Simulation of potato tuber performance under climate change conditions using the WOFOST model

After wheat, rice and corn, potato is the fourth product in the food basket of human societies, which shows the importance of its role in providing protein and food needs of people. Potatoes contain large amounts of vitamins A, B1, B2, and C along with potassium, phosphorus, and calcium, and consumption of 300 grams of potatoes tuber per day provides more than 50% of the human need for vitamin C and potassium. The World Food and Agriculture Organization (FAO) has introduced potatoes as a product that provides future food security in the world, and the country of Iran, having a diverse climate and with an annual production of five million tons of potatoes, ranks thirteenth in the production of this product in the world. So, in this research, the consequences of climate change on the yield of potato tuber, the statistics of the production values, and the area under cultivation of potatoes were evaluated.

The dada of Ardabil, Tabriz, Shahrekord, Hamadan, Jiroft, Kohnouj, Manojan, Isfahan, Sanandaj, and Shiraz stations in the statistical period of 1982–2015 were used. The mentioned regions are among the main centers of potato production in the Iran and account for 62.7% of the cultivated area and 68.3% of the country's potato production. In this research, the production and performance data of 10 meteorological stations inside Iran were used and the data of the fifth report of the Representative Concentration Pathway scenarios (RCPs) have been used to evaluate the consequences of climate change on potato tuber yield.In order to prepare field data for calibration and determining the validity of the WOFOST model in potential conditions (conditions without water and nutrients restrictions, disease, and weed control) the data of the Agricultural Research Stations of Hamedan, Isfahan, Ardabil, Shiraz, Tabriz, Shahrekord, Jiroft, Kohnouj, Manojan and Sanandaj were collected during 2010-2014 and used as a base or monitoring period.This information includes planting date, phenological stages from planting to germination, germination to flowering, and germination to physiological maturity of the potato plant in each of the regions.The SDSM statistical model was used for downscalling of CanESM2 model data, and the WOFOST model was used to simulate the performance of the potato tuber. In order to calibrate the WOFOST model using field data, the phenological and performance data in the years of 2010, 2011, and 2013 were used to calibration of the model, and from phenological and performance data in the years of 2012 and 2014 were used to determine the validity of the model. In order to calibrate and determine the validity of the WOFOST model in the study areas, it is first necessary to adjust the potato plant parameters (coefficients) for different climatic conditions. Therefore, the creation of the plant file, which is the most sensitive part of WOFOST model calibration, was done for tuber performance. In this study, WOFOST version 7.1 was used. This model uses parameters and functions to describe the effects of temperature, radiation and water stress on the main processes of crop growth. The outputs of the model include the final yield, leaf area index, crop growth rate, dry matter accumulation rate in each organ, and development stages with one-day intervals. The calibration of the model and the determination of the coefficients of the model were done in such a way that the expected performance of the tuber was simulated by the WOFOST model. Parameters of specific leaf area as a function of development stage (SLATB), maximum leaf CO2 assimilation rate as a function of development stage of the crop (AMAXTB), efficiency of conversion into leaves (CVL), efficiency of conversion into roots (CVR), efficiency of conversion into stems (CVS) and Lower threshold temperature for emergence (TBASEM), which were necessary for the simulation of gland function, were calibrated and determined based on the permissible range of model coefficients changes and using resources. Then, in order to simulate the potential performance of the potato plant for all 10 study areas and every five years, with conventional planting dates and also with a range of planting dates (three dates) a total of 150 loads were executed.

According to the obtained results, the average yield for the 10 studied stations was 46190 kg/ha for the calibration stage and 44434 kg/ha for the validation data. In the model implementation stage, the average yield during five years for the 10 studied stations was 45919 kg/ha for calibration and 43813 kg/ha for the validation stage by the model. Therefore, by evaluating the statistical indicators, the WOFOST model has high accuracy for simulating performance in all regions.

The results of the simulation of the yield of potato tuber under the future climate conditions indicated a decrease in yield in the studied stations. Thus, the greatest decline in performance was observed in the period of 2071-2099 and under the RCP8.5 scenario. In the studied areas, on average, the highest yield decrease was related to Ardebil station with 2397 kg/ha and Hamedan with 1817 kg/ha, and the lowest decrease in the yield of potato belonged to Sanandaj station with 813 kg/ha and Isfahan station with 982 kg/ha. On average, in the 10 studied areas, a decrease in potato tuber yield was observed by 1436 kg/ha. Reducing the length of the potato ripening period has the biggest contribution in reducing the potato tuber yield. The results obtained from the simulation of the yield of potato tuber showed that in parallel with the increase in temperature, the yield of potato also decreases, so that for one degree Celsius increase in the average annual temperature, the yield of potato will decrease by 3.77%.