Abstract:
Factors affecting the rate of evaporation from soil or water bodies can be broadly divided
into two groups, meteorological factors and surface factors. The variation of evaporation
and other climatic parameters related to it such as temperature, solar radiation, bright
sunshine hour, wind speed, air pressure, relative humidity and rainfall of Bangladesh is the
main objective of this study. Data of twenty five meteorological stations of Bangladesh
have been collected to achieve the objectives. The source of these climatic data is the
Bangladesh Meteorological Department (BMD).lt is identified from analysis that in
Bangladesh higher rate of evaporation occurs from the month of March to May and in
April it is maximum. Minimum evaporation rate occurs in the months of December and
January. In Bangladesh temperature is maximum in April or May. The variation of
temperature is little from April to September. Temperature begins to decrease from the
month of October and falls at lowest in January. It is also observed from the analysis that
in most of the locations of Bangladesh the solar radiation is high in the month of April and
May and low in the month of December and January. From the analysis it is found that air
pressure is minimum in the month of June and in some locations in July. On the other
hand the air pressure is maximum in December and in few locations it is in the month of
January. The variation of air pressure is not that much significant. In Bangladesh the
average annual humidity is 79%. In most of the locations maximum amount of humidity
occurs in the month of July and minimum amount of humidity occurs in March. Average
maximum length of bright sunshine hour occurs in the months between January and April
as these months are almost cloud free. Minimum length of sunshine hours in all the
locations was observed in July. From the analysis, it is found that maximum rate of rainfall
occurs in the months of June and July at the time of full monsoon. Average annual
maximum rainfall occurs in Sylhet (4100 mm) and minimum rainfall occurs in Rajshahi
(1527 mm). The average annual total rainfall of the country is 2354 mm. It is also obtained
from the study that in most of the locations wind speed is maximum in the month of April
and minimum in the month of November.
The correlation coefficient between evaporation rate and the other climatic parameters are
determined in the study. The correlation coefficient (r) between evaporation rate with solar
radiation, temperature and wind speed is large and positive. It means evaporation rate is increased with the increasing value of radiation, temperature and wind speed. On the other
hand the value of r between evaporation rate and bright sunshine hour, relative humidity
and pressure is small but in case of bright sunshine hour and rainfall it is positive and in
case of relative humidity it is negative. Again the value of r between evaporation rate and
bright sunshine hour is positive and medium.
Four approaches i) Energy Balance Method ii) Aerodynamic Method iii) Combined
Aerodynamic and Energy Balance Method and iv) Priestly-Taylor Method have been used
in this study for the computation of evaporation rate and their performance were
evaluated. The best result was obtained from the Energy Balance method. Then Priestly-
Taylor Method, Combined Method and the Aerodynamic Method respectively performed
well. None of the above mentioned methods gives reasonable results in Bangladesh.
