HOW L-THIA Estimate NPS Pollutant Loadings using Event Mean Concentration
Kyoung Jae Lim and Bernard Engel
Figure 1 shows how the L-THIA model estimates the runoff volume and nonpoint source pollutant loadings within the ArcView L-THIA extension. Runoff volume is computed by runoff depth multiplied by cell area. Cell size is read directly from the runoff depth grid. Event Mean
Concentration (EMC) data are used to predict NPS pollutant masses by multiplying the runoff volume of a grid cell by the EMC value and converting units. In the WWW-based L-THIA
(http://www.ecn.purdue.edu/runoff/lthianew), NPS pollutant masses are computed by multiplying runoff depth for a land use by the area of that land use and the appropriate EMC value and converting units. EMC data (Baird and Jennings, 1996) is shown in Table 1.
Combine two maps
Assign
Curve Number
HD
Res.
Land use
4001 79
A
Hydrologic soil group
Combined Grid Curve Number
Map
Using Long-Term
Daily Precipitation Data
Assign
Runoff Depth
Assign NPS EMC for each Land use
Compute Vol.
using cell size
0.85
Runoff Depth Map
2.4
100
NPS Pollutant
Map
Runoff Volume
Map
Figure 1. L-THIA GIS Overview
The Event Mean Concentration (EMC) data used was compiled by the Texas Natural
Resource Conservation Commission (Baird and Jennings, 1996). Numerous literature and existing water quality data were reviewed by the Baird and Jennings (1996) with respect to eight categories of land use and several parameters. Land use categories defined were: (1) industrial;
(2) transportation; (3) commercial; (4) residential; (5) agricultural cropland (dry land and irrigated); (6) range land; (7) undeveloped/open; and (8) marinas. The total pollutant load for
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various NPS pollutants divided by runoff volume during a runoff event yielded the Event Mean
Concentration. However, some pollutant concentrations vary with time for rainfall events, so flow-averaged sample values were used as Event Mean Concentrations in these cases.
Therefore, EMCs should be reliable for determining average concentrations and calculating constituent loads. However, additional efforts to improve estimates of EMCs are desirable
(Baird and Jennings, 1996).
The advanced version of L-THIA ( http://www.ecn.purdue.edu/runoff ) permits users to provide their own EMC values if data are available to do so. The ArcView L-THIA extension also permits EMC values to be replaced, however knowledge of the ArcView Avenue language is required. An alternative within ArcView is to compute the masses using location specific
EMC values and the runoff volume map within Arcview’s mapcalculator.
Table 1. Event Mean Concentration by land use classifications from Baird and Jennings (1996)
NPS Pollutant
Total Nitrogen (mg/L)
Total Kjeldahl Nitrogen
(mg/L as N)
Nitrate+Nitrite (mg/L)
Total Phosphorus (mg/L)
Dissolved Phosphorus
(mg/L)
Suspended Solids (mg/L)
Dissolved Solids (mg/L)
Total Lead (μg/L)
Total Copper (μg/L)
Total Zinc (μg/L)
Total Cadmium (μg/L)
Total Chromium (μg/L)
Total Nickel (μg/L)
BOD (mg/L)
COD (mg/L)
Residential
1.82
1.5
0.23
0.57
0.48
80
0.75
2.1
10
41
134
9
15
25.5
49.5
Commercial
Land use classification
Indus
-try
Transi
-tion
Mixed
1.34 1.26 1.86 1.57
1.1
0.26
0.32
0.11
55.5
185
13
14.5
180
0.96
10
11.8
23
116
1.0
0.3
0.28
0.22
60.5
116
15
15
245
2
7
8.3
14
45.5
1.5
0.56
0.22
0.1
73.5
194
11
11
60
1
3
4
6.4
59
1.25
0.34
0.35
0.23
57.9
157
12
13.9
141
1.05
5.5
7.3
17.2
67.5
Agricultural
4.4
Range
0.7
1.7
1.6
1.3
---
107
1225
1.5
1.5
16
1
10
---
4.0
---
0.2
0.4
0.01
---
Oil and Grease (mg/L) 1.7 9 3 0.4 3.5 --- ---
REFERENCES
Baird, C., and M. Jennings, 1996. Characterization of Nonpoint Sources and Loadings to the
7.5
---
6
1
1
245
5.0
10
0.5
---
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Corpus Christi Bay National Estuary Program Study Area, Texas Natural Resource Conservation
Commission.
Engel, B. A. 1997. GIS-based CN Runoff Estimation, Agricultural and Biological Engineering
Departmental Report, Purdue University.
If you need details, please send email to Prof. Bernie Engel ( engelb@purdue.edu
) or
Kyoung Jae Lim ( kjlim@purdue.edu
).
.
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