Study on spatial and temporal variation of evaporation and related climatic parameters

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.