MODIS and AMSR-E vegetation indices for land surface phenology studies in the Eurasia semi-arid grain belt

Alterations in land surface phenology (LSP) can affect land-atmosphere exchanges of energy and water. Monitoring and modeling LSP can be accomplished using image time series from remote sensing product streams. From MODIS onboard the Terra and Aqua platforms, these products and derivatives include, NPP, LAI, fPAR, vegetation indices, (NDVI, WDRVI, EVI/EVI2), and indicators of vegetation wetness (NDWI, LSWI). From AMSR-E onboard the Aqua platform, the principal product is the vegetation water content derived from multifrequency microwave brightness temperatures. Here we investigate the synergistic use of MODIS and AMSR-E products for LSP studies. Optical vegetation products are restricted for daytime acquisition and do not provide information on the diel canopy energy and moisture exchanges. Passive microwave remote sensing, on the other hand, senses the "cool earthlight" - the energy emitted at longer wavelengths (0.34 4.3 cm) during both day and night. The sensitivity of microwave emittance to the moisture status of vegetation has been amply demonstrated. We construct LSP models based using various MODIS products and indices and for the period 2003-2005 we assess (1) the value at the beginning of the observational period, (2) the seasonal peak, (3) the quantity of accumulated growing degree- days at the peak value, and (4) the seasonal dynamic range in value. We evaluate how the AMSR-E vegetation water content (VWC) product that is available at coarse spatial but finer temporal resolutions corresponds to the LSP models based on MODIS data. The region is interest is the semi-arid grain belt of Eurasia that covers about 200 million hectares and extends eastward from eastern Ukraine across southern Russia and northern Kazakhstan and northward of the Irano-Turanian deserts to the foothills of the Tian-Shan and the Altai Mountains. The western portions of this region served as the breadbasket of the Russian Empire. There are three principal grain crops grown within the region: barley, winter wheat, and spring wheat. Cropping predominantly follows a rain-fed fallow rotational system. Differential LSPs are observed across the region and the diel difference of the VWC is able to track significant changes in LSP at a quicker tempo than MODIS products.