Three strategies are used individually or in combination to get around these problems, with varying degrees of success. First, when feasible, variables such as precipitation, river discharge, and wind speed are measured at specific locations (“point” measurements). Second, remote sensing methods can provide information on the spatial distribution of key variables. These methods measure over large areas, but the measurements represent averages over some space and time “window”. Remote sensing can be ground based (e.g., radar estimates of rainfall), or based on aircraft or satellites. Third, models are used to interpolate between point measurements (e.g., precipitation), estimate unmeasured quantities based on measured ones (e.g., chlorophyll concentrations from certain wavelengths of light, or evaporation from wind speed, temperature, and relative humidity) and to predict hydrological conditions under a hypothetical future combination of land use, land cover, and climate.
Even with these tools, the field suffers great limitations in many areas of measurement. For example, for most aquifers there are no accurate estimates of recharge, especially their spatial and temporal resolution. Likewise, accurate measurements of the spatial distribution of snow water storage are virtually impossible to make in many areas due to extreme topography and/or limited access.
Out of interest in these issues and their implications, in 2005 the National Research Council’s (NRC) Water Science and Technology Board (WSTB) formed the Committee on Integrated Observations for Hydrologic and Related Sciences to examine the potential for integrating new and existing spaceborne observations with complementary airborne and ground-based observations to gain holistic understanding of hydrologic and related biogeochemical and ecological processes and to help support water and related land resource management. Funding for this effort (or for its parent standing committee, the Committee on Hydrologic Science [COHS]) was received from the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), the U.S. Army Corps of Engineers (USACE), the National Oceanic and Atmospheric Administration (NOAA), the Nuclear Regulatory Commission, and the Environmental Protection Agency (EPA). The full Statement of Task is shown in Box S-1. This report offers a comprehensive view of the current state of integrated observing for hydrology and the related sciences, with a particular emphasis on sensing.