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.

Runoff and soil erosion are known to cause a degradation in soil and water quality. Six natural runoff plots (three 10 m long and three 30 m long) were established on 6% uniform slope area for the study of P and N losses associated with runoff and soil erosion in northern Iraq. The soil at the site belongs to the Calciorthid suborder which dominates in the low rainfall zone of northern Iraq. Runoff, erosion, and associated P and N losses, were recorded from these plots for three rainfall seasons. Results illustrated that eroded sediment is always rich in available P and inorganic N compared to the original soil. Concentrations of soluble P and soluble N in runoff illustrated significant variability both between storms and between seasons. Both sediment-bound P and soluble P were significantly correlated with the ratio of runoff to rainfall. Pertes en phosphore et en azote dues au ruissellement et à l'érosion dans le nord de l'IrakRésumé On sait le rôle du ruissellement et de l'érosion des sols dans la dégradation des sols et de la qualité des eaux. Six parcelles d'étude du ruissellement naturel (trois de 10 m de long et trois de 30 m de long) de pente égale à 6% ont été mises en place en vue d'étudier les pertes en P et N dues au ruissellement et à l'érosion, sur un soussol calcaire très courant dans la région peu arrosée du Nord de l'Irak. Le ruissellement, l'érosion et les pertes en P et N ont été mesurés durant trois saisons des pluies. Les résultats ont montré que les sédiments provenant de l'érosion sont beaucoup plus riches en P et en N que le sol original. La concentration du phosphore mobilisable et de l'azote minéral dans les eaux de ruissellement présente une importante variabilité selon les averses et saisons étudiées. Aussi bien le P dissous que le P particulaire sont significativement corrélés au rapport du ruissellement aux précipitations.