Direct Estimation of Total Evaporation from a Southern African Wetland 1

A.D. Donkin, J.C. Smithers, S.A. Lorentz, R.E. Schulze 2

Abstract

The field measurement of all the components of the water balance of wetlands is important to understanding the hydrological processes occurring in wetlands. The total evaporation and groundwater components of the wetland water budget have been shown in the literature to be difficult components to measure accurately and form the emphasis of this project. The Ntabamhlope wetland, situated in western KwaZulu/Natal, South Africa, has an extensive hydrological network for monitoring surface flow and meteorological data and is thus an ideal site to measure all components of the wetland water budget. A method of estimating total evaporation by making use of the diurnal fluctuations in the water table within the wetland was adopted and results are presented. Detailed water retention curves of the soils at two locations and down through the soil profile were determined. A method developed at Colorado State University for determining these retention curves was used to characterise the water retention properties of the unsaturated layers. These characteristic curves were used to determine the amount of water released from the vadose zone as a result of total evaporation. Retention curves were plotted for initial and final groundwater levels each day. The area between these curves represents the volume of water per unit area released from the vadose zone for the particular day of interest. Results are presented for two vegetation regimes within the wetland for the 1992/1993 season. These total evaporation estimates are compared with the Penman equation and measured A-Pan data. The results are further verified by comparison with the residual of the wetland water budget. To complete the water budget, estimates of groundwater recharge and change in storage were made using hydraulic gradients and water table level fluctuations within the wetland respectively. This residual term from the water budget is shown to be unacceptably large during certain months, but the long term residual over the one year period of study is less than 1% which is well within acceptable limits. The method of estimating total evaporation used in this study is inexpensive, simple to install and monitor and may therefore be easily applied to other wetlands.

1. This paper is part of a project funded by the Water Research Commission entitled "Hydrological Systems Model Development" at the Department of Agricultural Engineering at the University of Natal, Pietermaritzburg, South Africa. Funding was also received from the Foundation for Research Development. The Computing facilities provided by the Computing Centre for Water Research and the assistance of its staff is gratefully acknowledged. Work documented in this paper contributed towards a M.Sc.Eng. project.
2. A.D. Donkin, Graduate Research Assistant, J.C. Smithers, Senior Researcher, S.A. Lorentz, Senior Researcher, R.E. Schulze, Professor of Hydrology, Agricultural Engineering Department, University of Natal, Pietermaritzburg, South Africa.