MECHANISTIC PROCESS-LEVEL CROP SIMULATION MODELS FOR ASSESSMENT OF AGRICULTURAL SYSTEMS
Location: Crop Systems & Global Change
Title: Simulation of potato gas exchange rates using SPUDSIM
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 7, 2010
Publication Date: March 1, 2010
Citation: Fleisher, D.H., Timlin, D.J., Yang, Y., Reddy, V. 2010. Simulation of potato gas exchange rates using SPUDSIM. Agricultural and Forest Meteorology. 150:432-442.
Interpretive Summary: Mathematical crop models have been developed over the past forty years for the primary purpose of helping farmers manage their field operations. Farmers, crop consultants, and scientists have used models in many ways to help answer questions such as: 1) when is/are the best times to fertilize/irrigate a crop so as to maximize yield and reduce resources, 2) when is/are the best times to plant and/or harvest my crop, and 3) will this variety of crop grow at a profit in my field? In spite of the efforts made in improving these crop models, there still exists the need to make their predictions more accurate. One way to accomplish this is to add more science to them. A new potato model called SPUDSIM was developed that incorporates new procedures and mathematics based on recent scientific findings. In order to test how accurate the new model was, SPUDSIM predictions were compared with the results from several experimental datasets. The data covered a wide range of temperatures similar to conditions in which potatoes are grown in the United States. The comparisons between the simulated and experimental values were favorable, and indicated SPUDSIM will be an improvement over existing potato models. The results of this study suggest SPUDSIM will be able to help farmers with their operations as well as scientists and food policy planners interested in studying possible climate change effects and conservation practices.
SPUDSIM was developed from the model SIMPOTATO to incorporate mechanistic approaches for simulating photosynthesis and canopy growth and development needed to improve modeling accuracy for studies involving nutrient/water stress and climate change. Modifications included routines for simulating individual leaf appearance rates and leaf expansion as a function of leaf physiological age and plant assimilate status. Coupled sub-models for leaf level photosynthesis, transpiration, and stomatal conductance were used to replace the older radiation use efficiency approach. A radiation transfer routine that estimated diffuse and direct-beam photosynthetically active radiation for sunlit and shaded leaves was also added. During each time increment, net photosynthetic rate was estimated for sunlit and shaded leaf area. Photosynthate was partitioned among leaves in the canopy according to leaf age, potential expansion, and plant assimilate status. Assimilate allocation to branches, roots, and tubers proceeded according to partitioning coefficients defined in the original model, SIMPOTATO. Remaining photosynthate was stored in the canopy and, when accumulated over a threshold amount, reduced leaf-level photosynthetic rate via feedback inhibition. Whole plant gas exchange and harvest data from SPAR (soil-plant-atmosphere research) chamber experiments conducted at USDA-ARS Beltsville, MD were used to evaluate SPUDSIM predictions over a broad range of temperatures from 12.6 to 32.3 degrees Celsius (24hour average basis). An additional independent SPAR chamber dataset was used to parameterize SPUDSIM crop coefficients. Root mean square error (RMSE) was less than 0.29 moles of carbon dioxide per square meter per season for seasonal daily net assimilation rates and indices of agreement (IA) were 0.80 and higher except at the 32.3 degrees Celsius study (0.62). Comparison of canopy photosynthetic rates at four different days indicated the model slightly under-predicted leaf area early in the season and over-predicted later in the season. IA and RMSE for leaf-level photosynthetic rates were above 0.88 and less than 1.6 micromoles of carbon dioxide per square meter per second respectively for all studies except the 32.3 degrees Celsius (0.61 and 3.8 micromoles of carbon dioxide per square meter per second). Dry matter predictions fell within two standard deviations of measured values for most plant organs at harvest. Overall, these results indicated that SPUDSIM accurately captured potato growth and development responses over a wide range of temperatures and will be suitable for a variety of applications involving complex soil-plant-atmosphere relationships.