Measuring and estimating evapotranspiration in a raingarden

Abstract

Raingardens are one of several versatile low impact design approaches, and they can provide a variety of worthwhile benefits related to the processes of evapotranspiration. There is currently no specific content for estimating ET in raingarden design guides in New Zealand. This thesis aims to find out the most reliable quantification method for estimating evapotranspiration in a raingarden.This research is based on hydrological and meteorological field experiments. The Hooton Reserve Raingarden (located in Albany, Auckland, New Zealand) was selected as the research site. Three methods were used: Bowen ratio energy balance method (BREB), water balance method, and agriculture-based equations. Data collection lasted for one year, from August 2016 to July 2017. Finding an appropriate research site was not easy, mainly because of the criterion of suitability for energy balance estimation. Data quality control is critical when using the BREB method, and data was excluded from further analysis if the Bowen ratio between -1.3 and -0.7 or the wind direction was from 315 to 360 or 0 to 135 degrees. Both the energy fluxes and the average Bowen ratio value on a high radiation day were about twice those on a low radiation day. Radiation and relative humidity are the two most significant meteorological variables affecting actual ET. Use of the water balance method was problematic because of uncontrollable environmental elements and equipment failure. Six agriculture-base equations were investigated for estimating ET in raingardens in the Auckland region. Over the one-year research period, radiation-based equations performed better than the temperature-based equations, and the performance was better in the dry season than in the wet season. The Makkink and the FAO56 Penman-Monteith equations were found to be the best of the six equations. The Makkink equation is simple so it can be used for raingarden preliminary design. For more detailed design, FAO56 Penman-Monteith equation can be used, with a crop coefficient for the whole year of 0.92.