; Wagner W., Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B
Fig. 4. Comparison of Hydrograph at main outlet (Mundali):
Calibration period
the period of 1972-85 and 7.47 mm in 1985-03) which is 4.53%
of the flow in 1972. It may be concluded that a decrease in
forest cover by 5.71% in the Mahanadi river basin has caused
the river flow to increase by 4.53%. This is quite a significant
amount in terms of volumetric rise (3514242122 m 3 ). Table 2
summarizes the predicted incremental changes in runoff (mm)
at outlet by season (e.g. JFM refers to cumulative rise for
January, February and March). In figure 7, monthly
hydrographs for 1972 and 2003 are presented to see the changes
that have taken place. A plot of relative percentage difference in
runoff (from 1972 to 2003) over 1972 is also shown in the same
figure. The rise in percent runoff was prominent during May,
June, Oct and November months. The decrease in runoff from
1972 to 2003 may be due to reverse trend in landcover
conversions and/or human activities.
Though the agreement between observed and simulated
discharges is good, under-estimations and over-estimations are
inherent in the simulation. This is because of the fact that VIC
simulates naturalized flows and the observed discharge used for
validation is biased and affected by human interventions. Model
performance showed good agreement at Mundali inspite of a
large reservoir since calibration was performed at this outlet. It
may be seen from the simulation results that model has
generally overestimated (S>0) during months of June, July and
under-estimated during August and September. The possible
reason may be initial reservoir storage in June-July due to which
observed flows are less as compared to simulated whereas
observed flow exceeds once the reservoir capacity is filled (in
Aug, Sept.). It may be concluded that the agreement between
and observed and simulated hydrological components is largely
dependent on the hydrological and landcover conditions in the
basin and model assumptions. The synoptic view and
landuse/landcover conditions of various sub-basins are shown in
the Figure 2. The landcover classes are same as shown for
whole of the Mahanadi basin (Fig. 1) with Mundali as an outlet.
Fig. 5. Simulated and observed monthly hydrographs at outlet
In summary, a decrease in natural cover of forest over time has
caused a significant rise in streamflows and particularly surface
runoff. Removal of forest cover is known to increase
streamflow as a result of reduced évapotranspiration. Base-flow
is expected to decrease while surface runoff increases owing to
the decrease in infiltration and hence groundwater recharge
processes. Urban expansion and intensive cultivation will
loosen the soil leading to soil loss (soil erosion) due to high
flows. Urbanization also tends to decrease infiltration rates and
increase extents of impervious surfaces, although the area over
which such changes have occurred is a small fraction of the
total basin area. The VIC model, being physically based,
distributed, macroscale model is particularly suitable for
studying climate and landcover change scenarios and their
implications on hydrological processes at regional and global
scale over long time frames.
Q-1972
4.2 Effect of landcover changes on streamflows
4.2.1 Historical and current hydrological simulation using
VIC model: Simulation was done for year 1972 and 1985 after
calibration and validation of the VIC model for 2003. Only the
vegetation cover and related parameters were changed in the
simulations; the model meteorological forcings and soil
parameters were kept same for both the current and historical
scenarios. In this way, the effects of vegetation change on basin
hydrology were isolated from the effects of climate variability.
4.2.2 Trend of changes in streamflows: Streamflows for year
1972, 1985 and 2003 were compared to look for the changes
that have taken place due to change in landcover in the
Mahanadi river basin. Monthly discharges were found to be
varying significantly as compared to daily flows. Fig 6 shows a
scatter plot of monthly flows (mm) for 2003 and 1972, events
above the slope line indicates an increase in river flow. A rise of
24.44 mm in the annual discharge is predicted at Mundali outlet
of the Mahanadi basin from 1972 to 2003 (16.97 mm being in
Fig.6. Comparison of streamflows for 1972-2003 at Mundali
Table 2. Changes in runoff by sub-basin and season (in mm)
Stations
JFM
AMJ
JAS
OND
Annu
al
%
increa
se
j | of?
Mundali
0.027
1.79
16.97
5.64
24.44
4.53
3.51 x
10 6