Summary of Land Cover Maps
49
Figure 4.10 Twenty years simulated hydrograph in Palu River. Wet season high flow; in order to analyze the seasonal stream flow, period
of April - June and October – December was selected as the wettest months, while the peak on May and November. January-March and August-September were
considered as driest month. During the wet season, rainfall increased and reached the peak on May and November. Due to the increasing of rainfall intensity, large
amount of stream flow was generated from surface runoff. During the 1990 to 2009 period, the stream flows fluctuated about 73 to 288 m
3
s, and reach the peak flow on May 2007 as 2007 big flood was occurring in Palu.
Figure 4.11. Variations between high wet seasons flow, low dry season flow and peak to low ratio.
5 10
15 20
25 30
35 40
50 100
150 200
250 300
350
P mm Discharge m
3
s
Years
50 100
150 200
250 300
350
1990 1991
1992 1993
1994 1995
1996 1997
1998 1999
2000 2001
2002 2003
2004 2005
2006 2007
2008 2009
Di sch
a rge
M
3
s
Years
Wet Dry
Ra\o
50
Dry season low flow; period of January-February, and August-September is state as the dry season. During this period, the rainfall decreased and reaching
the minimum value on September as the end of dry season. As shown on the Figure 6, the stream flow on Palu River during dry season is fluctuated from 5 to
67 m
3
s with the average value about 35 m
3
s. Peak to low flow ratio; on the watershed system, usually the stream
channel will show the speed response of the stream channel that can be quantitatively measured as ratio between the peak flow and low flow Q
P
Q
B
. The speed response is a function of the rain intensity and water spring on the channel.
The speed response of the stream channel can be categorized into 3 classes White, 1993; 1-3 of Q
P
Q
B
is categorized to very low response, 10 is medium speed response, and 100 very fast speed response. Based on the ratio between
seasonal peak flow and low flow is obtained peak to low flow ratio 9.7 or equal to 10, its mean the stream channel on Palu Catchment can be categorized into
medium speed responses. 4.2.4 Hydrological Response to Land Cover Changes.
Three multi temporal satellite imageries were analyzed to obtain the land cover maps. These land cover maps were used to determine the curve number as a
function of land-cover, slope, and soil to describe transformation of surface runoff to stream hydrograph in a watershed. The series of hydrograph were analyzed to
assess the stream responses to the land-cover changes in Palu catchment. Its came from three time series land cover maps land cover on 1990, 2001, and 2009 that
used to determine input variables to HEC-HMS model to simulate its stream flow, this allows studying the impact of land cover changes to the river discharges.
In analyzing the impact of the land-cover changes to the stream system in a watershed, there are some assumptions were made; all parameter except the
curve number are maintain to remain no changes during the simulation. Rainfall data of 2007 were used as input to HEC-HMS model to simulate the stream
impact of the land cover changes. Here the rainfall data on 2007 was selected due to completeness record from all selected rain gauge in the catchment.