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PART FOUR: PHYSICAL CHARACTERISTICS OF THE TAMARUGAL PAMPA

CHAPTER 14 - Climate

The typical Tamarugal Pampa climate is normal desert (B.W.) (Almeyda, 1950), distinctive in its clearcut, severe and marked climatic traits. This is not a hot desert area. Indeed, the inland pampas are over 1 000 m high, so that altitude intervenes as a mitigating factor. Most mean monthly temperatures are below 18° C (CORFO, 1965).

Lack of information makes it difficult to give a detailed description of the area. One of the most striking features is the low relative humidity and high luminosity of the unusually clear air. Relative humidity is low during the daytime and fairly high at night. There is a marked temperature variation and lack of rainfall, rain being very rare in the Pampa. To summarize: nocturnal temperatures are low and may drop to freezing at any time of the year; temperature variations are surprisingly high, given the lack of wind. Mean relative humidity is very low and dryness reigns supreme (51%); cloudiness is extremely rare or nil (1.5%) (CORFO, 1965). (See Annex II which summarizes climatic date for Canchones and Pintados.)

Though daytime relative humidity varies from 10–30%, there are days when 80–100% peaks are hit during the night, yet there is no rain. Heavy mist is rare, being recorded only some six to ten times during the months of December to February, coinciding with the period of heavy rainfall in the Cordillera and Altiplano -- a phenomenon known as “Bolivian winter”.

Temperatures range from absolute minimum of -5° C to -12° C to absolute maximums of 35° C and 36° C, with an average of 250 clear days a year and 550 cal/cm2/day. Mean monthly evaporation from the soil is 260 m3/ha in areas with tree vegetation, and 50 m3/ha within the forest (Sudzuki, 1975).

Annex II gives the mean monthly temperature for the Canchones Agricultural Station (20° 25' L.S. and 69o 35' W) (readings for the years 1965, 1970 and 1971), and the minimum and maximum, absolute minimum and maximum, and daily variations observed with particular regularity. The mean minimums for the three years are 4.78° C, 5.0° C and 3.8° C, respectively. The same occurs for the maximum temperatures which exhibit very little variation - 28.92° C and 31° C for the years observed. This station is located at an elevation of 1 300 m above sea level. The mean annual temperature is 16.7° C. The hottest month is February with 21.2° C and the coldest is June with 12.9° C. The mean annual variation is 8.3° C -- a marked variation from the coastal climate. Monthly temperatures are somewhat high, the maximum matching the sun's position at the zenith in its return from the tropical latutides in the months of December, January and February. Still, very abrupt swings in temperature are recorded for this sector, as can be seen in Annex II.

Wind readings for Canchones bring out the prevalence of the west winds, but there must be a daily alternance of breezes, such as the one described for the Pica area, and which are caused by the land heating up during the day (Bruggen, 1929). The marked variation is caused not so much by the daytime heating of the land as by its intense cooling at night. In the winter months, the maximum is observed to go as high as 30° C; indeed, nocturnal temperatures are always near zero and drop below zero in the months of May, June, July, August and September, with their respective mean maximums of 31° C, 27° C, 33° C, 29° C and 30° C.

During the period when rainfall readings were being taken by Almeyda (1946, 1950), rainfall was virtually non-existent in Canchones or Pintado (Annex II); nonetheless, at higher altitudes, rainfall plays an important ecological role of special significance for the hydrological cycle of the place. For instance, rain falls in the altiplano mostly in February, March and April, with mean monthly figures as high as 130 mm. This phenomenon (“Bolivian Winter”) varies in intensity from year to year. Starting and stopping dates also vary, for example, the rainy season may begin in November, or in January or February. Exceptionally, rain may fall during the winter months, with readings as high as 300 mm of rainfall in the most northern part of the Chilean-Bolivian altiplano (El Tarapacá, 1952; CORFO, 1965).

CHAPTER 15 - Geomorphology

Background

Chile's First Region conforms, macro-structurally, to a configuration of three major physical-geographic units: i) coastal ranges; ii) Central Depression: and iii) Andes range. The Tamarugal Pampa is an enclave in the Central Depression, running from latitudes 17° 45' S and 21° 20'.

It is interesting to trace the metamorphological changes which have occurred since the beginning of the Quaternery, when the great rains began which caused erosion and sedimentation of huge quantities of debris brought down from the eastern highlands into the Central Depression.

Drainage of the Tamarugal Pampa is entirely underground, with the sole exception of the great floods which occur during some summers in the cordillera area and which are caused by the displacement and intensification of the continental low-pressure centre. This is one of the principal, transcendent, present-day morphodynamic processes (IREN, 1976). Underground circulation is interrupted by the coastal range which acts as an underground barrier, forcing the groundwater to rise. Added to this phenomenon are the affects of capillary movement due to high-surface evaporation.

The circulation of water underground through strata rich in salts, together with the high rate of evaporation, explain the presence of active or partially active salt deposits on the western edge of the Central Depression (IREN, 1976).

Lastly, the floods appearing in certain years in the Central Depression form soft, cone-shaped deposits over the sedimentation glacis of the Tamarugal Pampa. Granulometry varies from fine sand to silt, differing from the sediments of the glacis itself, which include fine and coarse sands, pebbles and clastic material in alternating banks or strata ranging in average thickness from 20 to 30 cm. It is common to find the surface paved with thick gravel due to the action of the wind which sweeps away the finer fractions and creates, in exchange, a few rare dune areas in the Huara interior and the Camarones Pampa.

To summarize, IREN (1976) offers the following geomorphological conclusions:

  1. The geological cast of the region is the result of processes operative during the Tertiary and Quaternary which, in part, affected the pre-existing rocks and formations.

  2. The main phenomena or processes affecting the morphodynamics of the region are volcanic activity, the fill, or fills, of the Andes cordillera and the pluvioglacial periods.

  3. Present-day morphodynamics are confined to two major processes: a) the great summer floods produced by changes in continental atmospheric pressures which cause heavy rain in the Andean area; and b) the deflation and corrosion caused by wind.

  4. The presence of active salt flats in the Tamarugal Pampa is clear evidence of the hydrogeological potential of the upstream areas.

  5. The morphological evidence shows a priori that a relationship can be established between the surface run-off areas of floods and the presence of groundwater, since the main active salt deposits of the Tamarugal Pampa lie at the foot of the great alluvial cones whose waters infiltrate as they flow down the cone.

Geomorphological evolution

This is a coherent territory, interrupted in its north-south development and imprisioned between the coastal range on the west and the Andean foothills on the east. The many streams draining the slopes of the Andean precordillera flood onto the central pampa, laying down their alluvial debris. Most of these temporary watercourses fade away as they enter the pampa, which acts as a giant trap absorbing the products of the intensive erosion wearing away the Jurassic, Cretaceous and volcanic Andean Quaternary beds.

The salt deposits are found mostly in the southernmost part of the pampa, and cover a total estimated area of 3 750 km2.

The surface of the major salt flats now identifiable (which cover some 1 700 km2), is covered with calcium sulphate and NaCl salts, from the evaporation of a broad lake which once covered this part of the Tamarugal Pampa.

The presence of fresh water flooding the Guantajaya mines in the coastal range at Iquique and the layer of fresh water over salt along the coast, bear witness to the fact that in this way the flow of these fossil valleys is maintained at the expense of the waters from the high plains.

It can be deduced from the foregoing that, while the Tamarugal Pampa buries itself beneath successive waves of alluvian deposits moving towards even more marked aridity, there is increased potential to establish traps on the western edge of the coastal range to capture water circulating over ancient run-off beds before it becomes contaminated with sea water.

A gradual drying up of the climate has also been noted, as is demonstrated by deep cuts in the subsoil. Indeed, the oldest cones are the broadest and the poorest in salt because previously the climate was wetter. Present-day and geologically-recent fills are not very developed and are thicker and coarser due to a lack of run-off in the absence of abundant rainfall.

The origin of salt deposits and the formation of salt flats or crusts are closely related to these happenings. The salt originated in the volcanic activities of the Old Quaternary, a wetter climate than today's, which made possible the distribution of these chemical sediments throughout the northern territories, concentrating deposits in the depressions which formed as a result of local tectonic activities. Today, groundwater movement has replaced the surface run-off of these older times and now salts imprisoned in the subsoil can rise thanks to the extreme aridity of the air which acts as a suction pump on the moisture of the underground system. This phreatically-generated mechanism overturns the soil particles or surface debris, causing various kinds of crusts to form. Among these, according to the aforementioned authors, are “mottled” fields, polyganal crusts and columnar uprisings.

Lastly, the Loa River marking the southern border of the Tamarugal Pampa is the only drainage system with permanent (though spasmodic) surface run-off. With a flow of barely 0.9 m3/s (or 0.03 litres/second for an area of one square kilometer), it still reaches the sea by using the natural communication offerered by a very narrow, epigenetic valley more than 500 m deep.

CHAPTER 16 - Soils

Use of the soils of the First Region for agroforestry is confined to small, usually scattered, areas. With its deserts, vast concentrations of salt, few and feeble water courses, and its highly distinctive bioclimatic conditions, this territory is among the world's most arid.

The Tamarugal Pampa has the following physiogeographic sub-units: 1) desert-climate saline sediment depressions; 2) desert-climate alluvial sediment surfaces; 3) desert-climate inclined, dissected plains; 4) desert-climate sandy surfaces laid down by the wind, and 5) farming areas in desert near gorges, on slopes and hills (IREN, 1976).

According to Wright (1963), the soils of the arid zone of northern Chile can be divided into large groups: 1) true desert soils, and 2) semi-desert soils.

The Central Depression is cut by some alluvian and colluvial formations, usually saline. Their soil profiles have no clear development, and often exhibit extensive patches of solonchack. Following a north-south line on the western edge of the meseta, between latitudes 20o and 25o S, are the saltpeter or nitrate soils. These are not really soils but rather beds which develop in obedience to chemical enrichment by different kinds of salts, among which soidum nitrate is prominent. Sodium nitrate, known as natural Chilean nitrate, is made up of thirty chemical elements plus nitrogen and have been called vital impurities for their beneficial effect on crop yields (Corporación de Ventas de Salitre y Yodo de Chile, 1965).

Two clearly defined and different sectors stand out in the Tamarugal Pampa. The first is the higher, eastern part with its coarse, premeable materials; the second, the lower, western sector with its finer materials and depressions now occupied by salt deposits.

The eastern part of the Tamarugal Pampa is a large piedmont made from the union of alluvial fans formed at the outlets of streams leaving the Andes. The sediments exhibit rough stratification, being generally course, medium and fine sands separated by thin saline strata. Occasionally, some sands with high salt content are found -- these are usually extraordinarily hard. Soils are deep, stratified, sandy, greyish in colour, unstructured with simple, single, non-plastic, non-adhesive grains. These soils are flat or slightly inclined, with good or excessive drainage; alkaline-saline with low or very low natural fertility; extremely variable rooting capacity. The piedmont ends on the western side of the Tamarugal Pampa and the fill materials are finer though sand and silt still predominate.

There were some lakes in the western sector which later turned into salt flats where clay and silty materials predominate. These are stratified and covered by a crust of salt ranging in thickness from a few centimetres to one meter or more. They are usually sodium, calcium, magnesium and potassium salts, and deliquescent, which gives the impression that the ground is always wet. The groundwater level in the centre of these salt flats ranges from 1.30–1.40 m deep - clayey, silty materials, greyish or reddish-grey, heavily mottled.

The soil panorama of the Tamarugal Pampa shows a very regular graduation. Soils and sands with gravels occur in fringe formation along the foot of the Andes, varying gradually across the plain to fine, silty sand, silty loam, loamy, silty clay, until the point where silty clay soils are found. These latter have been deposited in “Lacustrian areas” at the foot of the inner slopes of the coastal range.

A general reconnaissance of soils of the First Region was made by IREN (1976) at a scale of 1:500 000. Cartographic details were obtained from field data backed by photo-interpretation of ERTS satellite pictures, blown up to 1:500 000 in bands 5 and 7 (visible and infra-red wavelengths, respectively).

The following is a description of tamarugo-associated soils, summarized by Wright (1963):

Series: Tamarugo

This is an elongated fringe of developed soil in the Tamarugal Pampa down to the Quebrada de Teliviche, and near Pintados on the southern side. The soils are made up of recent sediments in alternate formation. The area was originally occupied by open tamarugo forest. Elevation 1 100–1 200 m. Relief: flat. Markedly stratified soil profiles with alternate horizons of silt and sand.


Active salt deposit in Pintados, Refresco property (above). Saline crust which has been separated before planting takes place (below).

  0–20 cmGrey sand (10 YR 5/1, moist), and light grey (10 YR 7/2, dry) silt in plates 1.0–2.5 cm thick; friable, silt horizons very weakly cemented, but sand horizons friable and separated; simple granular structure; non-plastic (silt horizons very slightly plastic).
20–70 cmGrey (10 YR 5/1, moist); coarse, platy loamy-sandy; friable; simple platy structure; not sticky and non-plastic when moist; lower boundary unknown (pH 8.0).
70 cm +(100 cm +) light grey (10 YR 7/2) to grey (10 YR 5/1) coarse, platy sandy loam, but slight development of fragile pan in platy clay; not sticky, non-plastic when moist; lower boundary unknown (pH 8.0).

Desert soils with salt crusts

These are the remainder of salt deposits found on both sides of the Pan-American highway between Iquique and Pintados (Cruce) and other sectors of Tarapacá Province. Elevation ranges from 950–1 050 m. This is usually flat land with very irregular micro-relief; no natural vegetation, but a few carob trees have been planted on these soils.

  0–2 cmSurface layer of impregnated white crystals, giving a reddish-brown colour to the sand and fine silt; reddish brown (2.5 YR 3/2 when moist faintly reddish, 2.5 YR 5/2 when dry), duly cemented into crusts which roll or curl up into polygonal shapes; non-calcareous (pH 8.4), abrupt boundary.
  2–35 cmDark reddish brown (2.5 YR 3/4 when moist) and reddish brown (2.5 YR 5/4 when dry), predominantly sand with thin beds of silt and clay; firm to friable, platy; loose, sandy patches with single-grain structure; white calcium carbonate nodules (effervescent with HCl) in the lower part of the horizon; not sticky and slightly plastic when wet; clear boundary (pH 8.2).
35–56 cmDark reddish brown (2.5 YR 3/4 when wet) and reddish brown (5 YR 5/3 when dry) silty loam to loamy silty clay; firm, friable; slightly developed, platy structure, breaking under pressure; single grain and agranular; slightly sticky; moderately to very plastic when wet; calcareous; diffuse boundary (pH 8.2).
56–106cmReddish brown (5 YR 4/3 when wet); light reddish brown (5 YR 6/3 when dry); silty clay; very firm; massive structure (almost prismatic) difficult to break into fine blocky and coarse grains; various successive structures of slightly yellowish needles, crystal-like, in lower horizon (about seven, 2–5 cm layers or strata); moderately sticky and highly plastic when wet; undefined horizon (p.H 8.4) and crystal strata (pH 7.8).
106 cm +Reddish brown (5 YR 4/3 when wet); light reddish (5 YR 6/3 when dry); silty clay; very firm; massive structure, difficult to break into large blocky and coarse granules; slightly sticky and very plastic; boundary not visible (pH 8.4).

Soil analysis

Representative samples were taken of forest soils and terrain in the southern part of the Bellavista salt flat in the Victoria area to determine their chemical characteristics. Table 27 gives the findings (CORFO, 1971a).

TABLE 27: Findings of chemical analysis of four soil samples in the Salar de Bellavista, Victoria (CORFO, 1971a)

Sample No.pHK × 10-3NaCaMgSO4ClB
16.761421,280325104362,060-
37.341201,360107  82531,5706.4
57.98  17   140  1253656   1605.5
10  7.301161,070106228511,550-

In trials aimed at introducing tamarugo in areas with apparently extreme salinity, CORFO (1970) studied the effect of sulphur additives to correct excess salinity. (Table 28).

Table 28: Percentages of tamarugo plant losses observed in the Salar de Bellavista after nine months in soils with sulphur-base correctives and organic matter (CORFO, 1970)

TreatmentPlant losses (%)
Control24
Organic corrective (guano)10
10 gr sulphur corrective  5
20 gr sulphur corrective  8

CHAPTER 17 - Hydrology

The main purpose of a study undertaken in the Tamarugal Pampa by the Department of Water Resources of CORFO was to make a groundwater balance of the system to determine the amount of subterranean flow by means of the hydrological characteristics. The factors analyzed were: depth, chemical quality and displacement of groundwater, i.e. direction of flow, charge and discharge zones for groundwater and for confined groundwater. The action of aquifers containing alluvial fill from the pampa and their capacity to transmit and retain water were also studied by calculating the so-called coefficients of transmissibility and storage of aquifers (CORFO 1971a).

Both working wells and observation wells were sunk up to 1968. Details are found in Table 29.

TABLE 29: Working wells and observation wells sunk by CORFO for the study on aquifer recharge in the Tamarugal Pampa

PlaceWorking (deep) wells
(No.)
Observation wells
(No.)
Zapiga and Obsipo salt flats520
Pintados18  143  
Bellavista and Sur Viejo530
Llamara529
Total33  222  

The findings show that recharge in the Tamarugal Pampa aquifer comes from the various mountain streams in the study area, ranging from the Estanilla in the south to the Guatacondo in the north. Streams located between the Retamilla and the Aroma discharge part of their waters northwards towards the Jaspampa, which takes them out of the study area of the Zapiga and Obsipo salt flats (directly west of them). Another part flows south towards the Salar de Pintados.

Groundwater discharge is produced by evaporation in the various salt flats and by evapo-transpiration from plants, by pumping of wells and “norias” (a type of well) and by movement of water towards other zones not included in the study.

Hydrological characteristics of the groundwater of the various sections of the Tamarugal Pampa observed at six salt flats and in the Huara sector can be seen in Table 31.

TABLE 31: Depth of groundwater table and chemical quality of seven wells in the Tamarugal Pampa

Salt flat or areaDepth of groundwater table (m)Chemical quality of groundwater (p.p.m.)
Extreme valuesGreatest values(%)Extreme valuesGreatest values(%)
Zapiga4 – 206 – 16   700 –   2 500   800 –   2 500
Obispo8 – 208 – 162 000 – 20 0002 000 – 10 000
Huara20 – 80  40 – 80  1 000 –   4 0001 000 –   2 500
Pintados2 – 202 – 10   500 –   4 0001 000 –   2 500
Sur Viejo12 – 25  12 – 20  1 000 – 10 000   500 – 20 000
Llamara0 – 250 – 153 000 – 16 0003 000 –   9 000
Bellavista2 – 202 – 18   700 – 80 000   500 – 20 000

The Pintados and Bellavista salt flats are located in the plains area. Their origin is attributed to the evaporation of water as a result of capillary rise of groundwater and from masses of water lying on the surface (lakes). This basin possesses the following characteristics:

Total area17 080 km2
Perimeter    705 km
Elevation  2 300 m
Capacity index  1.51
Gradient index  0.11
Drainage density  0.30
Maximum elevation  5 750 m (Cerros de Quinsachata)
Minimum elevation     950 m (Cerro Gordo)

The following background was available in drawing up the groundwater balance (IREN, 1976):

a)Input:  
 Intercepted volume1,363.4 × 106 m3 
 Input from basin    24 × 106 m3 
b)Output:  
 Evaporation from salt flats23,300 ha × 1,000 m3=23 300 000 m3
 Evaporation from “bofedales”740 ha × 2,500 m3=  1 850 000 m3
 
Evapo-transpiration and consumption of tamarugo and carob
61,782 ha × 89.95 m3=89 950 000 m3

Outcome from tamarugo and carobs was determined on the basis of experiments on evapo-transpiration done by the Agricultural Department of CORFO.

The groundwater balance of the watershed takes into consideration the entry and exit of water from the area identified. The entry is generally made up of the volume intercepted by the basin, i.e. the quantity of rainwater falling onto the watershed. This volume, modified by the run-off coefficient, makes up the water input to the area. Water exiting is basically due to evaporation from the surface of lakes, flood plains and “bofedales”, from salt flats and evapo-transpiration from phreatophytic plants and crops (IREN, 1976).

Groundwater balance of the watershed

A study, carried out in 1975 on the Tamarugal Pampa by CORFO's Division of Water Resources, found that this was a hydrological system in imbalance, an imbalance which might increase significantly.

IREN (1976) agrees that there is an imbalance between the recharge and discharge of this hydrogeological system. Discharge is estimated at a constant flow of 1 343 lt/sec., with effective recharge estimated at 257 lt/sec., which means that storage variation is equivalent to 1.091 lt/sec. of continuous flow. The groundwater balance deficit calculated for the Tamarugal watershed system is thus 27.3 × 106 m3 (in volume), calculated from the equivalent in storage of 856.6 lt/sec. of equivalent flow, thus confirming that an imbalance does, in fact, exist.

One measure intended to partly offset evaporation by lowering the water table is reforestation with tamarugo and other trees. Structural conditions and a study of the wells indicate that underground water begins to circulate north and south, as of latitude 19o 50' S, roughly speaking. The isophreatic curve (average water table) at this latitude is 1 130 m above sea level, a difference of 60 m compared with the elevation of the Tamarugal Pampa (1 190 m); towards the Quebrada de Tiliviche sector, it is 1 110 m, and towards the Salar de Bellavista sector, 950 m (IREN, 1976).

The water-table depths identified indicate that for depths of 0.6 m, 6–10 m, and 10–44 m, the corresponding areas are 270 km2, 366 km2 and 130 km2. This means that an area of approximately 63 000 ha lies over a water table less than 10 m deep.

The chemical quality of groundwater in the Tamarugal Pampa ranges from highly saline (5 000 ppm) to good (500 – 1 200 ppm). The highest saline content is found where the groundwater table lies nearest the surface.

Castillo (1966) suggests that the withdrawal of water by forests has improved the chemical quality of water lying beneath these forests. The hypothesis is that by lowering the water table (as does vegetation generally), contamination with the top, saline strata and evaporation towards the surface of the soil are avoided.


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