Reservoir storage is essential for developing dependable water supplies and is a major component of the river system water budget. The storage contents of reservoirs fluctuate greatly with variations in water use and climatic conditions that range from severe multiple-year droughts to floods. Water surface evaporation typically represents a major component of the reservoir water budget. This thesis investigates the effects of evaporation and potential reductions in this evaporation on the water supply capabilities of reservoirs in Texas. As part of this research, a literature review based assessment of capabilities for reducing reservoir evaporation using monolayer films and other methods was performed. The literature review assessment provides an overview of past evaporation suppression studies performed using monolayer films and other evaporation suppression technologies including water shades and covers. The overview provides a summary on monolayer film application techniques, environmental impacts, operational and material costs, evaluation methods, and achievable evaporation reduction rates. This research project quantifies the impact of reservoir evaporation on water supply availability/reliability by using the Texas Water Availability Modeling (WAM) System which consists of the Water Rights Analysis Package (WRAP) and 21 sets of WRAP input files covering the 23 river basins of the state, a geographic information system (GIS), and contains over 8,000 water rights permits, which include 3,435 reservoirs. The impact of evaporation on water supply availability/reliability was evaluated by performing several analyses in which evaporation rates are reduced by specified percentages starting when storage levels drop below certain trigger percentages of reservoir storage capacity. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/149357