latitudinal and seasonal variation in calculated ultraviolet-b irradiance for rice-growing regions...

Phorochembtry and Photobiology Vol. 54, No. 3, pp. 411-422, 1991 Printed in Great Britain. All rights reserved

0031-8655191 $03.00+0.00 Copyright 0 1991 Pergamon Press plc

LATITUDINAL AND SEASONAL VARIATION IN CALCULATED ULTRAVIOLET-B IRRADIANCE FOR

RICE-GROWING REGIONS OF ASIA* D. BACHELET't, P. w. BARNES', D. BROWN^ and M. BROWN^

'ManTech Environmental Technology, Inc., US EPA Environmental Research Laboratory, 200 SW 35th Street, Corvallis OR 97333, USA and ZCollege of Oceanography, Oregon State

University, Corvallis OR 97331, USA

(Received 21 December 1990; accepted 5 March 1991)

Abstract-Ultraviolet-B (UV-B, 280-320 nm) irradiance was calculated for more than 1200 sites in Asia to characterize the spatial and temporal variation in the present UV-B climate for rice-growing regions. The analytical model of Green el al. (Phorochem. Photobiol. 31, 59-65, 1980) was used to compute UV-B irradiance for clear skies using satellite-observed ozone column thickness and local elevation data. Ground-based observations of cloud cover were then used to approximate the average effect of cloud cover on UV-B irradiance using the approach of Johnson et al. (Phorochem. Photobiol. 23, 179-188, 1976). Over the geographic range of rice cultivation, the maximum daily effective UV- B irradiance (UV-B,,), when weighted according to a general plant action spectrum, was found to vary approx. 2.5-fold under both clear and cloudy sky conditions. Under clear skies, the timing of maximum solar UV-B,, changed with latitude and varied from February-March near the equator to July-August at temperate locations. Cloud cover was found to alter the season of maximum UV-B,, in many tropical regions, due to the pronounced monsoonal climate, but had little effect on UV-B seasonality at higher latitudes. Under a climate resulting from a doubling of atmospheric carbon dioxide, estimated UV-B using predicted cloud cover was found to change by up to 17% from present conditions in Thailand. Both latitudinal and seasonal variation in solar UV-B radiation may be important aspects of the UV-B climate for rice as cultivars differ in sensitivity to UV-B and are grown under diverse conditions and locations.

INTRODUCTION

Rice (Oryza sativa L.) is the primary food crop for over 50% of the world's population (van Keulen, 1977) and is grown around the globe under a diver- sity of climates. Although rice originated in tropical regions with a strong monsoonal climate (Chang, 1976; Yoshida, 1977), present-day rice is grown from Uruguay, South America, at 3.5" S latitude, to northeastern China, at 50" N, and from below sea level in Kerala, India, to above 2000 m elevation in Kashmir (Huke, 1976; Yoshida, 1981). Over this

*The research described in this article has been funded by the U.S. Environmental Protection Agency (EPA). This document has been prepared at the EPA's Environmen- tal Research Laboratory in Corvallis, Oregon, through contract #68-C8-0006 to ManTech Environmental Technology, Inc. It has been subjected to the Agency's peer and administrative review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

tTo whom correspondence should be addressed. $Abbreviations: C, cloud cover; CDIAC, Carbon Dioxide

Information Analysis Center; GCM, general circulation model; GIs, geographic information system; GISS, Goddard Institute of Space Studies; GFDL, Geophysi- cal Fluid Dynamics Laboratory; NCAR, National Center for Atmospheric Research; OSU, Oregon State University; TOMS, total ozone mapping spectrometer; UV-B, ultraviolet-B radiation (280-320 nm); UV-B,,, biologically effective ultraviolet-B irradiance.

range, climatic variation is extreme and a number of investigators have related regional rice production to spatial and temporal variation in tem- perature, rainfall, insolation and growing season length (Matsushima, 1970; van Ittersum, 1972; Mur- ata, 1975; van Keulen, 1977; Angus and Zandstra, 1980; Yao and Le Duc, 1980; Agrawal and Jain, 1982; Terjung et a l . , 1984).

Because of the extensive geographic range of rice, large differences in solar ultraviolet-B radiation (UV-B, 28C-320 nm)$ potentially exist between rice-growing regions. Over the globe, surface UV- B irradiance varies with latitude, elevation and season; at high-elevation equatorial locations, the total daily effective UV-B radiation can be 7-fold higher than at low elevation high-latitude sites (Caldwell et af., 1980). Ultraviolet-B radiation is known to influence plant growth and development (Caldwell 1981) and UV-B-sensitivity varies both within and among species (Van et al . , 1976; Teramura and Murali 1986). Plants native to high latitudes are often more sensitive to UV-B injury than those originating from lower latitudes where solar UV-B irradiance is higher (Caldwell et a l . , 1982; Barnes et al . , 1987). Rice includes more than 80,000 different cultivars, and cultivars have been found to vary in their sensitivity to UV-B radiation (Coronel et al . , 1980; P . Barnes, V. Coronel, Q. Dai, S. Maggard and B. Vergara, unpublished). Recent evidence of a general global decline in stratospheric ozone

411

412 D. BACHELET et al.

(NASA, 1988; Watson et al . , 1990) has heightened concern over the potential consequences of the resultant increased UB-V irradiation for higher plants, including rice (Caldwell et al . , 1989; Tevini and Teramura, 1989). However, under anticipated ozone depletion, the change in UV-B at a particular location would still not surpass the differences in UV-B which presently exist over the globe (Madronich et al . , 1989). Characterization of the solar UV-B gradient over which rice is presently grown may aid in interpreting the responses of indi- vidual cultivars to UV-B under current and future conditions.

Cloud cover can modify the solar ultraviolet climate (Bener, 1964; Frederick et al . , 1989) and changes in cloudiness could affect the average UV- B irradiance at a given location more than ozone depletion (Frederick and Lubin, 1988). In many tropical locations, rice is grown year-round with each crop experiencing distinctly different solar radiation regimes due to seasonal differences in cloud cover (Yoshida, 1977). Under these conditions, cloud cover might also be expected to alter the seasonality of UV-B radiation. The time of year when solar UV-B is at a maximum could be an important aspect of the UV-B climate for rice because the different developmental stages of plants may vary in their sensitivity to UV-B (Teramura and Sullivan, 1987).

CLouds influence solar UV radiation in complex ways and have been found to either increase or decrease UV irradiance depending on whether they obscure the sun or not (Bener, 1964; Caldwell, 1968; Frederick and Lubin, 1988; McCormick and Suehrcke 1990). Efforts have been made to simulate instantaneous solar UV irradiance under complete and uniform cloud cover (e.g. Frederick and Lubin, 1988), but it has proven much more difficult to predict UV irradiance under partial cloud cover. For some situations, it may be appropriate to use climatological approaches which approximate the effect of long-term average cloud cover on inte- grated daily or monthly UV radiation (e.g. Freder- ick et al., 1989). Indeed, several investigators have developed simple correction factors to account for the effect of average cloud cover amounts on effective UV radiation (e.g. Bjorn, 1989, and references cited therein) and estimates from this approach have been found to compare well with ground-based measurements in tropical locations (Ilyas, 1987).

The overall objective of this study is to characterize the solar UV-B climate that may presently exist for rice-growing regions of Asia, where over 90% of the world's rice is produced. Specifically, we wish to describe the potential gradient in solar UV-B over which rice is grown and evaluate whether seasonal patterns of cloud cover might alter UV-B with respect to clear-sky conditions. In contrast to more developed regions (e.g. Scotto et al., 1988), a ground-based monitoring network for solar

UV radiation does not exist in Asia. Thus, we calculate the theoretical UV-B irradiance for clear skies with an existing solar UV-B model (Green et al., 1980) for over 1200 meteorological stations located throughout Asia, and approximate the average effect of clouds using the approach of Johnson et al. (1976) for long-term estimates of cloud cover. In addition, we evaluate whether projected cloud cover under global climate resulting from elevated atmospheric carbon dioxide might alter seasonal UV-B patterns in Thailand, the largest rice exporter in the world.

MATERIALS AND METHODS

Geographic extent of the study. The region studied extends from 20" E to 140" E longitude and from 20" S to 60" N latitude and includes all or part of the following countries: China, Hong Kong, Taiwan, Korea, Japan, Kampuchea (Cambodia), Vietnam, Laos, Myanmar (Burma), Thailand, Malaysia, Indonesia, Brunei Darussa- lam, Singapore, The Philippines, Nepal, India, Bangla- desh, Pakistan, Afghanistan, Bhutan, Sri Lanka, and The Maldives (Fig. 1). Rice is grown throughout this region, but production is limited by climatic conditions in many locations. According to Huke (1976), the core area for rice production (Region 1) includes locations where climatic conditions are favorable for rice growth such that crop failures are minimal and, when they do occur, they are generally due to insect pests and plant diseases. A tran- sitional area (Region 2) includes locations where one climatic factor (often rainfall) can be insufficient to support a rice crop but high yields can still be obtained given proper management (e.g. irrigation). A third region (Region 3) includes areas where unfavorable climatic conditions often reduce rice yields and production depends heavily on management practices. No appreciable rice production occurs beyond Region 3. For this study, we were interested in characterizing the UV-B climate both within and among these climatic regions. Thus, UV-B irradiance was calculated for over 12.00 sites located throughout Asia (833 US Air Force airfield meteorological stations and 441 National Center for Atmospheric Research (NCAR) weather record stations). Data from these stations were used to construct regional maps of UV-B (see below) and selected locations were then chosen to illustrate representative climatic and UV-B conditions for each agroclimatic region. For the studies examining cloud cover under a doubling of atmospheric CO,, UV- B irradiance was calculated for representative mainland (Chiang Mai) and peninsular (Songkla) sites in Thailand (Fig. 1).

Calculations of ultraviolet-B. Surface solar irradiance under clear sky conditions was calculated at 2 nm intervals over the UV-B waveband using the empirical model of Green et al. (1980). The algorithms in this model have been widely used by biologists to estimate UV-B spectral irradiance for terrestrial (e.g. Caldwell et al . , 1980; Bjorn and Murphy, 1985) and marine environments (e.g. Baker et al., 1980). In comparison to detailed radiative transfer models (e.g. Frederick and Lubin, 1988), input parameters for the Green model are few (elevation, latitude, time of year, surface albedo, column ozone thickness and relative aerosol conditions) and are readily obtained or approximated. For all our calculations, we asssumed aerosols characteristic of rural environments (aerosol coef- ficient = 2; Green, 1983) and of average height distribution (Model C; Green ei al., 1980). A UV albedo of 3% was assumed, which is representative of green vegetated surfaces (Green, 1983). Elevation and latitude data were obtained for each reporting station and monthly ozone

Ultraviolet-B irradiance in Asia 413

REGION 3

Figure 1. Location of meteorological stations (solid circles) used for the calculation of UV-B radiation and boundaries of the three major climatic regions for rice (adapted from Huke (1976); see text for

details). Within each region, representative cities used in Fig. 3 are depicted with triangles.

thickness was obtained from satellite measurements (see below).

The calculated UV-B spectral irradiance was weighted for biological effectiveness (normalized to unity at 300 nm) using a general plant action spectrum [Caldwell, 1971, as formulated by Green et al. (1974) and described in Caldwell et al., 19831. This action spectrum has a slope intermediate between Setlow (1974)'s average action spectrum for biological effects involving DNA and the Robertson-Berger meter spectral response curve (Green and Miller, 1975), and is widely used to approximate the general effectiveness of UV-B on plants (Coohill, 1989). Effective UV-B irradiance was computed every 30 min and then summed over a day to obtain the daily biologically effective UV-B irradiance (UV-B,,). Calculations were made for a single day in the middle of each month of the year.

We approximated the average effect of clouds on daily UV-B,, using the approach of Biittner (1938) as described in Johnson et al. (1976). This approach estimates UV-B irradiance under average cloud conditions (F,) as a func- tion of clear sky UV-B irradiance (F,) and the number of tenths of the sky covered by clouds (C) and has been successfully applied by Ilyas (1987) when comparing predicted and measured broad-band UV in tropical Asia:

FJF,, = 1 - 0.056C

Databases and database management. Satellite instru- ments have provided continuous global records of total column ozone since 1978 and have been normalized by comparison with nearly coincident ground-based Dobson measurements in the Northern hemisphere (NASA, 1988). Ozone data \Version 5 Total Ozone Mapping Spec- trometer (TOMS)] for 1982 were chosen for UV calcu-

lations because satellite and ground-based measurements for that year have been found to differ by less than 1% (P. Newman, NASA-Goddard, personal communica- tion).

Monthly cloud cover estimates were obtained from ground level observations averaged between 1971 and 1981 (Hahn et al., 1988). Data manipulations that generated these data are extensively documented in Hahn et al. (1988) and Warren et al. (1986)'s atlas of the global distribution of cloud cover over land. In general, present GCM simulated the observed cloudiness rather poorly (Hahn et al., 1988). However, they indicate that future climate change will include changes in cloudiness which could affect UV-B irradiance. We used the calculated average relative cloud cover obtained for climate conditions under both low [300 ppm in Geophysical Fluid Dynamics Lab- oratory (GFDL), 315 ppm in Goddard Institute of Space Studies (GISS) and 326 ppm in Oregon State University (OSU)] and doubled atmospheric CO, concentrations from 3 General Circulations Models (GCM): OSU (Schlesinger and Zhao, 1989), GFDL (Wetherald and Manabe, 1986) and GISS (Hansen et al., 1984). Rather than focusing on the actual prediction of future cloud cover, we were interested in the agreement between the models on the direction of the change in cloud cover and in the potential magnitude between current and doubled CO, conditions and the consequent change in UV-B climate.

Observed cloud cover (daily averages of observations made every 3 h) was obtained from CDIAC (9-track magnetic tape) as a data grid with a resolution of 5" latitude X 5" longitude (Warren et al., 1986), whereas GCM predicted cloud cover was obtained from NCAR (9-track magnetic tape) as grids of various resolutions (4.44" x 7.5"


for the GFDL, 7.83" X 10.0" for the GISS and 4" x 5" for the OSU model). The TOMS ozone data were obtained from NASA as a 2.5" latitude X 5" longitude grid. All data were entered and processed in a Geographic Information System (GIS) environment (ARCIINFO software); by overlaying the various grids onto the map of Asia using the GIS software, each meteorological station was associated with an array of input data to be used by the model (i.e. latitude, elevation, monthly ozone thickness, observed monthly cloud cover, and predicted monthly cloud cover). Spatial interpolation was then used within ARC/INFO to create maps of simulated UV-B distribution over Asia.

RESULTS

Latitudinal and elevational trends in ultraviolet-B irradiance

The calculated annual maximum daily effective UV-B irradiance (UV-BBE) over Asia varied both with latitude and elevation [Figs. 2(a) and (b)]. Under clear sky conditions, UV-BBE generally decreased with increasing latitude [Fig. 2(a)] and ranged from 11 kJ m-* d-' at potential rice-growing locations (below 2000 m elevation) near the equator (e.g. Djakarta in Fig. 3) to near 4 kJ m-2 d-l at locations of similar elevation in northern Japan and China (e.g. Sapporo in Fig. 3). The highest UV-BBE levels (up to 13.5 kJ m-2 d-2) were evident in mountainous areas in New Guinea, central Asia (the Himalayas and the Tibetan Plateau), Afghanistan and the southern tip of India, though rice is likely not grown in these locations.

Under conditions of average cloud cover, the relative latitudinal gradient in UV-BBE was similar to that under clear skies, though the absolute differences in UV-BsE were less [Fig. 2(b)]; maximum UV-BBE varied from 3 to 7 kJ m-2 d-l at lower- elevation sites over Asia with the highest levels (ca 8.5 kJ m-2 d-l) evident in the higher elevation regions described above. In general, maximum UV- BBE varied more among locations in climatic regions 2 and 3 than in the core region (Region 1) (Figs. 1 and 3). In region 1, maximum UV-BBE ranged from 9 to 10 kJ m-2d-' under cloudless conditions and 6-7 kJ m-* d-' for conditions of average cloud cover. Maximum UV-BBE under clear sky and cloudy conditions ranged from 7 to 11 and 4 to 7 kJ m-2 d-l, respectively, for Region 2, and from 6 to 9 and 3 to 8 kJ m-2d-1, respectively, for Region 3 (Table 1).

Seasonal patterns in ultraviolet-B irradiance

Both the annual variation in UV-BBE and the time of the year of maximum UV-BBE varied with latitude (Figs. 3 and 4). Annual variation in UV- BBE under clear skies was greater in temperate regions than in the tropics, due primarily to larger annual variation in column ozone thickness and solar zenith angles (Fig. 3). For all locations, clouds

tended to dampen the seasonal changes in UV-BBE (Table 1).

Equatorial sites (e.g. Songkla and Djakarta) exhi- bited distinct bimodal patterns in UV-BBE over the year in contrast to the single seasonal peak at higher latitudes (e.g. Calcutta and Seoul) (Fig. 3). For clear-sky conditions, the month of maximum UV- BBE generally occurred later in the year the more northern the location; peaks in UV-BBE occurred early in the year (February and March) for locations south of the Equator and then shifted from March to August over a relatively narrow latitudinal band near the Tropic of Cancer (Fig. 4). Maximum UV- BBE at higher latitude locations (Region 3) occurred in July and August.

Cloud cover shifted the time of year when maximum UV-BBE occurred in most tropical locations but had less of an effect on the seasonality of UV-B in temperate zones (Figs. 3 and 4). The most striking changes were in the southernmost locations (e.g. Sumatra and New Guinea), where UV-BBE maxima were shifted from February- March to September-October, and in India and southeast Asia, where there was a shift in seasonal peaks from July-August to March-May (Fig. 4).

Influence of climate change on cloud cover and ultraviolet-B irradiance in Thailand

Average cloud cover for northern (Chiang Mai) and southern (Songkla) locations in Thailand under present-day climatic conditions (1 x C02) was best represented by the GFDL GCM with substantially lower cloud cover predicted by the OSU GCM [Figs. 5(a) and (b)]. Estimated UV-BBE levels were close to those generated using observed cloud cover data when GFDL-predicted cloud cover data were used (minima on average 6% higher and maxima on average 7% higher), but substantially higher when using cloud cover predicted by the OSU GCM (minima on average 22% higher and maxima on average 31 YO higher). Seasonal patterns of cloud cover and UV-BBE were represented equally well by all GCM except the GISS GCM which missed the seasonality in both stations [Figs. 5(a) and (b)].

The OSU GCM predicted substantial increases in average cloud cover under conditions of doubled atmospheric COz [Figs. 5(a) and (b)]. This increase in cloud cover was predicted to be highest during winter months at lower latitudes (up to 60% in Chiang Mai and up to 40% in Songkla), and resulted in a decrease in the estimated maximum UV-BBE by up to 10% in Chiang Mai and 5% in Songkla. The largest predicted change in maximum UV-BBE (+ 17% in Chiang Mai and 13% in Songkla) was calculated using GFDL predicted cloud cover assuming a doubling of C02. Both GISS and GFDL predicted fluctations in the seasonal patterns in cloud cover and, thus, in the amount of seasonality


Table 1. Maximum daily effective UV-B irradiance under cloudless skies and average cloud conditions, and month of the year of its occurrence, in nine representative meteorological stations (see Fig. 1 for

location) in the 3 major rice growing regions of Asia

Under cloudless skies Under average cloud conditions

City

Peak UV-B Time of Peak UV-B Time of dose Year dose Year

Country (J m-z d-' 1 (3 m-2 d-I )

Region 1 Songkla 7.10" N, Thailand Phnom Penh 11.55" N, Cambodia Calcutta 22.40" N, India

Region 2 Djakarta 6.11" S, Indonesia Quezon City 14.40" N, Philippines Seoul 37.34" N, Korea

Region 3 Karachi 24.54" N, Pakistan Quiemo 38.10" N, China Sapporo 43.03" N, Japan

10020 9420 9311

11573 9365 6697

8921 9005 5661

March April July

February August July

July July August

6170 645 1 6414

5974 6441 4243

7583 6478 3410

March March May

March April August

May July August

of UV-BsE (from 12% to -10% in Songkla), especially at lower latitudes (Figs. 5(a) and (b)].

DISCUSSION

Agroclimatic classifications describe existing regional climatic conditions in relation to the environmental requirements of crops. For rice, climatic classification have been developed for parts of south and southeast Asia (Oldeman 1975, 1977; Oldeman et al . , 1979; Manalo 1977) and Asia at large (Huke, 1976, 1982), though the UV-B climate has not previously been considered in any of these classifications. While UV-B radiation represents a minor component (< 1%) of the total solar shortwave radiant energy which reaches the earth, this radiation can have important photobiological effects on plants (Caldwell, 1981). Ultraviolet-B radiation, at levels that are similar to or less than those in the tropics, has been found to affect growth (Becwar et al., 1982), photosynthesis (Tevini et al., 1989), pollen germination (Flint and Caldwell, 1984) and interspecific competition (Barnes ef al . , 1988) in some terrestrial plants. Enhanced levels of UV-B have been found to alter growth (P. Barnes and B. Vergara, unpublished) and reduce photosynthesis (Van et al . , 1976) in rice under laboratory conditions, but it is unknown if growth and productivity of rice are affected by either ambient or enhanced UV-B radiation under field conditions.

Our results indicate that maximum daily effective UV-B irradiances (UV-BBE) can differ by up to 2.5-fold over the geographic range of rice in Asia (Fig. 2 ) . This gradient is primarily due to latitudinal differences in stratospheric ozone, the principal atmospheric constituent which attenuates UV radiation, and prevailing solar angles, affecting the

extent to which solar radiation is filtered by the atmosphere (Caldwell et al., 1980). Solar UV-BBE increases with elevation, but this change is second- ary in importance to the latitudinal effects (Caldwell et a l . , 1980). Over this latitudinal gradient the shorter wavelengths of UV-B undergo the greatest relative change. Thus, the steepness of this gradient is heavily dependent upon which action spectrum is used to calculate the effective UV-B radiation. If shorter wavelengths are more effective, such that the action spectrum exhibits a steeper slope, the gradient in effective UV-B would be larger than that reported here. This gradient would, however, be essentially non-existent if a relatively flat action spectrum were used (Caldwell et al., 1986). While action spectra specific for rice have not been developed, it does appear that a relatively steep action spectrum, such as Caldwell's (1971) gen- eralized plant action spectrum, may be a good approximation for higher plants (Caldwell et al . , 1986).

Climatic conditions which are most favorable for rice production are found in south and southeast Asia (Regions 1 and 2 in Fig. 1). Many locations throughout these regions experience a strong monsoonal climate with distinct wet and dry seasons (e.g. Calcutta in Fig. 3). While potential UV-BBE in these locations is calculated to be uniformly high throughout much of the year, the actual UV-BBE may be considerably lower due to extended periods of high cloud cover, especially during the monsoons (Fig. 3). At Los Banos, The Philippines, the measured total shortwave radiation during the wet (monsoon) season is, on average, about 50% of maximum values observed in the dry season (Angus and Manalo, 1979). For Quezon City, The Philip-


Region 1

SONOKLA (Thailand) 7.10 N, 4m PHNOM PENH (Kampuchea) 11.55 N. 12m

MONTH

CALCUTTA (India) 22.40 N, 10m

Region 3

KARACHI (Pakistan) 24.54 N, 3m OEM0 (China) 38.10 N. 1248111 SAPPORO (Japan) 43.03 N. 17m

MONTH MONTH MONTH

-€+ UV-BBE(clear sky) * UV-BBE(clouds)

0 Ozone thickness - Cloud Cover

Figure 3. Monthly average ozone thickness (atm. cm), cloud cover and daily effective UV-B irradiance (UV-BBF, kJ m-* d-') under cloudless skies and average cloud conditions, for 9 meteorological stations in Asia representative of the three rice climatic regions described by Huke (1976) (see Fig. 1).

(RCC, relative cloud cover; OT, ozone thickness.)

pines, we estimate that the average cloud cover in September (80%) can reduce daily UV-BsE up to 46% relative to cloud-free conditions and 36% with respect to the maximum daily UV-BsE under cloudy conditions during the dry season (Fig. 3). Under similar cloud cover (83%) in Malaysia, Ilyas (1987) reports a 100% reduction in both calculated and measured UV radiation (295-390 nm) relative to clear skies. Thus, throughout much of south and southeast Asia, it seems likely that clouds might substantially reduce the average solar UV-B for much of the year. As a result, rice crops that are grown at different times of the year could potentially experience appreciable differences in UV-B exposure. For example, at Quezon City, The Philip- pines, the effective UV-B would be 22% higher for

the dry season crop (January-June) than for the wet season crop (June-December) (Fig. 3). However, under partial cloud cover, rice could still experience brief, but intense, periods of UV-B even during the wet season should clouds not obscure the sun. Moreover, periods of relatively high UV-B would also likely occur at different stages of plant development for wet and dry season crops. Should growth stages of rice differ in sensitivity to UV-B (e.g. Teramura and Sullivan 1987), the relative effectiveness of similar, short-term UV-B exposures might well be different for the wet and dry season crops.

Relatively few measurements of surface UV radiation have been reported for tropical Asia (Ishiki et al., 1980; Ilyas and Barton, 1983; Srivastava 1984; Srivastava et al., 1984, 1989; Kumar et al., 1989),

Ultraviolet-B irradiance in Asia

Figure 2. Maximum daily effective UV-B irradiance (UV-B,,, kJ m-* d-I) over Asia under cloudless skies (a) and average cloud conditions (b).

417

418 D. BACHELET e/ a/.

Figure 4. Month of maximum daily effective UV-B irradiance over Asia under cloudless skies (a) and average cloud cover conditions (b).

Ultraviolet-B irradiance in Asia

( A ) CHIANQ MA1 1 X C02 CHIANG MA1 1 x C02

419

UV-B BE (KJ m-2 d-1) 10

( 1 1

8 - + + + +

Cloud Cover 100,

t I + + + 2 ~ ' ' ' ' ' ' ' ' " ' 0 ~ ' ' " ' " ' ' ' J

1 2 3 4 6 0 7 0 0 10 11 12 1 2 3 4 6 0 7 0 0 10 11 12 MONTHS MONTHS

-CDIAC + OSU -*- QFDL -QIaB CHIANG MA1 2 x C02

Percent change In UV-B BE 20

16 (3) ;F

-16 ' " " " " " 1 1 2 3 4 6 8 7 8 0 x) 11 12

MONTHS

+ OSU - * -QFDL -CJ-QISS

( B ) SONQKLA 1 x C02

UV-B BE (KJ m-2 d-1) 10

(1 ) + c

-CDIAC + OSU .* QFDL -GIs6 2 x c 0 2

Percent Change in Cloud Cover 00 t (4 )

1 2 3 4 6 I 7 0 0 x) 11 12

MONTHS

+ OSU -*-QFDL -CJ-QlSS

SONGKLA 1 x C02

Cloud Cover 100 1

+ I 1 2 3 4 6 e 7 I 0 10 11 12

MONTHS

-CDlAC + OSU -*-QFDL -QIS8

2 x c 0 2

1 2 3 4 6 I 7 3 0 10 11 12 MONTHS

-CDlAC + OSU -* .QFDL *QlS0

2 x c 0 2

Percent Change in Cloud Cover 00

40 I Percent Change In UV-B BE

20 ( 4 )

- 1 6 1

-1

1 2 3 4 6 6 7 0 0 10 11 12

MONTHS

+ OSU -*-QFDL +QlSS

- 4 0 L i -60 1 2 3 4 6 I 7 0 0 10 11 12

MONTHS

+ OSU -*- QFDL +QlS3

Figure 5. Calculated daily effective UV-B irradiancc (UV;B,,, kJ m-'d-') in (A) Chiang Mai (18.47" N ) and (b) Songkla (7. lo" N) , Thailand, for observed (CDIAC, Carbon Dioxide Information Analysis Center) and predicted (GCM-general circulation model) cloud cover. Three GCM outputs were used for this analysis: OSU, GISS and GFDL. (AliBl): Daily effective UV-B irradiance calculated using observed (CDIAC) and predicted (OSU, GISS, GFDL) cloud cover assuming current atmospheric C 0 2 concentration. (AZIBZ): Percent changc from current atmospheric C 0 2 conditions in daily effective UV-B irradiance Calculated using predicted (OSU, GISS, GFDL) cloud cover assuming an instantaneous doubling of atmospheric C02 concentration. (A3/B3): Observed (CDIAC) and predicted (OSU, GFDL, GISS) cloud cover assuming current atmospheric CO? concentration. (A4iB4): Percent changc in predicted cloud cover (OSU, GFDL, GISS) assuming

an instantaneous doubling of atmospheric CQ concentration.


and differences in measurement techniques, spectral sensitivity of sensors, and time-course of data acqui- sition preclude direct comparisons of these observations with our calculated data. Ultraviolet-B spectral irradiances calculated with the Green et al. (1980) model have been found to differ slightly from instantaneous measurements at some temperate locations (Bjorn and Murphy, 1985; Rundel, 1986; S . Flint, Utah State University, personal com- munication), though simultaneous measures of atmospheric ozone column thickness were not obtained. We would expect that our calculated UV- B irradiances may not correspond exactly to ground- based measurements due to short-term variation in atmospheric turbidity, air pollution, humidity and ozone thickness; however, on average, the effective UV-B irradiances which we report here should, at least in relative terms, adequately describe the range of UV-B conditions which could be expected over rice-growing regions of Asia. Indeed, for five sites spanning a range of latitudes (0"-70" N) and ele- vations (15-4400 m above sea level), Caldwell et al. (1980) found that measured seasonal maximum daily effective UV-B and that calculated with the Green et al. (1980) model varied by less than 10%.

The method that we used to approximate the effects of cloud cover on UV radiation employs only observed cloud cover and does not account for differences in cloud type, optical thickness or position in the sky. Also, the cloud cover estimates that we used were daily averages, and thus, our analysis does not account for any consistent variation in cloudiness which might occur during the day. Because of these deficiencies, substantial uncertainty is associated with the estimates of the daily effective UV-B which we report here under cloud-cover conditions. However, in a study similar to ours, Ilyas (1987) also used the approach of Johnson et al. (1976) and found that the calculated and measured UV radiation for Penang, Malaysia differed by less than 7% when averaged over the year. Good agreement in the seasonal dynamics of UV radiation in relation to cloud cover was also found between calculated and measured values. Thus, despite their simplicity, it does seem that climatological approaches, such as the one used here and elsewhere (e.g. Frederick et al. , 1989), may be useful in characterizing the long-term, average effects of cloud cover on the solar UV-B climate.

Continued depletion of stratospheric ozone (Solomon, 1990) will potentially result in an increase in UV-B at the earth's surface (Lubin et al. , 1989) but these increases could be countered if changes in cloud cover also occurred (Frederick and Lubin, 1988). In the present study, results from three different GCM suggest that cloud cover under future, elevated atmospheric COz concentrations may change appreciably from current conditions for locations in Thailand (Fig. 5). However, consider-

able uncertainty exists over the ability of general circulation models to predict regional changes in cloud cover under climate change (Cess et a l . , 1989; La Brecque, 1990). However, GCM (except the GISS) mostly agree on predicted cloud cover patterns for present climate conditions, but greatly dis- agree for doubled C 0 2 conditions (Fig. 5). Obvi- ously, if the timing and/or duration of the monsoons were altered under climate change, changes in a number of climatic factors, including solar UV-B radiation, would be expected.

Acknowledgements-We want to thank Sandy Azevedo for processing the cloud data into ARC/INFO format, Amos Eddy for providing Thailand weather stations names and locations, Paul Newman for providing TOMS 1982 ozone data. We also want to thank the reviewers of this paper for their helpful comments.

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