Modelling hourly rates of evaporation from small lakes

Abstract

Abstract. The paper presents the results of a field study of open water evaporation carried out on three small lakes in Western and Northern Canada. In this case small lakes are defined as those for which the temperature above the water surface is governed by the upwind land surface conditions; that is, a continuous boundary layer exists over the lake, and large-scale atmospheric effects such as entrainment do not come into play. Lake evaporation was measured directly using eddy covariance equipment; profiles of wind speed, air temperature and humidity were also obtained over the water surfaces. Observations were made as well over the upwind land surface.The major factors controlling open water evaporation were examined. The study showed that for time periods shorter than daily, the open water evaporation bears no relationship to the net radiation; the wind speed is the most significant factor governing the evaporation rates, followed by the landwater temperature contrast and the land-water vapour pressure contrast. The effect of the stability on the wind field was demonstrated; relationships were developed relating the land-water wind speed contrast to the land-water temperature contrast. The open water period can be separated into two distinct evaporative regimes: the warming period in the Spring, when the land is warmer than the water, the turbulent fluxes over water are suppressed; and the cooling period, when the water is warmer than the land, the turbulent fluxes over water are enhanced.Relationships were developed between the hourly rates of lake evaporation and the following significant variables and parameters (wind speed, land-lake temperature and humidity contrasts, and the downwind distance from shore). The result is a relatively simple versatile model for estimating the Correspondence to: R. J. Granger (raoul.granger@ec.gc.ca) hourly lake evaporation rates. The model was tested using two independent data sets. Results show that the modelled evaporation follows the observed values very well; the model follows the diurnal trends and responds to changes in environmental conditions.

Evaporation from Lake Mead, Nevada and Arizona, March 2010 through February 2012

“…Dry air advection from the adjacent desert surface increased e o -e a , and the rate of this increase was dampened during periods when the ASL was most stable. These results are similar to those found in other recent lake studies by investigators using the EC method (Blanken and others, 2003;Liu and others, 2009;Granger and Hedstrom, 2011).…”

“…As the ASL became more stable (z/L >> 0), turbulent exchanges were increasingly suppressed. In other words, when the atmospheric surface layer was stably stratified and a downward temperature gradient was in place, vertical wind velocities were inhibited and turbulent mixing and exchanges were suppressed (Mahrt, 1999;Granger and Hedstrom, 2011). Atmospheric stability and turbulent exchange suppression increased as the difference between T o and T a increased in the negative direction.…”

“…Even though Lake Mead is a large lake, from an evaporation perspective it is essentially a series of smaller interconnected lakes. Boulder Basin is probably characterized by a continuous boundary layer governed by upwind land-surface conditions (Granger and Hedstrom, 2011). Under these advective conditions, evaporation decreases with distance downwind of shore as the evaporating surface causes the air temperature to decrease and the humidity to increase until some equilibrium is reached (Weisman and Brutsaert, 1973;Finch and Hall, 2005).…”

“…Furthermore, even though the focus of this report is on monthly estimates, one significant advantage of the EC method over other methods is the ability to accurately measure daily and sub-daily evaporation. Processes that control open-water evaporation become more evident at these shorter time steps (Blanken and others, 2003;Liu and others, 2009;Granger and Hedstrom, 2011). A secondary and indirect method, the BREB, also was employed.…”

Modelling sub-daily latent heat fluxes from a small reservoir

“…Energy exchanges between the atmosphere and inland water bodies in the form of latent (LE) and sensible (H) heat fluxes can be ecologically and climatologically important at regional and global scales (Rouse et al, 2005;Long et al, 2007). However, the environmental factors that determine these exchanges, such as wind speed (u), humidity and atmospheric turbulence, can be substantially different over small water bodies than over larger lakes Granger and Hedstrom, 2011). …”