Determination of runoff coefficient for small watersheds within Tano Basin in the forest zone of Ghana

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Many of the small watersheds within Tano Basin of the Forest Zone of Ghana are ungauged. But for highway drainage design, the runoff data is needed. The problem then is how to determine the runoff coefficient of these small ungauged watersheds within the Tano Basin of Ghana. The objective of this study is to estimate the runoff coefficient and water yield per square kilometre for small watersheds within the Tano Basin of Ghana. To achieve this objective, the relationship between the direct runoff, the rainfall and the oatchment areas of the small watersheds within the Tano Basin was determined. The Runoff Coefficient for these watersheds was estimated from generated relationship and the water yield per square kilometre per year was estimated. The runoff values were lagged against the rainfall from one month to twelve months. One month lag-time gave the best R2 value, with Tanoso having an R2 value of 0.854 followed by Jumoro, 0.829; Sefwi-Wiawso, 0.775 and finally Hwidiem 0.732. It was found that the runoff coefficients using the direct runoff data of the various small watersheds of the Tano Basin are 10.4%, 4.2%, 8.5%, and 11.8% for Tanoso, Hwidiem, Sefwi-Wiawso and Jumoro respectively. It was also observed that the yield of water per square kilometre is 140,400m3km~2yr~], 56,700m3knf 2yr~!, 122,315m3km2y"1 and 172,870m km' yr" , for Tanoso, Hwidiem, Sefwi-Wiawso, and Jumoro respectively. The methodology used for this study can also be used in other basins to determine their runoff coefficients. Because the Lag-Times for all the various watersheds were found to fall within one month after rainfall, it is recommended that further study is conducted using the same methodology but with Daily Rainfall and Runoff data to determine the specific Lag-Times for the various watersheds.
A thesis submitted to the College of Engineering in partial fulfilment of the partial requirements of the degree of Master of Master of Science.