The Institute of Hydrology of the Slovak Academy of Sciences, Department of Soil Hydrology Impact of the initial condition on the simulation of water movement in variably saturated zone of soil (Is it necessary to take the temporal variability of hydro-physical properties, which describe the water flow in variably saturated soil, during modeled period into account?) May 1, 2007 Vladimir Mikulec

Solution of Richards equation using method of finite elements in model GLOBAL Upper boundary condition – (the climate and phenological parameters) Initial condition – (the moisture profile at the beginning and the hydro-physical parameters (C, WRC)) - Parameters of porous medium Output – (calculated moisture profiles) Bottom boundary condition – (the pressure head at the bottom of the soil profile or the depth of ground water level under the soil surface)

Examined locality

Initial and boundary conditions of simulation The upper boundary condition was constituted by the phenological parameters of corn and by the climatic characteristics measured at the meteorological gauge station Bratislava-airport The bottom boundary condition was defined by the time series of the pressure head measured at the examined locality in the depth of 115 cm The initial condition is represented by the moisture profile at the beginning of the simulation and also by the definition of modeled environment – variably saturated zone of soil. The modeled environment is defined by measurement of main hydrophysical properties of soil – saturated hydraulic conductivity (C) and water retention curve (WRC). The measurements conducted in soil layer 0 – 115 cm at the examined locality have shown that the soil profile consists of the set of roughly homogeneous soil layers. From the point of view of simulation, was therefore possible to define the simulated soil profile as homogeneous This study is focused on the impact of the temporal variability of values of C on the results of simulation of water movement in the variably saturated zone of soil.

Variants of simulation according to application of measured hydro-physical parameters In the first variant was the value of C applied as a constant counted from the measurements realized in the first day of simulation In the second variant was the value of C applied as a mean counted from the measurements realized in the whole modeling period And finally in the third variant, the values of C changed in time as they were counted for all days of sampling during whole modeling period z [cm] C17.3. C14.5. C30.5. C16.6. C25.6. C14.7. C4.8. C16.9. Cmean [cm.d-1] 0-115 4.65 7.75 8.17 4.26 4.86 13.22 16.08 14.00 8.08 z [cm] θs [cm3cm-3] θr θfc (pf=2.3) α [cm-1] n [-] 0-115 0.458741 0.028053 0.331666 0.029493 1.18682

The time change in modeled moisture profiles (θ [cm3cm-3]) during the time period 17.3. – 20.10.2003 (day 76 – 293) in the first variant of simulation (C17.3.)

The time change in modeled moisture profiles (θ [cm3cm-3]) during the time period 17.3. – 20.10.2003 (day 76 – 293) in the second variant of simulation (Cmean)

The time change in modeled moisture profiles (θ [cm3cm-3]) during the time period 17.3. – 20.10.2003 (day 76 – 293) in the third variant of simulation (C = f(t))

The time change in measured moisture profile (θ [cm3cm-3]) during the time period 17.3. – 20.10.2003 (day 76 – 293)

Conclusion Modeled and measured moisture profiles at the examined locality in the time period 17.3. – 20.10.2003 were compared using method of correlation. r  measured Cconst. Cmean C = f(t) 1   0.511087 0.540221 0.993866 0.552285 0.992747 0.996101 The correlation coefficients (r) between all variants of simulation were higher than 0.99. At the examined locality it is therefore fully sufficient to simulate the water movement with constant C counted from measurements performed at the beginning of simulation as it is shown in variant with Cconst.. Compared to the measurements, the best information about volumetric water content in studied soil profile gave the third variant of simulation (r = 0.55) followed by the second (r = 0.54) and first (r = 0.51). But the differences between outputs from all modeled variants are marginal in comparison with amount of effort invested into laboratory measurements of saturated hydraulic conductivity

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