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 (>1000 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_descr.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/flowing.html