Instruments and Systems: Monitoring, Control, and Diagnostics Annotation << Back Ground-Based Septometric Method for Measuring the Leaf Area Index of Vegetation F.G. AGAEV, M.E. OMAROV The article is devoted to the development of a new septometric method for ground-based measurement of the leaf area index of vegetation. The proposed method provides for ground-based photometric measurements of LAI without the need to measure the optical fl ux at the crown top level. To carry out measurements, a modified formula of Mensi and Saki is used, where extra-atmospheric solar radiation is used as incoming radiation. The objective of the study is to develop a method for determining LAI without measuring the optical radiation arriving at the plant crown, provided that there are a priori known data on the degree of correlation between LAI and the extinction coefficient. When carrying out calculations using these formulas, the data given in the corresponding tables can be used as the value of the extinction coeffi cient. It is shown that replacing the radiation arriving at the crown with the radiation arriving at the soil outside the crown projection contour allows developing a method for determining LAI, the implementation of which does not require measuring the radiation arriving at the crown. Formulas for calculating LAI are given for the case of positive or negative correlation between LAI and extinction coefficient. Keywords: septometer, optical measurements, leaf area index, vegetation crown, correlation. DOI: 10.25791/pribor.3.2025.1564 Pp. 26-30.