Radiation Model for Row Crops: II. Model Evaluation
- P. D. Colaizzi *a,
- R. C. Schwartza,
- S. R. Evetta,
- T. A. Howella,
- P. H. Gowdaa and
- J. A. Tolka
- a USDA-ARS, Conservation and Production Research Lab., P.O. Drawer 10, Bushland, TX 79012-0010. The USDA prohibits discrimination in all its programs and activities. The USDA is an equal opportunity provider and employer. The mention of trade names of commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA
Relatively few radiation transfer studies have considered the impact of varying vegetation cover that typifies row crops, and methods to account for partial row crop cover have not been well investigated. Our objective was to evaluate a widely used radiation model that was modified for row crops having sparse to full vegetation cover. The radiation model was combined with geometric view factors based on elliptical hedgerows that account for the spatial distribution of row crop vegetation, and this approach was compared with the more commonly used clumping index approach. Irradiance measurements included transmitted and reflected visible and shortwave, outgoing longwave, and total net radiation. The model used optimized parameters for corn (Zea mays L.), grain sorghum [Sorghum bicolor (L.) Moench], and cotton (Gossypium hirsutum L.). The elliptical hedgerow and clumping index approaches resulted in similar model agreement; however, the former resulted in up to 7.3 W m−2 smaller RMSE and up to 7.5 W m−2 smaller mean bias error compared with the latter. Both approaches resulted in similar model sensitivities to inputs, which varied ±25%. Calculated shortwave irradiance fluxes were most sensitive to leaf area index (LAI; –3.25), canopy width (–1.94), ellipsoid leaf angle parameter (–0.77), and visible leaf absorption (–5.54) when LAI = 2.95 m2 m−2, and visible soil reflectance (0.89) when LAI = 0.21 m2 m−2. Calculated outgoing longwave irradiance and net radiation were most sensitive to the soil directional brightness temperature (0.55 and –0.61, respectively) when LAI = 0.21 m2 m−2.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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