52
The forest cover changed from 71.3 in 1990 to 67 in 2001 and decreased significantly in 2009 to become 58.8 from total watershed areas.
Peak flow and base flow: Figure 4.13 shows the peak stream flow, that occurred from March to May. Land cover map of 2009 had the highest stream flow
compared with the other land cover map 1990 and 2001. This is consistent with the fact that the forest covers on the watershed decreased year by year. Where
land cover map of 2009 has the lowest forest cover area rather than the other land cover maps. This indicates that in 2009 much rainfall was converted into direct
run-off, causing peak flow.
Figure 4.13 Peak stream flow from three different land cover period and Base flow from filtering process for each land cover period.
Simulated hydrograph was compared quantitatively with the annual averages seasonal stream flow. The annual stream flow in 2001 increased about
4.5 from annual stream flow on 1990, and hydrograph on 2009 also increased about 8.2 compared with the annual stream flow in 2001.
50 100
150 200
250 300
350
Q m
3
s
LC_1990 LC_2001
LC_2009
50 100
150 200
250 300
350
Q m
3
s
Q_1990 Q_2001
Q_2009 BF_1990
BF_2001 BF_2009
53
4.2.5 Hydrological Response for some Land Cover Changes Scenarios.
To investigate the hydrological response in Palu catchment to the land cover changes, three scenarios were deployed and the hydrological model was
rerun under the same meteorological condition used previously. The first scenario was based on the provincial development plan for agricultural; second and the
third scenarios using the projected land cover area on 2020 and 2030. • Provincial development plan RTRW scenario 1
The spatial planning maps only consist of forest protection and development plan classes including protected forest, limited utilization forest,
preserves forest, and non-forest area. Limited utilization forest, preserves forest, and non-forest area are categorized into unprotected forest. The total areas of each
land cover types are: protected forest 1,634.47 km
2
, unprotected forest 1,377.58 km
2
Advanced evaluation of this map was done by comparing and intersecting it with the land-cover maps of 2009, and we found that almost all land cover
classes agricultural, build up area, shrub, barren land exist in both the protected area and unprotected forest.
Table 4.9 The distributions of existing land cover class on protected and unprotected forest in Palu catchment.
Land Cover Classes Area km
2
Protected Forest
Percentage Developed
Area Percentage
Agriculture Land 527.25
32.27 278.80
19.68 Barren Land
48.98 3.00
0.90 0.06
Build Up Area 51.21
3.13 7.67
0.54 Forest
816.55 49.98
976.63 68.95
Shrub Land 181.83
11.13 114.96
8.12 Water Body
8.02 0.49
37.39 2.64
Total Area 1633.84
1416.35
The distribution of each land cover class on the protected and developed area, large number of agricultural land 32.7 and build up area 3.13 are exist on
the protected area, while the actual forest is only 50 in the protected forest area.
54
As one of the objectives of protected forest as the buffer zone is to maintain the water absorption in a watershed, this can reduce the peak flow when
the rain falling down Hartanto, N. 2009. It can be done by apply a good agroforestry system and reducing the utilization on the area with high slope values
or on the hills. The land use evaluation both in protected and unprotected forest above are
used to develop scenario rules. To maintain the function of protected forest, all the actual forest located inside the protected and unprotected area remain no changes,
shrub land and barren land area inside the protected forest is changes to forest, agricultural land inside the protected forest changed into agroforestry system, and
build up area inside the protected forest remained as build up area. For the land cover class outside the protected forest area is maintained with no changes. The
distributions of reclassified land cover classes based on protected and unprotected forest area are shows in Table 4.10.
Table 4.10 Distributions of adjusted land cover classes based on protected and unprotected forest
Land Cover Classes Area km
2
Percentage
Agriculture Land 278.80
9.14 Agroforestry
527.25 17.29
Barren Land 0.90
0.03 Build Up
51.21 1.68
Build Up Area 7.67
0.25 Forest
2024.00 66.36
Shrub Land 114.96
3.77 Water Body
45.41 1.49
Total Area 3050.19
100.00
• Projected Land Cover on 2020 and 2030 scenario 2 and 3 The projected land cover area on 2020 and 2030 were calculated by using
trends equation. The calculation is a simple prediction by using simple regression equation. Since here only three point of land cover maps, the regression results
might less accurate. All the trends equation is served in Table 4.11.
55
Table 4.11 Simple trends equation to projecting the land cover areas in the future.
CLASS Equation
Area km2 in 2020
in 2030
CL. 1 Agricultural Land y = 239.58x + 53.86
1,012 33
1,252 41
CL. 2 Barren Land y = 2.1273lnx + 47.763
51 2
51 2
CL. 3 Build Up area y = 36.989e0.157x
69 2
81 3
CL. 4 Forest y = 2422.6e‐0.096x
1,650 54
1,499 49
CL. 5 Shrub Land y = ‐58.42x + 485.17
222 7
121 4
CL. 6 Water Body y = 0.4244lnx + 44.887
45 1
46 1
Total 3,050
100 3,050
100
The land cover class are maintain same as land cover class on previously, consists of agricultural land, barren land, build up area, forest, shrub land, and
water body. The land cover area calculation using those equation are exceed of total watershed area 29 km
2
on 2020 and 72 km
2
on 2030. To maintain the total land cover area of projected land covers on 2020 and 2030, the area of shrub land
was reduces to the rest of the other class area.
4.2.6 Impact of Land Cover Changes Scenarios on Stream Flows
Three hydrographs were simulated by using the same parameters in which was used during calibration and validation model. In this case, again the rainfall
data of 2007 was used as meteorological input. All these three scenarios were run using same parameters, except the curve numbers are based on those previous
land cover condition. The curve numbers of each scenario are shows on Table 4.12.
The simulated hydrograph obtain the peak flow information for three different land cover scenarios. First scenario RTRW values of peak flow result
are 257.82 m
3
s, 292.29 m
3
s for scenario 2 projected land cover on 2020, and 301.76 m
3
s for scenario 3 projected land cover on 2030. Table 4.12 Curve number value of each scenario
Basin Name Scenario
2020 2030
RTRW
Sub‐Basin 1 65
67 58
Sub‐Basin 2 69
70 67
Sub‐Basin 3 67
70 65
Sub‐Basin 4 68
72 65
Sub‐Basin 5 73
79 63
Sub‐Basin 6 61
64 59
Sub‐Basin 7 79
84 67
56
Figure 4.14 Simulated hydrograph using 2007 rainfall data for different land cover scenarios.
Simulation using these three different scenarios, the peak flow increased 13.37 during wet season when the first scenario compared with the second scenarios.
For the second and third scenario, the peak flow also increased about 3.24. The comparison between scenario-based hydrograph and simulated
hydrograph using existing condition of 2009 data, the first scenario using RTRW has decreasing the peak flow significantly from 288 m
3
s on 2009 become 258 m
3
s. This because of the forest cover area is much bigger on the RTRW than the existing condition on 2009, which is on 2009 the total forest covers is 1793.2 km
2
or 58.79 and 2024 km
2
or 66.36 by using the first scenario. In addition, with the addition of agroforestry system on the RTRW scenario also gives significant
impact to reduce the surface runoff compared with the traditional existing agricultural system; where agroforestry system covered about 527.25 km
2
17.29 and agricultural land covered about 278.8 km
2
9.14 of total catchment area.
For the second scenario by using projected land cover area on 2020 and 2030, the simulated hydrograph shows the increased peak flow compared with the
existing condition on 2009 from 288 m
3
s to 292 m
3
s on 2020 and 302 m
3
s on 2030. This because of the forest cover was decreased by 4 on 2020 and 9 on
2030 and also the increased of agricultural land from 26 on 2009 become 33
5 10
15 20
25 30
35 40
50 100
150 200
250 300
350
1 31
61 91 121 151 181 211 241 271 301 331 361
Preci p
it a
Uo n
mm
Di sch
a rges
m
3
s
Days
Precipita\on RTRW
LC_2020 LC_2030
