Standard relations to estimate ground cover and LAI of agricultural crops from reflectance measurements

Abstract

Ground cover and leaf area index (LAI) of agricultural crops are important variables for the calculation of intercepted solar radiation that drives crop growth. The estimation of ground cover and LAI was studied from reflectance measurements for potato, sugar beet, barley, wheat and oats. The data set was gathered during ten years of observations on a number of locations and spanned a range of cultivars, treatments, soil types, soil moisture regimes and growing conditions from severely stressed to near-potential growth. Standard relations were derived to estimate the percentage ground covered by the crop and the LAI from the ≪ weighted difference vegetation index ≫ (WDVI), calculated from nearinfrared and green canopy reflectance. The average estimation accuracy of percentage ground cover from WDVI was of the same magnitude as that of conventional methods, i.e. about 5 per cent (absolute value) is most cases. The average estimation accuracy of LAI from WDVI increased from about 0.15 at LAI of 1 to 0.6 at LAI of 4 (somewhat higher for potato at low LAI values), which is also of the same magnitude as that of conventional methods. These estimation accuracies were derived from the same (calibration) data sets that were used to derive the standard relations from. Only for ground cover of potato, the estimation accuracy was derived from a separate validation data set. The advantages of using reflectance measurements to determine LAI and ground cover are that it is a non-destructive, fast and easy measurement, has a high degree of objectivity, is representative of relatively large areas of a field, and can be recorded automatically. Finally, it is argued that a reflectance measurement (WDVI) itself may be more appropriate than ground cover or LAI to quantify interception of solar radiation by a crop canopy.