Recognising the need for hydrologic data in Thailand and the deficiencies of records from which urgently needed design information must be deduced, the variation of specific yield in the Upper Chao Phya River Basin is herein determined with the object of providing, within easy access to the investigator in a design or operational forecast study, a reasonable estimate of the mean flow at any point along the main river reaches. The figures arrived at, although wanting in extreme accuracy, may be used as a first approximation to the flow for the majority of water resources development purposes. The specific yield of the whole basin was found to be 7.81 lit/sec/km². In the upper reaches, the interaction of the various basin features caused deviations from an ideal pattern. A specific yield as low as 5.83 lit/sec/km² was recorded at the lower reaches of the Wang River, and an abnormal decrease of specific yield was noted in Chiengmai. Otherwise, the trend of decreasing specific yield with increasing area was apparent. Where departures were observed, local features appear to have caused the modifications. A maximum value of 21.95 lit/sec/km² was recorded in the upper reaches of the Nan River; the minimum is in the Wang. Seasonal variations appear to follow the monsoonal distribution of rainfall, with time-lag effects offsetting the distribution curve slightly in the lower reaches. The seasonal distribution curves were found to be peaked in the north-eastern catchment areas, flatter in the western regions. The peaks of the curves generally occur during the month of September.

A thesis submitted in partial fulfillment if the requirements for the degree of Master of Engineering in the SEATO Graduate School of Engineering, Bangkok, Thailand.

Thesis (M.Eng.) - SEATO Graduate School of Engineering