Develop an Empirical Model to Forecast Rainfall Intensity as a Function of Probability For Al-Diwaniyah City in Iraq

The study aims to develop an empirical model to predict the rainfall intensity in Al-Diwaniyah City, Iraq, according to a statistical analysis based on probability and the specific rainfall return period. Rainfall data were collected daily for 25 years starting in 2000. Daily rainfall data were converted to rainfall intensity for five duration periods ranging from one to five hours. The extreme values were checked, and data that deviated from the group trend were removed for each period, and then arranged in descending order using the Weibull formula to calculate the probability. Statistically, the model performance with a return period of two years is considered good when compared with observed results and other methods such as Talbot and Sherman with a coefficient of determination (R2) > 0.97 and Nash-Sutcliffe efficiency (NSE) > 0.80. The results showed that a mathematical equation was obtained that describes the relationship between rainfall intensity, probability, and rainfall duration, which can be used for a confined return period with a 50% probability. Therefore, decision-makers can rely on the model to improve the performance of the city's current drainage system during flood periods in the future.