This article demonstrates how currently available digital elevation (NASA SRTM; 30 m resolution) and hourly global precipitation data (rain, snow; 5 and 10 km resolution) can be used to hydrographically quantify the regional watershed management context across northern Chile. This is done through comprehensive derivations of flow direction, flow accumulation, flow channels, floodplain extent, depressions, and upslope watershed outlines. In turn, these derivations allow for estimating potential precipitation accumulations within any watershed, and turn these into subsequent storm-averaged discharge estimates at, e.g., at any road—flow-channel crossing points. This article elaborates on this by modelling and mapping hydrological conditions and subsequent storm damage at the regional scale and at select locations in terms of recent flood events on March 2015, May 2017, and June 2017. It was found that modelled flood extent and storm-estimated discharge volumes and rates generally conform to reported values including storm-caused damages within communities along the Huasco, Elqui, Limari, Copiapó and Salado rivers. This included the storm response assessment of six water reservoirs as these varied, as quantified, from normal (Puclaro, La Laguna, Cogoti), at capacity (La Paloma, Cogoli), and overflowing (Recoleta). The details of the local to regional assessments are presented in the form hydrologically explicit maps, figures and tables. Together, these attest to the general validity of the framework as introduced. Hydrometrically based stream-discharge calibrations would assist in further refining the approach, especially in terms of estimating local to regional run-off coefficients.
Arid conditions combined with occasional heavy rainfall events are leading to catastrophic flashfloods, as have been reported for Northern Chile [
In view of recurring storm events, Chilean government departments, communities and industries have become increasingly mindful in reviewing community locations, transportation routes, landuse, industrial locations and activities, and placement of hydrological infrastructure [
1) ensuring socio-economic sustainability;
2) reducing negative impacts on human health;
3) stabilizing access routes (roads, railway tracks, powerlines, pipelines);
4) stabilizing mine tailings, including unconsolidated debris and dust piles;
5) protecting water supplies and communities;
6) reducing soil and sediment contamination;
7) conservingsalar ecologies.
In reference to flood-related damages including mobilizations of contaminants, [
This article focusses on quantifying the hydrographic flow channel, depression and watershed context across northern Chile in a locally to regionally comprehensive manner. This was done by way of combining established digital elevation modelling routines with NASA’s globally available precipitation distribution data in order to estimate amounts of water received and flowing across storm-impacted watersheds. In reference to storm-impacted accounts and analyses, emphasis is on building a comprehensive overview on where and how much water could potentially flow towards and through communities across northern Chile for any particular precipitation event.
The Northern Chile precipitation events for March 25-27, 2015 and May 12, 2017 were captured for the Arica, Tarapacá, Antofagasta, Atacama and Coquimbo regions using NASA’s georeferenced NEX-GDD Precipitation [
The re-projected digital elevation model (DEM) served to delineate local to regional flow networks using the D8 algorithm to determine flow direction and flow accumulation from the filled DEM [
Depressions were located by subtracting the non-filled DEM from the filled DEM. All depressions with significant water retention capacity up to their pour points were mapped by extent, depth, volume, and upslope watershed areas. The DEM-derived flow-channel networks were obtaining from the flow-accumulation raster by setting upstream flow-accumulation thresholds at 40, 400, 4000 and 40,000 ha, followed by raster to polyline conversion. This was done to emulate the extent to which the flow networks would advance towards or retract from higher elevations towards depressions and the coast by season and climate.
The extent of depression-terminated flow channels was emulated stepwise using increasing water-retaining volume capacity thresholds from 0.1 ML to 10 GL (1 ML = 103 m3; 1 GL = 106 m3) in order of magnitude steps. The capacity of the depressions to retain water was estimated by summing the 30 m × 30 m pixel depths across each depression. Flow channels were set to terminate at the lowest point in depression unless ready to overflow at the depression outlets.
The maximum amount of water potentially flowing along the DEM-derived flow channels was estimated by assigning the raster-based NASA-precipitation or rain amounts to each DEM raster cell, and summing these amounts along each flow accumulation path towards each water-receiving depression including the Pacific Ocean. The resulting sum was corrected by the amount of water retained within each upslope depression. The sum of water so accumulating was divided by storm duration to estimate the storm-average flow rates per event in m3・sec−1.
In principle, the numbers should serve as first-approximation location and event-specific flow-severity metrics. Hydrometric adjustments would be needed to account for, e.g., run-off diminishing evapotranspiration losses, snow sublimation, and soil and groundwater storage. Further adjustments may be required to account for actual amounts of water retained by upstream water reservoirs prior to each storm event, because SRTM-captured reservoir elevations are specific to the year of SRTM-DEM data capture (≈2000).
The extent of soil moisture regime and floodplain zonations next to the DEM-derived flow channels was determined using the cartographic depth-to-water (DTW) algorithm [
1) Potential flood extents and related risks to settlements and infrastructure, especially where DTW < 1 m;
2) Extent of arable land and related hydric to xeric soil moisture regimes, by climate regime and elevation; in general, the hydric to xeric elevation decreases from the Coquimbo region northward, with increasing elevation, and with decreasing flow accumulation. For example, the extent of arable land along most flow channels at elevations <2000 m from Copiapó southwards extends up to about 40 m above the main floodplain channels. Further north, flow channels and floodplains remain mostly arid unless subject to storm events. During those events, flood heights were found to reach up to DTW = 8 m (see below).
Other datalayers (shapefiles) relevant for establishing hydrographic context were obtained from the internet, i.e., Open Street Map (OSM) for roads and streams [
The flowchart in
1) upslope basin areas (in km2);
2) percent of floodplain coverage within upslope basins;
3) amount of precipitation (snow, rain) received per upslope basin as derived from NASA’s precipitation rasters;
4) corresponding estimate for average flow rates per storm duration by locations, assuming no water retention or loss per storm duration.
March 2015 Flood Event, Northern Chile
Presented in
depressions; 5) the locations of cities, towns, and mining activities. Note that the precipitation rates for this event were highest at elevations at >2000 m, while coastal areas received no to little precipitation.
Some of the water-retaining depressions across Northern Chile are presented in
Due to prevailingly arid climate conditions, most of the flow channel networks across northern Chile are endorheic. Rio Loa, however, is exorheic due to its deeply incised groundwater-receiving flow channel. Elsewhere, flow channels
terminate in salars, among which the depression at San Pedro de Atacama is the largest across the region, estimated to fill its 3255 GL capacity by only 0.16% during the March 2015 event.
Figures 6-9 provide close-ups of the DEM-derived flow-channel and floodplain delineations for the areas around Alto de Carmen, Vallenar, Freirina, Copiapo, Diego el Almagro, Chanaral and Antofagasta, with average discharge estimates at select road-stream/river crossings. For Alto de Carmen, Vallenar and Freirina along Rio Huasco, floodwaters would have risen quickly along the low-lying portions of the floodplains owing to steep adjacent basin terrain (
At Copiapó, the devastating mud-carrying flashflood (
Further north at Chanaral, at the mouth of Rio Salado (
hour event duration is 159.9 m3・sec−1, i.e. about one tenth of the peak flow rate estimate in [
The observed flood extent at Diego de Almagro [
The coastal area around the city of Antofagasta (
Event | Region | City/Town | River/Stream | Upslope basin area | Floodplain extent per basin | Upslope precipitation | Upslope rain | Average flow rate* |
---|---|---|---|---|---|---|---|---|
km2 | % | 106 m3 (GL) | m3 sec−1 | |||||
24-26 March 2015: 3 major discharge peaks within 51 hrs across basin areas | Antofagasta | Calama | Rio Lao | 8602 | 5.50 | 85.4 | 300.1 | |
Taltal | Quebrada del Taltal | 5517 | 5.20 | 185.1 | 306.1 | |||
Atacama | Chanaral | Quebrada del Salado | 8390 | 3.52 | 97.7 | 159.9 | ||
Diego de Almagro | Quebrada del Salado | 2393 | 2.86 | 45.4 | 75 | |||
Paipote | Quebrada de Paipote | 6660 | 1.86 | 132.5 | 219.1 | |||
Copiapo | Rio Copiapo | 17018 | 0.71 | 345.2 | 570.8 | |||
Coquimbo | Freirina | Rio Huasco | 9499 | 1.22 | 310.0 | 512.5 | ||
Vallenar | Rio Huasco | 9438 | 0.70 | 291.5 | 482 | |||
Alto del Carmen | Rio Del Carmen | 3038 | 0.57 | 75.8 | 125.4 | |||
Alto del Carmen | Rio Huasco | 4105 | 0.85 | 177.3 | 293.2 | |||
12 May 2017: 1 major discharge peak within 24 hrs across basin areas | Coquimbo | Pisco Elqui | 21.7 | 0.50 | 2.4 | 0.6 | 7.0 | |
Vicuna | Rio Elqui | 5946 | 0.88 | 442.6 | 61.4 | 710.3 | ||
La Serena | Rio Elqui | 9407 | 1.14 | 670.2 | 223.7 | 2589.0 | ||
Ovalle | Rio Limari | 9357 | 1.35 | 972.8 | 308.0 | 3565.3 | ||
Salamanca | Rio Chopa | 2234 | 1.14 | 249.8 | 27.3 | 315.6 | ||
Illapel | Rio Illapel | 1913 | 1.14 | 197.7 | 71.8 | 830.5 | ||
8 June 2017: 1 major discharge peak over 24 hrs across basin areas | Antofagasta | Antofagasta | Quebrada Caracoles | 40.5 | 1.68 | 1.0 | 11.0 | |
Antofagasta | Quebrada La Negra | 2106 | 3.26 | 52.7 | 585.0 |
**Average flow rate, assuming that upslope precipitation amount is discharged (no soil storage, no evapotranspiration) during the 51 or 24 yr event durations. Peak flow rates could be twice as high, approximately, as estimated fer Chanaral (Wilcox et al. 2016).
falling into the Rio Loa and Rio Salado basins above Calama would have entered groundwater reservoirs, with groundwater levels at Calama gradually rising about 2 to 3 months after the event [
G W L r i s e ( 2015 ) = G W L r i s e ( 2001 ) × V p p t ( 2015 ) / V p p t ( 2001 ) , (1)
where GWLrise (2001) = 34 cm, Vppt (2001) = 31.6 GL, and Vppt (2015) = 85.6 GL (from
Of special note is the SRTM-DEM mapped depression above the Rio Loa gorge east of Calama (
May 2017 Flood Event: Coquimbo Region
The Atacama and Coquimbo regions were again affected by heavy rains on May 12, 2017, causing loss of life, substantial damage to homes and roads, community isolation, and flood-compromised water supplies. The NASA-captured GPM rain- and snow-precipitation pattern, depicted for rain mm in
The estimated maximum average discharge rates set in
Three of the water reservoirs within the Rio Limari, Rio Elqui and Rio Choapa watersheds are estimated to have accommodated their upstream inflow on May 12, 2017, with the Laguna Reservoir basin receiving only snow, as detailed in
June 2017 Flood Event, Antofagasta Region
Water Reservoirs | Long. | Lat. | Elev. | Reservoir capacity | Basin area | Rain | Max. inflow (24 hrs) | Water stored prior to flood event3 | Inflow on top of water stored | Reservoir functioning | |||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Decimal degrees | m | GL | km2 | mm | m3・sec−1 | GL | GL | Capacity % | Capacity % | ||||||
Rio Elqui | Puclaro | −70.861 | −29.994 | 435 | 210 | 3091 | 37 | 1043 | 116 | 55.0 | 45.0 | 87.9 | Normal | ||
La Laguna1 | −70.040 | −30.206 | 3141 | 40.1 | 0 | 0 | 0 | 0.0 | 58.1 | 58.1 | Normal | ||||
Rio Limari | La Paloma | −71.032 | −30.704 | 383 | 750 | 4683 | 35 | 1897 | 164 | 21.9 | 89.1 | 111.0 | At capacity | ||
Recoleta2 | −71.093 | −30.496 | 448 | 100 | 1570 | 45 | 818 | 71 | 70.6 | 89.4 | 160.0 | Overflow | |||
Cogoti | −71.091 | −31.026 | 634 | 150 | 1601 | 45 | 834 | 72 | 48.0 | 59.0 | 107.0 | At capacity | |||
Rio Choapa | Corrales | −70.914 | −31.911 | 692 | 50.6 | 270 | 18 | 57 | 4.9 | 9.7 | 60.8 | 70.5 | Normal | ||
1No discharge on account of snow only. 2Overflowed May 12, 2017, with partial collapse of an upper section. 3Amount of water stored, based on filled versus non-filled SRTM-DEM data collected in 2000.
Another severe rainfall occurred across the Antofagasta region on June 8, 2017. Details about the rainfall distribution of this event and the flow-channel pattern across the hill-shaded watershed areas upslope from Antofagasta are presented in
In quantitative terms, the DEM-derived estimates for the event-specific stream discharge rates (y, in m3・sec−1) across the Antofagasta watershed areas vary by minimum water-retaining depression volumes (x, in GL) as follows:
y = 307.9 + 34.8 log 10 x ; R 2 = 0.979 . (2)
For the northern portion, overall depression-based water retention would be small, and would therefore do not depend in x in a major way, i.e.:
y = 8.25 + 0.02 log 10 x ; R 2 = 0.10 (3)
These equations were obtained through GIS and regression analysis based on systematically determining y by decreasing the water-retention volume threshold per depression (x) from 10 GL to 0.1 ML. The largest water retaining volume capacity amounted to 0.28 GL in the northern section at the watershed divide leading eastward towards Salar del Carmen, and 4.22 GL in the southeastern floodplain at the watershed divide along the Pan American Highway, with Oficina Rosario located due east.
Further Comments
The SRTM-DEM derived flow networks not only conform to varying degrees with already mapped river and stream delineations (GIS-DIVA), but also extend these delineations with greater accuracy towards and upwards into the many valleys of the Andean mountain range towards the east. Typically, the channel-to-channel distance conformance between already mapped and the SRTM-DEM derived flow channels is <100 m, 8 times out of 10 (details not shown). There is also a close correspondence between image and DEM-derived floodplain extent with the threshold for upslope floodplain flow-initiation area set at 400 ha. Terrace heights above the flow-channels with the floodplains can be varied, as these increase with increasing flow accumulation, up to about DTW = 40 m. In addition, within the Coquimbo region, valley soils tend to support vegetation growth up to about DTW = 40 m within the Coquimbo region.
Through the overlays and processing of precipitation patterns, watershed basins, floodplains, depressions flow channels, roads and railways, it is now possible to estimate:
1) maximum amounts of water transmitted for the upslope watershed areas at any flow-channel point of interest;
2) storm-average maximum run-off rates at any flow-channel location per duration of storm event;
3) approximate flood extent within the lower lying floodplain portions, done by varying the DTW threshold away from the floodplain stream channels;
4) DEM-determined water retention capacities of, e.g., water reservoirs, open-pit mines, quarries, and tailing ponds.
For the context of any particular rainfall event, the information so derived can be used to gauge existing and required infrastructure requirements to withstand actual and projected storm events. Of general importance in this regard is determining the relationship between maximum average potential run-off rates per storm event and the corresponding water retention threshold based on, e.g., depressions and other water-retaining features within watersheds. For this purpose, and to achieve greater accuracies, it is necessary to determine basin-specific relationships between peak and average flow rates per storm duration by way of hydrometric calibrations. Doing so will assist in determining storm- and basin-specific run-off coefficients as they would change by storm event and by antecedent soil, groundwater, and reservoir conditions. For example, the June 2017 run-off coefficient pertaining to the Quebrada La Negra watershed south of Antofagasta was likely near zero, since the water would have mostly been depression-retained through soil and groundwater retention across the far-reaching upslope floodplain complex. Similarly, the March 2015 and May 2017 storm events may or may not have contributed water to Salar del Carmen east of Antofagasta.
Much additional progress in terms of hydro-spatial analyses will likely accrue through processing higher resolution DEMs, either obtained through fusing already existing DEM layers (e.g. SRTM, ASTER, elevation contours), and/or using airborne or satellite LiDAR-based bare-earth DEMs [
It is suggested that the DEM-based framework for guiding incoming precipitation through flow channels, floodplains, and depressions could find many practical as well as socio-economic applications to facilitate the planning and management of storm events and water supplies across northern Chile [
This work was supported through Forest Watershed Research Centre activities at the Faculty of Forest and Environmental Management, UNB, Fredericton, New Brunswick, Canada.
The authors declare no conflicts of interest regarding the publication of this paper.
Moran, G., Cuadra, P.P., Arp, J.-P. and Arp, P.A. (2018) Establishing a Hydrographic Framework for Watershed Management across Northern Chile. Journal of Geographic Information System, 10, 539-561. https://doi.org/10.4236/jgis.2018.105029