As noted by others, ...

As noted by others, well-designed dams can accomplish any or all of four objectives: (1) flood control, (2) irrigation, (3) drought augmentation, and (4) recreation. The design needs to consider all the needs of the population and the climate and geology of the area to best balance these objectives. Much of the effectiveness in drought reduction (also known as "low-flow augmentation" will depend on the topography of the dam inundation area (is it small and deep or large and shallow). This (together with humidity and geologic seepage factors) will determine how effective/economical drought reduction can be. Only a highly-experienced team of consulting engineers, hydrologists, geologists, planners, and economists can adequately evaluate your particular situation. That's why most large dam projects in the 3rd world require external funding for feasibility study, which can be several million dollars. The construction costs can go into the 100s of millions, with additional cost for irrigation canals, hydro-electric plants, etc. It can be a major undertaking, but it can change the lives of thousands or millions of people for the better. There is also a downside to the environment and relocation of indigenous people who must be re-located out of the inundation area.  Good luck. Where is your project?