## Abstract

Canopy resistance (rc), which represents the composite diffusive resistance to water vapor transfer from vegetation surfaces to the atmosphere, plays an important role in describing the water vapor and energy fluxes and CO _{2} exchange mechanisms and is an essential component of the complex ecophysiological and turbulent transport and evapotranspiration models. While one-step (direct) application of combination-based energy balance models (i.e., Penman-Monteith, PM) requires r _{c} to solve for actual evapotranspiration (ET _{a}), a remaining challenge in practical application of PM-type models is the scaling up of leaf-level stomatal resistance (r _{s}) to r _{c} to represent an integrated resistance from the plant community to quantify field-scale evaporative losses. We validated an integrated approach to scale up r _{s} to the canopy. Through an extensive field campaign, we measured diurnal r _{s} for a subsurface drip-irrigated soybean [Glycine max (L.) Merr.] canopy and integrated several microclimatic and in-canopy radiation transfer parameters to scale up r _{s} to r _{c}. Using microclimatic and plant factors such as leaf area index for sunlit and shaded leaves, plant height, solar zenith angle, direct and diffuse radiation, and light extinction coefficient, we scaled up soybean r _{s} as a primary function of measured photosynthetic photon flux density (PPFD). We assumed that PPFD is the primary and independent driver of r _{c}; hence, the scaling approach relied heavily on measured PPFD-r _{s} response curves. We present experimental verifications of scaled up r _{c} by evaluating the performance of the scaled up r _{c} values in estimating ET _{a}. In addition, we solved the PM model on an hourly time step using the scaled up r _{c} values and compared the PM-estimated ET _{a} with the Bowen ratio energy balance system (BREBS)-measured ET _{a}. The relationship between r _{s} and PPFD was asymptotic, and r _{s} showed strong dependence to PPFD, as PPFD alone explained 67% to 88% of the variability in r _{s}. Beyond a certain amount of PPFD (400 to 500 μmol m ^{-2} s ^{-1}), r _{s} became less responsive to PPFD. At smaller PPFD (0 to about 150 μmol m ^{-2} s ^{-1}) and greater r _{s} (>70 to 80 s m ^{-1}) range, r _{s} was very sensitive to PPFD. The r _{c-min}, r _{c-avg}, and r _{c-max} ranged from 42 to 104 s m ^{-1}, 69 to 183 s m ^{-1}, and 95 to 261 s m ^{-1}, respectively, throughout the season. The seasonal average r _{c-min}, r _{c-avg}, and r _{c-max} were 54, 92, and 129 s m ^{-1}, respectively. Canopy resistances were higher in early growing season during partial canopy closure, lower during mid-season, and high again in late season due to leaf aging and senescence. The ET _{a} estimates from the PM model using scaled up r _{c} values correlated very well with the BREBS-measured ET _{a}. The average root mean square difference (RMSD) between the BREBS-measured and PM-estimated ET _{a} was 0.08 mm h ^{-1} (r ^{2} = 0.91; n = 827), and estimates were within 3% of the measured ET _{a} on an hourly basis. On a daily time step, RMSD was 0.64 mm d ^{-1} (r ^{2} = 0.86; n = 83), and the estimates were within 4% of the measured data. The approach successfully synthesized the whole-canopy resistance for use in PM-type combination-energy balance equations by scaling up from rs using a straightforward model of in-canopy radiation transfer.

Original language | English (US) |
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Pages (from-to) | 141-154 |

Number of pages | 14 |

Journal | Transactions of the ASABE |

Volume | 54 |

Issue number | 1 |

State | Published - Jan 2011 |

## All Science Journal Classification (ASJC) codes

- Forestry
- Food Science
- Biomedical Engineering
- Agronomy and Crop Science
- Soil Science