HR: 13:40h
AN: H22E-01 INVITED     [PDF]
TI: Potential for Observing the Global Water Cycle from Space
AU: * Lettenmaier, D P
EM: dennisl@u.washington.edu
AF: University of Washington, Box 352700, Seattle, WA 98144 United States
AB: The fundamental challenge in observing the global water cycle is to understand where the water is, and where it is moving. Of course, the answer to the "where is it?" question is "mostly in the oceans and ice sheets", but when one considers relative mobility, the focus becomes somewhat different. From the standpoint of the land surface, moisture stored in the atmosphere, and especially that released from the atmosphere as precipitation and returned to it as evapotranspiration, are the largest fluxes over most of the globe. Runoff and streamflow, although usually a somewhat smaller fraction of precipitation than ET, are critically important, as they are the source of or contribute directly to much of the water available for human use. Water stored in the subsurface, especially as soil moisture, controls vegetation moisture stress, and hence ET under many conditions. Soil moisture "antecedent conditions" are well known to affect storm runoff production as well. Furthermore, groundwater and variations therein are important both for human use, and through interactions with soil moisture and streamflow, depending on the particular conditions. Current estimates of the terms in the global water cycle are based primarily on in situ observations. For instance, over land, most estimates of precipitation are primarily gage-based. The same is true of streamflow. ET is measured directly only at a relatively small number of sites; large area estimates are primarily based either on the difference between precipitation and runoff (ignoring storage change in the long-term mean) or atmospheric budget analyses. Soil moisture, groundwater, and snow storage (except glaciers) are generally not well enough measured to provide quantitative estimates, except through use of models. Of the major terms on the land surface branch of the global water cycle, most terms (precipitation, streamflow, soil moisture, and snow water storage) are more or less directly observable from space, albeit at different (and sometimes incompatible) time and space scales. ET is observable only somewhat indirectly, as is groundwater storage. Space-based observation of many, if not most, terms in the global water cycle has considerable advantages, and poses a number of challenges as well. A major advantage of space-based observation is the potential for a truly global observing strategy, free from logistical complications of in situ observations. On the other hand, some terms are not easily observable, if at all. The potential and challenges of a true space-based global water cycle observing system are discussed, with particular focus on the land surface, but considering other aspects of global moisture storage and flux as well.
DE: 1800 HYDROLOGY
SC: Hydrology [H]
MN: 2003 Fall Meeting