Seasonal snow cover decreases young water fractions in high Alpine catchments

Estimation of young water fractions (F_yw), defined as the fraction of water in a stream younger than approximately 2–3 months, provides key information for water resource management in catchments where runoff is dominated by snowmelt. Knowing the average dependence of summer flow on winter precipitation is an essential context for comparing regional drought severity and provides the hydrological template for downstream water users and ecosystems. However, F_yw estimation based on seasonal signals of stable isotopes of oxygen and hydrogen has not yet explicitly addressed how to parsimoniously include the seasonal shift of water input from snow. Using experimental data from three high-elevation, Alpine catchments (one dominated by glacier and two by snow), we propose a framework to explicitly include the delays induced by snow storage into estimates of F_yw. Scrutinizing the key methodological choices when estimating F_yw from isotope data, we find that the methods used to construct precipitation input signals from sparse isotope samples can significantly impact F_yw. Given this sensitivity, our revised procedure estimates a distribution of F_yw values that incorporates a wide range of possible methodological choices and their uncertainties; it furthermore compares the commonly used amplitude ratio approach to a direct convolution approach, which circumvents the assumption that the isotopic signals have a sine curve shape, an assumption that is generally violated in snow-dominated environments. Our new estimates confirm that high-elevation Alpine catchments have low F_yw values, spanning from 8 to 11%. Such low values have previously been interpreted as the impact of seasonal snow storage alone, but our comparison of different F_yw estimation methods suggests that these low F_yw values result from a combination of both snow cover effects and longer storage in the subsurface. In contrast, in the highest elevation, glacier dominated catchment, F_yw is 3–4 times greater compared to the other two catchments, due to the lower storage and faster drainage processes. A future challenge, capturing spatio-temporal snowmelt isotope signals during winter baseflow and the snowmelt period, remains to improve constraints on the F_yw estimation technique.     

Linking transit times to catchment sensitivity to atmospheric deposition of acidity and nitrogen in mountains of the western United States

Transit times are hypothesized to influence catchment sensitivity to atmospheric deposition of acidity and nitrogen (N) because they help determine the amount of time available for infiltrating precipitation to interact with catchment soil and biota. Transit time metrics, including fraction of young water (F_yw) and mean transit time (MTT), were calculated for 11 headwater catchments in mountains of the western United States based on differences in the amplitude of the seasonal signal of δ¹⁸O in streamflow and precipitation. Results were statistically compared with catchment characteristics to elucidate controlling mechanisms. Transit times also were compared with stream solute concentrations to test the hypothesis that transit times are a primary influence on weathering rates and biological assimilation of atmospherically deposited N. Results indicate that transit times in the study catchments are strongly related to soil, vegetation, and topographic characteristics, with barren terrain (bare rock and talus) and steep slopes linked to high F_yw and short MTT, whereas forest soil (hydrogroup B) was linked to low F_yw and greater MTT. Concentrations of silicate weathering products (Na⁺ and Si) were negatively related to F_yw and barren terrain, and positively related to MTT and forest soil, supporting the concept that weathering fluxes and buffering capacity tend to be low in alpine areas due to short transit times. Nitrate concentrations were positively related to N deposition, catchment slope, and barren terrain, and negatively related to forest, indicating that hydrologic and/or biogeochemical processes associated with steep slopes limit uptake of atmospherically deposited N by biota. Interannual and seasonal variability in transit times and source water contributions in the study catchments was substantial, reflecting the influence of strong temporal variations in snowmelt inputs in high‐elevation catchments of the western United States. Results from this study confirm that short transit times in these areas are a key reason they are highly sensitive to atmospheric pollution and climate change.     

Investigating young water fractions in a small Mediterranean mountain catchment: Both precipitation forcing and sampling frequency matter

The proportion of water younger than 2–3 months (young water fraction, F_yw) has become increasingly investigated in catchment hydrology. F_yw is typically estimated by comparing seasonal tracer cycles in precipitation and streamflow, through water sampling. However, some open research questions remain, such as: (i) whether part of the summer precipitation should be discarded because the high evapotranspiration demand, (ii) how well F_yw serves as a metric to compare catchments, and (iii) how sampling frequency affects F_yw estimates. To address these questions, we investigated F_yw in soil-, ground- and stream waters for the small Mediterranean Can Vila catchment. Rainfall was sampled at 5-mm intervals. Mobile soil water and groundwater were sampled fortnightly. Stream water was sampled depending on flow at variable time intervals (30 min to 1 week). Over 58 months, this sampling provided 1,529 δ¹⁸O determinations. Isotopic analyses results led us to include summer precipitation in the input signal. We found the highest F_yw in mobile soil waters (34%), while this was almost zero for groundwater except during wet periods. For stream waters, F_yw depended on the discharge variations, so that the flow-weighted young water fraction () was 22.6%, whereas the time-weighted F_yw was just 6.2%. Both and its discharge sensitivity (S_d) varied when different 12-month sampling periods were investigated. The young water fraction that would be obtained from a virtual thorough sampling () was estimated from the S_d and the observed stream flow. This showed an underestimation of by 25% for the frequent dynamic sampling and 66% for weekly sampling, due to missing high flows. Our results confirm that F_yw and its discharge sensitivity are metrics very sensitive to meteorological forcing during the analysed period. Thus, comparisons between catchments need long-term mean annual values and their variability. Our findings also support the dependence of F_yw estimates on the sampling rate and show the advantages of flow-weighted analysis. Finally, catchment water turnover investigations should be accompanied by the analysis of flow duration curves.     

Bedrock geology controls on catchment storage,mixing, and release: A comparative analysis of 16 nested catchments

The bedrock controls on catchment mixing, storage, and release have been actively studied in recent years. However, it has been difficult to find neighbouring catchments with sufficiently different and clean expressions of geology to do comparative analysis. Here, we present new data for 16 nested catchments (0.45 to 410 km²) in the Alzette River basin (Luxembourg) that span a range of clean and mixed expressions of schists, phyllites, sandstones, and quartzites to quantify the relationships between bedrock permeability and metrics of water storage and release. We examined 9 years' worth of precipitation and discharge data, and 6 years of fortnightly stable isotope data in streamflow, to explore how bedrock permeability controls (a) streamflow regime metrics, (b) catchment storage, and (c) isotope response and catchment mean transit time (MTT). We used annual and winter precipitation–run‐off ratios, as well as average summer and winter precipitation–run‐off ratios to characterise the streamflow regime in our 16 study catchments. Catchment storage was then used as a metric for catchment comparison. Water mixing potential of 11 catchments was quantified via the standard deviation in streamflow δD (σδD) and the amplitude ratio (A_S/A_P) of annual cycles of δ¹⁸O in streamflow and precipitation. Catchment MTT values were estimated via both stable isotope signature damping and hydraulic turnover calculations. In our 16 nested catchments, the variance in ratios of summer versus winter average run‐off was best explained by bedrock permeability. Whereas active storage (defined here as a measure of the observed maximum interannual variability in catchment storage) ranged from 107 to 373 mm, total catchment storage (defined as the maximum catchment storage connected to the stream network) extended up to ~1700 mm (±200 mm). Catchment bedrock permeability was strongly correlated with mixing proxies of σδD in streamflow and δ¹⁸O A_S/A_P ratios. Catchment MTT values ranged from 0.5 to 2 years, based on stable isotope signature damping, and from 0.5 to 10 years, based on hydraulic turnover.     

Isotopic evidence for widespread cold‐season‐biased groundwater recharge and young streamflow across central Canada

Transformations of precipitation into groundwater and streamflow are fundamental hydrological processes, critical to irrigated agriculture, hydroelectric power generation, and ecosystem health. Our understanding of the timing of groundwater recharge and streamflow generation remains incomplete, limiting our ability to predict fresh water, nutrient, and contaminant fluxes, especially in large basins. Here, we analyze thousands of rain, snow, groundwater, and streamflow δ¹⁸O and δ²H values in the Nelson River basin, which covers 1.2 million km² of central Canada. We show that the fraction of precipitation that recharges aquifers is ~1.3–5 times higher for precipitation falling during cold months with subzero mean monthly temperatures than for precipitation falling during warmer months. The near‐ubiquity of cold‐season‐biased groundwater recharge implies that changes to winter water balances may have disproportionate impacts on annual groundwater recharge rates. We also show that young streamflow—defined as precipitation that enters a river in less than ~2.3 months—comprises ~27% of annual streamflow but varies widely among tributaries in the Nelson River basin (1–59%). Young streamflow fractions are lower in steep catchments and higher in flatter catchments such as the transboundary Red River basin. Our findings imply that flat, lower permeability, heavily tiled landscapes favor more rapid transmission of precipitation into rivers, possibly mobilizing excess soluble fertilizers and exacerbating eutrophication events in Lake Winnipeg.     

Simulating wind-affected snow accumulations at catchment to basin scales

In non-forested mountain regions, wind plays a dominant role in determining snow accumulation and melt patterns. A new, computationally efficient algorithm for distributing the complex and heterogeneous effects of wind on snow distributions was developed. The distribution algorithm uses terrain structure, vegetation, and wind data to adjust commonly available precipitation data to simulate wind-affected accumulations. This research describes model development and application in three research catchments in the Reynolds Creek Experimental Watershed in southwest Idaho, USA. All three catchments feature highly variable snow distributions driven by wind. The algorithm was used to derive model forcings for Isnobal, a mass and energy balance distributed snow model. Development and initial testing took place in the Reynolds Mountain East catchment (0.36 km²) where R² values for the wind-affected snow distributions ranged from 0.50 to 0.67 for four observation periods spanning two years. At the Upper Sheep Creek catchment (0.26 km²) R² values for the wind-affected model were 0.66 and 0.70. These R² values matched or exceeded previously published cross-validation results from regression-based statistical analyses of snow distributions in similar environments. In both catchments the wind-affected model accurately located large drift zones, snow-scoured slopes, and produced melt patterns consistent with observed streamflow. Models that did not account for wind effects produced relatively homogenous SWE distributions, R² values approaching 0.0, and melt patterns inconsistent with observed streamflow. The Dobson Creek (14.0 km²) application incorporated elevation effects into the distribution routine and was conducted over a two-dimensional grid of 6.67 × 10⁵ pixels. Comparisons with satellite-derived snow-covered-area again demonstrated that the model did an excellent job locating regions with wind-affected snow accumulations. This final application demonstrated that the computational efficiency and modest data requirements of this approach are ideally suited for large-scale operational applications.     

Elevation‐dependent responses of streamflow to climate warming

Warming will affect snowline elevation, potentially altering the timing and magnitude of streamflow from mountain landscapes. Presently, the assessment of potential elevation‐dependent responses is difficult because many gauged watersheds integrate drainage areas that are both snow and rain dominated. To predict the impact of snowline rise on streamflow, we mapped the current snowline (1980 m) for the Salmon River watershed (Idaho, USA) and projected its elevation after 3 °C warming (2440 m). This increase results in a 40% reduction in snow‐covered area during winter months. We expand this analysis by collecting streamflow records from a new, elevation‐stratified gauging network of watersheds contained within high (2250–3800 m), mid (1500–2250 m) and low (300–1500 m) elevations that isolate snow, mixed and rain‐dominated precipitation regimes. Results indicate that lags between percentiles of precipitation and streamflow are much shorter in low elevations than in mid‐ and high‐elevation watersheds. Low elevation annual percentiles (Q₂₅ and Q₇₅) of streamflow occur 30–50 days earlier than in higher elevation watersheds. Extreme events in low elevations are dominated by low‐ and no‐flow events whereas mid‐ and high‐elevation extreme events are primarily large magnitude floods. Only mid‐ and high‐elevation watersheds are strongly cross correlated with catchment‐wide flow of the Salmon River, suggesting that changes in contributions from low‐elevation catchments may be poorly represented using mainstem gauges. As snowline rises, mid‐elevation watersheds will likely exhibit behaviours currently observed only at lower elevations. Streamflow monitoring networks designed for operational decision making or change detection may require modification to capture elevation‐dependent responses of streamflow to warming. Copyright © 2014 John Wiley & Sons, Ltd.     

Assessing basin storage: Comparison of hydrometric- and tracer-based indices of dynamic and total storage

Storage is a fundamental but elusive component of drainage basin function, influencing synchronization between precipitation input and streamflow output and mediating basin sensitivity to climate and land use/land cover (LULC) change. We compare hydrometric and isotopic approaches to estimate indices of dynamic and total basin storage, respectively, and assess inter-basin differences in these indices across the Oak Ridges Moraine (ORM) region of southern Ontario, Canada. Dynamic storage indices for the 20 study basins included the ratio of baseflow to total streamflow (baseflow index BFI), Q ₉₉ flow and flow duration curve (FDC) slope. Ratios of the standard deviation of the streamflow stable isotope signal relative to that of precipitation were determined for each basin from a 1 year bi-weekly sampling program and used as indicators of total storage. Smaller ratios imply longer water travel times, smaller young water fractions (F _yw, < ~2–3 months in age) in streamflow and greater basin storage. Ratios were inversely related to BFI and Q ₉₉, and positively related to FDC slope, suggesting longer travel times and smaller F _yw for basins with stable baseflow-dominated streamflow regimes. Inter-basin differences in all indices reflected topographic, hydrogeologic and LULC controls on storage, which was greatest in steep, forest-covered headwaters underlain by permeable deposits with thick and relatively uniform unsaturated zones. Nevertheless, differential sensitivity of indices to controls on storage indicates the value of using several indices to capture more completely how basin characteristics influence storage. Regression relationships between storage indices and basin characteristics provided reasonable predictions of aspects of the streamflow regime of test basins in the ORM region. Such relationships and the underlying knowledge of controls on basin storage in this landscape provide the foundation for initial predictions of relative differences in streamflow response to regional changes in climate and LULC.     

Bedrock infiltration and mountain block recharge accounting using chloride mass balance

Mountain front catchment net groundwater recharge (NR) represents the upper end of mountain block recharge (MBR) groundwater flow paths. Using environmental chloride in precipitation, streamflow and groundwater, we apply chloride mass balance (CMB) to estimate NR at multiple catchment scales within the 27 km² Dry Creek Experimental Watershed (DCEW) on the Boise Front, southwestern Idaho. The estimate for average annual precipitation partitioning to NR is approximately 14% for DCEW. In contrast, as much as 44% of annual precipitation routes to NR in ephemeral headwater catchments. NR in headwater catchments is likely routed to downgradient springs, baseflow, and MBR, while downgradient streamflow losses contribute further to MBR. A key assumption in the CMB approach is that the change in stored chloride during the study period is zero. We found that this assumption is violated in some individual years, but that a 5‐year integration period is sufficient. Copyright © 2011 John Wiley & Sons, Ltd.     

Controls of streamflow generation in small catchments across the snow–rain transition in the Southern Sierra Nevada,California

Processes controlling streamflow generation were determined using geochemical tracers for water years 2004–2007 at eight headwater catchments at the Kings River Experimental Watersheds in southern Sierra Nevada. Four catchments are snow‐dominated, and four receive a mix of rain and snow. Results of diagnostic tools of mixing models indicate that Ca²⁺, Mg²⁺, K⁺ and Cl^? behaved conservatively in the streamflow at all catchments, reflecting mixing of three endmembers. Using endmember mixing analysis, the endmembers were determined to be snowmelt runoff (including rain on snow), subsurface flow and fall storm runoff. In seven of the eight catchments, streamflow was dominated by subsurface flow, with an average relative contribution (% of streamflow discharge) greater than 60%. Snowmelt runoff contributed less than 40%, and fall storm runoff less than 7% on average. Streamflow peaked 2–4 weeks earlier at mixed rain–snow than snow‐dominated catchments, but relative endmember contributions were not significantly different between the two groups of catchments. Both soil water in the unsaturated zone and regional groundwater were not significant contributors to streamflow. The contributions of snowmelt runoff and subsurface flow, when expressed as discharge, were linearly correlated with streamflow discharge (R² of 0.85–0.99). These results suggest that subsurface flow is generated from the soil–bedrock interface through preferential pathways and is not very sensitive to snow–rain proportions. Thus, a declining of the snow–rain ratio under a warming climate should not systematically affect the processes controlling the streamflow generation at these catchments. Copyright © 2012 John Wiley & Sons, Ltd.     

Spatial GR4J conceptualization of the Tamor glaciated alpine catchment in Eastern Nepal: evaluation of GR4JSG against streamflow and MODIS snow extent

Snow and glacial melt processes are an important part of the Himalayan water balance. Correct quantification of melt runoff processes is necessary to understand the region's vulnerability to climate change. This paper describes in detail an application of conceptual GR4J hydrological model in the Tamor catchment in Eastern Nepal using typical elevation band and degree‐day factor approaches to model Himalayan snow and glacial melt processes. The model aims to provide a simple model that meets most water planning applications. The paper contributes a model conceptualization (GR4JSG) that enables coarse evaluation of modelled snow extents against remotely sensed Moderate Resolution Imaging Spectroradiometer snow extent. Novel aspects include the glacial store in GR4JSG and examination of how the parameters controlling snow and glacial stores correlate with existing parameters of GR4J. The model is calibrated using a Bayesian Monte Carlo Markov Chain method against observed streamflow for one glaciated catchment with reliable data. Evaluation of the modelled streamflow with observed streamflow gave Nash Sutcliffe Efficiency of 0.88 and Percent Bias of <4%. Comparison of the modelled snow extents with Moderate Resolution Imaging Spectroradiometer gave R² of 0.46, with calibration against streamflow only. The contribution of melt runoff to total discharge from the catchment is 14–16% across different experiments. The model is highly sensitive to rainfall and temperature data, which suffer from known problems and biases, for example because of stations being located predominantly in valleys and at lower elevations. Testing of the model in other Himalayan catchments may reveal additional limitations. © 2016 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.     

Evaluating the effect of snow and ice melt in an Alpine headwater catchment and further downstream in the River Rhine

Abstract

The runoff regime of glacierized headwater catchments in the Alps is essentially characterized by snow and ice melt. High Alpine drainage basins influence distant downstream catchments of the Rhine River basin. In particular, during the summer months, low-flow conditions are probable with strongly reduced snow and ice melt under climate change conditions. This study attempts to quantify present and future contributions from snow and ice melt to summer runoff at different spatial scales. For the small Silvretta catchment (103 km²) in the Swiss Alps, with a glacierization of 7%, the HBV model and the glacio-hydrological model GERM are applied for calculating future runoff based on different regional climate scenarios. We evaluate the importance of snow and ice melt in the runoff regime. Comparison of the models indicates that the HBV model strongly overestimates the future contribution of glacier melt to runoff, as glaciers are considered as static components. Furthermore, we provide estimates of the current meltwater contribution of glaciers for several catchments downstream on the River Rhine during the month of August. Snow and ice melt processes have a significant direct impact on summer runoff, not only for high mountain catchments, but also for large transboundary basins. A future shift in the hydrological regime and the disappearance of glaciers might favour low-flow conditions during summer along the Rhine.

Citation Junghans, N., Cullmann, J. & Huss, M. (2011) Evaluating the effect of snow and ice melt in an Alpine headwater catchment and further downstream in the River Rhine. Hydrol. Sci. J. 56(6), 981–993.     

Water isotopes and hydrograph separation in different glacial catchments in the southeast margin of the Tibetan Plateau

Snow and glaciers are known to be important sources for freshwater; nevertheless, our understanding of the hydrological functioning of glacial catchments remains limited when compared with lower altitude catchments. In this study, a temperate glacial region located in the southeast margin of the Tibetan Plateau is selected to analyse the characteristics of δ¹⁸O and δD in different water sources and the contribution of glacier–snow meltwater to streamflow. The results indicate that the δ¹⁸O of river water ranges from ?16.2‰ to ?10.2‰ with a mean of ?14.1‰ and that the δD values range from ?117.0‰ to ?68.0‰ with a mean of ?103.1‰. These values are more negative than those of glacier–snow meltwater but less negative than those of precipitation. The d ‐excess values are found to decrease from meltwater to river to lake/reservoir water as a result of evaporation. On the basis of hydrograph separation, glacier–snow meltwater accounts for 51.5% of river water in the Baishui catchment in the melting season. In the Yanggong catchment, snow meltwater contributes 47.9% to river water in the premonsoon period, and glacier meltwater contributes only 6.8% in the monsoon period. The uncertainty in hydrograph separation is sensitive to the variation of tracer concentrations of streamflow components. The input of meltwater to a water system varies with local climate and glacier changes. The results confirm that hydrograph separation using water isotopes is valuable for evaluating the recharge sources of rivers, especially in ungauged glacial regions. This study provides insights into the hydrological processes of glacial catchments on the Tibetan Plateau, which is important for water resource management.     

Streamflow generation in semi-arid,glacier-covered,montane catchments in the upper Shule River,Qilian Mountains,northeastern Tibetan plateau

Streamflow generation was investigated using isotopic and geochemical tracers in semiarid, glacier-covered, montane catchments in the upper Shule River, northeastern Tibetan Plateau. Samples from stream water, precipitation, glacier meltwater, and groundwater were collected at the Suli and Gahe catchments along the Shule River, with an area of 1908 and 4210 km², respectively. The samples were analysed for stable isotopes of water and major ions. Results of diagnostic tools of mixing models showed that Ca²⁺, Mg²⁺ and Cl⁻, along with δ¹⁸O and δ²H, behaved conservatively as a result of mixing of three endmembers. The three endmembers identified by the mixing analysis were surface runoff directly from precipitation, groundwater, and glacier meltwater. Streamflow was dominated by groundwater, accounting for 59% and 60% of streamflow on average in the Suli and Gahe catchments, respectively, with minimum groundwater contribution in July (47% and 50%) and maximum contribution in October (69% and 70%). The contributions of surface runoff were slightly higher in the Suli catchment (25%) than in the Gahe catchment (19%). However, the contributions of glacier meltwater were higher in the Gahe catchment (21%) compared to the Suli catchment (17%), as a result of a higher percentage of glacier covered area in the Gahe catchment. This difference followed well the non-linear power–law trend of many glacier-covered catchments around the world. As glacier retreat continues in the future, the reduction of streamflow in glacier-covered upper Shule catchment likely will be accelerated and possibly elsewhere in the Tibetan Plateau. This study suggests that it is critical to define the turning point of an accelerated reduction in glacier meltwater for glacier-covered catchments around the world in order to better assess and manage water resources.     

Streamflow sensitivity analysis to climate change for a large water‐limited basin

Partitioning precipitation (P) between streamflow (Q) and actual evapotranspiration (E_a) on a basin scale is controlled by climate change in combination with catchment characteristics. Fu's formulation of the Budyko framework was used to estimate Q as a function of two meteorological variables, P and potential evaporation (E_p), and one adjustable parameter reflecting characteristics of catchment conditions (ω). Results show that ω reflects the impacts of catchment characteristics on the partitioning of P between Q and E_a for the different water yielding regions. As predicted, Q was more sensitive to P than to comparable changes in E_p for the whole of the Yellow River Basin (YRB), a water‐limited basin, while it was shown to be highly sensitive to changes in P, E_p, and ω in the low water yielding region (LWYR) of the basin, followed by YRB and the high water yielding region of the basin. The high sensitivity of Q to P, E_p, and ω in LWYR indicates that the management of catchments within these zones is critical to the management of overall basin flow, mitigating impacts of climate change on Q. The Budyko framework, incorporating the adjustable parameter ω, outlines interactions between Q, climate, and characteristics specific to different water yielding regions. It also provides a new approach in understanding hydrological process response to climate change. Due to the obscure physical attributes of ω, an explanation of the parameter using soil or vegetation characteristics will aid in the understanding of the eco‐hydrological behaviour of catchments and help to provide more detailed catchment management options for which to mitigate climate change with respect to concerns regarding agricultural water usage. Copyright © 2013 John Wiley & Sons, Ltd.     

A conceptual model of water yield effects from beetle‐induced tree death in snow‐dominated lodgepole pine forests

In regions of western North America with snow‐dominated hydrology, the presence of forested watersheds can significantly influence streamflow compared to areas with other vegetation cover types. Widespread tree death in these watersheds can thus dramatically alter many ecohydrologic processes including transpiration, canopy solar transmission and snow interception, subcanopy wind regimes, soil infiltration, forest energy storage and snow surface albedo. One of the more important causes of conifer tree death is bark beetle infestation, which in some instances will kill nearly all of the canopy trees within forest stands. Since 1996, an ongoing outbreak of bark beetles (Coleoptera: Scolytidae) has caused widespread mortality across more than 600,000 km² of coniferous forests in western North America, including numerous Rocky Mountain headwaters catchments with high rates of lodgepole pine (Pinus contorta) mortality from mountain pin beetle (Dendroctonous ponderosae) infestations. Few empirical studies have documented the effects of MPB infestations on hydrologic processes, and little is known about the direction and magnitude of changes in water yield and timing of runoff due to insect‐induced tree death. Here, we review and synthesize existing research and provide new results quantifying the effects of beetle infestations on canopy structure, snow interception and transmission to create a conceptual model of the hydrologic effects of MPB‐induced lodgepole pine death during different stages of mortality. We identify the primary hydrologic processes operating in living forest stands, stands in multiple stages of death and long‐dead stands undergoing regeneration and estimate the direction of change in new water yield. This conceptual model is intended to identify avenues for future research efforts. Copyright © 2012 John Wiley & Sons, Ltd.     

Sensitivity of hydrological variables to climate change in the Haihe River basin,China

Using the defined sensitivity index, the sensitivity of streamflow, evapotranspiration and soil moisture to climate change was investigated in four catchments in the Haihe River basin. Climate change contained three parts: annual precipitation and temperature change and the change of the percentage of precipitation in the flood season (P_f). With satisfying monthly streamflow simulation using the variable infiltration capacity model, the sensitivity was estimated by the change of simulated hydrological variables with hypothetical climatic scenarios and observed climatic data. The results indicated that (i) the sensitivity of streamflow would increase as precipitation or P_f increased but would decrease as temperature increased; (ii) the sensitivity of evapotranspiration and soil moisture would decrease as precipitation or temperature increased, but it to P_f varied in different catchments; and (iii) hydrological variables were more sensitive to precipitation, followed by P_f, and then temperature. The nonlinear response of streamflow, evapotranspiration and soil moisture to climate change could provide a reference for water resources planning and management under future climate change scenarios in the Haihe River basin. Copyright © 2011 John Wiley & Sons, Ltd.     

Simulating Groundwater-Streamflow Connections in the Upper Colorado River Basin

In mountain, snow driven catchments, snowmelt is supposed to be the primary contribution to river streamflows during spring. In these catchments the contribution of groundwater is not well documented because of the difficulty to monitor groundwater in such complex environment with deep aquifers. In this study we use an integrated hydrologic model to conduct numerical experiments that help quantify the effect of lateral groundwater flow on total annual and peak streamflow in predevelopment conditions. Our simulations focus on the Upper Colorado River Basin (UCRB; 2.8 × 10⁵ km²) a well-documented mountain catchment for which both streamflow and water table measurements are available for several important sub-basins. For the simulated water year, our results suggest an increase in peak flow of up to 57% when lateral groundwater flow processes are included—an unexpected result for flood conditions generally assumed independent of groundwater. Additionally, inclusion of lateral groundwater flow moderately improved the model match to observations. The correlation coefficient for mean annual flows improved from 0.84 for the no lateral groundwater flow simulation to 0.98 for the lateral groundwater flow one. Spatially we see more pronounced differences between lateral and no lateral groundwater flow cases in areas of the domain with steeper topography. We also found distinct differences in the magnitude and spatial distribution of streamflow changes with and without lateral groundwater flow between Upper Colorado River Sub-basins. A sensitivity test that scaled hydraulic conductivity over two orders of magnitude was conducted for the lateral groundwater flow simulations. These results show that the impact of lateral groundwater flow is as large or larger than an order of magnitude change in hydraulic conductivity. While our results focus on the UCRB, we feel that these simulations have relevance to other headwaters systems worldwide.