ESTIMATION OF THE SPATIO-TEMPORAL HETEROGENEITY OF RAINFALL AND ITS IMPORTANCE TOWARDS ROBUST CATCHMENT SIMULATION, WITHIN A HYDROINFORMATIC ENVIRONMENT. by K.Umakhanthan School of Civil and Environmental Engineering Faculty of Engineering University of New South Wales Sydney, Australia June 2002 This thesis is submitted to the University of New South Wales in fulfilment of the requirements for the degree of Doctor of Philosophy UNIVERSITY OF NEW SOUTH WALES Trim page to Thesis/Project Report Sheet guide lines Surname or Family Name: Umakhanthan First Name: Kanagaratnam Other name/s: Abbreviation for degree as given in the University calendar: PhD School: Civil and Environmental Engineering Faculty: Engineering Title: ESTIMATION OF THE SPATIO-TEMPORAL HETEROGENEITY OF RAINFALL AND ITS IMPORTANCE TOWARDS ROBUST CATCHMENT SIMULATION, WITHIN A HYDROINFORMATIC ENVIRONMENT Abstract 350 words maximum: Presented in this dissertation is an investigation of the spatial and temporal heterogeneity of rainfall and the influence of this on the robustness of predictions obtained from a Catchment Modelling System (CMS). Proposed in this dissertation is a methodology to investigate the degree of variability of rainfall in the spatial and temporal dimensions. Improved estimates of the spatially distributed with smaller time step hyetographs suited especially the urban catchments were obtained and importance of a more detailed rainfall model is highlighted towards a more robust prediction from CMS. The study identified both spatial and temporal semi-variograms, which were produced by plotting the semi- variance of gauge records in space and time against distance and time respectively. As the results of the investigation on the developed semi-variogram approach, real storm events were categorised as being High Spatial-High Temporal (HS-HT); High Spatial-Low Temporal; (HS-LT); Low Spatial-High Temporal (LS- HT); and Low Spatial-Low Temporal variability. A Comparatively detailed rainfall distribution model in space and time was developed within the GIS. The enhanced rainfall representation in both space and time scale is made feasible in the study by the aid of the powerful spatial analytic capability of GIS. From this model, improved estimates of the spatially distributed with smaller time step hyetographs suited for especially the urban catchments could be obtained. The importance of the detailed space-time rainfall model in improving the robustness of runoff prediction of CMS was investigated by comparing error parameters for predictions from CMS using alternate rainfall models, for various degrees of spatio-temporal heterogeneity events. From the investigations made, it was found that the spline surface rainfall model, which considered the spatial and temporal variability of the rainfall in greater detail than the Thiessen rainfall model resulted in predicted hydrographs that more closely duplicated the recorded hydrograph for the same parameter set. The degree of this improvement in the predicted hydrograph was found to be dependent on the spatial and temporal variability of the storm event. The analysis is based on the real events recorded from the Centennial Park Catchment (1.3 km2) and the Upper Parramatta River Catchment (110 km2) in Sydney, Australia. DECLARATION RELATING TO DISPOSITION OF PROJECT REPORT/THESIS I AM FULLY AWARE OF THE POLICY OF THE UNIVERSITY RELATING TO THE RETENTION AND USE OF HIGHER DEGREE PROJECT REPORTS AND THESES, NAMELY THAT THE UNIVERSITY RETAINS THE COPIES SUBMITTED FOR EXAMINATION AND IS FREE TO ALLOW THEM TO BE CONSULTED OR BORROWED. SUBJECT TO THE PROVISIONS OF THE COPYRIGHT ACT 1968, THE UNIVERSITY MAY ISSUE A PROJECT REPORT OR THESIS IN WHOLE OR IN PART, IN PHOTOSTATE OR MICROFILM OR OTHER COPYING MEDIUM. I ALSO AUTHORISE THE PUBLICATION BY THE UNIVERSITY MICROFILMS OF A 350 WORD ABSTRACT IN DISSERTATIONS ABSTRACTS INTERNATIONAL (APPLICABLE TO DOCTORATES ONLY) ....................................................... ....................................................... ....................................................... SIGNATURE WITNESS DATE THE UNIVERSITY RECOGNISE THAT THERE MAY BE EXCEPTIONAL CIRCUMSTANCES REQUIRING RESTRICTIONS ON COPYING OR CONDITIONS ON USE, REQUESTS FOR RESTRICTION FOR A PERIOD OF UP TO 2 YEARS MUST MADE IN WRITING TO THE REGISTRAR. REQUESTS FOR A LONGER PERIOD OF RESTRICTION MAY BE CONSIDERED IN EXCEPTIONAL CIRCUMSTANCES IF ACCOMPANIED BY A LETTER OF SUPPORT FROM THE SUPERVISOR OR HEAD OF SCHOOL. SUCH REQUESTS MUST BE SUBMITTED WITH THE THESIS/PROJECT REPORT. FOR OFFICE USE ONLY DATE OF COMPLETION OF REQUIREMENT FOR AWARD: REGISTRAR AND DEPUTY PRINCIPAL THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS This thesis is dedicated to the memory of my beloved father, the late Mr.S.Kanagaratnam (1938-1993) This is the proper plan of study: Reading, Reflection and Regular application in life. Study is Work. Inquiry into the value and applicability of what is studied is Worship. The experience of validity and value of the practice is Wisdom. True education is not for a mere living, but for a fuller and meaningful life. - Bagawan Sri Sathya Sai Baba CERTIFICATE OF ORIGINALITY I hereby declare that this submission is my own work and to the best of my knowledge it contains no material previously published or written by another person, nor material which to a substantial extent has been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgment is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged. Signature: ……………………………………. Date: …………………………….……… ABSTRACT Rainfall is a natural process, which has a high degree of variability in both space and time. Information on the spatial and temporal variability of rainfall plays an important role in the process of surface runoff generation. Hence it is important for a variety of applications in hydrology and water resources management. The spatial variability of rainfall can be substantial even for very small catchments and an important factor in the reliability of rainfall-runoff simulations. Catchments in urban areas usually are small, and the management problems often require the numerical simulation of catchment processes and hence the need to consider the spatial and temporal variability of rainfall. A need exists, therefore, to analyse the sensitivity of rainfall-runoff behaviour of catchment modelling systems (CMS) to imperfect knowledge of rainfall input, in order to judge whether or not they are reliable and robust, especially if they are to be used for operational purposes. Development of a methodology for identification of storm events according to the degree of heterogeneity in space and time and thence development of a detailed spatial and temporal rainfall model within a hydroinformatic environment utilising real-time data has been the focus of this project. The improvement in runoff prediction accuracy and hence the importance of the rainfall input model in runoff prediction is then demonstrated through the application of a CMS for differing variability of real storm events to catchments with differing orders of scale. The study identified both spatial and temporal semi-variograms, which were produced by plotting the semi-variance of gauge records in space and time against distance and time respectively. These semi-variograms were utilised in introducing estimators to measure the degree of heterogeneity of each individual storm events in their space and time scale. Also, the proposed estimators use ground based gauge records of the real storm events and do not rely on delicate meteorological interpretations. As the results of the investigation on the developed semi-variogram approach, real storm events were categorised as being High Spatial-High Temporal (HS-HT); High Spatial-Low Temporal; (HS-LT); Low Spatial-High Temporal (LS-HT); and Low Spatial-Low Temporal variability. i Abstract A comparatively detailed rainfall distribution model in space and time was developed within the Geographical Information Systems (GIS). The enhanced rainfall representation in both space and time scale is made feasible in the study by the aid of the powerful spatial analytic capability of GIS. The basis of this rainfall model is an extension of the rainfall model developed by Luk and Ball (1998) through a temporal discretisation of the storm event. From this model, improved estimates of the spatially distributed with smaller time steps hyetographs suited for especially the urban catchments could be obtained. The importance of the detailed space-time rainfall model in improving the robustness of runoff prediction of CMS was investigated by comparing error parameters for predictions from CMS using alternate rainfall models, for various degrees of spatio- temporal heterogeneity events. Also it is appropriate to investigate whether the degree of this improvement to be dependent on the variability of the storm event which is assessed by the adopted semi-variogram approach. From the investigations made, it was found that the spline surface rainfall model, which considered the spatial and temporal variability of the rainfall