Context

The Petorca River Basin is an emblematic case study of the lack of water security, environmental injustice, and inequality.

For over a decade, rainfall in the area has not reached normal levels, affecting the available water flow for human consumption and agricultural use. In 2018, more people had to receive potable water through water trucks. However, some publications blame not only the drought but also the agro-export model that has altered the region’s productive structure.

To address the water shortage issues in the basin, the government has implemented various measures, including strict control of wells from which water is extracted, the formation and empowerment of farmer monitoring boards to observe and demand compliance with the law, and the allocation of a budget of US$ 173,000,000 for the construction of the Las Palmas reservoir, with a capacity of 55 million cubic meters, which is set to become operational in 2024.

This case study aims to understand the water crisis that has unfolded in Petorca over the past decade, quantifying the spatial and temporal variation of the main hydrological processes in the basin. Additionally, the institutional response to water scarcity problems will be evaluated. Finally, we aim to identify the minimum requirements that must be addressed to overcome this crisis in the context of climate change adaptation.

Description

The Petorca River Basin is located in the Valparaíso Region, between parallels 33.00 and 33.05ºS and between meridians 71.06 and 70.06ºW, covering parts of the Petorca and La Ligua communes. This basin is bordered to the north by the Choapa River Basin and to the south by the La Ligua River Basin. It stretches from the Andes Mountains to the Coastal Range (33.00ºS - 33.08ºS), with a general NE-SW orientation and a length of about 90 km in that direction, while its average width in the N-S direction is approximately 20 km (DGA, 2006). The basin has a total area of 1969.5 km². Elevations within the basin reach up to 3721 meters above sea level at its highest point, while the defining gauging station is located at 126 meters above sea level.

Elevation map of the Petorca River Basin in Longotoma

Figure 1. Elevation map of the Petorca River Basin in Longotoma.

The main sub-basins of the Petorca River are the Pedernal River in Tejada, the Sobrante River in Piñadero, and the Petorca River in Hierro Viejo, which can be seen in Figures 2, 3, and 4.

The Pedernal River sub-basin in Tejada is the smallest, with an area of 81.1 km² and elevations ranging from 1372 to 3485 meters above sea level.

The sub-basin has a snow-rain hydrological regime, characterized by higher flows in the spring months due to snowmelt. During the winter season (June, July, and August), high precipitation occurs compared to other months, but with lower flow compared to the spring season (September, October, November), when precipitation decreases as summer approaches, but flows increase due to snowmelt.

Elevation map of the Pedernal River Sub-basin in Tejada

Figure 2. Elevation map of the Pedernal River Sub-basin in Tejada.

On the other hand, Figure 3 shows the Sobrante River sub-basin in Piñadero, which originates from the Sobrante Lagoon, located at about 3240 meters above sea level (DGA, 2006), and is classified as a basin with a low level of human intervention (Muñoz et al., 2020). This basin has an area of 241.1 km², with elevations ranging from 1176 to 3721 meters above sea level.

This sub-basin also has a snow-rain hydrological regime, similar to the Pedernal River sub-basin in Tejada, characterized by higher flows in the spring months compared to winter flows.

Elevation map of the Sobrante River Sub-basin in Piñadero

Figure 3. Elevation map of the Sobrante River Sub-basin in Piñadero.

Finally, the Petorca River sub-basin in Hierro Viejo, shown in Figure 4, includes the two previously mentioned sub-basins, making its total area 947.2 km², with elevations ranging from 453 to 3721 meters above sea level. The main tributaries are the Pedernal River and the Sobrante River.

This sub-basin has a snow-rain hydrological regime, influenced by the regimes of the Pedernal River in Tejada and the Sobrante River in Piñadero, as they are part of the area of the Petorca River sub-basin in Hierro Viejo.

Elevation map of the Petorca River Sub-basin in Hierro Viejo

Figure 4. Elevation map of the Petorca River Sub-basin in Hierro Viejo.

Hydrography

The Petorca River originates in the Andes Mountains, at the confluence of the Pedernal River (to the north) with the Sobrante River (to the south), near the town of Chincolco (650 m above sea level). After a journey of approximately 100 km, flowing southwest with a slope of 3.22%, it empties into the sea at a place called Las Salinas de Pullally, near the mouth of the La Ligua River. Most of the tributaries are short, especially those originating from the southern slope, corresponding to the Sobrante River. The main tributaries of the Pedernal River are La Tejada and Chacalo streams, while for the Sobrante River, the main watercourses are La Laguna stream (where the Laguna del Sobrante is located), Los Nacimientos stream, and El Chacay stream. In its lower course, the Petorca River receives waters from Las Palmas and Ossandón streams, along with several other streams, including Castro, El Bronce, and Ñipa. Figure 1 shows the main watercourses of the Petorca River Basin.

The primary use of its waters is for irrigation in the Petorca Valley (BCN).

Climate

The Köppen-Geiger climate classification for the period 1980–2016 presents six climate types: BSk, CSb, Csc, Dsb, Dsc, and ET (Beck et al., 2018). The lowest part of the basin (44–1316 m above sea level) is characterized by a cold semi-arid steppe climate (BSk). As the elevation increases (up to 2565 m above sea level), a long, dry summer with mild temperatures begins (Csb, Csc). Above 2565 m, a colder but still dry climate (Dsb, Dsc) starts, and above 3694 m, tundra-type polar climates (ET) dominate, which typically feature low precipitation and average temperatures below 10°C year-round, dropping below 0°C in winter (see Figure 5).

For the period 2071–2100, a significant change in the climate of the basin is projected. In higher areas, the tundra climate (ET) will disappear, and temperatures will rise, leading to a drier climate with less snow accumulation in the upper areas. In the lower part of the basin, temperatures will increase, resulting in an arid, warm-to-hot climate, which will reduce the areas previously characterized by a cooler, semi-arid climate.

In the context of the Petorca River Basin at Longotoma, the following climate types have been identified for the present and future periods:

  • BSk: Cold steppe climate, characterized by cold, dry winters and warm summers. Precipitation is generally scarce, and the vegetation is adapted to drought conditions.
  • Bsh: Cold arid steppe climate, similar to BSk but with even less rainfall, resulting in a more arid and dry environment.
  • Bwh: Cold desert climate, with cold, dry winters and warm summers. Precipitation is extremely scarce year-round, creating a desert environment.
  • Bwk: Cold desert climate with cold winters, similar to Bwh but with colder winters and cooler summer temperatures.
  • Csb: Temperate climate with dry, warm summers. Similar to the Csa climate, this type has dry, warm summers, but temperatures may be more moderate. Rainy seasons are concentrated in winter, with more noticeable drought in summer. Common in coastal regions with maritime influence.
  • Csc: Temperate climate with dry, cool summers. Summers are dry and cool rather than warm. Temperatures tend to be more moderate year-round, with more precipitation in winter and less in summer. Characteristic of areas with less oceanic influence.
  • Dsb: Cold climate with dry, warm summers. Similar to BSk, but with colder winters and lower temperatures. Precipitation is low throughout the year, but summers are brief and warmer than in Dsc climates. Found in cold areas with maritime influence.
  • Dsc: Cold climate with dry, cool summers. Similar to Dsb, but summers are cooler rather than warm. Precipitation is low year-round, with shorter and cooler summers. Found in cold areas with maritime influence.
  • ET: Polar tundra climate, found in polar regions with extremely low temperatures year-round. Summers are very short and cold, with tundra vegetation adapted to the cold climate.

These climate categories are essential for understanding and describing the distinct characteristics of the region’s climate, both in the present and in future projections. Analyzing climate trends is vital for decision-making in planning and adaptation to potential climate changes.



Figure 5. Köppen-Geiger climate classification map of the Petorca River Basin in Longotoma for the present (1980–2016) and future (2071-2100). Source: Beck et al., 2018.

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Figure 6. Spatial distribution of precipitation in the Petorca River Basin. Prepared from CR2METv2.5 data.

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Figure 7. Spatial distribution of the Maximum Temperature in the Petorca River Basin. Prepared from CR2METv2.5 data.

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Figure 8. Spatial distribution of the Minimum Temperature in the Petorca River Basin. Prepared from CR2METv2.5 data.

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Figure 9. Climogram of the Petorca River Basin. Prepared from CR2METv2.5 data.

Land Use

87% of the watershed is covered by shrubland and bush-type vegetation, largely due to the semi-arid climate present in the region. In some specific areas, such as the lower part of the watershed, there are high-altitude desert areas with sparse or no vegetation, similar to the high-elevation zones.



Figure 10. Land use map of the Petorca Basin in Longotoma. Source: Ceballos et al., 2018.

Table 1. Descriptive table of land cover in the Petorca River Basin in Longotoma.

Descriptive table of land cover in the Petorca River Basin in Longotoma



Soil Type

Figure 11 presents the soil characterization based on depth, showing that at depths between 0 and 5 cm, the soil is predominantly sandy loam (LoSa), with a high sand content (80%), but also containing 15% silt and 10% clay. Additionally, there is a significant percentage of sandy loam soil (SaLo), containing 65% sand, 25% silt, and 10% clay.

As soil depth increases (from 5 to 15 cm), a reduction in sandy loam (LoSa) is observed, while sandy clay loam (SaClLo) increases. The latter consists of 35% sand, 35% silt, and 30% clay. This results in a decrease in sand and an increase in silt compared to surface soil.

At depths between 15 and 30 cm, a significant reduction in sandy loam (SaLo) is noted, with an increase in sandy clay loam (SaClLo). Additionally, in the lower part of the basin, sandy loam (LoSa) reappears.

Soil type map of the Petorca River Basin in Longotoma

Figure 11. Soil type map of the Petorca River Basin in Longotoma. Source: Galleguillos et al., 2022

Geology

The lower part of the basin features volcanic and marine sedimentary sequences from the Jurassic period. In the middle section, sedimentary and volcanic sequences from the Lower Cretaceous period dominate, with andesite and basalt formations, which are prominent in the coastal mountain range between the Atacama and Metropolitan regions. Finally, in the upper part of the basin (sub-basins of the Sobrante River in Piñadero and the Pedernal River in Tejada), continental volcanic-sedimentary sequences prevail. At the highest point of the Sobrante River sub-basin, volcanic rocks generated in an intrusive environment are found, while at the confluence of the Pedernal River with the Sobrante River, continental sedimentary and volcanic sequences are observed (Peña, 2021).

Water Resources

Water use in the basin is mainly allocated to agriculture, which is developed in areas near river valleys, with avocado and lemon plantations prevailing (Peña, 2021). This is largely due to land purchases at low prices since the 1990s, which led to an excessive granting of provisional water rights by the DGA, resulting in a constant increase in water use and over-exploitation of the basin.

Due to constant water extractions by agricultural entrepreneurs, Petorca is currently experiencing a progressive water crisis (Panez-Pinto, Faúndez & Mansilla, 2017). It is projected that drought scenarios will persist and worsen (Morgan, 2020).

Currently, much of the Petorca population relies on water delivery trucks for their supply. This is due to several factors, including the uninterrupted Mega Drought since 2010, which has caused an annual decrease in precipitation, river flows, reservoir volumes, and groundwater levels (Garreaud et al., 2017).

Monitoring Points

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Figure 12. Precipitation monitoring points.

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Figure 13. Soil moisture monitoring points.

Precipitation Intensity-Duration Curves

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Figure 14. Intensity-Duration curve for Pedernal Station - Quebrada Huinganes (PP01).

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Figure 15. Intensity-Duration curve for Pedernal Station - Quebrada Lagunitas (PP02).

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Figure 16. Intensity-Duration curve for Quebrada Palmitas Station (PP03).

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Figure 17. Intensity-Duration curve for Quebrada El Ajial Station (PP04).

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Figure 18. Intensity-Duration curve for El Sobrante Station (PP05).

Water Rights

According to the General Directorate of Water (DGA, 2023), responsible for granting Water Rights (DAA), a total of 2,053 DAAs are registered in its database for the Petorca River Basin, all of which correspond to consumptive rights.

Photos

PP01 - Pedernal [Qda. Huinganes]
PP01 - Pedernal [Qda. Huinganes]
 PP02 - Pedernal [Qda. Lagunitas]
PP02 - Pedernal [Qda. Lagunitas]
 PP03 - Qda Palmitas
PP03 - Qda Palmitas
 PP04 - Qda El Ajial
PP04 - Qda El Ajial
 PP05 - El Sobrante
PP05 - El Sobrante
 SM01 - Qda El Ajial [Peumo]
SM01 - Qda El Ajial [Peumo]
 SM02 - Qda Palmitas
SM02 - Qda Palmitas
 SM03 - Qda El Ajial [Palto]
SM03 - Qda El Ajial [Palto]
 SM04 - Morro de Chivato [Pedernal]
SM04 - Morro de Chivato [Pedernal]
 SM05 - El Sobrante:
SM05 - El Sobrante:
Fieldwork in the study area

Relevant references: