The assessment of surface water and groundwater resources is the most important component in the long-term planning of an integrated water management scheme, since it provides information on the sustainable average available input on the supply side, its hydro-chemical properties as well as its temporal variability. Generally, two main methodologies are employed to quantitatively determine available water resources. These are:

• Direct measurement of discharge (surface / groundwater / wastewater) components and water budgeting.
• Forward modeling with hydrological models.

Forward modeling with process models is a powerful tool for the determination of surface water and groundwater discharge, since, once calibrated and validated, they allow for the prediction of the temporal and spatial variability of the discharge components. However, they require a large amount of detailed spatially distributed data on the catchment, e.g. hydrological soil and vegetation characteristics, relief, etc. that are not readily available.

Selected results:

• Mean long-term precipitation is relatively uniform (approx. 400–550 mm a⁻¹) - surface runoff and groundwater recharge exhibit larger differences
• Lowest fraction of surface runoff was measured to be about 1% (Wadi Auja, hydrological years of 2011–2013), due to a high infiltration capacity of the karstified limestone and dolomite outcrops
• Other catchments: surface runoff fractions between 3–7%
• Groundwater recharge ranges between 13% and 33%
• Recharge: highly variable for individual years, mainly due to the overall annual precipitation depth and different annual rates of evapotranspiration

(Wolf & Hötzl 2011, Klinger et al. 2014, Klinger et al. 2015)