Abstract In an earlier study, irrigation water use in the Del Reguero watershed (DRW) was assessed using various water management indices. In this study, the phosphorus (P) transport dynamics in the irrigation return flows were analyzed. Phosphorus fertilization practices were determined through interviewing farmers, and P loads were monitored from October 2007 to September 2009. In surface drainage waters, the average annual total phosphorus concentration (TP) was 0.112 mg L −1. Total dissolved P (TDP) represented the dominant P form (93% of the TP concentrations) indicating that subsurface flow was the dominant pathway for P transfer. Drainage water was classified as hypertrophic during 2008 and eutrophic during 2009. Fertilizer contributed the most to the final result of P mass balance by contributing 98% of the total P inputs. P crop needs were exceeded by 16% leading to an excess of 43.8 kg P ha −1 year −1. The exported mass of P was 240 kg in 2008 and 228 kg in 2009, and the TP loss was 205 g P ha −1 for 2008 and 195 g P ha −1 for 2009. The amount of TP load in the Del Reguero stream was considered small with regard to total phosphorus input (0.23%). However, the amount of TP load in the Del Reguero stream was considered significant in regard to surface water eutrophication. Because the majority of the TP load existed in the dissolved form (TDP = 93% of TP), a large portion of the P losses from the system was available for algae growth, thereby, enhancing eutrophication.