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282 (mm) 250 200 ~ ,g . 150 . . .c ;; ~ 100 c c .. J. lAVABRE ET AL. HYDROLOGICAL RESPONSE OF A MEDITERRANEAN BASIN 283 (mm) AIUBAIID BASIN MONTHLY MEDIANS (1967.89) 50 170 . 136 . Rainfall (mm) !fa Runoff (mm) 102 68 34 o .. Aug. Sept Oct. Nov. Dee. Jan. Feb. Mar. Apr. May June July Fig. 5. Median monthly precipitation and runoff during the reference periD<! (J9~7-1989). o contributions to reinitiate the permanent flow regime (to refill the initial hydrological deficit (DE)). T?e initi~l deficit, which is related to the necessary soil recharge, has been studIed dUrIng the reference period and linked to the duration of the non. permanent flow period (DUR is the number of days without permanent fiow from the start of the hydrological year). The regression equation DE (mm) = 1.07 DUR (number of days) + 166 (3) has been established. This equation provides a good estimate of DE (R = 0.915; u". = 19 mm). The fixed term (l66mm) IS related to the storage capacity and the variable ter~ represents the influence of the delay in reaching the pennanent flow regIme; the later the permanent flow is attained, the higher the deficit to be. refilled is, since it implies- that the intermediate precipitations are in fact lost by evapotranspiration (very high during this period, see Table 5). For this year, despite some floods at the start of October, the flow was not permanent until 12 October. Thus eqn. (3) predicts an initial deficit of242mm IOstead of the 165mm of accumulated rainfall recorded until this date Again, a complementary estimate leads us to conclude that there ~as a reduction of the losses in 1990. In this case the difference between predicted and observed deficit is about 77 mm (32% of reduction), and again this dIfference c~nnot be explained by the normal hazard fluctuations (it is more than .t~ree times greater than the arcs of the equation). Note, however, that this deficit IS only related to the initial losses, and that this figure of 77 mm is only o 400 800 1000 1200 (mm) 200 600 Fig. 4. Relation between annual base flow and tOla) flow for the Rimbaud basin (1967-1990). Total annual flow is 28.4%. This value is inside the range of ratios observed during the refer~ ence period (between 7% in 1969 and 36% in 1989). When we compare total annual base flowlo total runoff (see Fig. 4) we should point out that the 1990 value is slightly higher than the upper envelope of the reference period values. Therefore, we cannot make conclusions about an anomaly and we will suppose that the increase in AR is produced on both components. Monthly analysis The median values of monthly precipitation and runoff for the reference period are shown in Fig. 5 (median values are usually used owing to the asymmetry of the runoff). The precipitation distribution shows the classical representation of the Mediterranean climate: a dry period during the summer months but a rainy winter and spring. As for the runoff, it should be noted that: (I) The precipitation distribution shows good accordance with the pluvio- metry from January urhil the summer; (2) the flow is not permanent and disappears during the dry period; (3) at the start of the hydrological year, the basin needs high rainfall