Predicting rainfall-runoff erosivity for single storms in northern Iraq

Estimation of soil loss from single storm events is essential for basin planning and nonpoint source pollution control. Rainfall-runoff erosivity factors for single storm events in northern Iraq were derived from the basic theory of soil detachment and transport. The factors derived are a single parameter which includes runoff depth only and a factor which combines runoff depth and peak rate. These two erosivity factors gave satisfactory results when tested using data from natural runoff plots collected at different sites in the region. The same data were then used to test the applicability of various established erosivity factors. The two derived erosivity factors were superior to any known erosivity factor tested in the region. These include factors currently in use in Universal Soil Loss Equation (USLE) applications in different parts of the world. Prédiction de l'érosivité d'une averse dans le nord de l'Irak Résumé L'estimation des pertes en sol dues à une averse est essentielle à la planification au niveau du bassin versant et à la maîtrise des pollutions diffuses. Les facteurs érosifs relatifs aux averses dans le nord de l'Irak ont été déduits de la théorie classique concernant l'arrachement et le transport de particules de sol. Il s'agit d'une part d'un paramètre relatif à la hauteur d'eau ruisselée et d'autre part d'un paramètre combinant hauteur d'eau ruisselée et débit de pointe. Ces deux facteurs explicatifs, appliqués à des hydrogrammes naturels recueillis en différents sites de la région, fournissent des résultats satisfaisants. Les mêmes données ont également été utilisées pour tester l'applicabilité de différents facteurs d'érosion établis antérieurement. Les deux facteurs que nous avons dégagés se sont révélés supérieurs à tout autre jeu de facteurs testés dans la région, y compris ceux de l'équation universelle de perte en sol (Universal Soil Loss Equation, USLE) appliquée dans différentes parties du monde. I N T R O D U C T I O N Water erosion in the semiarid region of northern Iraq is characterized by a large variability in the erosive storms. Hence, event based rainfall-runoff erosivity factors are essential for accurately predicting erosion and sediment yield in the region. In addition, estimation of soil loss from single storm events is often required in analyses such as nonpoint source pollution and the selection of a proper design storm. Open for discussion untii 1 April 1995 536 M. H. Hussein et al. The subject of rainfall-runoff erosivity has been studied in different regions of the world. Among erosivity factors currently in use are the EI30 index (rainfall energy x maximum 30 min intensity) (Wischmeier & Smith, 1978), the Vr Im index (rainfall amount x maximum intensity) (Lai, 1976) and the E > 25 index (total kinetic energy of all the rain falling at more than 25 mm h") (Hudson, 1981). Other researchers have added a runoff erosivity factor to the rainfall erosivity index to obtain a more accurate representation of the erosion process (Onstad & Foster, 1975; Foster et al., 1977). Hussein (1986) published a rainfall erosivity map for Iraq. This map, however, assumes that the £730 index is a valid erosivity index for the region. No attempt has been made in the past to test the validity of any of the previously mentioned erosivity indices in the region. The purpose of this study was to use the fundamental theory of water erosion to derive suitable rainfall-runoff erosivity factors for northern Iraq. Available soil loss data in the region were then used to test the validity of the derived erosivity factors and other known erosivity factors in estimating soil loss.