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-Loading to 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 = N f + N p + N rsd + N n – N plt – N det - N oth 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 2 nd layer can cause groundwater contamination

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

Results continued…

Sensitivity Analysis

Discussion Highly sensitive parameters: 1> Leaching coefficient 2> Soil bulk density 3> Soil water content after gravitational water drained from bottom Layer 4> Soil water content at 15 bar of bottom Layer 5> Initial organic matter Sensitive parameters: 1> Mineralization rate of soil organic matter 2> 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 !