<p>I think there are a few things to consider. First, a drainage basin is defined as the area upstream of the point to which all precipitation converges. Flow does not converge at the outlet - flow converges at the lake. Outlets don't contain any additional information of the upstream area draining into the lake. This implies that to find the area draining <em>into a lake</em>, you would necessarily have to proceed beginning from <em>each and every inlet</em>. </p>

<p>Here's another way to see this. Approximately 20% of all land drains to lakes with no outlets. These are referred to as <em>endorheic</em> lakes or <em>endorheic</em> basins. Even though there is no outlet, there is absolutely still a definable drainage basin area.</p>

CC BY-SA 3.0
2017-07-30T23:00:16.980Z
Good point about the endorheic basins, in that case they must be delineated based on **a** or **every** inlet.
cr0
CC BY-SA 3.0
2017-07-30T23:01:16.837Z
"Outlets don't contain any additional information of the upstream area draining into the lake." But if all the inlets of the lake drain into the lake, wouldn't the outlet the lake drains into capture the drainage basins of all of those inlets?
cr0
CC BY-SA 3.0
2017-08-01T01:02:03.080Z
Basically I was trying to say that lakes don't contain any "additional" information over the outlet. But I now I need to think about this for a bit, because it might be a little more nuanced than I made it out to be. For instance, it's not clear to me whether the lake area itself should be included in the calculation of drainage area.
Z W
CC BY-SA 3.0
2017-08-01T16:54:51.783Z
Right. I'm thinking the lake should be included, as a way to 1) ensure all areas draining into the lake are included, not just its major inlet, and 2) to represent the entire "domain" of the lake. For example, let's say we were delineating political boundaries according to watersheds. For the watershed of a lake, how would you delineate that watershed? You'd want to include the lake itself as well, since anything happening in or on the lake affects the lake as a resource and as a contributor to all downstream of the lake.
cr0
CC BY-SA 3.0
2017-08-02T21:02:50.963Z
Yup i think you're right. And i think what this implies is that there is no "right answer", or that it's dependent upon the question. In your example of delineating political boundaries, I agree %100 with you. And as an example of where the lake should not be included is if you're trying to determine the relationship between lake size and drainage area. You'd think this would be discussed in textbooks but... i have yet to find it mentioned!
Z W
CC BY-SA 3.0
2017-08-03T00:11:13.297Z
For my purposes, I would include the lake in the catchment and calculate the drainage basin at the outlet. For example, If I'm considering how the lake responds to rainfall, I need to include the rain that falls on the lake. Besides it is easier ;-)
haresfur
CC BY-SA 3.0
2017-08-03T00:13:55.553Z
For an endorheic basin, I would trick the GIS by defining the cell at the lake centroid as having a slightly lower elevation (unless you have the actual lake-bed topography).
haresfur
CC BY-SA 3.0
2017-08-03T00:20:19.327Z
Yet another comment, in response to political boundaries. You might be able to do this if you are establishing the boundaries for the first time, but your results likely won't match established boundaries since you likely aren't using the exact same data and analysis method (originally probably a topo map and a pencil). There are other nuances, for example the State boundary between New South Wales and Victoria Australia is defined so that the Murray River is in NSW - the boundary doesn't run down the centre of the river.
haresfur
CC BY-SA 3.0
2017-08-03T12:44:59.663Z
Good points @haresfur. Interesting method defining the lake centroid as having the lowest elevation, or just grabbing the lowest elevation point in the lake. I've never delineated a basin using that as the 'pour point' but I suppose that would accomplish what I'm interested in: capturing the entire 'response area' affecting the lake.
cr0
CC BY-SA 3.0
2017-07-30T22:01:31.723Z
2017-07-30T22:01:31.723Z
Z W
<p>I am interested in the area draining into and affecting a particular lake. Delineating this drainage basin, should I use the lake's inlet or outlet to best capture the relevant drainage basin? I think including the lake is valuable in this exercise, as events on or in the lake itself also affect the lake.</p>

<p>I'd think outlet, but want to double check with you all. To complicate things, most lakes have more than one inlet and outlet. I guess I'd go with the lowest outlet in order to avoid missing any land which may drain into the lake.</p>

CC BY-SA 3.0
2017-07-29T20:07:36.093Z
Few lakes actually have more than one outlet.  Otherwise, if you use an inlet, you'd just get the drainage basin for that inlet.
jamesqf
CC BY-SA 3.0
2017-07-30T18:07:48.817Z
I was wondering about that, how many lakes actually have more than one outlet. If most have only one significant outlet, than I guess using that single significant outlet as the delineation point for the entire watershed affecting that lake would be most appropriate.
cr0
CC BY-SA 3.0
2017-08-03T00:26:14.613Z
It would be rare for a lake to have more than one outlet, but I think I stumbled across one on Google Earth in Nunavut Canada. An additional complication would be when a new outlet forms when lake level rises. That would be hard to handle in a GIS analysis, but probably isn't necessary to worry about.
haresfur
<hydrology><gis><watershed>
To delineate the drainage basin for a lake, would the pour point be the inlet or the outlet?
cr0
# To delineate the drainage basin for a lake, would the pour point be the inlet or the outlet?

(Asked by: username_0 on 2017-07-28)

I am interested in the area draining into and affecting a particular lake. Delineating this drainage basin, should I use the lake's inlet or outlet to best capture the relevant drainage basin? I think including the lake is valuable in this exercise, as events on or in the lake itself also affect the lake.

I'd think outlet, but want to double check with you all. To complicate things, most lakes have more than one inlet and outlet. I guess I'd go with the lowest outlet in order to avoid missing any land which may drain into the lake.

**Tags:** hydrology, gis, watershed

> **Comments:**
> **username_1**: Few lakes actually have more than one outlet.  Otherwise, if you use an inlet, you'd just get the drainage basin for that inlet.  
> **username_0**: I was wondering about that, how many lakes actually have more than one outlet. If most have only one significant outlet, than I guess using that single significant outlet as the delineation point for the entire watershed affecting that lake would be most appropriate.  
> **username_2**: It would be rare for a lake to have more than one outlet, but I think I stumbled across one on Google Earth in Nunavut Canada. An additional complication would be when a new outlet forms when lake level rises. That would be hard to handle in a GIS analysis, but probably isn't necessary to worry about.  

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## 1 answers

### Answer by username_3 on 2017-07-30. Score: 2 | Accepted answer

I think there are a few things to consider. First, a drainage basin is defined as the area upstream of the point to which all precipitation converges. Flow does not converge at the outlet - flow converges at the lake. Outlets don't contain any additional information of the upstream area draining into the lake. This implies that to find the area draining *into a lake*, you would necessarily have to proceed beginning from *each and every inlet*.

Here's another way to see this. Approximately 20% of all land drains to lakes with no outlets. These are referred to as *endorheic* lakes or *endorheic* basins. Even though there is no outlet, there is absolutely still a definable drainage basin area.

> **Comments:**
> **username_0**: Good point about the endorheic basins, in that case they must be delineated based on **a** or **every** inlet.  
> **username_0**: "Outlets don't contain any additional information of the upstream area draining into the lake." But if all the inlets of the lake drain into the lake, wouldn't the outlet the lake drains into capture the drainage basins of all of those inlets?  
> **username_3**: Basically I was trying to say that lakes don't contain any "additional" information over the outlet. But I now I need to think about this for a bit, because it might be a little more nuanced than I made it out to be. For instance, it's not clear to me whether the lake area itself should be included in the calculation of drainage area.  
> **username_0**: Right. I'm thinking the lake should be included, as a way to 1) ensure all areas draining into the lake are included, not just its major inlet, and 2) to represent the entire "domain" of the lake. For example, let's say we were delineating political boundaries according to watersheds. For the watershed of a lake, how would you delineate that watershed? You'd want to include the lake itself as well, since anything happening in or on the lake affects the lake as a resource and as a contributor to all downstream of the lake.  
> **username_3**: Yup i think you're right. And i think what this implies is that there is no "right answer", or that it's dependent upon the question. In your example of delineating political boundaries, I agree %100 with you. And as an example of where the lake should not be included is if you're trying to determine the relationship between lake size and drainage area. You'd think this would be discussed in textbooks but... i have yet to find it mentioned!  
> **username_2**: For my purposes, I would include the lake in the catchment and calculate the drainage basin at the outlet. For example, If I'm considering how the lake responds to rainfall, I need to include the rain that falls on the lake. Besides it is easier ;-)  
> **username_2**: For an endorheic basin, I would trick the GIS by defining the cell at the lake centroid as having a slightly lower elevation (unless you have the actual lake-bed topography).  
> **username_2**: Yet another comment, in response to political boundaries. You might be able to do this if you are establishing the boundaries for the first time, but your results likely won't match established boundaries since you likely aren't using the exact same data and analysis method (originally probably a topo map and a pencil). There are other nuances, for example the State boundary between New South Wales and Victoria Australia is defined so that the Murray River is in NSW - the boundary doesn't run down the centre of the river.  
> **username_0**: Good points @username_2. Interesting method defining the lake centroid as having the lowest elevation, or just grabbing the lowest elevation point in the lake. I've never delineated a basin using that as the 'pour point' but I suppose that would accomplish what I'm interested in: capturing the entire 'response area' affecting the lake.  

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