Precipitation Variability and Regionalization Over the Southern Altiplano, Bolivia

Precipitation over Southern Altiplano 149 Precipitation variability and regionalization over the Southern Altiplano, Bolivia Ramiro Pillco1,2, Cintia Bertacchi Uvo1 and Lars Bengtsson1 1 Department Water Resources Engineering, Lund University, Sweden 2 Institute of Hydraulic & Hydrology, San Andres Major University, Bolivia Contact author: Cintia B. Uvo Dept. Water Resources Eng./Lund University, Box 118, 221 00 Lund, Sweden Phone: +46 46 2220435, Fax: +46 46 2224435, Email: [email protected] Abstract: This is a first attempt of a regional precipitation analysis over the Bolivian Altiplano. The study region is the semi-arid Poopó and Uru-Uru lake basin, in the southern tip of the Altiplano. Fifteen rain gauges and three wind stations are located within and around this basin were the source of data used to perform a regionalization of the precipitation within the basin. The regionalization evidences four main zones of homogeneous precipitation within the basin: a) the mountainous region at the east of the basin, where precipitation is enhanced by the moist continent to the east; b) the northernmost tip of the basin, where precipitation is influenced by the larger amounts of precipitation from the region around Lake Titicaca; c) the central and southern zones, that are part of the wide flat area of basin. This is a very dry area which precipitation seems to be influenced mainly by westerly winds; and d) the north-western part of the basin which precipitation seems to be largely influenced by the presence of Lake Poopó, despite being located at about 50 km from it. Keywords: Precipitation variability; precipitation regionalization; Altiplano; Lake Poopó; Lake Uru-Uru Introduction The climate of the Altiplano, the wide region in the Central Andes, has been the interest of several studies in the latest two decades. Its precipitation is concentrated during the austral summer months (December to March), when about 70 % of the precipitation occurs (Garreaud et al., 2003). Characteristically, the Altiplano presents strong temporal and spatial precipitation variability, droughts and floods are common resulting on high 150 Ramiro Pillco, Cintia Bertacchi Uvo and Lars Bengtsson socio-economic costs. Spatially, precipitation decreases from 1100 in the northern parts to less than 200 mm in the south and from east to west (Figure 1). This spatial variation of precipitation is related to the topography and to the availability of moisture, provided by the Titicaca Lake in the north and by the continental source of moisture in the east. (Garreaud et al. 2003; Pillco and Bengtsson, 2006) The timing and intensity of the rainy season over the Altiplano has been associated by several authors to large-scale circulation such as the Atlantic Intertropical Convergence Zone – ITCZ (Ronchail, 1995, Garreaud et al. 2003), the El Niño- Southern Oscillation – ENSO (Thompson et., 1984; Acei- tuno, 1988; Lenters and Cook, 1999; Vuille et al., 2000a; Arnaud et al., 2001; Francou et al., 2003), the Bolivian High (Vuille, 1999; Lenters and Cook, 1999; Garreaud, 1999, Garreaud et al. 2003), and the South Atlantic Convergence Zone – SACZ (Lenters and Cook, 1995, 1999). Intra-seasonal variations of precipitation are frequently associated to the availability of moisture in the region, brought by the zonal wind aloft over the mountains, from the continent to the east (Garreaud and Aceituno, 2001; Garreaud et al., 2003). Figure 1. Isohyets for aver- age annual precipitation (TDPS, 1993) at the Titi- caca-Desaguadero-Poopó- Salinas (TDPS) system with location of the Poopó/Uru- Uru Lakes basin in the south-east of the region. Precipitation over Southern Altiplano 151 The rainfall mechanisms over the Altiplano as a whole, have been thor- oughly studied before and are reviewed by Garreaud et al. (2003). Few studies, however, have been developed for sub- regions within the Altiplano, due mainly to the lack of a dense regional data network over the region. The objective of this work is to de- fine the rainfall spatial and temporal variability within the Poopó/Uru-Uru Lake basin, located in the southeastern tip of the Altiplano (Figure 1). This basin has a total area of 24013 km2. A network of 15 rain gauges and three short-term wind stations within and around the Poopó/Uru-Uru lake basin were available for the development these analyses. The Poopó/Uru-Uru lake basin The Poopó/Uru-Uru lake basin is joined to the eastern mountain ranges of Cen- tral Andes, between 18o 30’ and 19o 15’ S, Figure 2. The Poopó and Uru-Uru lake basin and 66o 50’ and 67o 15’ W with an aver- with topography and location of the rain gauge network. age elevation of 3700 m a.s.l. (Figure 2). The mountains occupy about 40 % of the basin area and are mostly located in the eastern part of the basin. The remaining area is flat, and a part of it is occupied by two shallow lakes: Lake Poopó and Lake Uru-Uru that occupy about 15 % of the total basin during their maximum lake-level. Lake Poopó has a surface area of about 3000 km2 at spill-over and less than half of that during the dry season. It may dry out after consecutive drought years as observed from 1994 to 1997. The main sources of water to this lake are the River Desaguadero, which carries the outflow from Lake Titicaca, regional intermittent rivers and rainfall. Lake Uru-Uru is smaller and intermittent. At its maximum depth (about 0.75m) its surface area is 152 Ramiro Pillco, Cintia Bertacchi Uvo and Lars Bengtsson about 300 km2. During wet periods it may happen that the lakes join into a single one (Pillco and Bengtsson, 2006). Figure 2 shows the topographic map of the basin together with the distribution of the available rain gauge stations. The basin extends along the eastern ranges of Andes, where the mountainous formation is the most enhanced. Three topographic structure zones are distinguished over the basin: a) The eastern mountain region. This is a part of Central Andes where elevations can reach up to 5400 m a.s.l. It is the origin of most of the regional intermittent rivers that flows into the lakes. Most of precipitation within the basin falls in this region due to orographic effect. b) The small western mountain region that reaches up to 4800 m. c) The flat area of elevation around 3700 m a.s.l., extending from north to south, where the Poopó and Uru-Uru lakes are located. Figure 1 presents the yearly precipitation field over the Titicaca, Desagua- dero, Poopó and Coipasa Salar region, which comprises about 75 % of the Bolivian Altiplano (TDPS, 1993). Over the Poopó/Uru-Uru basin it is evi- dent a negative gradient of precipitation from north to south and east to west, changing from 500 to 200 mm. Pillco and Bengtsson (2006) describe two climate zones over the basin: semi-arid in the northernmost and central parts of it and arid in the southern parts . Data Within and around the Poopó/Uru-Uru lakes basin, 15 time series of monthly precipitation are available from the Servicio Nacional de Hidrom- eteorología de Bolivia (SENAMHI). Five gauges are located nearby the lake shore, eight along the flat area of the basin; and two at high altitudes (Table 1, Figure 2). The series were homogenized by the SENAMHI. Missing values were filled based on simple arithmetic mean, taking into account the precipitation observed on two or three adjacent rain gauges of similar altitudes. The original time series of Pazña and Challapata were extended to the period 1960 – 2001 by means of linear regression. For Pazña, the extension was made based on the Oruro series and for Challapata, on the Quillacas one. The period of October 1975 to September 1985 was chosen for the analysis of rainfall distribution and for regionalization analysis, as, following Precipitation over Southern Altiplano 153 suggestion from Lau and Sheu, (1988), during this period all stations had less then 10 % missing data. All time series were standardized previous of use in multivariate analysis. Monthly data were subsequently grouped in totals for hydrological years (October to September), totals for the wet season (December to March) and totals for the dry season (April to November). The highest yearly precipitation corresponds to Uncia with 592 mm, placed at the eastern mountains, and the lower to Uyuni with 201 mm, placed in the flat and arid zone (Table 1, Figure 2). Hourly wind measurements are available from Oct/01/2006 to Dec/31/ 2006 for three sites: Oruro, Salinas and Turco, the latest one located 50 km west from Corque, at similar latitude. The ENSO phenomenon was represented by the Oceanic Niño Index (ONI) and by the NINO3.4 Index, both provided by the Climate Predic- tion Center, USA (http://www.cpc.noaa.gov/data/indices). Table 1. Location and altitude of available precipitation stations used during this work. Information about total annual precipitation during hydrological years (Oct. – Sep.), precipitation during wet (Dec. – Mar.) and dry (May – Apr.) seasons, as well as years of record are also provided. Precipitation Altitude Latitude Longitude wet- dry- Record N Stations Annual m a.s.l. (º) (º) season season years mm mm mm 1 Condoriri 3750 17º 31’ 67º 14’ 442 312 121 70-95 2 Eucaliptus 3728 17º 36’ 67º 31’ 461 342 108 76-01 3 Caracollo 3770 17º 38’ 67º 13’ 434 323 109 76-86 4 Oruro 3702 17º 58’ 67º 04’ 478 351 122 60-02 5 Pazna 3710 18º 36’ 66º 56’ 484 356 120 60-95 6 Challapata 3720 18º 53’ 66º 47’ 397 299 93 60-95 7 Uncia 4420 18º 30’ 66º 05’ 592 439 128 70-84 8 Corque 3929 18º 21’ 67º 41’ 520 446 64 75-97 9 San Jose 3850 18º 36’ 67º 52’ 448 385 52 75-95 10 Andamarca 3740 18º 46’ 67º 30’ 244 198 40 60-95 11 Orinoca 3780 18º 58’ 67º 15’ 367 303 64 75-95 12 Quillacas 3749 19º 14’ 66º 57’ 277 215 56 60-02 13 Salinas 3860 19º 38’ 67º 41’ 274 251 25 75-85 14 Uyuni 3653 20º 27’ 66º 43’ 201 178 21 75-02 15 Potosi 4070 19º 35’ 65º 45’ 393 299 83 75-02 154 Ramiro Pillco, Cintia Bertacchi Uvo and Lars Bengtsson Analyses and Methods Spatial Rainfall Distribution The spatial interpolation of the 15 rain gauges over the Poopó/Uru-Uru basin was inferred by means of Kriging.

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