The implementation of Colorado Regulation 85 (5 CCR 1002-85) in 2012 has increased awareness of potential agricultural nonpoint source impacts on water quality in Colorado. The USDA Natural Resources Conservation Service (NRCS) enacted the Environmental Quality Incentives Program (EQIP) under the 1996 Farm Bill to assist producers in implementing conservation practices to address natural resource concerns; however, no research has estimated the water quality impacts in largely irrigated, arid/semiarid regions. The present study applied the USDA NRCS Soil and Water Assessment Tool (SWAT) to model edge-of-field losses of total nitrogen (N) and total phosphorus (P) before and after the implementation of EQIP-funded practices (e.g., reduced tillage, irrigation systems, and conservation buffers) between 2008 and 2018. It is the first study of its kind to analyze conservation practice effectiveness on irrigated fields and the first to utilize a novel method for assigning NRCS practice data to fields while maintaining landowner privacy. Because of data privacy requirements and limited measured data, the results provide estimated impacts on edge-of-field losses (which includes surface runoff and percolation). The modeling analysis estimated reductions for total N from 6% to 24% (average = 22%) and for total P from 10% to 34% (average = 22%). This represents an estimated total N reduction of 602 Mg (664 tn) and a total P reduction of 39 Mg (43 tn) over the 11-year period from EQIP-funded practices implemented on 134,000 ha (332,000 ac) in four major Colorado watersheds. The findings indicate the potential for EQIP-funded conservation practices to significantly reduce nutrient losses from irrigated, arid/semiarid agricultural lands. Additionally, this novel modeling effort, which utilized private data to the extent possible while maintaining privacy requirements, represents an important scientific advancement for agricultural nonpoint source policy evaluation.