Sacramento%20Soil%20Moisture%20Accounting%20Model%20(SAC-SMA)

1
Sacramento Soil Moisture Accounting Model
(SAC-SMA)

• Tanya Hoogerwerf

2
Overview

• Spatially-lumped continuous soil moisture
  accounting model
• Ideal model for the simulation of large-scale
  (gt1000 km2) basins
• Takes mean precipitation, evaporation and
  temperature as input

3
Input

• Calibrate by adjusting baseflow, tension water
  capacities and runoff simulation parameters
• Point or areal estimates of historical
  precipitation, temperature, and potential
  evaporation

4
Input (2)

• Topography
• Soil characteristics
• Location of important features such as reservoirs
  and river junctions

5
How the SAC-SMA Model Works

6
How the SAC-SMA Model Works (2)

• Each basin is represented vertically by two
  zones
• An upper zone (short- term storage capacity)
• A lower zone (bulk of the soil moisture and
  longer groundwater storage)

7
Soil Zones
http//meteora.ucsd.edu/knowles/html/land/mod_des
cr.html
8
How the SAC-SMA Model Works (3)

• Each layer models
• Tension water elements (water bound by adhesion
  and cohesion, extracted only by
  evapotranspiration)
• Free water elements (free to move under
  gravitational forces, may be depleted by
  evapotranspiration, percolation, horizontal flow)

9
Soil Moisture Budget

• W(t) the soil water content at time t 
• P(t) the mean precipitation over area A
• E(t) the mean evapotranspiration over area A 
• R(t) the net streamflow divergence from area A
• G(t) the net groundwater loss (through deep
  percolation) from area A

http//www.cpc.ncep.noaa.gov/soilmst/paper.html
10
Soil Moisture Budget (2)

• The streamflow divergence R(t) consists of a
• surface runoff component S(t) and a subsurface
  (base flow) runoff component B(t)
                  R(t) S(t) B(t).     
• Wmax is a measure of the capacity of soils to
  hold water in millimeters

http//www.cpc.ncep.noaa.gov/soilmst/paper.html
11
Soil Moisture Budget (3)

• G(t) is groundwater flow
• E(t) is estimated in this model as follows
• (Ep potential evapotranspiration rate in mm per
  month)

12
Soil Moisture Budget (4)

• Ep (potential evaporation)
• Depends mainly on the net radiative heating on
  the surface
• Can be estimated from pan evaporation
• Thornthwaite's method (1948)based on observed
  air temperature and duration of sunlight

13
SAC-SMA Model Parameters

• http//www.crh.noaa.gov/ncrfc/doc/calibration/flow
  ing.html