), where monitoring is "*a continuous process of examining progress made in planning and implementing climate adaptation*" [51], reporting is "*the process by which monitoring and/or evaluation information is formally communicated, often across governance scales*" [52], and evaluation is "*a systematic and objective assessment of the effectiveness of climate adaptation plans, policies and actions, often framed in terms of the impact of reducing vulnerability and increasing resilience*" [51].

Therefore, it is necessary to generate the appropriate analysis models and methodologies to predict trends, capture biophysical impacts and possible variations of climate change [53, 54]. In addition, it is necessary to incorporate socioeconomic elements within the analysis of ecological systems with the purpose of carrying out a sustainable management of the goods and services provided by these ecosystems [55, 56]. The alteration of the river flow regime generally is caused by human activity, an aspect that requires of the studies with a multidisciplinary approach to the analysis of the problem of global change in freshwater systems [57, 58]. In particular, regulation by interbasin transfers, dams, withdrawals, and land cover change are the main human intervention agents [59].

*Water Availability for the Environmental Flow in Two Rivers of Mexico under Climate Change DOI: http://dx.doi.org/10.5772/intechopen.104881*

The adaptation to climate change makes necessary the determination of environmental flows in rivers so as to establish the change in water consumption for the population, agricultural activities, and industry and electricity generation, among others. All this is to compensate for variations in annual precipitation by the planning of the water resource security through different actions (transfer of industries to regions of greater humidity, change in the morphology of the cities to compensate for floods, availability of water for irrigation and flood control). These changes have important consequences on economic activities, population health, the ecosystem, and biodiversity [60]. In this sense, it is necessary to generate the tools for ecological, socioeconomic, and political analysis in order to achieve the rational use of aquatic resources in rivers regulated by dams [61]. In Mexico, there is little limnological information available on the country's freshwater systems and the effect that climate change is having on water quality and pollution. In this sense, in the present study, a comparative analysis of the variation of the water availability is applied through percentage of precipitation in the rivers of the Yautepec and Cuautla subbasins for the base period (preimpact) and subsequent period (postimpact) to determine the change in the availability of water in the riparian ecosystem.
