1. ReNuMa: Nitrogen Dynamics
MODELLING DISCHARGE AND NITROGEN FLUXES FROM LARGE WATERSHEDS IN THE NORTHEASTERN UNITED STATES
D. P. Swaney1*, R. W. Howarth1 , A. E.. Galford1, E.W. Boyer2,
C.L. Goodale1 and R.M. Marino1
1Cornell University, Ithaca, NY USA
2University of California at Berkeley, Berkeley, CA USA *(
Water &
wetland
Forest &
shrubland
Urban &
barren
ReNuMa: Nitrogen Dynamics
Human
Waste
Direct addition
(load) to streams
In-stream
denitrification
Threshold response in DIN
concentrations following
Aber et al (2003)
Direct addition (load)
to streams
Direct addition to streams
Riverine
DIN flux
concentrations similar to
Billen &Garnier (2000)
Onsite
treatment
Wastewater
Treatment
Plants
Atmos
Deposition
Manure
Fertilizer
Agricultural
N-Fixation
Landuses
Landscape
& other losses
ReNuMa: Hydrological Dynamics
Daily
Temperature
Daily
Precipitation
 ABSTRACT
Much recent work has suggested that nitrogen fluxes to the coastal zone depend on interactions between input loading rates, climate, and landscape processes. Direct relationships between nitrogen inputs and riverine fluxes have been shown with a relatively simple nitrogen accounting methodology for 16 large watersheds in the northeastern US (Boyer et al., 2002). More recently, statistical models have shown that the average response of these watersheds per unit nitrogen load is strongly related to precipitation or hydrology (Howarth et al., in press). To extend this approach to examine the temporal response of these systems in response to climate and land use change, we are developing a simple Regional Nutrient Management model (ReNuMa) based on watershed-scale water balances and statistical relationships between N loads and responses. For the 6- year period examined so far, interannual discharge is well predicted over the range of watersheds studied. Nitrogen fluxes vary across watersheds in response to anthropogenic sources, including point sources, atmospheric deposition and fertilizer use.
Snowpack
Evapotranspiration
Snowmelt
Barren
Wetland
Vineyards/
orchards
Row crops
Open Water
Forest
Shrubland
Urban
Streamflow
SCS runoff equation
for each cover type
Shallow flow/runoff
Unsaturated zone
Annual DIN flux in 16 large Northeastern US watersheds
Baseflow
Saturated zone
Across all watersheds, simulated annual DIN fluxes also match observed values reasonably well. Some individual watersheds exhibit significant bias above or below
observed fluxes, but generally
correlate with annual changes variations reasonably well.
Human waste contributes N through sewers and septic system effluent. N deposition
traverses lakes and wetlands without retention, but exhibits a threshold landscape response in forests. Agricultural N sources (fertilizer, manure and fixation) also exhibit a threshold response due to retention (ie landscape denitrification, etc). The proportions of in-river denitrification are based on estimates in Van Breemen et al., 2002)
(1st order
linear reservoir)
Annual streamflow in 16 large Northeastern US watersheds
Simulated annual DIN fluxes improve as more sources are
added. Here, the scenarios
show the increase in several
goodness-of-fit measures
as more terms are included
in the model. Point sources alone (#5) underpredict the DIN load, but still
exhibit a significant
R2 (ie a linear
relationship with
observations).
Adding a fixed
contribution from
Preliminary comparison of simulated vs observed annual
DIN fluxes for the 12 watersheds with adequate
observations of DIN; ENS = Nash-Sutcliffe efficiency;
n=6 years*12 watersheds =72)
Run# Run description Bias R2 ENS
1 Baseline (all sources)
2 Pt sources + agriculture
+ N dep (no forest)
3 Pt sources + agriculture
4 Pt sources + agriculture
without load response
5 Pt sources only
16 watersheds in the Northeastern
USA in which Net Anthropogenic
Nitrogen Inputs (NANI) have been
related to average riverine N fluxes over
the period (Boyer et al., 2002).
We are extending the analysis to simulate
seasonal and annual streamflow and
nitrogen fluxes using the ReNuMa model.
Examples of threshold responses in agricultural and forest systems in the literature. Left: response of [NO3] to fertilizer loads in agricultural leachate (Billen & Garnier, 2000); Right: response of [NO3] to N deposition in forest leachate (Aber, et al., 2003). We use a similar parameterization to estimate landscape response from these land cover types.
References
The predecessor of ReNuMa is the Generalized Watershed Loading Function Model (GWLF):
Haith, D. A., Shoemaker, L. L Generalized watershed loading functions for stream flow nutrients. Water Resources Bulletin 23(3): A spreadsheet-based version of the model can be found on the web at:
Other references
The 2928 weather stations in the National Climate Data
Center network for Northeastern states were identified
( To select
candidate stations for each watershed, Thiessen polygons
for the network were generated using ArcView™ 3.2.
Stations with polygons intersecting a watershed and
with >95% complete records (daily temperature and
precipitation) were averaged to obtain representative
weather data for each watershed. Missing temperature
data for each station were replaced with averages of the
records preceding and following the missing interval;
missing precipitation values were replaced with zero.
agricultural lands greatly improves the agreement, and adding a load-dependent
contribution improves it further. Direct N deposition and the response of forests
also improves the agreement with observed fluxes across watersheds.
Aber, J. D., C. L. Goodale, S. V. Ollinger, M.-L. Smith, A. H. Magill, M. E. Martin, R. A. Hallett, and J. L. Stoddard Is Nitrogen Deposition Altering the Nitrogen Status of Northeastern Forests? Bioscience 54(4):
Billen, G. and J. Garnier Nitrogen transfers through the Seine drainage network: a budget based on the application of the ‘Riverstrahler’ model. Hydrobiologia. 410: 139–150.
Boyer, E.W., C. L. Goodale, N. A. Jaworski and R. W. Howarth Anthropogenic nitrogen sources and relationships to riverine nitrogen export in the northeastern U.S.A. Biogeochemistry 57/58:
Howarth, R.W., D.P. Swaney, E.W. Boyer, R.M. Marino, N. Jaworski and C.L. Goodale. The influence of climate on average nitrogen export from large watersheds in the Northeastern United States. Accepted for publication in Biogeochemistry.
Van Breemen, N., E.W. Boyer, C.L. Goodale, N.A. Jaworski, K. Paustian, S.P. Seitzinger, K. Lajtha, B. Mayer, D. Van Dam, R.W. Howarth, K.J. Nadelhoffer, M. Eve, and G. Billen Where did all the nitrogen go? Fate of nitrogen inputs to large watersheds in the northeastern U.S.A. Biogeochemistry 57/58: 267–293.
Ongoing work and future directions
Our work to date has focused on developing parameterizations of biogeochemical responses related to landuse/landcover:
Atmospheric deposition
Landscape level N retention of agricultural N sources (fertilizer, manure, N-fixation)
Nitrogen retention and losses (DIN) from forests
In-stream and landscape denitrification
We are currently working on refining the model parameters for individual watersheds to reduce bias at this scale, as well as developing alternative parameterizations of processes which incorporate results from smaller-scale models. Additional processes to be considered will include:
Phosphorus losses from P-saturated soils
Soil erosion and sediment transport
Minor adjustments to evaporative cover factor
for the Androscoggin and Susquehanna rivers were the only changes made to baseline parameter values for the watersheds. While
individual watersheds may exhibit some bias above or below observations (ie USGS annual streamflows ), agreement over all years and across all watersheds was generally good.
Acknowledgements
This work has been supported by an EPA STAR grant, “Developing regional-scale stressor models for managing eutrophication in coastal marine ecosystems, including interactions of nutrients, sediments, land-use change, and climate variability and change,” EPA Grant Number R830882, R.W. Howarth, P.I.