1. Introduction

Mountain watersheds are rather complex hydrological systems that provide water resources to downstream communities for irrigation, industrial activities, human consumption, and ecosystem sustainability [1]. Considering that 40% of the world population are dependent on mountainous regions for its water supply [2], a proper understanding of the hydrological processes of mountain systems is critical to ensure the sustainable development of mountain communities.

In mountain watersheds, water can be stored and released by a combination of hydrological components that may include glaciers, snowpack, lakes, and groundwater. Of these storage components, the less understood is groundwater, which in many cases is neglected. However, in some areas, groundwater release can be the only available water source for local communities during the dry season.

Worldwide, there is a consensus that in central Chile, climate change will affect the dynamics of glaciers and snowpack, increasing the amount of melting in spring and early summer and reducing the amount of melting in late summer and early autumn, which is low-flow season in Mediterranean climate areas, like central Chile [3–5]. Thus, groundwater storage and liberation will be more important in terms of the resilience of mountain communities to climate variability, especially in mountainous areas where the presence of fractured porous rock systems produces conditions for the maintenance of minimum flow due to the liberation of groundwater [6–9].

In the Andean watersheds of the Central Valley of Chile (33.5°–41.5°S), there is little information about the role that fractured porous rock groundwater systems play in the generation of streamflow, mainly because most research has been focused on snow hydrology, as snowmelt drives the streamflow generation in central Chile during spring and summer [3–5]. However, in south central Chile (36°–41.5°S), where the Andes Mountains are lower than 3000 [masl], snowmelt ends in mid-January and until the beginning of the rainy season in mid-April; streamflow in the rivers depends on base flow generated by groundwater exfiltration. Therefore, understanding groundwater recharge, storage, and release processes becomes critical to manage mountainous hydrological systems and therefore to protect water resources.

On the other hand, it is important to highlight that Chile, during the last 20 years, has experienced an increase in income levels, which produced a strong demand for second homes, especially in high-demand tourism areas, especially in the central area of the country. One of these tourism areas is the Renegado Valley, which is associated with a world-class ski center that also has hot springs, the "Termas de Chillán" complex. At the end of the last century, the valley was part of a large farm that was exploited for forestry and cattle feeding. In the 1990s, the land was divided, and the tourism-related development process began, resulting in a community that stretches along the 30-km mountain valley without any planning for both drinking and wastewater.

This chapter presents the results of research work that had as an initial objective the study of the hydrology of the Renegado Valley, as water availability was identified as a key limitation for the further development of the area. Therefore, the initial research question was why does the Renegado Creek exhibit a permanent shortage of streamflow during the dry season?

The answer to that question is rooted in the hydrogeological characteristics of the Renegado Valley [10–12]. But that answer, which will be presented in this chapter, raised a second question about the vulnerability of the drinking water sources of the communities that are being developed along the mountain valley.
