Soil moisture is one of the important physical state variables of the hydroclimatic system because it affects the partitioning of energy and water balances at the Earth's surface. It influences evapotranspiration, albedo, and thermal fluxes, and recent studies have shown a positive correlation between soil moisture levels and the probability of precipitation occurrence (Salvucci et al., 2002). The Global Soil Moisture Data Bank contains the largest collection of in situ observational data (Robock et al., 2000), and satellite retrievals of surface soil moisture by both active and passive sensors are increasingly available. However, Reichle et al. (2004) found a large number of land surface models produce widely different global-scale soil moisture output using identical meteorological forcing inputs, and they conclude that errors in present global-scale soil moisture observations and modeling datasets are so large that a universally agreed climatology cannot be identified with confidence.
An estimate of mean annual global soil moisture variation derived from the NCEP-NCAR Reanalysis data (Kalnay et al., 1996) is shown in Figure 7.20. At the global scale, soil moisture variability is related to fields of precipitation and evapotranspiration. Soil moisture is highest in the equatorial latitudes where precipitation is abundant. It decreases poleward toward the Tropics as precipitation decreases and evapotranspiration increases. This pattern is most evident in Africa. In North America, Europe, and Asia, the soil moisture pattern displays greater variability and is more complex as the precipitation and evapotranspiration fields are complicated due to topographic factors and seasonally varying atmospheric influences. Nevertheless, the general pattern is that the latitudes between 0° and 20° S have the greatest soil moisture values, and the region between 40° N and 60° N has the greatest quantity of moisture stored in the soil due to the vast expanse of land area at these latitudes.
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