Low (web) Quality - BALTEX

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Streamflow in the upper Mississippi river basin as simulated by SWAT
driven by 20C results of NARCCAP regional climate models
E. S. Takle, M. Jha, E. Lu, R. W. Arritt, W. J. Gutowski, Jr., and the NARCCAP Team
Iowa State University, Ames, IA 50011 USA, gstakle@iatate.edu
1. Introduction
Major hydrological quantities in the Upper Mississippi River
Basin (UMRB) in the last two decades of the 20C are
evaluated with the Soil and Water Assessment Tool
(SWAT) in comparison with observed streamflow. Inputs to
SWAT are provided by the daily meteorological quantities
from observed at weather stations in the basin and from
daily meteorological conditions simulated by a subset of
regional climate models reporting to the archive of the North
American Regional Climate Change Assessment Program
(NARCCAP, 2009) driven by reanalysis boundary
conditions. Results show that regional models correctly
simulate the seasonal cycle of precipitation, temperature,
and streamflow within the basin. Regional models also
capture interannual extremes represented by the flood of
1993 and the dry conditions of 2000.
2. Soil and Water Assessment Tool
The SWAT (Arnold and Fohrer, 2005) is a physically based,
continuous time, long-term, watershed scale hydrology and
water quality model. SWAT version 2005 was used for this
analysis. Meteorological input to SWAT includes daily
values of maximum and minimum temperature, total
precipitation, mean wind speed, total solar radiation, and
mean relative humidity. The hydrologic cycle as simulated
by SWAT at the HRU level is based on the balance of
precipitation, surface runoff, percolation, evapotranspiration,
and soil water storage. SWAT takes total daily precipitation
from models or observations and classifies it as rain or snow
using the average daily temperature. When climate model
output is provided, SWAT uses only total liquid
precipitation and does its own partitioning to rain or snow.
Snow cover is allowed to be non-uniform cover due to
shading, drifting, topography and land cover and is allowed
to decline non-linearly based on an areal depletion curve.
Snowmelt, a critical factor in partitioning between runoff
and baseflow, is controlled by the air and snow pack
temperature, the melting rate, and the areal coverage of
snow. On days when the maximum temperature exceeds
0ºC, snow melts according to a linear relationship of the
difference between the average snow pack maximum
temperature and the base or threshold temperature for
snowmelt. The melt factor varies seasonally, and melted
snow is treated the same as rainfall for estimating runoff and
percolation. Further details can also be found in the SWAT
User's manual (Neitsch et al., 2002).
3. Regional Climate Models
A preliminary analysis is presented of three regional climate
models reporting to the NARCCAP archive: MM5I run at
Iowa State University, RCM3 run by the University of
California – Santa Cruz, and the WRFP model run at the
Pacific Northwest National Laboratory. The 20C
simulations of NARCCAP consist of simulations over North
America for 1980-2004 driven by NCEP reanalysis data at
lateral boundaries.
4. Preliminary Results
Mean monthly precipitation and streamflow (Fig 1) for
these three models demonstrate skill in simulating
seasonal distributions and also interannual variability.
a)
b)
c)
d)
Figure 1. RCM/SWAT simulations of a)
temperature, b) precipitation, c) monthly
streamflow, and d) interannual streamflow.