The Impact of Vegetation Conditions on Land-Atmosphere Interactions During an Anomalously Wet Period

During the summer of 2007, Oklahoma experienced its second wettest summer (June-August) on record whereby heavy, persistent rains marked the period and flooding devastated parts of the state. In June alone, some areas of Oklahoma received over 400 mm of precipitation, shattering climatological records for the month. Due to the large quantity of precipitation during the summer of 2007, soil moisture conditions were extremely moist which impacted the land surface state across the region.

During the late spring 2007, eddy covariance flux towers were deployed in portions of central Oklahoma as part of 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 and continuously collected observations of the surface energy balance prior to, during, and following the CLASIC period. Using 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, soil moisture, and evapotranspiration (ET) were analyzed for the period spanning May to September 2007. At locations consisting of rangeland surface conditions, ET changed little through the duration of the warm season and maintained a consistent evaporative fraction of approximately 0.8. This result demonstrates an extreme benchmark for ET in the region given the extreme nature of the precipitation anomaly (i.e., the wettest summer on record for central Oklahoma). At the cropland sites, rapid native vegetation growth occurred at some locations due to the plentiful soil moisture following the harvest of winter wheat and local ET increased dramatically to values similar to the rangeland sites. To examine areal extent of the vegetation change over the agricultural areas and its impact on ET, surface reflectance data from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra and Aqua satellites was utilized as part of the study. When compared with other like warm-season periods from 2000-2010, the MODIS data revealed significant increases in vegetation compared to other years including over the agricultural areas. The impacts of the vegetation changes coupled with the moist soil conditions yielded an anomalous impact to the overall surface energy budget, ET, and associated land-atmosphere interactions.