The Goda massif (Figure 1) is located to the west of the city of Tadjourah. This steep area consists of hills and narrow valleys. It is bordered to the south-east by the littoral and is limited to the north by the plateau of Dalha at the village of Adaylou. On the northern and western slopes of Day Mountain, which peaks at 1800 m, the altitude of the massif decreases steadily. The massif is limited to the south and west by Lake Assal (−150 m asl) and the fault zone of Makarassou. The central slope of the massif is home to Day’s primary forest and consists of three exoreic watersheds (Darriyou, Magalé, Asleyi). The relief is particularly steep with dips reaching more than 35˚ on the descent of Day Mountain.

Precipitations over the massif of Goda are higher than the average over the territory of Djibouti (150 mm/year). Indeed, more than 350 mm/year of rainfall are recorded on the high zones. The precipitation decreases until reaching 200 mm/year on the coast near the city of Tadjourah (Figure 1).

The Goda massif is located on the terminal part of the Danakil block bounded to the north in Eritrea by Precambrian sedimentary rocks [22] [23] . The tectono-volcanic activities following the opening of the Red Sea [24] [25] [26] set up

the volcanic rocks of the Goda massif. The oldest geological unit is the formation of the Mabla rhyolites dating of the Miocene [27] . This unit outcrops on the central and northern part of the massif. Normal faults of N160 or N50 direction intersect the series of Mabla rhyolites.

The Mabla rhyolites are covered to the west by Dalha basalts (4 - 9 Ma) and stratoid basalts (1.25 - 3.5 My) [28] . Dalha basalts are weakly faulted and subhorizontal whereas the stratoid basalt series is affected by normal N-S main faults. There are some outcrops of the Gulf basalts (1.25 - 3 My), of the Asal series basalts [29] and of the Mabla basalts, which are a contemporary series of the Mabla rhyolites. Sedimentary formations (<2 My), composed of detrital materials, are mainly located on the coast, but can also be intercalated in volcanic formations (Figure 2).

The aquifer system of the Goda massif is defined by three hydrogeological units consisting of Dalha basalts, Mabla rhyolites and sedimentary formations. On the top of the massif, in the village of Day, two boreholes drilled until depths of 323 and 358 m allowed to estimate their thickness (locally) to 300 m. The total thickness of the Mablas rhyolite is not known. Electric soundings performed at the mouths of the Darriyou and Magalé rivers have shown that the total thickness of the sediments could exceed 300 m.

Over the whole Goda massif, 38 water points are available with piezometric data. They include 19 wells, 10 boreholes and 9 springs (Table 1). Pumping tests were carried out on only 5 boreholes, 4 of which are located in the Dalha basalts and only one in the sedimentary formations (Table 2). The water points are not evenly distributed over the study area. They are mainly concentrated in the central part of the massif and along the coast.

The Goda aquifer system is conceptualized with two layers: i) a top layer corresponding to a relatively productive zone with a thickness of 300 m; and ii) a semi-permeable lower layer that corresponds to a relatively unproductive zone in relation with the progressive clogging of deep fractures with a thickness of 1000 m. The top of the upper layer is defined by the DEM (Figure 1).

The model domain is limited to the south-east by the coastline where a fixed head of 0 m is set, to the south-west by Lake Assal where a fixed head of -150 m is set; to the northwest by a head contour line of 800 m, 30 km northwest from the summit of the Goda, where a fixed head of 800 m is set.

The area of the domain is 1800 km² and a regular discretization with a mesh size of 500 m × 500 m (11,600 mesh) has been considered. The boundary conditions are shown in Figure 3.

A constant hydraulic conductivity (K) of 10⁻⁸ m∙s⁻¹ is assigned to the lower layer, considered to be semi-permeable. Lower values of K do not significantly modify the simulated heads, but increase the calculation times. For the upper layer, three zones of hydraulic conductivity are defined on the basis of the geological map (basalts, rhyolites and sedimentary formations) (Figure 4).

In the first modeling scenario, a diffuse recharge is considered according to three zones defined on the basis of the isohyets (>350 mm∙y⁻¹; 250 to 350 mm∙y⁻¹; >250 mm∙y⁻¹) (Figure 5). In the second scenario, recharge is applied only to the model cells corresponding to the drainage network. The three zones defined based on the isohyets are associated with different degrees of the network development and with distinct indirect recharge values (Figure 6). To prevent the accumulation of water above the topographic surface, mesh drainage at the level of the DEM is carried out where necessary.

To identify the values of both hydraulic conductivity and recharge on the different zones defined above, a GLUE (Generalized Likelihood Uncertainty Estimation) approach is implemented. This method considers that several sets of input parameters can result in a concordance between the simulated and observed values of a given variable. The strategies adopted are the following:

1) 10,000 sets of parameters (hydraulic conductivities ranged from 10⁻⁴ to 10⁻⁸ m∙s⁻¹ and recharges ranged from 0 to 400 mm∙y⁻¹ for each of the different zones

defined in Section (3.2) are generated randomly according to a Uniform distribution;

2) a model performance indicator is defined, based on the Root Mean Square Error (RMSE) between the measured and simulated hydraulic heads on the 39 control wells;

3) 10,000 simulations (with each of the parameter sets) are run and the 250 sets of parameters resulting in the lowest RMSEs are kept to establish probability densities.

The differences between observed and simulated heads for the 250 selected simulations vary significantly according to the control wells. These differences must be compared with the uncertainty in the topography, in particular due to the change in DEM resolution from 30 m to 500 m in modeling. Indeed, in zones with high vertical differences, this uncertainty reaches ± 100 m (Figure 7). The probability densities obtained for K indicate that volcanic formations are

less permeable than sedimentary formations. The K values obtained for the Dalha basalts range from 10⁻⁸ m∙s⁻¹ to 10⁻⁶ m∙s⁻¹ with a modal value of 10⁻⁷ m∙s⁻¹. The K values obtained for Mabla rhyolites are generally lower, with a modal value of 10⁻⁸ m∙s⁻¹. The K values obtained for sedimentary formations are higher than 5 × 10⁻⁷ m∙s⁻¹ and two modal values are distinguished, 10⁻⁶ and 5 × 10⁻⁴ m∙s⁻¹ (Figure 8). The probability densities obtained for the diffuse recharge indicate that this parameter is lower for the upper zone (values ranged from 10 to 30 mm∙y⁻¹; modal value of 20 mm∙y⁻¹) than for the middle zone (values higher than 20 mm∙y⁻¹; modal value of 40 mm∙y⁻¹). For the low zone, not any clear trend can be identified (Figure 9).

Overall, the same trend as in the first scenario is observed for the second scenario, but on average the hydraulic conductivity values are slightly lower than the previous ones. Indeed, values between 3.4 and 8.5 × 10⁻⁸ m∙s⁻¹ for volcanic formations and a value of 3.4 × 10⁻⁶ m∙s⁻¹ for sedimentary formations are obtained (Figure 10). For this configuration, since the recharge is assigned only to the drainage network, the values required to adjust simulation and observation are

high. The upper zone receives 52.4 mm∙y⁻¹ and the middle and low zones 123.7 and 198.2 mm∙mm∙y⁻¹ respectively (Figure 11).