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Water Harvesting for Afforestation: I. Efficiency and Life Span of Asphalt Cover¹

ABSTRACT

The water required to irrigate greenbelts being established around cities in Iran is a major cause of concern in a country with insufficient supplies. Methods of afforestation which do not require irrigation must be found. One such method is to concentrate water, received by a watershed without vegetation, for distribution over a smaller area where plants are growing by making portions of the watershed area impervious to water through application of asphalt. This principle was tested on runoff plots 2 m wide and 10 m long. Runoff from each plot was measured to study its variations as a function of age of asphalt cover and rainfall amount and intensity. The concept was further tested by constructing 2-m wide terraces along contour lines at 5-m intervals on a hillside with a slope of about 30%. Tree species, ecologically adapted to the area, were planted on the terraces. Calibrated gypsum blocks were placed at depths of 15, 30, 45, and 60 cm in the terraces and at depths of 10 and 30 cm in the watershed area between the terraces to measure soil water potentials.

As the asphalt membrane aged, its efficiency in inducing runoff decreased. The runoff efficiency was nearly 75% initially, but decreased to 25% after 4 years. A regression equation was obtained correlating the runoff efficiency with age of the asphalt membrane. Freezing and thawing of the soil, growth of plants, and shrinking and swelling of clays resulted in the deterioration of the membrane. It remained effective for about 5 years.

The runoff from the asphalt-sprayed microwatersheds resulted in high soil water contents of the terraces, with average potentials of –0.4, –7.7, and –2.5 bars during 1972, 1973, and 1974 where runoff was received, corresponding to –7.7, –23.8, and –21.8 bars on control plots.

The technique is being developed as an alternative to the irrigation practices now in use for the establishment of greenbelts. It will result in savings of water, which in some localities could amount to 5,000 m³/ha/year, an amount sufficient to satisfy the needs of 50 people per year.

¹ Contribution from the Research Institute of Forests and Rangelands, 569 Pahlavi Avenue, Tehran 15, Iran and the Department of Soil Science, Oregon State Univ., Corvallis, OR 97331.

² President and Research Scientists, Research Institute of Forests and Rangelands, Iran, and Professor, Department of Soil Science, Oregon State Univ.

Received for publication October 3, 1977. Accepted for publication April 4, 1978.