Two nearby M dwarf binaries from the Two Micron All Sky Survey

We report the discovery of two binary M dwarf systems in the immediate solar neighbourhood using the Two Micron All Sky Survey (2MASS). The first is an M6.5 companion to the nearby G star HD 86728 (Gl 376). The known properties of HD 86728 indicate that the M dwarf (Gl 376B) is old, metal-rich and only 14.9 parsec away. The M dwarf is highly active, with both H α and X-ray emission. Thus, Gl 376B offers the opportunity to study an old, bright, active M dwarf with known metallicity, age and luminosity. We show that it is probable that Gl 376B is itself an unresolved pair. The other system consists of an M6.5 and an M8 dwarf with 14.5 arcsec separation. We estimate a distance of ∼16 parsec for this very low-mass pair. Stronger activity is observed in the M6.5 dwarf, supporting evidence that chromospheric activity is weakening near the hydrogen-burning limit.     

The Seasonal Prediction of the Exceptional Yangtze River Rainfall in Summer 2020

During June and July of 2020, the Yangtze River basin suffered from extreme mei-yu rainfall and catastrophic flooding. This study explores the seasonal predictability and associated dynamical causes for this extreme Yangtze River rainfall event, based on forecasts from the Met Office GloSea5 operational forecast system. The forecasts successfully predicted above-average rainfall over the Yangtze River basin, which arose from the successful reproduction of the anomalous western North Pacific subtropical high (WNPSH). Our results indicate that both the Indian Ocean warm sea surface temperature (SST) and local WNP SST gradient were responsible for the westward extension of the WNPSH, and the forecasts captured these tropical signals well. We explore extratropical drivers but find a large model spread among the forecast members regarding the meridional displacements of the East Asian mid-latitude westerly jet (EAJ). The forecast members with an evident southward displacement of the EAJ favored more extreme Yangtze River rainfall. However, the forecast Yangtze River rainfall anomaly was weaker compared to that was observed and no member showed such strong rainfall. In observations, the EAJ displayed an evident acceleration in summer 2020, which could lead to a significant wind convergence in the lower troposphere around the Yangtze River basin, and favor more mei-yu rainfall. The model forecast failed to satisfactorily reproduce these processes. This difference implies that the observed enhancement of the EAJ intensity gave a large boost to the Yangtze River rainfall, hindering a better forecast of the intensity of the event and disaster mitigation.     

A meta-analysis of global crop water productivity of three leading world crops (wheat,corn, and rice) in the irrigated areas over three decades

ABSTRACT

The overarching goal of this study was to perform a comprehensive meta-analysis of irrigated agricultural Crop Water Productivity (CWP) of the world’s three leading crops: wheat, corn, and rice based on three decades of remote sensing and non-remote sensing-based studies. Overall, CWP data from 148 crop growing study sites (60 wheat, 43 corn, and 45 rice) spread across the world were gathered from published articles spanning 31 different countries. There was overwhelming evidence of a significant increase in CWP with an increase in latitude for predominately northern hemisphere datasets. For example, corn grown in latitude 40–50° had much higher mean CWP (2.45?kg/m³) compared to mean CWP of corn grown in other latitudes such as 30–40° (1.67?kg/m³) or 20–30° (0.94?kg/m³). The same trend existed for wheat and rice as well. For soils, none of the CWP values, for any of the three crops, were statistically different. However, mean CWP in higher latitudes for the same soil was significantly higher than the mean CWP for the same soil in lower latitudes. This applied for all three crops studied. For wheat, the global CWP categories were low (≤0.75?kg/m³), medium (>0.75 to <1.10?kg/m³), and high CWP (≥1.10?kg/m³). For corn the global CWP categories were low (≤1.25?kg/m³), medium (>1.25 to ≤1.75?kg/m³), and high (>1.75?kg/m³). For rice the global CWP categories were low (≤0.70?kg/m³), medium (>0.70 to ≤1.25?kg/m³), and high (>1.25?kg/m³). USA and China are the only two countries that have consistently high CWP for wheat, corn, and rice. Australia and India have medium CWP for wheat and rice. India’s corn, however, has low CWP. Egypt, Turkey, Netherlands, Mexico, and Israel have high CWP for wheat. Romania, Argentina, and Hungary have high CWP for corn, and Philippines has high CWP for rice. All other countries have either low or medium CWP for all three crops. Based on data in this study, the highest consumers of water for crop production also have the most potential for water savings. These countries are USA, India, and China for wheat; USA, China, and Brazil for corn; India, China, and Pakistan for rice. For example, even just a 10% increase in CWP of wheat grown in India can save 6974 billion liters of water. This is equivalent to creating 6974 lakes each of 100?m³ in volume that leads to many benefits such as acting as ‘water banks’ for lean season, recreation, and numerous ecological services. This study establishes the volume of water that can be saved for each crop in each country when there is an increase in CWP by 10%, 20%, and 30%.     

Impact of stratospheric variability on tropospheric climate change

An improved stratospheric representation has been included in simulations with the Hadley Centre HadGEM1 coupled ocean atmosphere model with natural and anthropogenic forcings for the period 1979–2003. An improved stratospheric ozone dataset is employed that includes natural variations in ozone as well as the usual anthropogenic trends. In addition, in a second set of simulations the quasi biennial oscillation (QBO) of stratospheric equatorial zonal wind is also imposed using a relaxation towards ERA-40 zonal wind values. The resulting impact on tropospheric variability and trends is described. We show that the modelled cooling rate at the tropopause is enhanced by the improved ozone dataset and this improvement is even more marked when the QBO is also included. The same applies to warming trends in the upper tropical troposphere which are slightly reduced. Our stratospheric improvements produce a significant increase of internal variability but no change in the positive trend of annual mean global mean near-surface temperature. Warming rates are increased significantly over a large portion of the Arctic Ocean. The improved stratospheric representation, especially the QBO relaxation, causes a substantial reduction in near-surface temperature and precipitation response to the El Chichón eruption, especially in the tropical region. The winter increase in the phase of the northern annular mode observed in the aftermath of the two major recent volcanic eruptions is partly captured, especially after the El Chichón eruption. The positive trend in the southern annular mode (SAM) is increased and becomes statistically significant which demonstrates that the observed increase in the SAM is largely subject to internal variability in the stratosphere. The possible inclusion in simulations for future assessments of full ozone chemistry and a gravity wave scheme to internally generate a QBO is discussed.     

Streambed restoration to remove fine sediment alters reach‐scale transient storage in a low‐gradient fifth‐order river,Indiana, USA

Hyporheic restoration is of increasing interest given the role of hyporheic zones in supporting ecosystem services and functions. Given the prevalence of sediment pollution to waterways, an emerging restoration technique involves the removal of sediment from the interstices of gravel‐bed streams. Here, we document streambed sediment removal following a large, accidental release of fine sediment into a gravel‐bed river. We use this as a natural experiment to assess the impact of fine sediment removal on reach‐scale measures of transient storage and to document the responses of reaches with contrasting morphology (restored vs. unrestored) to changing discharge one‐field season. We conducted a series of conservative solute tracer experiments in each reach, interpreting both summary statistics for the recovered in‐stream solute tracer time series. Additionally, we applied the transient storage model to interpret the results via model parameters, including a Monte Carlo analysis to measure parameter identifiability and sensitivity in each experiment. Despite the restoration effort resulting in an open matrix gravel bed in the restored reach, we did not find the significant differences in most time series metrics describing reach‐scale transport and transient storage. We hypothesize that this is due to enhanced vertical exchange with the gravel bed in the restored reach replacing lateral exchange with macrophyte beds in the unrestored reach, developing a conceptual model to explain our findings. Consequently, we found that the impact of reach‐scale removal of fine sediment is not measureable using reach‐scale solute tracer studies. We offer recommendations for future studies seeking to measure the impacts of stream restoration at the reach scale.     

Flood‐plain responses to contemporary climate change in small High‐Arctic basins (Svalbard,Norway)

Changes in the supply of water and sediment to high‐latitude rivers related to contemporary climate change and glacier fluctuations largely determine the activity of fluvial processes. This study reconstructs fluvial dynamics since the end of the Little Ice Age (LIA) in two small, partially glaciated basins in the southern part of Spitsbergen, Svalbard Archipelago. We use a combination of aerial photograph interpretation, field mapping and dendrochronological analysis. Sequences of abandoned channels and glacifluvial terraces are distinctly visible in middle and lower parts of the Brattegg and Arie basins in this area. The advance of glaciers during the LIA in the upper part of the basins led to the development of a braiding pattern and to channel aggradation corresponding to the highest glacifluvial levels. The decreasing activity of these braidplains occurred at the turn of the 19th and 20th centuries, immediately prior to a significant incision period. A second generation of braided channels developed during the first half of the 20th century. Ice‐marginal lake development, less input of fine‐grained sediment to the river channel, and fast incision began from the second half of the 20th century onward. During the last two decades, the main fluvial response to the climatic warming has been contraction of flow within a narrower channel and the abandonment of braidplains. The increased lateral erosion and rate of downcutting and the formation of the most downstream reaches of the modern valley bottom occurred in the 1980s and 1990s. This process was intensified under flood conditions generated by extreme rainfall events. These micro‐scale (small partially glaciated basins) observations concerning the changes of the activity of fluvial processes since the end of LIA may be helpful for the reconstruction of past fluvial changes over longer time scales.     

Major shoreline retreat and sediment starvation following Snowball Earth

While cap dolostones are integral to the Snowball Earth hypothesis, the current depositional model does not account for multiple geological observations. Here we propose a model that rationalises palaeomagnetic, sequence‐stratigraphic and sedimentological data and supports rapid deglaciation with protracted cap dolostone precipitation. The Snowball Earth hypothesis posits that a runaway ice‐albedo can explain the climate paradox of Neoproterozoic glacial deposits occurring at low palaeolatitudes. This scenario invokes volcanic degassing to increase atmospheric greenhouse gases to a critical threshold that overcomes the albedo effect and brings the planet back from the ice‐covered state. Once this occurs, Earth should shift rapidly from a snowball to an extreme greenhouse. However, cap dolostone units overlying glacial sediments, typically interpreted as transgressive deposits, exhibit multiple magnetic reversals indicating they accumulated in >10⁵ years. By reviewing modern post‐glacial systems, sequence stratigraphic concepts and principles of sedimentology, we suggest that cap dolostones are not restricted to the transgression but rather represent sediment starvation following a major landward shoreline migration associated with the demise of Snowball Earth. Thus, the duration in which cap dolostone accumulated is not directly coupled to the timescale of the Snowball Earth deglaciation.     

Streamflow permanence in headwater streams across four geomorphic provinces in Northern California

Mountainous headwater streams represent a substantial proportion of the global stream network. These small streams may flow episodically, seasonally, or perennially, providing diverse values and services. Given their broad importance and growing pressures on terrestrial and aquatic resources, we must improve our understanding of the drivers of flow permanence to facilitate informed land and water management decisions. We used field observations from >10 cross-sections in each of 101 non-fish bearing, headwater streams across four geomorphic provinces in Northern California to quantify flow permanence and network connectivity during the summer low flow period in 2018. At each stream cross-section, we noted the presence or absence of streamflow and used this information to classify streams as perennial (continuous streamflow in all cross-sections) or non-perennial and connected (surface water in the most downstream cross-section) or disconnected. At each cross-section, we also quantified channel size (width and depth) and grain size. We coupled field observations with geospatial data of catchment physiography, hydrology, and climate in random forest models to investigate controls of flow permanence and network connectivity. Potential drivers of flow permanence or network connectivity included in our models were channel geometry, grain size, slope, aspect, elevation, annual and seasonal precipitation, air temperature, and topographic wetness index. We found more perennial streams in the Klamath Mountains and Sierra Nevada than in the Cascades and N. Coast regions. Streams in the Klamath were the most connected followed by streams in the N. Coast, Sierra Nevada, and Cascades. The most important variables for predicting flow permanence were channel grain size, winter 2018 precipitation, and drainage area. Comparatively, the most important variables for predicting network connectivity were winter and spring 2018 precipitation, grain size, and bankfull depth. Our study illustrated the complexity of the processes that drive flow permanence and highlighted the uncertainty in projecting the precense of water in streams across diverse regions.     

Magneto-atmospheric waves from a localized source

A technique developed by Lighthill for finding the asymptotic solution of an inhomogeneous wave equation with constant coefficients is applied to the study of wave propagation in magneto-atmospheres. The geometry of the wave number surface plays an important role in determining the generation and propagation of various types of magneto-atmospheric waves from a localized forcing region. Examples of these wavenumber surfaces are exhibited for various magnetic field strengths and wave frequencies. The asymptotic far field is tabulated for a time-harmonic, spatially gaussian, localized forcing term.