View
219
Download
0
Tags:
Embed Size (px)
Citation preview
Alan F. HamletDennis P. Lettenmaier
JISAO Center for Science in the Earth System Climate Impacts Group
and Department of Civil and Environmental EngineeringUniversity of Washington
October, 2004
Weekly and Daily Climate Change Streamflow Scenarios and Estimates
of Changing Crop Water Demand
http://www.hydro.washington.edu/Lettenmaier/Presentations/2004/hamlet_daily_flow_crop_et_2004.ppt
Problem:
1) Monthly naturalized streamflow observations are frequently available for a large number of sites over long periods of time, but availability of weekly and daily observations is typically very limited.
2) Monthly climate change scenarios are useful, but for many studies weekly or daily flows are required (e.g. flood control).
Methods are needed to:
•Produce weekly and daily “observed” records that are consistent from monthly naturalized data.
•Produce climate change scenarios at weekly and daily time step that are consistent with observed data sets.
Observed Monthly AverageStreamflow Data
SimulatedDaily Streamflow
Data
Simulated Weekly Streamflow
Data
Reconstructed “Observed”Daily
Record
Reconstructed “Observed”Daily
Record
VIC Hydrologic ModelDriven by Observed
Temperature and Precipitation
Data
Monthly average value comes from observed data. The daily and weekly time history come from the simulations.
Short Time Step Streamflow Reconstruction Process
0
10000
20000
30000
40000
50000
60000
1 62 123 184 245 306 367 428 489 550 611 672 733 794 855 916 977 1038 1099 1160 1221 1282 1343 1404 1465 1526 1587 1648 1709 1770
obs week
adj vic week
0
5000
10000
15000
20000
25000
30000
35000
40000
1 62 123 184 245 306 367 428 489 550 611 672 733 794 855 916 977 1038 1099 1160 1221 1282 1343 1404 1465 1526 1587 1648 1709 1770
obs daily
adj vic daily
Reconstructed Naturalized Weekly and Daily Flows at Palisades Dam for 1958-1992
Str
eam
flow
(cf
s)
Weekly Flow 1958-1992
Daily Flow 1958-1962
Bias Corrected Monthly or WeeklyClimate Change
Streamflow
SimulatedDaily Streamflow
Data
Daily Time StepStreamflow ScenarioVIC Hydrologic Model
Driven by Climate Change
Temperature and Precipitation
Scenario
Monthly or weekly data comes from bias corrected simulation. The daily time history used to construct the daily data comes from the simulations.
Short Time Step Climate Change Streamflow Scenario
0
100000
200000
300000
400000
500000
600000
19
28
19
29
19
30
19
31
19
32
19
33
Str
eam
flo
w (
acre
-ft/
wee
k)
"observed"
mpi2040
Weekly Climate Change Scenario for Palisades(MPI 2040 “warm and dry”)
Long-Term Estimates of Potential Evapotranspiration from a Reference
Crop
Problem:
Quantitative, spatially-explicit estimates of evaporation from irrigated crops are needed for:
•Estimates of future surface water diversions and return flows as a function of climate, irrigation technology, crop type, etc.
•Estimating losses from aquifers due to groundwater pumping for irrigation and aquifer recharge due to surface water application.
Methods:
A well-tested and frequently used method is to estimate the “potential evaporation” (PotET) from a well-watered reference crop (e.g. mature alfalfa), and then relate this to the PotET for other crops using linear factors that vary with crop type and season:
Actual Crop PotET = Kc * (PotETref)
(where Kc varies with date and actual crop)
PotETref is often estimated by the Penman Monteith equation. See e.g. :
http://www.cprl.ars.usda.gov/wmru/pdfs/982123.pdf
Conceptual Diagram of the Penman Monteith Approach
“Aerodynamic Resistance”Wind SpeedCrop Height
“Canopy Resistance”Stomotal Resistance
Leaf Area Index
“Surface Energy”Incoming Solar Radiation
Outgoing Longwave
“Vapor Pressure Deficit”Temperature
Relative Humidity
Potential Evapotranspiration
Gridded DailyPrecipitation and Temperature
Records1915-2002
VICHydrology Model
Daily Time Series of
Estimated Reference
Crop PotET1915-2002
Schematic Diagram of Simulation Tool for Producing Long Records of PotET
Seasonal Cycle of PotET for a Single Grid Cellin the Snake River Plain
0
1
2
3
4
5
6
7
8
9
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51
Water Year Week
Avg
Po
tET
(m
m)
1950
1994
Average July PotET for Alfalfa Reference Crop
Potential ET (mm)
Precipitation Fraction, 2020s
0.5
0.75
1
1.25
1.5
1.75
J F M A M J J A S O N D
Frac
tion
hadCM2
hadCM3
PCM3
ECHAM4
mean
Delta T, 2020s
-1
0
1
2
3
4
5
J F M A M J J A S O N D
De
gre
es
C
hadCM2
hadCM3
PCM3
ECHAM4
mean
Delta T, 2040s
-1
0
1
2
3
4
5
J F M A M J J A S O N D
De
gre
es
C
hadCM2
hadCM3
PCM3
ECHAM4
mean
Precipitation Fraction, 2040s
0.5
0.75
1
1.25
1.5
1.75
J F M A M J J A S O N D
Fra
ctio
n
hadCM2
hadCM3
PCM3
ECHAM4
mean
Four Delta Method Climate Change Scenarios for the PNW
~ + 1.7 C ~ + 2.5 C
Somewhat wetter winters and perhaps somewhat dryer summers
Average July PotET over the Southern Plain Region Current Climate vs. MPI2040 scenario
Current Climate MPI2040
PotET (mm/day)
Trends in July Avg PotET over the Southern Plain Region from 1915-2002
y = -0.0061x + 7.3589
5
5.5
6
6.5
7
7.5
8
8.5
1915
1920
1925
1930
1935
1940
1945
1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
Ref
eren
ce C
rop
Po
tET
(m
m/d
ay)
jul
Linear (jul)
Conclusions
Long-term gridded temperature and precipitation records can be used to drive hydrologic models to simulate potential ET for a reference crop.
Simple experiments in which the temperature is perturbed while other explanatory variables remain about the same suggest that crop water demand ought to be going up over time as the region warms.
The long term historic simulations, however, show that the trends are downward over time. One possible explanation for these trends is associated with increasing night time temperatures, which indicate that atmospheric moisture content is systematically increasing. This reduces the vapor pressure deficit and the incoming solar radiation.
These results suggest that changes in relative humidity, cloudiness, and wind may play a more dominant role than temperature alone in controlling ET. If so, more sophisticated methods for evaluating the effects of changing climate on these variables will be needed to better assess the potential changes.