Agricultural and Forest Meteorology 100 2000 309–322
Modelling long-term transpiration measurments from grassland in southern England
R.J. Harding
a,∗
, C. Huntingford
a,1
, P.M. Cox
b
a
Institute of Hydrology, Crowmarsh Gifford, Wallingford, Oxon OX10 8BB, UK
b
Hadley Centre, Meteorological Office, Bracknell RG12 2SY, UK Received 2 March 1999; received in revised form 14 October 1999; accepted 18 October 1999
Abstract
Observations of latent heat, sensible heat and momentum flux from eddy correlation equipment are presented, along with approximately weekly measurements of soil moisture content. The measurements were made over a pasture field in the south
of England for 3 years, covering two dry years, 1995 and 1996 and a more normal year, 1997. The data show that, overall, approximately 80 of the measured net radiation was used for evaporation. During July and August of the two dry years, soil
moisture deficits of over 150 mm developed within the top 1400 mm, and at these deficits the evaporation was reduced. During the summer months, overall between 71 and 76 of the net radiation was used for evaporation. During the winter months,
the latent heat flux exceeded the net incoming radiation. An energy closure better than 10 suggests that the measurements were well founded. The surrounding trees may, however, have influenced the momentum exchange; the estimated roughness
length of 38 mm is considerably more than the commonly accepted 10 of vegetation height.
This extensive dataset provides an excellent test for the current UK Meteorological Office Surface Energy Scheme MOSES when applied to this site, the results of which are presented here. In its unoptimised form, the model can explain 72 of the
variance of the hourly evaporative flux data, and with excellent agreement with the soil moisture measurements. Optimisation can improve the model fit, but only up to a maximum of 76. The largest remaining errors occur during mid-summer when
high temperatures, vapour pressure deficits and soil moisture deficits occur. ©2000 Published by Elsevier Science B.V. All rights reserved.
Keywords: Evaporation; Stomatal conductance; Soil moisture; Grassland; SVAT modelling
1. Introduction
One of the primary uses of Global Circulation Models GCMs is to predict global fields of near
surface temperature and humidity. The accuracy of such predictions depends critically upon correct pa-
rameterisations of the partitioning of net radiation
∗
Corresponding authors. Tel.: +44-01491-838800; fax: +44-01491-692424.
E-mail address: rjhceh.ac.uk R.J. Harding.
1
Co-corresponding author.
into thermal energy and latent heat fluxes at the land surface. Such surface fluxes influence the generation
of rainfall and the development of overall circulation patterns. Surface energy partitioning is highly depen-
dent upon stomatal opening, itself a function of local microclimate and soil status. Thus accurate descrip-
tions of both stomatal conductance and soil moisture evolution are essential.
The dependences of stomatal conductance on envi- ronmental conditions have been extensively described.
Jarvis 1976 derived functions to describe the ob- served variations of conductance in terms of soil leaf
0168-192300 – see front matter ©2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 9 9 0 0 1 5 2 - 5
310 R.J. Harding et al. Agricultural and Forest Meteorology 100 2000 309–322
water potential, vapour pressure deficit, temperature and photon flux density for several coniferous forests.
Based on the analysis of Jarvis, Stewart 1988 derived a similar description in terms of variables generated
by a meteorological model e.g. soil moisture, air tem- perature, vapour pressure deficit and solar radiation.
Similar formulations have been produced for conifer- ous forest Prince and Black, 1989; Dang et al., 1997,
for tropical rain forest Dolman et al., 1991; Wright et al., 1996, for savannah Huntingford et al., 1995 and
for grassland Stewart and Verma, 1992. Huntingford and Cox 1997 confirmed, through the use of uncon-
strained neural network fitting techniques applied to data from Thetford Forest, UK, that these functional
dependences are of the correct form to describe the variation of stomatal conductance.
The JarvisStewart functions are, however, empiri- cal descriptions of complex physiological responses.
Recently, more mechanistic, coupled models of pho- tosynthesis and transpiration e.g. Collatz et al., 1992
have been included within land surface schemes of GCMs. Typical of these is the MOSES Meteoro-
logical Office Surface Energy Scheme model of the UK Meteorological Office. The canopy conductance
sub-model within MOSES was developed using data from the Kansas grassland, FIFE sites, a C
4
grass, Cox et al., 1998. MOSES has also been tested suc-
cessfully for a savannah site within HAPEX-Sahel Blyth et al., 1999. This paper describes a similar
but more extensive testing and calibration of MOSES on a 3 year run of evaporation and soil moisture
data taken over a pasture grassland in the UK a C
3
grass. These observations have provided the op- portunity to compare the performance of the land
surface model in different, contrasting years and to check the applicability, and thus predictive ability, of
a calibration made with one year’s data in subsequent years.
2. Data
