In the following, the flux comprising mountain-front recharge is denoted as QMFR . The reference cross section for assessment of MFR is the (assumed) mountain front line. The spatial references or balance areas for its assessment are defined α-cuts of the Fuzzy Re-charge Areas for the Maawil ‘plume’ (see section 7.3.1). Especially for the application of the conceptual hydrologic model (see 7.3.5), but also for the long-term average approach, response units with distinct parameterisation have to be determined (see section 7.3.2). In addition to the actual recharge in the mountain catchment, water use in mountain oases has to be considered (see section 7.3.3).
Based on steady state groundwater modelling, an upstream inflow to the groundwater model domain on the Batinah plain QGWM of 68 mio m³/a was computed (Walther et al., 2012). This can be considered as a reference value for long-term average QMFR.
7.3.1 Data Processing of Fuzzy Recharge Areas
In the following, the procedure to derive Fuzzy Recharge Areas as outlined in section 5.1 is applied to the potential groundwater basin of the Maawil ‘plume’. Based on the discus-sion of recharge mechanisms in section 7.1, Figure 7.18 shows the data base to derive the Fuzzy Recharge Areas for the Maawil ‘plume’.
The outer boundary shows the assumed maximum extent of the underground catchments, both for the eastern Maawil ‘plume’, and the western Farah ‘plume’. Together, they repre-sent that part of the Jebel Akhdar of which the subsurface water potentially drains north-ward to the Batinah plain. In the north, this area is either limited by an assumed mountain front line or outcrops of the Samail Nappe ophiolites, which are supposed to be secondary in this context. To the east and south, the area is limited by the border of the Hajar Unit. In general, a degree of membership of µ(x,y) = 0 is assumed for the outer boundary. A value of µ(x,y) = 1 was only assigned to the mountain front line in the alluvial valley around Afi, the opening towards the alluvial basin aquifer.
The portions of the (surface) drainage basins of Wadi Taww and Wadi Maawil outside of the ophiolites are assumed to drain completely to the Maawil ‘plume’ (µ (x,y) = 1), while the drainage basin of Wadi Farah is assumed not to contribute at all (µ(x,y) = 0).
Figure 7.18: Outer and inner boundaries for derivation of Fuzzy Recharge Areas
The northern end of the presumed east-west divide (with µ(x,y) = 0.5) south to the Frontal Mountains lies between the two areas, covered with sub-recent alluvium, which spreads either further to the west or to the east. To the south, it follows in first instance the (local) drainage divides. Finally, its southern end reaches into the Saiq plateau. Here, it is as-sumed, that direct recharge and subsurface drainage to the Maawil ‘plume’ predominate compared to drainage according to the surface drainage network. Although they follow to some degree the topography, the supporting lines for ‘increased’ or ‘decreased drainage toward Maawil’ are quite subjectively which has to be considered in evaluation of the re-sulting water yields. This corresponds to the statement of Jacobs (2007), whereupon fuzzi-fication is quantifuzzi-fication at the same time. On the Saiq plateau, the inner boundaries are assumed to follow distinct isolines to describe the potential extent of the groundwater ba-sin, which is related to certain α-cuts. Based on isotopic evidence, the contribution of the areas above 2200 m a.s.l. to the Maawil ‘plume’ is quite solid. The 1800-isoline (‘pre-sumed divide’ with µ(x,y) = 0.5’) is completely within the low sloped plateau area, which shows the highest δ18O-values. Thus, it is assumed, that this area contributes mainly to the Maawil ‘plume’. Beyond that line, however, the steep southern slopes are starting – where surface runoff and, thus, indirect recharge is supposed to dominate and subsurface drainage to the north is more and more unlikely.
Based on these outer and inner boundaries, a Triangulated Irregular Network (TIN) was interpolated. Finally, this TIN was converted to an ascii raster file with a spatial resolution of 1 x 1 km². It is illustrated in Figure 7.19. With regard to water balance assessment for different α-cuts, the location of oases were included.
Figure 7.19: Fuzzy Recharge Areas of the Maawil ‘plume’
7.3.2 Determining response units
The raster based framework (section 2) allows defining hydrogeologic response units (HGRU) to distinguish zones with distinct response functions (for the conceptual hydro-logic model) or assumptions on recharge rates (for the long-term average considerations).
Especially with regard to the conceptual hydrologic model, the primary goal is to delineate zones with distinct characteristics regarding recharge mechanisms and recharge flow paths.
The degree of distinction depends on available data or expert knowledge of the catchment characteristics and reference data for calibration. An increasing number of response units are beneficial as long as they are accompanied by an increase of reliable information. In this case study, detailed field surveys, like, for example, carried out extensively by Lange (1999) in a similar context are lacking. Observed hydrographs of aflaj do not represent the total catchment area, but only unknown sub-catchments. Available reference data is limited to a single long-term average value (see above). Thus, a low number of 3 response units was defined. In addition to their names, Table 7.8 shows criteria for their delineation based on available geo data. The highlighted criteria were finally used for data processing.
In the case of the alluvial valleys, the spreading of (recent and sub-recent) alluvium
corre-slopes with outcrop of rocks (see section 7.3.5 including Table 7.13). Thus, the prevalent geology is finally used as a classification criterion instead of the slope. The ‘slopes’ are the remainder, i.e. those raster cells, which do not belong to the other classes with clear selec-tion criteria.
Table 7.8: Definition of response units
ID Name slope altitude prevalent geology
1 quaternary low to mean (≤ 15 %) < 1800 m a.s.l.
Quaternary (sub-recent and (sub-recent alluvium or slope colluvium) 2 slopes steep to very steep (> 30 %) arbitrary