SPATIAL AVERAGING BOWEN RATIO SYSTEM : DESCRIPTION AND LYSIMETER COMPARISON

Bowen ratio energy balance (BREB) measurements near a spatially divergent surface may not correctly partition energy fluxes when simply sampling over time. Thus, a Bowen ratio system was designed and constructed to perform spatial sampling and then evaluated by comparison to a precision weighing lysimeter at Bushland, Texas. Results from a comparison of BREB measurements with lysimeter ET measurements on a day following irrigation showed that daily BREB latent heat flux (lE) underestimated daily lysimeter lE by 8%. However, during the daylight period (sunrise to sunset), BREB lE underestimated lysimeter lE by 1.4%; lysimeter lE was overestimated by 0.2% during the period 0630 to 1845 h. Linear regression of BREB lE on lysimeter lE for the duration of the comparison (5 June to 7 June) produced a slope equal to 1.01, coefficient of determination equal to 0.95 and a standard error of estimate equal to 36.5 W m–2. Operation of the system at Fort Collins, Colorado, during an entire growing season showed that the system was functional over the long-term period with minimal maintenance.