Evapotranspiration in a subtropical wetland savanna using low-cost lysimeter, eddy covariance and modelling approaches

Evapotranspiration (ET) constitutes the largest loss of water from subtropical grassland and wetland ecosystems, yet data in much of the world have high uncertainty at the landscape scale as there is little information on plant water use. Additionally, anthropogenic alterations to grasslands are a major threat globally and alter ecosystem water use, but the impact of these changes is often unquantified. A major reason for this is the complexity and expense of field-based ET quantification methods such as agricultural lysimeters and eddy covariance systems. Accurate measurements of ET are critical for sustainable water management. This study developed two different low-cost lysimeters-weighing-type and water level-based, to measure ET under partly controlled conditions for single species as well as mixed grassland and wetland communities. Lysimeters were placed in an open-sided, transparent-roofed shade-house and watered periodically. ET values were then compared with (i) actual ET from an eddy covariance tower onsite, (ii) vapour transport-based reference ET models (FAO Penman-Monteith, modified Turc and Abtew simple radiation models), and (iii) reference ET data customized for vegetation types from the Florida Automated Weather Network (FAWN). Both weighing-type and water level lysimeters yielded seasonal patterns and annual magnitudes similar to other ET methods. Annual ET measurements from single-species weighing-type lysimeters (ranging from 881 to 1278 mm for four plant species, n = 5 per species, 20 in total) and water level lysimeters (1085 +/- 306 mm, plant community average, n = 31) were about 10-20% lower than reference ET models (similar to 1300 mm) and similar to FAWN data (1050 mm). Actual ET from eddy covariance was 722 mm for 10 months (missing data for February and March), while weighing lysimeter measurements for the dominant grass Paspalum notatum was 885 mm for the same 10 months. While rainfall exclusion and watering can result in ET differing from actual field conditions, low-cost lysimeters can provide an independent measure of ET for comparison with regional ET models/remote sensing data lacking field validation. Thereby, low-cost lysimeters are potentially useful for water resources and ecosystem management in data-poor regions of the world.

Automated summary

This study developed two low-cost lysimeters to measure ET under partly controlled conditions for single species as well as mixed grassland and wetland communities, providing critical information for water resources and ecosystem management in data-poor regions of the world.