Download Hydrological and geomorphological consequences of the extreme

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Hydrological and geomorphological consequences of the
extreme precipitation event of 24–26 March 2015, Chile
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Teresa Jordan , Rodrigo Riquelme , Gabriel González , Christian Herrera , Linda Godfrey , Steve Colucci , Jorge
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Gironás León , , Carolina Gamboa , Javier Urrutia , Lorenzo Tapia , Karen Centella , Hector Ramos
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Cornell University, Ithaca, NY 14853 USA
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Universidad Católica del Norte, Antofagasta, Chile
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Centro Nacional de Investigación para la Gestión Integrada de Desastres Naturales, Macul, Chile
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Rutgers University, Piscataway New Jersey 08854 USA
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Departamento Ingeniería Hidráulica y Ambiental, Pontifícia Universidad Católica, Santiago, Chile
contact email: [email protected]
Abstract. In the hyperarid Atacama Desert, all precipitation
events afford valuable opportunities to examine the shortterm responses of the landscape, surface water and aquifer
systems, ecosystem and soils to rare water input. The
precipitation event of 24-26 March 2015 impacted Chile
and Argentina between 22° and 32°S, an area exceeding
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200,000 km . The greatest precipitation occurred in the
highest regions of the Domeyko Range and the Main
Andean Range, where it widely exceeded 50 mm. A
secondary focus of precipitation was an area along the
coast from Antofagasta to Taltal. North of 27°S,
reconnaissance observations revealed remarkably little
erosion or deposition, with notable exceptions along the
Río Salado and the Río Chaco, where destruction of human
constructions resulted because these rivers funnel water
from the eastern highlands. North of 27°S, zones of low
surface slope and with weakly consolidated cover materials
absorbed the rainfall. In contrast, surfaces within the area
of heavy precipitation that were dominated by steep slopes
and bare rock experienced surface runoff.
Keywords. Atacama desert, extreme rain
geomorphology, Chile, flooding, upper level low
event,
1 Introduction
Precipitation over an area exceeding 200,000 km2 affected
three Chilean regions (Antofagasta, Atacama, Coquimbo)
and neighboring highlands in Argentina from March 24-26,
2015. Chilean communities located within and at the
mouths of canyons whose rivers are fed by precipitation in
the Andes suffered catastrophic flooding. Outside of those
canyons, highways and pipelines were less damaged.
Surprisingly, over the northern half of the affected area,
the extreme rainfall produced little impact to the land
surface and to coastal cities.
Rainfall and snow on the continental surface impacted an
area from north of Tocopilla to south of Illapel (~22°–
32°S), from the Pacific shoreline on the west to east of the
Andean crestline along the Chilean-Argentine border (Fig.
1). This rain event spanned six major geomorphological
zones (Coastal Escarpment, Coastal Cordillera, Central
777 Figure 1. Landforms and locations (squares) of precipitation
data for 24-26 March 2015 (data in mm). Lines of equal
precipitation (green) drawn based on the control points and
general topographic form. White dashed lines mark
physiographic boundaries. Data from Dirección Meteorológica de
Chile, Dirección General de Agua, Universidad Católica del
Norte, Weather Underground, Antofagasta Minerals, SQM,
Minera Meridian, and authors. Snow cover from MODIS image.
1 SIM 12 LOS ALUVIONES DE ATACAMA, CONTEXTO, CAUSAS Y EFECTOS
Depression, Domeyko Range, Pre- Andean basins, and
Main Andean Range) and at least two broad categories of
distinctive soils. The opportunities to examine both
geomorphological
consequences
and
potential
groundwater
recharge
motivated
an
integrated
investigation of the event.
This paper reports the results of field reconnaissance
(Coastal Cordillera and Central Depression, 29 March;
Central Depression, Domeyko Range and Pre-Andean
basins, 2-5 April), soil moisture studies (see also Tapia et
al., this Congreso), and compilation of meteorological data.
2 Atmospheric Conditions
The 24-26 March 2015 precipitation event was a synopticscale weather system centered in the eastern Pacific Ocean
region. Figure 2A shows an atmospheric pressure map at a
middle horizon in the atmosphere, with closed pressure
contours that
3
Spatial
Precipitation
Patterns
of
Cumulative
Reported rainfall varied from ~10 mm at the coast to >85
mm in the eastern mountains. Snow fell over a wide area
above 3600 m (Fig. 1, cross-hatchured). The elevation of
the rain-snow threshold increased from south to north.
Data for rainfall were compiled from meteorological
stations maintained by government agencies, mining
companies, and university programs (Fig. 1). Three other
control points for liquid precipitation amount come from
rain capture vessels placed (August 2014) in and near the
Alto de Varas (Fig. 1, red polygon). Based on the control
points and on interpreted relationships between
precipitation totals and topographic form, we interpolated
spatially to demonstrate regional variations in total
precipitation (Fig. 1, green lines).
The region between 24°–27°S received the most
widespread heavy rainfall (50–90 mm), spanning from the
Pacific coast to the highest parts of the catchment basins
that drain westward. To the south, rainfall in excess of 50
mm was recorded only in the eastern mountains above
~3000 m.
4 Temporal Pattern of Rainfall and River
Flow
Figure 2. Height above sea level, in meters, of the (A) 500-mb
pressure surface and (B) 850 mb surface at 8 PM EDT on March
24, 2015. At that time, rain had begun onshore. Blue arrow shows
surface wind direction.
For stations reporting hourly rainfall near the coast (Taltal),
in the Central Depression (El Peñon mine) and in northern
parts of the Main Andean Range, the precipitation spanned
approximately 48 hours and displayed multiple peaks (Fig.
3). For the coastal location with the greatest total
precipitation (Taltal, ~67 mm), the rain fell mostly during
a 24 hour period, with individual hourly precipitation as
great as ~11 mm/hr. El Peñon mine, where total rainfall
(38 mm) was typical of a wide region west of the
Domeyko Range (Fig. 1), experienced 5 separate hours
during which the rate of rainfall was >2 mm/hr. In the
Main Andean Range farther south (Pastillo, Fig. 3), the
rain began earlier, rates of 4–8 mm/hr fell during 5 hours,
and additional rain fell on March 27.
indicate a cold upper level low centered near 30°S, 75°W
(also referred to in the media as a cut-off low). Figure 2B
shows the corresponding pressure variations at a lower
height in the atmosphere, with a more longitudinally
extensive low pressure zone that stretched from 15°S to
34°S. The winds on the east side of this low altitude, low
pressure zone blew from the north toward the south along
the coastal region, advecting warm and humid air from the
tropical Pacific (near 15°S) to northern Chile.
Figure 3. Hourly precipitation totals and discharge curve for Río
Copiapó at DGA station Pastillo, for 24 – 26 March, 2015. Taltal
(25.4°S) is a coastal site, the El Peñon mine (24.4°S) is in the
Central Depression, and Pastillo (28°S) is in the Main Andean
Range.
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AT 3 geología del cuaternario y cambio climático
An example of river response is the Copiapó River in the
Main Andean Range (Figure 3; DGA Pastillo gauging
station). The flows recorded at times of peak discharge are
minima, as the river gauge design was exceeded (personal
communication, Dirección de Obras Hidráulicas). The
river responded to the first rain pulse with a modest
increase in flow that first crested 9 hours after the peak
rainfall. In response to onset of the second rainfall peak,
the discharge increased markedly after only 4 hours,
suggestive of more efficient hillslope runoff from the
already wet slopes.
5 Spatial variability of impacts
Reconnaissance of the Coastal Cordillera near Antofagasta,
Taltal and Chañaral (23°40’S–26°20’S), as well as
reconnaissance from 22°30’–25°30’S of the Central
Depression, Domeyko Range, and Pre-Andean Basins
revealed very little evidence of surface runoff. Except in
the major river valleys, puddles of surface water
accumulated only adjacent to or within areas of human
disturbance (e.g., roads; off-road vehicle trails) of the
landscape.
This generality has two important exceptions. First, in
canyons that drain the Domeyko Range, there was
widespread evidence of high discharge and local flooding.
Nevertheless, not all discharge within secondary streams
reached the trunk streams, as it progressively infiltrated the
bed. In some minor canyons draining the Alto de Varas,
surface flow ceased within a few kilometers west of the
mountain range. Second, in regions of steep slope within
the Alto de Varas (Fig. 1, red polygon), hillslopes with a
thin soil cover displayed freshly activated surface rills, and
runoff is inferred to have occurred across neighboring
bedrock surfaces.
Absent evidence of surface runoff, the precipitation is
inferred to have infiltrated the near-surface materials.
Infiltration to tens of centimeters depth by nine days after
the rain ceased was confirmed for 9 hand-dug soil pits.
These ~60 cm deep pits include seven located in the
eastern Central Depression, one in the Alto de Varas, and
one in the Pre-Andean Basins region (Tapia et al., this
Congress).
We anticipated that there would be a difference between
the relative degrees of surface runoff versus water
infiltration of areas covered by calcium sulfate soils
compared to areas with immature clay-based soils or active
siliciclastic deposits. In the hyperarid regions north of
27°S, calcium sulfate- dominated soils are widespread in
the Central Depression and Coastal Cordillera (Fig. 4,
west of yellow line) (Ewing et al., 2006). The field
reconnaissance revealed that the precipitation response of
surfaces in both regions was infiltration, irrespective of
whether the surface was mantled with gypsum-anhydrite
779 soil or with siliciclastic sediment or with incipient soils of
clay minerals.
6 Discussion
Surface runoff led to the devastating floods in cities and
villages that are located very close to river beds in the
major valleys. Those valleys are the long-term landscape
consequence of repeated major rainfall events. The region
with highest rainfall totals included eastern catchments that
supply streamflow to severely damaged cities: the Río
Chaco (Taltal), Río Salado (Chañaral) and Río Copiapó
(Copiapó).
The extent of surface runoff correlates to the surface slope.
Greater extents of bedrock outcrop and a general lack of
development of soils typify areas of steep surface slope.
North of 27°S, surface slopes of 10°–25° are common in
the eastern Domeyko Range (Fig. 4). South of 27°S,
surface slopes that exceed 10° dominate all areas from the
Pacific coast to the Andean crest line. Our observations
and the extent of flooding in the Río Salado valley are
consistent with the operation during the March 2015 event
of a slope-dependent differentiation between rain
infiltration and overland runoff. We infer that the majority
of the landscape with slopes <7° (Fig. 4, pale blue, blue
and green) absorbed the rain into weakly consolidated
surficial materials, whether they are sedimentary materials,
clay-rich soils, or gypsum-anhydrite soils. Apparently
there was a threshold of surface slope above which there
was extensive surface runoff into channels, and some of
those channels collected the storm waters into the major
rivers. Although we estimate that much of the landscape
with surface slopes >10° (orange, red, pink; Fig. 4)
experienced surface runoff, we do not know the slope
corresponding to the runoff threshold.
Human disturbance (e.g., paving; movement of vehicles;
construction activity) decreases the permeability of the
layers of unconsolidated sediment or soil. Even in areas
where infiltration was the dominant natural response to the
March rain fall, runoff occurred in off-road vehicle tracks.
Both the 24-26 March 2015 precipitation event and the 1718 June 1991 Antofagasta-Taltal extreme rain event
occurred during weak El Niño states, but they differed in
two ways. First, the rate of precipitation differed: in June
1991, 24 mm of rain fell in 2–3 hours in Antofagasta
(Vargas and Ortlieb, 2000) whereas in March 2015 more
rain fell but over a protracted period of time (Fig. 3, Taltal).
This distinction seems to have enabled more effective
infiltration in March 2015, avoiding formation on the
Coastal Escarpment hillslopes of destructive debris flows
like those of 1991. Second, the season of the event was
cold in 1991 but warm in 2015. Typical cut-off lows
impact northern Chile (Vuille and Ammann, 1997;
3 SIM 12 LOS ALUVIONES DE ATACAMA, CONTEXTO, CAUSAS Y EFECTOS
There is considerable interest in the prediction of
precipitation patterns under 21st century conditions of
rising global temperature, and in prediction of the
hydrological consequences. To make useful predictions
requires knowledge of the historical hydrometeorology
regime as well as the historical stream hydrology. For the
Atacama Desert there exist abundant data from which to
derive a statistically robust description of the typical
climate, to characterize its seasonal weather conditions,
and to describe seasonal variations in stream discharge.
But precipitation is not a part of those typical conditions,
and data with which to characterize past precipitation
extreme events are very sparse. There have been very few
documented rainfall events (e.g., Ortlieb [1995] found
record of ~50 extreme precipitation events between 1776–
1992 in the Atacama between 18-24°S). Consequently, the
characteristics of a typical rainfall event cannot be
described in a statistically meaningful manner. Whereas
24-26 March 2015 provides a singular event with very
useful information, neither it nor other well documented
precipitation events (e.g., 17-18 June 1991) provides a
sufficient template against which to predict changes in the
precipitation regime under future climate scenarios.
Acknowledgements
We thank CONICYT Anillos de Investigación en Ciencia
y Tecnología 2012, and CONICYT FONDAP 2011
Nº15110017 center CIGIDEN for support. Antofagasta
Minerals, SQM, Minera Meridian, and José Cerdo M.
(UCN) provided data. Mauricio Pulgar and Ian del Río
(UCN) provided technical assistance.
References Cited
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