CSAWAC 43 (5) 621-786 (2015) · Vol 43 · No · May 2015 CLEAN Soil Air Water Renewables Sustainability Environmental Monitoring Focus Issue: Water Management for Agriculture and Energy Security in Asia | 2015 www.clean-journal.com 652 Nguyen Hong Quan Günter Meon Institute for Environment and Resources, Vietnam National University, Ho Chi Minh City, Vietnam Leichtweiss Institute for Hydraulic Engineering and Water Resources, University of Braunschweig, Braunschweig, Germany Research Article Nutrient Dynamics During Flood Events in Tropical Catchments: A Case Study in Southern Vietnam Assessing surface water quality variation as well as chasing water pollution sources is essential for water quality management However, for existing conditions in developing countries, this assessment may not be done properly in many affected catchments due to limited data and lacking of tools In particular, pollutant transport from the catchment to its river system during flood events needs quantification and is the aim of the study For this, a combined water quality monitoring and modeling approach is proposed The study was exemplarily performed for a typical ungauged medium scale catchment located in the southern area of Vietnam The available budget allowed at least a limited monitoring of nutrients and driven parameters (e.g., flow, sediment) These data were used to, in total, successfully calibrate the complex Hydrological Simulation ProgramFortran (HSPF) model The results lead to three main conclusions: (1) the contributions of point and diffuse sources to nutrient loadings could clearly be identified with the help of monitoring; (2) water quality sampling during flood events is critical to assess pollution sources, especially, diffuse ones However, just a monitoring of data alone is not adequate to interpret the observed concentrations; modeling is required (3) Despite of the limited amount of data, which could be recorded and processed during the study, a representative catchment modeling during floods could be performed It delivered essential information for linking pollution sources with water quality data Furthermore, the limits of an application of the complex HSPF model under given conditions were shown Keywords: HSPF model; Point and diffuse sources; Tapioca; Tropical regions; Water quality monitoring Received: April 9, 2013; revised: August 27, 2013; accepted: September 18, 2013 DOI: 10.1002/clen.201300264 Introduction Water pollution is one of the challenging problems in water resources management At different impact levels, this problem still exists in both developed and developing countries [1, 2] For example, in Vietnam, surface water pollution at river basin scale stays at the highest priority to be concerned, for example, said by Mr K N Pham, Minister of the Ministry of Environment and Resources, that “the Vietnam Environment Administration must concentrate well in management and pollution control” (www.vea.gov.vn) National and international agencies, scientists as well as other water resource stakeholders have been looking for water pollution reduction solutions Typically, for example, is the Clean Water Act 1972 for the implementation of the total maximum daily load [3] in the Correspondence: Dr N H Quan, Department of Natural Resources Management, Institute for Environment and Resources (IER), Vietnam National University of Ho Chi Minh City, 142 To Hien Thanh, District 10, Ho Chi Minh City, Vietnam E-mail: hongquanmt@yahoo.com Abbreviations: HSPF, Hydrological Simulation Program-Fortran; LSUR, length of overland flow plane; LZSN, lower zone nominal storage; NSE, Nash–Sutcliffe efficiency; PBIAS, percent bias; RMSE, root mean square error; SLSUR, slope of overland flow plane; TSS, total suspended solid; US EPA, United State Environmental Protection Agency; UZSN, upper zone nominal storage © 2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim United States Other examples are the Drinking Water Directive, the Water Framework Directive in European countries [4], the Vietnam Law on Environment Protection [5] However, achievement to proposed objectives (i.e., good water quality status) is questioned due to, for instance, complexity of diffuse pollution [6] Research in nutrient dynamics during flood events is limited in literature, especially at medium-sized catchments ranging from 10 to 100 km2 The most concerned reason is the lack of monitoring data [7, 8] This problem is increasingly recognized in the scientific community, for example, during the Predictions in Ungauged Basins program [9] In the developing countries, for example, Vietnam, available observations from responsible agencies are often limited in temporal scale (e.g., from four to twelve times per year) Finer data resolution is sometimes available at field scale (few hectares) or at a few small catchments (