Land Use Change and Its Effect on Water Quality: A Watershed Level BASINS-SWAT

Model in West Georgia

Gandhi Raj BhattaraiDiane Hite

Upton Hatch

Prepared for presentation at the Alabama Water Resource Conference and Symposium, Orange Beach, Alabama, October 12-14, 2005

Introduction

• Non-Point Source Pollution (NPP) is caused by the movement of water over and through the ground

• NPP threatens majority of the water bodies in the US

• Increasing urbanization coupled with increasing use of fertilizers and chemicals in agricultural lands create significant challenges to maintain water quality

• Biophysical water quality models facilitate the spatial analysis of sources and effects of point and non-point pollutants with reference to their origin and locations

• Level of nitrogen, phosphorus and sediment loads are estimated to help in formulating control policies

Objective

• Find the relationship between land use change and water quality by simulating levels of nitrogen, phosphorus and sediment with two contrasting land use scenarios over time

The Modeling Approach

The BASINS framework

(Better Assessment Science Integrating Point and Non-point Sources)

• Provides a centralized platform for data extraction and descriptive analysis

• Helps in setting up individual watershed based models

• Includes four specific watershed level biophysical models for the estimation of in-stream and watershed loading and transportation (QUAL2E, PLOAD, HSPF and SWAT)

The Modeling Approach

Soil and Water Assessment Tool (SWAT)

• Soil and Water Assessment Tool (SWAT) is integrated in components of the BASINS model

• The program integrates ArcView Geographic Information System interface to derive the model input parameters and simulation

• Inputs are DEM, digital land use maps, soils maps, historical temperature and precipitation data, management parameters etc.

• Starts with hydrological boundary delineation

(a) (b)

(c) (d)

Figure 2. Automatic watershed delineation using the DEM and NHD stream network; (a) National Elevation Model data loaded, (b) watershed area and NHD networks loaded as focus area and burn-in option, (c) digitized stream networks and nodes created, (d) sub-basins created and parameters calculated.

(a) Reclassified land use (e.g., NLCD 2001) (b) Reclassified state soils map

Figure 3. Land use and soil reclassification and spatial overlay

Unique hydrological response units (HRU) are created by overlaying land use and soils maps; e.g. 5 land use classes and 3 different soils can produce 5x3 = 15 HRUs with unique land use-soil combinations

Limitations of the study

• Left out small land cover areas in unique uses (model limitation)

• Lack of model calibration and validation (data limitation)

5 0 5 Miles

Main Watershed

Study area sub-watershed

County Boundary

Reach File V1

Legend

Stream Network (NHD)

N

EW

S

Location of Study Area in West Georgia

(HUC #0331000212)

Fulton

CobbPaulding

Carroll

Douglas

Coweta

Heard

Meriwether

Troup

TalbotHarris

Muscogee

Randolph

Chambers

Lee

HUC # 03310002

Weighted Land Use Share in Harris County, GA

Land Use Share Broad Land Use Class 1992 2001 Change* Developed 1.0% 6.2% 515.9% Agricultural 8.2% 16.6% 101.8% Forest 86.6% 70.7% -18.3% Other 4.2% 6.5% 54.3% * Area weighted change

Land Use Change in Mulberry Creek Subwatershed1992 - 2001

(a) Reclassified NLCD-1992 (a) Reclassified NLCD-2001

Developed LandAgricultural LandForest LandOther Land

Subwatershed Boundary

Legend

Results

Description of watershed

Number of sub-basins : 331

Mean Elevation of sub-basins : 216 m.a.s.l.

Average size of sub-basins : 176.3 hectares

Hydrological Response Units NLCD-92 NLCD-01

# HRUs created 2089 2158

# HRUs per Sub-basin 6.3 6.5

Average size of HRU (ha.) 27.9 27.0

Effects of Land Use Change on Water Quality (60 years Annual Average)

NLCD 1992 NLCD 2001 Variable

Mean S. E. Mean S.E.

Change

%

Precipitation (mm) 1277.60 207.86 1277.60 207.86 n/a

Water Yield (mm) 243.15 93.17 251.87 94.20 3.6%

Sediment (t/ha) 13.60 8.49 16.13 9.22 18.7%

Nitrogen (kg/ha) 2.07 1.20 2.23 1.19 8.0%

Phosphorus (kg/ha) 0.24 0.14 0.27 0.14 9.0%

Average Annual Sediment Yield (t/ha)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

1950 1960 1970 1980 1990 2000

Year

Sed

imen

t (t

/ha)

.

LU-1992 LU-2001

`

Average Annual Organic Nitrogen Runoff (kg/ha)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

1950 1960 1970 1980 1990 2000

Year

Nitr

ogen

(kg/

ha)

.

LU-1992 LU-2001

`

Average Annual Organic Phosphorus Runoff (kg/ha)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1950 1960 1970 1980 1990 2000

Year

Pho

spho

rus

(kg/

ha)

.

LU-1992 LU-2001

Conclusion

• Land use change from 1992 to 2001 affected water quality:

• Average annual nutrient runoff increased by 8% for nitrogen and 9% for phosphorus

• Average annual sediment loadings increased by 19%

• Water yield in main channel increased by 4% suggesting less ground water recharge

• Less vegetative cover, more impervious surfaces, and increased agricultural land caused less percolation and higher runoff with higher nutrient runoff & sedimentation

Conclusion

• Simulation using two land use maps suggest– Using a fixed set of land use data for a long period of SWAT

modeling may not yield precise results as changes in land use causes significant changes in water quality result

– However, the same result indicates that SWAT can precisely predict the effects on water quality over time, if intermediate land use maps are used for comparative studies