Groundwater flow regime under natural conditions as

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Journal of
Arid
Environments
Journal of Arid Environments ] (]]]]) ]]]–]]]
www.elsevier.com/locate/jnlabr/yjare
Groundwater flow regime under natural
conditions as inferred from past evidence
and contemporary field observations in a
semi-arid basin: Cuenca de la Independencia,
Guanajuato, México
Ignacio Navarro de Leóna,, Jaime Gárfias-Solizb,
Jürgen Mahlknechtc
a
Facultad de Ciencias de la Tierra, Universidad Autónoma de Nuevo León,
Carr. Linares—Cerro Prieto km. 8, Linares, CP 67700 Nuevo León, México
b
Centro Interamericano de Recursos de Agua, Universidad Autónoma del Estado de México,
Cerro de Coatepec S/N, Toluca, CP 50130 Estado de México, México
c
Centro de Estudios del Agua, Instituto Tecnológico de Estudios Superiores de Monterrey,
Avenida Eugenio Garza Sada # 2501, Col. Tecnológico, Monterrey, CP 64849 Nuevo León, México
Received 15 October 2004; received in revised form 10 March 2005; accepted 20 April 2005
Abstract
This study was carried out in the Cuenca de la Independencia, a semi-arid basin in Central
Mexico. The objective is to describe the main features of a groundwater flow regime under
natural conditions, based on groundwater discharge manifestations. Information obtained
from paleoecological, paleontological, archaeological and historical data suggests that, prior
to heavy development (starting in the 1950s), the hydrogeologic regime was characterized by a
larger groundwater availability in a more humid and colder climate. Manifestations associated
to groundwater discharges are springs, lagoons, wetlands, saline soils, chalcedony deposits,
phreatophytes, thermalism, and artesianism. The different types of manifestations and their
position in the basin indicate the influence of groundwater flow systems hierarchically nested,
forming concentric zones at ground level. The groundwater flow regime corresponds to a
Corresponding author. Fax: +52 821 2142020.
E-mail address: inavarro_23@yahoo.com (I. Navarro de León).
0140-1963/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jaridenv.2005.04.003
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classical gravity-induced flow system with generation of local, intermediate and regional
patterns. Integrating several types of data to establish the flow geometry and its dynamics has
proven a useful tool to increase understanding of the original groundwater regimes. This
approach can also be applied in other over-exploited semi-arid basins.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Groundwater flow regime; Paleohydrogeology; Regional groundwater flow system; Central
Mexico; Semi-arid basin; Guanajuato
1. Introduction
In Mexico, arid and semi-arid climates cover more than half of the territory being
located mainly in the Altiplano Mexicano (Mexican Highlands) in the central and
northern part of the country. The annual rainfall in these areas is 20% of the
national total. The population is about 77 million producing 84% of the gross
national product (CNA, 2002). The physiographic conditions, the population
distribution, and the development of the economic activity generate contrasting
regions of hydraulic resource availability. Groundwater is the main (often only)
water resource in semi-arid basins. In about a 100 Mexican basins, the excessive
groundwater extraction during the last three decades has caused important
alterations in the hydrogeological balance. These alterations are made obvious by:
(a) the fast lowering of the water level; (b) the continuous reduction of spring and
river discharges; (c) a significant deterioration of the ecosystems that depend on the
shallow phreatic level; (d) remarkable changes in the hydrochemical quality that
affect agricultural activity and human consumption; and (e) increasing intensity and
extent of the subsidence phenomenon. This has brought about complex environmental, economic and social problems (CNA, 2002). The state of Guanajuato
has historically been known for its outstanding agricultural production, but at the
same time as the second largest groundwater consumer at national level
(2723 Mm3 year1). Additionally, it is the state that faces the most serious
problems derived from over-exploitation (SAGAR, 1999).
The Cuenca de la Independencia (CI) is located in the most arid region in the state
of Guanajuato (UNAM-CODER, 2000) (Fig. 1). The first deep wells were drilled
during the 1930s, and since then the number has increased to more than 2500 wells,
mainly used for irrigation purposes (CEASG, 1999). In the last 30 years, the
groundwater extraction rates have greatly surpassed the average recharge volume,
causing important alterations in the hydrogeologic regime. At present, the predevelopment conditions of groundwater flow can only be inferred through the
groundwater manifestations mentioned above. The description can be used to
improve the understanding of flow dynamics in the basin. This enables to analyse
hydrogeologic information and historical piezometric data, as well as to explain the
occurrence of similar phenomena in other semi-arid basins. Furthermore,
conceptualization of groundwater flow through the basin (conceptual model
development) mainly determines the interpretative and predictive capabilities of
numerical models (Anderson and Woessner, 1992).
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Fig. 1. Map of Mexico indicating the location of the State of Guanajuato and the Cuenca de la
Independencia.
The purpose of this research is to describe the main features of a hydrogeologic
regime under natural conditions. This was done by analysing distribution and
characteristics of relict manifestations associated to groundwater discharges. These
manifestations are springs, wetlands, phreatophytes, saline soils, and mineral
deposits, as well as the existence of thermalism and artesianism phenomena. The
results not only demonstrate the utility of various investigative approaches in
evaluating the dynamic nature of groundwater flow systems in a regional context,
but also provide insight into some of the factors that control groundwater overexploitation effects on ecosystem deterioration.
2. Study area
2.1. Basin description
The CI covers a total area of 6840 km2 and is located in the northeastern portion
of the state of Guanajuato (Fig. 1). The basin is composed of a plain with an
elevation that ranges from 1900 to 2100 masl (meters above sea level), surrounded by
mountains reaching 2850 masl. It has a major axis of 135 km in NNW-SSE direction
and a minor axis of 76 km oriented NNE-SSW.
Three climate types predominate in the area with variations in temperature and
winter precipitation. Climatic differentiations are caused by humid air coming from
the Pacific Ocean and the area orography. In the S and SW of the Sierra de
Guanajuato prevails a warm climate; in the Sierra de Guanajuato and Sierra de San
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Fig. 2. Perspective view to the north of the CI showing the main physiographic and geomorphic features.
It also indicates the location of cities and sites referred in text. The cities are: San Felipe (SF), Dolores
Hidalgo (DH), San Miguel de Allende (SMA), San Diego de la Unión (SDU), San Luis de la Paz (SLPz),
Doctor Mora (DM), and San José Iturbide (SJI).
José Iturbide a subhumid condition is observed; and in the lower parts of the north
and west of CI the climate is dry. Meteorological information obtained from 60
stations, with an average of 30-year registration (INIFAP, 1998), indicates a mean
precipitation of 576 mm year1. The highest rainfall registered is in the Sierra de
Guanajuato (4800 mm year1) and the lowest (o300 mm year1) in the northeast
of the basin area. Additionally, the annual mean temperature is 16 1C
(range ¼ 9–25 1C), with a mean potential evaporation of 1828 mm year1. The
average infiltration rate is 25 mm year1 according to chloride mass balance results
reported by Mahlknecht et al. (2004).
Predominating soils are phaeozems and vertisols covering approximately 80% of
the surface. Luvic phaeozems are present from the central part down to the southern
border of the basin. Haplic phaeozems are dominant in the north and southeast.
There are pine-oak forests in the mountains, grasslands and desertic shrubs down in
the plain. Lately, natural vegetation has largely been removed from the plains for
farmland activities.
The CI is located in the southern part of the physiographic province of Mesa
Central specifically in the Llanuras y Sierras del Norte de Guanajuato subprovince.
The main geomorphologic features are mountain of tectonic-volcanic origin
dominated by erosional processes. The plateau origin is tectonic-alluvial and
fluviolacustrine, depending mostly on accumulation processes (Fig. 2). It is a system
of valleys interconnected by stretchings, hills, and small gullies.
2.2. Geology
Three geological provinces intersect at the CI, the Sierra Madre Occidental
(SMOc), Sierra Madre Oriental (SMOr), and the Transmexican Volcanic Belt
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(TMVB) (Ferrari et al., 2000; Aranda–Gómez et al., 2003). The oldest rocks in the
area are of volcanic, plutonic and vulcanosedimentary origin of late Jurassic and
early Cretaceous ages. In late Cretaceous a thick sequence of limestones, sandstones,
and marls was deposited (SMOr). The sequences are exposed in the east (Sierra de
Mineral de Pozos) and southeast (Sierra de los Cuarzos) of the basin. During late
Cretaceous and early Tertiary (middle Miocene), voluminous volcanism of
intermediate to silicic composition (SMOc) took place in the north and west of
the CI (Sierra del Cubo and Sierra de Guanajuato). However, late Tertiary is
characterized by the eruption of important volume of andesitic, rhyolitic, and
basaltic rocks (TMVB). In the south and southeast of the CI, three big
stratovolcanoes were emplaced and caused the definitive hydrogeologic closing of
the basin. The episodic tensional events occurred during Tertiary provoked
differentiated sinking of tectonic blocks. The depressions formed were simultaneously filled with volcanoclastic sediments under a terrigenous and fluviolacustrine
environment. At the foot of the mountains, the sequence is composed of breccias and
partially consolidated conglomerates, with blocks and gravels in a matrix of medium
to coarse sands. The plain consists of gravels, sands, silts, clays and volcanic ashes.
The thickness varies some meters at the foot of the mountains and 100–460 m in the
center of the basin. The evolution of the basin and a detailed geologic description is
presented by Alanis-Ruiz (2002).
2.3. Hydrology and hydrogeology
The eastern border of the CI represents a continental divide, where the runoffs are
split up toward the Gulf of Mexico (Panuco River) and into the Pacific Ocean
(Lerma and Santiago rivers). The Laja River is the most important stream and the
only of perennial type. The average runoff recorded at the basin outlet is
258 Mm3 year1 (CNA, 1997). The drainage network is mainly of dendritic type.
The hydrogeology consists of three hydrostratigraphic units: (a) basal unit of low
permeability that performs as an aquiclude; composed of sedimentary, volcanosedimentary and plutonic rocks of late Jurassic and early Cretaceous; (b) fractured
aquifer made up of marine sedimentary sequences of the late Cretaceous and all the
later volcanic emplacements; and (c) porous aquifer composed of clastic sediments
filling the tectonic depressions and forming a vast plain. The hydraulic conductivity
mean value for both aquifers is 2.01 and 4.21 m d1, respectively. Total groundwater
recharge calculated in 1991 by water balance approach is estimated in
170 Mm3 year1 (CNA, 1992).
3. Materials and methods
Sedimentary basin research during last four decades has shown that, under gravity
force influence, water flows through a geologic medium in spatially defined patterns
(Ground Water Flow Systems, GWFS). Three flow system types are recognized: (a)
local, when recharge and discharge areas are contiguous; (b) intermediate, when
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there is one topographic elevation, or more, between recharge and discharge areas;
and (c) regional, when the recharge area is located at the border of the basin and
discharge area is situated in the lowest part. GWFS generate diverse types of surface
manifestations: hydrological, hydraulic, chemical, mineralogical, biological (vegetation), edaphological, geomorphological, etc. (Ingebritsen and Sanford, 1998; Tóth,
1999). Groundwater flow characteristics are unique in each basin and the
manifestations and phenomena produced are diverse in their forms and intensities.
Thus, there is no specific methodology for general applicability to analyse and
describe hydrogeologic regimes. Methods to investigate the hydrogeologic environment most commonly used (GWFS domain delineation and characterization)
include field observations, historical records, hydrogeochemistry, isotopic analysis,
numerical modeling and remote sensing techniques (Durazo and Farvolden, 1989;
Salama et al., 1994; Edmunds et al., 1999; Batelaan et al., 2003).
The methodology applied in this research involved three stages. First, a
compilation, generation and analysis of basic information was carried out. These
activities included gathering information from historic hydrogeological data and
from thematic maps of the state of Guanajuato (SRH, 1970; SPP, 1980).
Information on springs was initially obtained from a set of geographic maps and
inventories provided by local, state, and national water supply management
authorities (SRH, 1970; INEGI, 1981; SARH, 1981; CEASG, 1999). The analysis
of digital elevation models provided drainage network through automated
generation and basin delimitation from slope analysis. Also, runoff and accumulation areas were located using the methods described by Martz and Garbrecht (1992).
The Landsat-7 TM satellite images and aerial photographs (1:70,000 scale) were used
to identify geomorphic and geologic features, ancient drainage networks, saline soil
coverage along with ancient lagoons and bogs. The remote sensing methodology
used to locate recharge and discharge areas is reported by Salama et al. (1994). The
information was incorporated into a geographical information system (GIS) to
facilitate its handling. Secondly, an analysis of historical data and evidence search
about specific aspects related to groundwater discharge manifestations was done
(SRH, 1970). The activities taken into account were the examination of historical
archives and information exchange with specialists of other disciplines in order to
document manifestation sites. The localities documented include fossiliferous areas,
vestiges of early human settlements and ancient waterworks like hand-dug wells,
irrigation channels and aqueducts. The third stage consisted of fieldwork in order to
make an inventory of groundwater discharge manifestations. This comprised direct
observation, detailed description and georeferenced location with a handheld global
positioning system. This stage was facilitated by the area toponymy, because many
places are allusive to these manifestations.
4. Results and discussion
In a general way, the role of groundwater flow in a regional context has been
recognized as a geologic agent, acting continuously during hundreds or thousands of
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years, producing surface manifestations (Tóth, 1999). However, these inferences
have been obtained from observations in basins with cold and humid climates, where
groundwater availability is larger than in arid and semi-arid regions (Edmunds et al.,
1999). Taking into account that the climate is one of the factors on which the
number, magnitude and diversity of the manifestations depend, it would be logical to
deduce that evidence and manifestations in semi-arid regions were less remarkable or
impressive than in the most humid climatic regimes. The climatic variability and the
tendencies in the global climate change are also recognized, along with their timescales which are over hundred and thousands of years (O’Hara et al., 1994; Eckhardt
and Ulbrich, 2003). However, the noticeable increase in the groundwater abstraction
rates started in the 1950s and the effects on the ecosystem deterioration were
accelerated since the 1970s (SRH, 1970; SARH, 1981; CNA, 1992; CEASG, 1999).
Under this working hypothesis, initially old evidence was first investigated in order
to confirm the existence of a humid climate with ecosystems different from the
present. Paleoclimatic conditions suggesting humid and savanna-type environments
in the CI was supported by the fossil record and the existence of paleomeanders
forming a complicated drainage network as revealed by satellite images and aerial
photographs (Carranza-Castañeda and Ferrusquı́a-Villafranca, 1978). Also, archaeological evidence points out that abundant water availability was determinant
for the settlement of an indigenous group in the CI more than two millennia ago
(Nieto-Gamiño and Brown, 2001).
With the arrival of the Spaniards and later conquest of El Bajı́o (Lowlands),
during the XV and XVI centuries, surface water and springs were decisive to
determine the commended land extensions or given in graces to the settlers (Wright,
1998). In the case of the CI, groundwater discharges were not only considered for
land allotment, but rather they influenced directly on the foundation place of some
villages. For example, the original foundation of San Miguel village took place in the
proximity of a spring called Yzquinapan, that means ‘water of dogs’ in Náhuatl
language.
The oldest known map of the western portion of the basin is shown in Fig. 3. This
pictorial document, dated in 1580, is commonly referred to as ‘Pintura de San
Miguel y San Felipe’. The document was part of the Relaciones Geográficas de la
Nueva España and probably related to the ‘Chichimeca Códice’, whose text was
strayed (Nieto-Gamiño, pers. comm.; Wright, 1998). The map was manufactured on
a mixture of paper and maguey. At present, the original document is in the Sala de
Consejo de la Real Academia de Historia de Madrid. Undoubtedly, the document
mentioned above is the most important testimony describing the geographical,
economic, social and environmental aspects of the region at the time. The river and
springs are shown as strategic resources in the incipient colonization and regional
development. San Miguel and San Felipe represented the most important population
centers (Fig. 3 (1–2)). These sites played a decisive role in the colonization of
northern New Spain, whose expansion was accelerated by the discovery of silver
veins in Zacatecas (1546). A route running parallel to the Laja River was readily
established (Fig. 3 (3)). This road from Zacatecas to Mexico City was known as
‘Ruta de la Plata’ (Silver Road) (Fig. 3 (4)). There were numerous springs in San
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Fig. 3. Map of the western portion of CI, known as ‘Pintura de San Miguel y San Felipe’ (1580). (1) San
Miguel, (2) San Felipe, (3) Laja River, (4) ‘Ruta de la Plata’, (5) springs, (6) pine/oak forests, (7) Altepec or
‘Cerro de Agua’, (8) Water mill or batan. Original map measures 82 61 cm.
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Felipe valley locally referred to as ‘ojos de agua’ (water eyes) (Fig. 3 (5)). Also,
forests occupied the surrounding mountains (Fig. 3 (6)). The springs at the foot of
the mountains (Fig. 3 (7)) were denominated ‘Altépec’ that means ‘water hill’ in
Náhuatl language. In 1552, a water mill or ‘batan’ was built by friars (Fig. 3 (8)), and
was initially used for grain milling (Wright, 1998).
At the end of the XVI century, the hacienda (homestead) property system
substitutes the commendations and graces as land ownership and water control
system in New Spain (von Wobeser, 1989). During more than three centuries, from
the end of the XVI to the beginning of XX century, several haciendas were settled in
the CI for a diverse range of activities. The haciendas were responsible for
developing hydraulic infrastructure, building irrigation channel systems as well as
hand-dug wells. From the hydraulic point of view, these old wells represent the first
direct and quantitative evidence of groundwater level conditions before severe
exploitation. A total of 80 ancient wells were documented in the area. The depth
ranges from a couple of meters to 40 m, with an average of 25 m. Today most of
these wells have dried up. The depth well is an indication of the ancient groundwater
level condition if we consider that, once the water level was reached, it was not
possible to dig more than a few decimeters. The obtained phreatic level information
derived from these wells can be used for numeric flow model calibration under
steady-state conditions, and it is useful for setting up initial conditions for transient
flow numeric simulations (Navarro de León et al., in preparation).
4.1. Biological and hydraulic manifestations
The prevailing savanna environments during the XVI century in the CI and El
Bajı́o were documented by Butzer and Butzer (1997). The Laguna Seca and San Luis
de la Paz valleys had abundant wetlands and ponds representing groundwater
discharge. Also, Dı́az de Gamarra, in his 1777 description of La Villa de San Miguel
El Grande y su Alcaldı́a Mayor, mentions the presence of plants growing near
springs. These phreatophytes are known as guau (Parthenocissus quinquefolia) and
rejaldar (Thevetia thevetioides) (AMSMA, 1994). Nevertheless, the most typical
phreatophyte is the ahuehuete (Taxodium mucronatum), commonly known in the
region as ‘sabino’ or ‘cypress’. The historical archives of the CI cities mention
ahuehuetes growing in spring and stream borders (Butzer and Butzer, 1997).
However, in the 1970s, these phreatophytes dried up due to water level lowering
caused by the intensive groundwater pumping (CEASG, 1999).
The spring documentation provided information about the number, characteristics and distribution of groundwater discharge. The springs were categorized in
perennial and seasonal, current and extinct, with thermal and cold discharges,
discharging inward and outward the basin. A total of 201 springs were identified, 113
of them discharge inside the basin. At present, the number of active springs has
diminished notably, with only 44 having permanent discharges; 91 are active during
the rain season; 66 have dried off during the last years. The relation of height and
distance for each spring to the nearest divide is shown in Fig. 4. A major density of
springs is observed between 2050 and 2150 masl heights. This is due to a change in
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Fig. 4. Spring distribution according to height and divide distance. The springs are supplied by local (L),
intermediate (I) and regional (R) flow systems.
the slope and hydraulic properties, when water passes through the fractured-porous
aquifer limit. Furthermore, the horizontal distribution differences are evident,
suggesting that discharges are controlled by GWFS geometry. Practically, 90% of
the springs are supplied by local flow systems. The mountain springs (35%) have
fluctuations in their discharges, depending on the precipitation regime. The springs
in the plain (65%) generally show a smaller annual variation in their discharge rates,
although these rates have diminished since the porous aquifer was exploited
intensively. Spring discharge rates has been reported by SRH (1970), SARH (1981),
CNA (1992) and CEASG (1999). The morphological characteristics of the
surrounding mountains are a decisive factor for the spring distribution (Fig. 5). In
the Sierra de Guanajuato, the springs are equally distributed on both sides of the
divide. By contrast, the springs are absent inside the northeast divide, where all
springs discharge toward the Panuco River. This situation reverses up in the north
where almost all springs discharge inside the basin.
4.2. Mineralogical and edaphological manifestations
Saline soils cover important extensions in north and northeast Atotonilco. There
are also saline soils in the San Jose Iturbide–Doctor Mora and Laguna Seca valleys
covering small areas. The salt deposited in soils is mainly composed of sodium
carbonate and correlates with the groundwater geochemical evolution in the CI
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Fig. 5. Groundwater flow system domain and manifestation areas in CI.
(Mahlknecht et al., 2004). Groundwater in recharge areas, where local flow systems
dominate, are calcic-bicarbonate type. In contrast, a sodic-bicarbonate domain is
observed in discharge areas where the three flow systems converge.
The Atotonilco area contains a chalcedony deposit as a result of groundwater
discharge with high silica content. The silica origin can be explained because of the
silicate mineral dissolution present in volcanic rocks. The accumulation of
chalcedony confirms the hydrogeochemical modeling results that characterize the
groundwater in this area as chalcedony-oversaturated (Mahlknecht et al., 2004).
Several decades ago the Atotonilco and San Miguel groundwater discharges, as well
as the Laja River flow, were abundant compared with the present conditions. It was
almost unnecessary to drill deep wells for irrigation or water supply. However, when
they were drilled, some wells presented the artesianism phenomenon, even in the
mid-1970s.
4.3. Natural hydrogeologic conceptual model
The types of manifestations and their positions inside the basin suggest the
presence of three areas with defined characteristics (Fig. 5):
Area I.
This area is located at the foot of the mountains, in a mean distance of
5 km from the basin border, including the granular and fractured aquifer
limit. Four places are characterized by cold springs and phreatophytes:
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Area II.
Area III.
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the San Diego community south of San Jose Iturbide; the oriental section
of the San Luis de La Paz municipality; the northeast of San Felipe city
and the Puerto de Nieto community in San Miguel de Allende. Saline
soils at the center of San Jose Iturbide–Doctor Mora valley, are a result
of a local flow system discharge.
This area covers most of the central plain, which includes the Central
Graben and Laguna Seca valley. In the northern part of the latter, there
is an area with manifestations such as lagoons, wetlands, saline soils and
phreatophytes. Today only saline soils are observed.
It is situated at the center of the basin between Dolores Hidalgo and San
Miguel, having an area of approximately 400 km2, representing the most
important area of manifestations. In this region, the ground was prone to
floods, with widespread bogs and phreatophytes. The saline soils covered
important extensions alternating with chalcedony deposits. The
groundwater is thermal with temperatures close to 40 1C. The
artesianism was a common phenomenon in most of the wells.
The spring discharges were abundant, but they have recently dried out.
Some wetlands have disappeared and artesianism has decreased.
The manifestation areas correspond to the influence of three flow patterns or
systems. Area I, near the basin border, with cold springs of variable discharges
throughout the year, is controlled by local flow systems. The presence of lagoons,
wetlands and saline soils in area II, suggests a constant groundwater supply coming
from a deep intermediate flow system. Finally, thermal springs, artesianism, salt
deposits and chalcedony accumulation in area III, are produced by discharge of
deeper groundwater with high temperatures and mineral contents. These manifestations are typical of a regional groundwater flow system. In area II, a local flow
contribution is possible, whereas in area III the participation of local and
intermediate flow depends on the system’s geometry.
A conceptual model is proposed based on geomorphologic and hydrogeologic
basin characteristics, as well as on groundwater flow evidence and surface
manifestations. The movement of groundwater in the CI is a classical gravityinduced flow system. Recharge takes place in the mountainous areas and
groundwater flows to discharge in diverse regions of the basin, depending on the
geologic and topographic situation. A section delimited by the points ABCD is
shown in Figs. 5 and 6. The Sierra de Guanajuato (between points AB) represents the
groundwater divide and recharge areas are also located there. Groundwater flows
through three systems: one local flow that discharges near point A and deeper
intermediate and regional systems continuing toward the southwest to the El Bajı́o
region. Toward the interior of the basin, water recharged from the Sierra de
Guanajuato generates triple flow systems that discharge in the region indicated by
point B and toward the basin outlet. In the oriental part (point D), a continental
groundwater divide is located separating flows toward the Panuco River and inside
the basin. The water recharged in the proximities of this divide, discharges through a
local system in San Jose Iturbide–Doctor Mora valley. In the Laguna Seca valley,
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Fig. 6. Natural hydrogeologic conceptual model for CI. The groundwater flow is a classical gravityinduced flow system. The flow is generated in the basin borders and discharge in the lowest part of basin,
describing defined patterns and producing surface manifestations.
another system of local type system is generated. The water from both valleys
discharges in the center of the basin (point B) through an intermediate system and
flow continues toward the south, in the same direction as the superficial flow. Point B
is the region of convergence for the three flow systems. These interpretations are
supported by modeling studies (Navarro de León et al., in preparation) that quantify
many aspects of the groundwater regime described only qualitatively in this paper.
The conceptual model proposed with GWFS domain, describing surface
concentric patterns, has important implications for groundwater resource management and protection. The GWFS are dynamic entities operating in diverse space and
time-scales, having an order and distribution that correspond to these scale domains.
The local flow systems are the most dynamic and react more rapidly to changes
induced by recharge or extraction. For this reason, they are regarded as seasonal
resources, while the other systems are considered secular resources (Tóth, 2000).
5. Conclusion
Evidence and groundwater flow manifestation analyses have provided information about the hydrogeologic regime under natural conditions in the CI. The
paleoclimatic, paleontologic, archaeologic and historic evidence indicate that in the past
there were conditions of larger water availability. Savanna-type environments prevailed
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in the basin with abundance of surficial water and groundwater. The excellent
ecological conditions allowed human groups to settle down in the lowest areas of the
basin more than 2000 years ago. Groundwater discharge sites also played an important
role in the foundation of the first colonial cities in the region almost 500 years ago. The
groundwater shallow levels in the plain facilitated the rudimentary perforation of wells
which supplied water for the haciendas during the centuries XVI–XIX.
The manifestations associated to groundwater discharge are springs, lagoons,
wetlands, bogs, saline soils, chalcedony deposits, artesianism, thermalism and
phreatophytes. The types of manifestations and their positions in the basin suggest
the existence of concentric areas influenced by three GWFS: (a) area I, located in the
mountains and within the plain limits with cold springs and phreatophytes; (b) area II,
located in the central plain, where lagoons, bogs, saline soils and phreatophytes were
abundant; and (c) area III, located from the basin center to the basin outlet, in addition
to the manifestations previously mentioned, where chalcedony deposits along with
thermalism and artesianism phenomena are located. In these three areas, the GWFS
contributes with dominance of local, intermediate and regional order, respectively.
The hydrogeologic regime in the CI is characteristic of a classical flow system
controlled by gravity force. Groundwater recharge takes place near the mountainous
areas and discharge in diverse basin regions, depending on the geologic and
topographic situation. Manifestations of groundwater discharge indicate that the
GWFS convergence coincides with the basin center and continues in the same
direction as the surface flow to the basin outlet in the south.
Acknowledgments
The first author shows his appreciation to Universidad Autonóma de Nuevo León
for the support granted through the Programa de Mejoramiento Académico del
Profesorado (PROMEP). The access to relevant information and studies has been
made possible by the Consejo para el Desarrollo del Noreste de Guanajuato
(CODEREG), the Comisión Estatal del Agua de Guanajuato (CEAG), the
Comisión Nacional del Agua (CNA) and the Universidad Nacional Autónoma de
México (UNAM). We thank Soledad Medina for the bibliographical and
documental investigation. We also thank to Oscar Carranza, Jaime Durazo and
Luis Felipe Nieto for their valuable comments and contributions on paleontology,
hydrogeology and archaeology, respectively. Thanks to Manuelita González and
Fernando Velasco for their comments and suggestions in an earlier version of the
manuscript. Thanks are also given to Esteban Jobbágy and an anonymous referee,
whose valuable comments substantially improved the manuscript.
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