Integrated Hydrologic Modelling of Groundwater-Surface Water Interactions in Cold Regions

Groundwater-surface water (GW-SW) interaction, as a key component in the cold region hydrologic cycle, is extremely sensitive to seasonal and climate change. Specifically, the dynamic change of snow cover and frozen soil bring additional challenges in observing and simulating hydrologic processes under GW-SW interactions in cold regions. Integrated hydrologic models are promising tools to simulate such complex processes and study the system behaviours as well as its responses to perturbations. The cold region integrated hydrologic models should be physically representative and fully considering the thermal-hydrologic processes under snow cover variations, freeze-thaw cycles in frozen soils and GW-SW interactions. Benchmarking and integration with scarce field observations are also critical in developing cold region integrated hydrologic models. This review summarizes the current status of hydrologic models suitable for cold environment, including distributed hydrologic models, cryo-hydrogeologic models, and fully-coupled cold region GW-SW models, with a specific focus on their concepts, numerical methods, benchmarking, and applications across scales. The current research can provide implications for cold region hydrologic model development and advance our understanding of altered environments in cold regions disturbed by climate change, such as permafrost degradation, early snow melt and water shortage.

On the similarity of hillslope hydrologic function: a process-based approach

Hillslope similarity is an active topic in hydrology because of its importance to improve our understanding of hydrologic processes and enable comparisons and paired studies. In this study, we propose a holistic bottom-up hillslope similarity classification based on a region’s integrative hydrodynamic response quantified by the seasonal changes in groundwater levels. The main advantage of the proposed classification is its ability to describe recharge and discharge processes. We test the performance of the proposed classification by comparing it to seven other common hillslope similarity classifications. These include simple classifications based on the aridity index, topographic wetness index, elevation, land cover, and more sophisticated machine-learning classifications that jointly integrate all these data. We assess the ability of these classifications to identify and categorize hillslopes with similar static characteristics, hydroclimatic behaviors, land surface processes, and subsurface dynamics in a mountainous watershed, the East River, located in the headwaters of the Upper Colorado River Basin. The proposed classification is robust as it reasonably identifies and categorizes hillslopes with similar elevation, land cover, hydroclimate, land surface processes, and subsurface hydrodynamics (and hence hillslopes with similar hydrologic function). In general, the other approaches are good in identifying similarity in a single characteristic, which is usually close to the selected variable. We further demonstrate the robustness of the proposed classification by testing its ability to predict hillslope responses to wet and dry hydrologic conditions, of which it performs well when based on average conditions.

Sources and Sinks of Carbon in Boreal Ecosystems of Interior Alaska : A Review

Boreal ecosystems store large quantities of carbon but are increasingly vulnerable to carbon loss due to disturbance and climate warming. The boreal region in Alaska and Canada, largely underlain by discontinuous permafrost, presents a challenging landscape for itemizing carbon sources and sinks in soil and vegetation. The roles of fire, forest succession, and the presence/absence of permafrost on carbon cycle, vegetation, and hydrologic processes have been the focus of multidisciplinary research in boreal ecosystems for the past 20 years. However, projections of a warming future climate, an increase in fire severity and extent, and the potential degradation of permafrost could lead to major landscape and carbon cycle changes over the next 20 to 50 years. To assist land managers in interior Alaska in adapting and managing for potential changes in the carbon cycle, this paper was developed incorporating an overview of the climate, ecosystem processes, vegetation, and soil regimes. The objective is to provide a synthesis of the most current carbon storage estimates and measurements to guide policy and land management decisions on how to best manage carbon sources and sinks. We provide recommendations to address the challenges facing land managers in efforts to manage carbon cycle processes. The results of this study can be used for carbon cycle management in other locations within the boreal biome which encompasses a broad distribution from 45° to 83° north.

Assessment of Streamflow Predictions Generated Using Multi-model and Multi-precipitation Product Forcing

This study assesses streamflow predictions generated by two distributed hydrologic models, the Hillslope Link Model (HLM) and the National Water Model (NWM), driven by three radar-based precipitation forcing datasets. These forcing data include the Multi-Radar Multi-Sensor (MRMS), and the Iowa Flood Center’s single-polarization-based (IFC-SP) and dual-polarization-based (IFC-DP) products. To examine forcing- and model-dependent aspects of the representation of hydrologic processes, we mixed and matched all forcing data and models, and simulated streamflow for 2016–2018 based on six forcing-model combinations. The forcing product evaluation using independent ground reference data showed that the IFC-DP radar-only product’s accuracy is comparable to MRMS, which is rain gauge-corrected. Streamflow evaluation at 140 U.S. Geological Survey (USGS) stations in Iowa demonstrated that the HLM tended to perform slightly better than the NWM, generating streamflow with smaller volume errors and higher predictive power as measured by Kling-Gupta Efficiency (KGE). The authors also inspected the effect of estimation errors in the forcing products on streamflow generation and found that MRMS’s slight underestimation bias led to streamflow underestimation for all simulation years, particularly with the NWM. The less biased product (IFC-DP), which has higher error variability, resulted in increased runoff volumes with larger dispersion of errors compared to the ones derived from MRMS. Despite its tendency to underestimate, MRMS showed consistent performance with lower error variability as reflected by the KGE. The dispersion observed from the evaluation metrics (e.g., volume error and KGE) seems to decrease as scale becomes larger, implying that random errors in forcing are likely to average out at larger scale basins. The evaluation of simulated peaks revealed that an accurate estimation of peak (e.g., time and magnitude) remains challenging, as demonstrated by the highly scattered distribution of peak errors for both hydrologic models.

Thermal Imaging Shows Submarine Groundwater Discharge Plumes Associated with Ancient Settlements on Rapa Nui (Easter Island, Chile)

Submarine groundwater discharge (SGD) is an important component of many coastal environments and hydrologic processes, providing sources of nutrients to marine ecosystems, and potentially, an important source of fresh water for human populations. Here, we use a combination of unpiloted aerial systems (UAS) thermal infrared (TIR) imaging and salinity measurements to characterize SGD on the remote East Polynesian island of Rapa Nui (Easter Island, Chile). Previous research has shown that coastal freshwater seeps are abundant on Rapa Nui and strongly associated with the locations of ancient settlement sites. We currently lack, however, information on the differential magnitude or quality of these sources of fresh water. Our UAS-based TIR results from four locations on Rapa Nui suggest that locations of variably-sized SGD plumes are associated with many ancient settlement sites on the island and that these water sources are resilient to drought events. These findings support previous work indicating that ancient Rapa Nui communities responded to the inherent and climate-induced hydrological challenges of the island by focusing on these abundant and resilient freshwater sources. Our results highlight the efficacy of using UAS-based TIR for detecting relatively small SGD locations and provide key insights on the potential uses of these water sources for past and current Rapa Nui communities.

Streamflow indices to identify catchment drivers of hydrograph

Streamflow indices are flow descriptors that quantify the streamflow dynamics, which are usually determined for a specific basin and are distinct from other basin features. The flow descriptors are appropriate for large-scale and comparative hydrology studies, independent of statistical assumptions and can distinguish signals that indicate basin behavior over time. In this paper, the characteristic features of the hydrograph's temporal asymmetry due to its different underlying hydrologic processes are primarily highlighted. Streamflow indices linked to each limb of the hydrograph within the time-irreversibility paradigm are distinguished with respect to its processes driving the rising and falling limbs. Various streamflow indices relating the rising and falling limbs, and the catchment attributes such as climate, topography, vegetation, geology and soil are then correlated. Finally, the key attributes governing rising and falling limbs are identified. The novelty of the work is on differentiating hydrographs by their time irreversibility property and offering an alternative way to recognize primary drivers of streamflow hydrographs. A set of streamflow indices at the catchment scale for 671 basins in the Contiguous United States (CONUS) is presented here. These streamflow indices complement the catchment attributes provided earlier (Addor et al., 2017) for the CAMELS data set. A series of spatial maps describing the streamflow indices and their regional variability over the CONUS is illustrated in this study.

The impact of soil development, rainfall intensity and vegetation complexity on subsurface flow paths along a glacial chronosequence of 10 millennia

Hydrologic processes play an important role in the hydro-pedo-geomorphological feedback cycle of landscape evolution. Soil properties and subsurface flow paths change over time, but due to lack of observations important hydrologic processes such as water flow paths are often not properly considered in soil and landscape evolution studies. We investigated the evolution of subsurface flow paths during landscape development in the calcareous glacier forefield at the Griessfirn in the Swiss Alps. While the main focus was on flow path evolution and the formation of preferential flow paths with soil development, we also looked at the impact of irrigation intensity and vegetation complexity (in what way does the vegetation complexity defined by degree of vegetation cover and functional diversity at each age class relate to subsurface structures and flow path initialization?). We chose four glacial moraines of different ages (110, 160, 4 900, and 13 500 years) and conducted dye tracer experiments with Brilliant Blue (4 g l−1) on three plots at each moraine. The three plots at each age class differed by their degree of vegetation complexity (low, medium, and high) and each was further divided into three equal subplots where dyed water was applied with three different irrigation intensities (20, 40, and 60 mm h−1) and an irrigation amount of 40 mm. Dye pattern characteristics in form of volume density and surface area density were derived by digital image analysis and compared via statistical analysis.Volume density was used to classify the observed dye patterns into specific flow type categories. The effect of soil formation and thus changing soil characteristics on flow types were investigated by the analysis of structural and textural parameters in form of grain size distribution, porosity, bulk density, and loss on ignition. A change in flow types with increasing moraine age was observed from a rather homogeneous matrix flow to heterogeneous matrix and finger flow. Along the soil chronosequence, a reduction in particle sizes and an ongoing vegetation development resulted in an accumulation of organic matter in the topsoil and an increase in water storage capacity (decrease in bulk density and increase in porosity). Differences in irrigation intensity only had an effect on flow types at the oldest moraine, where the frequency of finger flow decreased with increasing irrigation intensity. A relation between vegetation complexity and flow types was only observed at the older moraines, which had a dense vegetation cover. With increasing vegetation complexity the proportion of preferential flow paths in form of finger flow also increased.

Extreme melt season ice layers reduce firn permeability across Greenland

Surface meltwater runoff dominates present-day mass loss from the Greenland Ice Sheet. In Greenland’s interior, porous firn can limit runoff by retaining meltwater unless perched low-permeability horizons, such as ice slabs, develop and restrict percolation. Recent observations suggest that such horizons might develop rapidly during extreme melt seasons. Here we present radar sounding evidence that an extensive near surface melt layer formed following the extreme melt season in 2012. This layer was still present in 2017 in regions up to 700 m higher in elevation and 160 km further inland than known ice slabs. We find that melt layer formation is driven by local, short-timescale thermal and hydrologic processes in addition to mean climate state. These melt layers reduce vertical percolation pathways, and, under appropriate firn temperature and surface melt conditions, encourage further ice aggregation at their horizon. Therefore, the frequency of extreme melt seasons relative to the rate at which pore space and cold content regenerates above the most recent melt layer may be a key determinant of the firn’s multi-year response to surface melt.