Vízügyi Közlemények, 1998 (80. évfolyam)
2. füzet - Nováky B.-M. a. Al-Yezidi-A. A. Al-Shami: Az átlagos évi lefolyás és területi alakulása a Jemeni Köztársaságban
348 Nováky В. M. Ahmed Al-Yezidi—A. Ahmed Al-Shami Annual mean runoff and its areal distribution in the Republic of Yemen by Dr. Béla NO VÁK Yhydrologi.it, Mohamed Ahmed AL-YEZIDI and Ali Ahmed AL-SHAMI Options of mapping annual mean (specific) runoff are shown for the severely data lacking situation of Yemen. The climate of Yemen is dry tropical/subtropical and moderately warm sub-humid in the mountain ranges of West-Yemen. On the basis of classification using the ratio of precipitation to potential evaporation P/E^, the following areas were specified: hyper-arid (P/E^, 0.03); arid (0.03 E^,, 0.25) and semiarid (0.25 E po l .50) (Figure I.). There are 19 monitoring stations in the country the data of whose can be used for estimating the annual mean runoff ( Table I. ). Reliability of these estimates is, however, low, due to the shortness of the records and the statistical homogeneity cannot be analyzed. Decreasing trend can be observed (Figure 2.) in the time series of annual runoff of the Wadi Zabid. This can be explained by the decreasing precipitation and increasing irrigation. Annual mean discharge varies with the size if the catchment area (Figure 3.). 13 regions can be distinguished on the basis of the relationship and considering the areal differences of the climate. The annual mean discharge is well related to the size (order) of the stream section as calculated by the Shreve method (Figure 4.). The relationship was determined on the basis of the data of wadies Surdud and Zabid and on that of the Hungarian river Zala, considered as analogous one. The relationship is suitable for estimating the hydrological profile of annual runoff as well (Figure 5.). The precipitation vs. runoff relationship is linear for precipitation lower than 500 mm. In this domain the runoff coefficient is constant and its value is 5%. The linear relationship in this domain is supported by the Schreiber-Budiko (2) formula. The aridity coefficient a/P a of this fonnula shows random variation and is not dependent on precipitation (Figure 6.). The average value of the aridity index is 2.90 and the Schreiber-Budiko formula can be rewritten as shown in Eq. 5. The linear character of this equation is obvious. For the higher domain of precipitation data the precipitation vs. runoff relationship is generally non-linear, as suggested by the data of the relevant literature. The Schreiber-Budiko formula was used for describing this relationship. This relationship was used for calculating runoff and evaporation, defined as the difference between precipitation and runoff, for three values of the potential evaporation r/L (2,000,2,500 and 3,000 mm). Figure 7 shows the distribution of precipitation among evaporation and runoff (the structure of the water budget) for three selected values of the potential evaporation. The water budget structure characteristic to Hungarian watersheds is also shown for two temperature values (—6 and +10 °C). In Yemen the annual mean temperature is 20-25 "C in the areas with potential evaporation of 2,000-3,000 mm. The non-linear Schreiber-Budiko fonnula substantial underestimates the value of runoff in the precipitation range 0.00-500 mm, independently of the selected r/L value. The precipitation-runoff relationship was finally accepted, after certain speculative considerations, as shown in Figure 8. In the precipitation range lower than 550 mm the linear relationship, as derived on the basis of measurement data, was accepted, while the Schreiber-Budiko formula was used above 1000 mm precipitation, fixing the potential evaporation value as 2,500 mm. In the precipitation domain of 550-1,000 mm the transitional part of the curve between the linear and non-linear ranges was estimated by interpolation. The precipitation-runoff relationship of Figure 8 wax used for the construction of the map of annual specific runoff (Figure 9.), making use of the map of mean annual precipitation. This runoff map is most likely the first of its kind in Yemen.
