The allocation of water resources is typically accomplished within the framework of water allocation systems (WAS). In general, a WAS sets priorities, applies rules, and organizes responses to a range of water allocation scenarios. This research presents a comprehensive study of water allocation strategies and provides a conceptual framework of principles and guidelines for designing, assessing, implementing and supporting WAS. The voluminous compilation of international treaties and conventions, interstate compacts, intrastate administrative documentation, and scientific/engineering literature was researched in order to identify different water allocation strategies and mechanisms. From this analysis eight fundamental areas of WAS were identified: water rights, determination of water allotment, administrative systems, reservoir storage considerations, system reliability, multiple uses, instream flow requirements, and drought management. The systematic scrutiny of these eight areas at the international, interstate, and intrastate levels defined the conceptual framework for assessing WAS. The Texas experience with regard to its Water Availability Modeling system is also reviewed with particular emphasis on the application of the Water Rights Analysis Package (WRAP) model in supporting water allocation efforts. The Lower Rio Grande WAS was used as a case study to demonstrate how the principles presented in the conceptual framework can be used to assess water allocation issues and identify alternative strategies. Three WRAP simulation studies utilizing several components of the conceptual framework were performed in order to assess the Lower Rio Grande WAS. The simulations focused on three of the major water allocation issues of the Texas Rio Grande: reallocation among uses, instream flow requirements, and drought management. The simulations showed several deficiencies in the Lower Rio Grande WAS, particularly regarding the size of the domestic-municipal-industrial (DMI) reserve and its effect on the reliability of other uses. The simulation results suggest that water from the DMI can be liberated to be used by irrigators and to support environmental flows without affecting the reliability to municipal users. Several strategies were proposed that can potentially improve the overall efficiency of the system. Nonetheless, implementing new strategies and water allocation policies in the Lower Rio Grande WAS would require considerable changes in regulation policies.