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Oregon State University (1976)

Soil surface application of asphalt in afforestation in semi-arid zones of Iran


Titre : Soil surface application of asphalt in afforestation in semi-arid zones of Iran


Université de soutenance : Oregon State University

Grade : Doctor of Philosophy (PhD) 1976

Résumé partiel
The water required to irrigate greenbelts being established around Iranian cities is a major cause of concern in a country with insuffi cient supplies. Alternative methods of afforestation which do not require irrigation must be found. One such method is to concentrate water received by a watershed without vegetation on a smaller area where trees are planted. This can be accomplished by making portions of the surface of the watershed area impervious to water through application of asphalt. This principle was tested by constructing two m wide terraces on contour lines at five m intervals resulting in a watershed to spreading area ratio of 1.5. A special formulation of asphalt was sprayed onto the surface of the microwatersheds at the rate of one liter/m in December 1969. Seedlings of Robinia pseudacacia L., Cupressus arizonica G., and Fraxinus rotundifolia tree species commonly used in irrigated afforestation projects, were planted in March 1970 on the terraces. Asphalt treatment did not result in a significant increase in the survival of the tree seedlings because 23.4 mm of rain fell in July 1970. This is a rare event for the Tehran environment. The increases in the growth of height, crown cover, and stem cross section due to the asphalt treatment during five growing seasons for Robinia pseudacacia L. were 61.5, 61.4, and 53.0 percent, respectively ; for Cupressus arizonica G. these were 14.6, 15.4, and 31.6 percent, respectively ; for Fraxinus rotundifolia Mill. these were 29.4, 79.2, and 23.9 percent, respectively. Runoff plots two m wide and ten m long were constructed at the site and sprayed with the asphalt. Runoff from each plot was collected in a container to study its variations as a function of time and rainfall amount and intensity. Calibrated gypsum blocks were placed at depths of 15, 30, 45, and 60 cm in terraces and at depths of 10, 20, and 30 cm between terraces to measure soil water potentials. A regression equation was developed correlating the runoff coefficient with the age of the asphalt-soil membrane, amount of rainfall, and intensity of rainfall. The coefficient was negatively correlated with the age of the membrane (r = -0.658**). As the membrane aged, its efficiency in inducing runoff decreased. Sub freezing temperatures, growth of vegetation, and expanding clays accelerated deterioration of the membrane. Extrapolation of the regression line indicates that the membrane is effective only for five years. The runoff from the asphalt sprayed microwatersheds resulted in high soil water potentials in the spreading basins


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