University of Twente Student Theses


Analysis of Nutrient Dynamics in Roxo Catchment Using Remote Sensing Data and Numerical Modeling

Bihawala Vithanage, Imesh Chanaka (2009) Analysis of Nutrient Dynamics in Roxo Catchment Using Remote Sensing Data and Numerical Modeling.

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Abstract:The water quality in catchments is influenced by the complex combinations of land use, point sources combined with weather and other natural and human influences. Agricultural non point source pollution is usually considered a major cause of water quality deterioration in larger agricultural catchments. Farmers usually tend to use more fertiliser with the intention of making more harvest without considering the optimum doses, and their effects on the environment. Excess nutrients leached through the watershed and are collected on the downstream water bodies, making them eutrophic, being excessive algae and/or plant growth due to an abundance of nutrients. The excess growth of algae and phytoplankton has various deleterious effects of the water storage and distribution systems like clogging of filters, reduction of dissolved oxygen (DO), unpleasant taste and odour. Some algae species (e.g., blue green algae) may be toxic to fish, animals (like birds) and even humans (Chapra, 1997). In addition ammonia (NH3), nitrates (NO3) and nitrites (NO2) can be harmful when present in excessive amounts in water. In this study, our aim was to research the hydrological and environmental processes associated with Nitrogen (N) and Phosphorous (P) compounds and their dynamics in the hydrological system of the Roxo catchment. A model is developed that simulates the catchment flow hydraulics and the water quality in the catchment and Roxo reservoir. A special effort was put in place to model the effects of the Beja city waste water treatment plant (WWTP) as a test case for the model. This WWTP is an identified nutrient point source in the upper catchment. The hydrological, physical and biochemical water quality processes have been developed using Duflow Modeling Studio (DMS) including its Rainfall Runoff component model (RAM), based on linear reservoir theory. RAM elements (runoff areas) permit simulation of runoff hydrograph at the detailed sub-catchment level and permit to simulate distributed nutrient source (N, P) apportionment of agricultural areas to streams. Because no direct stream flow gauging station was present existing in the upper Roxo, a reservoir water balance technique was used to estimate daily catchment stream flow to the reservoir. An extensive data set was available for this purpose. The hydrological model calibration and validation was based on this, was judged satisfactory. During summer periods, some small negative inflows were computed. These small estimation errors were considered due the precision in the level measurements, as well as uncertainties in some water abstraction values (extra unregistered withdrawals) or losses (evaporation, groundwater) from the reservoir especially during summer periods. The flow calibration of DMS was finally performed using cumulative decadal inflow values. Besides running of the DMS model using interpolations from daily rain gauge data, we have run the model with remote sensing derived rainfall data (i.e. the Meteosat Multi-sensor Precipitation Estimate or MSG-MPE). We used a density of 18 data extraction points to represent catchment rainfall inputs. A simulation period starting from Jan 2007 to May 2008 with daily time steps was used for the purpose, and was a function of the availability of the MSG satellite rainfall data products. The study showed that the MSG daily rainfall data is not properly correlated with the daily gauge precipitation records. However when compared with the entire period, the cumulative result showed good agreement with the ii aggregated gauge station total rainfalls. The hydrologic responses produced using MPE showed that there are discrepancies with runoff hydrographs and this can be explained by the occasional high magnitude rainfalls recorded in the MPE that are significantly different from the gauge records. After calibration and validation of the flow model, and comparison of the rainfall runoff response using ground gauged rainfall and satellite precipitation inputs, nutrient export from the agriculture areas, and the impact of a point source (i.e., the WWTP) was simulated. These first results were interpreted and compared to observed stream and reservoir water quality data. Although, initial water quality modeling results are encouraging, and confirmed we were able to simulate the magnitudes and local spatial and temporal variations of the nutrient processes, much more model and parameter uncertainties need to be eliminated before the water quality model can be judge as appropriate for the upper Roxo catchment and the reservoir system. This can be obtained through further study and research.
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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