Watershed Delineation Procedure 1. Prepare the DEM: - all grids should be in the same projection; if not, then reproject (or define and project); if in UTM, all grids must be in the same zone (if not, then reproject and perform calculations in another projection) - when mosaicing multiple DEMs, use the Mosaic to New Raster tool and specify 'Blend' under Mosaic Method, in order to reduce edge effects on subsequent surface analysis - a single 10-meter 7.5' quadrangle was used in this example; the watershed of interest is: 2. First, correct the DEM for imperfections by filling locally closed depressions: - open ArcToolbox Spatial Analyst Tools Hydrology Fill - specify the input DEM and an output raster name - leave the Z Limit blank (default) in order to fill all closed depressions 1 (approx. execution time is 9 minutes for twelve 10-meter DEMs mosaiced together) 3. Calculate the flow direction raster: - open ArcToolbox Spatial Analyst Tools Hydrology Flow Direction - specify the input surface raster (the output of the Fill tool) - specify an outpur raster name - check the Force all Edge Cells box and leave the Output Drop Raster blank (default) (approx. execution time is 0.5 minutes for twelve 10-meter DEMs mosaiced together) 4. Calculate the flow accumulation raster: - open ArcToolbox Spatial Analyst Tools Hydrology Flow Accumulation - specify the input flow direction raster (the output of Flow Direction) - specify an output raster name - leave the Input Weight Raster blank (default) (approx. execution time is 2.5 minutes for twelve 10-meter DEMs mosaiced together) - the result is shown below, with channels of highest relative flow in white; a hillshade of this raster clearly shows the dendritic drainage pattern: 1 in karst or other sink terrains, the Z limit has to be determined relative to the DEM's raster quality to allow for localized drainages; in low-relief terrain, this may require an iterative adjustment to override incorrect filling of low depressions (see Step 7).
5. Create a point feature of "pour points" (locations to which watersheds drain): - create a new point-feature shapefile in ArcCatalog in the same projection as the DEMs being analyzed; bring it into ArcMap, start editing, choose Create New Feature and specify the Target Feature; use the pencil tool to define a point at the most downstream point in the watershed of interest - add additional points whose x, y ranges bracket the watershed(s) of interest (the Watershed tool will truncate its analysis at the x, y extents defined by all pour points) - while still in Edit mode, choose Modify Feature and reposition each pour point exactly in the middle of the nearest high-flow cell by zooming in on the flow accumulation raster (Note: don't use the hillshade raster to guide repositioning, it's not as accurate) - edit the shapefile's attribute table to assign a unique ID value to each pour point (otherwise, all watersheds will be assigned the same Value number and can't be segregated later); save edits, stop editing - the result is a series of pour points that define and enclose the watershed(s) of interest: <= additional pour points used to define a watershed adjacent to the one of interest - Note: ArcToolbox's Snap Pour Point tool could be used to precisely locate pour points, but unless you have a huge number of pour points, zooming in and editing point locations individually is the quickest approach
6. Finally, delineate the watershed(s): - open ArcToolbox Spatial Analyst Tools Hydrology Watershed - specify the input flow direction raster (the output of Flow Direction) - accept the Pour Point field default (ID, for a point shapefile) - specify an output raster name - the result is a temporary raster classified into as many categories as there are unique ID values in the pour point shapefile: - as a quick check on the results, run the Basin tool in Hydrology to calculate all possible watersheds in the DEM; these boundaries should match those calculated by Watershed exactly (doing so may indicate if / where adjacent watersheds need to be delineated or where larger watersheds could be broken into subbasins). 7. Manual readjustment for incorrect delineations: - in low-relief terrain (such as the ESRP), it may not be possible to correctly discriminate between watersheds separated by very low topographic divides after filling closed depressions. For example in an area east of American Falls reservoir, the watershed of interest drains an area to the east of a low shield volcanic complex, but filling causes the divide to be breached when pour points are set at the extents of the area of interest: - in such cases, it may not be possible to use the Z Limit factor to optimally fill a DEM's closed depressions without affecting such subtle topographic divides
- the solution is to adjust the pour points' x,y extents to selectively delineate each watershed of interest (possibly having to repeat the process for some watersheds that cannot otherwise be separated in one step); in this example, pour points were relocated to the x,y extents of the watershed of interest, resulting in a correct delineation: 7. Convert and clean up the output: - open ArcToolbox Conversion Tools From Raster Raster to Polygon - specify the input raster (the output from the Watershed tool) - accept the default Field (Value) - specify an output file name - the result is a group of polygons corresponding to the input raster; select those that are of interest and export as a final shapefile:
8. Other manipulations on delineated watersheds A. Calculate watershed areas: - calculate areas with ArcToolbox Spatial Statistics Utilities Calculate Areas (alternatively, the accumulated flow at a watershed's pour point represents 1% of the calculated area, so these values could be used to estimate relative areas) B. Calculate amount of precipitation on a watershed: - first, decide if multiple subbasins are to be combined into one; if so, use the Merge tool (ArcToolbox Data Management General), then edit one field (e.g., gridcode) so that polygons to be combined share the same value - use the Dissolve tool (ArcToolbox Data Management Generalization) to combine polygons having the same value in the edited field - using the Intersect tool (ArcToolbox Analysis Overlay), specify the Input features as the output of Dissolve and the precipitation polygon feature 2, specify No FID under Join Attributes and accept remaining defaults - run the Calculate Area utility to calculate the areas of the intersected isohyetal polygons; for example, in the Ross Fork area, first merging then dissolving multiple subbbasins and intersecting the output with 30-year mean annual P results in: - with an attribute table (after removing unwanted fields) that contains an F_Area field: - multiply the F_Area field by the Range field (watch out for units!) and sum across all records. The result is the total amount of precipitation received by the watershed annually. In this example, doing so in Excel results in the following table: 2 The 30-year mean annual precipitation coverage distibuted by InsideIdaho / Idaho Dept. of Water Resources is a polygon coverage that is convenient for performing subsequent watershed calculations.
C. Refine the precipitation calculation: - because of evapotranspiration (ET), less of the precipitation that falls on lowland areas contributes to runoff and groundwater storage; for example, in SE Idaho, areas that recive less than 15 inches of precipitation annually generate very little or no sustained runoff/groundwater recharge. One way to adjust for this effect is determine which isohyetals (usually at higher altitudes) contribute the most runoff/storage - for example, if it is assumed that only areas receiving more than 20 inches per year contribute significantly to runoff/storage, then only those rows in the Excel table that have P>20" would be summed; doing so indicates that annual contribution to the this particular watershed would be 30% lower (16.2 inches/yr) - a more refined approach would involve creating one or more polygons based on topographic or other criteria that define zones of relative ET loss, run the Intersect tool for each zone, calculate revised areas and create new Excel tables with which to prorate total annual precipitation by the proportion of estimated ET loss in each area