Determining Near-Surface Soil Heat Flux Density Using the Gradient Method: A Thermal Conductivity Model–Based Approach

In the gradient method, soil heat flux density at a known depth G is determined as the product of soil thermal conductivity l and temperature T gradient. While measuring l in situ is difficult, many field studies readily support continuous, long-term monitoring of soil T and water content u in the vadose zone. In this study, the performance of the gradient method is evaluated for estimating near-surface G using modeled l and measured T. Hourly l was estimated using a model that related l to u, soil bulk density rb, and texture at 2-, 6-, and 10-cm depths. Soil heat flux Gm was estimated from modeled l and measured T gradient (from thermocouples). The Gm results were evaluated with heat flux data GHP determined using independent measured l and T gradient from heat-pulse probes. The l model performed well at the three depths with 3.3%–7.4% errors. The Gm estimates were similar to GHP (agreed to within 15.1%), with the poorest agreement at the 2-cm soil depth, which was caused mainly by the relatively greater variability in rb. Accounting for temporal variations in rb (with core method) improved the accuracies of l and Gm at the 2-cm depth. Automated u monitoring ap- proaches (e.g., time domain reflectometry), rather than gravimetric sampling, captured the temporal dynamics of near-surface l and G well. It is concluded that with continuous u and T measurements, the l model–based gradient method can provide reliable near-surface G. Under conditions of soil disturbance or deformation, including temporally variable rb, data improves the accuracy of G data.