Environmental and Social Impact Assessment - Gibe III

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Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 Rainfall (mm/month) Rainfall (mm/month) 300.00 250.00 200.00 150.00 100.00 50.00 0.00 250.00 200.00 150.00 100.00 50.00 0.00 Rainfall at Wolkite Jan Feb Mar Apr Mag Jun Jul Ago Sep Oct Nov Dec Figure 5.5: Monthly rainfall at Wolkite station (1985-2004) Rainfall at Bonga Jan Feb Mar Apr Mag Jun Jul Ago Sep Oct Nov Dec Figure 5.6: Monthly rainfall at Bonga station (1953-2003) CESI SpA - Mid-Day International Consulting Engineers Page 101
Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 Runoff data Another fundamental hydrological parameter was considered in the elaborations reported in the following paragraphs: runoff. Unfortunately not as many gauging stations as rainfall ones are present in the basin under investigation, but few of them at located at key sites, thus assuming particular importance. Gauging stations are mainly maintained by the Hydrology Department of the Ethiopian Water Resources Authority which processes and archives data. The longest record is that of the Abelti station on the Gibe River, starting in 1963. The following table reports the available hydrometric data: Station Table 5.11: Runoff stations of the EWRA and periods of observation Region Basin Longitude Latitude Location Catchment Observation Elevation masl CESI SpA - Mid-Day International Consulting Engineers Page 102 Area km 2 Mean elevation m asl Max elevation m asl 1 GIBE nr LIMU GENET GIBE 36°56' 08°06' - 533 - - 14 84-04 SEKA GIBE 36°45 07°36' - 280 - - 24 80-03 3 GIBE river nr. ABELTI KEFA GIBE 37°35' 08°14' 1100 15746 2030 3386 40 63-04 4 BIDRU AWANA nr. SEKORU KEFA GIBE 37°24' 07°55' 1500 41 2262 2536 20 81-03 5 BULBULE nr. SERBO KEFA GIBE 37°02' 07°34' 1700 301 1977 2837 8 86-93 6 MEGECHA nr. GUBRIE KEFA GIBE 37°28' 08°11' 1500 286 1750 2000 32 67-04 7 GILGEL GIBE nr. ASENDABO KEFA GIBE 37°11' 07°45' 1650 2966 2200 3359 27 67-93 8 REBU river nr. WOLKITIE SHEWA GIBE 37°47' 08°21' 1500 480 2067 2882 23 67-89 9 WABI nr. WOLKITIE SHEWA GIBE 37°46' 08°15' 1700 1866 2487 3719 38 67-04 10 GOGHEB river nr. ENDIBIR SHEWA GIBE 37°54' 08°06' - 190 - - 32 67-99 11 GOJEB nr. SHEBE GOJEB 36°23' 07°25' 1080 3356 1976 3358 36 70-05 12 GECHA nr. BONGA 36°13' 07°17' - 175 - - 23 82-04 13 SHETA nr. BONGA 36°14' 07°17' - 191 - - 25 80-04 It should be noted that for the station of Asendabo the runoff time series was derived from the Gilgel Gibe Hydroelectric Project, Hydrological Report, and November 1995. Most of the stations show a sufficiently long period of record. In some cases of particular importance (Abelti and Wabi), as will be shown in the following paragraphs, the infilling of missing data was necessary to obtain 40 years long time series. The Serbo station, with an 6 years long period of record, was excluded from the elaborations. As it can be seen the Abelti station on the Gibe River has particular importance as it represents alone a considerable percentage of the entire area under investigation (almost 46%). Most of the gauging stations listed in Tab. 3 are located upstream Abelti. Also the Wolkite station on the Wabi River is of great importance, as it covers an area, downstream Abelti, of 5% of the total area. The same consideration can be made for the Shebe gauging station on the Gojeb River (10% of the total area), but to enlarge the area covered by this station the runoff time series at the Gojeb Dam Site OM19 location was considered (15% of the total area), as reported in the Gojeb Medium Hydropower Project, Pre-Feasibility Study, Vol.1 - Executive Summary, Vol. 3 - Annex A, Hydrology, Oct 1996. The data reported for OM19 were suitably infilled for the years following 1996 according to the Shebe runoff, following the deterministic correlation existing between these two sites. n. year Period
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