Modeling Nitrogen Loading to the Groundwater in Response

to Land Use Change

By

Dibyajyoti (Diby) Tripathy ABE 527 (Spring’ 04)

Introduction

• Land use change can affect both surface water and groundwater quality

• Impacts of land use change on groundwater has received less attention

• Given the alarming rate of land use change globally, it is important to understand the linkage between land use change and nutrient (e.g., Nitrogen) loading to groundwater

Objectives

• Model Nitrogen loadings to surface water (or, runoff) in response to land use change

• Model how much of Nitrogen in the runoff is likely to contaminate groundwater if an orchard grass strip is used as a detention base

Models

Two models are used is this project:

1> Long-Term Hydrologic Impact Assessment (L-THIA)

2> Grazing Simulation Model (GRASIM)

(Both the models are developed at Purdue University and have WWW based interfaces)

Models continued…

L-THIA

N-Loadingto Runoff

GRASIM

N-Leaching to Groundwater

Models continued..

• L-THIA Model formed the basis for estimating Nitrogen loading to the runoff

• It is an empirical model that estimates runoff and pollutant loadings annually

• I have modified the model (a spread sheet version) to estimate daily Nitrogen Loadings

L-THIA Model

Models continued..

• GRASIM was used for estimating Nitrogen leaching to the groundwater through an orchard grass strip

• It is a comprehensive grazing model that predicts standing biomass, soil moisture, drainage, and Nitrogen leaching under pasture

• Nitrogen available for leaching is computed as: NAL = Nf + Np + Nrsd + Nn – Nplt – Ndet - Noth

• The processes involve: nitrification, mineralization, uptake, volatilization, denitrification, and leaching

GRASIM Model

Methods• A hypothetical land use scenario (consisting of meadows,

commercial, and high density residential land) in a 10 acre area in State College, Centre County, PA, was considered

• Precipitation data (1994), curve numbers (CN) for specific land use categories and hydrologic soil groups, and the area for each land use were used to calculate the daily runoff volume

• Finally, amount of N Loading (daily) to the surface water was determined by multiplying Event Mean Concentration (EMC) value for Nitrogen with the runoff volume

Methods continued…

• Daily N-loadings in the study area for 1994 were then incorporated into GRASIM such that it mimicked application of manure over a meadow

• Soil and nutrient parameters were adjusted accordingly to reflect the conditions of the study area

• Finally, total Nitrate leaching to the groundwater below the root zone was calculated

Results

• Amount of Nitrate leaching to groundwater is found to be 2.0 kg/ha for the given scenario

• A strong positive correlation is found between rainfall, N loading to runoff, and Nitrate leaching to groundwater

• Orchard grass strip is found to be an effective, low cost, and environment friendly measure to reduce groundwater contamination due to N

Results continued… Total amount of Nitrate leached from the top layer (30

cm depth below ground surface) is 57.5 kg/ha

Nitrate leaching from top layer can’t cause any groundwater contamination

Total amount of Nitrate leached from the bottom layer (30 cm to 70cm depth below surface) is 2.0kg/ha

Nitrate leached only from 2nd layer can cause groundwater contamination

Results continued…

A strong correlation between rainfall, N loading to runoff, and Nitrate leaching to groundwater

N Loading (kg/ha) from L-THIA: 1994, Center county, PA

0

0.5

1

1.5

2

2.5

3

1 2 3 4 5 6 7 8 9 10 11 12

Months (1: January; 12:December))

N L

oad

ing

:kg

/ha

Rainfall Distribution (monthly) in 1994, Center Cnty, PA

0

1

2

3

4

5

6

7

8

9

1 2 3 4 5 6 7 8 9 10 11 12

Months (1: January; 12:December)

Rai

nfa

ll (

in)

Nitrate Leaching (monthly) Below Root Zone in 1994, Center Cnty, PA (from GRASIM)

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7 8 9 10 11 12

Months (1: January; 12:December)

Nit

rate

:kg

/ha

Results continued…Total Available N Vs. Toal N Leaching to Groundwater

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1

2

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5

6

7

8

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13

14

0 50 100 150 200 250 300 350

Julian Days

Nit

rog

en

(k

g/h

a)

Total N Availabe

Total N Leaching

Sensitivity Analysis

Discussion• Highly sensitive parameters:

1> Leaching coefficient2> Soil bulk density3> Soil water content after gravitational water drained from bottom Layer4> Soil water content at 15 bar of bottom Layer5> Initial organic matter

• Sensitive parameters: 1> Mineralization rate of soil organic matter2> Initial nitrate in the bottom layer

Conclusion• It is possible to combine two very different models

- L-THIA and GRASIM - to estimate possible N loading to groundwater due to land use change

• Amount of Nitrate leaching to groundwater is found to be 2.0 kg/ha for the given scenario

• Due to it’s ability in substantially reducing N leaching, Orchard grass strips can be used as effective, low cost, and environment friendly measure for preventing groundwater contamination due to N

Conclusion continued…

• GRASIM results vary considerably with slight change in values of following parameters: – Leaching coefficient– Soil bulk density– Soil water content after gravitational water

drained from bottom Layer– Soil water content at 15 bar of bottom Layer– Initial organic matter

• Thus accurate and field specific values must be used to get more realistic N leaching estimations

Thank You !