Estimating ET for a Mixed Oak Forest Using Two Methods: Eddy-Covariance and the Soil Water Budget

We examined the evapotranspiration (ET) of a mixed Oak forest stand located within a woodland preserve in NW Ohio using two independent methods. The depth to ground water (DGW) in 2005 to 2007 ranged from 1.35 m at peak recharge (early to mid-May), to below 3.0 m during the driest period near the end of the growing season. The DGW during peak recharge was more shallow in 2007 than the other two years, and while the slopes of the ground water decline during the growing season were similar in 2005 and 2006 (-9 mm and -6 mm per day, respectively), the rate of decline was very steep in the first half of the 2007 growing season (-14 mm per day). Daily ET estimated from the eddy-covariance measurements (EC-ET) averaged 1.6 mm per day during 2005 to 2007. Growing season averaged EC-ET values (May-Oct) ranged from 0 to 5.32 mm per day, being highest in 2007 (2.5 mm per day), and lowest in 2005 (2.2 mm per day). The average growing season temperatures and relative humidity were also highest in 2007 when the highest precipitation and most shallow DGW values were recorded. Finally, weekly EC-ET values were highly correlated with the ET values estimated by the variation in DGW over the same 7 day period (GW-ET). This correlation was highest for 2005 (r=-0.77, p<0.05) and lowest for 2006 (r=-0.55, p<0.05). The correlations increased slightly when the 7 day sums of through-fall precipitation were added to the ground water sums. Soil water storage is included in the final GW-ET estimate, and lateral flows will be estimated from the change in water-table recession.