More Like this

1. Evolution of river regimes in the Mekong River basin over 8 decades and the role of dams in recent hydrological extremes

   https://doi.org/10.5194/hess-28-3347-2024  

   Dang, Huy; Pokhrel, Yadu (January 2024, Hydrology and Earth System Sciences)

   Abstract. Flow regimes in major global river systems are undergoing rapid alterations due to unprecedented stress from climate change and human activities. The Mekong River basin (MRB) was, until recently, among the last major global rivers relatively unaltered by humans, but this has been changing alarmingly in the last decade due to booming dam construction. Numerous studies have examined the MRB's flood pulse and its alterations in recent years. However, a mechanistic quantification at the basin scale attributing these changes to either climatic or human drivers is lacking. Here, we present the first results of the basin-wide changes in natural hydrological regimes in the MRB over the past 8 decades and the impacts of dams in recent decades by examining 83 years (1940–2022) of river regime characteristics simulated by a river–floodplain hydrodynamic model that includes 126 major dams in the MRB. Results indicate that, while the Mekong River's flow has shown substantial decadal trends and variabilities, the operation of dams in recent years has been causing a fundamental shift in the seasonal volume and timing of river flow and extreme hydrological conditions. Even though the dam-induced impacts have been small so far and most pronounced in areas directly downstream of major dams, dams are intensifying the natural variations in the Mekong's mainstream wet-season flow. Further, the additional 65 dams commissioned since 2010 have exacerbated drought conditions by substantially delaying the MRB's wet-season onset, especially in recent years (e.g., 2019 and 2020), when the natural wet-season durations are already shorter than in normal years. Further, dams have shifted by up to 20 % of the mainstream annual volume between the dry and wet seasons in recent years. While this has a minimal impact on the MRB's annual flow volume, the flood occurrence in many major areas of Tonlé Sap and the Mekong Delta has been largely altered. This study provides critical insights into the long-term hydrological variabilities and impacts of dams on the Mekong River's flow regimes, which can help improve water resource management in light of intensifying hydrological extremes. 

   more » « less
2. Increasing Alaskan river discharge during the cold season is driven by recent warming

   https://doi.org/10.1088/1748-9326/acb661  

   Blaskey, Dylan; Koch, Joshua C.; Gooseff, Michael N.; Newman, Andrew J.; Cheng, Yifan; O’Donnell, Jonathan A.; Musselman, Keith N. (February 2023, Environmental Research Letters)

   Abstract Arctic hydrology is experiencing rapid changes including earlier snow melt, permafrost degradation, increasing active layer depth, and reduced river ice, all of which are expected to lead to changes in stream flow regimes. Recently, long-term (>60 years) climate reanalysis and river discharge observation data have become available. We utilized these data to assess long-term changes in discharge and their hydroclimatic drivers. River discharge during the cold season (October–April) increased by 10% per decade. The most widespread discharge increase occurred in April (15% per decade), the month of ice break-up for the majority of basins. In October, when river ice formation generally begins, average monthly discharge increased by 7% per decade. Long-term air temperature increases in October and April increased the number of days above freezing (+1.1 d per decade) resulting in increased snow ablation (20% per decade) and decreased snow water equivalent (−12% per decade). Compared to the historical period (1960–1989), mean April and October air temperature in the recent period (1990–2019) have greater correlation with monthly discharge from 0.33 to 0.68 and 0.0–0.48, respectively. This indicates that the recent increases in air temperature are directly related to these discharge changes. Ubiquitous increases in cold and shoulder-season discharge demonstrate the scale at which hydrologic and biogeochemical fluxes are being altered in the Arctic. 

   more » « less
3. Are historical trends in weather consistent with model predictions in the Central United States?

   https://doi.org/10.1002/jeq2.70066  

   Baffaut, Claire; Metz, Megan; Moriasi, Daniel; Malone, Rob; Witthaus, Lindsey; Wacha, Ken; Goslee, Sarah; Hsieh, Hsun‐Yi; Robertson, G Philip (August 2025, Journal of Environmental Quality)

   Abstract Model projections predict increasing temperatures and precipitation change in many locations in the Central United States. To provide perspective on what these trends might bring relative to what has already happened, we compared historical temperature and precipitation change with what models from the Coupled Model Intercomparison Project (CMIP6) predict. The analysis focuses on regions represented by five long‐term agroecosystem research sites along a latitudinal transect from Michigan to Iowa, Missouri, Oklahoma, and Mississippi. We analyzed trends in long‐term records (≥50 years) of precipitation and temperature data at annual and monthly scales using indicators that characterize extreme and average temperature and rainfall amounts. Results show that temperatures have changed from 1900 to 2020, more for minimum (0.1°C–0.3°C decade⁻¹) than maximum (−0.1°C–0.2°C decade⁻¹), more for winter (−0.1°C–0.3°C decade⁻¹) than summer (−0.1°C–0.1°C decade⁻¹), and more often in the north than in the south. Except in Mississippi, annual precipitation has increased at rates of 25 mm decade⁻¹or greater over 1950–2020, but monthly trends were inconsistent. Projected trends suggest continued temperature increases, highlighting the urgent need for research on management systems that are resilient to such increases. 

   more » « less
4. Shifted sediment-transport regimes by climate change and amplified hydrological variability in cryosphere-fed rivers

   https://doi.org/10.1126/sciadv.adi5019  

   Zhang, Ting; Li, Dongfeng; East, Amy E; Kettner, Albert J; Best, Jim; Ni, Jinren; Lu, Xixi (November 2023, Science Advances)

   Climate change affects cryosphere-fed rivers and alters seasonal sediment dynamics, affecting cyclical fluvial material supply and year-round water-food-energy provisions to downstream communities. Here, we demonstrate seasonal sediment-transport regime shifts from the 1960s to 2000s in four cryosphere-fed rivers characterized by glacial, nival, pluvial, and mixed regimes, respectively. Spring sees a shift toward pluvial-dominated sediment transport due to less snowmelt and more erosive rainfall. Summer is characterized by intensified glacier meltwater pulses and pluvial events that exceptionally increase sediment fluxes. Our study highlights that the increases in hydroclimatic extremes and cryosphere degradation lead to amplified variability in fluvial fluxes and higher summer sediment peaks, which can threaten downstream river infrastructure safety and ecosystems and worsen glacial/pluvial floods. We further offer a monthly-scale sediment-availability-transport model that can reproduce such regime shifts and thus help facilitate sustainable reservoir operation and river management in wider cryospheric regions under future climate and hydrological change. 

   more » « less
5. Direct Versus Indirect Tree Ring Reconstruction of Annual Discharge of Chemora River, Algeria

   https://doi.org/10.3390/f11090986  

   Meko, David; Touchan, Ramzi; Kherchouche, Dalila; Slimani, Said (September 2020, Forests)

   Annual river discharge is a critical variable for water resources planning and management. Tree rings are widely used to reconstruct annual discharge, but errors can be large when tree growth fails to respond commensurately to hydrologically important seasonal components of climate. This paper contrasts direct and indirect reconstruction as statistical approaches to discharge reconstruction for the Chemora River, in semi-arid northeastern Algeria, and explores indirect reconstruction as a diagnostic tool in reconstruction error analysis. We define direct reconstruction as predictions from regression of annual discharge on tree ring data, and indirect reconstruction as predictions from a four-stage process: (1) regression of precipitation on tree rings, (2) application of the regression model to get reconstructed precipitation for grid cells over the basin, (3) routing of reconstructed precipitation through a climatological water balance (WB) model, and (4) summing model runoff over cells to get the reconstructed discharge at a gage location. For comparative purposes, the potential predictors in both modeling approaches are the same principal components of tree ring width chronologies from a network of drought-sensitive sites of Pinus halepensis and Cedrus atlantica in northern Algeria. Results suggest that both modeling approaches can yield statistically significant reconstructions for the Chemora River. Greater accuracy and simplicity of the direct method are countered by conceptual physical advantages of the indirect method. The WB modeling inherent to the indirect method is useful as a diagnostic tool in error analysis of discharge reconstruction, points out the low and declining importance of snowmelt to the river discharge, and gives clues to the cause of severe underestimation of discharge in the outlier high-discharge year 1996. Results show that indirect reconstruction would benefit most in this basin from tree ring resolution of seasonal precipitation. 

   more » « less