A Conceptual Model of the Aquifer of the Yucat´an Peninsula
6.5 Hydrological Balance of the Underground Aquifer
The Yucat´an peninsula has a humid tropical climate. The average annual tem-perature is 25C. The months with the highest temperature are July and August;
those with the lowest are December and January. The thermal regime is very stable throughout the year. The combination of high temperatures with abundant vege-tation results in the evapotranspiration of about 85% of precipivege-tation. Therefore, approximately 15% of precipitation infiltrates into the groundwater.
Annual rainfall ranges from less than 800 mm in the northwest to 1,300 mm along the eastern coast and 1,700 mm on the island of Cozumel (Figure 6.4). Ap-proximately 90% of annual rainfall occurs between May and October. The average annual precipitation along the eastern coast of the Yucat´an peninsula is 1,200 mm, and that of Progreso is 500 mm (SARH, 1989), reflecting a nonuniform spatial distribution of rainfall.
The main rainy season occurs from June to September. However, annual pre-cipitation levels are highly variable spatially as well as both within and between years. In general, yearly precipitation levels are negatively correlated to popula-tion density and consequently to water extracpopula-tion levels. However, this situapopula-tion is partly compensated by underground water flows such as that flowing from the state of Quintana Roo to the state of Yucat´an.
1395° 1326°
Figure 6.4. Precipitation isobars (in mm).
The evapotranspiration values (Figure 6.5) used to calculate the aquifer’s water balance were those reported in the evapotranspiration and water deficit map of the Yucat´an peninsula published by INEGI (1983).[17] Infiltrated water was calculated as the difference between rainfall and evapotranspiration.
Most of the Yucat´an peninsula has no surface streams because of the prevailing low slopes, the shallowness of the soil, the proximity of the water table to the sur-face, and, above all, the high permeability of the underlying rocks. Consequently, the underground aquifer is the only source of fresh water for human activities. By and large, the aquifer is replenished by rainfall, which infiltrates into the aquifer rapidly.[18] Water inputs to the peninsula’s aquifer also include underground water inflows, predominantly in the state of Quintana Roo.[19] The aquifer’s main water outflows are transpiration; evaporation; water extracted for agricultural, industrial, and other purposes; and water flows discharged to the sea. The hydrological bal-ance of the underground basin for a given time period is calculated as the difference between water inflows and water outflows.
1626° 1503°
Figure 6.5. Evapotranspiration isobars (in mm).
Table 6.3. Current underground water usage in the three states of the Yucat´an peninsula (million m3/year).
Water use Campeche Quintana Roo Yucat´an
Agriculture 228.04 46.98 300.00
Households and urban services 75.97 99.24 235.00
Industrial 9.76 3.66 16.00
Other 0.33 0.40 19.00
Total 314.10 150.28 570.00
Source: Authors’ calculations.
One of the most important water outflows from the aquifer is the water used for the peninsula’s different development activities. Table 6.3 shows the annual volume of water used in each of the three states.
Water volumes of rainfall, infiltration, and evapotranspiration as well as those of underground water inflows and outflows were used to establish the water balance
Table 6.4. Current hydraulic balance for the states of Campeche, Quintana Roo, and Yucat´an, and for the Yucat´an peninsula (million m3/year).
Inflows Outflows
Campeche
Precipitation 74,712.22 Evapotranspiration 63,698.66
Underground flow to Yucat´an 150.00
Flow to the sea 10,549.56
Underground water extraction 314.00
Total 74,712.22 74,712.22
Quintana Roo
Precipitation 62,888.53 Evaportranspiration 48,902.12
Underground flow 2,485.00 Flow to Yucat´an 1,350.00
Rio Hondo’s base flow 1,500.00
Flow to the sea 13,471.13
Underground water extraction 150.28
Total 65,373.53 63 373.53
Yucat ´an
Precipitation 44,877.00 Evapotranspiration 35,902.00
Underground flow 1,500.00 Flow to the sea 9,905.00
Underground water extraction 570.00
Total 46,377.00 46,377.00
Yucat ´an peninsula
Precipitation 182,477.75 Evaportranspiration 148,502.78
Underground flow 2,485.00 Rio Hondo’s base flow 1,500.00
Flow to the sea 33,925.69
Underground water extraction 1,034.28
Total 184,962.75 184,962.75
Source: Author’s calculations.
of the aquifer in natural conditions (Table 6.4). The volume of water extracted was then subtracted to determine water availability.
The annual replenishment volume can be found by adding the figures for flow to the sea and underground water extraction, both in the outflows column of Table 6.4.
For the state of Campeche, this volume is about 10,863 million m3, the volume of water extracted is approximately 314 million m3. Therefore, the yearly availability of water in the state’s aquifer is approximately 10,549 million m3. Only 2.89% of the aquifer’s water for the state of Campeche is being used.[20]
The annual water replenishment volume in the state of Quintana Roo is ap-proximately 13,621 million m3; the volume of water extracted annually is about 150 million m3. Therefore, the water available in the aquifer amounts to roughly 13,471 million m3/yr. Approximately 1.1% of the aquifer’s water for the state of Quintana Roo is currently being used.
The annual water replenishment volume in the state of Yucat´an is roughly 10,475 million m3. The volume of water extracted amounts to about 570 mil-lion m3. Therefore, the water available in the aquifer in the state of Yucat´an is approximately 9,905 million m3/yr. Approximately 5.4% of the aquifer’s water is being used.
The annual water replenishment volume for the Yucat´an peninsula is roughly 34,960 million m3. The volume of extracted water is about 1,034 million m3/yr.
Therefore, the water availability in the peninsula’s aquifer is approximately 33,925 million m3/yr. In general, about only 3% of the aquifer’s water is being used (Ta-ble 6.4). Exploitation of the aquifer is highest in the state of Yucat´an and lowest in the state of Quintana Roo.
The water availability data referred to above suggests that more detailed stud-ies of the Yucat´an peninsula’s aquifer are needed, encompassing more information about the dynamics of current and likely future water usage and its relation to both environmental and water quality characteristics.
Notes
[1] Using the origin of the constituent geological materials as a basis for their categoriza-tion, Mexico’s aquifers include, among others, a group constituted by unconsolidated and sedimentary rocks as well as a group composed of volcanic rocks, in addition to the aquifer of the Yucat´an peninsula. The first group may occur in the Pacific Ocean, and in the Gulfs of California, Tehuantepec, and Mexico; the second may be found in the central part of the country and in the states of Sonora, Chihuahua, Baja California, Baja California Sur, and Tamaulipas (CNA, 1994).
[2] Translator’s note: As we are dealing with an aquifer, information concerning the peninsula’s geological characteristics is crucial.
[3] Although some hydraulic heads and underground flows have been assessed for dif-ferent geological materials of the Yucat´an peninsula, calculation of a more accurate balance of the underground aquifer has been hampered by the lack of hydrometric and piezometric information.
[4] Translator’s note: The Chich´en Itz´a Formation is a group of mainly limestone rocks from the Eocene that were deposited in a single, permanent marine basin without great variation in sedimentary conditions. Lithological and microfaunal content dif-ferentiate the three members (Weidie, 1974).
[5] Translator’s note: “The lower beds of the formation are represented by coquinas, which have a total thickness of less than 1 meter, overlain by yellowish, hard, massive limestones with mollusks, madrepores, and Peneroplidae. Above these are yellowish to reddish yellow and locally white, more or less hard, nodular, impure, arenaceous limestones, which may alternate with yellowish marls, sands, and sandstones. The upper levels . . . are represented by yellowish to white, hard limestone with arenaceous interbeds” (Weidie, 1974:6).
[6] Translator’s note: “These limestones consist of cream-colored coquinas with a porous cryptocrystalline calcareous matrix, which are strongly weathered locally. They con-tain large quantities of mollusk shells” (Weidie, 1974:6).
[7] The limestones’ aquifer-related characteristics originate from the rocks’ secondary porosity; generally, the limestones’ primary porosity is at a low or medium level.
[8] Translator’s note: Hardening of surface limestone is widespread on the Yucat´an peninsula during the dry season, when upward-moving pore water saturated with cal-cium carbonate (CaCO3) re-precipitates calcite in the near surface zone (Isphording, 1974).
[9] Translator’s note: “Soft sascab has been used by the Maya for centuries . . . as a source of lime to soften corn, for plaster, and as a raw material for cement” (Isphording, 1974:79).
[10] Translator’s note: This set of lagoons and wetlands is of paramount importance for the peninsula’s biological diversity.
[11] Translator’s note: The biodiversity changes triggered by this impact may be related to the appearance of mammals, and therefore of Homo sapiens, on Earth.
[12] Different hydraulic conductivity values have been determined for different parts of the Yucat´an peninsula’s underground aquifer. For instance, M´endez Ramos (1991) used a hydraulic conductivity value of 0.064 m/sec in a mathematical model of the M´erida aquifer. Modeling the aquifer of the northwestern part of the Yucat´an penin-sula, Mar´ın (1990) used ak-value of 1.0 m/sec for a high- and 0.1 m/sec for a low-permeability layer with the latter underlying the former. Reeve and Perry (1990) determinedk-values between 0.0003 and 0.5 m/sec in a site located north of M´erida near Chuburna Puerto. Mart´ınez Guerra (1990) usedk-values ranging from 0.001 to 0.01 m/sec in a mathematical model of the island of Cozumel. Gonz´alez Herrera (1984) determinedk-values between 0.000001 and 0.005 m/sec in the laboratory us-ing rock samples obtained at depths up to 80 m deep in the city of M´erida. Villasuso et al. (1984) and Villasuso (1990) applied a similar method in the field for assessing hydraulic conductivity in wells. Theirk-values ranged from 0.00032 to 0.0087 m/sec.
Back and Lesser (1981) determined ak-value of 0.01 m/sec.
[13] Three karstic ducts of preferential underground water flows were recently determined using geophysical records; they are located at depths of 8–12 m, 20–22 m, and 28 m.
These ducts were associated with previous positions of the water table and with vari-ations in the sea level during the Pleistocene (Buckley et al., 1994).
[14] The levels of underground water, measured in the northwestern part of the state of Yucat´an, range between 0.45 m above the average sea level near Chuburna and 2.1 m above sea level in Sotuta, in the central part of the state. The variation of the water table between the dry and rainy seasons ranged between 0.05 m and 0.6 m in a two-and-a-half-year study that undertook such measurements (Mar´ın, 1990).
[15] These values were obtained from chloride variation curves (Villasuso et al., 1984).
[16] Translator’s note: Waste disposal represents both a hydrologic and a health problem, particularly in the northern and northwestern parts of the peninsula, where water ex-traction needs are highest because of population density and where precipitation is lowest. In some cases, this situation is further aggravated by the proximity to the sea.
Water-borne diseases are common. Likewise, the conjunction of population density and the development of tourism activities increases the likelihood of future problems regarding the availability of good quality water.
[17] Monthly and annual evaporation data, based on daily measurements in class A evap-orimeters, are available from meteorological stations on the Yucat´an peninsula. How-ever, the determination of a water balance for the regional aquifer requires evapotran-spiration data. Such data generally are not available directly. However, they can be estimated using site-specific data for climatological variables such as solar radiation, relative humidity, wind speed, and temperature. The method used here to estimate evapotranspiration was that developed by Thornwaite in 1948.
[18] Translator’s note: It is not uncommon to find mixed areas of shallow soils and areas of bare soil. In any case, it is the permeability of the rocks at the surface or underneath the soil that is central to the rapid infiltration of rainfall.
[19] The underground water inflow from outside the peninsula to the state of Quintana Roo amounts to approximately 2,485 million m3/yr (Lesser, 1980).
[20] Translator’s note: Because the dynamic features of the aquifer have not yet been prop-erly assessed, at this stage it is difficult to determine the practical significance of the
“low” percentage usage arrived at in these balances. For instance, 3% utilization of the aquifer, as low as it seems, may pose stability problems for the aquifer given that at some points the layer of fresh water is highly variable, thin, or highly susceptible to pollutants or to increases in extraction volumes and flows, and therefore to intrusion of saline water.
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