Hydrologic data, including rainfall and runoff data, have been collected on experimental watersheds operated by the U.S. Department of Agriculture Agricultural Research Service (USDA-ARS) in southern Arizona since the 1950s. These data are of national and international importance and make up one of the most comprehensive semiarid watershed data sets in the world. The USDA-ARS Southwest Watershed Research Center has recently developed an electronic data processing system that includes an online interface (http://tucson.ars.ag.gov/dap) to provide public access to the data. The goal of the system is to promote analyses and interpretations of historic and current data by improving data access. Data are collected from sensors in the field and are transmitted to computers in the office. The data are then processed, quality checked, and made available to users via the Internet. The publicly accessible part of the system consists of an interactive Web site, which provides an interface to the data, and a relational database, which is used to process, store, and manage data. The system was released to the public in October 2003, and since that time the online data access Web site has received more than 4500 visitors.