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HYDROLOGICAL PROCESSESHydrol. Process.16, 30193035 (2002)Published online 17 September 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1086

Groundwater recharge through an alluvial fan in theAtacama Desert, northern Chile: mechanisms,

magnitudes and causes

John Houston*Av. Las Condes, 10373, Of. 60, Santiago, Chile

Abstract:The Chacarilla fan in the Atacama Desert is one of several formed in the Late Miocene at the foot of the Pre-AndeanCordillera overlying the large, complex, Pampa Tamarugal aquifer contained in the continental clastic sediments of thefore-arc basin. The Pampa Tamarugal aquifer is a strategic source of water for northern Chile but there is continuingdoubt over the resource magnitude and recharge. During January 2000 a 1 in 4 year storm in the Andes delivered a34 million m3 flash flood to the fan apex where c . 70% percolated to the underlying aquifers. Groundwater rechargethrough the fan is calculated to be a minimum of 200 l/s or 6% of the long-term catchment rainfall. These figures aresupported by hydrochemical data that suggest that recharge may be 9% of long-term rainfall. Isotopic data suggestgroundwater less than 50 years old is transmitted westward through the permeable sheetflood sediments of the fanoverlying the main aquifer. Analysis of this and other events shows that the hydrological system is non-linear withpositive feedback. The magnitude of groundwater recharge is dependent on climatic variations, antecedent soil moisturestorage and changes in channel characteristics. Long-term declines in groundwater level may partly result from climaticfluctuations and the causes of such fluctuations are discussed. Copyright 2002 John Wiley & Sons, Ltd.

KEY WORDS recharge; runoff; precipitation; water resources; alluvial fans; Atacama Desert; Chile

INTRODUCTION

The Atacama Desert of northern Chile is one of the most arid zones in the world, but groundwater resources

exist. However, there is continuing controversy over the size of these resources and whether any recharge

takes place under current climatic conditions. Houston et al. (2001) have suggested that the resource in

the Pampa Tamarugal could be large, but its development is partly constrained by the lack of any reliable

quantification of its recharge. There is general consensus that recharge is currently taking place (DGA, 1987;

JICA, 1995) but the magnitude and mechanism(s) have largely been the subject of conjecture rather than

proof and calculation. It has been suggested, for example, that fresh, apparently recent, groundwater in the

center of the Pampa Tamarugal may originate through a system of faults and deep fissures connected with

Altiplano aquifers (Galli and Dingman, 1962; Margaritz et al., 1990). On the other hand, Grilli et al. (1999)provide convincing hydrochemical and isotopic evidence that recharge takes place at relatively shallow levels

as a result of infiltrating runoff from the Pre-Cordillera.

Mountain-front recharge through alluvial fans is well documented (e.g. Simmers, 1997) but rarely quantified,

and is a function of fan geology and morphology, as well as channel and flow characteristics (Issar and

Passchier, 1990; Sorman and Abdulrazzak, 1997):

1. infiltration rates are highest at the fan apex, where coarsest sediment is deposited, and are proportional to

the saturated vertical hydraulic conductivity;

* Correspondence to: John Houston, Av. Las Condes, 10373, Of. 60, Santiago, Chile. E-mail: [email protected]

Received 16 May 2001

Copyright 2002 John Wiley & Sons, Ltd. Accepted 15 October 2001

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3020 J. HOUSTON

2. infiltration rates increase with increasing depth to water table and hence increasing groundwater gradient;

3. infiltration rates increase with stream-flow velocity (probably as a result of scour), volume (owing to

increased cross-sectional area) and duration;

4. infiltration is greatest in active channels and least in overbank flooded areas.

This paper examines a specific mountain-front recharge event, which proves the mechanism and provides a

first-order quantification of the processes. It is shown that such events are intermittent and therefore average

values become meaningless; evaluations must take place over many years and take into account many factors,

including possible variations in geology (channel characteristics) and climate within the evaluation horizon.

THE FORE-ARC BASIN OF NORTHERN CHILE

The Pampa Tamarugal of northern Chile (Figure 1) is part of a continental fore-arc basin, which extends

from 18 to 24 S between the Coastal Cordillera in the west and the Pre-Andean Cordillera in the east.

It is nearly 600 km long and varies between 20 and 60 km wide at an average elevation of 1000 m. The

Tertiary sedimentary infill of the fore-arc basin represents a very large, complex aquifer system of strategic

significance to northern Chile. The provincial capital of Iquique depends entirely on groundwater supplies,

and several smaller towns, many mines and agriculture are also heavily dependent on groundwater resources

from the Pampa Tamarugal.

The basin started to form in the early Oligocene, when plate convergence rates slowed creating a

transtensionalextensional environment after the Eocene Incaic phase of compressional deformation (Pardo-

Casas and Molnar, 1987; Mpodozis and Ramos, 1989). The fore-arc basin is a complex asymmetric graben

bounded by two major NS fault zones, to the west the Coastal Range Fault Zone (CRFZ) and to the east

the Pre-Cordillera Fault Zone (PCFZ). Both show evidence of normal faulting as well as lateral displacement,

which, on the PCFZ started as dextral strike-slip in the Eocene and reversed in the late Oligocene (Reutter

et al., 1996). On the northern CRFZ, anticlockwise block rotations may be the cause of lateral displacement(Taylor et al., 1998; Taylor, personal communication 2001). Conjugate oblique NESW and NWSE faults

70W

20S

22S

CHILE

70W

20S

22S

Rio Loa

Rio

Loa

QUILLAGUA

J8

UJINA

COLLACAGUA

CERRO GORDO

3000m

IQUIQUE

FOREARC

BASIN

PRE-CORDILLERA

FAULT ZON E

Figure 1. Location map of the Pampa Tamarugal fore-arc basin between fault lines and the Chacarilla catchment and fan (outlined), showingplaces mentioned in the text

Copyright 2002 John Wiley & Sons, Ltd. Hydrol. Process. 16, 30193035 (2002)

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GROUNDWATER RECHARGE 3021

cut the fore-arc creating a series of rhomb-shaped pull-apart basins (Jensen et al., 1995). At least four major

basins have been identified, infilled with up to 1000 m of continental deposits (Houston et al., 2001).

The lowermost sediments of these basins are coarse conglomerates and gravels of the Sichal and Altos

de Pica Formation (Table I), eroded from the adjacent uplifting Coastal Range and Pre-Cordillera. At an

early stage, endorreic lacustrine and evaporitic sediments began to be deposited in the south. Coarse clastic

sediments continued to be deposited over wide areas throughout the Miocene. Episodes of volcanic activity

took place at several times throughout the Miocene, producing andesitic tuffs and ignimbrites from eruptive

centres in the Pre-Cordillera and Cordillera to the north and east of the Pampa Tamarugal (Galli and Dingman,

1962).

Towards the end of the Miocene, as the basins began to fill up, a series of large alluvial fans developed

along the east of the fore-arc basin as discrete architectural elements (Miall, 1996; Kiefer et al., 1997). These

were associated with a final lacustrine phase, which produced fine-grained and evaporitic lake sediments of

the Soledad Formation. Since the Pliocene only minor alluvial and evaporitic sediments have been deposited

at various locations throughout the fore-arc basin.

THE CHACARILLA CATCHMENT

The Quebrada Chacarilla is located on the east side of the Pampa Tamarugal (Figure 1). It is a perennial

river in its descent from the Cordillera, but elsewhere it is ephemeral. It has a catchment area of 1235 km2,

largely above 3000 m and discharges at the foot of the Cordillera to an alluvial fan, which extends into the

Pampa Tamarugal over an area of 684 km2. A hypsometric curve for the basin (Figure 2a) demonstrates that

nearly 90% of the basin is at elevations greater than 3000 m, extending up to 4520 m. The drainage basin is

sixth-order (based on 1 : 50 000 topographic maps) with a mixed trellis and dendritic pattern and a drainage

density varying from 01 to 37 km/km2, averaging 16 km/km2 (Figure 3).

The geology of the catchment (Figure 4 and Table I) is comprised of impermeable Palaeozoic and Mesozoic

bedrock overlain by the Huasco Ignimbrite and permeable, clastic, continental sediments of the Altos de Pica

Formation. To the east of the catchment, Quaternary strato-volcanoes have punched through the underlying

strata to form the highest peaks of the Western Cordillera. The Huasco Ignimbrite occupies large areas of

the central and northern part of the basin and has a relatively low drainage density compared with the clastic

sediments in the southern part of the catchment.

The river profile (Figure 2b) shows a steep descent in a canyon, in places over 500 m deep, down the

Pre-Cordillera to the fan system on the Pampa at elevations of around 1000 m. There are two major terraces

within the canyon. The upper terrace is 2040 m above the canyon floor. It is debris covered and clearly

ancient and inactive and may correlate with the presumed fan-base unconformity. At a height of 25 m is a

more recent terrace, which is contiguous with the downstream fan surface and represents the current floodplain

for extreme events. The currently active channel is cut into this terrace/floodplain.

THE CHACARILLA FAN

The alluvial fan is similar to the Arcas fan located to the south (Kiefer et al., 1997), although with a

smaller volume and different sedimentology, presumably owing to the lower permeability (more erosive)

Arcas catchment area (Blair, 1999). The Chacarilla fan cone has a maximum radius of nearly 40 km and

a flow expansion angle of only 60. Both the main fan and the recent, active channel are asymmetrically

displaced towards the north, as indeed are almost all the major fan systems exposed along the Cordillera.

Houston (2001) has suggested that this is a result of the sinistral movement on the PCFZ during the Miocene.

This model of fan formation explains why the flow expansion angle for the Chacarilla is small compared with

the Arcas fan and why the latter is apparently so much larger with a fan to catchment ratio of 1 03 (compared


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3022 J. HOUSTON

Table I. Stratigraphy of the Chacarilla area, based largely on Dingman and Galli (1965), Vergara and Thomas

(1984), Mpodozis and Ramos (1989) and Jensen (1992)

Epoch/

Period

Map

symbol

Formation/

description

Lithology Thickness

(m)

QS

QF

Salar deposits Saline alluvial deposits 0100

Alluvium Clay, sand and gravel 0100Pleistocene

StratovolcanoesAndesitic to basaltic

lavas and tephra01500

Late Pliocene Diaguita uplift and drainage incision

Alluvial fan Gravel, sand and clay 01000+

Late MioceneQuecha phase of compressional deformation

Member 5Sandstone and

conglomerate

QV

QAL

QTA5

QTA1

QTA2

QTA3

QTA4Member 4

Huasco Ignimbrite

Andesitic to dacitic

ignimbrite

Member 3Sandstone with

conglomerates

Member 2Andesitic ignimbrites and

tuffs

EarlyMiddle

Miocene

Altos

de

Pica

Member 1

735

TOM3 Sichal Massive gravels 1000+

Oligocene

Transtension/extensional formation of major basins

Eocene +Intrusive granodiorite and

quartz monzonite

Empexa

ChacarillaJ/K

Longacho

Marine sandstones,

shales, and volcanics12753470

Cretaceous

Jurassic

Gondwana Cycle of magmatic intrusion along Coastal Range andformation of back-arc basins

Permian + Intrusive granite

Permian

CarboniferousPZC Collahuasi

Rhyolitic to dacitic

volcanics >3200

Carboniferous

OrdovicianFamatinian Cycle of accretionary tectonics

Conglomerates and

sandstones


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1000

2000

3000

4000

5000

0 100 200 300 400 500 600

Area (km2)

Elevation(masl)

fan apex

0

1000

2000

3000

4000

5000

0 50 100 150

Distance from fan toe (km)

Elevation(masl)

Quebrada Caya(S tributary)

Quebrada Chara(N tributary)

watershed profile

fan apex

PCFZ

a

b

PCFZ

Figure 2. Hypsometric curve of the Chacarilla catchment (a) showing the control exerted by the Pre-Cordillera Fault Zone (PCFZ), and(b) profile of the watershed, quebradas and fan

with 055 for the Chacarilla), as there is no major fan to the south of Arcas to cover its southern part, whereas

all the fans to the north are partly covered by the adjacent fan to the south.

The Chacarilla fan is a Type II of Blair and McPherson (1994), being dominated by planar sheetflood

couplets of gravelsand or gravelclay (Figure 5). Couplets range from cm to m scales. The gravel beds

consist of subrounded clasts with a typical maximum diameter of c. 20 cm at fan apex, decreasing down

fan. The gravel beds are clast- or matrix-supported with some crude, thick cross-stratification and occasional

grading. The overlying finer grained, waning member of the couplet usually represents less than 20% of thepaired couplet thickness and is composed of finely stratified sand, silt or clay. These sheetflood deposits have

wide, belt-like geometries that can be traced over many hundreds of metres and are cut by occasional shallow,

erosive based, gravel filled, avulsion channels. Rare, overbank mudflows may be observed, and there are

occasional weakly developed palaeosols suggesting subaerial exposure and erosion during fan formation. Distal

fan areas are dominated by a sand skirt that probably interfingers with the uppermost sediments of the Altos de

Pica Formation or the Soledad Formation. The whole area is overlain by the feather edge of later palaeolake

sediments. The Thematic Mapper (TM) satellite imagery reveals the presence of a palaeospring line along the

toe of the Chacarilla fan, suggesting that this once represented the eastern shoreline of Palaeolake Soledad.

The fan has a slope that decreases from 2 at the apex to 05 in the distal sections. The currently active

channel has been incised by as much as 80 m at the fan apex and so has considerably lower gradients


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3024 J. HOUSTON

7740

7.700

1000

4

60

5

00

7740

4000

7.700

460

500

3000

4000

4000

2000

Figure 3. Drainage density of the Chacarilla catchment (UTM grid, 20 km squares, light grey 3 km/km2)

(002 D 11 at the fan apex). This channel is c. 100 m wide at the fan apex and becomes braided in the distal

fan sections. Recent mudflows are largely constrained to this channel, although crevasse splays are common,

and are dominated by silt and clay fractions with very little sand fraction.The age of the Chacarilla fan has not been determined directly but it is stratigraphically equivalent to the

Arcas fan that was formed between 73 (02) Ma and 68 (0.2) Ma (Kiefer et al., 1997).

REGIONAL PRECIPITATION AND THE STORM EVENT OF JANUARY 2000

Despite the general lack of precipitation in the Atacama Desert, the flanking Andes receive significant

precipitation during the austral summer (DecemberMarch) as a result of intense convection over Amazonia

coupled with strong low-level easterly winds, which generate significant precipitation on the eastern slope of

the Andes and over the Altiplano (Fuenzalida and Rutllant, 1986; Garreaud, 1999).

The Atacama Desert, on the western slope of the Andes, lies in a well-developed rainshadow, and as

a result there is a rapid decline in rainfall as air masses move west and descend. Recent reanalysis of 35

precipitation gauges maintained by the Direccion General de Aguas (DGA) between 18 and 24 S for theperiod 19751991 provides the following precipitation model for elevations between 20005000 m

MAR D e00012A

where MAR is the mean annual (OctoberSeptember) rainfall, and A is the altitude in metres.

Above average precipitation occurred during the wet seasons in 1999 and 2000. In particular the storm

events of January 2000 as recorded by the rain gauges at Ujina and Collacagua produced peak daily intensities

of 196 and 268 mm with a return period of c. 4 years (Figure 6). Ujina and Collacagua are two DGA

stations located at 4220 m a.s.l., 25 km south of the Chacarilla catchment and at 3990 m a.s.l., 40 km north,

respectively.


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Q L

QTA5

QTA5

QF

QTTA5

QTA4

QS QV

QV

QV

QS

PZC

J K

PCFZCFZPCFZCFZ

7.7407.740

7.7007.700

5

00

5

4

60

460

Figure 4. Geology of the Chacarilla catchment and adjacent areas superimposed on the Thematic Mapper (TM) satellite image. Modifiedfrom Dingman and Galli (1965) and Vergara and Thomas (1984)

Figure 5. Section through the fan apex sediments dominated by laterally extensive sheetflood facies with vertical permeabilities much lessthan horizontal at fan scales. Palaeosols arrowed. Vertical scale bar equals 1 m

Using these two stations, event models can be generated that are applicable to the Chacarilla catchment.

Double mass analysis of daily rainfall for the two stations between 1995 and 2000 shows that the difference

between the two stations is much less during heavy, intense storms than during lighter events, and reduces over


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3026 J. HOUSTON

0

10

20

30

Dailyrainfall(mm)

Event 224Jan :26.8 mm

Event 1

13Jan :19.6 mm

JanJul 1999 2000 Jun

1

10

100

1000

Annualmaxmonthlyrainfall(m

m)

Jan 2000

4areturn period

0.99 0.95 0.9 0.8 0.6 0.4 0.2 0.1 0.05 0.01

Exceedence probability

b

a

Figure 6. Mean daily rainfall for 19992000 (a) and the frequency of annual maximum monthly rainfall for 19702000 (b) at Ujina (4200m a.s.l.) and Collacagua (3900 m a.s.l.)

Table II. Event and long-term rainfall calculated for the Chacarilla catchment above 1400 m

Event 1 Event 2 1999 2000 Mean annualrainfall

Weighted mean depth (mm) 21 38 110 91Time span (days) 5 10 43

Volume (million m3) 26 47 136 112

the duration of the storm event. Daily amounts differ by an average of 03 mm and a maximum of 11 mm.

The difference is reduced for 10-day amounts, to an average of 02 mm and a maximum of only 3 mm.

This indicates strong spatial and temporal coherence at the catchment scale for significant storm events, and

therefore the two-station average provides a reliable record for the Chacarilla catchment between 3900 and

4200 m a.s.l. Despite the strong spatial coherence at this elevation, the rainshadow described above requires

that a storm event model takes elevation effects into account. Factoring the mean daily rainfall data at Ujina

and Collacagua according to the precipitationelevation model above and based on the hypsometric curve

gives total catchment rainfall (see Table II).

THE JANUARY 2000 FLOOD HYDROGRAPH

The lower part of the Chacarilla canyon was visited in the low flow season of October 1999 in order to obtain

water samples for a regional study being undertaken by Nazca S.A. A follow-up visit in March 2000 revealed


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Figure 7. Rio Chacarilla channel at the fan apex on 8 March 2000, showing (a) the incised bankfull channel (c. 4 m deep, c. 20 m wide)resulting from flash floods, with gravel bar bedforms, overbank mudflows (two episodes) and point bars, (b) detail of the offlappingoverbank mudflows, with the waning deposits of both floods showing differential raindrop intensities, and (c) bank scars resulting from

antidune standing wave scour (c. 4 m high and c . 10 m wavelength)

that a significant flood event had taken place in the interim (see Figure 7). Major changes had taken place to

the active channel morphology, a new mudflow had been deposited throughout the canyon and at least 20 km

out on to the fan along the course of the active channel. Considerable scour had taken place in some reaches

of the canyon, leaving behind the outline of antidune standing waves, and at the fan apex rearrangement of

the gravel bed of the channel had taken place prior to deposition of the mudflow. The overbank mudflow

shows two episodes, which are most likely correlated with differential timing of the flood surge from each

major tributary. The mudflows both display rain pitting, so that rainfall was still occurring at 1400 m a.s.l.

soon after peak flow, an indication that the response time of the catchment must be short because the rainfall

event lasted only a few days (see below).


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3028 J. HOUSTON

Based on the slope area method (Benson, 1968), it is possible to estimate the peak discharge. Cross-

and longitudinal profiles were surveyed of a uniform reach of the channel, to the topmost level of the fresh

overbank deposits. The cross-profile was split into six sections varying from 2 to 7 m wide and up to 4 17 m

deep with resistance coefficients estimated for the bed of each section using both the Chezy equation and

Manningsn (Chow, 1959). Using this information, the peak flow is calculated as 450 (50) m3/s.

The dominant process in transferring intense storm precipitation to the Rio Chacarilla, an arid catchment

with steep slopes and sparse vegetation, is likely to be Horton overland flow (Dingman, 1994). Some losses

are to be expected, however, largely owing to infiltration in permeable soils. Such infiltration is subsequently

either evaporated, passed to the river as interflow or percolates to aquifers as recharge. As the Chacarilla

catchment area is separated from the discharge area by the impermeable canyon, the flood peak at the

fan apex will result solely from Horton overland flow plus any baseflow discharge from permeable areas

in the catchment. Baseflow measured at the end of the dry season (October 1999) accounted for 15 l/s,

less than the error in the estimate of the flood peak and can be safely ignored for the purposes of this

calculation.The hydrograph generated by Horton overland flow from the storm event can be estimated using the

US Soil Conservation Service (1972) method to determine the unit hydrograph. This approach has been

widely used and is appropriate for the level of information available. The SCS method allows estimation

of the time to peak discharge from the centroid of the rainfall event (Tpk) based on soil characteristics

and catchment morphology. A value of 121 h has been calculated for Tpk, which is in agreement with

observations for similar catchments in the Calama basin during the 2001 flood events. As Qpk is known,

a degree of calibration is given to the computed unit hydrograph. By integrating the area under the

resulting hydrograph it is possible to obtain a reliable estimate of the total flood volume for each event

(Table III).

The computed flood volumes are high but not unrealistic for flash floods. Runoff coefficients are

consequently also high owing to the intense nature of the storm, but decline as the duration of the storm

event decreases in intensity and duration. This decrease in runoff coefficient in relation to the time spaninvolved indicates a non-linear catchment response, which is common in arid environments. Comparable

runoff coefficients with non-linear behaviour were found by Evenari et al. (1971) in the Negev Desert

of Israel, where long-term rates were around 15 20%, whereas during major floods this increased to

5070%.

THE GROUNDWATER RESPONSE TO THE STORM RUNOFF

The alluvial fan of the Chacarilla constitutes a discrete element in the Pampa Tamarugal aquifer system

(differentiated by its geometry, stratigraphical position, porosity and permeability characteristics) despite

being in overall hydraulic connection with the rest of the basin. The active channel close to the fan apex

represents the main recharge area, with the horizontal sheetflood sediments facilitating transmission down

gradient.

Table III. Storm and flood volumes calculated by the US Soils ConservationService method for the Chacarilla catchment

Event 1 Event 2 1999 2000total (dailyequivalent)

Storm volume (million m3) 26 47 136 (33)

Flow volume (million m3) 74 268 344 (08)Runoff coefficient 029 057 023


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long term recession

17.7cm/a

39.0

39.5

40.0

40.5

41.01995 1996 1997 1998 1999 2000 2001

Depthtowater(m)

0.5

0.3

0.1

0.1

0.3

0.5

1995 1996 1997 1998 1999 2000 2001

Detrendedwaterlevel

fluctuations(m)

Meandailyrainfall(mm

)

recessioncurves

a

b

0

5

10

15

20

25

30

Figure 8. Monthly groundwater levels at J8 during the period 1995 2000, (a) showing the superimposed linear regression and (b) detrended

water level fluctuations compared with mean daily rainfall at Ujina and Collacagua and the estimated recession curves

It is fortunate that the DGA monitoring well J8 penetrates the distal part of the Chacarilla fan, at a radialdistance of 33 km from the apex. Data for this well is continuous from 1995 to date with the exception of

a six-month gap in 199798. Since 1995, water levels in this well show an overall decline (see Figure 8a)

in common with many other monitoring wells in the Pampa Tamarugal, which generally is attributed to

overexploitation. Nevertheless, short-term fluctuations are visible and are most pronounced in 1999 and

2000. Such fluctuations are the result of recharge to the aquifer through the alluvial fan. If it assumed that

the long-term decline is the result of overexploitation, and this trend removed, then the residuals represent

groundwater recharge (water level rises) and subsequent recession (water level declines). When compared with

annual rainfall (Figure 8b) confirmation is obtained that recharge is associated with high intensity rainfall,

which causes flash floods and flood water infiltration.

Groundwater rises owing to recharge took place between February and November 1999 and between January

and June 2000. The rise started immediately after the rainfallrunoff peak owing to infiltration from flood

water, which reached the distal parts of the fan and continued for 59 months owing to the lag time in thepassage of groundwater through the fan, which was infiltrated near the fan apex.

In order to calculate the volume of recharge, several assumptions and calculations must be made. Firstly,

the total rise resulting from groundwater recharge is calculated by extending the prior recession. Secondly, it

is assumed that the rise of water level in J8 can be applied to the whole of the fan area. This is a conservative

assumption because J8 is in the distal fan area and any rise here will be a minimum. Furthermore, the exposed

fan area is a minimum because it is overlain by separate fan sediments from the south. Finally, the specific

yield (effective porosity) of the sediments must be known.

A pumping test was carried out on J8 and several other wells in alluvial fan sediments of the Pampa

Tamarugal by JICA (1995). Reanalysis of their data shows that these wells (and in particular J8) exhibit a

typical dual porosity response (Gringarten, 1982). This is typical for such alluvial fan sediments (Beard and


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3030 J. HOUSTON

Weyl, 1973; Galloway and Sharp, 1998a,b). The reanalysis provided the following parameter values

fissure permeabilityKf D 5 m/day

fissure storativitySf D 0005

matrix storativitySm D 008

In these layered sediments, where horizontal permeability is greater than vertical permeability, specific yield

is equivalent to matrix porosity and a value of 008 is considered conservative for such sediments (Johnson,

1967).

Then recharge is given by

RECH D WL CRAfan Sy

where, RECH is the volume of groundwater recharge, WL is the rise in groundwater level, R is the extrap-

olated prior groundwater recession, Afan is the area of the fan and Sy is the specific yield or effective porosity.

The results of the recharge volume calculations are given in Table IV. Note that it is possible to consider the

groundwater recharge event as either the result of storm-runoff events 1 and 2 or the water year 19992000.

The results are also provided for 19981999 for comparison.

The recharge volume is very significant. An examination of the recharge coefficients is instructive; recharge

coefficients are higher than normally might be expected, with event ratios being higher than annual values

confirming the non-linear catchment response. Furthermore, the higher recharge coefficient for 19981999

compared with 19992000, despite a lower total recharge volume, probably is related to antecedent moisture

in the catchment, because the years prior to 19981999 received relatively little rainfall-runoff-recharge and

hence soil moisture storage was depleted allowing a greater percentage infiltration.

The balance of the flood volume is evaporated over a period of days from temporary surface water ponds

created within the active channels of the fan and overbank flooded areas. As a check on the validity of

the recharge component it is possible to estimate the number of days required to evaporate the standing

water. For events 1 and 2, the flood volume amounts to 342 million m3

and recharge has been calculatedas 246 million m3, requiring the evaporation of 96 million m3. The area of the active channels and over-

bank flood zones has been measured as 65 km2 and summertime open water evaporation rates in the Pampa

Tamarugal are c. 14 mm/day (DGA, 1987). Thus, a period of 105 days would be required to evaporate the

standing water, which is consistent with observations carried out in the field.

HYDROCHEMICAL AND ISOTOPIC EVIDENCE AND THE RECHARGE MECHANISM

Two additional, independent lines of evidence support the recharge mechanism and magnitude determined

from hydrological data. Firstly, the chloride mass balance technique (Allison and Hughes, 1978; Houston,

Table IV. Water level rise in well J8 and calculation of recharge to the Chacarillafan

19981999 19992000

Events 1C2 Total

WL (m) 032 030 030R (m) 010 015 015

Afan (km2) 684 684 684

Sy 008 008 008Volume of groundwater

recharge (million m3)175 246 246

Recharge/runoff Not known 072 070Recharge/rainfall 025 034 018


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1990; Kruseman, 1997) has been shown to provide reliable quantification of long-term average recharge rates.

A simplified version of this method is given by

ClRF MAR D ClGW RECH

where ClRF is the chloride concentration in rainfall and Cl GWis the chloride concentration in groundwater, and

MAR and RECH are as defined previously. This formula is readily rearranged to make the recharge coefficient

(RECH/MAR) equal to the ratio between the chloride content of rainfall and groundwater. A recent study of

the chloride concentration in rainfall in the Altiplano has been conducted by Nazca SA and it can be shown

that mean values are around 12 mg/l. The chloride content of groundwater in J8 is around 128 mg/l (JICA,

1995), leading to an average long-term recharge coefficient of 009. This may be compared with a long-term

recharge coefficient of 006 determined from the year 2000 recharge, based on a 4 year return period and

mean annual rainfall for the Chacarilla catchment.

Secondly, as part of the same programme, a series of rainfall, baseflow and groundwater samples have beenanalysed for tritium concentration. The results of these studies will be reported elsewhere, but values within

or adjacent to the Chacarilla catchment are shown in Table V (note that Cerro Gordo is another well located

in fan sediments in the west of the Pampa Tamarugal).

There is a progressive decline in tritium content from rainfall, through baseflow in Quebrada Caya and the

Rio Chacarilla, to groundwater from wells on the west side of the Pampa Tamarugal. Using the radioactive

decay equation and adjusting precipitation values for known prior levels in the Altiplano (International Atomic

Table V. Tritium content and calculated mean residence time (MRT)for samples within and adjacent to the Chacarilla catchment

Tritium (TU) MRT (years)

Rainfall 367 021 0Quebrada Caya 134 019 39Rio Chacarilla 113 02 42Cerro Gordo well 073 016 50

WESTERN CORDILLERA

Rainfall

Runoff

Fan

rechangeFresh, recentfast moving water

Evaporation

COAST

RANGE

SALAR DE

BELLAVISTA

PAMPA TAMARUGAL

Stratified, oldslow moving water

PCFZ

CRFZ

69.5 W 69.0 W

SL

1000

3000

4000

5000

2000

0 10 20

Km

Early-Middle Mioceneclastic+volcanic sediments

Late Miocenefan sediment

Mesozoic basement++ +

++

+ +

+ +

+ +

+ +

+ +

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+ +

+

+++

++

+

+

+

+

+

+

+

+

++

Saline, residualstagnant water

Figure 9. Section through the Pampa Tamarugal and Chacarilla fan showing the inferred groundwater system and recharge pathway


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3032 J. HOUSTON

Energy Agency, Global Network for Isotopes in Precipitation (GNIP) database) it is possible to calculate mean

residence times (MRT) for the baseflow and groundwater. The MRT increase from a minimum of 39 years for

groundwater at high elevations within the Chacarilla catchment to 50 years for groundwater on the western

side of the Pampa Tamarugal. Such relatively low MRT confirm that recharge is taking place under current

climatic conditions and suggest that recharge entering the fan near its apex may be transmitted across the

Pampa Tamarugal through the near-surface fan sediments to reach the far western side relatively rapidly

(Figure 9). Hydraulic calculations based on data from the pumping test (see above), confirm that groundwater

may be transmitted from the fan apex to distal areas in 2030 years.

DISCUSSION

Rainfall in the Atacama Desert of northern Chile is highly variable and the non-linear response of the

hydrological system indicates a positive feedback, such that the causal mechanisms of rainfall variation willhave a correspondingly greater impact on groundwater recharge. It is interesting to assess possible causal

mechanisms for these variations. Rainfall in the Altiplano is linked with the El Nino Southern Oscillation

(ENSO) phenomena (e.g. Vuille, 1999) with La Nina years tending to produce higher rainfall. Rainfall also has

been linked to solar activity (e.g. Herman and Goldberg, 1978), with years of low solar activity being associated

with increased rainfall in the middle latitudes of western South America (Clayton, 1923) and a possible reason

for this has been proposed by Svensmark and Friis-Christensen (1997). Low solar activity allows increased

cosmic radiation to reach the atmosphere, which tends to increase cloudiness and precipitation. Data from

Ujina and Collacagua (Figure 10) provide support for the concept that either or both ENSO or solar activity

may have an impact on rainfall in the Altiplano region. Further investigation of rainfall variation and its

causes are clearly warranted.

Over the long term it can be anticipated that clusters of wet years will enhance runoff and recharge, whereas

conversely periods of drought will lead to reduced flow in rivers and groundwater recession. The cumulativerainfall departure shown in Figure 11 indicates that the late 1970s and mid-1980s were periods of above

average rainfall, whereas the late 1980s and 1990s have been a period of relative drought. It is interesting

to speculate whether all or part of the groundwater level decline in the Pampa Tamarugal reported by JICA

(1995) throughout the late 1980s and 1990s and monitored by the DGA in well J8 during the late 1990s is

the result of climate fluctuations on decadal to century scales as opposed to overexploitation. Further research

on these aspects are urgently required.

0

50

100

150

200

1970 1980 1990 2000

Meanmonthlyrainfall(mm)

4

3

2

1

0

1

2

3

4

Solaractivit

y

(standardisedsunspotnumber)El Nino

La Nina

solar activity

rainfall

Figure 10. Mean monthly rainfall at Ujina and Collacagua over the period 1970 2000 compared with solar and ENSO activity. Solar activitydata from World Data center-A, Boulder, CO and ENSO data from Climate Prediction Center, NOAA


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200

0

200

400

600

800

1970 1975 1980 1985 1990 1995 2000Cu

mulativedeparturefromm

eanrainfall(mm)

39.0

39.5

40.0

40.5

41.0

DepthtowaterinwellJ8(m)

rainfall

water level in J8

Figure 11. Cumulative departure from the mean rainfall at Ujina and Collacagua compared with groundwater levels in the Pampa Tamarugal.All data provided by the Direccion General de Aguas, Santiago, Chile. Data J8 plotted at same scale but adjusted for elevation

Not only are temporal variations in recharge important, but spatial variations also. Runoff from high-level

catchments clearly concentrates recharge through alluvial fans. As it is known that alluvial fans tend to be

dominated either by relatively impermeable debris flow sediments or more permeable sheetflood sediments

(Blair and McPherson, 1994) it can be expected that these different fan types will also display different

recharge characteristics. Further research is urgently required on the many different fans and their infiltration

characteristics throughout the Pampa Tamarugal and elsewhere in northern Chile.

CONCLUSIONS

A major flash flood took place in the Chacarilla catchment in January 2000 that can be quantified and followed

in its path from storm through runoff to recharge allowing a valuable insight into the associated hydrological

processes.

The Chacarilla hydrological system is composed of a high-altitude catchment area on both permeable and

impermeable rocks, a transfer channel through a deeply incised bedrock canyon, and a lower discharge zone

into the permeable alluvial fan. The fan is comprised of two main elements: a recharge zone close to the apex

extending along the active channel, and a transfer zone through the mid-sections of the fan.

Recharge takes place through this system, with water being transferred rapidly to the west through the

aquifer. Recharge is intermittent, related to significant flash floods draining the high-level catchment during

intense storm events.

The catchment response is non-linear with positive feedback, so that higher intensity, shorter duration eventslead to increased runoff and recharge coefficients. The 19992000 events are calculated to have generated

around 25 million m3 recharge. As this event has a return period of only c . 4 years, minimum average annual

recharge amounts to the equivalent of around 200 l/s.

Recharge mechanisms that are intermittent and non-linear are not readily quantifiable for groundwater

resource evaluation. They require an analysis of many factors, including possible variations in physical and

climatic parameters, over many years. Calculations based on average conditions are bound to be suspect,

although hydrochemical and isotopic methods do offer some hope of providing instantaneous answers.

The analysis of the January 2000 flood in the Quebrada Chacarilla allows a better understanding of the

recharge mechanisms and magnitudes, with a consequential increase in the reliability of groundwater resource

evaluation.


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ACKNOWLEDGEMENTS

Funding for this study was provided by Nazca SA. The Direccion General de Aguas and DSM Minera S.A.

supplied data. Ultra-low-level tritium analyses were carried out by the Environmental Isotope Laboratory of

the University of Waterloo in Canada.

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