Location: Hydrology and Remote Sensing LaboratoryTitle: On the use of a water balance to evaluate inter-annual terrestrial ET variability
Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2015
Publication Date: 5/27/2015
Citation: Han, E., Crow, W.T., Hain, C., Anderson, M.C. 2015. On the use of a water balance to evaluate inter-annual terrestrial ET variability. Journal of Hydrometeorology. 16:1102-1108. doi: 10.1175/JHM-D-14-0175.1.
Interpretive Summary: A critical part of forecasting water resource availability for agricultural applications is calculating the surface water balance. That is, determining what fractions of rainfall is: 1) diverted into runoff, 2) contributes to increased soil moisture or 3) is evaporated back into the atmosphere. This paper develops a new remote-sensing-based approach for evaluating the quality of water balance calculations and applies it within a series of hydrologic basins within the central United States. Results in the paper demonstrate 1) the importance of inter-annual variations in surface water storage on the water balance for agricultural areas of the central United States (especially for relatively humid watersheds in the South Central and Southeast United States) and 2) the ability of existing land surface models to accurately capture these inter-annual water storage variations. Based on these conclusions, the paper provides an enhanced view via of inter-annual variations in water cycle components within agricultural areas of the Central United States and will ultimately lead to enhanced water resource monitoring in this region.
Technical Abstract: Accurately measuring inter-annual variability in terrestrial evapotranspiration (ET) is a major challenge for efforts to detect inter-annual variability in the hydrologic cycle. Based on comparisons with annual ET values derived from a terrestrial water balance analysis, past research has cast doubt on the ability of existing products to accurately capture inter-annual ET variability. Using a variety of terrestrial ET estimates, this analysis re-examines this conclusion and finds that estimates of inter-annual ET variations obtained from a land surface model are more strongly correlated with annual ET independently acquired from thermal-infrared remote sensing (or the spatial interpolation of flux-tower observations) than annual ET amount derived from water balance considerations. This tendency is attributed to significant inter-annual variations in terrestrial water storage neglected by the water balance approach. Overall, results demonstrate the need to re-assess perceptions concerning the skill of annual ET estimates derived from land surface models and demonstrate the value of accurate remotely-sensed ET products for the validation of inter-annual ET variations.