The Spatial and Temporal Variability of the Evaporative Fraction and Soil Moisture During a Period of Historic Precipitation

During the summer of 2007, the state of Oklahoma experienced its second wettest summer (June-August) on record. This period was marked by heavy, persistent rains and flooding that devastated parts of the state. In June alone, portions of Oklahoma received over 400 mm of precipitation, shattering climatological records for that month. Typically during the summer, Oklahoma receives little precipitation and soil moisture decreases. However, due to the large quantity of precipitation during the summer of 2007, soil moisture conditions were extremely moist which further impacted the partitioning the available energy at the surface to fluxes of latent and sensible heat.

During the late spring of 2007, ten eddy covariance flux towers were deployed across portions of central Oklahoma at four study sites within the Cloud and Land Surface Interaction Campaign (CLASIC). These eddy covariance flux towers were placed in areas with varying surface conditions, ranging from forest to pasture to agricultural fields. Prior to, during, and following the CLASIC period, observations of the surface energy balance were continuously collected including the variables of sensible, latent, and ground heat flux.

Using the data collected during CLASIC as well as Oklahoma Mesonet and Atmospheric Radiation Measurement (ARM) observations, the spatial and temporal variability of the surface energy budget and soil moisture were analyzed for the period spanning June to August 2007. In particular, the focus of the study was placed on the variability of (a) the evaporative fraction and (b) soil moisture. Time series plots revealed little variability in evaporative fraction throughout the period for sites located over pasture, native prairie, and forest. However, significant variability in evaporative fraction was found for sites located on agricultural fields due to rapid changes in vegetation conditions. To quantify the significance of the 2007 period, evaporative fraction and soil moisture data from the CLASIC period were compared with available data from the summers of 2002 to 2006 and 2008. This comparative analysis revealed significant differences in spatial and temporal variability of the evaporative fraction when compared to 2007.