9
Soil Moisture
Edwin T. Engman
Head, Hydrological Sciences Branch,
Laboratory for Hydrospheric Processes,
NASA/Goddard Space Flight Center
9.1 Introduction
Soil moisture is an environmental descriptor that integrates much of the land surface hydrology and is the interface between the solid earth surface and the atmosphere. By far the most important role for soil moisture is in controlling and regulating the interaction between the atmosphere and the land surface. The redistribution of solar energy over the globe is central to studies in climate and weather.
Water serves a fundamental role in this redistribution through the energy associated with evapotranspiration, the transport of atmospheric water vapor, and precipitation. Residence times for atmospheric water is on the order of a week and for
soil moisture from a couple of days to months, emphasizing the active nature of the
hydrologic cycle. Understanding the importance of the land-surface hydrology to
climate has emerged as an important research area since the mid 1960s when researchers at the Geophysical Fluid Dynamics Laboratory placed a land hydrology
component into their general circulation model (GCM) (see Manabe et aI., 1965;
Manabe, 1969).
The role of soil moisture is equally important at smaller scales. Recent studies
with mesoscale atmospheric models have similarly demonstrated a sensitivity to
spatial gradients of soil moisture. Chang and Wetzel (1991) have concluded that
the spatial variations of vegetation and soil moisture affect the surface baroclinic
structures through differential heating which in tum indicate the location and intensity of surface dynamic and thermodynamic discontinuities necessary to develop severe storms. An analysis of the 1988 drought in the mid-western United
States by Atlas et aI. (1993) has shown that these conditions could be modeled
accurately only when the soil moisture values were realistic. A reanalysis (Beljaars
et aI., 1996) of the conditions leading to the 1993 floods in the United States illustrated improved precipitation forecasts, both in quantity and location, when realistic soil moisture values were used in the model.
Soils, their properties and their spatial distribution all reflect the historical hydroclimate processes that formed the soils and control the current land surface
response to meteorological inputs, i.e., precipitation and potential ET. Soil moisture patterns, both spatial and temporal, may be the key to understanding the spatial variability and scale problems that are paramount in scientific hydrology. Operational flood forecasting is based on limited measurements of rainfall and river
G. A. Schultz et al. (eds.), Remote Sensing in Hydrology and Water Management
© Springer-Verlag Berlin Heidelberg 2000
Soil Moisture
Edwin T. Engman
Head, Hydrological Sciences Branch,
Laboratory for Hydrospheric Processes,
NASA/Goddard Space Flight Center
9.1 Introduction
Soil moisture is an environmental descriptor that integrates much of the land surface hydrology and is the interface between the solid earth surface and the atmosphere. By far the most important role for soil moisture is in controlling and regulating the interaction between the atmosphere and the land surface. The redistribution of solar energy over the globe is central to studies in climate and weather.
Water serves a fundamental role in this redistribution through the energy associated with evapotranspiration, the transport of atmospheric water vapor, and precipitation. Residence times for atmospheric water is on the order of a week and for
soil moisture from a couple of days to months, emphasizing the active nature of the
hydrologic cycle. Understanding the importance of the land-surface hydrology to
climate has emerged as an important research area since the mid 1960s when researchers at the Geophysical Fluid Dynamics Laboratory placed a land hydrology
component into their general circulation model (GCM) (see Manabe et aI., 1965;
Manabe, 1969).
The role of soil moisture is equally important at smaller scales. Recent studies
with mesoscale atmospheric models have similarly demonstrated a sensitivity to
spatial gradients of soil moisture. Chang and Wetzel (1991) have concluded that
the spatial variations of vegetation and soil moisture affect the surface baroclinic
structures through differential heating which in tum indicate the location and intensity of surface dynamic and thermodynamic discontinuities necessary to develop severe storms. An analysis of the 1988 drought in the mid-western United
States by Atlas et aI. (1993) has shown that these conditions could be modeled
accurately only when the soil moisture values were realistic. A reanalysis (Beljaars
et aI., 1996) of the conditions leading to the 1993 floods in the United States illustrated improved precipitation forecasts, both in quantity and location, when realistic soil moisture values were used in the model.
Soils, their properties and their spatial distribution all reflect the historical hydroclimate processes that formed the soils and control the current land surface
response to meteorological inputs, i.e., precipitation and potential ET. Soil moisture patterns, both spatial and temporal, may be the key to understanding the spatial variability and scale problems that are paramount in scientific hydrology. Operational flood forecasting is based on limited measurements of rainfall and river
G. A. Schultz et al. (eds.), Remote Sensing in Hydrology and Water Management
© Springer-Verlag Berlin Heidelberg 2000