Annual Streamflow Research Articles

Diel streamflow cycles suggest more sensitive snowmelt-driven streamflow to climate change than land surface modeling does

Abstract. Climate warming will cause mountain snowpacks to melt earlier, reducing summer streamflow and threatening water supplies and ecosystems. Quantifying how sensitive streamflow timing is to climate change and where it is most sensitive remain key questions. Physically based hydrological models are often used for this purpose; however, they have embedded assumptions that translate into uncertain hydrological projections that need to be quantified and constrained to provide reliable inferences. The purpose of this study is to evaluate differences in projected end-of-century changes to streamflow timing between a new empirical model based on diel (daily) streamflow cycles and regional land surface simulations across the mountainous western USA. We develop an observational technique for detecting streamflow responses to snowmelt using diel cycles of incoming solar radiation and streamflow to detect when snowmelt occurs. We measure the date of the 20th percentile of snowmelt days (DOS20) across 31 western USA watersheds affected by snow, as a proxy for the beginning of snowmelt-initiated streamflow. Historic DOS20 varies from mid-January to late May among our sites, with warmer basins having earlier snowmelt-mediated streamflow. Mean annual DOS20 strongly correlates with the dates of 25 % and 50 % annual streamflow volume (DOQ25 and DOQ50, both R2=0.85), suggesting that a 1 d earlier DOS20 corresponds with a 1 d earlier DOQ25 and 0.7 d earlier DOQ50. Empirical projections of future DOS20 based on a stepwise multiple linear regression across sites and years under the RCP8.5 scenario for the late 21st century show that DOS20 will occur on average 11±4 d earlier per 1 ∘C of warming. However, DOS20 in colder watersheds (mean November–February air temperature, TNDJF<-8 ∘C) is on average 70 % more sensitive to climate change than in warmer watersheds (TNDJF>0 ∘C). Moreover, empirical projections of DOQ25 and DOQ50 based on DOS20 are about four and two times more sensitive to climate change, respectively, than those simulated by a state-of-the-art land surface model (NoahMP-WRF) under the same scenario. Given the importance of changes in streamflow timing for water resources, and the significant discrepancies found in projected streamflow sensitivity, snowmelt detection methods such as DOS20 based on diel streamflow cycles may help to constrain model parameters, improve hydrological predictions, and inform process understanding.

Hydrologic regime alteration and influence factors in the Jialing River of the Yangtze River, China

Under the influence of climate alteration and human activities, the hydrological regime of rivers is changing dramatically, which has become a hot spot for water environment research. In this study, we quantitatively assessed the degree of hydrological variability of the Jialing River and the contribution of climate alteration and human activities to hydrological alterations using the ecohydrological indicator range of alteration (IHA-RVA) method and Budyko hypothesis formulations. The results showed that (1) The average annual runoff and the precipitation of Jialing River showed a decreasing trend, the potential evapotranspiration showed an increasing trend. (2) Compared with before the hydrological situation changed, the degree of alteration in the annual extreme streamflow is 31%, which is a low degree of alteration; the degree of alteration in monthly streamflow, annual extreme flow magnitude, extreme flow ephemeris, and streamflow alteration frequency are 51%, 43%, 54%, and 64% respectively, which are all moderate degrees of alteration; the overall hydrological alteration is 50%, which belongs to moderate alteration. (3) The contribution of precipitation, potential evapotranspiration, and human activities to the runoff alteration is 61%, − 16%, and 55%, respectively. This study provides corresponding references for ecological restoration and sustainable development of the Yangtze River Basin in China.

Assessment of hydrological response to multiyear drought: Insights from lag characteristics and shift magnitude

AbstractThe analysis of the hydrological response to meteorological drought events can lead to a deeper understanding of alterations in hydrological processes that can be used to help maintain environmental flow (e‐flow) in aquatic ecosystems. In this study, we first identified an extreme meteorological drought event and then assessed its hydrological response. Monthly precipitation anomalies were used to identify meteorological drought, while the Lyne–Hollick method (LH filter) was used alongside the baseflow recession constant (calculated using the Automatic Baseflow Identification Technique [ABIT]) to isolate baseflow in the Baiyangdian basin. Results showed that an extreme meteorological drought event occurred in August 1996, continuing for a total of 127 months, which resulted in prolonged hydrological drought (i.e., streamflow and baseflow). A time lag was generally observed between the end of the meteorological drought event and the end of the hydrological drought event, where baseflow recovery lagged behind streamflow (runoff) recovery. Additionally, drought led to an unstable shift in the precipitation‐streamflow relationship that subsequently led to a reduction in the annual streamflow of coinciding annual precipitation compared with the historical relationship. Drought also led to a longer response between precipitation and runoff events. Results from this study show that the hydrological response to meteorological drought event is important for water resource management and for addressing effects related to climate change, especially extreme drought events.

How Far Can Nature-Based Solutions Increase Water Supply Resilience to Climate Change in One of the Most Important Brazilian Watersheds?

Water resources are paramount for the maintenance of the Earth’s system equilibrium; however, they face various threats and need increased conservation and better management. To restore water resources, nature-based solutions can be applied. Nevertheless, it is unclear which solution promotes greater water supply resilience: restoring riparian vegetation, improving management practices in key areas for water recharge, or both? In addition, how significant are these results in the face of climate change effects? To answer this, we used the SWAT (Soil and Water Assessment Tool) model to simulate and compare four different land use scenarios under three climate conditions (i.e., observed climate and two of the IPCC’s future climate projections). Focusing on key areas contributed more to increasing water supply resilience than forest restoration. Applying both solutions, however, yielded the greatest increases in resilience and groundwater recharge and the greatest decreases in surface runoff and sediment loads. None of the solutions caused a significant difference in streamflow and water yield. Furthermore, according to both of the IPCC climate projections evaluated, by the end of this century, the average annual streamflow will be lower than the historical mean for the region. Climate adaptation strategies alone will be insufficient to ensure future water access, highlighting the need for implementing drastic mitigation actions.

Physically-based distributed modelling of the hydrology and soil erosion under changes in landuse and climate of a humid tropical river basin

The effects of landuse/landcover (LULC) and climate change on hydrology and soil erosion processes are of major concern, especially in the humid tropics. In this study, an evaluation of these changes is performed in a humid tropical catchment (Vamanapuram river basin, India) using a physically based distributed model, SHETRAN. The past landuse maps and climate data from six fine resolution NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP) are used to force the SHETRAN model. A comparison of the change in the rate of soil erosion and hydrological responses during the future climate scenarios (near: 2021–2050 and far: 2071–2100) with respect to the historical period (1980–2005) is conducted.The isolated effects of landuse, climate variability and the combined effect caused change in streamflow (1.1%, 7.9%, 9.0% respectively) and sediment load (−10.4%, 1.7%, −10.5% respectively) in the past. An ensemble mean of general circulation model (GCM) projections showed that mean temperature and average annual precipitation under representative concentration pathway RCP 4.5 (8.5) scenarios for near and far futures increased from the historic period by 0.51 °C (1.6 °C), 0.6 °C (3.2 °C) and by 9% (6%), 15% (22.8%), respectively. Under RCP4.5(8.5), the average annual streamflow increased gradually from the near to far future, by 3.2% (−1.8%) and 13.6 % (22.1%) whereas, the projected sediment load a showed change of −21.05% (−26.63%) in near future while far future indicated change by −11% (4%). The SHETRAN model has been found to be effective in evaluating climate change impacts on hydrology and sediment yield and is useful for future river basin management.

Trend Detection in Annual Streamflow Extremes in Brazil

Changes in streamflow extremes can affect the economy and are likely to impact the most vulnerable in society. Estimating these changes is crucial to develop rational adaptation strategies and to protect society. Streamflow data from 1106 gauges were used to provide a comprehensive analysis of change in eight different extreme indices. The modified trend-free prewhitening and the false discovery rate were used to account for serial correlation and multiplicity in regional analysis, issues shown here to distort the results if not properly addressed. The estimated proportion of gauges with significant trends in low and high flows was about 23% and 15%, respectively. Half of these significant gauges had more than 60 years of data and were associated with changes greater than 5% per decade. A clear spatial pattern was identified, where most increasing trends in both low and high flows were observed in Southern Brazil, and decreasing trends in the remaining regions, except for the Amazon, where a pattern is not clear, and the proportion of significant gauges is low. Results based only on gauges unaffected by reservoirs suggest that reservoirs alone do not explain the increasing trends of low flows in the southern regions nor the decreasing trends in high flows in the remaining hydrographic regions.

Meandering Characteristics of the Yimin River in Hulun Buir Grassland, Inner Mongolia, China

The evolution of meandering rivers continues to attract considerable attention in research and for practical applications, given that it is closely associated with the safety of river systems and riparian zones. There has been much discussion regarding the various channel planform features exhibited by meandering rivers under different river systems and riparian conditions. The Yimin River is a good example and is located southeast of the Hulun Buir Grassland, which is characterised by a fragile ecosystem and little anthropological activity along with active flow during the non-frozen season from May to November each year and relatively low sediment discharge compared with the Yellow River and Mississippi River. Improved analysis of the evolution of the Yimin River from 1975 to 2019 can support increased local species diversity and more effective flood risk and river management. With the combined Google Earth Engine (GEE) platform and the Geographic Information Systems (GIS) technique, remote sensing images, including Landsat images and global surface water data, are used to analyse the channel planform features of the freely meandering river channel in the middle and lower Yimin River. The results show that the percentage of low sinuosity channel bends was higher than that of high-sinuosity bends. Although the bends with an amplitude greater than 0.48 km and sinuosity greater than 2.3 have an evident upstream-skewed trend, the main channel planform features were downstream skewed with 1499 such bends. The river system conditions in the Yimin River, including lower sediment discharge and vegetation cover, are conducive to the development of downstream-skewed bends. The high-sinuosity bends were found to have a relatively larger ratio during 1981–2000, a period with higher mean annual streamflow compared with other time periods.

Variable Streamflow Response to Forest Disturbance in the Western US: A Large‐Sample Hydrology Approach

AbstractForest cover and streamflow are generally expected to vary inversely because reduced forest cover typically leads to less transpiration and interception. However, recent studies in the western U.S. have found no change or even decreased streamflow following forest disturbance due to drought and insect epidemics. We investigated streamflow response to forest cover change using hydrologic, climatic, and forest data for 159 watersheds in the western U.S. from the CAMELS data set for the period 2000–2019. Forest change and disturbance were quantified in terms of net tree growth (total growth volume minus mortality volume) and mean annual mortality rates, respectively, from the U.S. Forest Service's Forest Inventory and Analysis database. Annual streamflow was analyzed using multiple methods: Mann‐Kendall trend analysis, time trend analysis to quantify change not attributable to annual precipitation and temperature, and multiple regression to quantify contributions of climate, mortality, and aridity. Many watersheds exhibited decreased annual streamflow even as forest cover decreased. Time trend analysis identified decreased streamflow not attributable to precipitation and temperature changes in many disturbed watersheds, yet streamflow change was not consistently related to disturbance, suggesting drivers other than disturbance, precipitation, and temperature. Multiple regression analysis indicated that although change in streamflow is significantly related to tree mortality, the direction of this effect depends on aridity. Specifically, forest disturbances in wet, energy‐limited watersheds (i.e., where annual potential evapotranspiration [PET] is less than annual precipitation) tended to increase streamflow, while post‐disturbance streamflow more frequently decreased in dry water‐limited watersheds (where the PET to precipitation ratio exceeds 2.35).

Event and Catchment Controls of Heavy Tail Behavior of Floods

AbstractIn some catchments, the distribution of annual maximum streamflow shows heavy tail behavior, meaning the occurrence probability of extreme events is higher than if the upper tail decayed exponentially. Neglecting heavy tail behavior can lead to an underestimation of the likelihood of extreme floods and the associated risk. Partly contradictory results regarding the controls of heavy tail behavior exist in the literature and the knowledge is still very dispersed and limited. To better understand the drivers, we analyze the upper tail behavior and its controls for 480 catchments in Germany and Austria over a period of more than 50 years. The catchments span from quickly reacting mountain catchments to large lowland catchments, allowing for general conclusions. We compile a wide range of event and catchment characteristics and investigate their association with an indicator of the tail heaviness of flood distributions, namely the shape parameter of the GEV distribution. Following univariate analyses of these characteristics, along with an evaluation of different aggregations of event characteristics, multiple linear regression models, as well as random forests, are constructed. A novel slope indicator, which represents the relation between the return period of flood peaks and event characteristics, captures the controls of heavy tails best. Variables describing the catchment response are found to dominate the heavy tail behavior, followed by event precipitation, flood seasonality, and catchment size. The pre‐event moisture state in a catchment has no relevant impact on the tail heaviness even though it does influence flood magnitudes.

Does the recent afforestation program in Ethiopia influenced vegetation cover and hydrology? A case study in the upper awash basin, Ethiopia

The Ethiopian government planned afforestation programs in the past decades whereas more attention was given to tree plantation since the 2010 year. However, the effectiveness of the afforestation programs and its impacts on vegetation cover and hydrology has not been well studied. This study aims to assess the recent campaigned afforestation program and its impact on vegetation cover and hydrology in the upper Awash basin, Ethiopia. Landsat 8 images of 2013–2020 years were used to calculate the NDVI for the upper Awash basin to assess trends in vegetation greenness for the basin. Moreover, observed streamflow and precipitation datasets of the basin were collected and used for assessing the impact of the afforestation on hydrology. The study result showed decreasing NDVI values despite the afforestation programs in the upper Awash basin. This shows either afforestation rate was less than the deforestation rate or the tree plantation campaign was not effective in the basin. In addition, the campaign based tree plantation focused on the number of tree planted not on how many trees are grown. On the other hand, mean annual precipitation and streamflow were generally increased from 2013 to 2020 in the upper Awash basin. Declining NDVI values but increasing mean annual precipitation in the Awash basin indicated that the declining vegetation was attributed to anthropogenic effects. The increasing streamflow during the same time could be due to the increasing mean annual precipitation. Moreover, the decreasing vegetation cover might have contributed for the increasing streamflow through increasing surface runoff and decreasing transpiration. However, further research is required to assess the precise impacts of afforestation on vegetation cover and hydrologic processes. Generally, the study result showed that the focus of afforestation should be on tree growing than on tree plantation alone.

Metrics Assessment and Streamflow Modeling under Changing Climate in a Data-Scarce Heterogeneous Region: A Case Study of the Kabul River Basin

Due to many uncertainties in hydrological data and modeling, the findings are frequently regarded as unreliable, especially in heterogeneous catchments such as the Kabul River Basin (KRB). Besides, statistical methods to assess the performance of the models have also been called into doubt in several studies. We evaluated the performance of the Soil and Water Assessment Tool (SWAT) model by statistical indicators including the Kling-Gupta efficiency (KGE), Nash–Sutcliffe efficiency (NSE), and the coefficient of determination (R2) at single and multi-outlets in the KRB and assessed the streamflow under changing climate scenarios i.e., Representative Concentration Pathways (RCP) 4.5 and 8.5 (2020–2045). Because of the heterogeneous nature of the KRB, NSE and R2 performed poorly at multi-outlets. However, the KGE, as the basic objective function, fared much better at single-outlet. We conclude that KGE is the most crucial metric for streamflow evaluation in heterogeneous basins. Similarly, the mean and maximum annual streamflow is projected to decrease by 15.2–15.6% and 17.2–41.8% under the RCP 4.5 and 8.5, respectively.

Evaluation of InVEST’s Water Ecosystem Service Models in a Brazilian Subtropical Basin

The biophysical modeling of water ecosystem services is crucial to understanding their availability, vulnerabilities, and fluxes. Among the most popular models, the Integrated Valuation of Ecosystem Services and Trade-offs (InVEST) models stand out. While many studies have used them, few have assessed their performance. This study evaluates the performance of InVEST’s Seasonal Water Yield, Nutrient Delivery Ratio, and Sediment Delivery Ratio models in a subtropical basin in southeastern Brazil on temporal and spatial scales, using 39 years of streamflow data, 29 for total phosphorus and total nitrogen, and 19 for total suspended solids. Statistical indicators R2, PBIAS, and NSE, were also calculated. The performance of the models varied according to the type of simulated WES and analysis scales used, with the Seasonal Water Yield model demonstrating the best performance and effectively representing the spatial and temporal variability of the average annual streamflow. All models performed well in simulating long-term mean values when compared to observed data. While one should bear in mind the study’s limitations, the results indicate that the models perform well in terms of relative magnitude, although their application in studies involving water-resource management and decision making is limited.

Increasing and decreasing trends in extreme annual streamflow in the Godavari catchment, India

Increasing and decreasing trends in extreme annual streamflow in the Godavari catchment, India

Effect of climate change on streamflow in the Gelana watershed, Rift valley basin, Ethiopia

Abstract Climate change is one of the current global threats and the topmost challenges. This study aims to investigate the climate change effect on streamflow in the Gelana watershed using the soil and water assessment tool (SWAT) model for three consecutive periods of 2031–2050, 2051–2070, and 2071–2090. Climate variables were downscaled from two regional climate models (RCMs) (RACMO22T and RCA4) from CORDEX-Africa under representative concentration pathway (RCP4.5 and RCP8.5) scenarios. RCMs were evaluated using four statistical indicators and performed very well. Power transformation and distribution mapping methods were used to correct biases of precipitation and temperatures, respectively. The 19 SWAT model-sensitive parameters were transferred from the gauged donor watersheds to the ungauged watershed outlet by using the principal component analysis coupled with the stepwise multiple linear regression. The ensemble mean of RCMs revealed that the maximum and minimum temperatures and potential evapotranspiration were predicted to increase up to 3.48 °C, 4.19 °C, and 17.85%, respectively, in the period of 2071–2090 under the RCP8.5 scenario. These changes translate to possible reductions in the mean annual rainfall and streamflow up to 15.12 and 44.14%, respectively, with a consequent higher decline of surface runoff by 22.23%, groundwater by 42.54%, and water yield by 35.89% in the period of 2051–2070 under the RCP4.5 scenario. The projected rainfall and streamflow are expected to face a higher decline in wet seasons. Detections of trends in hydro-climatic variables were performed by using the Mann–Kendall test. Hence, these projection scenarios should be of interest to river users and water resource managers in the Gelana watershed.

Evaluating long‐term One‐Way Atmosphere‐Hydrology simulations and the impacts of Two‐Way coupling in four mountain watersheds

AbstractJoint hydrologic‐atmospheric model frameworks offer novel insights into the terrestrial hydrologic cycle and the potential for improved predictive capabilities for stream discharge and other hydrologic fluxes. In this study, we examine both one‐ and two‐way coupled integrations of the Weather Research and Forecasting (WRF v3.8.1) atmospheric and WRF‐Hydro (v5.0) hydrologic models for four 1000–2000 km2 snow‐dominated mountain watersheds (1500–2100 m mean elevation) in Idaho's Rocky Mountains. In watersheds where anthropogenic withdrawals are minimal (3 of 4 watersheds), we simulate stream discharge with high confidence (KGE > 0.63) for a 20 year period in the uncoupled scenarios, and find that WRF winter precipitation accumulations have less than 15% average error for all but two of the fourteen comparison NRCS Snotel sites. However, annual streamflow biases are highly correlated (r2 > 0.8 in some cases) with the annual errors in WRF cold‐season precipitation, suggesting that process representation of winter orographic precipitation limits hydrologic predictability. In the second part of the study, we evaluate the potential for ‘two‐way’ model coupling to influence hydrologic predictability by examining a 2 month case‐study period with active spring season convective precipitation. We quantify the impacts of resolving hillslope‐scale soil water redistribution on the ABL, and find that while resolving overland and saturated subsurface soil moisture flow influences soil moisture distributions and surface energy fluxes, the impact on precipitation is non‐systematic, as precipitation is generally atmospherically controlled during the study period. Consequently, future efforts should focus on improving winter orographic process representation, as streamflow is highly sensitive to errors in these processes.

Influence of warming and atmospheric circulation changes on multidecadal European flood variability

Abstract. European flood frequency and intensity change on a multidecadal scale. Floods were more frequent in the 19th (central Europe) and early 20th century (western Europe) than during the mid-20th century and again more frequent since the 1970s. The causes of this variability are not well understood and the relation to climate change is unclear. Palaeoclimate studies from the northern Alps suggest that past flood-rich periods coincided with cold periods. In contrast, some studies suggest that more floods might occur in a future, warming world. Here we address the contribution of atmospheric circulation and of warming to multidecadal flood variability. For this, we use long series of annual peak streamflow, daily weather data, reanalyses, and reconstructions. We show that both changes in atmospheric circulation and moisture content affected multidecadal changes of annual peak streamflow in central and western Europe over the past two centuries. We find that during the 19th and early 20th century, atmospheric circulation changes led to high peak values of moisture flux convergence. The circulation was more conducive to strong and long-lasting precipitation events than in the mid-20th century. These changes are also partly reflected in the seasonal mean circulation and reproduced in atmospheric model simulations, pointing to a possible role of oceanic variability. For the period after 1980, increasing moisture content in a warming atmosphere led to extremely high moisture flux convergence. Thus, the main atmospheric driver of flood variability changed from atmospheric circulation variability to water vapour increase.

Trends in the Frequency of Water and Heat Stress in Mid-Latitude North America since 1980

The water-energy balance of many mid-latitude watersheds has been changing in recent decades due to global warming. These changes manifest themselves over both long timescales (e.g., hydrologic drought) and short timescales (e.g., agricultural drought) and may be ameliorated or exacerbated by vegetative response. We apply a Budyko framework to assess short-term response to long-term trends in water and heat stress (HS) across mid-latitude North America. Using high-resolution meteorological data and streamflow records, we calculate the frequency of HS every year since 1980 for every gaged watershed with adequate data (n = 1528). We find that HS has become more frequent in most watersheds in the western US, New England, and southeastern Canada. However, we find that HS has become less frequent in the Midwest and the relatively humid eastern US. By assessing the relationship between trends in HS frequency and proximate forcing variables (annual PPT, annual streamflow, minimum and maximum daily temperatures, actual evapotranspiration, and potential evapotranspiration), we find that these trends in HS frequency are primarily driven by meteorological forcings rather than vegetative response. Finally, we contextualize our findings within the Budyko framework, which assumes a landscape in equilibrium with its climate, with the implication that these trends in HS are only likely to be realized after local vegetation has adapted to new meteorological norms.

Stability and variability of long‐term streamflow and its components in watersheds under vegetation restoration on the Chinese Loess Plateau

AbstractAssessing ecological restoration effects on hydrological regimes is important for water resources management, especially under semi‐arid conditions. Here, the stability and long‐term variability of streamflow components were analyzed for three less forested watersheds (LFWs, vegetation coverage 16–23% in 1970) undergoing major vegetation restoration (57–80% coverage in 2019), and for two largely forested watersheds (FWs, 65–68% cover in 1970 increasing to 88–92% in 2019) within the Beiluo River Basin on the Chinese Loess Plateau. Daily flow data for each watershed and year were normalized by rainfall to eliminate effects of non‐stationary rainfall. Mean annual streamflow totals for the ~60‐year study period were 25.1–34.1 and 21.6–48.1 mm y−1 for the LFWs and FWs, respectively. Average contributions of baseflow to total streamflow were 32–44% for the LFWs and 58–61% for the FWs. Mann‐Kendall tests showed significant decreasing trends for annual streamflow and stormflow (0.23–0.54% y−1) from the LFWs throughout. Mean streamflow from the LFWs between 2000 and 2019 decreased by 58% compared to the pre‐2000 period, while the average contribution of stormflow decreased from 66% to 35% (2010s). However, winter baseflow increased with time since start of restoration. Conversely, streamflow (components) for the FWs showed only slight fluctuations in decadal, annual and seasonal trends. Both total streamflow and stormflow exhibited strong power relationships with vegetation coverage, illustrating a tendency towards stable flow regimes for vegetation coverage >60–70%. Large‐scale vegetation restoration has fundamentally changed amounts and temporal distribution of streamflow (components) on the Loess Plateau mainly by regulating stormflow. After 20 years of restoration, the trend and proportion of streamflow components from the LFWs approximated those of the FWs. These results underpin the planning of sustainable management of natural resources vis‐á‐vis socio‐economic development during long‐term ecological restoration on the Loess Plateau.

Controls on Stream Water Age in a Saturation Overland Flow‐Dominated Catchment

AbstractWater age and flow pathways should be related; however, it is still generally unclear how integrated catchment runoff generation mechanisms result in streamflow age distributions at the outlet. Here, we combine field observations of runoff generation at the Dry Creek catchment with StorAge Selection (SAS) age models to explore the relationship between stream water age and runoff pathways. Dry Creek is a 3.5 km2catchment in the Northern California Coast Ranges with a Mediterranean climate, and, despite an average rainfall of ≈1,800 mm/yr, is an oak savannah due to the limited hillslope water storage capacity. Runoff lag to peak—after initial seasonal wet‐up—is rapid (∼1–2 hr), and total annual streamflow consists predominantly of saturation overland flow, based on field mapping of saturated extents and an inferred runoff threshold for the expansion of saturation extent beyond the geomorphic channel. SAS modeling based on daily isotope sampling reveals that streamflow is typically older than 1 day. Since streamflow primarily consists of overland flow, a significant portion of overland flow must not be event‐rain but instead derive from older, nonevent groundwater returning to the surface, consistent with field observations of exfiltrating head gradients, return flow through macropores, and extensive saturation days after storm events. We conclude that even in a watershed fed primarily by overland flow, runoff is primarily not composed of event water. Our findings have implications for the interpretation of stream chemistry and the assumptions built into widely used hydrograph separation inferences, namely, the assumption that overland flow consists of new (event) water.

An Assessment of the Impacts of Snowmelt Rate and Continuity Shifts on Streamflow Dynamics in Three Alpine Watersheds in the Western U.S.

In semiarid to arid regions of the western U.S., river flow availability and variability are highly subject to shifts in snow accumulation and ablation in alpine watersheds. This study aims to examine how shifts in snowmelt rate (SMR) and snow continuity, an indicator of the consistent existence of snow on the ground, affect snow-driven streamflow dynamics in three alpine watersheds in the U.S. Great Basin. To achieve this end, the coupled hydro-ecological simulation system (CHESS) is used to simulate river flow dynamics, and multiple snow metrics are calculated to quantify the variation of SMR and snow continuity, the latter of which is measured by snow persistence (SP), snow residence time (SRT), and snow season length (SSL). Then, a new approach is proposed to partition streamflow into snow-driven and rain-driven streamflow. The statistical analyses indicate that the three alpine watersheds experienced a downward trend in SP, SRT, SSL, and SMR during the study period of 1990–2016 due to regional warming. As a result, the decrease in SMR and the decline in snow continuity shifted the occurrence day of 25% and 50% of the snow-driven cumulative discharge, as well as peak discharge, toward an earlier occurrence. Moreover, the magnitudes of snow-driven annual streamflow, summer baseflow, and peak discharge also decreased due to the declined snow continuity and the reduced SMR. Overall, by using multiple snow and flow metrics, and by partitioning streamflow into snow-driven and rain-driven flow via the newly proposed approach, we found that SMR and snow continuity determine the streamflow hydrographs and magnitudes in the three alpine watersheds. Given that warming can significantly affect snow dynamics in alpine watersheds in semiarid to arid regions, this has important implications for water resource management in the snow-dominated region when facing future climate warming.