Title: Nitrogen dynamics and land use in the Xochimilco urban wetland ecosystem, Mexico City - A model approach
Author: Isabel Wallnöfer
Supervising Institution: IHE Delft - Institute for Water Education
The Xochimilco urban wetland in Mexico City is a dynamic system, which has undergone continuous land use change throughout its long history. Documented presence of humans in the wetland reaches back to pre-Hispanic times. It is mainly composed of small artificial islands and a surrounding canal system. These islands, called chinampas, host a type of agriculture that is renowned for its sustainable method of nutrient recycling within the system. This agricultural system is recognized by UNESCO as a cultural heritage site. In addition the wetland is a Ramsar site due to its ecological importance. Despite its protected status, several developments such as urbanization, intensification of agricultural production and groundwater abstraction have led to deterioration of the water quality in the canals of the wetland. The water quality is currently classified as eutrophic with high nitrogen and phosphorous concentrations. The concentrations of nitrogen in the canal water show a pattern that is related to land use. This leads to the assumption that there is the connection between these two factors. The investigation of the relationship between different land use types and nitrogen dynamics in the Xochimilco urban wetland ecosystem aims to improve the understanding of the system and to develop a strategy for assessing nitrogen input and output to and from the Xochimilco urban wetland. Understanding nitrogen dynamics is a step towards sustainable management and conservation in this urban wetland system to ensure the provision of ecosystem services. Therefore this research integrates the targets of the Sustainable Development Goals predominantly goals 6 and 15. To shed light on the relationship between nitrogen input and anthropogenic activities in the Xochimilco urban wetland, this complex system was categorized into different land uses. Each land use was described to indicate activities, which could lead to nitrogen input into the aquatic system. Most nitrogen inputs were thereby related to diffuse pollution and agricultural activities, especially intensive greenhouse agriculture. A first attempt was made to translate the properties of this complex system into a system dynamics model using the StellaTM modelling software. The purpose of the model was to facilitate the understanding of nitrogen dynamics in the Xochimilco urban wetland and to serve as a basis for further investigations on causes and effects of nitrogen pollution in Xochimilco. A better understanding of the nitrogen dynamics can help with regards to management incentives. The model showed promising outputs, which be related to realistic environmental and water quality conditions in the Xochimilco wetland.