Water is an integral part of agricultural practices, with agriculture being the largest user of surface water in the United States. Agriculture's reliance on surface water is strengthening as climate change and growing populations are stressing irrigated croplands. This surface water is primarily stored by a complex network of dams, but despite our reliance on surface water for irrigation, we lack a spatially detailed record of irrigation dam command areas. Therefore, we assigned irrigation command areas to the approximately 1,100 large irrigation dams in the continental United States by combining a tiered assignment strategy with field level infrastructure and agricultural data. We showed that these large irrigation dam command areas are responsible for 10.7 million acres of cropland. This translates to approximately 13.3 billion dollars of crops annually that depend on these large irrigation dams for water. The high-resolution, crop specific assignment of these command areas allows for water scarcity assessments that can be used for better water management decisions to address the changing environmental conditions and public demand pressuring the nation's agriculture and water infrastructure. / Master of Science / Agriculture depends heavily on surface water stored by large dams, especially in the face of climate change and population growth. However, the spatial distribution of croplands that rely on these dams is poorly known. We mapped the irrigation command areas of around 1,100 large dams in the continental US using a combination of field-level data and a tiered assignment method. We then traced the agricultural products from these areas to their domestic and international markets. We found that large dams support 10.7 million ares of cropland and 13.3 billion dollars of crop value annually in the US. Our high-resolution mapping of irrigation command areas enables more accurate assessments of water scarcity and reveals the local and distant connections between water infrastructure and its users.