Estimate the “Dissolved N Concentration in Runoff” value for Cropland

[mmcfarland]

Calculate the “Dissolved N Concentration in Runoff” value (first position [NitrConc] on Line 40 of “*.gms” file (see “MapShed GMS File Key.doc” for further detail). This particular value is currently set by default as 2.9.

Procedure:

• Use the reclassified “USDA Croplayer” in combination with the 2011 NLCD layer to calculate this value( Reclassify and Ingest CropLand raster #1757 )

Reclassed CDL Value	Dissolved N Value
1	1.63
2	2.35
3	2.86
4	0.70
5	0.52
6	1.10
7	1.68
8	2.60
9	0.36
10	2.56

• Calculate the new NitrConc value in the gms file as the weighted “Dissolved N” values from the above table. For each of the new categories above, calculate

% of New CDL Category * Total area of NLCD value of 82 in AOI * Dissolved N` values in the above table

And then calculate the weighted average.

For example, if the cropland area in a given AOI has a “New CDL Category” distribution of 30% = 1, 25% = 4 and 45% = 8, then the weighted average value would be computed as:

NitrConc = (0.30 * 1.63) + (0.25 * 0.70) + (0.45 * 2.60) = 1.83