A novel strategy for estimating groundwater recharge in arid mountain regions and its application to parts of the Jebel Akhdar Mountains (Sultanate of Oman)

In arid regions, mountain catchments are the major contributor to the total natural water yield. Due to generally low groundwater tables, sub surface underflow referred to as mountain-front recharge is important in distincti on to the surface runoff at the mountain front. The extent of the groundwater basin is hereb y often vague. Approaches to assess mountain-front recharge are mostly based on groundw ater data and integrate over time and space. This, however, cannot provide prognostic and time-dependent estimates of subsurface inflow to the adjacent alluvial basin aquifer. Consequently, the proposed strategy builds on rainfall based approaches. Temporal and s patial resolution is in this case mostly limited by data scarcity regarding hydrological cha racteristics of the catchment area and high-resolution rainfall data. The proposed novel strategy combines three approach es to tackle these challenges. A newly developed conceptual hydrologic model provide s time-dependent estimates based on fully distributed monthly rainfall. For distinct response units and seasons, non-linear relationships between rainfall and recharge describ e the hydrogeologic response. The derivation of the response functions is based on a mass b lance and considers the principal recharge mechanisms. Parameterisation makes use of available expert knowledge on geomorphology and seasonal rainfall characteristics. A s an efficient tool to assess uncertainties, fuzzy arithmetic is used for complementary lo ng-term average water balance estimates. This technique allows considering fuzziness in rainfall input, crop water use in mountain oases, and best available assumptions on r echarge as portion of rainfall. Uncertainty regarding the potential, albeit unknown exte nt of groundwater basins is portrayed based on continuous surfaces which represent the de gr e of membership to a distinct geographical entity (termed as fuzzy regions). Distinc t subsets of these fuzzy regions represent potential groundwater basins for water balance asse ssm nt. The proposed strategy was applied on the large scal e in an arid karst mountain range in northern Oman. The two complementary assessment app roaches result in similar ranges of values. They are in good agreement with inversely c omputed inflow to a steady state groundwater model for the adjacent basin aquifer. T he results of the conceptual hydrologic model are confirmed by the plausibility of average recharge rates for distinct response units and seasons. This shows that less intense win ter rainfall contributes mainly to groundwater recharge. Uncertainties due to the vagu e extent of the groundwater basin are about 30 % of the total mean annual value. An optio n o mitigate this uncertainty is the complementary consideration of adjacent aquifer sys tem in future studies. Hydrogeologic survey and observation of groundwater levels in the alluvial basin aquifer in near distance to the mountains is a way to underpin these finding s i future studies. This recommenddation applies not only to the discussed study area , but also to mountain block systems in general.