Change Detection Land Cover Changes Assessment

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. 51 Figure 4.12 Simulated hydrograph from three different lands cover period. The hydrograph comparison by using land cover 1990, 2001, and 2009 was producing the same peaking time. It was occurred on day 136 in current simulation years. The peak value of each land cover years are 257.63, 268.92, and 290.58 m 3 s on respectively years. The total discharges volume of each simulation using different land covers maps was produce 3,422,287,000 m 3 on 1990, 3,574,677,000 m 3 on 2001, and 3,867,125,000 m 3 on 2009. From its simulation found that the discharge volumes were increased 152,390,000 m 3 during period 1990 to 2001 and 292,448,000 m 3 during period of 2001 to 2009. The contributions of each sub-basin to generate river discharge are difference among sub-basin. Generally, sub-basin 2 gave highest contribution with 31.1 of total discharges was generated, followed by sub-basin 1 with 30.4, then sub-basin 3 with 13.1, sub-basin 7 generate about 10 of total discharges, sub-basin 5 with 7.2, sub-basin 4 with 5.2 and sub-basin 6 with 3.2 respectively. Based the visual comparison of three hydrographs that came from three different land cover periods, the increasing stream flow happened during 2001- 2009 period. It can be compared with the land cover map on 1990-2001 and 2001 to 2009 period, where the most conversion from forest into other land use and changes from shrub land to agricultural land happened during 2001-2009 period. 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