Estimating evapotranspiration from a reed bed using the Bowen ratio energy balance method

An increase in demand for water for agricultural and domestic use, combined with new legislation regarding the water needs of natural habitats, has led to an increased requirement for accurate calculations of hydrological fluxes through wetlands. Evapotranspiration is one of the most important but least well understood fluxes in wetland hydrology. Research has been carried out on Stodmarsh National Nature Reserve in Kent, UK, a Ramsar site containing the largest reed beds in southern England. The objective was to quantify water loss through evapotranspiration on the site to allow more effective management of water levels and maintenance of maximum conservation potential for bird life. The Bowen ratio energy balance approach was employed, which is one of the most accurate ways of measuring evapotranspiration but has been rarely used on reed beds. These measurements were used with Penman-Monteith reference evapotranspiration in order to create crop coefficients. It was found that crop coefficients were inconsistent from day to day but were generally less than unity. The inconsistency was thought to be caused by variations in meteorological conditions: e.g. significant differences in crop coefficients were found between days with high radiation and a dry canopy, compared with days with low radiation and a wet canopy. Canopy interception of precipitation was particularly important, with crop coefficients being significantly higher on wet days, possibly due to the higher rates of evaporation of intercepted water due to the lack of stomatal resistance. Copyright (C) 2004 John Wiley Sons, Ltd.