Surface runoff and planting density relationship
The surface runoff, runoff production in slope, is related to the soil infiltration. On raining, the soil evaporation is too small to neglect due to the relatively low temperatures, greater air humidity and lower air vapor pressure deficit near the ground. Thus, the rainwater that reaches the soil surface through forest canopies infiltrate into the soil or run off as overland flow. In the period from April, 2002 to October, 2003, there were many greater rain events, but there were only five rain events generated runoff in our plots because of the gentle terrain, small rainfall intensity, higher planting density and higher cover degree of Caragana shrublands and the runoff amount ranged from to . The equation describing the relationship between surface runoff SR and precipitation outside caragana forest P2 was found to be (Guo & Shao 2004,2013), see table 3
SR = 0.01P2+ 0.1765
The runoff in Loess Plateau, China normally is infiltration-excess runoff production and it occurred when the rainfall intensity exceeds the infiltration rate (Guo and Shao 2013). Because vegetation cover can increase surface roughness and vegetation canopy interception reduce the kinetic energy of raindrops impacting the soil surface and the degree of aggregate breakdown and compaction causing surface sealing. The barrier effect of standing forest on runoff can increases the surface runoff resistance, thus vegetation reduces velocity of the surface runoff and increases the rainfall infiltration.
The surface runoff declined with increasing planting density, which is consistent with the reported by Zhang et. al. (2002). The relationship between surface runoff R and planting density PD can be can be written as follows (Guo & Shao 2013), see table 4
R = 78.454 – 16.899Ln (PD)
Where, R is surface runoff and PD is planting density