| NDVI,
or normalised difference vegetative index, is a measure of
the greenness of a crop. We can see intuitively when a
plant has died or stopped using water: they will be brown
or, if green, wilting. By comparison, healthy plants,
which are using water, will be a rich green colour. A
change in NDVI therefore equates to a change in plant
greenness and gives an early warning to factors such as
water stress, which can assist with pasture management. To determine NDVI, two sensors have been installed at each BIGG weather station, an ‘incoming radiation’ sensor and a ‘reflected radiation’ sensor. The ‘incoming’ sensor is mounted face up and measures red and infra-red radiation levels from the sky, while the ‘reflected’ sensor is mounted at an angle facing down on the pasture and measures red and infra-red radiation levels being reflected from the pasture. The ratio of the radiation differences between the two sensors is calculated as the NDVI. Plants appear green to us because they absorb the radiation which corresponds to red light: what we see is what does not get absorbed and hence gets reflected back to our eyes, which is predominantly red. In contrast, plants do not absorb infra-red radiation. So the amount of infra red radiation they reflect will remain constant. The incoming radiation sensors measure the level of infra-red and red radiation coming on from the sky. The reflected sensors, measure the levels being reflected by the plants. Because only the level of reflected red radiation changes in accordance with the level of greeness of the plant, we can then compare the relative ratios of the incoming and reflected radiation from the red and infra-red sensors. The result is the NDVI. We use the weather station to estimate reference crop evapo-transpiration (ETo). If we want to find the water use of a perennial pasture, we multiple the reference crop ET by a crop coefficient (Kc). But the lack of good quality crop coefficient data has in the past hampered our ability to use Evapo-transpiration figures to monitor plant water use and from that to estimate the yield loss due to water stress. In the main, Kc figures have only been published on a monthly basis and even then they are only available for a limited umber of plants. Fortunately for us, researchers have also found a very high correlation between NDVI and Crop Coefficient. So if we have the NDVI figure from our NDVI sensors, we can estimate the crop coefficient very easily. To obtain the NDVI for a given day, we average the readings over the period 11:00 to 2:00 pm. This is in turn multiplied by a constant to give us the Kc for that day. |
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