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.