Abstract:
Solar radiation is the dominant, direct energy input into the terrestrial ecosystem and it
affects all physical, chemical and biological processes, The sun exerts a natural
influence on the earth's atmosphere and climate. In order to understand mankind's roles
in climate change, the sun's impact must first be understood. For its importance in the
climate system, many countries in the world conducted stndies regarding solar
radiation. Such an attempt hID;been made in this study.
Sunshine dl.lHltiondata at mne meteorological stations (Dhaka, Rangpur, Rajshahi,
Sylhet, Bansal, Comilla, Khulna, Chittagong and Cox's Bazar) are analyzed to sec the
long-term changes and trends. The stations are selected such that they represent all
hydrological regions of Bangladesh and spread more or less unifonnly alI over the
country. The data used arc the Bangladesh Meteorologkal Department data spanning
from 1961 to 2006. The analysis is based on 36 len-day periods in a year. A decreasing
trend in sunshine duration is found at all selected stations and in almost all 10-oay
periods. The overall allllual decrease for the entire Bangladesh is found to be about OJ
hours a day in every 10 years. The decrease is the highest in the winter season
(November-February), Ihen in the summer (March-May) and lowest in the monsoon
(June-October). The highest arumal and seasonal decreases arc found at the Dhaka
station. The maximum decreasing rate (0.63 hours a day in every 10 years) is found in
the last lO-day period of Janmuy, the coldest month in the winter season. Annual
sunshine hour has dcerea.\ed by almost 20% from 1961 to 2006. Statistical significance
of the deerea.-;ingtrends was also tested at a 10%level of significance. 11was (ound that
the trends are statistically signilicant at 29, 19, 17, 15, 8, 5, 5, 5 and 3 ten-day periods,
out of 36, for Dhaka, BaTisal, Srlhet, Chittagong, Cox's Bazar, Rajshahi, Rangpur,
Khulna and Comilla ~tations, respectively.
To assess the impact of deerea;;ed sunshine hOI.lTon rice evapotranspiration, FAO
recommended CROPWAT sollwdl"e was used to calculate reference crop
evapotranspiration (E"r~).SlJllshilleduration .forthe years 1961 and 2006 was eMimated
by fitting 36 regression equations to 36 len-day average sunshine duration time series.
For the years 1961 and 2006, ETo was calculated with their respective sunshine
dur'ltion kceping other climatic panunclers (tcmpcrature, humidity and wind speed)
wlcbanged. Tt was found th'll thc annual ETohas declined by 6.1% fwm 1961 to 2006
dlle to the decline in sunshine duwtion. The ET0 values WOremultiplied by crop factor
(K,) for Bom rice to obtain rice evapotrunspiultion (ET,). As the crop factor is a fixed
factor, similar decreases in ETc were found.
To assess the impact of decreased sunshine honr on Boro rice yield, a v>idely used rice
growth model called ORYZA 2000 was nsed. The modcl was run with thc average
climatic data on temperature, humidity and vapor pressure but with ch'lllging sunshine
dumtion data for 1961 and 2006.11 was assumed that there are no watcr and nitrogen
deficits during the grov>ing period of rice (January-May). The results revcaled lh'lt the
Boro yicld m'lY h'lve decreased by 4.3% due to the decrease in sunshine duration from
1961 to 2006. As this study was bused on sceondary data and information 'lnd as many
input parameters needed in the model were not avail'lble for the rice varieties
commonly grown in B.mgladesh, the actual impact of solar dimming may be more than
that found from the model results. Further research is needed in this aspect.
