Westerly-triggered lake-effect snowfall enhanced with climate warming over the Tibetan Plateau.

Lake-effect snowfall (LES) occurs when cold air moves across open lakes. LES is expected to occur more frequently over the TP, due to the intensified lake expansion caused by intensified global warming. Thus, there is an urgent need to comprehensively assess the LES over the TP. Here, we revealed that the LES is triggered by westerly southward shift leading to the drop in air temperature and is positively correlated with lake area, wind speed and longitude across 12 large lakes (>300 km2) based on satellite observations and reanalysis data. Using a sensitivity model simulation, we determined that large lakes in the southern TP contributed to more than 50% of the snowfall in the downwind area in 2013. Projections indicate that the westerly-triggered LES will increase under the future RCP4.5 climate warming scenario, highlighting the importance of developing adaptive policies to address the growing risks associated with future LES.

Satellites reveal hotspots of global river extent change.

Rivers are among the most diverse, dynamic, and productive ecosystems on Earth. River flow regimes are constantly changing, but characterizing and understanding such changes have been challenging from a long-term and global perspective. By analyzing water extent variations observed from four-decade Landsat imagery, we here provide a global attribution of the recent changes in river regime to morphological dynamics (e.g., channel shifting and anabranching), expansion induced by new dams, and hydrological signals of widening and narrowing. Morphological dynamics prevailed in ~20% of the global river area. Booming reservoir constructions, mostly skewed in Asia and South America, contributed to ~32% of the river widening. The remaining hydrological signals were characterized by contrasting hotspots, including prominent river widening in alpine and pan-Arctic regions and narrowing in the arid/semi-arid continental interiors, driven by varying trends in climate forcing, cryospheric response to warming, and human water management. Our findings suggest that the recent river extent dynamics diverge based on hydroclimate and socio-economic conditions, and besides reflecting ongoing morphodynamical processes, river extent changes show close connections with external forcings, including climate change and anthropogenic interference.

Coulomb stress analysis for several filling and operational scenarios at the Grand Ethiopian Renaissance Dam impoundment.

Increased demand for power generation coupled with changing seasonal water uncertainty has caused a worldwide increase in the construction of large hydrologic engineering structures. That said, the soon-to-be-completed Grand Ethiopian Renaissance Dam (GERD) will impound the Blue Nile River in Western Ethiopia and its reservoir will encompass ~ 1763 km2 and store ~ 67 Gt (km3) of surface water. The impoundment will undergo maximum seasonal load changes of ~ 28 to ~ 36 Gt during projected seasonal hydroelectric operations. The GERD impoundment will cause significant subsurficial stresses, and could possibly trigger seismicity in the region. This study examines Coulomb stress and hydrologic load centroid movements for several GERD impoundment and operational scenarios. The maximum subsurficial Coulomb stress applied on optimally oriented fault planes from the full impoundment is ~ 186 kPa and over 30% of our model domain incurs Coulomb stresses ≥ 10 kPa, regardless of the impoundment period length. The main driver behind Coulomb stress and load centroid motion during impoundment is the annual, accumulated daily reservoir storage change. The maximum Coulomb stresses from the highest amplitude season of five long-term operational scenarios are around 36, 33, 29, 41, and 24% of the total maximum stresses from the entire GERD impoundment. Variations in annual Coulomb stresses during modeled GERD operations are attributed to the seasonal load per unit area, and partially to the initial seasonal water level. The spatial patterns and amplitudes of these stress tensors are closely linked to both the size and timing of GERD inflow/outflow rates, and an improved understanding of the magnitude and extent of these stresses provides useful information to water managers to better understand potential reservoir triggered seismic events from several different operational and impoundment strategies. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s12665-021-09591-w.

A Global Assessment of Terrestrial Evapotranspiration Increase Due to Surface Water Area Change.

Surface water, which is changing constantly, is a crucial component in the global water cycle, as it greatly affects the water flux between the land and the atmosphere through evaporation. However, the influences of changing surface water area on the global water budget have largely been neglected. Here we estimate an extra water flux of 30.38 ± 15.51 km3/year omitted in global evaporation calculation caused by a net increase of global surface water area between periods 1984-1999 and 2000-2015. Our estimate is at a similar magnitude to the recent average annual change in global evapotranspiration assuming a stationary surface water area. It is also comparable to the estimated trends in various components of the hydrological cycle such as precipitation, discharge, groundwater depletion, and glacier melting. Our findings suggest that the omission of surface water area changes may cause considerable biases in global evaporation estimation, so an improved understanding of water area dynamics and its atmospheric coupling is crucial to reduce the uncertainty in the estimation of future global water budgets.

The contribution of wind wave changes on diminishing ice period in Lake Pyhäjärvi during the last half-century.

To address the contribution of long-term wind wave changes on diminishing ice period in Northern European lakes, an in situ observation of wind waves was conducted to calibrate a wind-wave numerical model for Lake Pyhäjärvi, which is the largest lake in southwest Finland. Using station-measured hydrometeorological data from 1963 to 2013 and model-simulated wind waves, correlation and regression analyses were conducted to assess the changing trend and main influences on ice period. Ice period in Lake Pyhäjärvi decreased significantly over 51 years (r = 0.47, P < 0.01). The analysis of main hydrometeorological factors to ice period showed that the significant air temperature rise is the main contributor for the diminishing of ice period in the lake. Besides air temperature, wind-induced waves can also weaken lake ice by increasing water mixing and lake ice breakage. The regression indicated that mean significant wave height in December and April was negatively related to ice period (r = - 0.48, P < 0.01). These results imply that long-term changes of wind waves related to climate change should be considered to fully understand the reduction of aquatic ice at high latitudes.

Little impact of Three Gorges Dam on recent decadal lake decline across China's Yangtze Plain.

The ubiquitous lakes across China's Yangtze Plain (YP) are indispensable freshwater resources sustaining ecosystems and socioeconomics for nearly half a billion people. Our recent survey revealed a widespread net decline in the total YP lake inundation area during 2000-2011 (a cumulative decrease of ~10%), yet its mechanism remains contentious. Here, we uncover the impacts of climate variability and anthropogenic activities including i) Yangtze flow and sediment alterations by the Three Gorges Dam (TGD) and ii) human water consumption in agricultural, industrial, and domestic sectors throughout the downstream Yangtze Basin. Results suggest that climate variability is the dominant driver of this decadal lake decline, whereas studied human activities, despite varying seasonal impacts that peak in fall, contribute marginal fraction (~10-20% or less) to the interannual lake area decrease. Given that the TGD impacts on the total YP lake area and its seasonal variation are both under ~5%, we also dismiss the speculation that the TGD might be responsible for evident downstream climate change by altering lake surface extent and thus open water evaporation. Nevertheless, anthropogenic impacts exhibited a strengthening trend during the past decade. Although the TGD has reached its full-capacity water regulation, the negative impacts of human water consumption and TGD-related net channel erosion are already comparable to that of TGD's flow regulation, and may continue to grow as crucial anthropogenic factors to future YP lake conservation.

A Fast Algorithm to Estimate the Deepest Points of Lakes for Regional Lake Registration.

When conducting image registration in the U.S. state of Alaska, it is very difficult to locate satisfactory ground control points because ice, snow, and lakes cover much of the ground. However, GCPs can be located by seeking stable points from the extracted lake data. This paper defines a process to estimate the deepest points of lakes as the most stable ground control points for registration. We estimate the deepest point of a lake by computing the center point of the largest inner circle (LIC) of the polygon representing the lake. An LIC-seeking method based on Voronoi diagrams is proposed, and an algorithm based on medial axis simplification (MAS) is introduced. The proposed design also incorporates parallel data computing. A key issue of selecting a policy for partitioning vector data is carefully studied, the selected policy that equalize the algorithm complexity is proved the most optimized policy for vector parallel processing. Using several experimental applications, we conclude that the presented approach accurately estimates the deepest points in Alaskan lakes; furthermore, we gain perfect efficiency using MAS and a policy of algorithm complexity equalization.

Efficient meltwater drainage through supraglacial streams and rivers on the southwest Greenland ice sheet.

Thermally incised meltwater channels that flow each summer across melt-prone surfaces of the Greenland ice sheet have received little direct study. We use high-resolution WorldView-1/2 satellite mapping and in situ measurements to characterize supraglacial water storage, drainage pattern, and discharge across 6,812 km(2) of southwest Greenland in July 2012, after a record melt event. Efficient surface drainage was routed through 523 high-order stream/river channel networks, all of which terminated in moulins before reaching the ice edge. Low surface water storage (3.6 ± 0.9 cm), negligible impoundment by supraglacial lakes or topographic depressions, and high discharge to moulins (2.54-2.81 cm⋅d(-1)) indicate that the surface drainage system conveyed its own storage volume every <2 d to the bed. Moulin discharges mapped inside ∼52% of the source ice watershed for Isortoq, a major proglacial river, totaled ∼41-98% of observed proglacial discharge, highlighting the importance of supraglacial river drainage to true outflow from the ice edge. However, Isortoq discharges tended lower than runoff simulations from the Modèle Atmosphérique Régional (MAR) regional climate model (0.056-0.112 km(3)⋅d(-1) vs. ∼0.103 km(3)⋅d(-1)), and when integrated over the melt season, totaled just 37-75% of MAR, suggesting nontrivial subglacial water storage even in this melt-prone region of the ice sheet. We conclude that (i) the interior surface of the ice sheet can be efficiently drained under optimal conditions, (ii) that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and (iii) that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater export from the ice sheet to the ocean.

Rapid early development of circumarctic peatlands and atmospheric CH4 and CO2 variations.

An analysis of 1516 radiocarbon dates demonstrates that the development of the current circumarctic peatlands began approximately 16.5 thousand years ago (ka) and expanded explosively between 12 and 8 ka in concert with high summer insolation and increasing temperatures. Their rapid development contributed to the sustained peak in CH4 and modest decline of CO2 during the early Holocene and likely contributed to CH4 and CO2 fluctuations during earlier interglacial and interstadial transitions. Given the decreased tempo of peatland initiation in the late Holocene and the transition of many from fens (which generated high levels of CH4) to ombrotrophic bogs, a neoglacial expansion of northern peatlands cannot explain the increase in atmospheric CH4 that occurred after 6 ka.