r.wrat(1)		      Grass User's Manual		     r.wrat(1)



NNAAMMEE
       wwrraatt - Water Resource Assessment Tool



UUSSAAGGEE
       This module must be used interactively.



IINNPPUUTT MMAAPP CCOODDEESS
       elevation:	meters, as well as cell resolution


       rainfall maps:	100ths of an inch


       K factor:	K factor times 100  K of .35 = 35



IInnttrroodduuccttiioonn
       The  Water  Resource  Assessment  Tool  is a collection of programs run
       within the GRASS GIS. These programs are an aid	in  understanding  the
       nature  of runoff in a study area based on information typically avail-
       able for a GIS. The programs analyze the  terrain  to  define  drainage
       direction and areas, simulate runoff and peak runoff and model nonpoint
       source pollution and map contaminant source areas and contaminant path-
       ways.  This  users  guide  outlines  the  typical  analytical sequence,
       describes input data requirements and possible output. Some  suggestion
       for  interpreting output are included. It is assumed the user is famil-
       iar with the basics of GRASS.



IInnppuutt
       Map layers required: digital elevation model, land cover, soil maps  of
       hydrologic  soil group, soil texture, and erodiblity factor (K factor).
       Optional maps describe best managment practices for sediment,  nitrogen
       phosphorous and chemical oxygen demand (COD).



TTyyppiiccaall AAnnaallyyssiiss
       Three  interrelated  areas of analysis are available: terrain analysis,
       hydrology and pollution (see figure X1).  Because these areas  are  not
       distinct,  analysis  should  proceed in a logical order since some por-
       tions build on previous results. Terrain analysis and runoff generation
       must proceed routing peak discharge, which in turn must proceed contam-
       inant routing. The following steps provide a natural sequence of analy-
       sis through all of the tools.  However, a great deal of valuable infor-
       mation may be gained executing only a portion  of  the  available  pro-
       grams.  This is a tool kit, not a prescription for studying every area.



       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.





DDeeffiinniinngg tthhee ssttuuddyy aarreeaa
       GRASS looks at the world through a  rectangular	"window".  The	window
       should be set to include the entire area of interest with several cells
       to spare in all directions. Because of difficulty interpreting  terrain
       at  the edge of the window, the analysis moves in from the window edge.
       The extra area is particularly important if the top of a  watershed  of
       interest  is not marked by a steep decline into the adjacent watershed.


       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  informa-
       tion  about local contributions to the larger resource. However, calcu-
       lations 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.   How-
       ever,  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 accu-
       mulation 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 analy-
       sis from the beginning.


       Some of the output maps may be resampled to a smaller window or differ-
       ent  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.



TTeemmppoorraall ssccaallee
       Hydrologic simulations are based on single storms, presumably  one  day
       events. Actual or design storms may be employed.



TTeerrrraaiinn aannaallyyssiiss
       The actual analysis may begin using the digital elevation model to pro-
       duce an idealized elevation model.  Real data has, for a number of rea-
       sons,  sections	which  are difficult to drain. The idealized elevation
       model has no cells which can not drain to the edge of the window.  This
       assumes	complete surface drainage is possible. This condition is never
       completely true and in some regions, such as those with karst  topogra-
       phy,  so far from the case as to invalidate the peak discharge and con-
       taminant routing results.


       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.



HHyyddrroollooggyy
       Two programs contribute to the hydrology section: runoff and peak  dis-
       charge.	The  runoff section is based on the Soil Conservation Services
       Curve number method (Soil Conservation  Service,  USDA,	1971).	 Curve
       numbers,  from  0 to 100, describe the potential for runoff. Curve num-
       bers are inferred from the land cover map and  the  map	of  hydrologic
       soil  groups. A depth of precipitation for a design storm or a rainfall
       map is required to produce a runoff map.


       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 charac-
       teristics are interpreted from several all of the results from the ter-
       rain analysis. The resulting map  of  peak  discharge,  significant  by
       itself, is also used in the routing of contaminants.



PPoolllluuttiioonn
       Contaminant  sources are mapped as the contaminant originating per unit
       area.  Sediment, nitrogen, phosphorous and  COD	may  be  modeled.  The
       algorithms used to predict contaminant mobilization depend on the land-
       cover of the cell involved. The universal soil loss equation and  adap-
       tations	from AGNPS and CREAMS are employed for rural, agricultural and
       open lands. An algorithm from SWMM is adopted  for  urban  areas.  Land
       cover,  soil texture and erodibility factors, rainfall, runoff and peak
       discharge maps are the required inputs for this section.


       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  bal-
       ance  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.





SSoommee pprraaccttiiccaall ccoonnssiiddeerraattiioonnss::
       GRASS  only  stores integers, assuming values less then one to be equal
       to zero. Therefore small units of measure have been employed.  Rainfall
       and  runoff  are described in hundredths of inches, while quantities of
       sediment are in kilograms with nutrients in grams. Flows  are  measured
       in  cubic  feet	per  second.  The  mixing  of english and metric units
       employs those units in common usage in the U.S.	Maps may be  converted
       to  any	appropriate set of units using "r.mapcalc" to multiply the map
       by the appropriate conversion factor.



PPrreeppaarriinngg mmaapp llaayyeerrss
       GRASS associates numeric values with cells as their  attributes.  Some-
       times  these  numbers  have  real meanings, as in an elevation model in
       which the cell's value is its elevation. Some are more arbitrary  codes
       representing  qualitative  categories, such as landcover. The following
       tables provide guides to units and codes used  in  the  water  resource
       assessment  tools. The GRASS tool "r.reclass" allows speedy recoding of
       map layers to these codes.



IInnppuutt llaayyeerrss aanndd ddaattaa
EElleevvaattiioonn
       Values in the digital elevation model represent the altitude above mean
       sea level expressed in meters.



RRaaiinnffaallll
       Rainfall  maps  should  be  rainfall for a single day, measured in hun-
       dredths of an inch. Thus a one and a quarter  inch  rainfall  would  be
       represented  as	125.  The  GRASS  command  "s.surf.idw" can be used to
       interpolate a  rainfall	map  between  observation  stations,  however,
       s.surf.idw  does  not understand the effects of terrain on rainfall and
       will make significant mistakes in hilly country and  for  thunderstorms
       passing	through a widely scattered set of observation points. A design
       storm may be preferable for assessment purposes.



SSooiill,, tteexxttuurree
       Values are a code for the dominant texture.

	    1 = clay soils
	    2 = silt soils
	    3 = sandy soils
	    4 = peat
	    5 = water




SSooiill,, hhyyddrroollooggiicc ssooiill ggrroouupp
       Hydrologic soil group is primarily dependent on the soil texture.  How-
       ever  depth to bed rock and depth to water table may strongly influence
       a soil's classification.



	    1 = hydrologic soil group A
	    2 = hydrologic soil group B
	    3 = hydrologic soil group C
	    4 = hydrologic soil group D
	    5 = water




SSooiill,, eerrooddiibbiilliittyy KK ffaaccttoorr
       K factors are a decimal value greater then zero and typically less then
       .5.  Formally they represent the soil loss rate per unit of per erosion
       index unit for a specified test plot. (Agriculture Handbook Number  537
       "Predicting  Rainfall  Erosion  Losses",  USDA  1978) Information about
       local soils should be available from your local Soil Conservation  Ser-
       vice office. Because GRASS uses only integer values, multiply K factors
       by 100. Thus a K factor of .37 would be coded as 37. Use a code of  100
       for water.



LLaannddccoovveerr
       The  landcover map must be coded so the water resource assessment tools
       can recognize the landcovers indicated.	 The  following  table	illus-
       trates  the  15	landcover category codes expected.  When attempting to
       make the best fit of available data to this encoding  scheme,  remember
       to think of the areas both in terms of their ability to slow runoff and
       their contributions to water quality.  Some creative lies may be useful
       for special purposes.  For instance, a development built to the perfor-
       mance standard that peak discharge is not to exceed  a  field  in  good
       condition  could  be  coded as an old field for hydrologic analysis and
       built up for pollution analysis.



	  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


OOuuttppuutt MMaapp llaayyeerrss
       Many maps can be produced with the water resource assessment tools. The
       following  sections  outline  how these maps are prepared, the units of
       measure and codes employed along with some notes as to how they	should
       and  should  not be used. More complete information is available in the
       model documentation. The source code is also provided  and  contains  a
       great  deal of internal documentation. Output maps are presented in the
       approximate order in which they would be produced.


       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 fur-
       ther 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.



DDrraaiinnaaggee DDiirreeccttiioonn
       Cells in the drainage direction map are coded to indicate the neighbor-
       ing  cell into which they drain. The eight nearest neighbors are repre-
       sented by the integers 1 through 8 as depicted in figure X3.



				   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 low-
       est cell and draining all cells in that drainage. The  algorithm  moves
       up through the drainage one unit of elevation at a time until all adja-
       cent 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.



DDrraaiinnaaggee AAccccuummuullaattiioonn
       Values in the drainage accumulation map represent the number of	cells,
       including  the  cell  with the value, that drain through the cell. Thus
       the minimum drainage accumulation is one, while the maximum is the num-
       ber  of	cells in the largest watershed in the study area. The drainage
       accumulation map is used to find the watershed area for	each  cell  in
       the  study  area  and  to  guide  a search up the watershed to find the
       length of the longest stream. The drainage accumulation map can be used
       to  define the stream networks by reclassifying the cells below a mini-
       mum accumulated drainage to zero. The remainder represents the streams.
       Drainage  accumulation is drainage area represented in cells. Multiply-
       ing a drainage accumulation by the area of a cell yields  the  drainage
       area above that cell.



RRuunnooffff
       The  values  in the runoff map represent the depth of water expected to
       run off that cell, from the input rainfall, based on the soil and land-
       cover  maps  specified.	Values could range between zero and the amount
       of rain that fell on that cell.	Runoff, like rainfall, is measured  in
       hundredths  of  inches.	 A value of 75 represents .75 inchs of runoff.
       Runoff values can be multiplied by the area of each cell to give a vol-
       ume  of	runoff.  However,  care  should be taken when converting units
       because cell sizes in GRASS are defined in meters. Upgradient values in
       the  runoff  map  are summed during the peak discharge calculations and
       used in the contaminant source area and routing calculations.



PPeeaakk DDiisscchhaarrggee
       Peak discharge is the maximum rate at which water passes through a cell
       measured  in  cubic  feet  per second.  These estimates are based on an
       empirical relationship which describes the entire upstream  portion  of
       the  watershed.	If the entire upstream portion of the watershed is not
       within the current window, the model assumes the upstream edge  is  the
       top of the watershed. This is  wrong and estimates of peak discharge in
       streams which enter the window are meaningless as are routings of  con-
       taminants  based  on  peak  discharge. The calculations are retained so
       that local contributions from side streams and slopes may  be  modeled.
       These  local effects may be of substantial importance to those involved
       in the local area regardless of distant conditions.



CCoonnttaammiinnaanntt SSoouurrccee AArreeaass
       Sediment, nitrogen and phosphorus nutrients and COD can	be  simulated.
       The  algorithms	used to predict contaminant mobilization are dependent
       on the contaminant and the landcover class. In non urban areas sediment
       production  is  based on the universal soil loss equation. Nitrogen and
       phosphorous are modeled both in solution and associated	with  sediment
       in  rural  areas and only as a function of sediment in urban areas. COD
       is assumed to be soluble and is calculated by loading factors.


       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 gen-
       eration. 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.



RRoouuttiinngg ooff CCoonnttaammiinnaannttss
       Contaminants  are routed down stream assuming total conservation unless
       a best management practice map for the particular contaminant is  used.
       If  a  best  management practice map is used contaminants flux across a
       BMP is reduced by the percentage indicated.


       The output maps indicate the totak mass of contaminant passing  through
       a cell. These maps can be used as indicators of the contaminants deliv-
       ered 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.



IInntteerrffaaccee PPrrooggrraamm
       The  Water  resopurces assessment tool is run through an interface pro-
       gram invoked from the grass environment at any GRASS prompt  by	typing
       wrat. Work is organized in projects which should represent the investi-
       gation of a geographic area. The user interface is a  series  of  menus
       which  guide  the  user	through the program. A project file is created
       which keeps track of the options used and input and output  map	names.
       The  project  file  also  creates a database window based on the active
       window at the time a project is started.  The active window is returned
       to this active window for subsequent work sessions on the project.



FFiirrsstt MMeennuu
       At this menu the user can:


       1 start a new project

       2 start a project based on an existing project

       3 work on an existing project

       4 remove project files

       5 exit


       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 informa-
       tion. where appropriate the likely choices are supplied if not insisted
       upon.  The drainage direction map must be based on the idealized eleva-
       tion 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 rruunnooffff aannaallyyssiiss sseeccttiioonn 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 rain-
       fall map. The most difficult information requested of the user  is  the
       anticeedent moisture condition. This is a number between 1 and 3 inclu-
       sivly 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  con-
       ditions.   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 ccoonnttaammiinnaanntt sseeccttiioonn 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 foot-
       ings  for  comparison.  To  complicate  the  issue street sweeping also
       removes contaminants from road ways and so if the study area has a reg-
       ular  streeet  cla\aning  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.



SSuummmmaarryy
       The  tools  presented  here  run  within the GRASS GIS. A flow chart of
       input and analysis (figure X1) can be used as a guide for the order  of
       analysis. The system of units and means for encoding qualitative infor-
       mation are supplied for both  input  and  output  map  layers.  Careful
       preparation of input maps is strongly encouraged since this effort will
       likely save time and trouble later in the analysis.


       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  sce-
       narios.	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!
       AUTHOR	 Brian R. Brodeur    Cook College Remote Sensing Center


       _L_a_s_t _c_h_a_n_g_e_d_: _$_D_a_t_e_: _2_0_0_2_/_0_1_/_2_5 _0_5_:_4_5_:_3_4 _$



GRASS 5.0							     r.wrat(1)
