Numerical modeling of nutrient transport to assess the agricultural impact on the trophic state of reservoirs

A trophic state increase relates to surface water bodies nutrient enrichment, due to the chemical products used such as fertilizers in agriculture and residues from cattle raising activities. This research consists of nutrient transport numerical modeling to analyze the Betancí reservoir trophic state in Colombia; Water samples were collected to analyze total nitrogen, total phosphorus, ammonia, nitrates, nitrites, phosphates, chlorophyll-a, dissolved oxygen, BOD, COD, suspended solids, and water transparency. The water quality model MOHID Studio was implemented and after its calibration, scenarios of increase and decrease of nutrients and inflows to the reservoir through its main tributaries were simulated to reproduce the agricultural activity changes in the basin and the effects that would have on the reservoir's trophic state dynamics; Therefore, the Carlson Trophic Status Index was calculated for each case. The results show that variations in total nitrogen concentration and increased inflows present short-term consequences on the reservoir's trophic state. Increasing the incoming total nitrogen concentrations by 100% causes the reservoir to change from a light eutrophic to a hypereutrophic state. The results of this research provide a starting tool to water resources integrated management in reservoirs.