Watershed Modeling Using ArcView Timothy N. Loesch 2000 Minnesota GIS/LIS Consortium

Watershed Modeling Using ArcView

Timothy N. Loesch

2000 Minnesota GIS/LIS Consortium

Minnesota GIS/LIS Consortium

Spring Workshops 2000’Page 2

Watershed Modeling Workshop

Welcomes and Introductions Who the heck am I?

• Work in the DNR MIS Bureau - GIS Section

• First involved in GIS in 1985

• Interest in Education, Programming and Hydro

• e-mail - [email protected] Who the heck are you?

• Name, Rank & Serial number

• Background Bio Information...

• ArcView Experience?




Class Content Overview Hydrologic Modeling Process in GIS Overview of Digital Elevation Models Explanation of Processes and Procedures Attributes Derived from DEMs Implementation in ArcView




Class Timetable/Agenda 9:00 - 9:15 - Welcome and Introduction 9:15 - 10:00 - Hydrologic Modeling in GIS

• Description of terms

• Discussion of Digital Elevation Models (DEM) 10:00 - 10:15 - Preprocessing DEMs

• Filling Sinks

• Determining Sink Depth




Class Timetable/Agenda 10:15 - 10:30 - Break! 10:30 - 11:30 - Surface Parameters

• Flow Direction

• Flow Accumulation

• Flow Length 11:15 - 12:15 - Delineate Watersheds

• Delineation of Contributing Areas

• Delineation of Watersheds and Basins

• Summarizing Watershed Parameters




Class Timetable/Agenda 12:15 - 12:45 - Lunch 12:45 - 2:30 - Map Overlay and Predictive

Equations 2:30 - 2:45 - Break 2:45 - 4:00 - Extending the Hydrologic Model in

3D 4:00 Wrap-up and questions….




Hydrologic Modeling in GIS The Shape and characteristics of the earth’s surface

is useful for many fields of study. Understanding how changes in the composition of

an area will affect water flow is important!• What happens when residential development occurs?

• How does this affect the watershed?

• How can these affects be mitigated?– Best management Practices






Topographic Maps




Traditional watershed delineation has been done

manually using contours on a topographic map.

A watershed boundary can be sketched by starting at the outlet point and following the height of land defining the drainage divides using the contours on a map.

Outlet Point





Drainage system - The area upon which water falls and the network through which it travels to an outlet.

Drainage Basin - an area that drains water and other substances to a common outlet as concentrated flow (watersheds, basins, catchments, contributing area)

Subbasin - That upstream area flowing to an outlet as overland flow

Pour Point - A location at which the contributing area can be determined.

Drainage Divide - The boundary between two basins. This is an area of divergent flow.




Stream Network A stream network is composed of those areas where concentrated flow is occurring.

Stream Ordering Shreve - When two links intersect, their

magnitudes are added and assigned to the downslope link.

Strahler - Stream order only increases when streams of the same order intersect




Curvature - Curvature quantifies the shape of the surface to understand erosion and drainage process

• convex (bulging)• concave (bowing)

Profile Curvature - Curvature of a surface in the direction of slope.

• Concave - slow flow and deposition• Convex - Increasing slopes and erosion

Planform Curvature - Curvature of a surface perpendicular to direction of slope

• Convex - Divergent flow indicating ridges• Concave - Convergent flow indicates valleys




Raster Data Structure Much of the data we will use in this class will be

“Raster” data. Raster formatted data is much more suitable for

many types of landscape modeling, including hydrologic analysis.

Inputs such as elevation can only be processed as a raster data set

Raster is Faster, Vector is Corrector




Raster

Vector

Real World




Most Raster data is created by Rasterizing existing Vector data

Transparent grid is placed on map.

Each cell is assigned a numeric value according to the dominate attribute type found in cell.

Integer and Real values are valid




Digital Elevation Models (DEM) To do all of this stuff you need data. A DEM is a digital representation of the elevation

of a land surface. X,Y and Z value The USGS is the major producer of DEM’s in the

Nation



Watershed Modeling Workshop•DEM’s consist of an array of data representing elevation DEM’s consist of an array of data representing elevation

sampled at regularly spaced intervalssampled at regularly spaced intervals

XX

YY

ELEVATION VALUES



Watershed Modeling WorkshopThe elevation values of the DEM can be grouped into intervals. Each interval is then represented by a different gradient color.




Two Scales of DEMs Available for Minnesota 1:24,000 Scale

• Level 1 - 30 meter spacing– Errors up to 15 meters inherent in data– Developed using automated methods from air photos– Systematic errors evident as banding– Not appropriate for hydrologic modeling

• Level 2– Matches map accuracy of 1:24,000 scale quads– Developed by scanning published quads– Appropriate for hydrologic modeling

1 Degree (~250,000) scale - 93 meter spacing– Appropriate for regional analysis



Metro Area done by the Met CouncilFor more information look at:http://www.lmic.state.mn.us/bmap90/dem/dem.htm




Watershed Modeling WorkshopArcView Exercise #1 - Viewing DEMs1) Start ArcView by clicking on the ArcView Icon on the desktop that says HYDROMOD.APR

2) This opens an ArcView Project that we will be working in for the remainder of this workshop. It has the Spatial Analyst Extension Loaded and a couple of predefined Views and Themes.

3) Open the View called Digital Elevation Model.

4) This View contains a digital elevation model (DEM) for a small (7400 acres) watershed in Rice county Minnesota. This View contains 4 themes:

Watershed Basin - Watershed delineated in 1995Roads and Highways - From MNDOTStreams and Ditches - Digitized from USGS Digital Raster Graphics (DRGS)Original DEM - A hydrologically adjusted 1:24,000 scale Level 2 DEM from the USGS with a cell resolution of 10 meters.

5) Since we want all of this to be bounded by the incredible graphics you can create using GIS let’s start by creating a Hill-shaded view of the Original DEM Theme. To do this make the “Original DEM” theme active and then accessing the Compute Hillshade option in the SURFACE menu. Accept the default values of Azimuth and Altitude, then press OK to generate the grid. Once it’s done processing a new theme will be added to the view called “Hillshade of Original DEM”. Turn it on and view it. It’s a pseudo-3D perspective based on a light source in the upper left. Then Turn it off.

Now hide the legend of this theme by making it active and then using the THEME menu, Hide/Show Legend option.



Watershed Modeling WorkshopArcView Exercise #1 - Viewing DEMs6) We can now use the hillshade theme to act as a brightness theme for our elevation.To do this access the legend editor for the “Original DEM” theme. Then press the Advanced button in the lower left hand corner of the Legend Editor dialog. Fill outthis dialog as it appears to the right. Then press the OK button and Apply the legend.

NICE HUH?!

7) Turn on the other themes in your view (Roads and Highways, Streams and Ditches, and Watersheds). Thisshaded relief view is really a great way to view data. It adds a nice perspective to the landscape and when the watersheds and streams are displayed you can do some valuable visual analysis and interpretation.

8) Now add a couple more themes. There are two images in the C:\HYDROMOD subdirectory that you should add to this View. One is a Digital Orthophoto Quadrangle (DOQ.TIF) and one is a scanned 1:24,000 USGS quadrangle map called a Digital Raster Graphic or DRG (DRG.TIF). To add these to your View document use the VIEW menu, Add Theme option. Make sure you set the Data Source Type selection to IMAGE. Select the two images and then press the OK button.



Watershed Modeling WorkshopArcView Exercise #1 - Viewing DEMs

9)Edit the property sheets of these images by making them active and using the THEME menu, Properties option. Set the name of the DOQ to “Digital Ortho Photograph” and the name of the DRG to “Digital Raster Graphic”.

10) Then re-order these themes so that the Digital Ortho Photo theme is on the bottom of the table of contents and the Digital Raster Graphic theme is on the top.

11) Edit the legend of the Digital Raster Graphic Press the Colormap button as shown. Set the color of Class 1 to transparent. Then turn on the image and do some exploring by zooming in and out and panning around to learn more about our project area.

12) Now Save Your Project and minimize ArcView.

Colormap button

Class 1

Transparent



Elevation data drapedon Shaded Relief

Notice that this watershed is characterized by a flat upper end with few tributaries and a large valley, more relief, increased drainage in the lower stretches.




Preprocessing DEMs DEMs typically require some type of preprocessing

prior to hydrologic modeling to remove errors inherent in the data. This type of processing can greatly increase the accuracy of a DEM.

Methods of producing DEMs rarely include a “Drainage Enforcement” Algorithm to remove these errors.

Primary error found in DEMs are “Sinks”• A sink is an erroneous depression created by the

DEM interpolation routine




100 100 100 100 100

100 97 96 95100

1001009998100

100 100 100 100 101

Stream

100-meter Elevationcontour

Sink Area

100 Cell Elevation

Cells containing the contour are assigned the value of the contour,all other cells are interpolated. Sinks are always possible in areaswhere contours converge near a stream.

An example Sink




Preprocessing DEM’s (continued)

Drainage Enforcement • Using existing hydrographic features to remove

these errors is essential to produce accurate watershed basins.

• Sinks are usually small and cause drainage basins to be incorrectly delineated

• If a stream is not present elevations in sink areas are raised to elevation of saddle so that water can drain

• If a stream is present elevations of saddles are lowered to that of the sink so that water can drain.




Preprocessing DEM’s (continued)

ArcView can Fill sinks but not enforce other types of hydrologic information

ANUDEM program from Australian National University which is used to enforce known drainage patterns

Arc/Info has a routine called TOPOGRID which is an implementation of this software

• Must be careful not to remove real sinks• Lake areas should be flat!• Handles point or contour elevation data



The flat areas in this shaded elevation model are wetlands and lakes.





ANUDEM uses stream data to enforce drainage.

Stream data must have correct directionality and connectivity.




•This DEM is considered to be “Hydrologically

Adjusted”



Watershed Modeling WorkshopArcView Exercise #2 - PreProcessing DEMs1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start ArcView and Open the project which can be found in the C:\HYDROMOD folder.

2) Using the mouse, make the theme “Original DEM” active.

3) To remove sinks we will use the DNR HYDRO menu Fill Sinks option (do it now). This option will process the DEM and remove all sinks that are found. In this implementation of the fill routine you do not have any options for specifying size or area of sink to remove. It simply iterates through the data until all sink areas are removed.

4) You will be presented with a prompt “Create Sink Shapefile and Statistics?”. This will find the sinks that were filled and create a shapefile of them with statistics that represent the size and depth of the filled areas. Answer YES to the prompt. Again, ArcView will buzz a little and when done you will be asked to supply a name for this new file - Take the default name SINKPOLY.SHP and make sure it’s in the C:\HYDROMOD folder.

5) When complete, you will have two new themes added to your view document. A Grid theme called “Filled Original DEM” and a polygon theme called “Sinks with Statistics”. Turn off all your other themes and turn on the “Sinks with Statistics” shapefile. See anything? There’s one small polygon in the center of the watershed. To view the statistics, make the “Sinks with Statistics” active and then use the THEME menu, Table option to open the table. You will see that the sink is 700 square meters in size and has a depth of 1 meter.That’s pretty good but remember, this is a hydrologically adjusted DEM.



Watershed Modeling WorkshopArcView Exercise #2 - PreProcessing DEMs

6) Now turn off all your themes and turn on the “Filled Original DEM” theme and make it active. You probably don’t notice much here and it’s not the same color scheme as the original. Let’s create a new legend using the DNR Hydro menu, Interval Legend option.

Use an interval of 1, a Base value of 290 and a Max Value of 375and press the OK button.

Then select a color scheme of interest. One of my favorites is “Full Spectrum” but you can choose your own or try a few if you’d like

8) Wow, edit the legend and change the NO Data value color to transparent. Then use the THEME menu Hide/Show Legend option to hide that really long legend and make some room on our table of contents.Then, move the two new themes down a couple of notches on the Table of Contents so that you can display the vector themes (Roads and Watersheds) on top of them.

9) You now have a depressionless DEM, one that can be used for further Hydrologic Analysis. We want to save this file so, make the “Filled Original DEM” theme active and then use the THEME menu Save Data Set option to save this file to C:\HYDROMOD\DEMFILL.

10) Now save your project!



Generating Surface Parameters Flow across a surface will always be in the steepest

down-slope direction Known as “Flow Direction” this is the basis of all

further watershed modeling processes. Once the direction of flow is known it is possible to

determine which and how many cells flow into any given cell!

This information is used to determine watershed boundaries and stream networks.





Slope - Many of the parameters that we will discuss and generate in this workshop are based on slope.

Usually calculated on a 3x3 window with the center cell being the target cell.

Slope is calculated from the center cell to each of the 8 neighbors

Greatest slope is assigned to the center cell

Flow direction is that way…..

12 13 13

11 10

8 10 9

Target Cell

10



Generating Surface Parameters - Flow Direction In ArcView Spatial Analyst, the output of a Flow

Direction is a grid whose values can range from 1 to 255 based on the direction water would flow from a particular cell. The cells are assigned valued as shown below.


32 64 128

16 1

8 4 2Target Cell



Generating Surface Parameters - Flow Direction If a cell is lower than its eight neighbors, that cell is

given the value of its lowest neighbor and flow is defined towards this cell.

If a cell has the same slope in all directions the flow direction is undefined (lakes)

If a cell has the same slope in multiple directions and is not part of a sink the flow direction is calculated by summing the multiple directions




Flow Direction


100 100 100 100 94

100 97 96 95100

1001009998100

100 100 100 100 101

2 2 2 1 128

1 1 1 12864

3264128128128

128 64 64 32 80

Original Surface Flow Direction Surface



Flow Direction




Generating Surface Parameters - Flow Accumulation If we know where the flow is going then we can figure

out what areas (cells) have more water flowing through them than others.

By tracing backwards up the flow direction grid we can figure the number of cells flowing into all cells in a study area

Accumulated flow is calculated as the accumulated number of all cells flowing into each downslope cell.




Generating Surface Parameters - Flow Accumulation For an accumulation surface the value of each cell

represents the total number of cells that flow into an individual cell

Cells that have high accumulation are areas of concentrated flow and may be used to identify stream channels.




2 2 2 1 128

1 1 1 12864

3264128128128

128 64 100 32 80

Flow Direction Surface


0 0 0 0 18

0 3 8 150

00220

0 0 0 0 0

Flow Accumulation Surface




Flow Accumulation Surface



Generating Surface Parameters - Flow Length Another parameter that can be calculated is flow

length Flow length is defined as the length of the longest

flow path within a drainage basin or watershed. This measure is often used to calculate the time of concentration of a basin.

ArcView calculates the flow length for each cell within the basin. The cell values represent the length of flow to an outlet from that point.




2 2 2 1 128

1 1 1 12864

3264128128128

128 64 100 32 80

Flow Direction Surface

144.6 114.6 84.6 30 0

132.3 102.3 72.3 42.330

84.672.384.6114.6144.6

156.9 126.9 114.6 126.9 0

Flow Length Surface


Assuming 30 meter sq. cells



Flow Length Surface




The Whole Process - a Flowchart




Watershed Modeling WorkshopArcView Exercise #3 - Generating Land Surface Parameters1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start ArcView and Open the project which can be found in the C:\HYDROMOD folder. Make sure that the “Digital Elevation Model” View is open and active.

2)The first step in this process is to create the Flow Direction theme. We use an elevation theme as our base so make the “Filled Original DEM” theme active and then use the DNR HYDRO menu Flow Direction option to create this grid. The computer will whir for awhile and then add a new theme called “Flow Direction”. Turn this theme on. Notice the cell values in the legend. Remember that 1 represents east flow direction etc. You may also notice that this is really showing us the aspect of the landscape. Edit the legend for this theme and then press the Load button to load the legend file: C:\HYDROMOD\FLOWDIR.AVL. Use the defaults and apply the legend.

4) Now we’re ready to create the Flow accumulation grid. To do this, make the “Flow Direction” grid active and then use the DNR HYDRO menu Flow Accumulation option to create this theme. You will then be requested to enter a “Flow Accumulation Weight Grid”. We don’t have one at this time so press the CANCEL button. At this point the computer will process for a bit and then add a new theme to your View called “Flow Accumulation” (surprise).

5) Turn on this grid and have a look. All you really see in this display is one area that look like they could be stream channel. What you’re looking at here is an artifact of the legend. To create a better looking view of this surface we can modify the legend to show more of the concentrated flow network. To do this create a legend that has the following classes:

0 - 100100 - 1000010000 - 2500025000 - 5000050000 - 260000



Watershed Modeling WorkshopArcView Exercise #3 - Generating Land Surface ParametersThen colorize the classes and apply the legend. You should see a much better display of the drainage channels in the landscape. Let’s assume that any cell with less than a 100 cell accumulation is most likely overland flow so make this class transparent in your legend. Also make the NO DATA class symbol transparent.

6) Now let’s calculate flow length for every cell in the watershed. To do this make sure that the “Flow Direction” theme is active and then press the DRN HYDRO menu Flow Length option.

7) You will be asked if you want to calculate the flow length to an outlet. In other words, do you want ArcView to find an outlet and calculate flow length from there. The answer in this case is yes. ArcView will then create the flow length grid called “Downstream Flow Length” and add it to your view document.

8) Examine the grid. What does this tell you? Save your project……

9) Let’s look at how stream ordering can be created from this “Flow Accumulation” theme. To save some time I’ve already created these data sets. You will find two Grid Data sets in the C:\HYDROMOD subdirectory, one called STRAHLER and one called SHREVE. Add these two themes to your View Document. Notice the difference between the two systems, most notably the difference in maximum stream order. You could convert these to Vector representations of flow channels if desired.



Watershed Modeling WorkshopArcView Exercise #3 - Generating Land Surface Parameters

10) Another way to visualize flow paths is to create a vector version of them. Do this using the DNR Hydro menu Create Stream Network option. You will be required to enter a Flow Accumulation theme, a Flow Direction Theme, and a flow accumulation threshold. For the Flow accumulation threshold use a value of 25.

11) Save the file using the name provided.

12) Once complete a new theme will be added to your view called Stream Network Shape - (25). Turn it on. This shows you all flow paths that drain an area of 25 or more cells (250 square meters).

If you look close, you’ll see that these drainage paths follow the flow direction grid…..

13) If you’d like you can create another one of these and try a threshold of 10 and see what happens…

14) Save your project and minimize ArcView….



Generating Watersheds and Catchments Now you are ready to start delineating watershed

basins Watersheds or Catchment areas are the basis for

many hydrologic analysis Watersheds can be created for any spot in a dataset.

• Culverts, bridges, gauging stations etc.

• Points can be interactively selected by the user or you can specify a minimum catchment size.

• Points become the lowest point on the boundary of the watershed and are therefore referred to as pour points




Generating Watersheds and Catchments Pour points can be input as an entire theme or they

can be created by selecting a point on the screen. Using a Flow Accumulation Theme you can specify

a minimum sized watershed and let Spatial Analyst automatically generate watersheds for an entire area.

• Generally not useful for most purposes

• Arbitrarily assigns watershed outlets

• Has trouble assigning leftover areas….




Pour Point Contributing Area





Minimum Catchment Area = 5000 cells Minimum Catchment Area = 10000 cells



Watershed Modeling WorkshopArcView Exercise #4 - Watershed Delineation1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start ArcView and Open the project which can be found in the C:\HYDROMOD folder. Make sure that the “Digital Elevation Model” View is active and open.

2) What we’ll do first is create some watersheds for the entire study area based on a couple of points that are stored in a file. These points represent stream gauges on the watershed and can be found in the file C:\HYDROMOD\GAUGES.SHP. Add this theme to your project and turn it on.

3) To create the watersheds, make the “GAUGES.SHP” theme active and then use the DNR HYDRO menu Watershed from Points option. You will then be requested to supply the grid theme that represents the Flow Directions. Select the “Flow Direction” theme from the list and press the OK button.

You will be prompted with a “Conversion Extent” window. Set the Output Grid Extent to “Same as Original DEM” and set the “Output Grid Cell Size” to 10.

4) You will then be required to specify the Field in the GAUGES Shapefile who’s values will be used to assign ID’s to the new watersheds. Use the ID Field in this case.

5) Give the output file a name. Take the default in this case. The computer will crank the numbers for a while and then add a new theme to the view called “Calculated Watersheds”. You will see that ArcView has created 3 basins for this area based on the points in the file. To get a better view of them edit the legend and use the Advanced button to specify a brightness theme for this theme. Use the theme “Hillshade of Original DEM” as the brightness theme. Then apply the legend and view the data. Wow! What a country! Now turn off that theme.



Watershed Modeling WorkshopArcView Exercise #4 - Watershed Delineation

6) As discussed in the lecture, you can also interactively create watershed by selecting a point of interest. To do this we need to use the Watershed Tool (the tool with a “W” in it). Select the tool and then press the mouse on the View on any location on the map that you want to see a drainage area. You will be requested to select a Flow Direction and Flow accumulation grid once you select an area and a watershed will be generated and a theme added to your view document for that theme. Try it and turn on the theme.

7) Interpretations? What do you think? Did it produce the watershed you expected? This process is REALLY sensitive to where you place the point. If it’s not on a flow path, then it won’t create a watershed you expect. To help this process, Turn on the Flow Accumulation theme to show you where the flow paths are. You may need to zoom in quite a ways in order to select a point that falls on a flow path.

8) Try it for some other locations in the study area. Try creating a catchment area above a point where a road crosses a stream. In the words of the infamous Bart Simpson, “Cool Man”.

9) Try using the RainDrop Tool. This tool will follow the flow direction from the point chosen to the outlet. You can see where the water will flow if it fell on that spot. Use the RainDrop Tool (the Tool with an “R” in it) to select a point on the View, you will be requested to supply a Flow Direction Grid and the flowpath will be created.

10) Save your project and experiment a bit.



Summarizing Information for Watersheds. Once you have created watersheds for an area many times

you want to summarize other land surface variables that fall within the watersheds.

This type of Information derived from raw data increases your Understanding a system and how changes to that system (land use etc) affect the process.

Two types of reports can be created• Statistical summaries of numeric data (elevation, precip)

• Cross-tabulations of Nominal Data (land use, soil types) This can be done easily with ArcView.




Summarizing Information for Watersheds.


Statistical Summary

Cross-Tabulation



Summarizing Information for Watersheds. This type of information can then be used in reports or fed

to other analytical programs for further analysis.• Hydrologic models

– HEC, HydroCAD, SWRRB, others….

• Graphs, charts can be used to view data When using existing programs create a summary of

required inputs and see if you can generate them from the spatial layers available to you via GIS.

More and more is being done to interface GIS and Hydrologic models.










Watershed Modeling WorkshopArcView Exercise #5 - Summarizing Watershed1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start ArcView and Open the project which can be found in the C:\HYDROMOD folder.2) What we’ll do first is summarize some information by watershed. Let’s start with land use/cover. In the C:\HYDROMOD subdirectory you will see a GRID theme called LULCXRA3 that contains this information. Add this theme to the View document and then edit the legend and load the legend file C:\HYDROMOD\LULCXRA3.AVL. Then apply the legend and view the data.3) Since this is nominal data (the numbers represent land use classes) you can’t perform statistics, you can only do a cross-tabulation. To do this use the ANALYSIS menu Tabulate Areas option. We’ll use the LULCXRA3 Theme as our Row theme with the DESC field as the Row Field. We want to summarize by watershed so choose the “Calculated Watersheds” theme you created during Exercise 4 or as shown below.When you are done, press the OK button. At this time a table will be created that shows the results. Remember that the units here are in square meters.



Watershed Modeling WorkshopArcView Exercise #5 - Summarizing Watershed5) Close this table and make sure that your View document is active6) Now let’s create some statistics for the elevations within each of the watersheds. To do this make your “Calculated Watersheds” theme active and then use the ANALYSIS menu Summarize Zones option. You will be presented with two inputs you need to provide (see examples below) A) Pick Theme containing variable to Summarize: “Filled Original DEM” B) Select Statistic to Chart: Area7) A new table and a new chart will be created. What do you think? What could you do with these? Of course you would pretty them up and make them interpretable but for now, we’ll just move on...8) That’s all folks! Save your project and quit ArcView.




Map Overlay is the one of the bread-and-butter abilities of GIS

Cell-by-cell overlay is the process of taking two or more raster files and comparing the values of each of the files, cell by cell.

The result is a map that identifies map coincidence with the values of the output file based on some user-defined process.




Raster Files can also support “Mapwide Overlays” Take a mathematical equation and “plunge” it

through a set of registered map layers. Each map layer is considered a variable Each location is considered a case Each value considered a measurement

Very important stuff for modelers




Using the Flow Direction and Flow Accumulation processes it is easy to compute a mass balance for each cell in terms of: S = P - I - F - E where,

• S = surplus water per cell

• P = input precipitation

• F = infiltration

• E = Evaporation Cumulative flow over the net is then obtained by

accumulating S over the Flow Direction grid.




Predictive equations are another way to use map overlay.

The Water Available for Runoff equation is used by hydrologists use to calculate the amount of water that will be available for runoff for a specific rainfall event, based on the type of soil and land cover present at a location.

Using this information you could predict the amount of water available for runoff at every cell in a watershed

This could then be accumulated across a flow direction surface Very important to a road engineer who has to design a road that crosses that drainage. After all, if they put in a culvert that is too small there could be problems.



Watershed Modeling WorkshopArcView Exercise #6 - Calculating Water Available for RunoffPROJECT OVERVIEW

Let’s assume that you are managing a set of watersheds in Rice county and you wanted to determine the total amount of surface water runoff that could be potentially derived from these watersheds. One of the classic methods of producing this type of information is to use the Natural Resource Conservation Service’s (NRCS) “Water Available for Runoff” equation. This equation predicts the amount of water that will be available as runoff based on the land cover and soil conditions, present at a certain spot, based on a given rainfall amount.

That is, if I get an inch of rain, how much can I expect to produce as runoff on a cornfield in clay soils?

The landcover/soil information is based on a value called the Curve Runoff Number or CN. The CN is derived by examining the Hydrologic soil grouping of a soil and the landcover present. CN values range from 0 – 100 and are an indication of the ability of the soil/landcover mix to absorb moisture. Concrete has a value close to 100. Sandy soils covered by forbes have values in the 30’s.

These values are derived from tables produced by the NRCS that compare landcover with hydrologic soil groups. Hydrologic soil group values range from A – D. “A” soils are light, sandy porous, well drained soils while “D” soils are heavy, clayey, compact, and poorly drained.



Watershed Modeling WorkshopArcView Exercise #6 - Calculating Water Available for Runoff

To predict the total water available for runoff you need to use the following equation:

Q = (sqr(p – 0.2 * (1000/c – 10))) / (p + 0.8 * (1000/c – 10))

Where: Q = total water available for runoff p = precipitation

c = curve number

Thus, at a minimum we need two map variables, Precipitation and Curve number. Once we have these two grids the equation can be “Plunged” through these map layers to produce an output layer that shows the water available for runoff at each location in the watershed.

The flowchart on the next page illustrates the flow of control for this analysis. Flowcharts are useful because you can map out the process, the commands and files you will need, and those you will create, during the analysis.

Sometimes analysis can get confusing because of the number of files created during the process. This file proliferation can be hard to manage if you don’t have a plan. On-the-fly planning is not the best way.

As you go through the analysis, print the name of the files you create on the flowchart sheet. The documentation is important.





ArcView Exercise #6 - Calculating Water Available for Runoff

1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start ArcView and Open the project which can be found in the C:\HYDROMOD folder.

2)Close the “Digital Elevation Model” View and open the “Runoff View” and have a look. You will see three of the four themes that we need to do this project. The “Precipitation” theme represents rainfall from a particular storm, “Hydrologic Soil Group” represents the hydrologic soil group for the soils in the watershed and “Land Use and Cover” is the land cover in the basins. Look at the property sheets of these themes to get an idea of their sources and view the data to get an idea of their spatial distribution.

3) Look at the flow chart, the first thing we need to do is create the curve runoff number grid by combining the Hydrologic Soil Group and Land Use and Cover themes. To do this use the ANALYSIS: Map Calculator option. Enter the following map equation:

[Hydrologic Soil Group].Combine( { [Land Use and Cover] } )

This command creates a new grid by performing a spatial coincidence overlay analysis of two or more grids. The values in the new grid represent zones of unique combinations of values in the input grids. A new theme, called “Map Calculation 1” will be added to your View.





4) Open the table for this new theme by making the theme active and using the THEME: Table option. This table shows you that there are 28 unique combinations of Hydrologic soil groups and Landcover. Each of these soil/landcover combinations is then assigned a value of Curve runoff based on a table you can get from the NRCS.

Normally you would edit the table, add a new item called CN (for curve number) and then enter in the values based on the soil and landcover combination.

To save time I have already created this file so you can make this new theme active and then use the EDIT: Delete Theme command to remove the theme. Then add a GRID theme using the grid: C:\HYDROMOD\CN. Then edit the legend of this theme and create Unique Legend using the VALUE field. This field stores the values of curve number for each cell.

5) Now we’re ready to do the analysis. To do this we will use the ANALYSIS: Map Calculator option.You need to enter the equation for water available for runoff as:

( [Precipitation] - 0.2.AsGrid * (1000.AsGrid / [CN] - 10.asgrid)).sqr / ([Precipitation] + 0.8.AsGrid * (1000.AsGrid / [CN] - 10.AsGrid))

Be careful when you enter this equation. It has to be just right for it to work properly. Use the mouse to enter as much of this information as you can. You will need to add some parenthesis by hand to make it work.

Once you have entered the equation press the Evaluate command to execute the analysis.





6) The machine will chuck away for a while (if you entered the equation properly) and when complete will add a new theme to the view. This theme, called “Map Calculation 1”, contains cell’s whose values represent the total amount of water that will be available for runoff based on the input rainfall event.

Edit the properties of this theme and name it “Water Available for Runoff”. Edit the Legend and classify and symbolize as you see fit.

This grid, in itself, is an important piece of information. It is obvious that in the upper reaches of the watersheds there is a few areas that have a high potential for soil erosion because they have a high potential water available for runoff value.

7) Let’s take it a step further. Remember the Flow Accumulation grid that we created in earlier exercises. In those exercises we created the accumulation grid where the grid values represented the number of cells that would flow through it. Well, the Flow Accumulation command can take a weight grid and accumulate the values of the cells in the weight grid and assign that value to the cells in the output grid.

To create the Weighted Flow accumulation theme we need the Flow Direction theme that was created earlier in this Workshop. Let’s get that Theme. Open the View “Digital Elevation Model” and make the “Flow Direction” theme active and then use the EDIT menu Copy option to copy this theme to the ArcView Clipboard.





Now close the “Digital Elevation Model” View and make the “Runoff View” active and then paste the theme in the clipboard to this View using the EDIT menu Paste option.

Now, make the Flow Direction theme active and then run the DNR HYDRO: Flow Accumulation command. You will then be asked to provide an Accumulation weight grid. Use the “Water Available for Runoff Grid”.

The resulting theme is a grid whose cells are coded based on the accumulated flow as water available for runoff. Of course, this is a highly idealized view of the world and does not factor in things like friction, flow length, timing and ponding but it is a start.

8) Now close all of your open ArcView windows, save your project and minimize ArcView.





There are a variety of other types of hydrologic variables that can be generated using DEMs.

Flow Direction Grids are the basis for a wide range of dynamic modeling tools

These can be calculated using ArcView and Spatial Analyst

See van Deursen and Burrough (1998) for more information.




Wetness Index wetness index = ln(As / tanB),

where:• As = Contributing Catchment

Area in meters squared

• B = Slope of cell measured in degrees




The Stream Power Index w = As / tanB

This is directly proportional to the stream power

P = pgq tanB where:

• p = density of water,

• g = acceleration due to gravity

• q = overland flow discharge per unit width

An indicator of erosive power of overland flow




The Sediment Transport Index t = [As / 22.13]^0.6 * [sinB / 0.0896]^1.3

This index characterizs the process of erosion and deposition

Presents the effects of topography on soil loss

Can vary along the length of a stream




These tools are useful for quantifying the properties and characteristics of the watershed you happen to be investigating.

May help find areas for wetland restoration or areas where bank stabilization is needed

Relate stream processes to fish distributions Plan fish structures and habitat improvements Measure the effects of management practices on the

watershed.



ArcView Exercise #7 - Calculating More Land Surface Variables

1) ArcView should still be started and the project, HYDROMOD.APR should still be loaded. If not, then start ArcView and Open the project which can be found in the C:\HYDROMOD folder.

2) Make sure that the “Digital Elevation Model” View is open and active. We’ll experiment with some of the new tools we’ve discussed in the lecture part of this workshop. Rather than cluttering up an already cluttered view we’ll make a new view and copy the “Filled Original DEM” into that view. Do the following:

a) make the “Filled Original DEM” Theme activeb) Use the EDIT menu Copy option to copy the theme to the clipboardc) Make the Project Window active and create a new View.d) Use the EDIT menu Paste option to paste the theme to this new Viewe) Edit the View’s Property sheet and name the View “Extended Variables”

3) Now make the Wetness Index Map by making the “Filled Original DEM” Theme active and then accessing the HYDRO menu Wetness Index option. Your little silicon subordinate will dutifully start processing and when done, add a new theme called “Wetness Index” to your View. Open the Theme’s property sheet and view the comments, they tell you the equation used for this analysis….

Edit the legend to make it visible. You could generate some statistics from this baby…..




ArcView Exercise #7 - Calculating More Land Surface Variables

4) Now make the Stream Power Index Map. Do this by making the “Filled Original DEM” Theme active and then accessing the HYDRO menu Stream Power Index option. This will take a bit as the machine needs to calculate a few temporary variables. When done a new theme called “Stream Power Index” will be added to your View. Again, view the property sheet and see what equation was used…..

Edit the legend to make it visible.

5) Now make the Sediment Transport Index Map. Do this by making the “Filled Original DEM Theme active and then accessing the HYDRO menu Sediment Transport Index option. The machine will crunch away and create this derived product. Once it’s done it will add a new theme called “Sediment Transport Index”. View the Property sheet.

6) Now, close the open windows in ArcView, save your project and minimize ArcView





Where is this all going? More and more effort is being put into integration

of Hydrologic Models and GIS Several Web sites are available

• http://www.ce.utexas.edu/prof/maidment/

• http://www.ciesin.colostate.edu/USDA/hydrology/models_soft.html

• http://www.ecgl.byu.edu/wms.htm

• http://www.esri.com/news/arcuser/arcuser498/hydrology.html

• http://www.esri.com/library/userconf/proc98/PROCEED/TO400/PAP400/P400.HTM




Where is this all going? Right now most efforts are being put into software

that bridges the gap between existing software and GIS.




Several vendors and Universities are putting together integration Software

ESRI has a really good CD that comes with some training materials, software and data

Obtain this CD by contacting ESRI at your local office.




To Conclude This was a brief introduction to the Hydrologic

Tools available within ArcView Realm of Possibilities was the intent of this

workshop Much more can be done with this stuff, all it takes

is some imagination and ideas on how to apply this information.

Documents

Watershed Modeling Using ArcView Timothy N. Loesch 2000 Minnesota GIS/LIS Consortium