Saudi Arabia depends on desalinated seawater, treated and untreated ground and surface water as freshwater resources. Surface water is available in limited regions in Saudi Arabia, such as Al-Baha province. Surface water is the main freshwater resource for Al-Baha region through some dams’ reservoirs, such as Al-Aqiq reservoir. This work aims to assess the inflow (untreated) and outflow (treated) water quality (WQ) conditions of Al-Lahyan water treatment plant (WTP) at Al-Baha region for urban usage. For that, descriptive statistics and WQ index (WQI) approaches were developed, based on the drinking water Saudi standards. The statistical analysis of considered WQ records showed that some of raw water measurements were higher than the Saudi standards for drinking water. While for the produced water, limited records of nitrite, free chlorine and coliform were higher than the considered standards. According to the developed WQI, untreated raw water was ranked as “Poor” water quality for drinking, while the treated produced water was ranked as “Good”, which reflects the WTP performance. A restricted control should be applied on using of untreated water for drinking, in particularly for bacterial contamination. Modified WQI should be applied at WTPs to control the WTP efficiency. A water quality protection strategy for the reservoir should be designed and implemented for sustainable water uses.