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1. Alignment between water inputs and vegetation green‐up reduces next year's runoff efficiency

   https://doi.org/10.1002/hyp.15211  

   Newcomb, Sarah K; Van_Kirk, Robert W; Godsey, Sarah E; Kraft, Maggi (June 2024, Hydrological Processes)

   In the western United States, water supplies largely originate as snowmelt from forested land. Forests impact the water balance of these headwater streams, yet most predictive runoff models do not explicitly account for changing snow‐vegetation dynamics. Here, we present a case study showing how warmer temperatures and changing forests in the Henrys Fork of the Snake River, a seasonally snow‐covered headwater basin in the Greater Yellowstone Ecosystem, have altered the relationship between April 1st snow water equivalent (SWE) and summer streamflow. Since the onset and recovery of severe drought in the early 2000s, predictive models based on pre‐drought relationships over‐predict summer runoff in all three headwater tributaries of the Henrys Fork, despite minimal changes in precipitation or snow accumulation. Compared with the pre‐drought period, late springs and summers (May–September) are warmer and vegetation is greener with denser forests due to recovery from multiple historical disturbances. Shifts in the alignment of snowmelt and energy availability due to warmer temperatures may reduce runoff efficiency by changing the amount of precipitation that goes to evapotranspiration versus runoff and recharge. To quantify the alignment between snowmelt and energy on a timeframe needed for predictive models, we propose a new metric, the Vegetation‐Water Alignment Index (VWA), to characterize the synchrony of vegetation greenness and snowmelt and rain inputs. New predictive models show that in addition to April 1st SWE, the previous year's VWA and summer reference evapotranspiration are the most significant predictors of runoff in each watershed and provide more predictive power than traditionally used metrics. These results suggest that the timing of snowmelt relative to the start of the growing season affects not only annual partitioning of streamflow, but can also determine the groundwater storage state that dictates runoff efficiency the following spring. 

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2. Runoff Variability in the Truckee–Carson River Basin from Tree Rings and a Water Balance Model

   https://doi.org/10.1175/EI-D-23-0018.1  

   Meko, David M; Biondi, Franco; Taylor, Alan H; Panyushkina, Irina P; Thaxton, Richard D; Prusevich, Alexander A; Shiklomanov, Alexander I; Lammers, Richard B; Glidden, Stanley (January 2024, Earth Interactions)

   Abstract Regional warming and associated changes in hydrologic systems pose challenges to water supply management in river basins of the western United States and call for improved understanding of the spatial and temporal variability of runoff. We apply a network of total width, subannual width, and delta blue intensity tree-ring chronologies in combination with a monthly water balance model to identify droughts and their associated precipitationPand temperatureTfootprints in the Truckee–Carson River basin (TCRB). Stepwise regression gave reasonably accurate reconstructions, from 1688 to 1999, of seasonalPandT(e.g.,R²= 0.50 for May–SeptemberT). These were disaggregated to monthly values, which were then routed through a water balance model to generate “indirectly” reconstructed runoff. Reconstructed and observed annual runoff correlate highly (r= 0.80) from 1906 to 1999. The extended runoff record shows that twentieth-century droughts are unmatched in severity in a 300-yr context. Our water balance modeling reconstruction advances the conventional regression-based dendrochronological methods as it allows for multiple hydrologic components (evapotranspiration, snowmelt, etc.) to be evaluated. We found that imposed warming (3° and 6°C) generally exacerbated the runoff deficits in past droughts but that impact could be lessened and sometimes even reversed in some years by compensating factors, including changes in snow regime. Our results underscore the value of combining multiproxy tree-ring data with water balance modeling to place past hydrologic droughts in the context of climate change. Significance StatementWe show how water balance modeling in combination with tree-ring data helps place modern droughts in the context of the past few centuries and a warming climate. Seasonal precipitation and temperature were reconstructed from multiproxy tree-ring data for a mountainous location near Lake Tahoe, and these reconstructions were routed through a water balance model to get a record of monthly runoff, snowmelt, and other water balance variables from 1688 to 1999. The resulting extended annual runoff record highlights the unmatched severity of twentieth-century droughts. A warming of 3°C imposed on reconstructed temperature generally exacerbates the runoff anomalies in past droughts, but this effect is sometimes offset by warming-related changes in the snow regime. 

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3. Arctic hydrology: a water balance study of Imnavait Creek and Kuparuk River watersheds, Alaska

   https://doi.org/10.1139/as-2025-0033  

   Youcha, Emily K; Stuefer, Svetlana L (January 2026, Arctic Science)

   Increasing air temperatures are expected to change streamflow patterns in Arctic watersheds. However, the hydro-meteorological datasets necessary to evaluate these changes remain limited in the Arctic. We revised and updated a unique hydrologic dataset from Arctic Alaska with field measurements of continuous streamflow, precipitation, and evapotranspiration to identify changes in the hydrologic cycle. We analyzed water balance components and peak streamflow in Imnavait Creek, Upper Kuparuk River, and Kuparuk River during snowmelt and summer periods. Field measurements show that summer streamflow (mid-June to late-September) has increased in volume and magnitude. While snowmelt runoff still constitutes a sizable portion of annual runoff (28%–58%), the relative proportion of summer runoff is notably increasing. High interannual and spatial variability is observed for both rainfall and summer runoff, whereas comparatively lower interannual variability is observed for snowmelt runoff. This paper compares streamflow changes occurring at three watersheds of varying sizes and identifies reasons for observed changes using snow, rainfall, and evapotranspiration measurements. We also provide hydrologic analysis for design considerations at Arctic Alaska communities, the Dalton Highway, and the oil pipeline; as this infrastructure may be at risk of increased erosional damage due to more frequent and higher magnitude summer flow events. 

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4. Water balance response of permafrost-affected watersheds to changes in air temperatures

   https://doi.org/10.1088/1748-9326/ac12f3  

   Debolskiy, Matvey V; Alexeev, Vladimir A; Hock, Regine; Lammers, Richard B; Shiklomanov, Alexander; Schulla, Joerg; Nicolsky, Dmitry; Romanovsky, Vladimir E; Prusevich, Alexander (August 2021, Environmental Research Letters)

   Abstract Observations show increases in river discharge to the Arctic Ocean especially in winter over the last decades but the physical mechanisms driving these changes are not yet fully understood. We hypothesize that even in the absence of a precipitation increase, permafrost degradation alone can lead to increased annual river runoff. To test this hypothesis we perform 12 millennium-long simulations over an idealized hypothetical watershed (1 km 2 ) using a distributed, physically based water balance model (Water flow and Balance Simulation Model, WaSiM). The model is forced by both a hypothetical warming defined by an air temperature increase of 7.5 ∘ C over 100 years, and a corresponding cooling scenario. To assess model sensitivity we vary soil saturated hydraulic conductivity and lateral subsurface flow configuration. Under the warming scenario, changes in subsurface water transport due to ground temperature changes result in a 7%–14% increase in annual runoff accompanied by a 6%–20% decrease in evapotranspiration. The increase in runoff is most pronounced in winter. Hence, the simulations demonstrate that changes in permafrost characteristics due to climate warming and associated changes in evapotranspiration provide a plausible mechanism for the observed runoff increases in Arctic watersheds. In addition, our experiments show that when lateral subsurface moisture transport is not included, as commonly done in global-scale Earth System Models, the equilibrium water balance in response to the warming or cooling is similar to the water balance in simulations where lateral subsurface transport is included. However, the transient changes in water balance components prior to reaching equilibrium differ greatly between the two. For example, for high saturated hydraulic conductivity only when lateral subsurface transport is considered, a period of decreased runoff occurs immediately after the warming. This period is characterized by a positive change in soil moisture storage caused by the soil moisture deficit developed during prior cooling. 

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5. Surface‐Atmosphere Carbon Dioxide Exchange and Ecosystem Water Use Efficiency in a Multi‐Functional Perennial Polyculture Agroecosystem

   https://doi.org/10.1002/sae2.70182  

   Shrestha, Ojaswee; De_La_Roca, Gabriela; Ramsey, Philip W; Hoffman, Sophie; Stoy, Paul C (June 2026, Journal of Sustainable Agriculture and Environment)

   ABSTRACT Perennial polycultures, with a combination of perenniality and vegetation diversity, have carbon mitigation potential and water conservation benefits, via increased plant carbon uptake and improved ecosystem water use efficiency (EWUE). Few studies have assessed the coupled carbon and water balance in perennial polyculture agroecosystems that are not yet a widespread agricultural practice. We measured net ecosystem carbon dioxide (CO₂) exchange (NEE) and evapotranspiration (ET) using eddy covariance, partitioned measurements into gross primary productivity (GPP), ecosystem respiration (R_ECO), evaporation (E) and transpiration (T), to assess annual carbon sinks and EWUE in a fruit orchard with diverse forbs and grass understory cover in Southern California's Mediterranean climate. Seasonal trends showed peak CO₂uptake and EWUE from late spring through summer. The agroecosystem functioned as a modest yet statistically significant net carbon sink compared to a hypothesised net zero carbon balance (no carbon storage), sequestering 58 g C m⁻² year⁻¹(Z(standardised variability) = −2.41,p = 0.016) in 2022 (January–December) and 174 g C m⁻² year⁻¹(Z = −7.31,p < 0.001) in 2023 (January–October). Smaller net CO₂losses from January to March when trees were dormant underscored the critical role of understory vegetation during winter and early spring in maintaining the net annual carbon sink. Cumulative ET was 664 mm (2022) and 617 mm (January–October 2023) with T dominating E, particularly from April to July during peak tree activity. Monthly EWUE ranged from 1.4 to 4.7 g C m⁻² mm⁻¹H₂O. EWUE stayed above half of the maximum observed value throughout the tree dormant season, beyond October until apricot leaf‐out in April, due to photosynthetically active understory cover. Staggered complementary phenologies between trees and understory cover vegetation can extend the carbon uptake period, making perennial polyculture agroecosystems a net CO₂sink with high water‐use efficiency. 

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