Soil Moisture: Empirical Data and Model Results

A unique dataset of soil moisture in the upper 1-m soil layer at sites with natural plant cover in the Soviet Union is compared to simulations of soil moisture for the present climate by the Geophysical Fluid Dynamics Laboratory, Oregon State University, and United Kingdom Meteorological Office general circulation models. It is found that the present-day soil moisture regime is not well simulated by these models. Delworth and Manabe's hypothesis that the spectrum of time variations in soil moisture in the upper 1-m layer corresponds to a first-order Markov process with a decay time of the correlation function equal to the ratio of field capacity to potential evapotranspiration is empirically confirmed with this dataset. Analysis of measurement data over the 1972–1985 period reveals that the long-term trends of soil moisture north of 50°N are mainly due to increasing precipitation during this period of the same scale (1–3 cm/10 yr). The seasonal structure does not correspond to the “summer continental desiccation” scenario predicted by some climate models.