The Rio Grande River is considered as an over-appropriated river basin in Texas, where the number of permits to use surface waters exceed the amount of available water. Agricultural and municipal water supply and use in the Lower Rio Grande Valley (LRGV) are essentially dependent upon storage of the International Amistad and Falcon Reservoirs, which are owned and operated by the International Boundary and Water Commission (IBCW) based on provisions of the 1944 treaty between Mexico and the United States. The Texas share of the waters of the Rio Grande is allocated among numerous farmers, irrigation districts, and cities by a unique water rights permit system administered by the Rio Grande watermaster of the Texas Commission on Environmental Quality (TCEQ). The Rio Grande Water Availability Model (WAM) obtained from the TCEQ WAM System has a hydrologic period-of-analysis of 1940-2000. However, hydrology since 2000 includes the severe 2008-2014 drought and is important to the simulation study. The hydrologic period of analysis for the Rio Grande WAM was extended from 2001 to 2015 using Water Rights Analyses Package (WRAP) programs and methodologies. Extending the hydrologic period-of-analysis of the Rio Grande WAM to cover 1940-2015 was an initial major task in the research. A WRAP/WAM simulation combines natural hydrology represented by sequences of monthly naturalized streamflows and reservoir evaporation-precipitation rates for a specified hydrologic period-of-analysis, 1940-2015 in this study, with specified scenarios of water resources development, allocation, management, and use. Water availability is assessed based on supply reliability metrics and storage and flow frequency metrics computed from simulation results. Additionally, the Rio Grande WAM original 1940-2000 hydrologic period of analysis is extended to cover 1940-2015 and long-term simulations were performed to develop water supply reliability and storage frequency metrics for major water right groups, reallocation of municipal water rights in the Amistad-Falcon Reservoir system, and water planning scenarios including drought management. The Conditional Reliability Modeling (CRM) methods were applied to assess short-term water planning and management strategies for the LRGV along with the drought management scenarios were simulated to predict the likelihood of extended drought conditions based on beginning storage in the Amistad-Falcon Reservoir system. The reliability and exceedance frequencies of maximum end-of-month storage at Amistad and Falcon reservoirs were developed using CRM.