Linking evapotranspiration seasonal cycles to the water balance of
headwater catchments with contrasting land uses
Abstract
Land use affects evapotranspiration rates and is a primary driver of the
catchment water balance. The water balance of two catchments in
southeastern Australia dominated by either grazed pasture or blue gum (
Eucalyptus globulus) plantation was studied, focusing on the
patterns of evapotranspiration (ET) throughout the year. Rainfall,
streamflow, and groundwater levels measured between 2015-2019 were
combined to estimate annual ET using a water balance equation. In the
pasture, eddy covariance was used to measure ET from the catchment. Sap
flow measurements were used to estimate tree transpiration in May 2017
– May 2018 and Feb 2019 – Feb 2021 in two different plots within the
plantation. The tree transpiration rates were added to direct
evaporation, estimated as a percentage of annual rainfall, to calculate
ET from the plantation catchment. ET in the pasture showed strong
seasonal cycles with very low ET rates in summer and ET rates in spring
that were larger than the transpiration rates in the plantation, where
trees transpired consistently throughout the year. The estimated annual
ET from the water balance equation were comparable to ET estimated from
other measurements. In the pasture, ET on average accounted for 88% of
annual rainfall, while ET in the plantation was on average 93% of
rainfall, exceeding it in the years with annual rainfall lower than 500
mm. The difference between the ET rates in the plantation and the
pasture were approximately 30 to 50 mm y -1. The
larger ET rates in the plantation are reflected in a gradual decrease in
the groundwater storage. The differences in ET rates were thus enough to
cause a decrease in groundwater storage in the plantation, while the
groundwater levels in the pasture remained stable.