Hydrological Sciences- J'ournal-des Sciences Hydrologiques, 42(3) June 1997 391 A physically-related regional model for extreme discharges in Israel ISABELA SHENTSIS, ARIE BEN-ZVI & SOLOMON GOLTS Israel Hydrological Service, PO Box 6381, Jerusalem, 91063 Israel Abstract Israel is a small country which experiences wide variations in magnitudes of extreme discharges. A consistent model has been constructed for prediction of such discharges throughout the country. Extreme flow characteristics, geographical proximity, lithology, soils, and rainfall properties are the major factors in the delineation of relatively homogeneous regions within the country. For each region, discharge-area relationships are formulated in association with low exceedance probabilities. These relationships follow at-site predictions which have been prepared by fitting the log Pearson type III distribution to annual maxima series of peak discharges. For catchments larger than 100 km2 in area, the differences between the regional and the at-site predictions are small. Relatively high extreme discharges are found for the arid area where rainfall depth is low, for an area of steep slopes, for areas of low permeable lithology and soils, and for areas where the fraction of intense rainfall in the total depth of precipitation is high. For large arid catchments, the discharge- area relationships exhibit a negative trend. The model is simply applicable and appears suitable for other semiarid and arid areas. Modèle régional pour les débits de crues en Israël Résumé Israël est un petit pays qui, sur de courtes distances, connaît de grandes variations de topographie, de précipitations, de lithologie et par conséquent une grande variabilité des débits de crues. Un modèle destiné à estimer ces débits dans le pays a été établi. Les caractéristiques des écoulements extrêmes, la proximité géographique, la lithologie, les sols et les propriétés des précipitations sont les principaux facteurs permettant la délimitation de régions relativement homogènes à l'intérieur du pays. Pour chaque région, des relations débit-surface pour les faibles probabilités au dépassement ont été établies. Ces relations complètent des estimations fondées sur l'ajustement d'une loi Log Pearson 3 aux séries de débits annuels maximaux mesurés aux stations de jaugeage. Pour les bassins versants de plus de 100 km2, les différences entre les estimations du modèle régional et celles réalisées aux stations de jaugeage sont petites. Des débits extrêmes relativement importants peuvent être observés dans les régions arides où les précipitations sont faibles, les pentes sont abruptes, la perméabilité des sols est faible et où la fraction des pluies de forte intensité par rapport au total pluviométrique est élevée. Le modèle est d'application simple et convient aux régions arides et semi-arides. INTRODUCTION Israel is a small country which experiences wide variations over short distances in topography, precipitation, lithology, and soils, and consequently in magnitudes of extreme discharges. The southern half of the country is exposed to an arid climate, and the northern half to a semiarid Mediterranean climate. The summers are hot and dry and the winters are relatively cool and rainy. Runoff events occur in the Open for discussion until 1 December 1997 392 Isabela Shentsis et al. winter season only. Duration of direct surface runoff ranges from 0 to 1000 h in a year. A few rivers carry a perennial or a seasonal baseflow. Statistical predictions of extreme discharges were prepared in the past for limited regions only (e.g. Ben-Zvi, 1982, 1996; Cohen & Ben-Zvi, 1983; Garti et al., 1996). No consistent regional model was published for the entire area of Israel. The difficulties in the development of such a model stem from the wide differences in magnitudes of extreme discharges within short geographical distances. Ben-Zvi (1988) attributed the diversity in maximal observed discharges to watershed lithology and to mean annual depth of precipitation. The present work regionalizes extreme discharges for the entire area which is the responsibility of the Israeli Government (including the West Bank, the Gaza strip and the Golan Heights), and relates them to exceedance probability, geographical region, lithology, soils, and rainfall. The model was constructed in two stages: one fits a probability distribution to records at gauged sites, and the other draws regional relationships for at-site discharges associated with low exceedance probabilities. This approach differs from that of some other recently published models (e.g. Hjalmarson & Thomas, 1992; Farquharson et al., 1992; Zrinji & Burn, 1994) by avoiding the use of dimensionless distributions and the mean annual flood, and by considering the relative occurrence of years of no runoff. The regions are delineated with respect to discharge-area relationships rather than to shapes of at-site distributions of discharge. The direct presentation of end results appears comprehensible to hydrologists, engineers, and non-professional decisionmakers. DATA The present study employs annual peak discharges recorded at 67 hydrometric stations of the Israel Hydrological Service. Information on the stations and their catchments is presented in Table 1. Record lengths vary from 16 to 56 years, with a median at 38 years. Catchment areas vary from 10 to 3350 km2. Twentyfive catchments are smaller than 100 km2 while five catchments are larger than 1000 km2. The hydrometric stations are operated, and the records are processed, through standard procedures. Annual maximum discharges were abstracted from records of sufficient com­ pleteness and accuracy. Catchment areas were based upon 1:50 000 topographical maps of the Survey of Israel. The other influential properties are assumed to be the geographical location, lithology, soils and precipitation. The lithology was deter­ mined from the 1:500 000 geomorphological map prepared by Nir (1985) with additional references to maps developed by Amiran & Nir (1970), and Picard (1970a,b). The soil associations were determined from the 1:500 000 map produced by Dan et al. (1975). Information about precipitation was obtained from the Israel Meteorological Service and adapted to watershed delineation by routine procedures of the Hydrological Service. A physically-related regional model for extreme discharges in Israel 393 Table 1 Stations and watershed properties. Region Sub-region Stream Station Area Preeip. Years no. (km2) (mm) I c* Keziv Gesher Haziv 131 873 48 I c* Ga'aton Ben-Ami 41 746 27 I c* Bet Ha'emeq Shavei Tsion 72 755 47 I c* Hilazon Yas'ur 158 703 47 I c Nahalal Railroad 41 757 27 I M-N Ha'shofet Hazorea 12 700 30 I c Bet Lehem Kfar Yehoshua 19 625 36 I c Qishon Quarry 694 546 42 I M-N Tsipori Tel Alil 211 599 40 I M-N Daliya Bat Shlomo 42 662 40 I M-N Daliya Haifa Rd. 70 645 46 I M-N Taninim Amiqam 51 656 28 I M-N 'Ada Giv'at Ada 18 641 46 I M-N Barqan Kefar Glickson 29 647 29 I C 'Ada Binyamina Rd. 66 638 36 I c 'Iron Sha'ar Menashe 61 649 31 I c Hadera Gan Shmu'el 519 609 42 I c Alexander Elyashiv 492 618 55 I M Amud Tiberias Rd. 124 550 44 I M Tsalmon Tiberias Rd. 103 550 33 II M Kana Yarhiv 240 619 39 II M Shilo Nahshonim 357 613 44 II C Yarqon Herzliya Rd. 953 611 54 II C Ayalon Lod 135 540 39 II c Natuf El-Al Jcn. 251 575 53 II M Bet 'Arif Migdal Afeq 46 575 37 II C Ayalon Bet Dagan Rd. 526 577 39 II M Soreq Motsa 78 603 19 II M Soreq Har Tuv 245 590 33 II M Harel Kefar Uriya 13 530 37 II M Soreq Yesodot 405 574 51 II C Eqron Bet El'azari 62 536 44 III C Guvrin Shafir 204 468 45 III M Haela Tel Tsafit 286 507 34 HI C Haela Gan Yavne 423 500 21 III C Lakhish Yavne Rd. 992 470 47 IV Shiqma Tel Milha 38 311 32 IV Adorayim Railroad 207 371 36 IV Shiqma Beror Hayil 378 366 44 IV Gerar Re'im 658 276 32 V Besor Nitsana Rd. 133 107 44 V Be'er She va Zarnuq 405 250 25 V Be'er She va Be'er Sheva 1090 240 21 V Beqa Be'er Sheva 96 165 44 V Be'er Sheva Hatserim 1220 258 23 V Besor Re'im 2630 200 30 V Lavan Nitsana Rd. 192 92 33 V Tsin Har Medad 135 70 38 V Tsin Avdat Fall 233 80 41 continued... 394 habela Shentsis et al. Table 1 continued. Region Sub-region Stream Station Area Precip. Years no. (km2) (mm) VI* Tsin Masos 660 100 38 VI* Tsin Aqrabim 1130 100 30 VI Mamshit Oron Rd. 64 100 40 VI Neqarot Har Masa 697 60 36 VI Par an Bottleneck 3350 50 44 VII Snir Ma'ayan Baruh 526 1300 56 VII Hermon She'ar Yashuv 140 1100 54 VII Jordan Sede Nehemiya 800 900 49 VII 'lyon Metula 32 800 46 VII Dishon Metula Rd. 91 600 29 VII Hatsor Ayelet Hashahar 32 649 53 VII Jordan Obstacle Br. 1380 700 33 VIII Yardenon Lehavot Habashan 10 600 25 VIII Orevim Lehavot Habashan 40 600 34 VIII Meshushim Dardara 160 550 26 VIII Daga -200 Contour 104 550 17 IX Maliah Hamam 58 400 16 IX Qilt Yeriho 133 360 16 Legend: Area is catchment area; Precip. is mean annual depth of precipitation; Years is number of years on records; C is coastal sub-region; C* is northern sector of the coastal sub-region; M is mountainous sub-region; N is "Nari"; and V* is lower Tsin sub-region. Notes: Owing to data shortage some of the precipitation depths are grossly estimated. Region names appear in Table 2. AT-SITE PREDICTIONS In order to expedite preparation of the model, and to concentrate on the regional aspects, no preparatory study was carried out on the selection of the most appropriate distribution and fitting method.