**Ralph A. Wurbs**

Department of Civil and Environmental Engineering, Texas A&M University, College Station, TX 77843, USA; r-wurbs@tamu.edu; Tel.: +1-979-845-3079

Received: 3 September 2020; Accepted: 1 October 2020; Published: 4 October 2020

**Abstract:** Effective water resources managemen<sup>t</sup> requires assessments of water availability within a framework of complex institutions and infrastructure employed to manage extremely variable stream flow shared by numerous, often competing, water users and diverse types of use. The Water Rights Analysis Package (WRAP) modeling system is fundamental to water allocation and planning in the state of Texas in the United States. Integration of environmental flow standards into both the modeling system and comprehensive statewide water managemen<sup>t</sup> is a high priority for continuing research and development. The public domain WRAP software and documentation are generalized for application any place in the world. Lessons learned in developing and implementing the modeling system in Texas are relevant worldwide. The modeling system combines: (1) detailed simulation of water right systems, interstate compacts, international treaties, federal/state/local agreements, and operations of storage and conveyance facilities, (2) simulation of river system hydrology, and (3) statistical frequency and reliability analyses. The continually evolving modeling system has been implemented in Texas by a water managemen<sup>t</sup> community that includes the state legislature, planning and regulatory agencies, river authorities, water districts, cities, industries, engineering consulting firms, and university researchers. The shared modeling system contributes significantly to integration of water allocation, planning, system operations, and research.

**Keywords:** water allocation; planning; river/reservoir systems; water availability modeling
