Estimating rice potential yield and fertilizer requirements in Guilan province using GIS and crop modeling

Rational fertilizer management is crucial in the efficient use of resources that are basically non-renewable and that can have a great environmental impact when used without scientific basis. The availability of scientifically sound decision-making tools for rational fertilization  is scarce. FertiliCalc-Fertigate software is a program to determine the consumption of NPK fertilizers during the growing season in a cost-effective and sustainable way. Today, the geographic information system is widely used in spatial planning by determining the distribution of phenomena and combining maps and interpreting ecological data in different stages of planning. Also, the potential yield in an area can be estimated using field tests and simulation models. The ORYZA2000 model is one of the efficient models in investigating rice potential yield, which simulates the growth and development of rice plants under favorable conditions, water limitation and nitrogen limitation. In this study, an attempt was made to estimate the potential yield in Guilan province by integrating the ORYZA2000 model and geographic information system. Also, after determining the potential yield, the NPK fertilizer requirement estimated using FertiliCalc-Fertigate software.

This research was conducted to investigate the potential yield of rice in Guilan province using the ORYZA2000 model. After calibrating and validating the model at the field test level, the model was used to evaluate the potential yield of rice in 12 synoptic stations of Guilan province. The processing of Landsat 8 satellite images was used to separate rice fields in Guilan province and the studied area was separated using supervised classification. The estimation of potential yield in Guilan province was done by combining GIS environment and ORYZA2000 model. The amount of radiation for the whole area was calculated from the Points Solar Radiation function in GIS. Then, from the relationship between the amount of radiation received during the rice growing season and the potential yield estimated in the ORYZA2000 model, the potential yield was calculated and generalized to the whole area based on the agricultural land use of Guilan province. Fertilizer requirement of lands was calculated using FertiliCalc-Fertigate 1.0 software. In order to evaluate  the fertilizer requirement at the province level, first, 320 points were randomly selected in the rice cultivation area of the province and the fertilizer requirement of each point was determined based on the required information, including potential yield and soil information. Then the points were interpolated and the study results were presented in the form of fertilizer requirement maps.

The results showed that the amount of radiation in rice fields in the Guilan area, during the growing season, was between 2552 to 6259 MJ/m2 (average 4405 MJ/m2) in 2016 and from 2423 to 5337 MJ/m2 (average 3880 MJ/m2) in 2017. The lowest amount of received radiation was in the central areas of the basin, which can be due to the topographic conditions of the area. Using the regression relationship between radiation during the growing season and potential yield, a potential yield map of rice fields was prepared. Based on the results, potential yield in rice fields of Guilan province was between 4416 to 7038 kg/ha (with an average of 5160 kg/ha) in 2016 and between 4558 to 7180 kg/ha (with an average yield of 5302 kg/ha) in 2017. Based on these results, the combined approach of ORYZA2000 model and GIS has a good ability to simulate potential yield in the study area. Estimation the levels of fertilizer requirements in the rice fields of Gilan province showed that in order to achieve the potential yield, 262 to 274 kg/ha of potassium fertilizer, 116 to 171 kg/ha of nitrogen fertilizer, and 8 to 12 kg/ha of phosphorus fertilizer are needed. Based on the calculated fertilizer requirement, potassium has played the most important role in achieving the potential yield of rice in the province.

It seems that in many areas of Gilan province, the application of NPK levels in an inappropriate amount and time causes a decrease in rice yield. The results of this research can recommend the appropriate fertilizer consumption pattern to farmers through rice experts so as to achieve maximum yield and avoid problems such as phosphorus leaching due to excessive consumption.