The Full Picture

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• How is surface soil moisture dynamics linked with

that in the subsurface profile?

• Are Land Surface Models (LSMs) accurate enough

and unbiased enough to be used in data assimila-

tion?

Enhancing global in situ and satellite soil moisture

observations involves different types of problems. Some

specific considerations for improved in situ observations

include: taking advantage of existing infrastructure;

opportunity-based expansion strategy; striving for

consistency and standardization; linking long records

of points to short records of spatial description; exploit-

ing temporal stability; a single point or even a cluster at

a station will not necessarily provide the data needed

for satellite product validation of data assimilation; and

including but not being limited to five cm measurement

depth to link to satellite observations.

During the last few years, tremendous progress has

been made in understanding the necessity of a global

soil moisture observing system. The contributions of

international organizations such as GEWEX, IWGCO

and GEO, with the support of Space Agencies and with

the efforts of numerous individual scientists, has enabled

this progress in the realization of a global in situ soil

moisture network as part of a global soil moisture

observing system.

this global network of in situ soil moisture measurements and that

the space agencies will commit to the development of a quasi-oper-

ational soil moisture data product.

Coordination of these efforts is done through the International

Soil Moisture Working Group and is based upon the following guide-

lines:

A coordinated plan for soil moisture networks should be devel-

oped at both the national and international levels. A supersite

programme with high density measurements is required to provide

the comprehensive data sets needed for satellite sensor evaluation

and calibration, and to provide a basis for developing soil wetness

algorithms for satellite measurements and the evaluation of climate

model outputs.

Capabilities to measure soil moisture from space have been demon-

strated, but committed missions are needed to perfect these

measurements and to determine the utility of soil moisture measure-

ments derived from space at different spatial and temporal scales.

Vegetation cover can obscure the radiatiances arising from warm wet

soils. Research is needed to find better ways of removing the vege-

tation effect from the signal that is being used to derive soil moisture.

Finally, the relationship between surface soil wetness and deep soil

moisture profiles needs to be quantified or at least understood. The

measurements within the in situ network will need to measure soil

moisture at several depths to establish a soil moisture profile.

In situ networks will occupy a key position in integrated water

cycle understanding and quantitative prediction. Beside their role in

validating satellite retrieval algorithms, data from in situ sensors can

help address the following fundamental science questions about soil

moisture, the key variable state of the land branch of the water cycle:

• What is the soil moisture memory? How does it depend on

climate variability, climate persistence and soil hydraulic prop-

erties? How do vegetation characteristics affect this memory?

• How does soil moisture at the surface and below the surface

respond to precipitation events and restore during drydowns?

• What are the functional relationships between soil moisture and

evaporation? How does it depend on vegetation, soil type and

structure?

Photo: Tom Jackson & Gary Schaeffer

The pit in which the soil moisture probes have been installed to measure soil

moisture over three different depths

Photo: Tom Jackson & Gary Schaeffer

The measurement of soil moisture using the gravimetric method,

which is considered to be the standard measurement method for

soil moisture

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