Study on long-term soil moisture characteristics over Bangladesh and its relations to atmospheric variables

Abstract

Soil moisture is the water occupying pore spaces between soil particles, which plays an important role for hydrological cycle. The moisture for four different depths of soil in Bangladesh was analyzed using ERA-Interim reanalysis data from ECMWF (European Centre for Medium-Range Weather Forecasts). The dataset have approximately 80 km grid resolution and 60 vertical levels from the surface up to 0.1 hPa. The daily data was extracted and averaged for annually, monthly, and seasonally to analyze soil moisture in the soil layer-1 (0-7cm), soil layer-2 (7-28cm), soil layer-3 (28-100cm), and soil layer-4 (100-255cm) in Bangladesh for 36 years from 1979 to 2014. Soil moisture in each layer is gradually increasing in this region from 1979 to 2014 by 7.63%, 7.42%, 6.50% and 5%, respectively for soil layer-1 to layer-4. Soil moisture also increases vertically downward from the surface layer. The yearly average moisture differences between the successive layers are found to be 0.53%, 2.54% and 3.28%. The spatial variation shows that soil moisture is decreasing from the eastern toward western parts of Bangladesh up to the third layer where fourth layer maintains almost unique value in all over Bangladesh. The value of soil moisture provides an annual periodicity with maximum value of ~0.288 m3m-3 in all four layers on July and minimum value of ~0.18 m3m-3 in the first layer on March. May is found as a transition month where moisture in all the four layers are started to coincide with each other with the approximate value of 0.24 m3m-3. Differences in Soil moisture in the four layers is prominent in the dry season (November-May) with the values of 0.204, 0.207, 0.218 and 0.23 (m3m-3), respectively from the surface layer to the bottom layer. On the other hand, in the wet season (June-October), moisture reaches the saturated condition for all layers with a constant value of ~0.278 (m3m-3). Soil moisture contributes 56% of the total moisture in the wet season where 44% in the dry season in a year. North eastern side of Bangladesh holds 12.4% more moisture than the south western side in the surface layer. Atmospheric variables of evaporation minus precipitation (E-P), surface relative humidity and temperature are correlated with soil moisture in the all layers. The surface layer i.e. the first layer moisture indicates the strong negative relation with (E-P) and strong positive relation with surface relative humidity having the correlation coefficients of -0.87 and 0.88, respectively. The relations between soil moisture and atmospheric variables are decreasing from the surface layer to the bottom layer. The surface air temperature also proportionally affects the amount of soil moisture. The per degree changing of surface temperature change soil moisture amount significantly after two months later with an approximate value of 0.01 m3m-3.