  In this paper, we investigate quantitatively the coupling between gradients
of temperature and of chemical or water potential under steady state conditions
in the vapor phase. This coupling is important for the measurement and modeling
of the dynamics of water in unsaturated environments like soils and plants. We
focus on a simple non-equilibrium scenario in which a gradient of temperature
exists across an air-filled gap that separates two aqueous phases with no net
transfer of water. This scenario is relevant for measurements of the water
potential in environmental and industrial contexts. We use a new tool, a
microtensiometer, to perform these measurements. We observed variations of
water potential with difference of temperature across the air gap of
$-\SI{7.9(3)}{\mega\pascal\per\kelvin}$, in agreement with previous
measurements. Our result is close to a first order theoretical prediction,
highlighting that most of the effect comes from the variation of saturation
pressure with temperature. We then show that thermodiffusion (Soret effect)
coupled to natural convection could occur in our experiment and discuss how
these effects could explain the small discrepancy observed between measurements
and first order theoretical prediction.
