Whole building water modeling continues to gain traction in the building industry with the public release of ASHRAE 191P Standard for the Efficient Use of Water in Building Mechanical Systems and LEED Pilot Credit: Whole Project Water Use Reduction. As this new field develops, technical challenges and opportunities arise that highlight the growing need for accurate whole building water modeling tools and procedures. This paper will explore the current challenges in defining plumbing fixture usage, irrigation calculations, and rainfall patterns as well as current methods and data sources for each section.While energy modelers have many diversity schedules to use -- such as the ASHRAE handbooks, ASHRAE 90.1 reference guide, and the Labs21 schedules – water modelers don’t have a comparable set of resources for fixture usage.There are numerous methods for calculating evapotranspiration to determine irrigation water demand. Many methods such as Penman-Monteith and SLIDE have been adapted for use in landscape calculations, but this has limitations such as use for non-turf grass plants.Finally, rainfall data sources are varied and inconsistent across data sets. This inconsistency creates repeatability challenges for irrigation and water storage calculations.