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Induced seismicity is a limiting factor in the widespread deployment of Enhanced Geothermal Systems (EGS). Induced seismicity occurs not only during injection, but also after the stop of injection, with the largest magnitude earthquakes usually occurring after the stop of injection at different scales of time (hours to months). The post-injection large magnitude seismicity is counterintuitive and is still not well understood. Multiple mechanisms have been identified as triggering induced seismicity in EGS systems. Pore pressure increase due to fluid injection is the most commonly accepted triggering mechanism to explain induced seismicity. Yet, coupled poromechanical effects and static stress transfer are also important mechanisms in the process of fault reactivation. We study the combination of these different mechanisms by simulating coupled hydromechanical processes of the Deep Heat Mining Project at Basel, Switzerland (2006), a well-known case of short-term post-injection large-magnitude seismicity. We apply the material characteristics and stress conditions of the site to a simplified 2-dimensional fault network that is based on the monitored seismic events. In our model, pore pressure has a dominant effect on the triggering of the seismicity in the vicinity of the injection well, but its effect decreases away from the well as static stress transfer and poroelastic stressing gain importance. Poroelastic stress can have a stabilizing effect when its direction is opposite to the fault slipping orientation; but an abrupt shut-in, and consequently a quick poroelastic relaxation, reactivates the fault soon after the shut-in. Later post-injection induced seismicity is partially due to the post-injection diffusion of the pore pressure, but is mainly induced by constant static stress redistribution from reactivations of the different faults of the domain. Understanding the processes inducing seismicity, and especially the post-injection large-magnitude seismicity, should enable developing better strategies of shut-in to mitigate the risks of post-injection large-magnitude seismic events.

Although water table recessions associated with seasonal freezing have been widely reported, the conditions that groundwater could respond to seasonal freezing are poorly known. Based on 30 records of winter-time water table recessions reported in the literature, we find the mean water table recession increases from sand to loam to silt to clay. To obtain a quantitative understanding of the conditions of freezing-induced groundwater migration and the role of soil texture, we construct a one-dimensional SHAW model with a dynamic water table that could respond to winter-time atmospheric conditions. By simulating freezing-induced groundwater migration under a series of pre-freezing water table depths, soil textures and freezing indices, we find the extinction depth of freezing-induced groundwater migration, i.e., the limit water table depth beyond which there is negligible freezing-induced groundwater flow, increases from coarse to fine soils and increases with freezing index. Moreover, we find there is a good linear relationship between the distance from the freezing front to the water table corresponding to the extinction depth of freezing-induced groundwater migration and the distance from the land surface to the water table corresponding to the extinction depth of evaporation-induced groundwater migration for different soils. To demonstrate the importance of defining extinction depth, we also show the overestimation of freezing-induced water gain in the frozen zone by the widely used fixed water table condition when the water table depth is shallower than the extinction depth. This study has implications for future hydrologic and engineering studies in critical zones with a cold climate.

Groundwater is critical in sustaining streamflow, especially in headwater catchments, because of its ability to supply baseflow. In water-limited arid and semi-arid mountain environments, the need to characterize groundwater recharge and discharge has grown in tandem with demands to manage current and future water resources. However, studying groundwater in complex terrain is challenging due to limited field measurements. Nearly a decade of monitoring in Gordon Gulch in the Colorado Front Range provides an opportunity to study such an environment. The field data is used to parameterize and calibrate a groundwater flow model (MODFLOW-NWT). Model results reveal that groundwater is recharged primarily during one to two recharge periods each year, driven by spring snowmelt coupled with rain or by intense/prolonged summer rain. Gordon Gulch is a net gaining stream, with greater fluxes from groundwater to stream in lower Gordon Gulch and during springtime. Groundwater is discharged to the stream via long, deep flowpaths sourced from upper Gordon Gulch and from hillslopes, and via short, shallow flowpaths in lower Gordon Gulch. Modelled groundwater accounts for approximately 16 to 34% of baseflow in the stream. Using Gordon Gulch as a case study, this model and data analysis contribute to a larger effort to understand and constrain the mechanisms driving groundwater recharge and groundwater-stream exchanges in semi-arid, headwater catchments.

Induced seismicity is a limiting factor for the development of Enhanced Geothermal Systems (EGS) and has led to the cancellation of a few projects. Its causal mechanisms are not fully understood, especially those of post-injection seismicity. Here, we revisit the controversial case of the Basel EGS (Switzerland) to better understand the mechanisms that induced seismicity by simulating the hydro-mechanical response to hydraulic stimulation of a pre-existing fault network built on the basis of the monitored seismicity. Simulation results show that the faults located in the vicinity of the injection well fail during injection, triggered by pore pressure buildup coupled with poroelastic stressing, whereas distant faults are stabilized by poroelastic effects depending on the orientation. After injection stops, poroelastic stress relaxation leads to the immediate rupture of these previously stabilized faults. Shear-slip stress transfer, which also contributes to post-injection reactivation of distant faults, is enhanced in faults with slip-induced friction weakening. This work presents a modeling approach to understand the multiple processes leading to the rupture of pre-existent fractures in EGS reservoirs, which is key to improve our induced seismicity forecasting and mitigating capability.

A simple relation between pore pressure change and one-dimensional surface deformation is presented. The relation is for pore pressure change in a confined aquifer that causes surface deformation. It can be applied to groundwater models of any discretization and is computationally efficient. The estimated surface deformation from model results can be compared to observed surface deformation through geodetic techniques such as Differential Interferometric Synthetic Aperture Radar. Model parameters then are constrained using the observed surface deformation. The validity of this relation is shown through constraint of model parameters for surface uplift due to pore pressure increase caused by wastewater disposal injection.

Four large hydropower stations have recently been built downstream the Jinsha River in Southwestern China with a strong regional tectonic activity background. There is widely felt seismicity since ...

Arctic rivers export a large amount of organic carbon (OC) and mercury (Hg) to Arctic oceans. Because there are only a few direct calculations of OC and Hg exports from these large rivers, very little is known about their response to changes in the active layer in northern permafrost-dominated areas. In this study, multiyear data sets from the Arctic Great Rivers Observatory (ArcticGRO) are used to estimate the export of dissolved organic carbon (DOC), particulate organic carbon (POC), total mercury (THg) and methylmercury (MeHg) from the six largest rivers (Yenisey, Lena, Ob, Mackenzie, Yukon and Kolyma) draining to the Arctic Ocean. From 2003 to 2017, annual DOC and POC export to the Arctic Ocean was approximately 21612 Gg and 2728 Gg, and the exports of Hg and MeHg to the Arctic Ocean were approximately 20090 kg and 110 kg (0.002% of the total Hg stored in the northern hemisphere active layer). There were great variations in seasonal OC and Hg concentrations and chemical characteristics, with higher fluxes in spring and lower fluxes in winter (baseline). DOC and Hg concentrations are significantly positively correlated to discharge, as discharge continues to increase in response to a deepening active layer thickness during recent past decades. This study shows that previous results likely underestimated DOC exports from rivers in the circum-Arctic regions, and both OC and Hg exports will increase under predicted climate warming scenarios.

We report unique observations from drilling and hydraulic stimulation at a depth of approximately 4.3 km in two Enhanced Geothermal System (EGS) wells at the Pohang EGS site, South Korea. We surveyed drilling logs and hydraulic stimulation data, simulated pore pressure diffusion around the fault delineated by seismic and drilling log analyses, conducted acoustic image logging through the EGS wells, observed significant water level drops (740 m) in one of the two EGS wells, and obtained hydrochemical and isotopic variation data in conjunction with the microbial community characteristics of the two EGS wells. We discuss the hydraulic and hydrochemical responses of formation pore water to a few key seismic events near the hypocenter. We focused on how the geochemistry of water that flowed back from the geothermal wells changed in association with key seismic events. These were (1) a swarm of small earthquakes that occurred when a significant circulation mud loss occurred during well drilling, (2) the MW 3.2 earthquake during hydraulic stimulation, and (3) the MW 5.5 main shock two months after the end of hydraulic stimulation. This study highlights the value of real-time monitoring and water chemistry analysis, in addition to seismic monitoring during EGS operation.

Four large hydropower stations have recently been built downstream the Jinsha River in Southwestern China with a strong regional tectonic activity background. There is widely felt seismicity since the impoundment of the Xiluodu and Xiangjiaba reservoirs, increasing the public concern in this region. We begin with a criticality analysis of the faults near these reservoirs to quantify their susceptibility to triggered seismicity. Then we focus on the Xiluodu reservoir to investigate the correlation between the impoundment and seismicity nearby. We analyze the spatio-temporal distribution of seismicity near the Xiluodu reservoir, and identify the plausible rapid and delayed seismic response due to the impoundment. According to the impoundment record, we explicitly model the hydro-mechanical changes due to diffusion and reservoir water load, that is, pore pressure, elastic stress, and the resulting Coulomb stress. Our results show that the pore pressure changes can reach a level that may trigger fault reactivation and consequently, seismicity nearby. The water load can also induce the positive Coulomb stress changes on faults, depending on the fault orientation, which is especially important for understanding the earthquakes that occurred shortly after the impoundment and at more than 10 km distance from the reservoir. The combination of these two effects can induce positive Coulomb stress change over a larger area, which overlaps the majority of the events after the impoundment, including two M5+ events. While the causal relationship between the impoundment and seismicity warrants further analysis, we hope to inform the regional seismic impact of impoundment with this timely study., Journal of Geophysical Research: Solid Earth, 126 (9), ISSN:2169-9313, ISSN:0148-0227, ISSN:2169-9356

This commentary explores the challenges and opportunities associated with a possible transition of Water Resources Research to a publication model where all articles are freely available upon publication (“Gold” open access). It provides a review of the status of open access publishing models, a summary of community input, and a path forward for AGU leadership. The decision to convert to open access is framed by a mix of finances and values. On the one hand, the challenge is to define who pays, and how, and what we can do to improve the affordability of publishing. On the other hand, the challenge is to increase the extent to which science is open and accessible. The next steps for the community include an incisive analysis of the financial feasibility of different cost models, and weighing the financial burden for open access against the desire to further advance open science.

Causal mechanisms for fluid injection-induced earthquakes remain a challenge to identify. Past studies largely established spatiotemporal correlations. Here, we propose a multi-process causal mechanism for injection-induced earthquakes through a case study of the 2017 Mw 5.5 induced earthquake near Pohang Enhanced Geothermal System, Korea, where detailed hydraulic stimulation and on-site seismicity monitoring data provide an unprecedented opportunity. Pore pressure modeling reveals that pore pressure changes initiate seismicity on critically stressed faults and Coulomb static stress transfer modeling reveals that earthquake interactions promote continued seismicity, leading to larger events. On the basis of these results, we propose the following causal mechanism for induced seismicity: pore pressure increase and earthquake interactions lead to fault weakening and ultimately triggering larger earthquakes later in the process. We suggest that it is prudent that pore pressure change, initial seismicity locations, and Coulomb static stress transfer from seismicity earlier in the sequence are assessed in real-time., The authors here suggest a causal mechanism for injection-induced earthquakes. They further suggest pore pressure modeling as a practical alternative to direct in-situ pore pressure observation which can then be used for stress build-up monitoring.

Xiluodu is currently the third largest hydropower station in the world and situates on the upper Yangtze River in Southwestern China. The 285.5 m-high dam lies in the center of a relatively intact and stable tectonic block, triangulated by three large fault zones. The seismicity in the region increased markedly since the reservoir was first impounded in 2013. Previous studies suggest a strong spatial-temporal link between the seismicity and reservoir impoundment. This study attempts at conducting a quantitative analysis integrating the geological and engineering data to constrain the link between the impoundment and the seismicity, which could inform the future seismic evolution in the area.We first study the characters of the spatial activity of earthquakes in different periods to address the correlation between increased seismicity and reservoir impoundment. Since the impoundment, the earthquakes in this region can be plausibly separated spatially into two groups. The first group (including a ML5.4 and a ML5.5 event) is located within ~10km of reservoir, where a major fault zone is absent. Within this spatial range, earthquakes > ML 2.0 are rare three years prior to the impoundment, but more than 1000 events were detected between the initial impoundment in 2013 and September 2014 when the reservoir reached its peak level. Thereafter, the fluctuations of water level were accompanied by continuous seismicity, albeit at a considerably lower rate. The seismicity in this region is strengthened again in 2019. The other group of earthquakes are clustered with several mapped major fault traces. Some of these events quickly followed the water level fluctuation, while some were observed after significant delays. In general, the distances between locations of delayed events and the reservoir gradually increase with time. To address the influence of impoundment on seismicity, we analyzed the hydrologic and mechanical effects of the impoundment, i.e., the fluid pressure diffusion and the reservoir loading. We computed the spatiotemporal changes of Coulomb stress on known faults resulting from these two effects. The sensitivity analysis of hydraulic and mechanical parameters shows that the changes of Coulomb stress in the area could increase to a level that is relevant to reactivation of faults. While the relationship between the impoundment and increase seismicity warrants further analysis, we hope to inform the regional seismic impact by integrating in-situ stress state, fault geometries, and the coupled hydro-mechanical stress changes.

The Pohang quake shows the need for new methods to assess and manage evolving risk

The Raton Basin had the highest number of earthquakes in Colorado and New Mexico from 2008 to 2010. The rate of both wastewater injection and earthquakes in the basin increased dramatically starting in 1999 and 2001, respectively. We compare seismicity (ML 0.0–4.3) in the Raton Basin from 2008 to 2010 with the location of modeled pore pressure increases, estimated from cumulative wastewater injection volume beginning at the onset of well injection to present for all 28 injection wells in the basin. We find that modeled pore pressures in the New Mexico portion of the basin (above 0.08 MPa) reached that necessary to induce seismicity (0.01–0.1 MPa). We simulate a fault plane, 20 km long, inferred from seismicity in Vermejo Park (1355 of 1881 total earthquakes), in our model. We find that the relatively permeable fault allows pressures to migrate deeper into the basin at the onset of our study in 2008, providing an explanation for the observed seismicity in the basement. The Tercio lineament of earthquakes is similar to Vermejo Park fault in strike, but has fewer earthquakes (129) and is shorter in length (9 km). Seismicity in Vermejo Park occurs continuously, but earthquakes occur episodically in the remainder of the basin. The number of earthquakes we observe in seven seismic clusters increases as the cumulative injected volume from wells within 5 km increases. The modeled pore pressures, earthquake locations, and relationship between cumulative volume and number of earthquakes indicate that seismicity in the Raton Basin is likely induced.

Changes in streamflow have been observed at 23 stream gauges in central Taiwan after the 1999 M W 7.6 Chi–Chi earthquake. Post-earthquake increases, ranging from 58 to 833% in discharge, were recorded at 22 gauges on four rivers and their tributaries. The streamflow increase typically peaked in 2–3 days and followed by a slow decay for a month or more. An increased groundwater discharge to the river after the earthquake can be attributed to rock fracturing by seismic shaking as well as pore pressure rise due to compressive strain. A large decrease in discharge was recorded immediately after the earthquake at the gauge near the earthquake epicenter. Further analysis of long-term data indicates that the post-earthquake discharge at the gauge reduced to a level smaller than that at an upstream gauge for 8 months. Such a streamflow decrease might have been caused by a discharge to the streambed due to a co-seismic decrease in pore pressure induced by crustal extension during the rupture of the thrust fault.

Mitigation of injection-induced seismicity in Greeley, Colorado, is based largely on proximity of wastewater disposal wells to seismicity and consists of cementation of the bottom of wells to eliminate connection between the disposal interval and crystalline basement. Brief injection rate reductions followed felt events, but injection rates returned to high levels, >250,000 barrels/month, within six months. While brief rate reduction reduces seismicity in the short term, overall seismicity is not reduced. We examine contributions to pore pressure change by injection from twenty-two wells within 30 kilometers of the center of seismicity. The combined injection rate of seven disposal wells within 15 kilometers of the seismicity (Greeley Wells) is correlated with the seismicity rate. We find that injection from NGL-C4A, the well previously suspected as the likely cause of the induced seismicity, is responsible for ~28% of pore pressure increase. The other six Greeley Wells contribute ~28% of pore pressure increase, and the fifteen Far-field Wells between 15 and 30 kilometers from the seismicity contribute ~44% of pore pressure increase. Modeling results show that NGL-C4A plays the largest role in increased pore pressure, but shows the six other Greeley Wells have approximately the same influence as NGL-C4A. Furthermore, the fifteen Far-field Wells have significant influence on pore pressure near the seismicity. Since the main mitigation action of cementing the bottom of wells has not decreased seismicity, mitigation based on reduced injection rates and spacing wells farther apart would likely have a higher potential for success.

© 2019 Seismological Society of America. All rights reserved. On the afternoon of 15 November 2017, the coastal city of Pohang, Korea, was rocked by a magnitude 5.5 earthquake (Mw, U.S. Geological Survey). Questions soon arose about the possible involvement in the earthquake of the Republic of Korea's first enhanced geothermal system (EGS) project because the epicenter of the earthquake was located near the project's drill site. The Pohang EGS project was intended to create an artificial geothermal reservoir within low-permeability crystalline basement by hydraulically stimulating the rock to form a connected network of fractures between two wells, PX-1 and PX-2, at a depth of ∼4 km. Forensic examination of the tectonic stress conditions, local geology, well drilling data, the five high-pressure well stimulations undertaken to create the EGS reservoir, and the seismicity induced by injection produced definitive evidence that earthquakes induced by high-pressure injection into the PX-2 well activated a previously unmapped fault that triggered the Mw 5.5 earthquake. Important lessons of a general nature can be learned from the Pohang experience and can serve to increase the safety of future EGS projects in Korea and elsewhere.

Underground Thermal Energy Storage (UTES) has emerged in both specific applications and within energy policy literature as a promising technology for meeting thermal loads with locally collected and stored solar energy, as well as several other potential applications, such as time-shifting of grid-based wind and solar power to better align variable generation with loads. In Europe, UTES systems have experienced increased deployment in connection with district heating systems. But despite this academic attention and several demonstration projects, the commercial market viability of UTES systems has yet to be established in North America, and the finance world uses different conceptions of viability than engineering or academic studies. This study explores, through the conventions of finance and risk-mitigation, what capital costs North American UTES systems would need to exhibit to achieve market viability; which is to say, the investment cost at which a UTES system represents an attractive investment when compared with natural gas-based systems for the provision of residential space heating.

Although siting of thermal energy storage systems in the vadose zone may be beneficial due to the low thermal conductivity of unsaturated soils, water phase change and vapor diffusion in so...

We present an overview of the current status of unconventional energy development, particularly of shale gas, and underground CO2 storage as a measure to mitigate greenhouse gas increase in the atmosphere. We review their potential to induce seismicity, which has caused debates among related energy enterprises, engineers, researchers, and environmental and public communities regarding their potential hazards. Studies show that fracking can be a problem in that it consumes abundant water, but the seismicity induced by fracking has not yet been observed to induce many felt earthquakes. However, massive wastewater injection, a part of the unconventional energy development process has caused M5.0+ earthquakes in the past as well as several recent and ongoing cases of induced seismicity. Large-scale CO2 injection as a part of carbon sequestration efforts in the near future has a high risk of inducing large earthquakes. Therefore, injection operations related to both unconventional energy development and carbon sequestration should be optimized and managed to mitigate the likelihood of an induced seismic event.

Making quakes depends on injection rates Wastewater injection wells induce earthquakes that garner much attention, especially in tectonically inactive regions. Weingarten et al. combined information from public injection-well databases from the eastern and central United States with the best earthquake catalog available over the past 30 years. The rate of fluid injection into a well appeared to be the most likely decisive triggering factor in regions prone to induced earthquakes. Along these lines, Walsh III and Zoback found a clear correlation between areas in Oklahoma where waste saltwater is being injected on a large scale and areas experiencing increased earthquake activity. Science , this issue p. 1336 ; Sci. Adv. 10.1126/sciadv.1500195 (2015).

The Crescent Moon Spring is a unique scenic spot located in an aeolian sand environment of northwestern China. The water level of this spring has continuously declined in recent decades, causing both the government and many scholars great concern. To better understand its hydrologic structure, this study investigated the origin of the spring and its recharge sources based on the hydrochemical and isotopic (18O, 2H and 3H) composition of 56 water samples collected along the groundwater flow path. The results indicate the spring is a window into the groundwater table cut out of low-lying terrain in sand dunes, with the geographic and sedimentary conditions affecting its genesis and hydrochemistry. The spring and shallow sedimentary aquifers are recharged primarily by river water, while deep bedrock aquifers are recharged by deep lateral groundwater flow via fault zones in the piedmont of the Qilian Mountains. The results of recharge altitude calculations demonstrate a possible groundwater supply from deep bedrock aquifers to the south of spring. The recorded environmental 3H content varied from 11 to 40 T.U., indicating the groundwater is primarily recharged by glacier meltwater from modern precipitation, with only a small fraction from pre-modern times. End-member analysis using Cl- and δ18O shows recharge ratios of influent water from the Dang River and deep groundwater represent about 43 and 22% of the lake water, respectively, with artificial recharge accounting for the remaining 35%. Regional hydrogeological conditions, arid climate, and sand deposition have negative influences on the maintenance of lake water levels, but observed regional groundwater level declines resulting from the construction of the Dang River Reservoir, seepage from drainage canals, and excessive exploitation of groundwater are the most important factors contributing to the ecological crisis in the study area. These results provide insights into the origin and recharge sources of the Crescent Moon Spring that will guide efforts to preserve the water level.

The major processes controlling groundwater flow in intermountain basins are poorly understood, particularly in basins underlain by folded and faulted bedrock and under regionally realistic hydrogeologic heterogeneity. To explore the role of hydrogeologic heterogeneity and poorly constrained mountain hydrologic conditions on regional groundwater flow in contracted intermountain basins, a series of 3-D numerical groundwater flow models were developed using the South Park basin, Colorado, USA as a proxy. The models were used to identify the relative importance of different recharge processes to major aquifers, to estimate typical groundwater circulation depths, and to explore hydrogeologic communication between mountain and valley hydrogeologic landscapes. Modeling results show that mountain landscapes develop topographically controlled and predominantly local-scale to intermediate-scale flow systems. Permeability heterogeneity of the fold and fault belt and decreased topographic roughness led to permeability controlled flow systems in the valley. The structural position of major aquifers in the valley fold and fault belt was found to control the relative importance of different recharge mechanisms. Alternative mountain recharge model scenarios showed that higher mountain recharge rates led to higher mountain water table elevations and increasingly prominent local flow systems, primarily resulting in increased seepage within the mountain landscape and nonlinear increases in mountain block recharge to the valley. Valley aquifers were found to be relatively insensitive to changing mountain water tables, particularly in structurally isolated aquifers inside the fold and fault belt.

Wastewater disposal linked to earthquakes The number of earthquakes is increasing in regions with active unconventional oil and gas wells, where water pumped at high pressure breaks open rock containing natural gas, leaving behind wastewater in need of disposing. Keranen et al. show that the steep rise in earthquakes in Oklahoma, USA, is likely caused by fluid migration from wastewater disposal wells. Twenty percent of the earthquakes in the central United States could be attributed to just four of the wells. Injected fluids in high-volume wells triggered earthquakes over 30 km away. Science , this issue p. 448

Groundwater flow is an important control on subsurface evaporite (salt) dissolution. Salt dissolution can drive faulting and associated subsidence on the land surface and increase salinity in groundwater. This study aims to understand the groundwater flow system of Gypsum Canyon watershed in the Paradox Basin, Utah, USA, and whether or not groundwater-driven dissolution affects surface deformation. The work characterizes the groundwater flow and solute transport systems of the watershed using a three-dimensional (3D) finite element flow and transport model, SUTRA. Spring samples were analyzed for stable isotopes of water and total dissolved solids. Spring water and hydraulic conductivity data provide constraints for model parameters. Model results indicate that regional groundwater flow is to the northwest towards the Colorado River, and shallow flow systems are influenced by topography. The low permeability obtained from laboratory tests is inconsistent with field observed discharges, supporting the notion that fracture permeability plays a significant role in controlling groundwater flow. Model output implies that groundwater-driven dissolution is small on average, and cannot account for volume changes in the evaporite deposits that could cause surface deformation, but it is speculated that dissolution may be highly localized and/or weaken evaporite deposits, and could lead to surface deformation over time.

We studied the 24 year record of water level responses in Devils Hole, Death Valley National Park, NV, to dynamic crustal stresses from earthquakes. The continuous water level record exhibited 219 responses from earthquakes around the world, displaying hydroseismogram and coseismic offset types of response. We found that the water level in Devils Hole is extremely sensitive to earthquakes, and the seismic energy density required to initiate both hydroseismogram and coseismic offset responses is e ~ 10−6 J/m3, 2 orders of magnitude smaller than previously documented. Multiple mechanisms at Devils Hole may be responsible for observed water level responses to distant earthquakes. The hydroseismogram-type responses are best explained by poroelastic deformation, while coseismic offset responses are likely the result of localized permeability changes. This study could have implication to studying dynamic triggering of earthquakes, as remote earthquakes can lead to pore pressure changes and consequently effective stress changes in fluid-filled fault zones.

Summary In arid and semiarid regions, vegetation growth largely depends on groundwater, and causes diurnal fluctuations of shallow groundwater levels. Diurnal groundwater level fluctuations have been widely used to estimate groundwater evapotranspiration (ETG) in several methods. This study compared ETG estimated by three commonly used methods. A groundwater flow model was created to generate synthetic diurnal groundwater level fluctuations caused by a given evapotranspiration. The model also calculates the change in groundwater storage and net groundwater inflow at locations of observation wells. The White method, the Hays method, and the Loheide method were applied to estimate ETG with the model-generated diurnal groundwater levels. The comparison of the actual and estimated ETG revealed the accuracy of each method and indentified the applicability of the methods. When the recovery limb of the groundwater level hydrograph is nonlinear, these existing methods underestimate daily ETG. The Loheide method is comparatively better and can be improved by representing the rate of water table increase in the recovery limb of the hydrograph using an exponential equation. When the recovery limb of the groundwater level hydrograph is linear, all three methods can accurately estimate the daily ETG. The modified White method can provide hourly ETG estimates and is recommended for general use. In practical applications, the analysis of the shape of the water table recovery limb and the up and down gradient groundwater head differences can be used to identify the proper method for estimating ETG.

Seasonally frozen ground (SFG) and permafrost underlay approximately half of the land surface in the Northern Hemisphere. It is anticipated that climate warming will degrade both types of frozen ground, altering groundwater discharge to streams. While the effects of permafrost degradation on groundwater discharge have been analyzed, quantification of how groundwater discharge in degrading permafrost differs from that in SFG is lacking. This study simulates coupled groundwater and heat transport under freeze-thaw conditions for four representative hillslopes underlain by either continuous permafrost or SFG and compares groundwater discharge outputs under projected warming scenarios over decadal scales. Model results show that without warming there is more groundwater discharge in hillslopes with SFG than permafrost. After a century of warming, groundwater discharge increases for both kinds of frozen ground, but permafrost experiences a larger increase than SFG. These findings have implications for aquatic ecosystems and prioritizing water resource planning.