There are two major natural hazards that Cambodia is exposed to, namely, floods and droughts. Targeting the most vulnerable communes, this study aimed at estimating the costs of drought at the household level in the rural communes of Kampong Speu Province and assessing the costs and benefits of restoring a water reservoir as an adaptation option to deal with drought. Questionnaires were used to collect data from households for two rice ecosystems (totally rain-fed and with supplementary irrigation) in Kampong Speu. The study found that the expected loss from drought in a rain-fed farm was USD 53.47 per hectare compared to USD 23.01 per hectare for an irrigated farm. This study analyzed the costs and benefits of the restoration of a completely damaged reservoir. At a 6% discount rate, the repair of the reservoir will yield a net present value of about USD 914,835 with a benefit-cost ratio of 2.18. If the reservoir is restored, it will not only increase rice production, but will also stabilize food security in the area. Agricultural diversification, especially fish raising, animal raising, and vegetable planting will become more viable for farmers.