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Abstract Here we use Richards Equation models of variably saturated soil and bedrock groundwater flow to investigate first‐order patterns of the coupling between soil and bedrock flow systems. We utilize a Monte Carlo sensitivity analysis to identify important hillslope parameters controlling bedrock recharge and then model the transient response of bedrock and soil flow to seasonal precipitation. Our results suggest that hillslopes can be divided into three conceptual zones of groundwater interaction, (a) the zone of lateral unsaturated soil moisture accumulation (upper portion of hillslope), (b) the zone of soil saturation and bedrock recharge (middle of hillslope) and (c) the zone of saturated‐soil lateral flow and bedrock groundwater exfiltration (bottom of hillslope). Zones of groundwater interaction expand upslope during periods of precipitation and drain downslope during dry periods. The amount of water partitioned to the bedrock groundwater system a can be predicted by the ratio of bedrock to soil saturated hydraulic conductivity across a variety of hillslope configurations. Our modelled processes are qualitatively consistent with observations of shallow subsurface saturation and groundwater fluctuation on hillslopes studied in our two experimental watersheds and support a conceptual model of tightly coupled shallow and deep subsurface circulation where groundwater recharge and discharge continuously stores and releases water from longer residence time storage.
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IMPACT OF EXTREME PRECIPITATION EVENTS ON THE WATER TABLE AND GROUNDWATER RECHARGE
Extreme precipitation events (EPEs) play a crucial role in influencing soil water storage and groundwater recharge worldwide. With climate change, extreme precipitation events are expected to increase in intensity, creating an urgent need to examine their effects on water resources. On the surface, the wide-ranging impacts of EPEs are visible. These impacts can be described as destructive, causing mass flooding, property damage and putting lives at risk. Below the surface however, the impacts of EPEs on subsurface processes, are less clear and warrant urgent study. In this dissertation, I examine the impacts of extreme precipitation events on water table mechanics and groundwater recharge. I begin my study locally, by investigating the role of an extreme precipitation event that occurred along the Colorado Front Range in September 2013. The event quickly caused widespread flooding on the surface, but flood waters disappeared just as quickly. For many years, it remained unclear whether that EPE-water had infiltrated the soil and if so, for how long the EPE-water may have impacted local soil water storage and water tables. My first goal was thus to examine the subsurface hydrologic response to the 2013 Colorado EPE using an unsaturated-flow model and field data from a site that had experienced the event. I find that after the EPE, the water table at a field site remained elevated for at least 18 months after the event, while the soil water storage was higher than average for two water years after the event. Thus, infiltration from EPEs is present for much longer than flood waters, and may aid recharge. Having investigated the subsurface response to an EPE at a site, the next step was to expand the study to examine whether similar responses occurred across varying soil types and EPEs for other sites nationwide. I find that greater EPE amounts generally lead to higher water-table displacements, but that soil properties are also a strong control of displacement and determine the length of time needed for the water-table to recede after an EPE. Finally, I conduct a more comprehensive study where I investigate the response of twelve different soil types to EPEs of varying amounts and durations. I find that water-table response times are shorter with increasing EPE amount and that water-table response occurs much faster in coarser-grained soils (i.e., sand), while taking upwards of hundreds of days to respond in finer-grained soils. Water-table displacement is positively correlated with increasing EPE amount and poorly correlated with longer EPE duration. Soil properties appear to be the greater control in water-table recession time, despite EPE amounts. Finally, I calculated first-order recharge rates and found average recharge equaled 69% of the total, with the amount of total recharge primarily controlled by the amount of the EPE and the soil properties. In sum, EPEs can be beneficial for replenishing water storage below surface despite their destructive tendencies above surface.
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- Award ID(s):
- 1834290
- PAR ID:
- 10419641
- Date Published:
- Journal Name:
- Dissertation
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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