Estimating Load and Concentration of Total Nitrogen by Non Point Source in  Nooksack watershed

 

Joon Hee, Lee

 

Department of Civil and Environmental Engineering

Utah State University

 

Introduction/Method/Results/Conclusion/Future work/References

Introduction

In recent years, water supplies have increased in watersheds of Washington State.  Therefore, Washington State began to concern that insufficient in-stream flows remain for fish and other users.  In 1998, Washington State legislature passed the Watershed Management Act to encourage and provide some funding for local watershed planning (USGS, 2002).  This Watershed Management Act requires examination of Total Maximum Daily Load (TMDL) for non-marine water bodies in Water Resource Inventory Area No.1 (WRIA 1), which includes Nooksack watershed and some neighboring watershed located on northwest of Washington State.  For examination of TMDL, estimation of load and concentration of constituents from point source and non-point source is required (Watcom County, 2000).

Nitrogen is the most important constituent in water quality issue because nitrogen causes eutrophication.  Therefore, in this study, total nitrogen (T-N) load and concentration from non-point source in Nooksack watershed were estimated.  In addition, the effect of landuse change was evaluated.

 

Method

In order to estimate total nitrogen (T-N) annual load, export coefficient approach was used.  The export coefficient is a pollutant loading rate for some land use in unit area and unit time period (EPA, 2000).  The load is calculated by following equation.

LP = S (LPU x AU)

Where LP= Total nitrogen annual load (lb/yr)

           LPU= export coefficient (lb/ac/yr)

           AU=Area of land use type (ac)

The export coefficient depends on landuse.  For example, the total nitrogen export coefficients for agricultural and urban area are higher than forest (Table1). 

Table 1. T-N export coefficient

Landuse type*

LEVEL 2 **

T-N Export coefficient

(lb/ac/yr)*

Low density residential

Mixed urban or built-up

Other urban or built-up

4.43

Multi family residential

Residential

7.07

Commercial

Commercial and service area

Industrial commercial complex

9.48

Highway

Transport, Commercial, Utility

6.25

Industrial

Industrial

9.93

Open Land

Shrub & Brush range land

2.32

Wetlands

Forested wetland

Non forested wetland

4.90

Pasture

Cropland and pasture

5.60

Agriculture

Orchard, Grove etc.

15.65

Woodland

Deciduous forestland

Evergreen forestland

Mixed forestland

2.78

 

Others

2.20

* Supplied by North Florida Water Management district in EPA, PLOAD User manual.

**In this study, landuse shape file with LEVEL2 was used.  Therefore, T-N export coefficient values supplied for 10 landuse type were matched to LEVEL2.

 

In this study, annual run off was calculated by following equation

R= P-ET-U

Where R= Annual run off (in)

            P= Precipitation (in)

            ET= Evaporate transportation (in)

            U= Water use

 

After calculation of annual load and run off, total nitrogen concentration is calculated by following equation.

C= (LP x 454000)/(R x 0.0254x AU)

 Where C= Total nitrogen concentration  (mg/L)

            LP= Total nitrogen annual load (lb/yr)

            R=Run off (in)

  AU=Area of land use type (m2)

            454000= conversion factor (mg/lb)

            0.0254= conversion factor (m/in)

 

The T-N concentration from non-point source at outlet of each sub-watershed was estimated by following equation.

                                                                      S L

T-N concentration at outlet (mg/L) =    ---------

                                                                      S Q

Where L= T-N load from each land use in a watershed (or sub-watershed) in mg/yr

            Q= Run off from each land use in a watershed (or sub-watershed) in L/yr

 

          In order to estimate annual average T-N load and concentration, run off data, export coefficient data and a shape file including landuse data were used (Figure 1).  Run off data came from the water balance table by Utah Water Research Laboratory.  Averaging run off in inch for each sub watershed was used to calculate run off in m3.  Export coefficient data came from PLOAD user manual by EPA (EPA, 2000).  Land use shape file including area of each land use type was downloaded form EPA BASINS web site (EPA, 2002).  Runoff data and export coefficient data are inserted to the table of shape file for land use.  After inserting runoff data and export coefficient data, annual average total nitrogen load and concentration for each land use of each sub watershed were calculated by calculation function of the table in Arc Map (Figure2). 

Figure 1. Linking data to estimate T-N load and concentration

 

          In order to study the effect of landude change,  It was assumed that deciduous forest area at western part of North fork Nooksack sub watershed was converted to agricultural area.  After export coefficient for this area was change,  the T-N load and concentration at outlet of North fork Nooksack sub watershed were recalculated.

Figure 2. Table of landuse shapefile inserting export coefficient and run off data

 

Results

1) Area and landuse pattern

The total area of Nooksack watershed is 1935 Km2.  Five sub watershed, which are north fork Nooksack, middle fork Nooksack, south fork Nooksack, Nooksack river and Sumas river, were delineated (Figure 3).  North fork Nooksack  is biggest sub-watershed and sumas river is smallest sub watershed in Nooksack watershed (Figure 4).  In three eastern watershed , north fork, south fork and middle fork Nooksack sub-watershed,  much area are forestland and just small areas are agricultural and urban land (Figuer 5).  However, in western part, 52% of Nook sack river sub watershed and 86% of Sumas river sub watershed are Agricultural land.  Also, 12% of Nooksack river sub watershed and 6% of Sumas river sub-watershed are urban land (Figure 6)

Figure 3. Sub watershed in Nooksack watershed

 

Figue 4. Distribution area for each sub watershed

 

Figure 5. Landuse in Nooksack watershed

Figure 6. Landuse distribution for each sub watershed

 

2) Estimated annual average T-N load from non-point source

Total annual average T-N load caused by non-point source of entire Nooksack watershed was 1,537,029 lb/yr. Even though Nooksack river and Sumas river sub-watershed occupied just 31% of entire Nooksack watershed area, 45% of total Non-point source T-N release were caused by Nooksack river and Sumas river sub watershed (Figure 7).  This is because these two western sub watershed has high export coefficient due to agricultural activity such as use of fertilizer and urban activity.

Figure 7. T-N load distribution for each sub watershed

 

3) Run off

North fork, middle fork and south fork Nooksack sub watershed have much higher precipitation than Nook sack river and Sumass river sub watershed (Figure 8). Therefore, these three eastern sub watershed have higher runoff than western two sub watershed (Figure 9).

Figure 8. Annual average precipitation, 1961-1990 (USDA, 2002)

 

Figure 9. Average annual run off

Text Box: . precipitastion, 1961-1990 ,USDA,2002  


4) Estimated annual average T-N concentration from non-point source

          Almost Sumass river sub watershed and western part of Nooksack River had over 1.0mg /L T-N concentration from non-point source while T-N concentrations are below 0.6 mg N/L in almost north fork, middle fork and south fork Nooksack sub watershed (Figure 10).  This because Sumas river and western Nooksack river sub watershed have high T-N export coefficient due to agricultural landuse and low runoff. Estimated T-N concentration at outlet of Sumas river sub watershed was 1.2 mg N/L.  Therefore, even though estimated T-N concentration at outlets of three up stream sub watersheds are 0.16 – 0.22 mg N/L, the estimated T-N concentration at outlet of entire Nooksack watershed was 0.35 mg N/L (Figure 11).

Figure 10. Estimated T-N concentration from non-point source

 

Figure 11. Estimate T-N concentration at outlet of each sub watershed

5) The effect of land use change

          After the deciduous forest land at western north fork Nooksack sub watershed was converted to agricultural area, the T-N load from non-point source of north fork Nooksack sub watershed increased 10.8 % from 428,105 lb/yr to 474,388 lb/yr (Figure 12).  Since it was assumed that run off was not changed after landuse change, the T-N concentration also increased 10.8 % from 0.18 mg/L to 0.20 mg/L at outlet of north fork Nooksack sub watershed.  Since landuse changing area was small (22.2 km2, 3.5% of north fork Nooksack sub watershed area), land use change did not give large effect in T-N concentration.

Figure 12. Area of Landuse change

Conclusion

  1. In Nooksack River and Sumas River sub watershed, the dominant landuse is agriculture.  On the contrary, the dominant landuse in north fork, middle fork and south fork is forest land.
  2. Annual average T-N load caused by non-point source of entire Nooksack watershed was 1,537,029 lb/yr.  Even though Nooksack river and Sumas river sub watershed occupied just 31% of entire Nooksack watershed area, 45% of total Non-point source T-N release were caused by Nooksack river and Sumas river sub watershed.
  3. Annual average non-point source T-N concentrations of Sumas rive and Nooksack river sub watershed were higher than those of north fork, middle fork and south fork Nooksack sub watershed.  The non-point source T-N concentration at outlet of entire Nooksack watershed was 0.35 mg N/L.
  4. When deciduous forest land at western part of north fork Nooksack sub watershed was converted to agricultural area, T-N load and concentration increased 10.8%.

 

Future work

  1. The T-N load from point source should be combined with non-point source.
  2. Monthly pattern of T-N load should be investigated.
  3. Total Maximum Daily Load (TMDL) should be estimated.

References

1. EPA, BASINS,2002, http://www.epa.gov/waterscience/ftp/basins/gis_data/huc

2. EPA, 2000, PLOAD user manual

3. USDA, 2002, climate and map data, http://www.ftw.nrcs.usda.gov/prism/prism.html

4. USGS, WRIA 1, 2002. Watershed management project, http://wa.water.usgs.gov/wria1/Intro/intro.html

5. Watcom county, 2000,The watershed management act

6. USGS, 2002, Obtaining NHD, http://edcnts14.cr.usgs.gov/Website/nhdserver/viewer.htm