A mobile platform to constrain regional estimates of
Ray G. Anderson
, Michael L. Goulden
Department of Earth System Science, University of California at Irvine, Croul Hall, Irvine, CA 92697-3100, USA
Regional estimates of ET are needed for a variety of ecological,
environmental, climatological, and agricultural monitoring
and modeling applications. In agriculture, regional ET esti-
mates are critical for managing irrigation supplies and
practices to increase agricultural productivity with increas-
ingly scarce water supplies (e.g. Er-Raki et al., 2007; Akbari
et al., 2007). Numerous methods exist for determining
evapotranspiration (ET) and water use efﬁciency (WUE);
however, gaps remain in the spatial and temporal range of
existing approaches, especially at a regional scale.
Micrometeorological methods such as Eddy covariance (EC)
(Law et al., 2002), Bowen ratio energy balance (BREB) (Held
et al., 1990), and surface renewal (Paw and Brunet, 1991), use in
situ observations that can provide direct, site-speciﬁc esti-
mates of ET at high temporal resolution. However, spatial
heterogeneity and the relatively few number of sites prevent
direct scaling of these calculations to regional estimates. EC
can assess regional scale ($100 km
) ﬂuxes from an aircraft
agricultural and forest meteorology 149 (2009) 771–782
Received 7 March 2008
Received in revised form
12 October 2008
Accepted 31 October 2008
Water use efﬁciency
Regional estimates of evapotranspiration (ET) are needed for environmental analysis and
management purposes, yet can be difﬁcult to obtain. Current methods for determining
regional ET have spatial, temporal, methodological, and/or logistical limitations that affect
their usefulness. To address these gaps, we developed a surface mobile measurement
technique, the Regional Evaporative Fraction Energy Balance platform (REFEB), which
measures evaporative fraction (EF) and water use efﬁciency (WUE) using a truck operating
on a public right of way. REFEB can measure EF and WUE at 25 or more locations per day,
which allows for rapid, dense, and spatially distributed sampling of ﬁelds across a region.
We assessed the accuracy of the ﬁeld measurements of EF and WUE with REFEB by
comparing them to an Eddy covariance (EC) and Bowen ratio energy balance (BREB) tower.
This site validation showed that REFEB has error and uncertainty similar to previous BREB
approaches. We then used empirical relationships between ﬁeld measurements and remote
sensing vegetation indices to derive regional maps of EF. We combined these EF maps with
satellite observations of net radiation to derive monthly and annual calculations of ET at a
250 m resolution during calendar year 2004 in the Imperial Valley, California, a major
agricultural region that is dependent upon irrigation and which has a well constrained
water budget. We then summed ET for the Imperial Valley and compared the result to a
surface water budget based on irrigation and drainage measurements, which showed good
annual and seasonal agreement. These results indicate REFEB produces accurate ﬁeld
measurements of EF and WUE, which can be scaled to estimate regional ET at time scales
ranging from less than a week to annual sums.
# 2008 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +1 949 824 9273; fax: +1 949 8247 3874.
E-mail addresses: firstname.lastname@example.org (R.G. Anderson), email@example.com (M.L. Goulden).
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/agrformet
0168-1923/$ – see front matter # 2008 Elsevier B.V. All rights reserved.