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Even if average year rainfall is about 2515 mm in Taiwan, the usage of surface water resources is limited due to the cliffy terrain and weak geological structure, steep slope and short stream length, unequal rainfall distribution on space and time. So, reservoir storage becomes a reliable and effective function in most of public water supply. Serious storm can cause soil erosion in catchments and flows into reservoir. Turbidity is then increased and influences the water quality. Turbidity is the most important index for public water supply. High turbidity inflow causes harassment on processing of public water supply even need to cut off the water supply. For example, typhoon Herb (1996), Nari (2000), Toraji (2001) and Aere (2004) all caused severe water non-providing events. In order to avoid high turbidity water inflow, it is important to strengthen the catchment’s conservation, protect the water resources territory, and predict the inflow turbidity concentration before the treatment operation.
The GMDH (Group Method of Data Handling) algorithm of self-organization network is being used as basic configure in this paper to build up the turbidity predicting model with simply input/output observation data. The GMDH structure originated from the animal or plant evolution process in nature. The optimum high level nonlinear input/output network structure can be self organized by the procedure of variables combination in first layer then compete each other within multiple layers to constrain the error or the error converge no longer, then feed back to its original first layer combination unit to pick up those optimization variables. Stream inflow of Chia-Shian Weir in Chisan Chi and the relative hydrologic parameters has been provided as study sources for the turbidity prediction. Part of water resource of Nanhua reservoir comes from Chia-Shian Weir through a diversion tunnel. The inflow turbidity directly influences the water supply quality and probably reduces the capacity of Nanhua reservoir. The inflow turbidity of Chia-Shian Weir intake diverted prior into Nanhua reservoir and rain-storm quantity as well the discharge in Chisan Chi are most concerned for inflow diverted operation of Chia-Shian Weir. The daily hydrology data from May of 2000 to Dec. of 2004 such as rainfall of Chia-Shian station, discharge of Chisan Chi and turbidity of Chia-Shian Weir intake were used as the input of this GMDH turbidity prediction model. Then, use the GMDH algorithm to organize variables and result in the best (i.e., minimum estimation error) final relation to build up an optimum turbidity prediction model. The GMDH turbidity prediction model proposed in this paper has regressive mode also. The system can assess the estimate error if over the threshold and self-adjusted the original model by update the field input data which make the model possible to achieve long period prediction and accurate estimation.
The model construction procedure shows that GMDH algorithm is better than SGMDH (Stepwise regression GMDH) and data length of 70 is the best choice for model construction, among them the result of 2003 is outstanding. Having multiple tests, the best model construction by 2002 data is being selected as the final turbidity estimate model for Chia-Shian Weir. According to the treatment operation procedure of Taiwan Water Corporation, turbidity less than 500 NTU still can be accept to provide water supply normally. The 10% error (50 NTU) is selected to be the index of inflow turbidity prediction (90% confidence interval) and result show that most of estimate values are all within the confidence interval which indicate this turbidity prediction model with GMDH algorithm for turbidity estimation is useful. Also, this model updates the input data from the field observation while the estimate value is over the threshold of ±50 NTU to reset up the model by using the regressive structure for best prediction and tests show that the estimate value are within the confidence interval as well. The critical divert operation is another focal point in this paper. The divert operation opportunity should be in the period of concentration of 1850 mg/l and discharge about 180 cms in Chisan Chi corresponding 500 NTU turbidity in Chia-Shian Weir theoretically but the concentration of 1000 mg/l and discharge about 100 cms in Chisan Chi corresponding 250 NTU turbidity in Chia-Shian Weir be suggested as the critical divert operation for conservative consideration. Because of the 1~10 days prior predicting function, 250~500NTU turbidity can be easily estimated by this remarkable model so as to provide the corresponding critical divert operation period which made this GMDH turbidity prediction model be a practical tool for critical inflow diverting operation.
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