Hydrologic effects of dryland shrubs: defining the spatial extent of modified soil water uptake rates at an Australian desert site

Miniature cylinder infiltrometers were used to quantify the spatial variation of infiltration rates in the soils surrounding some Australian dryland shrubs of the genus Maireana. Infiltration rate (f, in mm/h) declined steeply with increasing distance from the stem (dsin cm), and showed no abrupt change at the position of the overhead canopy margin, whose mean location was 46 cm from the stem. Infiltration rates close to a stem were 3–5 times faster than the mid-interspace value. With data from 14 transects, totalling 76 infiltrometer tests, the radial trend was described by a significant (p=0•0001) power-function model: F=98•8 d−0.527s. The results indicate gradational change in infiltration rates, and are not consistent with the binary division of shrublands into ‘canopy’ and ‘interspace’ zones. Thus, for shrubs growing sufficiently close together, there may be no interspace soils in the gap between the two canopies. By defining the ‘zone of influence’ of a shrub as that area beneath and adjacent to the canopy and within which the water uptake rate is at least 10% greater than the shrub interspace rate (whose mean was 8•5 mm/h), it is shown that this zone is on average 3.3 times larger than that of the projected canopy area that has traditionally been used as a measure of plant influence. Implications for hydrologic modelling of dry shrublands are noted.