Transcript The NAM Model Evaporation Overland flow The excess rainfall is divided between overland flow and infiltration Overland flow The overland flow varies with the soil moisture: Overland flow The.

The NAM Model
Evaporation
Overland flow
The excess rainfall is divided between
overland flow and infiltration
Overland flow
The overland flow varies with the soil
moisture:
Overland flow
The overland flow varies with the soil
moisture:
Interflow
QIF
Groundwater recharge
Groundwater storage & baseflow
a) Standard linear reservoir
BF = H / CKBF for H  0
BF = 0
for H < 0
BF
H
CKBF
baseflow
water level in groundwater storage
reservoir time constant
b) Shallow groundwater reservoir
BF = (GWLBF0 - GWL) Sy / CKBF
BF = 0
for GWL  GWLBF0
for GWL > GWLBF0
Baseflow
Rainfall data
Required: catchment rainfall
Available: point rainfall, usually
Estimation of catchment rainfall:
– Thiessen
– Kriging and splines
– etc.
Data requirements:
• How many stations?
– Depends on catchment topography
– min. recommended: 1 per 1000 km2
•
Frequency of observation?
– Depends on regime and study objectives
– Flood forecasting: hourly or better
– water balance studies: daily
Potential Evaporation
Two possible data sources:
a) Pan evaporameter
Often a low data accuracy:
– dependent of the surroundings
– large variation in pan coefficient
– good maintenance important
Monthly data normally within 10% accuracy
b) Calculation from climate variables, e.g.
Penman-Monteith:
– net radiation
– wind speed
– temperature
– relative humidity
Daily or monthly data can be used
Generally more reliable than pan data
Selection of data type depends also on availability
Main NAM parameters
Umax
Maximum contents of surface storage
Evaporation; small peaks
Usual value: 10-25 mm
Lmax
Maximum contents of rootzone storage
Evaporation; water balance
Usual value: 50-250 mm
CQof
Overland flow coefficient
Divides excess rainfall in runoff and infiltration
Usual value: 0.01 - 0.99
TOF
Rootzone threshold value for overland flow
Delays overland flow at the beginning of a
wet season
Usual value: 0.0 - 0.7
Main NAM parameters
TG
Root zone threshold value for recharge
Delays groundwater recharge at the
beginning of a wet season
Usual value: 0.0 - 0.7
CKBF
Time constant for routing baseflow
Determines shape of baseflow hydrograph
Usual value: 500 - 5000 hours
CK1 ,CK2 Time constant for routing overland flow
Determines shape of peaks
Usual value: 3 - 48 hours
NAM calibration
3 - 5 years of observed runoff
Graphical comparison with simulation:
Statistics in file: RRstat.txt
Recommended procedure:
1. Initial, approximate adjustment of:
– Lmax and Umax for water balance
– CQof and CK1,2 for peaks
– CKBF for baseflow
2. Repeat for further parameter adjustment:
– identify a parameter change, likely to improve
the calibration
– change this parameter only
– make significant changes the first time