In this Commentary, we argue that it is possible to improve the physical realism of hydrologic models by making better use of existing hydrologic theory. We address the following questions: (1) what are some key elements of current hydrologic theory; (2) how can those elements best be incorporated where they may be missing in current models; and (3) how can we evaluate competing hydrologic theories across scales and locations? We propose that hydrologic science would benefit from a model-based community synthesis effort to reframe, integrate, and evaluate different explanations of hydrologic behavior, and provide a controlled avenue to find where understanding falls short.
MotivationThe discipline of hydrology continues to be an exciting field, with ongoing advances in field observational techniques, availability of global data products, and increasing computational power. Now, perhaps more than ever before, we are rising to the challenge of building models of everywhere [Beven, 2007]. Key efforts include building continental-domain hydrologic models for water security assessments [Schewe et al., 2014;Mizukami et al., 2015] and improving the representation of hydrologic processes in Earth System Models [Clark et al., 2015a]. These efforts require moving beyond the traditional tactics used in hydrology, such as detailed analysis and modeling of individual catchments, which makes it difficult to generalize results to large domains and other hydrologic regimes. Instead, hydrologic synthesis across space and across many elements of hydrologic theory is needed, in order to improve the physical realism and general applicability of hydrologic models, i.e., to improve hydrologic process representations across a large range of catchments . To this end, some have argued (somewhat optimistically) that advances in modern hydrologic modeling efforts are possible through progress on the following fundamental research challenges: identifying consistently observed behaviors across research watersheds, formulating the laws that govern macroscale hydrologic behavior, and unifying process explanations across watersheds in order to develop theory of hydrology at the catchment scale [e.g., Dooge, 1986;Sivapalan, 2005;McDonnell et al., 2007].The needs of the hydrologic modeling community as articulated in this way are admittedly sizeable and potentially insurmountable. This has led others to adopt a rather pessimistic view, doubting if it is even possible to generalize hydrologic behaviors given the unique character of individual basins [Beven, 2000]. This raises the question, do we now, and/or will we always, lack the necessary information on climate, topography, vegetation, soils, and subsurface structure required to develop powerful and exceptionless explanations? Put differently, are the problems of underdetermination so pronounced that we cannot move Key Points: We seek to increase the physical realism of hydrologic models through better way existing theory We seek to improve the way models are used to integrate and eva...