Use of Remotely Sensed Products to Derive Robust Global Maps of Soil Moisture/Evapotranspiration Coupling

The link between soil moisture and land surface evapotranspiration (ET) is a key component of multiple land-atmosphere feedback cycles. However, numerous factors other than soil moisture also impact ET and the strength of the relationship between soil moisture and ET varies significantly as a function of climate and land cover conditions. Therefore, an improved observational constraint on the strength of the (one-way) coupling between soil moisture and ET would be of great benefit for the enhanced representation of land-atmosphere feedbacks in coupled models. Recent advances in the remote sensing of both soil moisture and ET provide a potential path forward in this regard. However, the (remaining) presence of significant random errors in remotely sensed soil moisture and ET products degrades their ability to provide a useful coupling strength benchmark. Specifically, independent random errors impacting remotely sensed ET and soil moisture retrievals lead to sampled soil moisture/ET (one-way) coupling strength estimates that are biased low relative to reality.

This presentation will describe the development and application of a novel triple collocation (TC) technique to compensate remotely sensed coupling estimates for this low bias. The approach is based on the collection of three independent estimates of soil moisture (obtained from: radar remote sensing, passive microwave remote sensing and land surface modeling) and ET (obtained from: thermal infrared remote sensing, passive microwave remote sensing and land surface modeling) products. The TC approach provides the first reliable global map of summertime soil moisture/ET coupling strengths suitable for comparison against internal soil moisture/ET coupling strength estimates provided by land surface models. By rotating various soil moisture and ET products into (and out of) the TC analysis, we will demonstrate that TC-based coupling strength estimates are robust. Presented results will confirm that off-line land surface models generally over predict the strength of (one-way) soil moisture/ET coupling along transitional regimes between wet and dry climates (e.g., the North American Central Great Plains region).