1. 4.6 INTRODUCING ‘SWAM’ (SOIL WATER ACCOUNTING MODEL)

2. Soil Water Accounting Model (SWAM) Why SWAM?  Simulates soil moisture storage under different scenarios (number/ size of water turns) and for different soils  Tells you how much of the applied water will remain in the soil profile – by calculating the evaporation and the deep percolation losses  Easy to use (spreadsheet)  Similar results as more complex models such as SWAP

3. SWAM concepts Running water balance of the root zone. (Application of the water balance equation per time step). Uniform distribution of moisture content (θ) in the root zone with depth (z), or dθ/dz = 0 Percolation rate at lower boundary equals the hydraulic conductivity k in the root zone Darcy’s Law for vertical flow: Simplified Darcy’s law: Upward flux is negligible: groundwater table is deep

4. Computation procedure Water balance where Δt is the time step (one day), E is the evaporation flux, P is the precipitation flux and I is irrigation flux in cm d -1.

5. Computation procedure Saturation deficit Where S t is saturation deficit in cm at time t, Dr is the root Zone depth in cm, n is total soil porosity in %

6. Computation procedure Initial saturation deficit where S o is initial saturation deficit in cm, Dr is root zone depth in cm, n is total soil porosity in %, SMS 0 is initial soil moisture storage

7. Computation procedure Bare soil evaporation: Equation of Penman for open water evaporation E Pen E is bare soil evaporation, α is a reduction factor, which is a function of the matric pressure in the root zone h

8. Input Data Time invariable  Soil moisture characteristic θ(h)  Hydraulic conductivity relation k(θ)  Depth of the rootzone (maximum range?)  Length of the time step Time variable data  Initial saturation deficit?  Precipitation  Irrigation gift and interval  Potential evapotranspiration

9. Input data – example silt loam soil Matric pressure at the lower boundary of the root zone (hb) in cmpF = log |hb| Volumetric water content (θ) in cm 3 cm -3 Soil Moisture Storage (SMS) in the root zone in cm Saturation deficit (S) in the root zone in cm Hydraulic conductivity at the lower boundary of the root zone (kb) in cm d -1 Reference evaporation (ETo) in cm d -1 10.000.504100.80.0012.0000000.67 101.000.48897.63.2011.9000000.75 201.300.48697.23.609.3600000.83 311.490.48296.44.407.1300000.65 501.700.46893.67.204.4500000.00 1002.000.42384.616.201.2900000.00 2502.400.25551.049.800.0310000.00 5002.700.22444.856.000.0044000.00 10003.000.17535.065.800.0017000.00 25003.400.13226.474.400.0004600.00 50003.700.11222.478.400.0001800.00 100004.000.09619.281.600.0000670.00 160004.200.08717.483.400.0000340.00