沙漠研究 22-1, 303 -306 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－

Effect of Atmospheric Pressure on Evaporation in Central Ethiopia
Shinji SUZUKI*1), Masku DERESSA2), Atsushi SANADA3), Kenji NAKAMURA4),
Fumio WATANABE1) and Satoru TAKAHASHI1)
Abstract: The authors have implemented a meteorological observation in the Oromia State, central Ethiopia. The region islocated at the Great Rift Valley and classified as semi-arid area. Generally, the area is exposed to intense solar radiation, moderatewind, and low relative humidity resulting in high driving force of evaporation. Further, the authors suggest that low atmosphericpressure associated with high elevation would accelerate the evaporation ratio since water is vaporizable when the atmosphericpressure is reduced. The current study aims to monitor annual change in the atmospheric pressure and analyze the effect of it on
the evaporation ratio in central Ethiopia. The evaporation ratio was computed by the Bowen ratio energy balance method. The
atmospheric pressure in central Ethiopia was almost constant throughout a year ranging from 83.9 to 84.5 kPa independent of air
temperature and rainfall. Taking into account the actual atmospheric pressure measured at the experimental field, the evaporation
ratio computed by the Bowen ratio energy balance method was 4.5% larger than that calculated under the standard atmosphere.
Although this difference (4.5%) would seem small, this would cause considerable error (i.e. underestimation) when the amount ofevaporation is assessed cumulatively for longer period (ex. months or years). The results of the current study also suggest thathighlands in arid lands have a high potential to dry due to its low atmospheric pressure. This implies fragility of eco-systems inother similar environments and accounts in part for difficulty to reestablish forests in highlands of the arid lands once the forests arecleared.
Key Words: Heat budget, Highlands, Meteorology, Semi-arid region, Water resource

1. Introduction
Crop production systems in central Ethiopia are mostly
under rainfed condition. Although the mean amount ofannual rainfall is still much for crop growth (400 to 800 mm),the crop production systems often suffer from severe droughtstress resulting in unstable food security. Further, the forest
area in the country, which was reported as 40 to 65% in
hundreds years ago, decreased down to 2.2% over the past 60years due to consumption as a fuel and expansion of arable
land (Berry, 2003; EFAP, 1994; Hawando, 1997). Hence
unstable agricultural production in the region is exacerbated bythese severely degraded natural resources. The draught stressis in part due to erratic and fluctuated rainfall distribution
patterns in the region, while it is of great importance to clarify
reasons of causing draught in the region in order to consider
more suitable measures to mitigate degradation of natural
resources and improve crop productivity. The author’s
previous work (Suzuki et al., 2009a) reported that the area isexposed to intense solar radiation, moderate wind, and low
relative humidity resulting in high driving force of evaporationfrom soil surface. Further, the region is characterized in highaltitude (more than 1,500 m Alt.) indicating low atmosphericpressure. Depressions of the atmospheric pressure would
affect diffusion coefficient of gas including vapor. Thepsychrometric constant is a function of the atmosphericpressure (see eq. (5) in Materials and Methods), which is used
when an evaporation ratio is calculated by energy balancemethods. Therefore, the authors suggest that low atmosphericpressure associated with high elevation would accelerate theevaporation ratio. The current study aims to monitor annualchange in the atmospheric pressure and analyze the effect of iton the evaporation ratio in central Ethiopia after verifying aninstrument used.

2. Materials and Methods
The experimental field is located at Adami-tulu Agricultural
Research Center, Oromia region of central Ethiopia (N 7° 52’,E 38°43’, Alt. 1,650 m). The region is located in the Great
Rift Valley. Rainy season is during June to September, anddry season is other months. Soil texture of the experimental
field was Loamy Sand. The maximum air temperature
becomes over 30°C during Feb. to May and the minimum airtemperature decreases below 10°C during Dec. to Feb. with amean daily temperature through a year ranging from 18 to23°C. A mean annual solar radiation is considerably large* Corresponding Author: s4suzuki@nodai.ac.jp
Tokyo University of Agriculture, 1-1-1, Sakuragaoka, Setagaya, Tokyo, 156-8502, Japan.
1) Faculty of Regional Environment Science, Tokyo University of Agriculture, Japan

3) Faculty of International Agriculture and Food Studies, Tokyo University of Agriculture, Japan

2) Adami Tulu Agricultural Research Center, Oromia, Ethiopia.

4) NTC International, Co., Japan(22 MJ m-2 d-1) and a mean annual rainfall ranges from 600 to800 mm (Suzuki et al., 2009b). The experimental field is 70m × 70 m and the surface is flat with sparse shrub. Although
the experimental field is covered with local vegetation with aheight of around 0.3 m in rainy season, the vegetation withersand dies in dry season. In the center of the experimental field,a flat and bare observation site (10 m × 10 m) was established.
The atmospheric pressure was measured using a pressuregauge (U20-001-01, Onset Computer Co.) at the observationsite from Jan. 2009 to Jan. 2010 with 20 minute intervals. In
addition, air temperature at a height of 1.73 m above theground and rainfall were measured using a Temperature/RH
smart sensor (S-THB-M002, Onset Computer Co.) and a rain
gauge smart sensor (S-RGB-M0002, Onset Computer Co.),
respectively. Data of the air temperature and the rainfall wererecorded by the HOBO Weather Station (Onset Computer Co.)
with 10 minute intervals. The atmospheric pressure and theair temperature were averaged for 24 hours.
Dailysummations were derived for the rainfall.
Since the U20-001-01 was originally designed to measurewater level, a validity of the U20-001-01 to measure theatmospheric pressure was tested by comparison with two highprecision air pressure sensors, namely; a digital barometer(PTB210, VAISALA) and a handy manometer (PG-100-102VP, NIDEC Copal Electronics Co.). A comparison withthe PTB210 was conducted at a meteorological station
operated by the National Agricultural Research Center for
Hokkaido Region (N 42°53’, E 143°04’, Alt 93 m), located in
the central part of Tokachi District, Hokkaido, Japan. The
U20-001-01 was set close to the PTB210 and the data of theatmospheric pressure was recorded from Oct. 2009 to Jan.
2010 with 10 min intervals. The data were averaged for 24hours. Although the atmospheric pressure was suggested tobe lower than 90 kPa in central Ethiopia, the atmosphericpressure measured at the National Agricultural Research Centerfor Hokkaido Region ranged from 97 kPa to 101 kPa (seeResult and Discussion). Therefore the accuracy of theU20-001-01 to measure the atmospheric pressure lower than
95 kPa was tested at a laboratory of the Setagaya Campus ofTokyo University of Agriculture, Japan (N 35°38’，E 139°38’,Alt 57 m) using a system shown in Figure 1. By changing Hin Figure 1, decreased air pressure was established andmeasured by the U20-001-01 and the PG-100-102VP with 1min intervals.
In order to measure the evaporation ratio from bare soil
surface at the observation site of central Ethiopia, following
measurements were undertaken from 9th Dec. 2009 to 7th Jan.
2010, namely; a ground heat flux and a net radiation weremonitored using a heat-flux plate (HFT-1.1, REBS) installed ata depth of 0.02 m and a domeless net radiometer (NR-Lite,PG100102VP
U20-001-01

H
De-aired water

Fig. 1. Schematic diagram of the experiment to verify the accuracyof the water level logger (U20-001-01) by comparison with
PG-100-102VP.

Kipp & Zonen) at a height of 0.72 m, respectively (Suzuki etal., 2011). Both temperature and relative humidity weremonitored at two heights (0.30 m and 0.50 m above the groundsurface) using the S-THB-M002 and the HOBO Weather
Station with a solar radiation shield (RS3, Onset ComputerCo.). These data were recorded with 10 min intervals.
Daily summations were derived for the ground heat flux andnet radiation.
Daily averages were obtained for thetemperature and the relative humidity.
The evaporation ratio (E, mm d-1) was computed by theBowen ratio energy balance method as follows;
𝑅𝑛 − 𝐺𝐸=(1)𝑙(1 + 𝛽)𝜌𝑤
where Rn is the net radiation (J m-2 d-1), G is the ground heat
flux (J m-2 d-1), ρw is the density of water (=1,000 kg m-3), and𝑙 is the latent heat of water (J kg-1) given as;
𝑇1 + 𝑇2
𝑙 = 2.501 × 106 − 2.361 × 103 ∙ ��(2)
2
T1 and T2 are the temperatures (˚C) at each height. β is theBowen ratio which can be calculated as;(𝑇1 − 𝑇2 )𝛽=𝛾(3)(𝑒1 − 𝑒2 )where e1 and e2 are the vapor pressures (kPa) at each height andderived from (Campbell and Norman, 1998);
17.502𝑇𝑖 𝑅𝑅𝑖�
𝑒𝑖 = 0.611𝑒𝑒𝑒 �
𝑇𝑖 + 240.97 100
𝑖 = 1 and 2(4)where RHi is the relative humidity (%) at each height. γ is thepsychrometric constant (kPa K-1) and calculated as;
𝐶𝑝 𝑃(5)𝛾=0.622𝑙
where Cp is the heat capacity of air at constant pressure (J kg-1K-1) written by (Ham, 2005 p541);

Actual pressure (kPa)

105
100
9590
8580808590
95100
105
Measured by U20-001-01 (kPa)

Fig. 2. Comparison of the atmospheric pressures between datameasured by U20-001-01 and actual data measured by
PTB210 (open) and PG-100-102VP (closed). r 2 and RMSE
are coefficient determination and root mean square error,
respectively.

0.522(𝑒1 + 𝑒2 )+ 1�(6)
𝐶𝑝 = 1004.7 �2𝑃
P is atmospheric pressure (kPa). In many cases, γ is literallyconsidered as a constant (=0.066 kPa K-1) (Meyer, 1999;Dingman, 2002). This is because the γ is often dealt with
under the standard atmosphere (101.3 kPa and 15°C).
However, the γ is not strictly a constant, but a function of P.
In the current study, the evaporation ratio was calculated
substituting the atmospheric pressure measured at theobservation site for eq. (5) and compared with the evaporation
ratio calculated when P=101.3 kPa (i.e. standard atmosphericpressure).

3. Results and Discussion
The air pressure measured by the U20-001-01wasremarkably agreed with the data measured by both the PTB210
and the PG-100-102VP (Fig. 2). The coefficient ofdetermination (r2) and the root mean square error (RMSE)
were 0.9998 and 0.067 kPa, respectively (Fig. 2). The result
ensures validity of the U20-001-01 to measure the atmosphericpressure in central Ethiopia with noticeable precision.
The atmospheric pressure in central Ethiopia was almost
constant throughout a year of 2009 ranging from 83.9 kPa to84.5 kPa independent of air temperature and rainfall (Fig. 3).
Low atmospheric pressure observed in central Ethiopia is ofcourse resulting from high altitude of the region, however, it isof note that the atmospheric pressure in central Ethiopia isconsiderably smaller than the standard atmospheric pressure bymore than 15%. There were two rainfall events during theobservation period of the evaporation ratio (21 mm for 11th to12th Dec. 2009 and 18 mm for 29th to 30th in Dec. 2009) (Fig.
3). The evaporation ratio calculated in the current study
showed more than 4.0 mm d-1 after the two rainfall events, andimmediately decreased down to 2.5 mm d-1. This indicatesthat the evaporation ratio increases only when soil has muchwater after rainfall, while the evaporation ratio is impeded soon
due to decreases in the soil water content. A meanevaporation ratio during the observation period was 2.4 mm d-1(Fig. 4). Further, the evaporation ratio calculated using theatmospheric pressure measured at the observation site (actualevaporation ratio; Ea) was always higher than the evaporation
ratio calculated when P=101.3 kPa (EP=101.3 kPa). The Ea was
4.5% larger than the EP=101.3 kPa in average (Fig. 4). Although
the difference between Ea and EP=101.3 kPa (4.5%) would seemsmall, this would cause considerable error (i.e.
underestimation) when the amount of evaporation is assessedcumulatively for longer period (ex. months or years).

4. Conclusion
The current study quantitatively revealed how small theatmospheric pressure in central Ethiopia is compared with thestandard atmosphere. The results of the current study suggestthat assessments of the amount of evaporation in central
Ethiopia by the Bowen ratio energy balance method would
result in underestimation without taking into account thedepression of the actual atmospheric pressure. The results ofthe current study also suggest that highlands in arid lands havea high potential to dry due to its low atmospheric pressure.
This implies fragility of eco-systems in other similar
environments and accounts in part for difficulty to reestablish
forests in highlands of the arid lands once the forests arecleared.

Acknowledgement
Authors are grateful to Dr. Y. Iwata for his providing
opportunity of observation and data of the atmosphericpressure at Memuro. Appreciation is expressed to Dr.
Nishimaki and Messrs. K. Shiratori, Mr. I. Matsumoto and T.
Hotta for their encouragement and suggestions for the current
study. Messrs. M. Hattori, Y. Kobayashi, A. Kubo, Mses. K.
Iiduka, M. Sasaki, and students in the laboratory of Land andWater Engineering of the department of Bioproduction andEnvironment Engineering are also acknowledged for theirhelpful support to the research. The current study waspartially funded by the Research for Verification of the
Applicable Agricultural Techniques from the Japan
International Cooperation Agency (JICA) and the Research
Project of the Faculty of Regional Environment Science fromthe Tokyo University of Agriculture (TUA).

8483
82
25Rainfall (mm d-1)

6050204030

1520

Air temperature (℃)

Atmosphericpressure (kPa)

85100Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Date in 2009

Fig. 3. Changes in atmospheric pressure associated with air temperature and rainfall in central Ethiopia.
5.0

Evaporation rate (mm d-1)

4.5

Using actual atmospheric pressure
Using atmospheric pressure=101.3 kPa

4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
9-Dec

13-Dec 17-Dec 21-Dec 25-Dec 29-Dec
Date in 2009 to 2010

2-Jan6-Jan

Fig. 4. Changes in evaporation ratios calculated using standard atmospheric pressure (open) and atmospheric pressure measured at centralEthiopia (closed).

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