Comparison of Prognostic and Diagnostic Surface Flux Modeling Approaches over the Nile River Basin.

Regional evapotranspiration (ET) can be estimated using diagnostic remote sensing models, generally based on principles of energy balance, or with spatially distributed prognostic models that simultaneously balance both energy and water budgets over landscapes using predictive equations for land surface temperature and moisture states. Each modeling approach has complementary advantages and disadvantages, and in combination they can be used to obtain more accurate ET estimates over a variety of land and climate conditions, particularly for areas with limited ground truth data. In this study, energy and water flux estimates from diagnostic Atmosphere-Land Exchange (ALEXI) and prognostic Noah land surface models are compared over the Nile River basin between 2007 and 2011. A second remote sensing dataset, generated with Penman-Monteith approach as implemented in the Moderate Resolution Imaging Spectroradiometer (MODIS) MOD16 ET product, is also included as a comparative technique. In general, spatial and temporal distributions of flux estimates from ALEXI and Noah are similar in regions where the climate is temperate and local rainfall is the primary source of water available for ET. However, the diagnostic ALEXI model is better able to retrieve ET signals not directly coupled with the local precipitation rates, for example over irrigated agricultural areas or regions influenced by shallow water tables. These hydrologic features are not well-represented by either Noah or MOD16. Evaluation of consistency between diagnostic and prognostic model estimates can provide useful information about relative product skill, particularly over regions where ground data are limited or non-existent as in the Nile basin.

Figure 11. Daily average Latent Heat (W/m 2 ) from ALEXI (blue), Noah (orange), and MODIS (red), ALEXI net radiation (green) (W/m 2 ), surface temperature (K) change (DTRAD (brown)) ; to enhance patterns 5 times the actual values are plotted), and LAI (black) computed between 2007 and 2011. 1 May (DOY 121) and 10 June (DOY 161) are shown by two black vertical lines over Nile Delta plot.