Frequency Analysis of Annual One Day Maximum Rainfall at Amman Zarqa Basin, Jordan

Abstract

Water management and design of irrigation and drainage projects are based on extreme values rather than on average values. Annual daily maximum rainfall corresponding to return periods varying from 2 to 100 years is used by design engineers and hydrologists for economic planning, and design of minor and major hydraulic structures. This research aims at performing frequency analysis of annual daily maximum rainfall in Amman-Zarqa Basin (AZB) which is an important basin in Jordan.  Daily rainfall data at 22 stations distributed in Amman-Zarqa Basin with long time series (more than 40 years) were used for this purpose. For each station, the annual 1- day maximum rainfall data were extracted. Daily maximum values have then been statistically analyzed by RAINBOW software using two probability distribution functions, namely: Linear and log normal distributions. The goodness of fit for the selected distributions is tested using the Chi-square and the Kolmogorov–Simrnov tests at three significant levels (?=5%, 10% and 20%). The results of the goodness of fit indicate that the Log normal distribution provides a good fit to the rainfall data in the basin. Frequency analysis is then conducted to extract the magnitude of 1 day annual maximum rainfall corresponding to 2, 5, 10, 25, 50 and 100 yr return periods for the 22 stations in AZB. Analysis of rainfall regime would enhance the management of water to prevent floods and droughts as well as an effective design of drainage structures especially in relation to their required hydraulic capacity.

Keywords: Amman-Zarqa Basin, Extreme Events, Frequency Analysis, Probability Distribution, RAINBOW Software, Return Period