Journal of Resources Development and Management www.iiste.org ISSN 2422-8397 An International Peer-reviewed Journal Vol.32, 2017 101 Development of One Day Probable Maximum Precipitation (PMP) and Isohyetal Map for Tigray Region, Ethiopia Yohannes Gerezihier Gebremedhin 1 Shoeb Quraishi 2 Habtamu Itefa 3 1.Department of Natural Resources Management, College of Agriculture and Environmental Sciences, Adigrat University, Adigrat, 50, Ethiopia, (Lecturer)-author 2.Department of Soil and Water Engineering, Institute of Eriengineering, Haramaya University, Haramaya, 138, Ethiopia, (Professor Dr. Lecturer)-co-author 3.Department of Natural Resources Management, Institution of Science and Technology, Addis Ababa University, Addis Ababa, 16417, Ethiopia, (Assistant Professor, Dr. Lecturer) - co-author Abstract Water is a prime requirement for the existence of life; however uncontrollable amounts of water can adversely affects the survival of living beings. Due to wide range of precipitation variability, drought and extreme floods, the study of one day Probable Maximum Precipitation (PMP) for Tigray region is necessary. In an attempt to develop PMP and Isohyetal map for one day duration, using annual daily extreme rainfall series of 22 stations were subjected to statistical analysis using Hershfield formula adapted version of chow. Stations having inadequate daily records were identified and estimated using Normal Ratio Method, and Double mass curve was employed to check for the consistency of the data. An appropriate frequency factor (K m ) was displayed as a function of the mean of the annual maxima for rainfall observations and the PMP for one day duration, and the highest value of K m was found to be 5.91. It was found that PMP vary from 70.06 to 144.51 mm, and the ratio of estimated one-day PMP and highest observed rainfall varied from 1.04 to 1.42. To predict extreme daily rainfall for each station normal, log normal, log Pearson type-III and Gumbel probability distribution functions were used, and values were subjected to goodness of fit tests of chi-square, correlation coefficient and coefficient of determination tests to assess how best the fits had been. Results revealed that the log Pearson type-III distribution performed the best, with average return period 2.7*10 3 years. The ratio of one-day PMP to rainfall depth for frequencies return period of 5, 10, 50, 100, 1000 and 10000 year floods had been estimated and found to vary from 33.29 to 175.92 mm. The predicted PMP value to depths of various years return period ratios were computed and found to vary between 0.5132 and 2.712. Isohyetal map over Tigray region was generated by means of Arc Map10, IDW interpolation approach and the PMP Isohyetal lines were vary from 80 to 135mm. The high PMP Isohyetal values were observed in the Southern, Central and Eastern Zone and decreases in South-East Zone. For more reliable finding it is better to deal with uniformly distributed stations and larger update data as the climate pattern of the region is dynamic. Keywords: PMP 1 , Probability Distribution Function, Goodness of Fit Test, Return Period, Isohyetal Map, Tigray Region 1. Introduction Assessment of extreme precipitation is an important problem in hydrologic risk analysis and design. Water is a prime requirement for the existence of life; however uncontrollable amounts of water can adversely affect the survival of living beings (Wanniarachchi [1] and Wijesekera [2], 2012). Extreme natural environmental events, such as floods, rainstorms, high winds and droughts, have severe consequences for human society and cause significant damage to agriculture, ecology and infrastructure, disruption to human activities, injuries and loss of lives. Planning for weather-related emergencies such as design of engineering structures, reservoir management, pollution control, and insurance risk calculations, all rely on knowledge of the frequency of these extreme events. (Einfalt et al., 1998). Ethiopia has very diversified pattern in different climatic conditions, ranging from semi-arid desert type in low lands to humid and warm temperate type in the southwest, which means higher in the south west and gradually decreases to north east of the country. Rainfall is mainly controlled by the seasonal migration of ITCZ in associated with atmospheric circulation, as well as complex topography of the country (NMSA, 2004). Flooding is common in Ethiopia during rainy season between June and September and the major types of floods are flash floods and river floods (FDPPA, 2006). Flash floods are mainly linked with isolated, localized intense and short duration event caused by high peak discharge (Few, 2006). This type of flood could pose a big damage to human life and property because they occur suddenly with very high intensity (Greenough et al, 2001). There were flash floods in many parts of Ethiopia at different times. For example, due to the phenomenon of extreme rainfall events in Dere-Dawa in 2006 when 9,956 people were displaced and 256 were killed (FDPPA, 2007). In South Omo, Gode, Afar, Gambella, Oromia, Tigray and Amahra regions, nearly 118,000 1 Probable Maximum Precipitation-PMP