Climate change and recurrent droughts are increasing pressure on freshwater resources in Bulgaria, where potable water is still commonly used for urban landscape irrigation. This study evaluates alternative irrigation water supply strategies for two central parks in Gabrovo (a region with periodical water shortages) by combining rooftop rainwater harvesting (RWH) with alternative local sources. Irrigation demand was quantified from metered consumption (2020–2023), while RWH potential was assessed using long-term precipitation records and roof areas of the city’s central Library (600 m²) and Drama Theatre (650 m²). With a runoff coefficient of 0.9, harvested rainwater can cover at most about 29% of seasonal irrigation needs, indicating that standalone RWH is insufficient. Three hybrid options were therefore designed and compared via discounted life-cycle cost analysis over 30- and 50-year horizons, with varying discount rates (0, 4, 8%) and technological equipment replacement intervals (10, 15, 20 years): (1) RWH with potable network backup (RW+N); (2) RWH with supplementary intake from a drainage well in the Sinkevitsa riverbed (RW+W); and (3) RWH with supply from the nearby Sinkevitsa dam via a rehabilitated pipeline (RW+D). At realistic discount rates (4%), RW+W yields the lowest present value costs, fully eliminates potable water use, and offers robust drought resilience with manageable technical complexity, whereas RW+D is consistently the most expensive and operationally risky option. Furthermore, the parametric analysis highlights strong interactions among DR, analysis horizon and TER. At low DRs, shorter TERs significantly increase PV because multiple replacement cycles are only weakly discounted. At high discount rates, the influence of TER diminishes and OPEX-intensive configurations and RW+N can become comparatively more attractive. Beyond the local context, the proposed methodology can inform climate adaptation strategies in other European municipalities facing similar environmental and socio-economic challenges.