Dynamics of reconstructed “instrumental” and calculated data of the 131I specific activity in cultivated pasture grass and rainfall in the Piski district, the Ostroleka Area (exponential reduction rates for instrumental (lm = 0.19 day–1) and calculated (lc = 0.21 day–1) data for the 131I specific activity in grass). Data options for the 131I activity in pasture grass: Gr1 – reconstructed “instrumental” data for the cloud with recalculation; Gr2 – nonlinear interpolation of Gr1; Gr3 – reconstructed “instrumental” data for the cloud with instrumental data; Gr4 – direct calculation model; Gr5 – heterogeneous cloud model; Gr6 – homogeneous cloud model. Rainfall: Rain1 – weather data; Rain2 – reconstruction in an interval of 2.4 to 5.5 days after the accident for the cloud with recalculation.