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. 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. 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. Acknowledgments 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.