The model for seasonal dynamics of the normalized difference vegetation index (NDVI) for spring barley crops was developed. The research study was carried out in the Leningradskaya oblast in 2015–2021. Comparing the data on the NDVI measurements in a growing season with the model data can allow us to monitor and, if necessary, to adjust the conditions for crops with the agrotechnological techniques. The measurements were performed once a week on the monitored permanent experimental plots in two field experiments. The NDVI dynamics for the experiments with the spring barley mixed with perennial grasses and grown alone were different. In the first instance, its maximum value was recorded at calendar week 28 in the year, while in the second instance, it was recorded at calendar week 25. The difference indicated above was associated with the influence of perennial grasses neutralizing the effects of herbicide treatments performed in the experiments because of increasing their aboveground biomass. The NDVI seasonal dynamics models built based on the normalized long-term data preventing the random effects were found more reliable for a general analysis than the one-year data. In both cases of modeling the seasonal index variability relative to two growing spring-barley experiments, good results were obtained with the use of the 2nd-degree polynomial function (R2 = 0.97 and 0.80, p = 0.00003 and 0.008). In addition, fitting the model to the 3rd-degree polynomial resulted in increasing the forecast precision (R2 = 0.92) and reliability (p = 0.004) only for the second experiment. In the models describing the NDVI seasonal dynamics in certain years, the higher precision also corresponded to the 2nd-degree polynomial.
M. A. Fesenko (Mon,) studied this question.