Establishing permanent plant cover with economical efficiency and rural participatory is a way to

control water erosion of piedmonts. For this purpose, making rain water harvesting micro- catchments systems

along with increasing surface runoff coefficient is necessary especially during low daily rainfalls.

This research project established in Garacharyan station of Zanjan with regard to requirement of country

and province of Zanjan. Because Zanjan synoptic station is considered to be the index of semiarid climate

couditions, so the daily rain frequency is prepared for every month, 45 years statistied duration and for values

larger than 0, 1, 5 and 10 mm. The surface treatments of rain water harvesting plots were impediment,

semi impediment and natural with 4 blocks that established on a slope and measured the depth of daily

rainfall and caused runoff. The results show that daily rain frequency with depth equal to 1, 5 and 10mm

or larger, have average frequency of occurence of one time in one, three and 10 years, respectively, in warm

months of July August and September. The variance analysis, mean comparing and regression analysis

between depth of daily rainfall and runoff showed that the treatments have significant differences at 1%

level. So the threshold of daily rainfall for runoff occurrence due to treatments of impediment, semi impediment

and natural are 1, 4 and 7 mm and average runoff coefficients are 43.9, 12.15 and 3.04 percent,

respectively.