Developing an integrated catchment-scale modelling approach for supporting the sustainable management of water nutrient pollution from diffuse agricultural sources
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Water is one of our most vital natural resources for life sustaining and human's economic development and social well-being. Agricultural diffuse water pollution (ADWP), the biggest remaining problem of water pollution in the world, has been realised as a major threat for water quality and the implementation of the EU Water Framework Directive (WFD). Indicative estimates of the costs of water pollution from ADWP are about £225 M per year in the UK, whilst failure to meet the requirements of the EU WFD by 2015 may incur heavy fines. From the technical and scientific points of view, there are three major gaps, namely, "method and tool", "research scale" and "fundamental knowledge" gaps between current ADWP research and the successful implementation of the EU WFD. It is timely to develop integrated catchment-scale numerical modelling tools or methods to handle the ADWP problem at the catchment scale. This thesis describes the development of an integrated catchment-scale modelling approach, ICEMAN, for supporting the effective decision-making of the ADWP sustainable management at the catchment scale, thus helping the implementation of the EU WFD in handling ADWP. In order to quantitatively describe the nutrient process in the complete hydrological cycle, it is necessary to integrate numerical water models into ICEMAN to calculate or simulate the groundwater pollution pathway vulnerability, groundwater pollution risk, water balance in soil, nutrients biochemical cycling in soil, and surface water quality and quantity processes. This study showed that GIS and the Arc Hydro model consisting of the modules of data mining, hydrological analysis, and visualisation can facilitate the developing and applying the ICEMAN by providing data support and powerful functions of spatial analysis. The DRASTIC model was applied in the case study area - Upper Bann Catchment, Northern Ireland for assessing the groundwater pollution pathway vulnerability of general pollutants and pesticide. The results showed that DRASTIC is suitable to be introduced into the ICEMAN for catchment-scale groundwater pollution vulnerability assessment. However, DRASTIC has drawbacks in the groundwater pollution risk assessment, namely, having no risk concept and considering no pollutant dynamic nature with runoff. A D-DRASTIC approach was developed in this study for reliable groundwater pollution risk assessment from diffuse agricultural sources based on DRASTIC within an ArcGIS environment. D-DRASTIC overcomes the pitfalls of applying DRASTIC in groundwater risk assessment. The results of applying D-DRASTIC in the case study showed that D-DRASTIC is helpful in guiding the activities of groundwater pollution prevention at the catchment scale and can be used in the development of ICEMAN. A numerical catchment-scale surface water model capable of the simulation of ADWP is necessary in developing the ICEMAN method. A HSPF model was selected based on the review of popular surface water models and tested in the study area. The calibrated and validated HSPF model can well represent the characteristics of surface water quantity and quality in the study area. Climate change scenario evaluation results in five years showed that when the annual mean temperature increase 3 Celsius the mean yearly total runoff volume will decrease by 11% and the mean daily river flow of five years will decrease by 11%. The results showed that HSPF is a suitable model in simulating the diffuse source surface water pollution and can be integrated into the ICEMAN. ICEMAN was developed by integrating the models of Arc Hydro, DRASTIC, D-DRASTIC, HSPF into an ArcGIS environment. ICEMAN can describe the nutrient biochemical cycles in soil, whole hydrological quantity and quality processes, and groundwater pollution vulnerability and risk, by considering factors in the catchment ADWP process, namely, meteorology, nutrient loading from different land uses, nutrient biochemical cycling in soil, nutrient dynamic nature with runoff and interflow, topography, depth to water, net recharge, aquifer media, soil media, impact of the vadose zone media, hydraulic conductivity of the aquifer, and the relationships between soil water and groundwater. The results of applying ICEMAN in the study area showed that ICEMAN can well support the decision-making of the catchment ADWP sustainable management. In the study area, ICEMAN provides satisfied simulation of river flow and quality, groundwater pollution vulnerability and risk zones, and quantitative descriptions of ADWP process including nutrient biochemical cycle in soil and can help better understand the ADWP characteristics in a specific catchment. In addition, ICEMAN can evaluate the impacts of water management plans on water processes under the climate change. For example, when changing 20% farming land into forest land in the Gamble's Bridge watershed of the study area, the mean concentrations of nitrate, nitrite, NH4, and P04 in river will decrease by 19%, 33%, 31%, and 31% respectively. ICEMAN, transferable to other areas, can bridge gaps of "method and tool" and "research scale" in the implementation the EU WFD in handling ADWP and can act as an important complement of the River Basin Management Plans. This multi-disciplinary study may provide a good starting point for tackling ADWP at the catchment scale in an integrated, quantitative, and sustainable manner. Therefore, the results in this multi-disciplinary study are not only useful for better implementation of the EU WFD, but also helpful for tackling the ADWP problem outside the EU.