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A Distributed Surface Temperature and Energy Balance Model of a Semi-Arid Watershed

A simple model of surface and sub -surface soil temperature was developed at
the watershed scale ( -100 km2) in a semi -arid rangeland environment. The model
consisted of a linear combination of air temperature and net radiation and assumed:
1) topography controls the spatial distribution of net radiation, 2) near- surface air
temperature and incoming solar radiation are relatively homogeneous at the
watershed scale and are available from ground stations and 3) soil moisture
dominates transient soil thermal property variability. Multiplicative constants were
defined to account for clear sky diffuse radiation, soil thermal inertia, an initially
fixed ratio between soil heat flux and net radiation and exponential attenuation of
solar radiation through a partial canopy. The surface temperature can optionally be
adjusted for temperature and emissivity differences between mixed hare soil and
vegetation canopies. Model development stressed physical simplicity and commonly
available spatial and temporal data sets. Slowly varying surface characteristics, such
as albedo, vegetation density and topography were derived from a series of Landsat
TM images and a 7.5" USGS digital elevation model at a spatial resolution of 30 m.
Diurnally variable atmospheric parameters were derived from a pair of ground
meteorological stations using 30 -60 min averages. One site was used to drive the
model, the other served as a control to estimate model error.
Data collected as part of the Monsoon '90 and WG '92 field experiments over
the ARS Walnut Gulch Experimental. Watershed in SE Arizona were used to
validate and test the model. Point, transect and spatially distributed values of modeled surface temperature were compared with synchronous ground, aircraft and
satellite thermal measurements. There was little difference between ground and
aircraft measurements of surface reflectance and temperature which makes aircraft
transects the preferred method to "ground truth" satellite observations. Mid- morning
modeled surface temperatures were within 2° C of observed values at all but satellite
scales, where atmospheric water vapor corrections complicate the determination of
accurate temperatures.
The utility of satellite thermal measurements and models to study various
ground phenomena (eg. soil thermal inertia and surface energy balance) were
investigated. Soil moisture anomalies were detectable, but were more likely
associated with average near -surface soil moisture levels than individual storm
footprints.

Identiferoai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/617637
Date05 1900
CreatorsWashburne, James Clarke
ContributorsDepartment of Hydrology & Water Resources, The University of Arizona
PublisherDepartment of Hydrology and Water Resources, University of Arizona (Tucson, AZ)
Source SetsUniversity of Arizona
Languageen_US
Detected LanguageEnglish
Typetext, Technical Report
SourceProvided by the Department of Hydrology and Water Resources.
RightsCopyright © Arizona Board of Regents
RelationTechnical Reports on Hydrology and Water Resources, No. 94-050

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