Use of VEGETATION satellite imagery to map pasture quality for input to a methane budget of New Zealand

Abstract

To calculate an accurate methane budget of New Zealand, it is necessary to measure the spatial and temporal variation of metabolizable energy of pasture. The VEGETATION sensor, on board SPOT4 and SPOT5, provides imagery at appropriate spatial and temporal scales. Imagery can be composited over 10 days to remove cloud cover and then processed to remove artefacts associated with directional reflectance. One year of VEGETATION imagery from March 2000 through to February 2001 was processed and co‐acquired with metabolizable energy measurements from 17 farms spread throughout New Zealand. Metabolizable energy was related to the Normalized Difference Vegetation Index (NDVI) with a Loess regression model to produce monthly maps of metabolizable energy. If these maps were used to estimate average metabolizable energy of pasture in a national methane budget, then the accuracy would not be increased (the uncertainty of average metabolizable energy from the satellite based method is the same as the nominal uncertainty assigned from expert judgement). However, the satellite‐based method would lend statistical credibility, and give the potential for spatial and temporal disaggregation of the budget.