Procedia Environmental Sciences 24 ( 2015 ) 297 – 302

1878-0296 © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Selection and peer-review under responsibility of the LISAT-FSEM Symposium Committeedoi: 10.1016/j.proenv.2015.03.038

Available online at www.sciencedirect.com

ScienceDirect

The 1st International Symposium on LAPAN-IPB Satellite for Food Security and Environmental Monitoring

The utilization of global digital elevation model for watershed management

a case study: Bungbuntu Sub Watershed, Pamekasan Muhammad Taufik*, Yogrema Setyanto Putra, Noorlaila Hayati

Department of Geomatics Engineering, Institut Teknologi Sepuluh Nopember, Jl. Raya ITS 60111, Indonesia

Abstract

This paper reviewed mapping of Bungbuntu watershed using Aster GDEM and SRTM DEM. The main difference is Aster GDEM using thermal, whilst radiometric images and SRTM using radar wavelength. These DEMs were used to determine Bungbuntu watershed boundary using hydrology and geomorphology analyses. The results showed Aster GDEM and SRTM have significant different values compared to BPDAS (Government Field Data) but generally morphometric information was still in the same class range. However, DEM generation failed to determine channel network. DEM produced by SRTM had better elevation accuracy than Aster GDEM contrary to the resolution of elevation on each DEM source. © 2015 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of the LISAT-FSEM Symposium Committee.

Keywords: DEM; ASTER GDEM; SRTM; hydrology; geomorphology; watershed

1. Introduction

Watershed is a region of river and stream that have a function to save, keep and channelize water from rainfall to lake or sea naturally. It needs a proper management due to its function in production and protection of water resources from natural hazard such as flood and erosion. The quality of watershed management would influence watershed condition.

Watershed Management Agency (Badan Pengelolaan Daerah Aliran Sungai; BPDAS) is one of government institutions aims to control and manage watershed areas. One of the responsibilities is to understand topography on

* Corresponding author. Tel.: +62-31-592-9486; fax: +62-31-592-9487. E-mail address: taufik_srmd@yahoo.com / taufik_m@geodesy.its.ac.id

© 2015 Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Selection and peer-review under responsibility of the LISAT-FSEM Symposium Committee

298 Muhammad Taufi k et al. / Procedia Environmental Sciences 24 ( 2015 ) 297 – 302

certain areas by conducting watershed mapping. It includes region boundary mapping, environmental condition, land cover, and watershed morphometry. Watershed mapping is a main parameter used to assess the border of determining land cover condition and geomorphology on watershed areas. We used the regulation issued by ministry of forestry [3] to determine watershed boundary. It gives a reference and direction for all related institutes to comprehend the technique in determining watershed area.

Nowadays, advanced radar and remote sensing technologies has generated earth's surface model which is known as Digital Elevation Model (DEM) which covers the entire world. DEM describes relief surface on a digital format raster that has height value on each pixel. This DEM could be used as sources in the establishment of watershed management tools. Some of DEM products have been launched freely including DEM Shuttle Radar Topography Mission (SRTM) and Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model (ASTER GDEM). This research used morphometry BPDAS data as a reference for the result of DEM processing. As a case study, this research used sub watershed, Bungbuntu, Pamekasan.

2. Data and Method

This research conducted in Sub Watershed Bungbuntu, Tarokam, Pamekasan. It lies on 7,027o - 7,15o S and 113,438o - 113,529o E. According to statistical data of BPDAS, this a region is a watershed cultivation which have high population level as 137.277 people occupy this wathershed. Landsat 8 imagery, ASTER GDEM version 2, and DERM SRTM version 4 are used as primary data. While topography map from Geospatial Information Agency were used as secondary information..

This study is divided into 3 phases: processing of BPDAS data, processing of border and morphometry information from SRTM and ASTER GDEM, and 3d analysis map. We also determined channel networks using Stahler method. This method explains that the first order is the upper of river channel that does not have branches; the second order is where a region of the first order meeting with another first order; for the next level of order, it is also a region between previous orders are met. The main river was marked as the biggest number of order. Furthermore, we made watershed boundary based on the accumulation parameter trend and number of rivers. Pour points were marked as there is an activity of water trend in watershed. As it showed on Figure 1, these pour points have been laid along the river branches from downstream to other sub watersheds.

Fig. 1. Pour points on DEM SRTM (left) and ASTER GDEM (right)

299 Muhammad Taufi k et al. / Procedia Environmental Sciences 24 ( 2015 ) 297 – 302

3. Result and Discussion

3.1. Channel Network Establishment Based on processing result of SRTM and ASTER GDEM, we found that channel network derived from DEMs (Figure 2) were not compatible with the channel network model produced from topography map. According to BPDAS data, the channel network is created using topography map conducted by Geospatial Information Agency (2000) on 1:25.000 scale. From this channel network, we then calculated the length of major river which is 19.44 km and the total of length channel river that is 95.06 km.

Fig. 2. The comparison of channel network derived from SRTM (red), ASTER GDEM (brown) and topography map (purple)

3.2. Morphometry value based on BPDAS data and DEM processing

According to table 1, channel network generated from DEM has different value from those produced from

topography map in determining morphometry value on a watershed. Viewed this way we assumed that the the ministry of forestry regulation is better to determine watershed boundary. However, the topography mapcould be used as a basic reference on morphometry watershed calculation.

Based on SRTM DEM calculation, the watershed area is 46,521,368.4 m2 with perimeter of 40,329.3 m and the height of 0.1 length from main river which is 156 m, while the height of 0.85 length is 26 m. Meanwhile, based on ASTER GDEM, watershed area is 48204901,3 m2 with perimeter 45.099,4 m and the height of 0.1 length from main river is 171 m while the height of 0.85 length is 37 m.

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Table 1. The comparison of morphometry value from SRTM DEM, ASTER DEM and BPDAS data

Morphometry Parameter Morphometry value from ASTER GDEM

Morphometry value from DEM SRTM BPDAS Unit

Watershed area 48.205 46.521 48.138 Km2 Perimeter 45.099 40.329 41.412 Km River length 14.174 15.614 19.441 Km Watershed width 3.401 2.980 2.476 Km River slope 14.39 11.36 8.50 % Rc 0.30 0.36 0.35

River density level (Dd) 0.45 0.47 1.97 Km/Km2 River branch level 2 2.5 3.21

3.2. The comparison of the boundary of Bungbuntu watershed between SRTM, ASTER GDEM and BPDAS data

It needs some pour points to know the exact place of catchment area in determining watershed boundary. Those pour points are spread along flow accumulation points generated by flow direction (Fig. 2.). This processing is called as watershed area. Fig. 3 shows the comparison of the boundary of Bungbuntu watershed between SRTM, ASTER GDEM and BPDAS data.

Fig. 3. Bungbuntu Watershed Map derived from DEM SRTM (yellow), ASTER GDEM (green) and BPDAS data (red)

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From the watershed area calculation, DEM produced by ASTER GDEM has a closer value to BPDAS data.

However, the value area generated from ASTER GDEM does not show a good shape of Bungbuntu watershed area. Moreover, the figure of the upper main of the river derived from ASTER GDEM has great different with BPDAS data. On the other hand, DEM SRTM shows watershed figure similar to BPDAS especially in the upper course of river. Unfortunately, on the lower course of river part, DEM SRTM fails to have same shape as BPDAS watershed and it causes 1.62 km2 less value on DEM SRTM calculation. This analysis supports previous research from [5] who stated that it is difficult to determine watershed figure on slightly slope river area (Figure 4).

(a) (b)

Fig. 4. Comparison of watershed boundary on 3D modelling (a) the difference on upper course of river, (b) the difference on lower course of river

According to the calculation of perimeter on sub Bungbuntu watershed, DEM SRTM shows a closer perimeter

value to BPDAS. However, DEM SRTM has 1.08 km in length different to BPDAS, while ASTER GDEM has 3.69 km different to BPDAS. Inaddition, DEM SRTM also shows a closer value (0.36) to BPDAS (0.35) while ASTER GDEM could only calculate 0.3. Based on a few assessments, DEM SRTM has more similar shape to BPDAS compared to ASTER GDEM.

4. Conclusion

Based on morphometry of Bungbuntu sub watershed (Tarokam), both DEM SRTM and ASTER GDEM have significant different values to BPDAS data but SRTM has a closer value to BPDAS. In terms of processing of channel network, both DEMs have failed to represent the network because the resolution of both DEM is only 30 m for ASTER GDEM and 90 m for SRTM. That means if there is precise indentation on river, it could be recognized neither on ASTER GDEM nor SRTM. Nevertheless, these different values do not change the figure, slope and density level of watershed. Generally, DEM SRTM and ASTER GDEM still have similar shape to BPDAS data. Another factor causing the difference on determining watershed and channel network is the land cover on the area. It could influence vertically and horizontally the error position on DEM especially in high slope areas. Therefore, the more accurate of DEM resolution has the better result in determining watershed and channel network.

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