Measurement of grassland evaporation using a surface-layer scintillometer

M.J Savage,C Jarmain,G.O Odhiambo,M.G Mengistu,C.S Everso

doi:10.4314/wsa.v36i1.50901

Abstract

A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every 2 min for a path length of either 50 or 101 m, for more than 30 months in a mesic grassland in eastern South Africa. The SLS method relies on Monin-Obukhov similarity theory, the correlation between the laser beam signal amplitude variances and the covariance of the logarithm of the beam signal amplitude measured using 2 laser detectors. Procedures for checking SLS data integrity in real-time are highlighted as are the post-data collection rejection procedures. From the H estimates, using SLS and measurements of soil heat flux and net irradiance, evaporation rates were calculated as a residual of the shortened energy balance equation and compared with grass reference evaporation rates (ETo). Inconsistent hourly ETo values occur in the late afternoon due to the incorrect assumption that the soil heat flux is 10% of net irradiance. The SLS estimates of H and the estimates of evaporation rate as a residual compared favourably with those obtained using the Bowen ratio and eddy covariance methods for cloudless days, cloudy days and days with variable cloud. There was no evidence for the eddy covariance measurements of H being underestimated in comparison to the Bowen ratio and SLS measurements. On many days, the diurnal variation in SLS H was asymmetrical, peaking before noon.Keywords: energy balance, Bowen ratio, eddy covariance, grass reference evaporation, rejection criteria

Highlights

The possible prescription by government of methods for making a volumetric determination of water, for purposes of water allocation and setting charges in the case of activities resulting in stream flow reduction, is stated in the 1998 Republic of South Africa National Water Act
For positive dT/dz values, adjusted for the dry adiabatic lapse rate, where dT/dz is the profile air temperature gradient measured using Bowen ratio (BR) thermocouples, and corresponding to unstable atmospheric conditions, error-free data (EFD) > 25 % and lo > 2 mm for the 101-m path length and non-blank HSLS values, -Hday is displayed in the cell
For periods when the BR equipment was not available, the sign of HSLS was made equal to that of HEC. If both BR and eddy covariance (EC) measurements were unavailable, it was assumed that unstable conditions prevailed between 06:00 and 18:00 and stable conditions otherwise

Summary

Introduction

The possible prescription by government of methods for making a volumetric determination of water, for purposes of water allocation and setting charges in the case of activities resulting in stream flow reduction, is stated in the 1998 Republic of South Africa National Water Act. Point (single-level), profile and path-weighted atmospheric measurements have been used to estimate sensible heat flux H. Sensible heat flux is driven by vertical temperature differences between the canopy or soil surface and overlying air. Latent energy flux LE – from which evaporation rate may be calculated – is driven by vertical water vapour pressure differences between that which is measured just above the canopy or soil surface and that of overlying air. Point measurements of H = HEC and LE = LEEC are obtained by eddy covariance (EC), and path-weighted measurements of H = HSLS by scintillometry. All of these flux measurements have footprint representation.

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