Global Biomass Variation and Its Geodynamic Effects: 1982–98

Abstract

Redistribution of mass near Earth’s surface alters its rotation, gravity field, and geocenter location. Advanced techniques for measuring these geodetic variations now exist, but the ability to attribute the observed modes to individual Earth system processes has been hampered by a shortage of reliable global data on such processes, especially hydrospheric processes. To address one aspect of this deficiency, 17 yr of monthly, global maps of vegetation biomass were produced by applying field-based relationships to satellite-derived vegetation type and leaf area index. The seasonal variability of biomass was estimated to be as large as 5 kg m−2. Of this amount, approximately 4 kg m−2 is due to vegetation water storage variations. The time series of maps was used to compute geodetic anomalies, which were then compared with existing geodetic observations as well as the estimated measurement sensitivity of the Gravity Recovery and Climate Experiment (GRACE). For gravity, the seasonal amplitude of biomass variations may be just within GRACE’s limits of detectability, but it is still an order of magnitude smaller than current observation uncertainty using the satellite-laser-ranging technique. The contribution of total biomass variations to seasonal polar motion amplitude is detectable in today’s measurement, but it is obscured by contributions from various other sources, some of which are two orders of magnitude larger. The influence on the length of day is below current limits of detectability. Although the nonseasonal geodynamic signals show clear interannual variability, they are too small to be detected.

Estimating groundwater storage changes in the Mississippi River basin (USA) using GRACE

“…Adopting that view might be appropriate given that the technique described here supposes groundwater to be spatially continuous across the region of interest, and it would effectively eliminate surface water variations as a source of error. Rodell et al (2005) applied field-based relationships to satellite derived maps of leaf area index in order to produce monthly, global maps of vegetation biomass. They showed that seasonal and interannual biomass variations typically were smaller than the uncertainty in GRACE based hydrology.…”

GRACE satellite observed hydrological controls on interannual and seasonal variability in surface greenness over mainland Australia

“…After removing the effects of atmospheric surface pressure and ocean bottom pressures changes [Tapley et al, 2004] and considering the relatively small contribution of changes in vegetation biomass [Rodell et al, 2005], the variability in the resulting monthly gravity solutions results primarily from redistribution of terrestrial total water storage (TWS) [Rodell et al, 2004a]. For cases without large water storage changes within surface water reservoirs, changes in TWS are mainly driven by changes in soil moisture storage (SMS) and groundwater storage, suggesting the potential of using GRACE data to examine hydrological controls on vegetation dynamics.…”

Satellite-based estimates of groundwater depletion in India

“…Interannual variations in biomass have been shown to be well below the detection limits of GRACE 18 . The climate of the study region is warm and the terrain shallow; hence, snow is uncommon.…”