Evaporation from three water bodies of diļ¬erent sizes and climates: Measurements and scaling analysis
Evaporation from small reservoirs, wetlands, and lakes continues to be a theoretical and practical problem in surface hydrology and micrometeorology because atmospheric ļ¬ows above such systems can rarely be approximated as stationary and planar-homogeneous with no mean subsidence (hereafter referred to as idealized ļ¬ow state). Here, the turbulence statistics of temperature(T)and water vapor (q)most pertinent to lake evaporation measurementsover three water bodies diļ¬ering in climate, thermal inertia and degree of advective conditions are explored. The three systems included Lac LeĀ“man in Switzerland (high thermal inertia, near homogeneous conditions with no appreciable advection due to long upwind fetch), Eshkol reservoir in Israel (intermediate thermal inertia, frequent strong advective conditions) and Tilopozo wetland in Chile (low thermal inertia, frequent but moderate advection). The data analysis focused on how similarity constants for the ļ¬ux-variance approach, CT/Cq, and relative transport eļ¬ciencies RwT/Rwq, are perturbed from unity with increased advection or the active role of temperature. When advection is small and thermal inertia is large, CT/Cq <1 (or RwT/ Rwq >1)primarily due to the active role of temperature, which is consistent with a large number of studies conducted over bare soil and vegetated surfaces. However, when advection is signiļ¬cantly large, then CT/Cq >1 (orRwT/Rwq < 1). When advection is moderate and thermal inertia is low, then CT/Cq ļæ½1. This latter equality, while consistent with MonināObukhov similarity theory (MOST), is due to the fact that advection tends to increase CT/Cq above unity while the active role of temperature tends to decrease CT/Cq below unity. A simpliļ¬ed scaling analysis derived from the scalar variance budget equation, explained qualitatively how advection could perĀ¬turb MOST scaling (assumed to represent the idealized ļ¬ow state).
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