Fathom Geophysics Newsletter 26

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Our Capabilities: Water and land resources baseline studies

ENVIRONMENTAL baseline conditions such as the timing and distribution of rain, snow, vegetation growth, and lake-level changes can be systematically mapped, analyzed, and monitored using publicly available remote-sensed datasets and satellite imagery. We briefly describe how in this abstract of a full case study in Argentina.

A great deal of work goes into establishing the feasibility of a mineral project.

For instance, ascertaining the availability of an adequate ore-processing water supply and quantifying how nearby water resources and precipitation patterns typically naturally fluctuate over the span of many months and years are examples of some of the important steps involved in carrying out careful economic and environmental due diligence work. [1] [2]

Establishing environmental baselines in the region surrounding a given mineral project also allows land users to monitor continuing weather and vegetation patterns as they unfold — and therefore to better anticipate and quickly respond to situations such as the threat of a looming water-lean year.

When it comes to water-supply and land-use studies, satellite datasets can assist with, for example:

  • snowfall and rainfall estimation
  • lake level monitoring
  • vegetation monitoring
  • subsurface water detection
  • land disturbance evaluation
  • preliminary reservoir-site optimization analysis

These analyses can be done at individual-catchment scales or on a more regional scale. The Taca Taca case study we summarize here in this abstract is a catchment-scale example.

Taca Taca, northwest Argentina

Lumina Copper Corp's/First Quantum Minerals' Taca Taca project is a porphyry copper development project situated in northwest Argentina. It sits roughly 200 kilometers south of the triple junction of the Bolivian, Argentinian, and Chilean borders.

The Taca Taca area — located in the southern central Andes orogenic plateau at a latitude of about 24.5 degrees South — is physiographically stranded from moisture-laden weather systems that dominate regions further to the east. [3]

Regions and processes favorable for prolonged tectonic uplift, metallogenic activity, and good preservation of ore bodies tend to create relatively arid, low-erosion environments, thanks to the creation of vast, topographic rain shadows. [3] [4]

Such regions also generally involve a salt-pan-festooned plateau of thick sedimentary basins that are rather chaotically drained by local and internal water-transport features. This is in marked contrast to the organized stream networks found incising the land in more precipitation-rich regions.

It means that when considering mine development in semiarid highlands, it's important to understand the specifics of hydrological transport and storage.

Taca Taca catchments and data

For the purposes of water-resources study, the Taca Taca project area initially included four main catchments defined by the location of watersheds (Figure 1). The names of those main catchments are Pulares, Socompa, Pluma Verde, and Vega de Arizaro.

The project study area was then widened to include four additional, more distant catchments: Vega de las Burras, Vega de Cori, Vega de Chaschas, and Antofalla.

Figure 1Figure 1: Topographic map showing the Taca Taca water-studies catchments (outlined as black polygons), named Pulares, Socompa, Pluma Verde, Vega de Arizaro, Vega de las Burras, Vega de Cori, Vega de Chaschas, and Antofalla. Map data source: SRTM. To view a larger version, click on either the image or this text link.

As part of environmental monitoring work carried out in the Taca Taca region, we used remote-sensed data to directly estimate liquid and solid precipitation at locations throughout the study area. This information was then used in subsequent calculations by hydrology specialists to arrive at runoff estimates.

Among the data we examined for the Taca Taca case-study catchments were scenes from the multispectral satellites MODIS (Moderate Resolution Imaging Spectroradiometer), Landsat 5, and Landsat 8. MODIS Terra supplied daily snowfall-related data at a resolution of 500 meters, while Landsat supplied snowfall, lake-level, and vegetation-related data every 16 days at a resolution of 30 meters. We also used IMERG (Integrated Multi-satellitE Retrievals for Global precipitation measurement), along with the ALOS (Advanced Land Observing Satellite) World 3D 30m (AW3D30) digital surface model, which was used in runoff evaluation and reservoir site analysis. We used ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) thermal infrared data to highlight locales in the study area that show potential for hosting near-surface fresh water.

Precipitation-related datasets we also considered included CMORPH (Climate Prediction Center Morphing by NOAA), GSMaP (Global Satellite Mapping of Precipitation) by JAXA, and GPM (Global Precipitation Measurement) by NASA/JAXA, among others. [5]

At first, we worked with CMORPH data because at the time it had the most detailed resolution and was in greatest agreement with ground-based weather-station snow pillow data and satellite-detected snowfall data.

But from mid-2019 onwards, improvements to IMERG snow and rain estimates data led to it being our preferred dataset (as at the time of this writing). IMERG data are produced by a joint NASA/JAXA mission that combines passive microwave and infrared satellite measurements to estimate rainfall and snowfall. The IMERG algorithm takes the precipitation events identifiable in passive microwave data and interpolates between passive-microwave readings using infrared data that are collected by geostationary satellites that have continuous global coverage.

The resulting IMERG dataset yields global precipitation estimates every 30 minutes with a resolution of approximately 10 kilometers.

The existence of all of these datasets is very good news for those wanting access to potentially useful environmental-monitoring data. But in order for project managers to properly make use of all of this data, huge volumes of information need to be assessed, pulled in, sorted, calibrated, summarized, synthesized, and presented in a meaningful way.

In this abstract we summarize our approach to meaningful IMERG snowfall and rainfall data analysis — in particular, how these types of precipitation seem to be influenced by prevailing cyclical weather teleconnections (linkages).

Snowfall monitoring

Snowfall, in the parts of the world where it occurs, is a hydrological factor relevant to monitoring and forecasting amounts of surface-water runoff versus groundwater recharge, drought periods, and flood events.

Snowfall assessment is also of particular importance to projects situated in mid-latitude to high-latitude locations, and to projects in or near mountainous terrain.

As part of our Taca Taca case study, we compared remote-sensed precipitation estimates to ground-based weather-station snow-pillow and rain-guage data, and used the comparison results to calibrate the remote-sensed data.

Plots showing daily IMERG snowfall estimates summed across the four main Taca Taca study catchments (Pulares, Socompa, Pluma Verde, and Vega de Arizaro) reveal ever-changing timings and sizes of snowfall events during the period from 2000 to 2017 (Figure 2).

Figure 2Figure 2: TOP: Daily IMERG snowfall estimates (in rain-equivalent millimeters; orange plot-line) through time at the Taca Taca water-studies project. Plot shows summed estimates for the four main Taca Taca catchments (Pulares, Socompa, Pluma Verde, and Vega de Arizaro) and starts at 1 June 2000 and ends at 31 December 2017. BOTTOM: The same graph, this time also showing each year's total snowfall estimate (orange squares), centered above the study area's meteorological year, which we have defined to be November 1 through to October 31. Note the different x-axis scale. Data source: NASA/JAXA. To view a larger version, click on either the image or this text link.

On a year-by-year cumulative basis from calendar 2000 to calendar 2019, the Pulares catchment in the Taca Taca area's northernmost parts received the majority of detected snowfall (see black-outline polygon that is almost completely filled with orange and red pixels in Figure 3). This kind of geographic-dispersion information can be extremely useful to water-resources planner

Figure 3Figure 3: Cumulative distribution of IMERG snowfall estimates through time from calendar 2000 to 2019 at the Taca Taca water-studies project. A quantitative legend was produced to accompany this image, but has been omitted from this write-up. Reds and oranges indicate areas receiving greater snowfall amounts, while greens and blues indicate areas receiving lesser snowfall amounts. Yellows indicate areas receiving intermediate snowfall amounts. Data source: NASA/JAXA. To view a larger version, click on either the image or this text link.

Yearly cumulative plots of daily IMERG snowfall estimates detected over individual catchments can reveal further useful details (Figure 4).

Figure 4Figure 4: A comparison of yearly cumulative snowfall plots for selected calendar years. This figure shows some of the variability in the pattern of IMERG rainfall-estimate increments for the overall study period of 2000 to 2017 for the eight catchments at the Taca Taca water-studies project area. Plots show three examples of relatively high snowfall years (plots at left, showing the years 2005, 2011, and 2017), moderate snowfall years (plots at middle, showing the years 2002, 2012, and 2016), and relatively low snowfall years (plots at right, showing the years 2004, 2007, and 2009). Individual plot lines represent each of the catchments studied (olive green line is Antofalla, orange line is Pluma Verde, magenta line is Pulares, brown line is Socompa, lime green line is Vega de Arizaro, pink line is Vega de Chaschas, black line is Vega de Cori, and purple line is Vega de las Burras). Data source: NASA/JAXA. To view a larger version, click on either the image or this text link.

Overall, the IMERG data suggest that the southernmost catchments in the Taca Taca study area (Antofalla, Vega de Chaschas, and Vega de Cori) tend to remain relatively devoid of snowfall events.

Note that IMERG snowfall estimates may underestimate the true snowfall received at any given location.

Rainfall monitoring

In the same way that snowfall-monitoring data is relevent in applied-hydrology studies, rainfall is an another important factor to consider when making freshwater recharge estimates for given catchments in water resources-related environmental studies.

The Taca Taca case study area tends to receive most of its annual precipitation during the southern hemisphere's summer months of December, January and February.

Plots showing daily IMERG rainfall estimates summed across the four main Taca Taca study catchments (Pulares, Socompa, Pluma Verde, and Vega de Arizaro) reveal continually evolving growth and decay trends in rainfall events during the period from 2000 to 2017 (Figure 5).

Figure 5Figure 5: TOP: Daily IMERG rainfall estimates (in millimeters; blue plot-line) through time at the Taca Taca water-studies project. Plot shows summed estimates for the four main Taca Taca catchments (Pulares, Socompa, Pluma Verde, and Vega de Arizaro) and starts at 1 June 2000 and ends at 31 December 2017. BOTTOM: The same graph, this time also showing each year's total rainfall estimate (blue circles), centered above the study area's meteorological year, which we have defined to be November 1 through to October 31. Note the different x-axis scale. Data source: NASA/JAXA. To view a larger version, click on either the image or this text link.

On a year-by-year cumulative basis from calendar 2000 to calendar 2019, we see that the Pulares and Vega de las Burras catchments in the Taca Taca area's north hogged much of detected rainfall (see the two northernmost black-outlined polygons in Figure 6). Water-resources planners need to know which catchments tend to receive what levels of precipitation. And they need to keep track of any significant rainfall-distribution changes unfolding over time if they are to successfully respond to these kinds of evolving circumstances in project-critical water supplies.

Figure 6Figure 6: Cumulative distribution of IMERG rainfall estimates through time from calendar 2000 to 2019 at the Taca Taca water-studies project. A quantitative legend was produced to accompany this image, but has been omitted from this write-up. Reds and oranges indicate areas receiving greater rainfall amounts, while greens and blues indicate areas receiving lesser rainfall amounts. Yellows indicate areas receiving intermediate rainfall amounts. Data source: NASA/JAXA. To view a larger version, click on either the image or this text link.

Yearly cumulative plots of daily IMERG rainfall estimates detected over individual catchments can reveal further useful details (Figure 7).

Figure 7Figure 7: A comparison of cumulative rainfall plots for selected calendar years. This figure shows some of the variability in the pattern of IMERG rainfall-estimate increments for the overall study period of 2000 to 2017 for the eight catchments at the Taca Taca water-studies project area. Plots show three examples of relatively high rainfall years (plots at left, showing the years 2001, 2008, and 2012), moderate rainfall years (plots at middle, showing the years 2006, 2011, and 2017), and relatively low rainfall years (plots at right, showing the years 2004, 2009, and 2016). Individual plot lines represent each of the catchments studied (olive green line is Antofalla, orange line is Pluma Verde, magenta line is Pulares, brown line is Socompa, lime green line is Vega de Arizaro, pink line is Vega de Chaschas, black line is Vega de Cori, and purple line is Vega de las Burras). Data source: NASA/JAXA. To view a larger version, click on either the image or this text link.

Having access to environmental monitoring data (such as rainfall and snowfall estimates) by itself can be a very useful thing for mineral project planners and custodians.

But the value of such data access would be boosted if noticeable data trends or patterns can be explained by bigger-picture phenomena, such as meteorological patterns.

El Nino Southern Oscillation and Madden-Julian Oscillation

One well-studied meteorological phenomenon is the El Nino Southern Oscillation (ENSO) teleconnection.

The term teleconnection refers to patterns of meteorological fluctuations that are linked across various locations that are geographically separated. [6]

In the case of ENSO, meteorologically-relevant activity taking place in the tropics, such as vertical ocean circulation-induced periodicity in sea surface temperature fluctuations across the east-to-west span of the Pacific Ocean, ultimately affect storm tracks over distant mid-latitude regions, such as the Andes plateau in the vicinity of Taca Taca.

Another globally significant weather pattern that seems to influence Taca Taca area's precipitation supply is the Madden-Julian Oscillation (MJO).

The MJO describes the cyclic behavior of an eastward propagating region of enhanced equatorial rainfall over a warm oceanic pool as the pool's location moves from the Indian Ocean to the western Pacific Ocean, then to South America and Africa, over a period that can be as short as 30 days, or as long as 60 days. Outside of the region of enhanced rainfall is a region of suppressed rainfall.

The MJO's influence on rainfall enhancement and suppression extends to tropical and temperate South America, because the MJO creates high and low sea-level air pressure anomalies, along with clockwise and anticlockwise 200 hPa streamfunction anomalies that sweep around the entire equator and create poleward wave trains of anomalies too. [7]

The Bolivian High and Taca Taca rainfall

How exactly might the ENSO and MJO teleconnections be influencing precipitation in the Taca Taca region? The answer is the long-lived summertime meteorological feature known as the Bolivian High.

The broader central Andes region receives the majority of annual precipitation in December to March because these are the months when mid-level and upper-level easterly winds intensify. Their intensification is caused by the strengthening at this time of year of an anticyclonic eddy known as the Bolivian High. What strengthens this long-lived high is latent heat released during deep monsoon-related convection over the Amazon Basin. [8] [9]

The further south the Bolivian High sits, the more that enhanced easterly flow in the upper-air circulation over the central Andes is favored. This in turns favors greater entrainment of monsoonal Amazon Basin moisture into the central Andes, and greater levels of precipitation there. [10] [11]

The prevailing ENSO phase (La Nina, El Nino, or Neutral) has a significant impact on the shape, location and behavior of the Bolivian High. In turn this impacts how much or how little precipitation the Taca Taca region 'sees'.

In a La Nina summer, the cooler sea surface temperatures found in the eastern tropical Pacific Ocean means there is reduced meridional baroclinicity (i.e., less atmospheric instability) between tropical and subtropical latitudes, which leads to a weakening of the upper-air westerly flows above the central Andes. This permits the Bolivian High to venture further south than it might otherwise and to deliver more precipitation to the central Andes, all else being equal.

The converse situation happens in an El Nino summer, leading to below-average central Andes precipitation in that case. [10]

Taca Taca snowfall in winter and spring

The Bolivian High disappears in the southern hemisphere's winter, and upper-level westerly winds dominate South America during this period. [8]

In general, wintertime precipitation in central and southern Chile is associated with cold fronts arching equatorward from a surface low moving along the persistent storm track existing to the south of 40 degrees South. [12] [13] [14] [15]

Large springtime snowfall events also appear to be related to offshore surface pressure lows adjacent to the Chilean coastline at latitudes near Taca Taca. However, instead of being 'flung' northward from the south, the lows responsible for Taca Taca's spring snowfall seem to be 'injected' southward from the north.

Conclusions about Taca Taca

Overall conclusions we've made with the help of environmental-monitoring data collation and analysis include:

  • The Taca Taca study area receives much greater than average precipitation during its December-January-February wet season when the summertime monsoon-related Bolivian High establishes itself as an intense, compact, symmetrically-rounded anticyclonic eddy that is able to migrate as far to the south and to the west of the South American continent as possible. In such a scenario, the strong westerlies seen during the wintertime have eased off considerably. All of these characteristics tend to develop to the greatest extent during strong La Nina events, thanks to the counteracting effect of the strong trade winds traveling westward above the Pacific Ocean.
  • In contrast, markedly lower-than-average precipitation tends to be experienced in Taca Taca's wet season when the Bolivian High is a less intense, elliptical-looking eddy with a northwest-to-southeast-trending major axis that is restricted to locations over the South American continent that are noticeably further to the north and east. The strong westerly winds seen during the colder half of the year tend to continue relatively unabated. All of this tends to happen during strong El Nino events due to relatively weak trade winds prevailing above the Pacific Ocean.
  • Wet seasons in which neither a clear La Nina phase nor a clear El Nino phase is active — hybrid events — may involve moderate amounts of precipitation at Taca Taca. Generally speaking, we expect that if the hybrid event involves neutral ENSO indices that seem situated nearer to scientifically defined El Nino thresholds, then Taca Taca's cumulative rainfall total may skew towards a slightly-drier-than-average wet season. And if the hybrid event's indices seem nearer to the La Nina threshold, a slightly-wetter-than-average wet season may happen.
  • During Taca Taca's June-July-August winter, noteworthy snowfall events seem to be triggered by the combination of (1) arrival of strong and relatively moisture-laden corridor of high- and mid-level winds (sometimes referred to as an atmospheric river), (2) a fleeting north-directed and eastward-propagating incursion of frigid air originating from the regional-scale sinusoidal fluctuations (with a roughly 1-week-long peak-to-peak period) of the interface between the Antarctic circumpolar vortex and the southern hemisphere's mid-latitude system of westerly winds, and (3) development of a northward-propagating cutoff-low pressure system that reaches landfall at central Chilean latitudes. The frequency with which these two conditions co-occur seems quite low, which may explain why only a handful of noteworthy snowfall events occurs in any given relatively-snow-rich winter at Taca Taca.
  • Snowfall events occurring at Taca Taca early in the southern hemisphere's September-October-November spring seem to form under circumstances similar to wintertime snowfall events, but with one important difference: low pressure systems seem to originate from equatorial source regions.

Additional teleconnections may also be relevant to the weather prevailing on any given day, week, and month at the Taca Taca project area.

Atmospheric soundings catch teleconnections in action

Given that finalized IMERG global precipitation data involves (at the time of this writing) a roughly 10-week delay from when data are collected to when research-quality data are released by government agencies, the wind direction and wind speed data found in meteorological Skew-T plots showing upper-air atmospheric soundings (obtained from daily weather-sensor observations via radiosonde) may help project managers make day-to-day and week-to-week environmental monitoring and water-planning decisions during the interim.

Environmental monitoring and social license

As we've seen, remote-sensed data analysis can be used to augment data obtained from weather stations, hydrological geophysical surveys [16], and other hydrological studies.

This type of work is part of the collaborative environmental stewardship that mining sector stakeholders have been participating in as a sharer of the communal water supply. [17]

**Fathom Geophysics gratefully acknowledges John Dean of First Quantum Minerals Ltd for permission to discuss the Taca Taca case study and to present some results on environmental baseline conditions assessment, analysis, and interpretation.

References

[1] H. Punkkinen, L. Rasanen, U.-M. Mroueh, J. Korkealaakso, S. Luoma, T. Kaipainen, S. Backnas, K. Turunen, K. Hentinen, A. Pasanen, S. Kauppi, B. Vehvilainen and K. Krogerus (2016) "Guidelines for mine water management", VTT Technology, 266, 157 pages.

[2] J. Ossa-Moreno, N. McIntyre, S. Ali, J.C.R. Smart, D. Rivera, U. Lall and G. Keir (2018) "The hydro-economics of mining", Ecological Economics, 145, 368-379.

[3] M.R. Strecker, R.N. Alonso, B. Bookhagen, B. Carrapa, G.E. Hilley, E.R. Sobel and M.H. Trauth (2007) "Tectonics and climate of the southern central Andes", Annual Review of Earth and Planetary Sciences, 35, 747-787.

[4] H. Pingel, T. Schildgen, M.R. Strecker and H. Wittmann (February 2019) "Pliocene-Pleistocene orographic control on denudation in northwest Argentina", Geology, 47, 4, 359-362.

[5] R.J. Joyce, J.E. Janowiak, P.A. Arkin, P. Xie (2004) "CMORPH: A method that produces global precipitation estimates from passive microwave and infrared data at high spatial and temporal resolution", Journal of Hydrometeorology, 5, 487-503.

[6] S. Nigam and S. Baxter (2015) "Teleconnections", In: G.R. North, J. Pyle, and F. Zhang (editors), "Encyclopedia of Atmospheric Sciences", 2nd Edition, Volume 3, 90-109.

[7] A.J. Matthews, B.J. Hoskins and M. Masutani (2004) "The global response to tropical heating in the Madden-Julian oscillation during the northern winter", Quarterly Journal of the Royal Meteorological Society, 130, 1991-2011.

[8] J. Sulca, M. Vuille, Y. Silva and K. Takahashi (2016) "Teleconnections between the Peruvian central Andes and northeast Brazil during extreme rainfall events in austral summer", Journal of Hydrometeorology, 17, 499-515.

[9] J.D. Lenters and K.H. Cook (1997) "On the origin of the Bolivian High and related circulatoin features of the South American Climate", Journal of the Atmospheric Sciences, 54, 656-677.

[10] M. Vuille and F. Keimig (2004) "Interannual variability of summertime convective cloudiness and precipitation in the central Andes derived from ISCCP-B3 data", Journal of Climate, 17, 3334-3348.

[11] M. Vuille, D.R. Hardy, C. Braun, F. Keimig, and R.S. Bradley (1998) "Atmospheric circulation anomalies associated with 1996/1997 summer precipitation events on Sajama Ice Cap, Bolivia", Journal of Geophysical Research, 103, D10, 11191-11204.

[12] R. Garreaud (2013) "Warm winter storms in central Chile", Journal of Hydrometeorology, 14, 1515-1534.

[13] N. Quispe and G. Avalos (2006) "Intense snowstorm in the southern mountains of Peru associated [with] the incursion of cut-off low-pressure systems at upper level", Proceedings of 8th International Conference on Southern Hemisphere Meteorology and Oceanography, Brazil, 24-28 April 2006, Instituto Nacional de Pesquisas Espaciais (INPE), 1945-1958.

[14] R.D. Garreaud and P. Aceituno (2001) "Atmospheric circulation over South America: Mean features and variability", In: T. Veblen, K. Young, and A. Orme (eds) "The Physical Geography of South America", Chapter 2, 33 pages.

[15] M. Vuille and C. Ammann (1997) "Regional snowfall patterns in the high, arid Andes", Climatic Change, 36, 413-423.

[16] See, for example, discussion in: A. Binley, S.S. Hubbard, J.A. Huisman, A. Revil, D.A. Robinson, K. Singha and L.D. Slater (2015) "The emergence of hydrogeophysics for improved understanding of subsurface process over multiple scales", Water Resources Research, 51, 3837-3866.

[17] See for example, the principles discussed in: "Shared water, shared responsibility, shared approach: Water in the mining sector" (2017) by the International Council on Mining and Metals, and the International Finance Corporation (World Bank Group), 48 pages.

About Fathom Geophysics

In early 2008, Amanda Buckingham and Daniel Core teamed up to start Fathom Geophysics. With their complementary skills and experience, Buckingham and Core bring with them fresh ideas, a solid background in geophysics theory and programming, and a thorough understanding of the limitations of data and the practicalities of mineral exploration.

Fathom Geophysics provides geophysical and geoscience data processing and targeting services to the minerals and petroleum exploration industries, from the regional scale through to the near-mine deposit scale. Among the data types we work on are: potential field data (gravity and magnetics), electrical data (induced polarization and electromagnetics), topographic data, seismic data, geochemical data, precipitation and lake-level time-lapse environmental data, and remotely-sensed (satellite) data such as Landsat and ASTER.

We offer automated data processing, automated exploration targeting, and the ability to tailor-make data processing applications. Our automated processing is augmented by expert geoscience knowledge drawn from in-house staff and from details relayed to us by the project client. We also offer standard geophysical data filtering, manual geological interpretations, and a range of other exploration campaign-related services, such as arranging surveys and looking after survey-data quality control.