topic Re: How to calculate the discharge of the river in ArcGIS Spatial Analyst Questions

How to calculate the discharge of the river

Chang-HengYang — Mon, 11 Nov 2013 14:00:48 GMT

Hi all,

I am doing a small project related to the hydrology in New Mexico. My goal is to get the discharge of a targeted river. All I have is the DEM (Digital Elevation Model) of the area of interest. Please let me know if I could use the Hydrology tools in Spatial Analyst to solve this problem. Please also let me know If I need more data or other tools.

Many thanks,
Hank

Re: How to calculate the discharge of the river

NeilAyres — Tue, 12 Nov 2013 05:01:35 GMT

The hydrology tools would certainly be the place to start.
http://resources.arcgis.com/en/help/main/10.2/index.html#//009z0000004w000000
At the very minimum, to get your DEM to be hydrologically correct (everything flowing in the right direction, no sinks etc).
But the hydrological experts would probably need to use other tools like arcHydro or something.
But if all you're looking for is possible complete discharge, wouldn't a FlowAccumulation meet your needs.
Cheers,
Neil

Re: How to calculate the discharge of the river

XanderBakker — Tue, 12 Nov 2013 07:28:32 GMT

The discharge is normally calculated by multiplying the cross-sectional area of the river with the velocity of the water (m³/s).

The "Flow Accumulation" creates a raster of accumulated flow into each cell. A weight factor can optionally be applied.

If your weight map would be the amount of water that contributes to the discharge, then you could calculate it. Since you say you only have a DEM, this will be not enough to calculate the discharge.

Kind regards,

Xander

Re: How to calculate the discharge of the river

Chang-HengYang — Tue, 12 Nov 2013 14:43:44 GMT

Hi Xander and Neil,

Thank you for your information. I wonder if Xander could provide more detailed information on this sentence "If your weight map would be the amount of water that contributes to the discharge, then you could calculate it." I am a little confused how I calculate the discharge even if I could have the amount of water.

Thanks,
Hank

Re: How to calculate the discharge of the river

TimBarnes — Wed, 13 Nov 2013 01:31:55 GMT

Hi Xander and Neil,

Thank you for your information. I wonder if Xander could provide more detailed information on this sentence "If your weight map would be the amount of water that contributes to the discharge, then you could calculate it." I am a little confused how I calculate the discharge even if I could have the amount of water.

Thanks,
Hank

Discharge at a location will, of course, vary with rainfall (and time....and catchment characteristics*)- No rain = no flow/discharge.
Your weight map can be used to represent this rain. The default value is 1 which assumes the amount of 'water' applied at each cell is the same and therefore flow is hypothetical. However, if a rainfall event only affected half the catchment, then this weighting might give values of 1 for the area where rain fell and 0 for areas where it didn't etc meaning flow would only be present in cells below the rainfall extent.

* these characteristics are probably best calculated with other software.

Re: How to calculate the discharge of the river

XanderBakker — Wed, 13 Nov 2013 05:32:24 GMT

Discharge at a location will, of course, vary with rainfall (and time....and catchment characteristics*)- No rain = no flow/discharge.
Your weight map can be used to represent this rain. The default value is 1 which assumes the amount of 'water' applied at each cell is the same and therefore flow is hypothetical. However, if a rainfall event only affected half the catchment, then this weighting might give values of 1 for the area where rain fell and 0 for areas where it didn't etc meaning flow would only be present in cells below the rainfall extent.

* these characteristics are probably best calculated with other software.

As Tim already outlined the weight map can be used to distinguish between rain (1) and no rain (0) to obtain a "discharge". The discharge in that case will be the number of pixels that contribute to the discharge.

What I was referring at, is if you knew what amount of rainfall per time unit is contributed by each pixel, then this information as a weight would lead to the discharge of the river in a flow accumulation calculation.

This is only theoretical since it will be quite challenging to derive that kind of information. The schema below shows some of the aspects involved:

[ATTACH=CONFIG]29068[/ATTACH]
source: http://www.bbc.co.uk/scotland/education/int/geog/rivers/drainage/index.shtml

Kind regards,

Xander

Re: How to calculate the discharge of the river

MarkBoucher — Thu, 14 Nov 2013 16:28:19 GMT

The "Flow Accumulation" creates a raster of accumulated flow into each cell. A weight factor can optionally be applied.

Please understand that the numbers in the "Flow Accumulation" raster have little to do with the amount of water that is flowing off of the watershed. It is derived from the "Flow Direction" raster and is simply a count of the number of raster cells that are upstream of that cell (and maybe including that cell) and nothing more. It is a count and not an actual "flow" such as cubic feet per second or gallons per minute. It can represent the area if you multiply that grid's value by the area of the grid cells. The term "flow" comes from the fact that it is created using the flow direction grid. Noting the possible confusion would make me tend to want to rename it to "Cell accumulation count based on flow direction grid", but that's too long.

Estimating the flow rate (discharge) at a point in a pipe/creek/stream/river can be done several ways. There is the:

Rational Method (easy by hand),

Unit hydrograph methods (can be done by hand but is normally done using software such as HEC-HMS),

Other methods that require specialized software (such as that created by Vieux, Inc. http://www.vieuxinc.com/about.html).

Each method requires a different level of data for input and provides a different level of detail in the output. the Spatial Analyst tools Arc Hydro tools can be used to measure the inputs for several different methods.

The acceptable method is usually determined by the size and complexity of the watershed and sometimes the accuracy you need for your situation. The purpose (planning or design) will also determine if you need a back of the envelope number or a hydrograph with 5-min time steps. Normally, the local jurisdiction will have hydrology standards that they accept for the different purposes.

Re: How to calculate the discharge of the river

XanderBakker — Fri, 15 Nov 2013 06:40:08 GMT

Thank you Mark, for clarifying that one simply cannot obtain the discharge from a DEM and providing insights on how this should be done. +1 for that.

I guess I should have mentioned more clearly in my first post that it isn't possible to calculate the discharge applying a flow accumulation on the DEM.

Kind regards,

Xander

Re: How to calculate the discharge of the river

Chang-HengYang — Fri, 15 Nov 2013 16:32:04 GMT

Hi professionals,

I really appreciate your opinions. I will discuss with my coworkers to see if we could collect the necessary data and calculate the flow rate.

Thanks again,
Hank