IntroductionThese notes describe how to extract profiles of elevation and contributing area along streamlines from a digital elevation model (DEM) using ArcGIS 10 and Microsoft Excel. The general process involves deriving streamlines from a DEM, determining both elevation and number of contributing pixels at each point along stream lines and fine tuning, correcting and graphing of this data in Excel. It is possible to drive profiles of slop. Stream filw and stream power from the data derived here. Before commencing these steps there are several initial tasks that need to be completed, as described below.

1.1 Data and Software used SRTM DATA:SRTM 90m (3 arc second) data is distributed by continent and is available in 1 deg tiles on the SRTM FTP server (ftp://e0srp01u.ecs.nasa.gov/srtm/version2/). The address of a given tile is derived from the lat/long coordinates of the SW corner of that tile. The SRTM tiles are in WinZip format. SRTM data is also available in a web interface on the USGS Seamless Server (http://seamless.usgs.gov/ or http://srtm.csi.cgiar.org/).The SRTM can be accessed on internet free of charge. This section explains acquisition of data and processing for hydrological purpose using GIS tools, specifically, ArcGIS which is the most used GIS software.The SRTM is projected into a geographic coordinate system (GCS) with the WGS84 horizontal datum and the EGM96 vertical datum.

1.2 Arc GIS 10

2. Methodology - Derive a Drainage Network from DEM& Long Profile2.1 Processing for Hydrological Parameters DeterminationA filled DEM or elevation raster is void of depressions. A depression is a cell in an elevation raster that are surrounded by higher-elevation values, and thus represents an area of internal drainage. A sink (or depression) is a cell or a group of cells which is at a lower elevation than all its neighboring cells. If the cell has at least one cell adjacent to it, at a higher elevation, and no cells adjacent to it at a lower elevation then it is said to be in flow sink. Sinks occur due to error in measurement and are actually may not present. Hence sinks have to be removed. It is important to have a depression less DEM for all subsequent hydrological analyses. Areas of internal drainage can cause problems later in the watershed delineation process. Creating a depression less DEM.1. Open the Arc Tool box tool set Spatial Analyst Tools > Hydrology. This is where the surface hydrology tools are located.2. Open the Fill tool. The input surface is the DEM grid

Fill DEM tool in ArcGIS3. After a few minutes, a new layer, fill_dem1, will be added to the data frame.This is identical to the DEM raster, but any areas of internal drainage are filled in.

Filled DEM Show in ArcGIS4. Note the difference in the lowest elevation value in the legend; sink cells in the original data set have been filled in.5. Turn off display DEM layer from the data frame, since you will be working on the filled grid from this point on.

2.2 FLOW DirectionA flow direction raster shows the direction water will flow out of each cell of a filled elevation raster. The basic principal for assigning flow directions is given as: From a cell, water flows to the neighboring cell that has the highest positive distance-weighted drop. A widely used method for deriving flow direction is the D8 method. The D8 method assigns a cells flow direction to the one of its eight surrounding cells that has the steepest distance-weighted gradient . The method does not allow flow to be distributed to multiple cells. The D8 method produces good result in zones of convergent flows and along well defined valleys (Freeman 1991).For each cell C (at [i, j]) in the D.E.M., the following steps are performed:1. Calculating the distance weighted drop to neighboring cells:The distance-weighted drop to a neighboring cell is defined as(Value of the cell C Value of the neighboring cell)/Distance to the neighboring cell (d)For cells which are horizontally or vertically adjacent to C, d =1For cells which are diagonally adjacent to C, d = (2)2. The cell FD [i, j] is given the direction code of the neighboring cell with the largest positive weighted drop.A special case is when the largest positive weighted drop occurs in more than one direction. In this case, the cell FD [i, j] is given the sum of the direction codes of all the directions in which the largest positive weighted drop occurs. The resulting code is called a combined direction code.3. If the cell is a no data cell the Flow direction value is set as -1*10(4).After the above steps, some cells in FD have a combined direction code. These are the cells for which the largest positive weighted drop occurs in more than one direction. These cells are assigned a single direction code using a lookup table.Flow direction1. Open the Flow Direction tool. 2. The input surface is the filled DEM grid. 3. The output raster should be set to Folder

Flow Direction in ArcGIS4. Turn off display of the Filled DEM layer.

2.3. FLOW AccumulationA flow accumulation raster tabulates for each cell the number of cells that will flow to it. The flow accumulation value of a cell is the sum of the flow accumulation values of the neighboring cells which flow into it. The tabulation is based on the flow direction raster. A flow accumulation raster records how many upstream cells will contribute drainage.Flow accumulation1. Open the Flow Accumulation tool.2. Set the input flow direction raster to the output of the last task. 3. Set the output raster to Folder.

Flow Accumulation in ArcGIS4. The flow accumulation layer has a value for each cell; that value represents the number of cells upstream from that cell. Cells with higher values will tend to be located in drainage channels rather than on hillsides or ridges.5. Flow accumulations are important because they allow us to locate cells with high cumulative flow. After flow Acc will make

2.4Conditional File1 Open Spatial Analyst Tools Conditional Con2 conditional file con conditional file in first input Flow Direction file Second input will be flow accumulation file.3 Turn off the accumulation Layer.In map algebra tool Raster calculator is used to evaluate con >1000 is an arbitrarily high number representing large flows in the river bed. Depending on the size of area adjust this number.

Raster calculator in ArcGIS1. In the resulting Calculation layer, re-symbolize the 0 values to be No Color (i.e., transparent). Now the raster layer is showing the river course.2. Now a flow direction and a flow accumulation raster , as well a raster layer showing the river course.3. Turn off the Flow Direction layer.

2.5 Stream Order4. Open the Stream order tool bar5. Set the input resulting calculation layer6. Set the input flow direction raster to the output of the last task. A stream network can be derived from a flow accumulation raster. The derivation is based on a threshold accumulation value. A threshold value of 1000, for example, means that each cell of the drainage network has a minimum of 1000 contributing cells. Given the same flow accumulation raster, a higher threshold value will result in a less dense stream network. The resulting stream network from a threshold value should correspond to a network obtained from traditional methods such as from high-resolution topographic maps or field mapping.After a stream network is derived from a flow accumulation raster, each section of the stream raster line is assigned a unique value and is associated with a flow direction. A stream link raster therefore resembles a topology-based stream layer: the intersections or junctions are like nodes, and the stream sections between junctions are like arcs or reaches.

3 Steps for generating long profileMake sure that your DEM is projected in UTMExtract the trunk channel/ main stream from your drainage network. Perform the following steps to do the task:Create the source point. Run a cost path 3.1 Spatial Analyst Tools Distance Cost PathUse the source point as the Input raster or feature destination data.For the cost distance and cost backlink rasters, use the flow accumulation and flow direction rasters respectively. Specify the output as your channel.Now you want to extract the coordinates and elevation along this channel.

3.2 Spatial Analyst Tools Extraction Sample.Use your filled DEM, flow accumulation as the Input rastersSpecify channel as your Input location raster.Give a name for the Output table and do not forget to add the extension .dbf.

You can now open this file in an Excel sheet. The Sample tool runs from NW to SE rather than along your channel, so you will need to sort all the data by elevation before you can process this data. To do so, select all the columns, then go to Data Sort (in the main toolbar) and sort the data by elevation in the descending order.Get the distance between two adjacent points by using the distance formula. Get the cumulative distance. You can first convert this into km. Now plot the cumulative distance against the elevation.