Atmospheric Stability Effects on Penman-Monteith Evapotranspiration Estimates

Abstract

This study explores surface-layer stability effects on the Penman-Monteith (P-M) method, one of the most widely used methods to estimate evapotranspiration (ET). Stability correction is applied to the original (neutral stability) formula by using atmospheric exchange coefficients developed by LOUIS et al. (1981). First, the effects of stability on the P-M formula are explored theoretically. ET is then computed from field data using both P-M formulas and the values are compared to measured ET from Bowen-ratio and lysimeter data. Theoretical investigation of the P-M formula is performed by varying stability conditions for a range of vapor pressure deficit and canopy conductance. Infinitely large canopy conductance (zero canopy resistance) gives potential ET with results similar to MAHRT and EK (1984): the stability-dependent formula gives larger (smaller) potential ET than the original formula under unstable (stable) conditions. For finite canopy conductance, ET behaves differently from potential ET as a result of coupling between canopy and atmospheric conductances. Stability-dependent ET values become smaller than the original formula values under extreme unstable conditions (when atmospheric conductance becomes large compared to canopy conductance) because stability-dependent sensible heat flux is considerably larger than its neutral counterpart under same net input energy. Field data collected during both wet and dry growing seasons indicate that both P-M formulas track the Bowen-ET estimates and lysimeter measurements quite closely.

Simulation of semi-arid biomass plantations and irrigation using the WRF-NOAH model – a comparison with observations from Israel

“…Mahrt and Ek (1984) discuss the significance of stability correction in a study based on the Wangara experiment. Otles and Gutowski (2005) also discuss this issue in relation to a modelling study carried out within a semi-arid climate. They tested methods with and without stability correction, and obtained fairly similar results which corresponded closely with lysimeter and flux observations.…”

Comparison of Latent Heat Flux Using Aerodynamic Methods and Using the Penman–Monteith Method with Satellite-Based Surface Energy Balance

“…Mukammal et al [19] compared aerodynamic and energy balance techniques to estimate evapotranspiration from a cornfield where they discussed difficulties in using the aerodynamic method in tall crops. Results from Otles and Gutowski [18] showed that under extremely unstable boundary layer conditions, actual evapotranspiration values become smaller than estimated by the original PM formula, because under the same net input energy, the sensible heat flux is considerably larger than under neutral conditions. Thom and Oliver [7] extensively discussed the variation in estimated λE with the stability of the ratio of actual to neutral aerodynamic resistance.…”

“…The root-mean-square deviation (RMSD) between λE PM_∆ra and λE PM for the Landsat overpass time on 18 (Figure 7a,d,g,j), indicating strong correlation among estimates, but with strong biases (error) for some of the relaxations. Table 5 compiles summaries for λE PM_∆ra , λE PM_∆ and λE PM_∆RL for different land use class pixels where the percentage change is compared to λE PM based on the simulated T s from BAANS.…”

“…Table 5, Column 6 summarizes the impact of ∆ approx and the assumption of a neutral condition (λE PM_∆ra ) on the PM method. The maximum decrease in λE between λE PM_∆ra and λE PM was about 25 percent for desert pixels, and the maximum decrease was about six percent in the agricultural AOI on 18 Use of ∆ approx with stability correction in the PM equation is widely used for λE calculation. Figure 7b,e shows scatter plots of λE estimated using the PM with ∆ approx and stability correction (λE PM_∆ ) versus λE estimated by the fully and appropriately parameterized PM (λE PM ) for an agricultural AOI using Landsat-based and simulated T s , respectively, on 18 June 2008.…”

“…Otles and Gutowski [18] studied atmospheric stability effects on the PM method and compared the evapotranspiration from PM to the Bowen-ratio and lysimeter data. Mukammal et al [19] compared aerodynamic and energy balance techniques to estimate evapotranspiration from a cornfield where they discussed difficulties in using the aerodynamic method in tall crops.…”