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DROUGHT CHARACTERIZATION IN THE OJOS NEGROS REGION,
BAJA CALIFORNIA, MEXICO
Ampar V. Shetty, Ahmad Taher-Shamsi and Victor M. Ponce
Department of Civil and Environmental Engineering
San Diego State University
San Diego, California, USA
Rafael Garcia Cueto
Instituto de Ingeniería, Universidad Autónoma de Baja California
Mexicali, Baja California, Mexico
ABSTRACT
Drought phenomena in the Ojos Negros region, in Baja California, Mexico,
has been analyzed using the conceptual model developed by Ponce et al. (2000).
Climatological data for ten stations encompassed within the study region were used
in the analysis.
Drought intensity, duration and frequency extracted from the data were compared with
model predictions.
Event drought intensity varied from 0.61 to 1.23, with an average of 0.83.
Drought duration varied from 2 to 3 yr, with an average of 2.46 yr.
Drought frequency varied from 3.5 to 4.83 yr, with an average of 3.96 yr.
Drought intensity vs exceedence probability has also been determined.
INTRODUCTION
Drought is a normal and recurrent feature of the climate. It occurs in virtually
all climatic zones, and its characteristics vary significantly among regions.
Drought differs from aridity in that drought is temporary; aridity is a permanent
characteristic of regions with low rainfall.
Drought produces a complex web of impacts which spans many sectors of the economy,
reaching well beyond the area experiencing direct drought.
This complexity exists because water is essential
to society's ability to produce goods and services.
The intensity, duration, and frequency of droughts
have a major influence on society.
Dracup et al. (1980) have studied drought characteristics
in terms of severity, duration, and magnitude, wherein magnitude
is defined as the ratio of severity over duration.
According to Ponce et al. (2000), intensity is akin to severity;
therefore, intensity is equal to magnitude times duration. T. C. Sharma (1997)
has derived probabilistic relationships for duration and intensity to estimate the
largest drought severities for design return periods for use in the design of storage structure.
The relations between drought intensity, duration and frequency can be
analyzed using the conceptual model developed by Ponce et al. (2000),
shown in Table 1.
The conceptual approach is applicable to subtropical and midlatitudinal regions,
and is limited to meteorological droughts lasting at least one year.
In the Ponce et al.
model, the climate types, which encompass the climatic spectrum
from superarid to superhumid, are defined in terms of the ratio
of mean annual precipitation Pma
to (mean) annual global terrestrial
precipitation Pagt.
The ratio Pma /
Pagt =
1 represents the middle of the climatic spectrum.
The conceptual model is also defined in terms of the ratio of
annual potential evapotranspiration Eap
to mean annual precipitation Pma.
The ratio Eap /
Pma = 2
describes the middle of the climatic spectrum.
To complement the description, the length of rainy
season Lrs
is also indicated.
The drought duration varies between 1 yr at the extremes of the
climatic spectrum, and 6 yr at the middle.
For any year for which P is the annual precipitation,
drought intensity is defined as the ratio of the deficit
(Pma - P)
to the mean (Pma).
For any one year, an intensity of [(Pma - P) /
Pma] = 0.25 is classified as moderate;
0.5 is severe, and 0.75 is extreme.
For drought events lasting more than one year,
intensity is the summation of the individual annual intensities.
Therefore, the longer the drought duration, the greater the intensity.
Extreme drought intensities
are generally associated with droughts of long duration.
The dry periods (droughts) are generally followed by corresponding wet periods.
Therefore, the drought recurrence interval (i.e., the reciprocal of the frequency)
is always
greater than the drought duration.
Drought recurrence intervals increase from 2 yr
on the dry side of the climatic spectrum (superarid) to 100 yr on the wet side
(superhumid).
THE OJOS NEGROS REGION
The Ojos Negros valley and environs are located in Baja California, Mexico,
about 80 km south of the U.S.-Mexico
border, as shown in
Fig. 1.
Recorded human settlement
originated with the discovery of gold mining in 1870. Later, the local economic
activities
shifted first to animal husbandry and then to agriculture.
At present, agriculture is the most important activity in the valley.
The products of its irrigated agriculture, which
include alfalfa, green onions, onions, watermelons and other staples, are
commercialized on
both sides of the U.S.-Mexico border.
The Ojos Negros valley is drained by Ojos Negros creek, a tributary of El Barbon wash.
The drainage area of Ojos Negros creek is 173.4 km2,
while that of El Barbon wash is
1094 km2 (Ponce et al., 1999).
The valley is a natural depression of
roughly rectangular shape, bounded by mountain ranges on all sides,
of approximately 113 km2
and contained within 31o 52'
and 31o
57' north latitude and 116o 12' and
166o 19' west longitude.
The terrain elevation
varies from 720 m at the entrance to the valley,
on the northeastern edge, close to
El Barbon wash, to 670 m at
the mouth of the valley,
in the vicinity of the junction between
Ojos Negros creek and El Barbon wash. The average elevation is around
705 m, and the valley slopes generally from east to west, between 0.5 and 1 percent.
CLIMATOLOGY
The Ojos Negros region has an arid climate,
with rainy season in the winter (November to April) and extreme
temperature variability. The climate is characterized as cold
and humid in the winter, and hot and dry in the summer.
The ecosystem associated with this type of climate and geographical location
is commonly referred to as "Mediterranean,"
which prevails throughout coastal northern Baja California and coastal
Southern California.
The study area is that encompassed within ten climatological stations located
between 31o 47' 03"
and 32o 11' 43" North Latitude and
116o 09' 54" and
116o 45' 16" West
Longitude (Fig. 2).
The length of the record varied from 21 yr in Valle de
San Rafael to 72 yr in Ensenada.
The following climatic variables were measured daily:
(1) precipitation, (2) actual temperature, measured at 7 am, (3)
maximum daily temperature, and (4) minimum daily temperature.
The distribution of monthly temperature throughout the year
is unimodal, i.e., there is typically only one maximum and one
minimum. Mean annual temperature varies from
12.5 oC to 18.6oC.
The mean temperature of the
hottest month (July/August) varies from 19.8oC to
26.3oC. Conversely,
the mean temperature of the coldest month (January) varies
from 6.6oC to
13.6oC.
Mean annual precipitation varies from 235 mm to 517 mm, with about 20 percent
occurring in the wettest month (January), and 0.25 percent in the
driest month (June).
The precipitation is markedly seasonal, with most of the precipitation
occurring in the November-April period. The number of
days per year when rainfall exceeds 0.1 mm (an indicator of the number
of storms per year) ranges from 20 to 40.
DROUGHT CHARACTERIZATION
Drought characterization in the Ojos Negros region
has been performed using the conceptual model developed by Ponce et al. (2000).
Drought intensity, duration and frequency for the ten climatological stations selected
for this study are
shown in Table 2.
Drought intensity (per event) varies from 0.61 to 1.23, with an average of 0.83.
Drought duration ranges from 2 to 3 yr, with an average of 2.46 yr.
Drought frequency ranges from 3.50 to 4.83 yr, with an average of 3.96 yr.
The longest mean drought duration occurred at Boquilla de Santa Rosa,
El Pinal and San Juan de Dios Norte (3 yr), and the shortest at Ojos Negros (2 yr),
with an average of 2.46 yr.
For all stations, event intensities were
ordered and divided into the following categories,
(based on an average duration of 2.5 yr):
(1) moderate (less than 0.625), (2) severe (0.625-1.25), and (3) extreme
(greater than 1.25).
Fig. 3 shows the
histogram of event intensities, with 48% moderate, 34% severe, and 18% extreme.
Drought duration and frequency were compared using predicted and measured values,
as shown in Fig. 4.
It is seen that model predictions agree reasonably well with measured data.
Fig. 5 shows drought intensity vs
probability of exceedence. The lognormal curve fitting shows a very high coefficient of determination (0.95).
SUMMARY AND CONCLUSIONS
Drought characterization has been performed for the Ojos Negros region
and its vicinity, in Baja California, Mexico, close to the U.S.-Mexico border.
A conceptual model developed by Ponce et al. (2000) was used in the
characterization.
Climatological data for ten (10) stations, with record length varying
from 21 to 72 yr, were analyzed for drought intensity, duration, and frequency.
Intensity (per event) varied from 0.61 to 1.53, with an average of 0.83.
Duration ranged from 2 to 3 yr, with an average of 2.46 yr.
Frequency ranged from 3.33 to 4.83 yr, with an average of 3.96 yr.
Based on the characterization,
the study region is classified as arid
according to drought duration (2-4 yr) and frequency (3-6 yr).
Moderate drought events (intensity less than 0.625) have a 48% probability of occurrence;
severe events (intensity between 0.625 and 1.25) have a 34% probability; and
extreme events (intensity more than 1.25) have a 18% probability.
The model predictions agree reasonably well with the
values obtained from the climatological record.
The distribution of drought intensity of the region is lognormal.
ACKNOWLEDGEMENTS
The climatological data on which this study was based
was obtained from the Comision Nacional del Agua,
Gerencia Regional de Baja California, Mexicali, Baja California, Mexico.
REFERENCES
Dracup, J. A., K. S. Lee, and E. G. Paulson, Jr. 1980.
"On the statistical characteristics of drought events."
Water Resources Research, 16(2), 289-296.
Ponce, V. M., R. P. Pandey, and S. Kumar.
1999. "Groundwater recharge by channel infiltration in El Barbon basin,
Baja California, Mexico." Journal of Hydrology, 214, 1-7.
Ponce, V. M., R. P. Pandey, and S. Ercan. 2000.
"Characterization of drought across the climatic spectrum."
Journal of Hydrologic Engineering, ASCE, 5(2), 222-224.
Sharma, T. C. 1997. "A drought frequency formula." Hydrological Sciences Journal 42(6), 803-814.
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