Measuring sensible heat flux in plastic mulch culture with aerodynamic conductance sensors

Black plastic mulch is commonly used in horticultural systems, but it may c omplicate the sparse crop energy balance by contributing to within-row adve ction, A steady state, heated foil technique was used to approximate the ae rodynamic conductances to heat transport (g(h)) of bare soil and black plas tic mulch for 33 days. The test site was a field of 0.7 m wide raised beds covered with black plastic mulch and separated by 0.8 m wide strips of bare soil, with no crop present. From numerous point estimates of g(h) and meas urements of the temperature difference between the surface and the air, the sensible heat flux (H) was calculated independently for bare soil and plas tic mulch. Conductance values ranged from 8 to 23 mm s(-1) and no differenc e occurred between the mean g(h) for mulch (17.0 mm s(-1)) and that for bar e soil (17.8 mm s(-1)). The H estimated from conductance data was strongly Linearly related, in a 1 : 1 ratio, to the H determined by independently so lving the energy balance of plastic mulch. The conductance sensor method wa s used subsequently to estimate H from bare soil (H-soil). Sensible heat fr om plastic mulch (H-mulch) is the major source of H in the field because it is primarily a function of the net radiation of the plastic. Whereas H-mul ch was always negative, with daily maxima consistently approaching -400 W m (-2), H-soil varied between -200 and +50 W m(-2) according to surface wetne ss. Latent heat fluxes from the bare soil were <100 W m(-2) when the surfac e was dry, and up to -400 W m(-2) when the surface was wet. Managing the su rface wetness of bare soil in a mulched field will not affect the energy ba lance of the mulch surface per se, but may reduce within-row advection, whi ch is potentially detrimental to seedlings and transplants in plastic mulch systems. (C) 1999 Elsevier Science B.V. All rights reserved.