Flat plate solar collectors (FPSC) remain a cost-effective option for harnessing solar energy, yet their performance is strongly influenced by climatic conditions and system design. In high-altitude semi-arid regions such as Abha, Saudi Arabia, conventional FPSCs experience reduced output due to moderate irradiance and significant heat losses. To address these limitations, a modified collector was developed by integrating a layer of dark natural pebbles on the absorber plate and side-mounted polished aluminum reflectors. The experimental system was installed and monitored under real outdoor conditions, and its performance was compared against a baseline collector. Thermal efficiency was determined using first-law analysis, while supporting simulations were carried out using ANSYS Fluent to validate heat transfer trends and temperature distributions. The baseline collector exhibited the lowest efficiency, averaging similar to 0.45. The pebble-modified collector showed clear improvement, reaching an average of similar to 0.54, owing to enhanced heat absorption and thermal storage that stabilized outlet temperatures. The combined pebble-reflector system achieved the highest performance, with efficiencies between 0.55 and 0.75 and a mean of similar to 0.66. Reflectors increased incident solar flux and raised absorber temperatures, while pebbles acted as a sensible heat reservoir, extending useful operation into late hours. Uncertainty analysis confirmed the robustness of the measurements, with efficiency errors within +/- 2-3%. The results demonstrate that low-cost, locally available materials can significantly enhance FPSC efficiency in challenging climates. The integration of optical concentration and thermal storage strategies provides a practical pathway toward sustainable and high-performance solar water heating.