Determining Runoff Coefficient for Different Surfaces of Catchment Systems to Harvest Rain Water for a Dryland Region

The Global warming and climate change caused changes in rainfall patterns. Instead of snow، rainfalls are as precipitations which are not useful since usually they are lost as runoff or flood. Thus، proper recovery of precipitation especially snow and rain is very important، particularly in arid and semi-arid areas. By establishing rainwater harvesting systems with high runoff coefficients and storage of the runoff we can produce sustainable conditions to meet the water requirement of the plants and forest trees in the dryseasons. For this purpose، three treatment procedures including intact natural surfaces، areas covered with gravel، and use of plastic cover with gravel were selected and repeated thrice. The areas of each constructed experimental plot were 4m2squared shape، and their average slope was 11. 5-18%. The results obtained from 16 occasions of rainfall showed that the recovered runoff obtained from natural surfaces with gravel، intact natural surfaces and plastic cover with gravel treatments were 7، 12 and 48 % of the rainfall، respectively. The rainfall thresholds to initiate runoff flows in the treatments were obtainedto be 4. 4، 3. 5 and 2. 7 mm، respectively. The analysis of variance showed that the runoff from treatments were statistically significant at P<0. 01. The mean comparison revealed that the highest runoff was produced by plastic surfaces with gravel treatment and this system is suggested as an appropriate option to harvest rainwater as a result of this study.