Soil moisture is a measure of how much water is available to the gridcell's vegetation. Although too little moisture will obviously hamper the vegetation's growth, too much water will in fact have the same effect. A balance is therefore needed to insure proper growth. Usually this balance is defined as field capacity, which is the amount of water in the soil after two days free drainage.
Willmott, Cort J., and Matsuura Kenji (2001): Terrestrial Water Budget Data Archive: Monthly Time Series (1950-1999). Available: this address.
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