Trend Analysis of Nutrient Concentrations and Loads in Surface Water in an Intensively Fertilized Watershed

Abstract Nutrient concentrations and loads from 1988 to 1997 at the surface‐water outlet of a heavily fertilized watershed in the Brittany region, west of France, were analyzed for trends. The 1240‐ha watershed is characterized by milk production and an increasing indoor pig and poultry production. A nonparametric statistical analysis performed on mean monthly and mean annual data detected no trend in nitrate and total P concentrations and loads from 1988 to 1997. When comparing data from the same month for each year of the study, a decreasing trend was detected in the November nitrate flow adjusted concentrations, and in the October maximum nitrate flow adjusted concentrations. An increasing trend was detected in the August total P flow adjusted concentrations. No trend was detected in the monthly and annual nitrate and total P loads. A multiple regression analysis showed that the annual nitrate concentration is positively correlated to the annual rainfall and to the percent area of corn ( Zea mays L.). A nutrient global mass balance was constructed for 1988, 1991, and 1994 to explain the lack of an increasing trend in nutrient concentrations and loads despite an intensification of the pig production during these years. Results from this analysis showed that farmers have compensated for increased on‐farm production of nutrients from animal manure by purchasing less fertilizers; therefore, the total nutrient budget remained relatively stable.