Abstract. Flow in complex karst aquifers is challenging to
conceptualize and model, especially in poorly investigated areas, in
semiarid climates, and under changing climatic conditions; however, it is
necessary in order to implement long-term sustainable water management practices.
Thus, the objectives of this work were to propose a calibration approach based on
time series analyses for a karst aquifer and to assess the impact of
climate change on spring discharge. Based on more than 3 years of
high-resolution continuous monitoring, a semi-distributed lumped model was
calibrated and validated for the Qachqouch karst spring, north of Beirut
(Lebanon). Time series analyses and decomposition of spring hydrographs
revealed that the system has a high regulatory function, with considerable
storage capacity providing stable flow (minimum flow of 0.2 m3 s−1)
during the dry season and with flow rates exceeding 10 m3 s−1 during the
wet season, which is similar to other karst aquifers in the region. Based on this
detailed understanding of the hydrodynamics of the system, the model
geometry and parameters were validated. Three linear reservoirs were
implemented to reproduce the combined contribution of the different flow
components of the system. A satisfactory simulation (Nash–Sutcliffe
efficiency coefficient, NSE, of 0.72) of the measured spring flow rates was obtained after
calibration. Climate change conditions (+1 to +3 ∘C warming,
−10 % to −30 % less precipitation annually, and the intensification of rain
events) were added to a baseline climatic year to produce scenarios of
expected spring flow responses. Results show that the Qachqouch karst
aquifer is sensitive to decreasing rainfall, which is associated with more
pronounced recessions, with flow rates decreasing by 34 % and 1-month
longer dry periods. Because of the limited influence of snow on the spring
flow rate, a warming climate has less impact on spring flow conditions than
a reduction in precipitation. Although the model shows that increasing
rainfall intensity induces larger floods, recessions, and shorter low-flow
periods, the real impact of high-intensity precipitation events remains
uncertain, as the model does not account for complex unsaturated and
epikarstic processes. This work shows that calibrating a semi-distributed
lumped model using time series analyses can be an efficient approach to
improve simulations of complex karst aquifers, thereby providing useful models
for long-term sustainable water management.