Abstract
Snow is an important water resource, and climate warming has caused significant changes in snow. Over half of the global potable water supply is extracted from reservoirs, groundwater, or directly from rivers. River flows are sensitive to changes in precipitation partitioning due to climate warming, especially in regions that rely on snow and glacial meltwater. Many research investigations have focused on the trend of streamflow when there is less snow in the future (Horton et al. 2006; Moore et al. 2007; Stewart et al. 2008; Berghuijs et al. 2014a; Godsey et al. 2014a; Foster et al. 2015; Zhang et al. 2015; Barnhart et al. 2016; Li et al. 2017; Musselman et al. 2017; Coppola et al. 2018; McCabe et al. 2018; Hammond et al. 2019; Barnhart et al. 2020; Milly and Dunne 2020; Liu et al. 2022b), but there is a lack of a systematic explanation, such as the runoff mechanisms explored in this research, to clarify the observed hydrological changes in snow-dominated regions. With this background,this research has drawn attention to the changes in the long-term average streamflow in snow-dominated places and aims to find possible explanations for the observed changes.
The first part of the study focuses on the seasonal streamflow responds to annual decreases in snowpack due to climate warming. We found that regions with higher annual snow fraction typically experience increased spring, summer, and potentially winter runoff which is the first key finding. Then we use a literature review, we summarized four runoff generation mechanisms to explain seasonal runoff changes resulting from decreased snow fraction. The runoff generation mechanisms are the water input exceeds threshold, demand-storage competition, water-energy synchrony, and energy partitioning. Water input exceeds threshold is the runoff only generated when the liquid snowmelt water input rates are higher than the surface infiltration rate. As climate warming progresses, the snowmelt rate will slow down, resulting in lower chances of exceeding the runoff
generation thresholds under this mechanism. Demand-storage competition mechanism is about the
competition between the atmospheric water deficit (evaporation) and catchment storage capacity. In a warmer world, the earlier and longer growing season, increases loss of soil moisture storage, and increase of evapotranspiration might mean that catchment storage capacity is not filled as early or as often, and so less recharge/runoff is generated. Under the water-energy synchrony mechanism that the runoff production is controlled by the degree of synchrony in water and energy availability. Climate warming has caused a decrease in snow, increasing the asynchrony between the timing of peak liquid water input and potential evapotranspiration, mainly due to the timing of the liquid water inputs. Energy partitioning involves the distribution of energy between snowmelt and evapotranspiration. With sufficient solar radiation due to climate warming, runoff could be reduced by either higher vegetation water consumption or increased evaporation.
The second part focusses on the practical application of the runoff generation mechanisms. Catchment scale observational data was applied to investigate the runoff generation mechanisms in three snow-dominated catchments in the Contiguous US. We found that the runoff generation in Reynolds Mountain East watershed and Sagehen Creek watershed is dominated by demand-storage competition mechanism and the energy partitioning mechanism. The soil frost in watershed 1 in Hubbard Brook watershed does not affect the runoff generation. This section could provide examples to researchers on how to utilize the methods outlined in the pervious chapter to identify the runoff mechanisms summarized in this thesis.
The last part we focus on assessing the models’ simulation skills for the runoff generation mechanisms. We found the CAMELS benchmark models applied in this research have different model components to reproduce different runoff mechanisms. SAC-SMA, VIC and mHM models have the model components to reproduce the water input exceeds threshold mechanism. SAC-SMA model, VIC and three FUSE model frameworks have the model components to reproduce the demand-storage competition mechanism. VIC model which has the energy balance equation within the model structure to reproduce the water-energy synchrony mechanism and energy- partitioning mechanism.
| Date of Award | 26 Jul 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Ross A Woods (Supervisor) |