The development and evaluation of a soil moisture index

Soil water is an important parameter in the hydrological balance and is essential for understanding land-surface interactions. While spatial coverage of soil water observations is poor across most of the United States, impedance probes have been used to measure soil water throughout Nebraska in the Automated Weather Data Network (AWDN) since 1999. With 8 years of continuous data at selected sites, we were able to derive a Soil Moisture Index (SMI) that is based on an actual water content (theta) and a knon field capacity and wilting point. The SMI is a continuous function and is scaled from 5.0 to -5.0, with 5.0 representing theta at field capacity and -5.0 representing theta at wilting point. Since the inception of the soil water network in 1999, periodic episodes of drought have affected all areas of Nebraska. Thus, the SMI was used in this study to analyse growing seasons (1 April to 1 October) from 1999 to 2006 at six selected sites throughout Nebraska. The SMI proved to be effective at depicting the onset of drought and at identifying soil recharge, or lack thereof, when heavy rainfall events followed a significant dry spell. Copyright (C) 2008 Royal Meteorological Society