Soil Moisture

Soil moisture values for the 1981-1988 period simulated by RegCM2 were compared with observations of soil moisture for the state of Illinois as reported by Hollinger and Isard (1994) and Robock et al. (2000). Composite monthly mean values for the 8-year period (Figure 15.5) for the top 10-cm layer show a winter-spring recharge of soil moisture and a growing-season drawdown. Simulation of the deep soil (figure not shown) reveals a persistent dry bias in the model, whereas the simulation of the top layer follows the observations much more closely, except for an early fall recharge rate that is too low. Interannual variability of moisture in the top layer (Figure 15.6) shows minima in three summers (1983, 1984, and 1988). Simulated moisture tends to capture dry and wet years but with lower interannual variability. There is no apparent tendency for long-term drift in simulated soil moisture. See Pan (2001a) for further details.

Figure 15.5 Monthly composites of observed and simulated volumetric soil moisture in top 10 cm for 1981-1988 period averaged over 19 observer sites in Illinois.

Figure 15.5 Monthly composites of observed and simulated volumetric soil moisture in top 10 cm for 1981-1988 period averaged over 19 observer sites in Illinois.

6 7 Month

6 7 Month

10 11

Month

Figure 15.6 Time series of observed and simulated monthly volumetric soil moisture in top 10 cm averaged over 19 sites in Illinois.

Month

Figure 15.6 Time series of observed and simulated monthly volumetric soil moisture in top 10 cm averaged over 19 sites in Illinois.

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