63
thickness. Clay lenses may produce local confinement in a few places, whereas the
aquifers may connect with each other under certain conditions (Mahlknecht et al.
2008). The climate in the area is semi-arid, semi-hot with summer monsoon rains
(García 2003), with highest temperatures occurring between June and August when
the mean temperature reaches 27 °C and the lowest temperatures occurring in
January when the mean temperature is 10 °C. The mean annual precipitation is
338 mm and the evaporation rate exceeds 2400 mm. The vegetation cover belongs to
scrubland and grassland that support livestock grazing. A small fraction of the land
is irrigated to grow crops such as pecan trees, alfalfa, and cotton.
Water Level (WL)
The evolution of the WL was measured in 2004, 2007, 2010, 2011, and 2015. We
complemented these data with those reported by CONAGUA from 1978 to 1999.
Historical water data, WL measurement, and water sample collection are described
in more detail in Alarcón-Herrera et al. (2013) and Reyes Gómez et al. (2015). The
WL measurements and water samples were obtained from 31 wells in TA, 14 in SD,
and seven in LH (see Fig. 4.2). The WL drop rate was calculated as the difference
between the oldest WL measurements minus the most recent value, divided by the
number of years between the two measurements. At the same time, WL was measured in seven of the wells (spring, summer, and fall 2015, fall 2016, and winter
2016–2017 in order to appreciate the seasonal variations in WL).
Changes in Land Cover Use (CLCU)
CLCU rates were determined at the scale of the Chihuahuan Desert, a region representative of arid and semi-arid zones of Mexico. Then, we proceeded in the same
way but considering surface cover of three semi-urbanized aquifers of the state of
Chihuahua, in the center of the Chihuahuan Desert. This way we were able to relate
changes in land use with the chemical quality of the water of a well network monitored in that zone of aquifers of Chihuahua. CLCU was determined from a 1:250,000
digital model obtained from Series II (INEGI, 1993) and V (INEGI, 2013). Land
use types were classified as: agricultural, urban, water bodies, and non- vegetated
land, while vegetation cover grown as a result of primary and secondary succession
was classified according to the following three general classes: (1) desert scrubland,
(2) grassland, and (3) others. Primary succession indicates vegetation that has suffered little alteration after it has been established and secondary succession an area
that has been disturbed by either natural or man-made processes.
CLCU layers were processed using the ArcGIS 10.3 TM software. The loss of
vegetation cover was determined after comparing areas that had been previously
assigned to a different class, either agricultural, urban, water body, or areas with no
4 Changes in the Vegetation Cover and Quality of Aquifers in the Drylands of Mexico…
thickness. Clay lenses may produce local confinement in a few places, whereas the
aquifers may connect with each other under certain conditions (Mahlknecht et al.
2008). The climate in the area is semi-arid, semi-hot with summer monsoon rains
(García 2003), with highest temperatures occurring between June and August when
the mean temperature reaches 27 °C and the lowest temperatures occurring in
January when the mean temperature is 10 °C. The mean annual precipitation is
338 mm and the evaporation rate exceeds 2400 mm. The vegetation cover belongs to
scrubland and grassland that support livestock grazing. A small fraction of the land
is irrigated to grow crops such as pecan trees, alfalfa, and cotton.
Water Level (WL)
The evolution of the WL was measured in 2004, 2007, 2010, 2011, and 2015. We
complemented these data with those reported by CONAGUA from 1978 to 1999.
Historical water data, WL measurement, and water sample collection are described
in more detail in Alarcón-Herrera et al. (2013) and Reyes Gómez et al. (2015). The
WL measurements and water samples were obtained from 31 wells in TA, 14 in SD,
and seven in LH (see Fig. 4.2). The WL drop rate was calculated as the difference
between the oldest WL measurements minus the most recent value, divided by the
number of years between the two measurements. At the same time, WL was measured in seven of the wells (spring, summer, and fall 2015, fall 2016, and winter
2016–2017 in order to appreciate the seasonal variations in WL).
Changes in Land Cover Use (CLCU)
CLCU rates were determined at the scale of the Chihuahuan Desert, a region representative of arid and semi-arid zones of Mexico. Then, we proceeded in the same
way but considering surface cover of three semi-urbanized aquifers of the state of
Chihuahua, in the center of the Chihuahuan Desert. This way we were able to relate
changes in land use with the chemical quality of the water of a well network monitored in that zone of aquifers of Chihuahua. CLCU was determined from a 1:250,000
digital model obtained from Series II (INEGI, 1993) and V (INEGI, 2013). Land
use types were classified as: agricultural, urban, water bodies, and non- vegetated
land, while vegetation cover grown as a result of primary and secondary succession
was classified according to the following three general classes: (1) desert scrubland,
(2) grassland, and (3) others. Primary succession indicates vegetation that has suffered little alteration after it has been established and secondary succession an area
that has been disturbed by either natural or man-made processes.
CLCU layers were processed using the ArcGIS 10.3 TM software. The loss of
vegetation cover was determined after comparing areas that had been previously
assigned to a different class, either agricultural, urban, water body, or areas with no
4 Changes in the Vegetation Cover and Quality of Aquifers in the Drylands of Mexico…