Calibration and Validation of WOFOST Model for Predicting the Phenology and Yield in Potato (Solanum tuberosum L.) Growing Regions in Iran

Introduction:

  Food and Agriculture Organization of the united nations (FAO) stated potato as a product supplier in the world's future food security and Iran with an annual production of 5 million tons of potatoes ranked eleventh in the world's total production. Future climate change may have strong influence on field crops including potato and evaluation of these effects is of great importance. Crop simulation models are known as powerful tools to study crop responses to the future climatic scenarios. However, such models should be calibrated and validated for local conditions before using in climatic studies at regional scale. WOFOST (WOrld FOod STudies) is a well- known crop simulation model and during the past two decades has been widely used in various studies. Simulation method of WOFOST is based on leaf photosynthesis and the assimilates are converted to dry matter of different plant organs using specific conversion factors, after subtraction of the calculated values of maintenance and growth respiration. The model also simulated phenological stages of crop based on accumulated degree days. For simulation of potential yield WOFOST inputs are daily weather data and crop specific parameters. In previous studies WOFOST model was calibrated for several crops such as wheat, corn and sugar beet in different parts of Iran but not for potato. The aim of this study was to calibrate and statistically validate the WOFOST model for predicting phenology and tuber yield of potato under potential growth conditions in different climatic regions of Iran.

Yield, phenological and weather data of major potato production regions of Iran (Hamedan, Ardabil, Isfahan, Ghorveh, Shiraz, Jiroft, Mashhad, Gorgan and Dezful) covering wide range of climatic conditions were collected from official databases and field observations for 5 years (2010-2014). The model was calibrated under potential production conditions for semi-late varieties (e.g. Agria) with the dataset of 3 years and the remaining 2-year data was used for model validation. Calibration was conducted using FSEOPT sub-program which optimizes the model parameters with the lowest deviation between measured and simulated yield and phenological variables.Predicted results of yield and phenological stages were tested against observed values during model validation. The model performance was statistically evaluated using coefficient of determination (R2), t-test, root mean square error (RMSE), normalized root mean squareerror (RMSEn), maximum error (ME) and coefficient of efficiency (E).

During the ccalibration, fewmodel parameters and functions including thermal time from emergence to initiation of flowering (TSUM1), thermal time from initiation of flowering to maturity (TSUM2), specific leaf area as a function of development stage (SLATB), lower threshold temperature for ageing of leaves (TBASE), maximum leaf CO2 assimilation rate as a function of development stage of the crop (AMAXTB) and air temperature affecting photosynthetic rate (TMPFTB) was amended. It should be noted that for regions with cold summers such as Ardabil, Oromieh or Sarab TSUM2 was set at 1580 °Cd which is relatively lower than 1789 °Cd used for other parts of the country. Validation of model with independent data showed a great compliance of simulation results with field observations. Average simulated tuber yield over all regions and the studied period was 52061 kg ha-1 that was reasonably close to the mean observed potato tuber yield of 50650 kg ha-1 and the same was obtained for phenological variables. RMSE for tuber yield was 2933 kg ha-1 and for time to emergence, flowering and physiological maturity were estimated as 1.6, 3.2 and 6.4 days, respectively. RMSEn for phenological stages such as days to emergence, flowering, physiological maturity and tuber yield were 9.5, 8.3, 5.6 and 5.8%, respectively showing good model accuracy.

Based on the results the WOFOST model will be able to simulate the yield and phenological stages of potato with an acceptable performance at different regions of Iran. The calibrated model can be successfully used for climate change impact studies and yield gap analysis of potato under wide range of climatic conditions over country. However, it seems that the model should be also assessed for other potato cultivars with different growth habits.