modeling nitrogen loading to the groundwater in response to land use change by dibyajyoti (diby)...
Post on 21-Dec-2015
214 views
TRANSCRIPT
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
0
1
2
3
4
5
6
7
8
9
10
11
12
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 !