WATER AVAILABILITY AND QUALITY VULNERABILITY AFTER CLIMATE AND LAND-COVER CHANGES IN AN ANDEAN VOLCANIC WATERSHED IN SOUTH CENTRAL CHILE

In Chile there is a consensus that climate change will affect the dynamic of snowpack, increasing the amount of snowmelt in spring and early summer and reducing the amount of snowmelt in late summer and early autumn (low flow season). Therefore, in volcanic dominant mountain watersheds, groundwater base flow will be a larger proportion of streamflow during the low flow season.

This article presents a study of the Diguillín watershed located in Central Chile (36,9° S) where geology is dominated by the Volcanic Complex Nevados del Chillan. The headwater of the Diguillin River has two main subwatersheds: Renegado creek and Alto Diguillín. Superficial water balance does not fit for both watersheds because there are a deficit/excess of superficial runoff. Renegado watershed soils are predominant sands over a basement composed by fractured rock; infiltration of rain and snowmelt predominates over superficial runoff, producing that about 5 m³/s of groundwater flows to the Diguillín River discharging in a cluster of springs located 3 km downstream of the superficial connection. Alto Diguillín streamflow is feeding by other cluster of springs that discharges the aquifers located inside the volcano. Therefore groundwater discharge becomes the mayor contribution of the Diguillin River during low flow seasonality, reducing the watershed vulnerability on water availability after modifications of snowmelt dinamic induced by climate change.

In Chile, land cover change has been driven by the increment on population income, which has induced an important expansion of second housing construction in the area of the Renegado watershed because it is an important tourism center for Sky and thermals water. There are now more than 1000 vacation houses and several resorts. Because the extensive use of septic tanks, located above the highly permeable soils that overlies the fractured rock aquifer, there is a concern on the impact on water quality at the Diguillín River of the land development. Pollutants from wastewater disposal system will move through the fractured system and discharge on the river. That will be especially critical during low flow season when groundwater discharge is the main contribution to streamflow. Therefore in spite that the vulnerability on water availability is lower, it is higher on water quality.