Soil heat flux calculation for sunlit and shaded surfaces under row crops: 1 - Model Development and sensitivity analysis

Authors: Colaizzi, Paul; Evett, Steven - Steve; AGAM, NURIT - Ben Gurion University Of Negev; Schwartz, Robert; Kustas, William - Bill

Submitted to: Agriculture and Forest Meterology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2015
Publication Date: 11/10/2015
Publication URL: https://handle.nal.usda.gov/10113/62036
Citation: Colaizzi, P.D., Evett, S.R., Agam, N., Schwartz, R.C., Kustas, W.P. 2015. Soil heat flux calculation for sunlit and shaded surfaces under row crops: 1 - Model Development and sensitivity analysis. Agriculture and Forest Meterology. 216: 115-128. doi: 10.1016/j.agrformet.2015.10.010

Interpretive Summary: Irrigation is important for crop production. However, water is becoming less available for this purpose. Therefore, there is greater need to manage and conserve irrigation water. One way of doing this is to know when and how much water to apply to crops. This can be difficult because crop water needs depend on many complex factors. Scientists from ARS (Bushland, TX), Beltsville, Maryland and Ben Gurion University of Negev addressed one of the more complicated factors that affect crop water needs. This is how much heat flows into the soil beneath a crop. For a developing crop, some of the soil may be sunlit, and some soil may be shaded. These change with time of day, time of year, and with the weather. This study developed a new and better method to calculate heat flowing into the soil beneath a crop. This will improve knowing when and how much water to apply to crops. This will help maintain or increase crop production while using less water for irrigation.

Technical Abstract: 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. Calorimetric G0 and calculated RN,S were related by assuming their normalized values were equal at 24 h time steps. 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. The method was tested for different interrow positions and row orientations, which was reported in a subsequent paper.