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PRECISION MAPPING

Design and Testing of a Global Positioning System-Based Radiometer for Precision Mapping of Pearl Millet Total Dry Matter in the Sahel

^a Inst. of Animal Production in the Tropics and Subtropics (480), Niamey, Niger
^b International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) Sahelian Center, B.P. 12404, Niamey, Niger
^c Unité décologie des grandes cultures, Université Catholique de Louvain, Place Croix du Sud, 2 Bte 11, 1348 Louvain-la-Neuve, Belgium
^d Inst. of Plant Nutrition (330), Univ. of Hohenheim, D-70593 Stuttgart, Germany, presently Inst. of Crop Science, Univ. of Kassel, D-37213 Witzenhausen, Germany

buerkert{at}wiz.uni-kassel.de

The nondestructive determination of plant total dry matter (TDM) in the field is greatly preferable to the harvest of entire plots in areas such as the Sahel where small differences in soil properties may cause large differences in crop growth within short distances. Existing equipment to nondestructively determine TDM is either expensive or unreliable. Therefore, two radiometers for measuring reflected red and near-infrared light were designed, mounted on a single wheeled hand cart and attached to a differential Global Positioning System (GPS) to measure georeferenced variations in normalized difference vegetation index (NDVI) in pearl millet fields [Pennisetum glaucum (L.) R. Br.]. The NDVI measurements were then used to determine the distribution of crop TDM. The two versions of the radiometer could (i) send single NDVI measurements to the GPS data logger at distance intervals of 0.03 to 8.53 m set by the user, and (ii) collect NDVI values averaged across 0.5, 1, or 2 m. The average correlation between TDM of pearl millet plants in planting hills and their NDVI values was high but varied slightly depending on solar irradiance when the instrument was calibrated. There also was a good correlation between NDVI, fractional vegetation cover derived from aerial photographs and millet TDM at harvest. Both versions of the rugged instrument appear to provide a rapid and reliable way of mapping plant growth at the field scale with a high spatial resolution and should therefore be widely tested with different crops and soil types.

Abbreviations: TDM, total dry matter • GIS, Geographic Information System • GPS, Global Positioning System • NDVI, normalized difference vegetation index • NIR, near-infrared • SAVI, soil adjusted vegetation index • LED, light emitting diode • V/F, voltage to frequency • UART, universal asynchronous receiver/transmitter • LCDs, liquid crystal displays • DAS, days after sowing

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