Abstract
Abstract. The Bowen ratio surface energy balance method is a relatively simple method to determine the latent heat flux and the actual land surface evaporation. The Bowen ratio method is based on the measurement of air temperature and vapour pressure gradients. If these measurements are performed at only two heights, correctness of data becomes critical. In this paper we present the concept of a new measurement method to estimate the Bowen ratio based on vertical dry and wet bulb temperature profiles with high spatial resolution. A short field experiment with distributed temperature sensing (DTS) in a fibre optic cable with 13 measurement points in the vertical was undertaken. A dry and a wetted section of a fibre optic cable were suspended on a 6 m high tower installed over a sugar beet trial plot near Pietermaritzburg (South Africa). Using the DTS cable as a psychrometer, a near continuous observation of vapour pressure and air temperature at 0.20 m intervals was established. These data allowed the computation of the Bowen ratio with a high spatial and temporal precision. The daytime latent and sensible heat fluxes were estimated by combining the Bowen ratio values from the DTS-based system with independent measurements of net radiation and soil heat flux. The sensible heat flux, which is the relevant term to evaluate, derived from the DTS-based Bowen ratio (BR-DTS) was compared with that derived from co-located eddy covariance (R2 = 0.91), surface layer scintillometer (R2 = 0.81) and surface renewal (R2 = 0.86) systems. By using multiple measurement points instead of two, more confidence in the derived Bowen ratio values is obtained.
Highlights
Evaporation is – after rainfall – the most important term of the hydrological water balance (Gleick, 1993)
One basic assumption is that the fluxes are one-dimensional only, with no horizontal gradients, and that measurement sensors are located within the equilibrium sub-layer, where fluxes are constant with height (e.g. Dyer and Hicks, 1970)
The lowest daytime averaged temperature was measured on 10 November and was 16 ◦C
Summary
Evaporation is – after rainfall – the most important term of the hydrological water balance (Gleick, 1993). Evaporation is a major term of the land surface energy balance, i.e. the latent heat flux. Knowledge on the consumptive use of water is key for water scarce areas, where consumptive use mainly consists of evaporation (Perry, 2007). An accurate estimation of the actual evaporation can be used for hydrological studies, environmental studies, irrigation studies, climate change studies, water accounting studies and solving international conflicts on the verification of water use. It is often difficult and costly to estimate the actual evaporation above land surfaces accurately. Actual evaporation is often estimated using numerical simulations of hydrological processes Schoups et al, 2008; Uhlenbrook et al, 2004), soil–vegetation–atmosphere-transfer processes Often require ground measurements for calibration (Xing et al, 2008)
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