Operational hydrology and water quality investigations of the stream-reservoir system in the Upper Pampanga River Project, Philippines

Abstract

The compilation, collection, analysis, and simulation of UPRP streamflow and water-quality information were undertaken in order to assess the expected levels and variability of the quantity and quality of the surface waters in relation to the meteorology and hydrology of the area--the intended objectives of the project--and the modes of system operation. Under the two major phases of the study, important aspects which were examined were the availability and adequacy of the data base, and the applicability of modeling schemes to simulate the observed properties of the data. The major objective in the operational hydrology phase was to test the applicability of operational hydrology in a tropical environment. A stochastic model was fitted to the streamflow data of the extended UPRP inflows in order to serve as a generator of synthetic monthly streamflows which are used as inputs to operational studies of the project. As an initial requirement, the historical record was augmented by means of regional estimation techniques and a statistical data augmentation scheme. The percent deviation method of data augmentation was selected and applied, based on the results of a comparison of four augmentation methods. Two normalizing transformations were applied on the augmented-historical data in order to express them as statistical variables amenable to synthesis. These were the logarithmic transformation and Harter's table interpolation scheme. A time series analysis applied on the normalized data prescribed the adoption of the separate-monthly Matalas model for synthesis. The application of the model yielded synthetic streamflows which satisfactorily reproduced the correlation coefficients, means, and standard deviations of the augmentedhistorical monthly flows. Model extensions and refinements were recommended in order to surmount model limitations. In general, the applicability of operational hydrology in a tropical environment has been demonstrated. The quality of the UPRP surface water was characterized in order to provide background information which is essential in determining the suitability of the water for fishery development in the reservoir, for irrigation, and for domestic and industrial use. The Pantabangan reservoir was characterized in terms of observed patterns of temperature, dissolved oxygen, and salinity. An annual cycle was observed which exhibits intermittently disturbed and moderate stratification in the wet season, full circulation and isothermal condition during the cool, windy, and dry months, and a short warm period favorable to stratification prior to the heavy rains. Atmospheric reaeration was identified to be the major contributor of dissolved oxygen to the reservoir water. The application of the Markofsky-Harleman predictive model for reservoir temperature and dissolved oxygen emphasized the need for substantial data on the hydrology, meteorology, hydrodynamics, and water quality in the reservoir which are required as model inputs. The best agreement between observed and predicted temperature and DO was obtained for cool, windy, and dry months of December to February. Limitations in the input data and in the assumed parameters were observed. The available and collected water-quality data in the reservoir and streams of the UPRP indicated a salinity level as low as or less than 100 ppm in the headwaters to as high as 300 ppm in the outflows from the irrigation service area. The surface water is essentially the alkaline-bicarbonate type. The heavy runoff during the wet season produces a dilution of the TDS, but causes the conveyance of considerable amounts of sediment to the reservoir and river channels. An approximate salt balance for the project area yielded estimates of net amounts of salts leached from the area per season.