rainfall maps: 100ths of an inch
K factor: K factor times 100 K of .35 = 35
Terrain Analysis Hydrology Pollution _________________ _________________ _________________ | | | | | | | Elevation | | Land Cover | | Land Cover | | Model | | | | | --------+-------- | Hydrologic | | Soils | | | Soil Group | | K factor | ________V________ --------+-------- | Texture | | | | --------+-------- | Idealized | | | | Elevation | | | | Model | | | | | | | | Drainage | | | | Direction | | | | | | | | Drainage | ________V________ | | Accumulation | | | | | | | Runoff Map | | | Slope | -+------+-------- | --------+-------- | | | | +---- | | --------+----(--------->|<----------------------- | | ________V________ _________________ |<---+ | | | | | | Contaminant | | Best | | | Source Areas | | Management | | --------+-------- | Practices | ________V________ | --------+-------- | | | | | Peak | ------------+------------ | Discharge | | ----------------- ________V________ | | | Contaminant | | Routing | ----------------- figure X1.
Routing routines assume that the entire watershed is within the current window. This is not always true or even practical. For instance, an analysis along the side of a long river will provide valuable information about local contributions to the larger resource. However, calculations in the long river itself can not take into account upstream effects outside the current window and are thus invalid. Yet those local contributions may be of considerable interest. Great care must be taken interpreting results of watersheds only partially contained within the operating window.
Cell resolution, the size of each cell, should be set as large as is appropriate for your purposes. In no case should the resolution be smaller than the spatial resolution of your best input layer. Doubling the cell length and width will cut computation times by 3/4s! The east-west resolution need not match the north-south resolution. However, large deviations from square cells will distort some of the neighbor concepts used by some algorithms to "feel their way" across the terrain.
Once the analysis is begun, the same window, including cell resolution, must be maintained throughout the project. The interface program tries to insure this. Changing the boundaries invalidates the drainage accumulation maps. Changing the cell resolution can have disastrous effects on the drainage direction map, such as creating dead end and circular drainages. The idealized elevation model will lose its "ideal" character if resampled. If the window must be changed, start the analysis from the beginning.
Some of the output maps may be resampled to a smaller window or different cell size after the analysis is finished. Contaminant source area maps are produced in units of mass per unit area and may be resampled. This may be useful for presentation of results or incorporation within another GIS analysis.
The idealized elevation model is used to determine the direction in which each cell will drain. This is done by searching the edge of the window for the lowest point and draining the watershed that leads there. Then the next lowest outlet is found and its watershed drained, and so on until the entire area is drained.
The drainage direction map is used to produce a drainage accumulation map. The values in a drainage accumulation map are the total number of cells including that cell, which drain through that cell. If multiplied by the area of a cell, the drainage accumulation map will yield the area of the watershed above and including each cell.
To this point the terrain analysis and the hydrology could progress independently. From this point on the distinctions between terrain analysis, hydrology and pollution, as separate lines of analysis begin to fade. Actually pollution analysis will always be dependent to some extent on hydrology but that will be dealt with in a later section. Peak discharge combines the results of the terrain analysis and runoff sections.
Peak discharge, the volume of water passing out of a cell per unit time, is calculated with an empirical relationship developed by Smith and Williams (1980) and employed in CREAMS and AGNPS. Watershed characteristics are interpreted from several all of the results from the terrain analysis. The resulting map of peak discharge, significant by itself, is also used in the routing of contaminants.
The final part of the analysis routes contaminants. It requires the results of the preceding sections. Sediment, nitrogen and phosphorous nutrients and COD may be modeled. The routing is done as a mass balance accounting for imports of contaminants into a cell, contaminants originating within the cell, losses to infiltration and deposition and exports to the next cell. Figure X2 is a schematic representation.
rainfall with contaminants | ______________ imports from / / Surface exports up gradient - / / - down gradient /_____________/ | losses to infiltration and deposition figure X2.
1 = clay soils 2 = silt soils 3 = sandy soils 4 = peat 5 = water
1 = hydrologic soil group A 2 = hydrologic soil group B 3 = hydrologic soil group C 4 = hydrologic soil group D 5 = water
1 = corn 2 = rye 3 = oats 4 = soybeans 5 = hay 6 = grass 7 = old field (grass) 8 = old field (shrub) 9 = pasture 10 = forest 11 = wetlands 12 = fens 13 = water 14 = built up 15 = barren
The idealized elevation file is a digital elevation file. Cell values represent elevation in meters. This elevation model will differ from the input DEM provided by the user, in that depressions in the data are filled in so that water landing anywhere on the idealized surface can flow to the edge. Although useful for analyzing terrain, and employed for slope measurements, the idealized elevation model is probably further removed from reality then the original data upon which it is based. The idealized elevation data may not be resampled to a different cell resolution and retain its desired "idealized" characteristics.
4 3 2 5 1 6 7 8 figure X3.Drainage direction is similar to aspect. However they are not exactly the same thing. In an aspect map, the cells at the bottom of a V shaped valley may face each other without draining into each other. This map is produced by searching the border of the window for the lowest cell and draining all cells in that drainage. The algorithm moves up through the drainage one unit of elevation at a time until all adjacent cells of that elevation are drained, trying to assign the most direct drainage direction. Then the next lowest undrained border cell is found and that watershed is drained until the entire map is drained. The routine sometimes has difficulty at the top of watersheds if the next valley does not clearly drop away.
Sediment loadings are expressed as kilograms per hectare. Nutrients and COD loadings are expressed as grams per hectare.
These loadings are a function of the rainfall, the soil, slope and landcover. They do not represent the natural scatter found in natural events. Results may be used as a relative indicator of contaminant generation. As such these maps may highlight those areas in a watershed that would benefit most from conservation efforts. Subtracting two source area maps generated for different scenarios would help locate the areas of greatest change.
The output maps indicate the totak mass of contaminant passing through a cell. These maps can be used as indicators of the contaminants delivered to any point down stream. Similar analysis for varying land cover regimes could be subtracted with "r.mapcalc" to find areas of greatest positive and negative change resulting from the changes in scenarios. Again these numbers represent relative indicators.
1 start a new project
2 start a project based on an existing project
3 work on an existing project
4 remove project files
The user is prompted for needed information such as the name of the project to work on with the option of listing project files if needeed. Once a project is selected the secon menu is offered. The User may do:
1 Terrain Analysis
2 Runoff Analysis
3 Contaminant Analysis
4 return to main menu
Choices 1, 2 or 3 lead to menus for each of those analysis.
In the Terrain Analysis the options are:
1 Create an idealized elevation model
2 Create a drainage direction map
3 Create a drainage accumulation map
4 Create a slope map
5 Return to the previous menu
In each case except option 5 the user is prompted for needed information. where appropriate the likely choices are supplied if not insisted upon. The drainage direction map must be based on the idealized elevation map. The drainage accumulation map needs a drainage direction map which should be the output map from option 2. In this way the user is guided along.
In the runoff analysis section there are only three options:
1 Create a runoff map
2 Simulate peak discharge
3 Return to the previous menu
In this section the options are not as straight forward and the user is coached along. The user must provide the names of several input maps and can obtain a listing by entering list instead of a map name where requested. The user must also choose between a design storm and rainfall map. The most difficult information requested of the user is the anticeedent moisture condition. This is a number between 1 and 3 inclusivly which describes the amount of moisture already in the soil which effects the amount of rain which can be absorbed during the current storm. 1 represents extreemly dry conditions and 3 extreemley wet conditions. The concept comes from the Soil Conservation Service curve number method of predicting runoff. The basic guidance from the SCS is offered with the request and a value of 2 neither wet or dry is the default. Peak discharge is quite sensative to thee anticeedent moisture condition so some care should be taken here.
The contaminant section has only three options as well:
1 Model Contaminant source areas
2 Rout contaminants through the watershed
3 Return to the previous menu
When modeling contaminant source areas the user may model just sediment or sediment and any combination of nitrogen, phosphorpus and COD. Because nutrients are associated with sediment sediment predictions are required for nutrient modeling. Most input for this section is straight forward except the number of days since the last significant rain. The amount of dirt available to wash off of urban areas accumulates between rainstorms. Rain storms wash urban areas clean. a default of 7 days is suggested because this puts urban and rural areas on roughly even footings for comparison. To complicate the issue street sweeping also removes contaminants from road ways and so if the study area has a regular streeet cla\eaning program that should be reflected by entering fewer dry days. Several minor rain storms, especially short hard rains will accomplish the same cleaning as a long large rain. SWMM assumes that a half inch of rain removes half of the available dirt and the next half inch removes half of what is left an so on. A worst case for urban areas is a rain ater an extended dry period. However the goal is to compare watersheds or source areas. Agriculturlal areas contribute the most poultion with an anticeedent moisture condition of 3 since that is when the most runoff and erosion takes place.
The ultimate utility of analyses performed with these water resource assessment tools is dependent on the interpretation of the user. Always keep in mind that these results are based on empirical relationships which represent expectations over a long term average. For this reason numerical results should be used only for comparison of areas and scenarios. Employing the entire set of tools from the tool kit may be inappropriate, and or redundant information.
You can't hurt the model by running it! So feel free to experiment!
Last changed: $Date: 2002/01/25 05:45:34 $