Abstract:
Hydrological models, used to study the behavior of river networks, have developed significant
progress over the past four decades. Among several distributed hydrological models,
Geomorphology Based Hydrological Model (GBHM) is utilized in this study. Hillslope elements
are the fundamental computational units of the model. The catchment is divided into
a number of flow intervals. Each interval is represented by its area and width function. This
scheme of topography representation has made GBHM suitable for complete hydrological
simulation of large river basins.
GBHM is used in this thesis to study the Surma-Meghna basin which is the longest
(669 km, inside Bangladesh) river system among the four major river systems of Bangladesh
and covers a large area. The system also drains out one of the world’s heaviest rainfall
areas (Cherapunji, Meghalaya, India). Therefore, study on the Meghna river basin allows
the understanding of the hydrological processes affecting a vast region of the country.
Observed rainfall data is used for simulation for benchmark of the model. The simulated
discharge is compared with the observed discharge. After verification of the model, it is
used to predict future discharge of the river network.
The simulated discharge hydrograph using TRMM measured rainfall data shows oscillations
around the observed discharge values due to the assumption made in GBHM. The
overshootings are from the over estimation of TRMM rainfall. So, a combined scheme is
used to minimize the effect of limitations of TRMM data. Observed rain gauge data are
used wherever available and TRMM data are used for the remaining area. Then, overshootings
are greatly reduced and the ratio of absolute error to the mean introduced by World
Meteorological Organization becomes 0.38 which is within the allowable error ratio limit
for large basins.
Finally, PRECIS predicted rainfall data is used for discharge estimation. The average
daily simulated discharge for the years 2001, 2031, 2061 and 2091 are 6370, 4050, 4650
and 7870 cumec, respectively. The predicted discharge values in 2031 and 2061 is less than
that of 2001 whereas for 2091 it is more than the other years resulting from the changed
precipitation pattern due to climate change. The trend analysis shows a continuous increase
in discharge. PRECIS prediction shows, climate change will cause intense precipitation in
future affecting the basin runoff and river discharge. Increased discharge may cause greater
number of flood events and large fluctuations in the discharge may cause water-stress in the
coming years.
