Jordan Journal of Civil Engineering, Volume 10, No. 2, 2016 - 209 - © 2016 JUST. All Rights Reserved. Modeling of Rainfall-Runoff Relationship in Semi-Arid Watershed in the Central Region of Jordan Nidal Hadadin Associate Professor, Department of Civil Engineering, The University of Jordan, Amman 11942 E-Mail: n.hadadin@ju.edu.jo ABSTRACT As a result of quick progression in computer and information technologies, computer modeling has become a vital tool in watershed research and management practices. Stanford Watershed Model (SWM) is an integrated physically based Watershed model that can be used to simulate water flow stream/canal network and overland runoff, interflow and evaporation by considering the interaction between surface water and sub- surface water. The objective of this study is to apply the SWM in order to estimate the rainfall-runoff relationship for Wadi Wala streamflow semi arid area with an average annual rainfall of about 300 mm/yr and a catchment area of 1800 km 2 . SWM has been widely accepted as a tool to synthesize a continuous hydrograph of hourly or daily streamflow. Sensitivity analysis, as well as trial and error adjustment techniques were used for the optimization of the number of parameters of the model. Comparing estimated and measured surface runoff for Wala valley indicated that the model is considerably efficient in predicting the total annual surface runoff from rainfall for similar watersheds. KEYWORDS: Watershed modeling, Watershed hydrology, Rainfall-runoff relationship, SWM, Jordan. INTRODUCTION Hydrological models can be classified into two categories: deterministic models and stochastic models. Deterministic hydrological models can be further classified into three main categories on the basis of the spatial representation: empirical models, lumped conceptual models and distributed models. Empirical models, also called black box models, treat watersheds as a single unit where the parameters and the input do not vary spatially within the basin and the basin response is evaluated only at the outlet. The lumped conceptual models, also called grey box models, use physically sound structures and equations together with semi-empirical ones (Refsgaard, 1996). Several models have been developed to estimate runoff at different comprehension levels and based on different numerical approaches using computer technology. Among these models, some emphasize water quantity, while the others focus on water quality. However, the increasing water resource problems and the recognition that the interactions of different components of hydrologic processes sometimes play an important role require more comprehensive management of water resources. An integrated description of the entire land phase of the hydrological cycle and an integrated description of water quantity, quality and ecology improved tools based on resonance scientific principles and efficient technologies are necessary to predict the relations among different hydrological variables. It is necessary to estimate the parameters from Accepted for Publication on 25/12/2014.