REFINING TOOLS FOR EVAPORATION MONITORING IN SUPPORT OF WATER RESOURCES MANAGEMENT by C. Jarmain1, C.S. Everson2, M.J. Savage3, M.G. Mengistu2, A.D. Clulow2, S. Walker4 and M.B. Gush2 1 CSIR Natural Resources and the Environment, P.O. Box 320, Stellenbosch, 7599, South Africa 2 CSIR Natural Resources and the Environment, c/o Soil-Plant-Atmosphere Continuum Research Unit, Agrometeorology Discipline, School of Environmental Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa 3 Soil-Plant-Atmosphere Continuum Research Unit, Agrometeorology Discipline, School of Environmental Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa 4 Soil, Crop and Climate Sciences, Faculty of natural and Agricultural Sciences, University of the Free State, P.O. Box 339, Bloemfontein, 9300 WRC REPORT NO 1567/1/08 ISBN 978-1-77005-798-2 JANUARY 2009 DISCLAIMER This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. REFINING TOOLS FOR EVAPORATION MONITORING IN SUPPORT OF WATER RESOURCES MANAGEMENT Acknowledgements The research in this project was funded by the Water Research Commission, for whose assistance we are sincerely grateful. We also wish to acknowledge contributions made by members of the steering committee: Dr R Dube Water Research Commission (Chairman) Dr GC Green Private (ex Water Research Commission, initial Chairman) Prof RE Schulze / Prof G Jewitt University of KwaZulu-Natal Prof GS Pegram University of KwaZulu-Natal Dr JE Hoffman University of Stellenbosch Dr PJT Roberts Private / Forestry Industry Dr D Terblanche South African Weather Services Mr M Warren Department of Water Affairs and Forestry Mr E Nel Department of Water Affairs and Forestry Mr O Wilson Department of Water Affairs and Forestry Mr J Bosch Formerly with CSIR Dr Theresa Volschenk Agricultural Research Council Prof N Jovanovic CSIR (Formerly with University of the Western Cape) Dr N Lecler / Dr A Singels South African Sugarcane Research Institute The following people provided valuable technical assistance to the experiments: Alistair Clulow, Lulethu Sinuka, Eric Prinsloo, Joshua Xaba, Lucas Ngidi and Vivek Naiken. The following students provided assistance to the various trials: Michael Mengistu, Nile Eltayeb, Angelo Mockie, Mpumelelo Shange, Moses Nape, Obed Phahlane and Manuel Mbuende. Various land owners are acknowledged for allowing field work to be conducted on their property. We are very grateful for this and the assistance they provided during the various trails. The South African Sugarcane Research Institute (SASRI) and specifically Mr Francois Olivier from the Komatipoort research station and Dr Neil Lecler from the SASRI offices at Mt Edgecombe are thanked for making the lysimetry total evaporation data collected at the Pongola research farm, and other information regarding the lysimetry trial, available. Research staff from the Pongola research farm is thanked for their general assistance while conducting the field work. page i REFINING TOOLS FOR EVAPORATION MONITORING IN SUPPORT OF WATER RESOURCES MANAGEMENT The managers of the Vumbuka Reserve at the AECI industrial complex at Umbogintwini are thanked for permission to conduct the research on their premises. Staff from the Department of Water Affairs and Forestry (DWAF) offices at Midmar Dam is thanked for making available the Symon’s pan evaporation data. The managers of the Midmar Resort, Howick are thanked for allowing project team members free entrance to the site. Mr Mat Jackson is thanked for arranging permission to work on the Midmar Dam premises. Dr Peter Dye and Dr Colin Everson are thanked for allowing integration of field work conducted during this project, with work conducted as part of other field experiments, specifically work conducted at the Ukulinga research farm, the Vumbuka Reserve at the AECI industrial complex and the Magoebaskloof Podocarpus research site. page ii REFINING TOOLS FOR EVAPORATION MONITORING IN SUPPORT OF WATER RESOURCES MANAGEMENT Executive Summary 1. MOTIVATION AND management applications for which BACKGROUND evaporation measurements/estimates are needed. The 1998 National Water Act, which Assessment of accuracy and precision proposes licensing of consumptive uses of requirements relating to evaporation water that result in streamflow reduction, measurement/estimation for various implicitly requires that such uses be water-resource management measured or estimated with an acceptable applications. degree of accuracy. This makes it important Assessment of appropriateness of to know the degree of accuracy and evaporation measurement/estimation precision that can be achieved when techniques for addressing a range of key estimating evaporation using currently water-resource management needs. available methods. Development of guidelines for the complementary use of measurement Most techniques for estimating evaporation and estimation techniques (in order, e.g., have been around since the late 1940s, but to meet calibration or verification only in the last 20 years has technology requirements). enabled them to come into their own as Development/refinement of evaporation reasonably affordable and practically measurement/estimation techniques for applicable methodologies. Their full key water-resource management potential has yet to be realized, especially in applications. South African circumstances. At the same Establishment of a sound basis for time new opportunities for improved capacity building and skills development measurement, such as through the relating to evaporation measurement complementary use of ground-based and and estimation. remote sensing techniques, especially for larger scale water resource related applications, also need to be explored. 3. METHODS This project followed on from a research A number of methods are used in South project by Savage et al. (2004) which Africa to estimate either evaporation, investigated the operational use of transpiration or total evaporation. This scintillometry for spatial estimates of project was aimed at testing the suitability evaporation of a grassland surface (WRC and accuracy of a variety of techniques for Report No 1335/1/04). total evaporation estimation, and also at suggesting improvements and/or changes in the application of these techniques for a 2. PROJECT OBJECTIVE range of land surfaces. Almost all of the evaporation methods used rely on the The project objectives were: simplified energy balance, Rn = LE + H + G, Classification and characterisation of where Rn is the net irradiance, LE the latent land uses/units and water-resource heat flux density associated with page iii REFINING TOOLS FOR EVAPORATION MONITORING IN SUPPORT OF WATER RESOURCES MANAGEMENT transpiration, soil evaporation and Jatropha surface, a tall narrow plantation of evaporation of water, H the sensible heat Podocarpus trees, an open sugarcane field, flux density and G the soil heat flux density. a grass/shrub mix, a Chromolaena stand, a When each term of the energy balance is tree/shrub mix and an open water surface. measured independently, and the equation In Case studies 4 to 7, the suite of different satisfied, energy balance closure is said to techniques were only tested during one be achieved. The closure discrepancy is window period or season. But, the different defined as (LE + H)/(Rn - G). techniques were tested in more than one season at both the Podocarpus and A range of methods were applied and Jatropha sites (Case studies 1 to 3). For evaluated, including the eddy covariance Case study 5, two different sites were used technique, the surface renewal method, the to test the suitablilty of the techniques to heat pulse velocity method and the estimate total evaporation from a short scintillometry method. The one-sensor eddy heterogeneous (species rich) and covariance system consisted of a three- aerodynamically rough surface. dimensional sonic anemometer. The two- sensor (full) eddy covariance system In Case study 1 the energy balance closure included an infra-red gas analyser for of an “orchard-like” surface consisting of absolute humidity measurement, for the young Jatropha trees was investigated. The direct estimation of total evaporation. The In Situ Flux systems and modified Applied surface renewal system consisted of a fine- Technologies Inc. open path eddy wire temperature sensor connected to a covariance systems were used for this datalogger programmed to collect high purpose. In addition, the sensible and latent frequency air temperature measurements. heat flux densities of the two open path eddy The scintillometer consisted of a laser beam covariance systems, were compared with (in the case of the surface layer flux estimates of the RM Young eddy scintillometer) or an infra-red beam (in the covariance (EC) system and the surface case of the boundary layer scintillometer) (SR) renewal system. Measurements were positioned horizontally over a distance, performed from 11 November to 2 above the canopy. The optical December 2005. characteristics of the atmosphere partly scatters the beam, allowing measurements In Case study 2 the suitability of a range of from which evaporation can be estimated. micrometeorological techniques in The heat pulse velocity method consists of a estimating total evaporation