Crop phenology based on MODIS satellite imagery as an indicator of plant available water content

Maintaining agricultural productivity into the future is one of the most important goals of our society. The world’s vast grain production areas in arid environments have a high sensitivity to climate change, thus demanding accurate future predictions. Management and decision making depends on an understanding of complex spatial and temporal interactions between rainfall, temperature and soils, but detailed soil information is extremely costly and generally unavailable. Crop phenology, the timing of plant growth and developmental stages, is the combined response to environmental factors such as soil, water and temperature. Crop phenology plays an important role in crop growth models and agronomic management. In Mediterranean environments like South Australia, water is the driving factor for crop growth. In these rain-fed farming systems, water availability largely depends on rainfall amount and seasonality and the capacity of the soil to retain rainfall inputs and hence make water available for crops. Plant Available Water Capacity of soil (PAWC) is an important measure of the spatial and temporal variability of crop growth and yield, but there is a paucity of detailed maps at a management relevant spatial resolution and extent. In this study, different phenological metrics derived from MODIS NDVI imagery were assessed over sites of known PAWC and cropping history. We used the metrics: • NDVI at the onset of greenness (Onset) and time of Onset (OnsetT) • NDVI at peak greenness (MaxV) and time of maximum greenness (MaxT) • The length of the growing season (WidthGS) • Rate of greenup (GURate) The green up rate of the curve (GU-rate) showed the most consistent difference between soils of different PAWC. The Rate of Green up is higher for the soil points with relatively low PAWC. Some of the variability in the difference between sites with low and high PAWC can be explained with rainfall amount and seasonality but the relationship is complex. The results indicate that the crop phenology derived from MODIS satellite imagery is consistently different and may therefore provide useful information about site conditions, which would allow improvements to the spatial detail in soil maps.Sofanit Araya, Bertram Ostendorf, Gregory Lyle, Megan Lewishttp://www.mssanz.org.au/modsim2013/index.htm