**1. Introduction**

For humanity, access to freshwater is a right and a necessity; however, the contamination and the increasing scarcity of this precious resource have alarmed the entire world. Currently, investigations executed around water knowledge are focusing on multiple subjects. One of them, the water cycle, is without objection a complex topic, and it has been studied for its undeniable connection with life. The water cycle is related to the atmosphere and the ground; the atmosphere is tied with the carbon cycle, the ground with the biota, and the system continues with multiple relationships that happen at the same time.

Moreover, added to the difficulty of modeling the water cycle and its relationship with biodiversity and climate; political, social, and economic issues need to be linked. Hydrological processes such as interception, evapotranspiration, and the effects of irrigation need a combined investigation of impacts such as the extraction of groundwater, changes in land use [1]; as well as to vapor pressure deficits, temperature changes, and extreme events (floods and droughts) [2]. Nonetheless, the impacts mentioned earlier have their own functioning, which have been aggravated seriously in recent years with the overexploitation of ecosystem resources and the anthropogenic activities that have ushered to irregular climate change.

Attempting to fathom and handle the problems related to the water cycle and ecosystems, the science of ecohydrology allows researchers to address the principle that water is a critical factor for the biosphere's functioning, bio productivity, and biodiversity-ecosystem services for society [3]. Ecohydrology aspires to integrate the knowledge and understanding of both hydrological and ecological processes that cooccur at different scales, considering ecosystems as the primary tool to increase water sustainability in basins [3]. "A key challenge faced by science in the 21st century is not only understanding the causal relationships between abiotic factors and biotic factors in all types of ecosystems, but also linking them to the dynamics of society" ([4] p. 396). Sometimes humans are narrow-minded, this further complicates noticing these connections, therefore the reality is oversimplified [5]. It exists a great necessity to change that idea and start to think systematically.

Since ecohydrology studies variables of its interest as a multidimensional phenomenon, composite indexes methodology could be a proper technique to be applied in this field. A composite index is a measure that determines and expresses the degree or state of a variable [6]. The variable is measured by employing a single score obtained by combining other values, sometimes through a direct sum or by more complex manners. The indexes and their distinct aspects are a bridge between specialists' work and their scope being understood by the common people.

Environmental, water or ecological indexes embody several variables for their calculation, and they are called indicators. Quantitative or qualitative indicators can represent the variables since they categorize a large amount of data. Indicators have been used as an instrument to estimate changes and risks, but lately, they have also taken importance in water and resources management [7] for government policies. Indicators need constant upgrades to generate better quality and trustworthiness of their results. One path for indicators' improvement is creating a scheme of criteria for selecting and assessing them along with designing the mediums to do it [8]. In this work, the authors believe that indicators are a viable method of assessing the complexity and depth interrelation of the freshwater system with the ecological medium; that is the main reason to review the published literature related to the subject. From examining the available indexes works with a systematic review, it is possible to find the right ecohydrological indicators to be the baseline for a new composite index.
