American Water Resources Association 2016 ANNUAL WATER RESOURCES CONFERENCE November 14‐17, 2016 Orlando, FL Tuesday, Nov. 15 1:30 PM – 3:00 PM SESSION 34: Advances in Hydrology Data Collection, Management and Modelling Potential Evapotranspiration as a Source of Uncertainty and Bias in Hydrologic Impact Analyses ‐ P.C.D. Milly, U.S. Geological Survey, Princeton, NJ (co‐author: K. A. Dunne) The diversity of commonly used potential evapotranspiration (PET) models contributes uncertainty in the estimation of hydrologic response to anthropogenic climate change. The temperature sensitivity of six commonly used PET equations (Hamon, Oudin, Penman‐
Monteith, Priestley‐Taylor, Samani‐Hargreaves, and Thornthwaite) is readily shown to vary by almost an order of magnitude, with energy‐ unconstrained (i.e., temperature‐based) methods showing the largest sensitivity. The change in annual multimodel (Coupled Model Intercomparison Project, Phase 5) PET under�Representative Concentration Pathway 8.5 from 1981‐2000 to 2081‐2100 is typically 10‐20% (20‐40%) in the low (high) latitudes according to the physics‐based Penman‐Monteith (ASCE Standardized Reference Evapotranspiration) equation, but 20‐40% (20‐80%) according to the empirical, temperature‐based Hamon equation. Radiation‐based Priestley‐Taylor changes are smaller than both of these, while empirical, temperature‐based Thornthwaite changes are larger than both. These differences in PET change translate to large differences in change of water availability; when combined with a form of the Budyko water‐balance relation, the PET methods predict a wide range of runoff changes. Furthermore, all PET methods result in bias that indicates drier conditions globally than those computed by the climate models themselves, and all PET methods overestimate the changes in actual evapotranspiration in non‐water‐ stressed seasons/regions relative to the changes in the climate models. We conclude that use of PET methods that are inappropriate for climate‐change applications is a source not only of uncertainty, but also of more drying than suggested by climate models, in hydrologic impact analyses. Incorporating Climate Change Impacts to Inland Hydrology ‐ Climate Hydrology Assessment Tool ‐ Peter Seman, USACE, Hanover, NH (co‐authors: B. Baker, J. Gade) The US Army Corps of Engineers (USACE) is taking steps toward developing policy and guidance around projected changes to climate hydrology and how these changes might affect water resources project planning, design, construction, operation and maintenance. The objective is to enhance our climate preparedness and resilience by incorporating relevant information about climate change impacts in hydrologic analyses for new and existing USACE projects. USACE projects, programs, missions, and operations have generally proven to be robust to the range of natural climate variability over their operating life spans. Recent scientific evidence shows, however, that in some places and for some impacts relevant to USACE operations, climate change is shifting the climatological baseline about which that natural climate variability occurs, and may be changing the range of that variability as well. This is important to USACE because the assumptions of stationary climatic baselines and a fixed range of natural variability as captured in the historical hydrologic record may no longer be appropriate for long‐term projections of the climatologic parameters, which are important in hydrologic assessments for inland watersheds. To aid in qualitative assessments of these changes we developed the Climate Hydrology Assessment Tool, a web‐based decision support system that helps staff to easily access and analyze trends in both historical and projected climate data. The system allows USACE districts across the country to develop repeatable analytical results using consistent information. In doing so, we reduce potential error and speed the development of information so that it can be used earlier in the decision‐making process. The system includes modules for consideration of both past (observed) changes as well as potential future (projected) changes in baseline or variability range over time. First‐order statistical analyses of climate data can be performed using standard methods to characterize climate data and identify statistically significant trends. It can be used to identify historical trends in instantaneous peak flows at streamflow gages as a proxy for understanding how regional flows have changed over the period of record. It can identify projected changes in annual maximum monthly flows for Hydrologic Unit Code (HUC‐4) watersheds. The tool steps users through the analysis process and supplies graphics suitable for use in a report. The intent is not to give quantitative design inputs, rather to help users gain a sense of the potential direction and general magnitude of changes. The hope is that these insights will help inform the broader decision making process for projects. More than Just a Rain Shadow Effect ‐ Stabilization Mechanism in Basin Water Circulation ‐ Hong‐Quan Zhang, The University of Tulsa, Tulsa, OK From the Tarim Basin in northwest China to the Great Basin in the west US, the arid climate in basins is commonly believed to be caused by the rain shadow effect of the surrounding mountain ranges blocking the rain‐producing weather systems. However, the historical records from different basins have shown steadiness of the water levels over extended times. Imagine on a raining day, if a bucket is placed outside, water accumulates and may even overflow. During sunny days, water level in that bucket will lower due to evaporation and even dry up completely. Would it be strange if the water level in the bucket does not change over many sunny and raining days? Yet this is exactly what happens in the basins around the world, including the two basins mentioned above. Then, what is the stabilization mechanism? The rain shadow effect cannot explain the phenomenon. This paper presents an explanation based on the water vapor mass transfer between basin and its outside atmosphere. For an endorheic (closed) basin, water precipitation rate must equal to water evapotranspiration rate in order to maintain a stable water level. There must be a System Equilibrium Evapotranspiration Rate (SEER), which supports a humidity distribution on top of the basin at equilibrium with the surrounding atmosphere. External weather systems may temporarily bring large amount of water into the basin. When the surface water in the basin is more than the equilibrium level, the System Evapotranspiration Rate (SER) will be higher than SEER. The air humidity above the basin will be higher than that of the surrounding atmosphere. Water vapor will escape from the basin to outside. As a result, the system water amount will be reduced to the equilibrium level. If the evaporable water amount in the basin is lower than its equilibrium level because of drought, the air humidity above the basin will be lower than that of the surrounding atmosphere. Water vapor will be transferred from outside into the basin. As a result, the system water amount will also increase back to the equilibrium level. This water circulation stabilization mechanism must exist in all basins around the world. Its strength depends on the geometry and scale of the basin. The higher the mountain ranges around the basin and the more regular the basin shape, the stronger the stabilization mechanism will be. The inside water circulation can be temporarily affected by the external weather events. However, like an invisible hand, this stabilization mechanism works continuously and draws the water circulation back to its equilibrium point. In this paper the stabilization mechanism in basin water circulation will be demonstrated through mass transfer between basin and outside atmosphere under different non‐equilibrium conditions. The relationship between SPR and SER is used to show the stability under positive or negative perturbation assumption.