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Abstract: The occurrence and consequences of soil water repellency (SWR) have been reported in many parts of the world, but little is known on the existence of SWR in Iran. The purpose of the present study was to investigate the persistence of SWR in 10 forestry sites under different plant species (Pinus taeda, Alnus subcordata, Pterocarya fraxinifolia, Sambucus ebulus, Boxus sempervirens, Parotia persica, Quercus castanefolia, Fagus orientalis, Acer sp. and sparse grass) in North of Iran at Guilan province. The soils examined in this study include a broad range of textures, organic matter contents and pH levels. The persistence of SWR at the surface of the soil in the undisturbed samples (0–5cm, n =256) was collected at the areas, was measured using the Water Drop Penetration Time Test (WDPT). In the field, the actual water repellency was determined using the WDPT test but the same test was used to determine the potential water repellency after drying process for several days at 25°C in the laboratory. A wide range of SWR classes (hydrophilic to extreme water-repellent soils) has been found in sandy soils under pine, Alnus vegetation and broadleaves forests with silty loam, silty clay loam and clay loam texture, while inexistent or slight water repellency has been observed in soils under pine forest with clay contents >35%. Sampling was performed within 2 seasons (Autumn and Summer) and actual water repellency was only observed in forests in the Summer. The results showed a positive correlation between LogWDPT and organic matter content with r=0.41 (p=0·01, n =256). Comparing all soil samples together, low coefficients, r=−0.35 and 0.27 (p=0.01) between LogWDPT with clay and sand percentage were determined respectively. Soil pH had no significant effect on soil water repellency when all soil samples were compared together but when sites were compared separately, a negative and significant relation was detected between pH and LogWDPT. In this research, maximum water repellency was found in sandy soil samples with 5.26–11.5% organic matter. Field observations imply that SWR is not a static soil property and could be observed in field when soil becomes so dry. The results show that soil organic matter content, texture and pH are important factors for developing SWR in soil. [Copyright &y& Elsevier]

• Some forest areas of northern Iran are characterized by extreme water repellency. • An adapted version of SWAP4 model used to compute soil water fluxes and runoff. • Water repellency decreases water infiltration and increases runoff generation. • Water repellency reduces evaporation from the soil surface. Water repellent or non-wetting soils show poor water infiltration, which leads to an increased runoff in both natural and agricultural setting. Some forest areas of northern Iran are characterized by extreme water repellency, which makes their soil hydrological response highly complex. A field experiment was carried out to study the effect of water repellency on water movement and runoff in coarse-textured soil covered in Pinus teada in northwestern Iran. An adapted version of the popular agrohydrological model SWAP (Soil, Water, Atmosphere and Plant) has been used to evaluate the modelling of soil water flow and runoff generation. Water repellency was accounted for by multiplying the soil water content and the unsaturated hydraulic conductivity of soil with the F factor. This factor is equal to the volumetric fraction of soil occupied by preferential flow paths. Twenty-seven rainfall experiments were carried out under artificial rainfall using soils with various wettability levels and various rainfall intensities (14 mm/h, 27 mm/h and 34 mm/h). The result show that during the summer and early autumn (May to October) the soil became extreme repellent and runoff was formed when the soils were dry (θ < 2 %) and the rain intensity was more than 27 mm/h. Also, we investigated the role of the repellent layer on evaporation losses. The results of laboratory and simulation data confirm that water repellency reduces evaporation from the soil surface. Comparing modelling results with field data shows that an assumption of uniform flow in water repellent soil leads to an overestimation of soil water content and an underestimation of runoff. After incorporating water repellency in SWAP with the mobile–immobile concept, the agreement between measured and model simulation data considerably improved. [ABSTRACT FROM AUTHOR]