oil heat flux calculation for sunlit and shaded surfaces under row rops : 1 . Model development and sensitivity analysis

Soil heat flux at the surface (G0) is strongly influenced by whether the soil is shaded or sunlit, and therefore can have large spatial variability for incomplete vegetation cover, such as across the interrows of row crops. Most practical soil–plant–atmosphere energy balance models calculate G0 as a function of either total (RN) or soil net radiation (RN,S). Even though RN,S includes sunlit and shaded conditions, this is seldom considered, even at spatial scales of a few m. In order to improve the utility of surface energy balance models designed for row crops at relatively small spatial scales, a method was developed to calculate G0 as a function of shaded, partially sunlit, or fully sunlit RN,S. Calculation of RN,S was derived using a geometric approach, and G0 was derived by the calorimetric method using measurements of soil temperature and volumetric soil water content under upland cotton (Gossypium hirsutum L.) over a wide range of canopy cover conditions. Calorimetric G0 and calculated RN,S were related by assuming their normalized values were equal at 24 h time steps and in phase. The method required only a single empirical parameter, which related the 24 h minimum G0 (G0,MIN) to the 24 h maximum RN,S (RN,S,MAX) as G0,MIN = a × RN,S,MAX, and a = −0.31 was found by simple linear regression (p < 0.01). Model sensitivity (SM) of calculated G0 was calculated for sparse, medium, and full canopy cover; nighttime, shaded, partially sunlit, and fully sunlit surface conditions; and north–south and east–west row orientations, where input values of agronomic, shortwave, and longwave input variables were varied ±25% of their base values. The method was most sensitive (1.0 < SM < 36) to canopy width, canopy height, leaf area index, row spacing, canopy and soil emittances, and canopy and soil temperatures for medium to full canopy cover. Also, there was generally greater sensitivity for shaded and partially sunlit surfaces compared with sunlit and nighttime surfaces, and north–south rows compared with east–west rows. However, the method had little sensitivity (SM usually <0.50) to input variables used to calculate the shortwave components of RN,S. Published by Elsevier B.V. Abbreviations: a, empirical constant used in surface soil heat flux model (no units); B apacity of the soil in layer j (J m−3 K−1); END, end period of the study during full or nea rganic matter at depth zj (m3 m−3); fSIS, fraction of shading of an interrow section (no u midnight to midnight) (W m−2); G0,MIN, minimum G0 over 24 h (midnight to midnight) (W uring intermediate canopy cover; NS, north–south crop row orientation; RN , total net ra midnight to midnight) (W m−2); RN,S,MIN, minimum RN,S over 24 h (midnight to midnight) and time i + 1 (s); TS,0, soil surface temperature (K); Ts,z , soil temperature at depth z (K); eat flux plates (m); G0,Zp , divergent heat flux into the soil layer between zp and soil su t depth zj (m3 m−3); , soil thermal conductivity (J s−1 m−1 K−1); b,zj , soil bulk density at Mention of company or trade names is for description only and does not imply endor ∗ Corresponding author. Tel. : +1 806 356 5763. E-mail address: paul.colaizzi@ars.usda. gov (P.D. Colaizzi). ttp://dx.doi. org/10.1016/j.agrformet. 2015.10.010 168-1923/Published by Elsevier B.V. EG, beginning period of the study during sparse canopy cover; Czj , volumetric heat rly full canopy cover; EW, east–west crop row orientation; fo,zj , volume fraction of nits); G0, soil heat flux at the soil surface (W m−2); G0,MAX, maximum G0 over 24 h m−2); GZ , soil heat flux measured at zp (W m−2); MID, middle period of the study diation (W m−2); RN,S , soil net radiation (W m−2); RN,S,MAX, maximum RN,S over 24 h (W m−2); SM , model sensitivity (no units); ti and ti+1, time at successive time steps zj , depth of the midpoint of soil layer j (m); zp , depth below the soil surface of soil rface (W m−2); zj , thickness of soil layer j (m); v,zj , volumetric soil water content depth zj (Mg m−3). sement by the USDA. The USDA is an equal opportunity provider and employer.