D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105 101
higher 8 than that for the Control High N treatment, whereas the WUE for the FACE Low N treatment was
again lower 6 than that for the FACE High N treat- ment. As in the first year, the effects on WUE due
to nitrogen level p=0.90 or due to CO
2
by nitrogen interaction p=0.25 were not highly significant.
4. Discussion
Soil water depletion measurements combined with FAO-56 basal crop coefficient procedures were used
to determine ET. It is important to note that the ET values derived for the various treatments in the study
have a degree of uncertainty due to potential errors made in measurements of soil water or in parameter es-
timates used in the crop coefficient procedures. Since soil water content measurement was the fundamental
measurement used in determining ET, it represented the most significant potential source of error. The esti-
mated accuracy of the water content measurement for the TDR system used in this study is ±2, accord-
ing to the manufacturer of the device. The estimated accuracy of the water content measured by properly
calibrated neutron moisture gauges has been often re- ported to be about ±2 Sinclair and Williams, 1979;
Haverkamp et al., 1984. The estimated accuracy of the neutron gauge used in this study ±2.4 was sim-
ilar to the reported accuracy. However, water content measurement may have resulted in ET accuracy less
than 2.4 because ET was primarily based on the dif- ference in the measured soil water between two suc-
cessive dates i.e. soil water depletion. Sinclair and Williams 1979 reported that the error associated with
the change in water content measured by a neutron gauge at a depth of 0.30 m was typically about 5 for
a properly calibrated gauge. In a more recent study, Evett et al. 1993 used a combination of TDR mea-
surements at the soil surface 0–0.40 m and neutron gauge measurements at deeper depths 0.4–2.0 m to
determine the change in water storage over a 16-day period for a winter wheat crop grown in precision
weighing lysimeters. Their results indicated excellent agreement 1 mm between the change in water stor-
age determined from soil water measurements and lysimeters. Since the wheat ET for the16-day period
was about 95 mm, the error in ET associated with the soil water measurement was less than 1. This error
was about one fifth of that realized when neutron scat- tering alone was used Evett et al., 1993, indicating
the great improvement in accuracy when the TDR is used to determine water content changes at the soil
surface. Since the ET in our study was determined us- ing the TDR and neutron scattering combination, we
estimate that the uncertainty in ET introduced during measurement of soil water depletion was no larger than
3. However, additional errors in the soil depletion determination of ET may have been introduced under
the assumption of zero water flux below the estimated crop rooting depth.
Any errors in ET determined during soil water de- pletion periods would also introduce errors in the sim-
ulated ET for soil wetting periods. Furthermore, the uncertainty in ET during soil wetting periods would
be expected to have been greater than 3, since crop, soil, and climatic parameters used in the crop coeffi-
cient evaluation could also introduce errors that might impact the resultant ET and soil evaporation values
during those periods. Hunsaker 1999 using the ap- proach presented in this study determined crop ET
for irrigated cotton in Arizona. In that 2-year study, comparison between predicted and measured ET dur-
ing soil wetting periods indicated that the error in ET based on the crop coefficient approach averaged about
9. Assuming a similar accuracy in ET during the soil wetting periods of the current study, we estimate
that the uncertainty in total ET for the various treat- ments in the study was probably greater than 5 but
less than 10.
Conceivably, real differences in the ET between CO
2
treatments could have been masked amid the potential errors noted above and therefore possibly
were not detected with the approach used in these studies. However, although differences in calculated
ET between CO
2
treatments were not detected, dif- ferences in ET between high and low soil nitrogen
treatments were large and statistically significant. It should also be pointed out that the total ET values of
601 and 595 mm in 1995–1996 and 1996–1997, re- spectively, that were obtained under ambient CO
2
and well-fertilized conditions were similar to the seasonal
wheat ET determined under those conditions in other studies in Arizona, e.g. 650 mm Erie et al., 1982 and
580–610 mm Hunsaker and Bucks, 1992.
When daily ET was accumulated for the entire sea- son, the total ET under High soil N was less for FACE,
102 D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105
about 3.7 and 4.0 less for the first and second years, respectively. However, during both seasons, the daily
cumulative ET for F–H typically lagged behind the cu- mulative ET for C–H by more than 4 until the final
2 weeks of the season. Thus it appears that the ET de- clined more rapidly for C–H than F–H during the end
of the season possibly due to the earlier senescence of the Control than FACE plants.
In two previous FACE wheat studies conducted dur- ing 1992–1993 and 1993–1994, treatments compara-
ble to the F–H and C–H were also imposed i.e. treat- ments under ample soil N and water. In the earlier
studies, the total cumulative wheat ET, determined us- ing a soil water balance approach Hunsaker et al.,
1996, was 4.5 and 5.8 less in FACE than Control in 1992–1993 and 1993–1994, respectively. Thus, in
all four studies with wheat, the total cumulative ET was decreased for FACE when water and nutrient in-
puts were adequate. In three of the four studies, the reduction was 4 or more.
Parallel investigations using a residual energy bal- ance approach to measure ET Kimball et al., 1995,
1999 in the same four FACE wheat studies were in general agreement with the soil water balance ET
results under well-watered and well-fertilized con- ditions. The energy balance approach determined a
4-year average reduction of 6.7 in wheat ET for the FACE treatment, although the reduction was less
than 6.0 in the 1995–1996 and 1996–1997 years. Considering the two independent investigations, it is
apparent that a modest reduction in the ET of wheat occurs under elevated CO
2
, when water and nitrogen are not limited.
In the present studies, the total ET under limited soil N was determined to be only 0.7 and 1.2 less for
FACE than for Control. However, seasonal trends re- vealed that the cumulative ET for F–L was 3–5 less
than that for C–L through late-March in both sea- sons. Following the final fertilizer application to the
Low N treatment in late-March of both years, the ET rate for F–L was increased relative to C–L through the
remainder of the seasons. Because the CO
2
enrich- ment effect produced larger and more vigorous plants
than those in the Control treatment, the FACE wheat plants under Low N may have been able to more fully
exploit the available nutrients during the late season, and thus consume more water than the Control dur-
ing that period. Although the 25 fA
PAR∗
data did not indicate an earlier crop senescence for the Control
than FACE under Low N, differences in senescence between Control and FACE may also have contributed
to the ET differences observed during the final weeks of the season.
The energy balance determinations of ET for the Low N treatments by Kimball et al. 1999 indicated
a 19.5 average reduction in ET for FACE for the 1995–1996 and 1996–1997 seasons, which was in con-
trast to the ≈1 cumulative reduction in ET deter- mined from the soil water depletion procedures in the
two seasons. A possible explanation for the difference in magnitude of the FACE ET reduction between the
two approaches was that the energy balance technique was used to determine the relative differences in ET
between the treatments but not the absolute cumulative values. Also, as discussed by Kimball et al. 1999,
there were differences in leaf area, canopy architec- ture, and leaf greenness between the CO
2
treatments, particularly under the Low N treatments. Thus, it is
conceivable that the lower leaf area in the Control plots under Low N resulted in the infrared thermometers to
have viewed warmer soil temperatures rather than leaf temperatures. Consequently, the sensible heat compo-
nent of the energy balance may have been overesti- mated in the Control Low N plots, which would result
in ET estimates that were too low. Similarly, if canopy coverage was overestimated for the Control Low N
plots, the ET determined from the soil water balance procedures may have also been too low. In this case,
the soil water evaporation component of ET for the Control Low N treatment would have been underes-
timated following irrigation and large rains resulting in underestimated total ET during the soil wetting pe-
riods. On the other hand, as indicated above, the soil water approach was subject to a possible accumula-
tion of errors that may have been more exaggerated under Low N than High N conditions.
Based on the total ET determined from the soil wa- ter balance procedures, the FACE treatment attained
water use efficiencies that were 19 and 23 greater, respectively, than Control under the ample soil N
conditions in 1995–1996 and 1996–1997. These in- creases in WUE for FACE were consistent with the
13 and 18 higher WUE attained for FACE under the well-watered and well-fertilized treatments in
1992–1993 and 1993–1994, respectively Hunsaker et al., 1996. In contrast, the 12 and 7 increases
D.J. Hunsaker et al. Agricultural and Forest Meteorology 104 2000 85–105 103
in WUE for FACE under the Low N treatment for the 1995–1996 and 1996–1997 seasons, respectively,
were considerably lower than the 15 and 24 in- creases in WUE for FACE under limited water con-
ditions in the 1992–1993 and 1993–1994 studies, respectively. These results suggest that the benefits of
higher CO
2
on the yield and water use efficiencies of wheat, which appear to be increased under water
stress, were greatly diminished when limited nitrogen supplies were encountered.
5. Conclusions
