GEOG 400, Advanced GIS, Fall 2020; Week 9 Lecture 1

Transcript

1. GEOG 400: Advanced GIS - Raster Week 9 – Lecture 1 Hydrologic Modeling, Part 1
2. GEOG 400: Advanced GIS - Raster An introduction to hydrologic modeling, or modeling the flow of water across the landscape using ArcGIS A hands-on exercise in using provided data (i.e., canned, not all sorts of screwed up) to walk through watershed delineation in ArcGIS and an introduction to USGS StreamStats Hydrologic Modeling, Part 1 A hands-on exercise in using freely-available data (i.e., probably just full of errors and weird stuff) to perform watershed delineation and hydrologic modeling in QGIS. TODAY NEXT LECTURE Like much of our terrain modeling work, hydrologic modeling begins with a digital elevation model THIS WEEK’S LAB
3. GEOG 400: Advanced GIS - Raster Hydrologic Modeling, Part 1 Hydrologic Modeling, Defined [anytime we say modeling, people tend to freak out] simplification of a real-world system (e.g., surface water, soil water, wetland, groundwater, estuary) that aids in understanding, predicting, and managing water resources. For GEOG 400, hydrologic modeling is predicting the flow of water, the locations of streams, and the size of watersheds from terrain data (DEMs)
4. GEOG 400: Advanced GIS - Raster Hydrologic Modeling, Part 1 What’s a Watershed? A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel.
5. GEOG 400: Advanced GIS - Raster Hydrologic Modeling, Part 1 What’s a Watershed? A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel.
6. GEOG 400: Advanced GIS - Raster Hydrologic Modeling, Part 1 What’s a Watershed? A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel.
7. GEOG 400: Advanced GIS - Raster Hydrologic Modeling, Part 1 What’s a Stream Network? A vector representation of streams across the landscape Stream order: - 1st order stream has no streams flowing into it - 2nd order stream forms when two 1st order streams meet - 3rd order stream forms when two 2nd order streams meet Mississippi River at Gulf of Mexico is 10th order
8. GEOG 400: Advanced GIS - Raster Hydrologic Modeling, Part 1 Why are these things important?
9. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no flow threshold
10. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance
11. Hydrologic Modeling • Flow direction • Typically the first step in the hydrologic modeling process • Focal analysis technique that determines, for each cell in a DEM, which neighboring cell has the maximum drop in elevation • Drop = ∆z / ∆d • If cell size is 1… • ∆d to N, S, E, and W is 1 • ∆d to NW, NE, SE, SW is 1.414 (√2) • Sort of like a simpler aspect NW N NE W target cell E SW S SE 3 x 3 focal neighborhood for flow direction calculation
12. Hydrologic Modeling • Flow direction • DropN = (100 – 103) / 1 = -3.00 • DropNE = (100 – 107) / 1.414 = -4.95 • DropE = (100 – 105) / 1 = -5.00 • DropSE = (100 – 100) / 1.414 = 0.00 • DropS = (100 – 97) / 1 = 3.00 • DropSW = (100 – 96) / 1.414 = 2.83 • DropW = (100 – 98) / 1 = 2.00 • DropNW = (100 – 99) / 1.414 = 0.71 99 103 107 98 100 105 96 97 100
13. Hydrologic Modeling • Flow direction • Maximum drop is directly south (3.00) • Therefore, the flow direction is to the south • In other words, if you were to pour water on the center cell, that is the direction that the water would flow • Of course in reality, water can flow in any direction (0°- 360°) • Limitation of gridded raster data S 3.00
14. Hydrologic Modeling • Flow direction • In ArcGIS, the flow direction values are encoded using the following scheme (8-bit): • E = 1 • SE = 2 • S = 4 • SW = 8 • W = 16 • NW = 32 • N = 64 • NE = 128 32 64 128 16 1 8 4 2
15. Hydrologic Modeling • Flow direction
16. Hydrologic Modeling • Flow direction • Flow Direction tool has a few important parameters • Option to force all edge cells outward • If unchecked, could introduce error since you’re not including a full neighborhood for flow direction calculation • Usually best to check
17. Hydrologic Modeling • Flow direction • Flow Direction tool has a few important parameters • Output drop raster • Flow direction : aspect :: drop : slope • Flow direction type • Three options, but let’s stick with “D8”, the default • The method discussed earlier
18. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance
19. Hydrologic Modeling • Sink • But what happens if the target cell is lower than all of the surrounding cells, or if two cells flow into one another? • This is known as a sink • It has no flow direction (internal drainage) • Other cells flow into it 105 103 107 102 100 105 103 101 104
20. Hydrologic Modeling • Sink • But what happens if the target cell is lower than all of the surrounding cells, or if two cells flow into one another? • This is known as a sink • It has no flow direction (internal drainage) • Other cells flow into it • In the Flow Direction raster, sinks are given different values
21. Hydrologic Modeling • Sink • Sometimes, sinks are good! • Real areas of internal drainage (ponds, lakes, etc.) • But often times, sinks are bad! • Artifact in the DEM data that will negatively affect your hydrologic modeling elevation profile with an artifact sink
22. Hydrologic Modeling • Sink • The higher the spatial resolution of your DEM, the more likely you are to find sinks sinks derived from lidar DTM (25 cm resolution) sinks derived from USGS seamless DEM (30 m resolution)
23. Hydrologic Modeling • Sink • Sink tool in ArcGIS • Very simple, one input (flow direction), one output (raster of sink cells)
24. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no
25. Hydrologic Modeling • Fill • Regardless of if the sinks are real or artificial, you often want to fill them in order to facilitate an accurate surface flow model on the landscape, small depressions will fill and overflow
26. Hydrologic Modeling • Fill • Thus, if sinks are found in your DEM (they almost always will…), you typically want to run the Fill tool • Notice the Z limit… Slide #26 the Fill tool identifies and fills sinks it also identifies and flattens peaks
27. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no
28. Hydrologic Modeling • Flow accumulation • With your depressionless DEM, you can then re-calculate flow direction • With the resulting flow direction raster, you can estimate flow accumulation • I know where the water’s going to flow, now I need to see where and how much it’s going to accumulate
29. Hydrologic Modeling • Flow accumulation • How flow accumulation works… • Calculates the accumulative number of cells flowing into each cell Slide #29 ↓ ↓ ↙ ↙ ← ← ← ← ← ← ↓ ↘ ↘ ↑ ↖ ↘ → → ↑ ↖ → → ↗ ↑ ↖ flow direction flow accumulation
30. Hydrologic Modeling • Flow accumulation • How flow accumulation works… • Calculates the accumulative number of cells flowing into each cell ↓ ↓ ↙ ↙ ← ← ← ← ← ← ↓ ↘ ↘ ↑ ↖ ↘ → → ↑ ↖ → → ↗ ↑ ↖ flow direction 0 0 0 1 0 24 22 19 16 0 0 0 0 13 0 1 0 2 11 0 0 3 4 0 0 flow accumulation
31. Hydrologic Modeling • Flow accumulation • How flow accumulation works… • However, you can also add a weight raster to control localized patterns of rainfall (or impermeability)
32. Hydrologic Modeling • Flow accumulation
33. Hydrologic Modeling • Flow accumulation hydrologic modeling does not work very well in areas of low relief – water doesn’t know which way to go!
34. Hydrologic Modeling • Flow accumulation • Although this output looks cool, you typically want to apply a flow accumulation threshold in order to define “stream” vs. “not stream” • There is no magic number for this • Depends on local climate, terrain, soil, etc.
35. Hydrologic Modeling • Flow accumulation • For example… • Streams defined by a flow accumulation threshold of: • 100 (Flow Acc > 100)
36. Hydrologic Modeling • Flow accumulation • For example… • Streams defined by a flow accumulation threshold of: • 100 (Flow Acc > 100) • 1000 (Flow Acc > 1000)
37. Hydrologic Modeling • Flow accumulation • For example… • Streams defined by a flow accumulation threshold of: • 100 (Flow Acc > 100) • 1000 (Flow Acc > 1000) • 10000 (Flow Acc > 10000) • Also depends on the level of detail you’re trying to extract • Ephemeral trickles? • Perennial rivers? • etc.
38. Hydrologic Modeling • Flow accumulation • This thresholding is accomplished using the Con tool
39. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no flow threshold
40. Hydrologic Modeling • Stream order • Once you have a streams raster, you can calculate stream order • The relative size or importance of a stream based on its number of tributaries • There are two main ways to calculate this: • Strahler • Stream orders only increase when streams of the same order intersect • e.g. if two 1st order streams intersect, it forms a 2nd order stream; if you 2nd order streams intersect if forms a 3rd order stream 1 1 1 1 1 1 1 2 2 2 2 2 3
41. Hydrologic Modeling • Stream order • Once you have a streams raster, you can calculate stream order • The relative size or importance of a stream based on its number of tributaries • There are two main ways to calculate this: • Shreve • Stream orders are additive • e.g. if two 1st order streams intersect, it forms a 2nd order stream; if you 2nd order streams intersect if forms a 4th order stream 1 1 1 1 1 1 1 2 3 4 2 3 7
42. Hydrologic Modeling • Stream order Strahler stream order
43. Hydrologic Modeling • Stream order Shreve stream order
44. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no flow threshold
45. Hydrologic Modeling • Stream to feature • If you want/need to convert your rasterized stream data into vector, you can use the Stream to Feature tool • Similar to Raster to Polyline, except specifically designed to better handle flow
46. Hydrologic Modeling • Stream to feature
47. Hydrologic Modeling • Stream to feature
48. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no flow threshold
49. Hydrologic Modeling • Stream link • The Stream Link tool allows you to generate a raster dataset where each stream segment between intersections has a unique value • Useful if you’re trying to create watersheds for each stream segment • We’ll discuss watersheds next…
50. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no flow threshold
51. Hydrologic Modeling • Watershed • To figure out the catchment basin for each stream, you can use the watersheds • The area within which water will flow into each individual stream
52. Hydrologic Modeling • Watershed • To figure out the catchment basin for each stream, you can use the watersheds • The area within which water will flow into each individual stream • Can be generated based only on higher-order streams as well…
53. Hydrologic Modeling • Watershed • These are examples of using the stream link outputs as the basis of your watershed delineation
54. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no flow threshold
55. Hydrologic Modeling • Snap pour point • But you can also use specific pour points • Let’s say you are sampling water at a specific location, and you want to know the entire land area (catchment basin, watershed) that is contributing runoff to your sample
56. Hydrologic Modeling • Snap pour point • You can use these pour points in the Watershed tool as well, but you first need to use the Snap Pour Point tool, in order to ensure that your sample location is precisely aligned with your Flow Accumulation data
57. Hydrologic Modeling • Watershed • Then, input these points to your Watershed tool, and run!
58. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no flow threshold
59. Hydrologic Modeling • Flow distance • Two additional tools in the Hydrology toolbox are Flow Distance and Flow Length • Flow Distance allows you to calculate the vertical and/or horizontal distance from the nearest stream • Vertical distance
60. Hydrologic Modeling • Flow distance • Two additional tools in the Hydrology toolbox are Flow Distance and Flow Length • Flow Distance allows you to calculate the vertical and/or horizontal distance from the nearest stream • Vertical distance • Horizontal distance
61. Hydrologic Modeling • Hydrologic modeling • The process for hydrologic modeling in ArcGIS… DEM flow direction sink are there any sinks? fill depressionless DEM flow accumulation stream order stream to feature stream link flow length snap pour point watershed flow distance yes no flow threshold
62. Hydrologic Modeling • Flow length • Flow Length, on the other hand, looks at the entire drainage system, and determines how far any given point is from a terminal outlet • e.g. if I spill toxic chemicals here, how long will it take to reach this sample location