Measurements of cloud water deposition on vegetation using a lysimeter and a flux gradient technique

The deposition of cloud droplets onto moorland vegetation has been measured using two independent methods. Vertical gradients in wind velocity and liquid water content (LWC) provided cloud deposition fluxes of typically 10 mg m^-2 s^-1 and deposition velocities (vg) in the range 21 to 39 mm s^-1 for droplets with a number mean radius in the range 6 to 7 μm. In these conditions, the aerodynamic resistance provided the major limitation to deposition rates contributing 60% of the overall transfer resistance. Simultaneous measurements of net water exchange between the atmosphere and the ground using a lysimeter showed that the bulk of the water (typically 80%) was deposited as a vapour flux onto frozen soil within the lysimeter. The vapour deposition continued to dominate the water flux measurements until the frozen soil thawed. The measurements show that cloud water deposition at Great Dun Fell (altitude 847 m asl) may increase annual wet deposited SO₄^2-, NO₃^-, H⁺ and NH₄⁺ by 12%, but if such high altitude sites were afforested, the increase would be 44%.