2. Methods and materials

#### 2.1 Study area

The Diguillín River watershed is located in central Chile at latitude 36.9°S and longitude 71.4°W (Figure 1a) and drains the southwestern section of the Nevados de Chillán volcanic complex, located in the Andes Mountains (Figure 1b and c). At the upper part of the watershed, there are two gauging stations that define the two sub-watersheds that are shown in Figure 1c: Alto Diguillín (207 km<sup>2</sup> ) which is controlled by the Diguillín en San Lorenzo (DSL) gauging station and Renegado Valley (127 km<sup>2</sup> ) which is controlled by the Renegado en Invernada (RI) gauging station.

## Andean Mountain Groundwater, Drinking Water Sources, and Vulnerability: A Case Study… DOI: http://dx.doi.org/10.5772/intechopen.85016

In the Alto Diguillín sub-watershed, there is a national protected area called Reserva Ñuble and some farms dedicated to forestry and cattle production; in contrast the Renegado Valley, as described before, has been intensively populated for second homes, due to the tourism value associated with the Volcan Chillán Ski Area and the existence of hot springs. Additionally, there is a marked difference between both sub-watersheds when the streamflows are compared. The Renegado Creek exhibits much lower values than the Alto Diguillín (Figure 2), more than can be easily explained based on watershed extent. In fact, the Renegado Creek exhibits lower specific flows (flow rate per unit of area), in comparison with those of the neighboring watersheds Alto Diguillín and Chillán (used for comparison in Figure 3), even when those rivers exhibit the same East to West orientation and a similar rainfall distribution. That lower specific flow is consistent with the water availability limitation for the development of the community along the valley, which is one of the research questions of this work.

#### Figure 1.

(a) Diguillín watershed location in South America; (b) the Biobío Region, showing the main cities of Concepción, Chillán, and Los Angeles; (c) location of the Renegado Creek and Alto Diguillín sub-watersheds at the upper section of the Diguillín watershed, the Agua Bonita location where a large cluster of springs flows to the Diguillín River, and also the location of gauging station: "Ch" is Chillán River, "RI" is Renegado en Invernada, and "DSL" is Diguillín en San Lorenzo.

#### Figure 2.

Average rainfall and streamflow measured at the Diguillín River at San Lorenzo and Renegado Creek in Invernada.

Figure 3.

Comparison of measured monthly specific streamflow for Alto Diguillín and Chillán Rivers with Renegado Creek.

#### 2.2 Field research

Due to the existence of several infrastructure projects that have been proposed for the Diguillín River watershed, previous studies were considered as the base for the initial characterization of the watershed. The study for irrigation planning conducted by the National Commission of Irrigation (CNR) in the Itata River basin, which concludes that the Diguillín River receives flow from groundwater discharge in the middle part of the watershed, which becomes relevant during the low-flow season between January and April [13], was particularly important. Hydrological data for the watershed (streamflow and rainfall) were collected from the database of the Chilean Water Authority (Dirección General de Aguas, DGA).

In addition, to incorporate local knowledge about the Diguillín River, a series of interviews of various stakeholders such as the river authority (Junta de Vigilancia), villagers of every sector, mountaineers, and sport fishermen was carried out in order to determine if the existence of springs that feed the Diguillín River was true.

The available geological information came from two principal publications that describe the geology of the upper part of the Diguillín River watershed [14, 15]. Both references explain the marked influence of the volcanic processes associated with the Nevados de Chillán Complex on the development of this watershed. The geological information [15] includes a geological map at 1:50.000 scale which was digitalized in a raster format and virtually mounted on Google Earth, using Global Mapper software.

As a complementary analysis for the identification of hydrological processes, a hydrogeochemical data analysis was performed. Samples of rain, snow, surface water, and springs collected from the Renegado, Diguillín, and Chillán Rivers during 2012 and 2013 were considered. Samples were chemically analyzed for mayor cations and anions (i.e., Na<sup>+</sup> , K<sup>+</sup> , Ca2+, Mg2+, Cl˜, HCO3˜, SO4 <sup>2</sup>˜) in the Laboratory of Soil and Plants Analysis of the University of Concepción. Additionally, concurrent samples were derived to the Chilean Commission of Nuclear Energy for the environmentally stable isotopes analysis ( 18O y <sup>2</sup> H). Further description about the technics used can be found in Arumí et al. [10]. Also, samples were analyzed for 222Rn by the Environmental Laboratory of University of La Serena using a Durridge RadH2O equipment [16].

The analysis of secondary information suggested the existence of a cluster of springs discharging into the Diguillín River, in a gorge located downstream of the confluence of the Renegado Creek and the Diguillín River. This sector was studied in detail by walking surveys, which allowed the identification of a 2-km section of the river with clusters of fractured rock-related springs that discharge to the

Andean Mountain Groundwater, Drinking Water Sources, and Vulnerability: A Case Study… DOI: http://dx.doi.org/10.5772/intechopen.85016

#### Figure 4.

(a) Large spring draining to the Diguillín River in Agua Bonita; (b) water is channeled through multiple small fractured rock springs (b); (c) springs are located at the base of cliffs of a gorge that can be reached only in summer conditions when the river flows are minimal; (d) typical fractured rock profiles observed along the Renegado sub-watershed at the location of the spring cluster.

Diguillín River (Figure 4) in a location locally known as "Agua Bonita" (Figure 1c). All the springs were located at the bottom of a hundred-meter-high cliff, in a very difficult-to-access area located along a gorge that can be reached only in summer when river flows are minimal.

Because locations were of difficult access, measurements taken in Agua Bonita were only possible at the end of the dry season (March 2012 and 2013). These measurements were carried out using the FlowTracker Acoustic Doppler Velocimeter, from SonTek. Streamflow was measured at the Diguillín River, above and below this 2-km section. It was found that the river flow increases from 2.5 to 7.4 m<sup>3</sup> /s; therefore, spring discharge was estimated as 4.9 m<sup>3</sup> /s.

### 2.3 Water balance analysis

The water balance was analyzed through a conceptual model approach to better understand the hydrologic behavior of the Renegado and Alto Diguillín watersheds [12]. The model simulates the rainfall-runoff and snowmelt-runoff processes. The rainfall-runoff component was modeled through a lumped model that considered the watershed as a double storage system: subsurface and groundwater. The snowmelt-runoff model calculates the snowfall based on precipitation above the zero-degree (base temperature at which melting starts) isotherm falling as snow. The melting calculations are performed based on the concept of the degree-day method [17]. Thus, the potential melting is estimated, and then based on the stored snow, the real melting is calculated. The model needs the rainfall and the potential evapotranspiration as inputs, and the output is the total runoff at the watershed outlet, including both subterraneous and direct runoff, the amounts of which are calculated through six calibration parameters, plus two for the input

modification (useful in the case of non-representative PM and PET data). Further description about the model, its implementation, and calibration can be found in [12].

The major findings in the water balance of the Renegado-Diguillín system were that the low specific flow condition at the Renegado Valley and the existence of the cluster of springs that flow into the lower Diguillín River suggest that a significant part of the base flow that is produced at the Renegado Valley is transferred through a subterranean connection to the spring cluster [12]. To reproduce such conditions, the Renegado-Diguillín model was modified by adding a groundwater connection, where a percentage of the Renegado base flow was transferred to the Diguillín watershed. This finding is consistent with an indirect estimation of groundwater storage evolution [11] based on recession flow analysis. In that work it was shown that whereas for the Upper Diguillín basin and the period 1961–2010, no increase or decrease trend in groundwater storage was detected; for the Renegado sub-basin, it was possible to observe a statistically significant decreasing trend in subsurface water storage.

This water balance analysis allowed the understanding of the observed condition, i.e., that the Renegado Creek presents lower specific flows than the Alto Diguillín River. By adding a groundwater connection between watersheds, it was possible to better simulate the monthly flows of these two basins. Thus, a main conclusion from these studies was that a groundwater contribution provided from the Renegado watershed to the Diguillín watershed was necessary to adequately reproduce the hydrogeological behavior of the Renegado-Diguillín hydrological system. After the calibration processes, it was possible to estimate that about 77% of the base flow is lost through groundwater seepage from the Renegado watershed. That flow was estimated to be around 4.6 m<sup>3</sup> /s, very close and on the same order of magnitude to the 4.9 m<sup>3</sup> /s measured at the springs cluster located in the Diguillín River.
