Journal cover Journal topic
Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
Hydrol. Earth Syst. Sci., 21, 1-22, 2017
https://doi.org/10.5194/hess-21-1-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Review article
02 Jan 2017
Rain or snow: hydrologic processes, observations, prediction, and research needs
Adrian A. Harpold1, Michael L. Kaplan2, P. Zion Klos3, Timothy Link3, James P. McNamara4, Seshadri Rajagopal2, Rina Schumer2, and Caitriana M. Steele5 1Department of Natural Resources and Environmental Science, University of Nevada, 1664 N. Virginia Street, Reno, Nevada, USA
2Division of Hydrologic Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada, USA
3Department of Forest, Rangeland, and Fire Sciences, University of Idaho, 875 Perimeter Drive, Moscow, Idaho, USA
4Department of Geosciences, Boise State University, 1910 University Dr., Boise, Idaho, USA
5Jornada Experimental Range, New Mexico State University, Las Cruces, New Mexico, USA
Abstract. The phase of precipitation when it reaches the ground is a first-order driver of hydrologic processes in a watershed. The presence of snow, rain, or mixed-phase precipitation affects the initial and boundary conditions that drive hydrological models. Despite their foundational importance to terrestrial hydrology, typical phase partitioning methods (PPMs) specify the phase based on near-surface air temperature only. Our review conveys the diversity of tools available for PPMs in hydrological modeling and the advancements needed to improve predictions in complex terrain with large spatiotemporal variations in precipitation phase. Initially, we review the processes and physics that control precipitation phase as relevant to hydrologists, focusing on the importance of processes occurring aloft. There is a wide range of options for field observations of precipitation phase, but there is a lack of a robust observation networks in complex terrain. New remote sensing observations have the potential to increase PPM fidelity, but generally require assumptions typical of other PPMs and field validation before they are operational. We review common PPMs and find that accuracy is generally increased at finer measurement intervals and by including humidity information. One important tool for PPM development is atmospheric modeling, which includes microphysical schemes that have not been effectively linked to hydrological models or validated against near-surface precipitation-phase observations. The review concludes by describing key research gaps and recommendations to improve PPMs, including better incorporation of atmospheric information, improved validation datasets, and regional-scale gridded data products. Two key points emerge from this synthesis for the hydrologic community: (1) current PPMs are too simple to capture important processes and are not well validated for most locations, (2) lack of sophisticated PPMs increases the uncertainty in estimation of hydrological sensitivity to changes in precipitation phase at local to regional scales. The advancement of PPMs is a critical research frontier in hydrology that requires scientific cooperation between hydrological and atmospheric modelers and field scientists.

Citation: Harpold, A. A., Kaplan, M. L., Klos, P. Z., Link, T., McNamara, J. P., Rajagopal, S., Schumer, R., and Steele, C. M.: Rain or snow: hydrologic processes, observations, prediction, and research needs, Hydrol. Earth Syst. Sci., 21, 1-22, https://doi.org/10.5194/hess-21-1-2017, 2017.
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Short summary
The phase of precipitation as rain or snow is fundamental to hydrological processes and water resources. Despite its importance, the methods used to predict precipitation phase are inconsistent and often overly simplified. We review these methods and underlying mechanisms that control phase. We present a vision to meet important research gaps needed to improve prediction, including new field-based and remote measurements, validating new and existing methods, and expanding regional prediction.
The phase of precipitation as rain or snow is fundamental to hydrological processes and water...
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