Spatial Hydrology of the Urubamba River System in Peru
Using Geographic Information Systems (GIS)

by Francisco Olivera, Ph.D.
Center for Research in Water Resources
University of Texas at Austin
January 1996

Macchu Picchu, one of the wonders of the Inca civilization,
is located few kilometers away from the Urubamba River.
(Photo by F. Olivera, 1989)


1. Introduction
2. Overview of the study site
3. Stream and Watershed Delineation
4. Using the Hydrologic Modeling Extension

1. Introduction

The purpose of this page is to give the reader a feeling of the potentiality of available-for-whole-world data, such as the USGS 30 arc-second digital elevation model (DEM) and the Digital Chart of the World (DCW), when used in combination with the PC version of ArcView 3.0 and its extension Spatial Analyst. The 30 arc-second DEM's (approximately 1 Km² square cells) for the entire world have been developed by the Earth Resources Observation Systems (EROS) Data Center of the United States Geological Survey (USGS), and can be downloaded from their internet site. Instructions for downloading, unzipping, and setting these DEM grids are available at How to Download a 30" DEM?. The Digital Chart of the World is a database of geographic features of the world, developed by the Environmental System Research Institute (ESRI).

The study site is the Urubamba River system located at the South-West of Peru in South America. Any region of the world could have been studied, and the case of the Urubamba river has been chosen more because of personal than of technical reasons. My familiarity with the region, undoubtedly, biased the selection of the study site, but that does not imply that knowledge of the area is required for identifying the basic spatial hydrologic features of the region.

This web-page does not attempt to be a complete analysis of the hydrology of the Urubamba River system or an ArcView tutorial, it is just a demonstration of how far one can get using basic and available data handled with ArcView 3.0 in a personal computer (IBM or compatible).

Before you start

Download the following files located at the anonymous ftp site in directory pub/gisclass/urubamba:

The pe30dem.asc is an ASCII version of the 30" DEM of Peru and it is a 16Mb file. If you don't have room for this on your class file space, you can find an imported version of this grid in Arc/Info format at /home1/alpha62/maidment/peru. Since ArcView can read files across directories you can use this file without actually copying it into your own directory.

You need to also have available the Spatial Analyst and Hydrologic Modeling extensions of Arcview 3.0. These are loaded on the Alpha machines in the LRC. To make these extensions available, start Arcview 3.0, click on the Project window to make it active, and then use File/Extensions to bring up the list of available extensions list. In the list of Available Extensions, click on the Spatial Analyst and the Hydrologic Modeling extensions to make them active. When you open a view, you'll see additional menu items than normal, including Analysis and Hydro. If in the Available Extensions list you did not find the Hydrologic Modeling extension, you can download the file hydro.avx from the same site as the other exercise files. This file must be put in the same place in the ArcView program files as the other extensions.

To import the ASCII file pe30dem.asc into a grid, open a new View window and make it active, then go to File/Import Grids..., select ASCII under Select import file type , select the file pe30dem.asc, and name it pe30dem. Be patient while the importing of this Grid occurs as it contains more than 3 million cells. The imported grid will not be added to the View. Also, import the ASCII file urgrid.asc into a grid called urgrid, in the same way as for pe30dem.asc.


Prepare the following maps:

All maps are included in this exercise. Include title, North arrow, scale, and legend if applicable.

2. Overview of the Study Site

Peru is located at the north and a half of South America's West coast. It extends from latitude 0° to 19°S, longitude 68°W to 82°W, and has an area of 1,285,215.6 Km² (496,251.7 sq-mi). Here is a view of the world seen from above Peru:

The coverage used to prepare this figure, called cntry94.shp, was obtained from ESRI Data & Maps supplied with the ArcView 3.0 code. The original coverage is in Geographic Projection and has been projected into Orthographic Projection with central meridian 75°W and reference parallel 10°S.

To create this View of the world from above Peru, click the View icon in the Project window, press the New buttom, go to View/Add Theme, select Feature Data Source under Data Source Types, and select the shapefile cntry94.shp. To change the name of the View to World, go to View/Properties and enter World in the Name slot. To display the View in Orthographic Projection, go to View/Properties, press the Projection button, check the Custom option, under Projection select Orthographic, and enter the values of the Central Meridian (-75.00) and Reference Parallel (-10.00) in decimal degrees. Further information on projections of the world and the method of presenting the world from space is presented in Exercise 3. Map Projections.

Topography of Peru

Create a new View and name it Peru (using the View/Properties menu box). Add the themes pe30dem (grid data source), and border.shp, roads.shp and cities.shp (feature data sources). You can find the pe30dem theme at /home1/alpha62/maidment/peru. You cannot just add these themes to the View World because these themes are in a different map projection, in planar coordinates, while the View of Peru from Space created earlier is in Orthographic Projection. All themes presented in a single view have to have the same map projection.

To display the topographic map of Peru with a nice color legend, click the theme bar in the legend area of the View Peru to make pe30dem active (the theme bar will raise), go to Theme/Edit Legend, press the Load button, and select the file topo_shd.avl. Here is the result of these actions:

Very nice!!

Add the grid theme urgrid to the view (if you don't find this theme it is probably because you omitted to converted from urgrid.asc earlier). You'll see that this grid is a portion of the pe30dem grid for the central part of Peru. Shade this grid with the topo_shd.avl legend file as described previously. Click off the pe30dem grid and use the zoom tool in the View window to zoom in on the urgrid region. Highlight the urgrid, click on the Info (i) tool, and then click on various points on the Grid. In the query response box that appears, you'll see a box called Identify Results with a right hand column containing two items value and count. The value is the elevation of the land surface in meters above mean sea level and count is the number of cells which have this value in the grid. Click on the Histogram button in the View window with the urgrid theme highlighted, and you'll see a histogram of the elevation values in the Urgrid.

Drainage Regions of Peru

Peru is crossed from North to South by the Andes, which contain its highest mountain, the Huascaran, 6,768 m (22,205 ft) high. Three major drainage systems are found in Peruvian territory:

Thus, in a nutshell, all the land to the West of the Andes drains to the Pacific Ocean, that to the East of the Andes drains to the Amazon river, and that of the Southern part of the Peruvian Andes drains to Lake Titicaca. The Urubamba drainage basin that we will study in this exercise, as part of the second water system, is a tributary of the Amazon River.

Hillshade Depiction of Topography

Another way of looking at topography is to view it as a hillshade, that is as it does when the sun casts light and shadows on a hilly region in relation to the angle of the sunlight and the nature of the topography.

To accomplish this, make the pe30dem grid theme active (use the urgrid theme if you are short of file space), go to Analysis/Compute Hillshade... , and enter the parameters or accept the default ones. A new Theme is created and added to the View Peru. You might have disk space problems because your are actually creating the hilshade grid. Click on the small square to its left to display it.

Roads and Cities of Peru

Add the Feature Data Source themes roads, cities,and border give more information about Peru. These data were selected from the themes contained in the Digital Chart of the World. Of the 2230 populated points included for Peru in the Digital Chart of the World, only 35 have been selected for display here. These cities include the capitals of the former departamentos (equivalent to provinces or states), the main ports, the cities with important economic activity and Pisac (of course!).

Here is a display of these themes:

You'll see that the roads theme contains roads which are outside the border of Peru. To select just those roads which are inside Peru, highlight the roads.shp theme, using Theme/Select By Theme which produces a dialog box, using intersect with the selected features of Border.shp. This takes a little while to select all the roads within the borders of Peru, which show up colored yellow. Once that is done, use Theme/ Convert to Shapefile to produce a new shape file of just the roads within Peru. Lets call this file peroads. Add this file to the View and click off the roads.shp file so that you display only the roads within Peru on your map.

Urubamba River System

The Urubamba River system is located in the southern part of Peru. A rectangular box of 410 Km (East -West) by 543 Km (North - South), that comprises the entire watershed, was selected for analysis purposes. Let's take a closer look at this area. Here is a location map of this analysis region, which shows the Urubamba River system as well the main cities of the country. Cities within the study area are labeled with their names.

The rivers of the Urubamba river basin were selected from a large set of rivers contained in the Digital Chart of the World, as shown in the following figure in which the Urubamba River and main tributaries are in dark blue, and the rivers of the study area are in light blue.

Topography of the Urubamba River Basin

Create a new View and name it Urubamba (using View/Properties menu box). Add, in this order, the themes ur_burn0.shp (Feature data source) and Grid data source pe30dem (or urgrid if you are short of file space). You will find yourself, zoomed-in to the Urubamba River watershed. Adjust the display area so that the Northern part of the drainage system ur_burn0.shp flows out of the window and that the entire drainage area fits within the window. Go to Analysis/Properties, and select Same as Dislay in the Analysis Extent slot. This has the effect of isolating the portion of the pe30dem or urgrid for study.

Change the legend of pe30dem (or urgrid if you choose to work with it) to topo_shd.avl as you did before.

Here is the result of these actions. In this image, the Urubamba River and its main tributaries are shown in blue.

3. Stream and Watershed Delineation

The standard methodology for delineating streams and watersheds from a raster digital elevation model (DEM) is based on the eight direction pour-point algorithm. This algorithm identifies the grid cell, out of the eight surrounding cells, towards which water will flow if driven by gravity. This methodology consists of:

An extra and prior process has been added to this methodology, and it consists of burning-in the digitized streams that have been observed in the field. This burning-in process consists of raising the elevation of all the cells but those that coincide with the digitized streams. By doing this, water is forced to remain in the streams once it gets there; however, it is not forced to flow towards them. Extensive experience at the CRWR has shown that the streams delineated using this improved methodology represent much better the real stream network. This process consists of:

After these three steps have been performed, the standard methodology described above is applied to the burned-DEM. A step-by-step description of the whole methodology is presented below.

Part I: Burning in the Streams

This part consists of modifying the DEM, by burning-in the streams and by filling the sinks, so that ArcView hydrologic functions can be implemented. Assuming that the name of the working View Window is Urubamba, and that it contains the ur_burn0 stream coverage and the pe30dem grid, to modify the DEM:

Part II: Delineating Streams and Watersheds

It is assumed that the working View is Urubamba, and that it contains the Filled Burndem grid-theme. The stream and watershed delineation itself is made by the Wshed script shown in the figure.

To run this script:

Here are some results of this processing.

Flow Direction Grid.
The Flow Direction grid is best displayed by editing the legend to show Legend Type: Graduated, Classification Field: Value, Classify: Equal Interval using 8 classes, and then typing the values 1, 2, 4, 8, 16, 32, 64, 128 in the Value column of hte Legend Editor. The shading shown in the picture was done using a Red Monochromatic Color Ramp with the shading reversed using the button at the bottom of the legend editor so that low numbers have a dark color and large numbers a light color.

Flow Accumulation Grid
The Flow Accumulation Grid is best displayed by editing the legend so as to show Legend Type: Graduated Color, Classification Field: Value, and Classify as Standard Deviation with Break Classes at 1/4 Std Deviation.

Stream Link Grid
The Stream link grid has a unique value for each stream segment. These can be illustrated by using Legend Type: Unique Value, Values Field: Value, and Color Scheme:Bountiful Harvest. If you use the Info tool and click on various cells within each stream link, you'll see how they have the same value, and how the value changes as you move from link to link.

Watershed Grid
The Watershed grid is displayed similarly to the stream link grid. If you click on cells of the watersheds in the same region as the stream links, you'll find that they have the same value because the value in the stream link is assigned to all cells in the watershed area that flows to that link.

  • Vectorize the Wshed grid by making it active, going to Theme/Convert to Shapefile, and writing a file name in the dialog box.
  • The script, as it looks in the figure, uses a threshold of 2500 grid cells, i.e., 2500 Km2. This number, though, can be changed manually editing the script according to the user needs.

    The streams and watersheds, of the Urubamba River basin, delineated with this methodology are shown in the figure. In this case, just the watersheds that drain the Urubamba River have been selected by highlighting the Wshed.shp theme, and using Theme/Select By Theme to Intersect with the streams coverage, Ur_burn0.shp, to produce the new theme Urwshed.shp shown in the picture.

    4. Using the Hydrologic Modeling Extension

    To take full advantage of the Hydrologic Modeling extension, including the use of the W (watershed) and R (flow path) buttons and the Hydro/Watershed function, the name of the flow-direction and flow-accumulation grids (Urfdr and Urfacshould be entered in the Hydro/Properties dialog box. Once the name of these grids has been entered, the buttons and the function become active.

    After making active the Filled Burndem grid-theme (regardless it is displayed or not), cliking the R button enables the flow-path function (the R button will remain depressed). Clicking on any point of the View display area will generate a flow-path line that runs from the point to its pour point or out of the analysis area. More than one point might be cliked and all the flow-paths are displayed at once. The following figure shows the results of applying the flow-path function to six points of the study area.

    Clicking the W button - again with the Filled Burndem grid-theme active - enables the watershed function (the W button will remain depressed). Clicking on any point of the View display area will generate a watershed grid for the selected point. The following figure shows the result of applying the watershed function on the most downstream point of the Urubamba, just before its confluence with the Ene River to form the Ucayali River. You have to be sure that you are clicking on a stream cell when you operate this function or else what you'll find is that you get a very small watershed from a point just next to the stream. Zoom in on the stream itself and display the stream grid Urstr so that you can see the cells that are actually on the stream.

    To Be Turned In: a map showing the delineated watersheds and streams of the Urubamba River, and a copy of the Avenue Scripts Burndem and Wshed that you used to do Parts I and II of the grid delineation process.

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