Sidewalk Effects on Soil Moisture and Temperature

Soils under sidewalks, unless severely compacted or poorly aerated, may be better environments for tree roots than surrounding soils, being both moister and warmer. However, few data have been collected to measure the effects of sidewalks on moisture and temperature. During 1982,13 arrays of gypsum blocks and 5 arrays of thermocouples were used to measure moisture and temperature in aclay loam soil undersidewalks and under sod near sidewalks. As part of a study testing procedures for reducing tree-root damage to sidewalks, the arrays were installed on the grounds of the Department of Environmental Horticulture, University of California at Davis. In each array sensors were placed at 3 depths (6,12, and 18 inches) at each of 4 positions (under center of walk, at edge of walk, and under sod at 18 and 36 inches from edge of walk), giving 12 sensors per array. Sidewalks were 36 inches wide and 3 to 4 inches thick. The study area was sprinkler irrigated at rates sufficient to maintain the sod in good condition.

Table 1 summarizes moisture levels and temperatures, in both cases measured in clear weather 2 days after sprinkler irrigation. The moisture measurements shown were made the morning of August 26,1982; the temperature measurements were made from 7:30 to 8:15 am, 11:30 am to 12:15 pm, and 6:30 to 7:10 pm on September 2, 1982. Analysis of variance showed soils significantly moister and warmer under sidewalks than under sod (P < .001). Soils under sod were significantly moister 18 inches from the sidewalk edge than 36 inches from the edge (P < .001). Temperature differences between soils 18 and 36 inches from the sidewalk edge were not significant (at P = .05).

The temperature patterns are as would be expected. But the moisture patterns go against common expectations that soils under impervious pavements will be drier than surrounding soils. However, results are consistent with Harris (1992) who noted that much moisture moves through soils as vapor. Moisture from warm soil apparently moves as vapor, condensing and concentrating under the walks as they cool at night and becoming a source that then influences soils at some distance.

• © 1994, International Society of Arboriculture. All rights reserved.