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Title:   Author  item
                                            Yilmaz, Mustafaitem
                                            Anderson, Marthaitem
                                            ZAITCHIK, B - Johns Hopkins Universityitem
                                            Crow, Wadeitem
                                            HAIN, C - National Oceanic & Atmospheric Administration (NOAA)item
                                            OZDOGAN, M - University Of Wisconsinitem
                                            CHUN, J - Johns Hopkins University
  Submitted to: American Geophysical Union
                                    Publication Type: Abstract Only
                                    Publication Acceptance Date: 10/18/2012
                                    Publication  Date: 12/3/2012
                                    Citation: Yilmaz, M.T., Anderson, M.C., Zaitchik, B.F., Crow, W.T., Hain, C., Ozdogan, M., Chun, J.A. 2012. Comparison of prognostic and diagnostic approached to modeling evapotranspiration in the Nile river basin [abstract]. 2012 Fall Meeting of the American Geophysical Union, December 3-7, 2012, San Francisco, CA.Interpretive Summary:
                                Technical Abstract: Actual evapotranspiration (ET) can be estimated using both prognostic and diagnostic modeling approaches, providing independent yet complementary information for hydrologic applications. Both approaches have advantages and disadvantages. When provided with temporally continuous atmospheric forcing data, prognostic models offer continuous sub-daily ET information together with the full set of water and energy balance fluxes and states (i.e. soil moisture, runoff, sensible and latent heat). On the other hand, the diagnostic modeling approach provides ET estimates over regions where reliable information about available soil water is not known (e.g., due to irrigation practices or shallow ground water levels not included in the prognostic model structure, unknown soil texture or plant rooting depth, etc). Prognostic model-based ET estimates are of great interest whenever consistent and complete water budget information is required or when there is a need to project ET for climate or land use change scenarios.  Diagnostic models establish a stronger link to remote sensing observations, can be applied in regions with limited or questionable atmospheric forcing data, and provide valuable observation-derived information about the current land-surface state.

Analysis of independently obtained ET estimates is particularly important in data poor regions. Such comparisons can help to reduce the uncertainty in the modeled ET estimates and to exclude outliers based on physical considerations. The Nile river basin is home to tens of millions of people whose daily life depends on water extracted from the river Nile. Still, the complete basin scale water balance of the Nile has been studied only a few times, and the temporal and the spatial distribution of hydrological fluxes (particularly ET) are still a subject of active research.  This is due in part to a scarcity of ground-based station data for validation.  In such regions, comparison between prognostic and diagnostic model output may be a valuable model evaluation tool.

Motivated by the complementary information that exists in prognostic and diagnostic energy balance modeling, as well as the need for evaluation of water consumption estimates over the Nile basin, the purpose of this study is to 1) better describe the conceptual differences between prognostic and diagnostic modeling, 2) present the potential for diagnostic models to capture important hydrologic features that are not explicitly represented in prognostic model, 3) explore the differences in these two approaches over the Nile Basin, where ground data are sparse and transnational data sharing is unreliable. More specifically, we will compare output from the Noah prognostic model and the Atmosphere-Land Exchange Inverse (ALEXI) diagnostic model generated over the ground truth data-poor Nile basin. Preliminary results indicate spatially and temporally consistent flux estimates for ALEXI and Noah over the  irrigated Delta region, while there are differences over river-fed wetlands.


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