H21L-01 INVITED
Linking Flow Processes to Landscape Characteristics through Isotope Hydrology in Semi- arid Catchments
While a lot is now known about hillslope behavior of many study sites around the world, the ability to generalize these findings for predictions in unmonitored sites remains difficult. This is largely due to the fact that the link between climate, hydrologic response and how the landscape is structured is poorly understood. Notwithstanding, such understanding is fundamental to advancing new hydrological theory and useful model structures that can be used in ungaged sites. In this paper we will present recent results from field investigations in two semi-arid environments in southwest USA: the Santa Catalina Mountains (SCM) near Tucson, AZ, and the Valles Caldera National Preserve (VCNP) near Los Alamos, NM. In SCM we have monitored the hydrological response in two hillslopes since 2006 using an array of hydrometric and hydrochemical instruments, in an attempt to estimate the mean transit time of water in those hillslopes. The two hillslope are different in their lithology (granite versus schist) and plan form (oval versus V-shaped). We find that the granite-oval hillslope has a mean transit time 5 times shorter than the schist-V-shaped hillslope. The parent material and the prevailing climate are responsible for very different soil characteristics and thus storage capacities, leading to important differences in transit time distributions. In VCNP we have designed an experiment that involves calculating transit times for a number of catchments that drain from a large dome called Redondo Peak. These catchments have different orientations and therefore receive different amounts of solar radiation. In general, we found that there was a correlation between mean transit times and aspect for these streams. At the same time, other topographic characteristics, which are typically considered as controls over catchment mean transit times, such as catchment area, elevation, and the ratio of flowpath length to slope gradient, exhibit limited predictive power with respect to mean transit times. The relationship between mean transit times and aspect suggests that in the Valles Caldera, transit times might be affected by a variety of features that are influenced by aspect, such as slope steepness, vegetation patterns, and soil depth.
H21L-02
A Groundwater Perspective on Streamflow Generation
Most streamflow researchers probably consider that they account adequately for groundwater contributions to streams, along with overland and near-surface soil and vadose zone flows. And groundwater can be identified as being the major source for some streams. But often such groundwater flows are seen as shallow, and flowing over "impermeable" bedrock. This work considers the contribution of deeper groundwater flows to streams. Possible methods of identifying the often subtle evidence of deeper groundwater input to streams are suggested, and some applied to the Pukemanga and Taupo Catchments in New Zealand. These catchments have runoff dominated by deep groundwater flow, but may not be unusual. How can we identify deeper groundwater flows to streams? Possibilities are: 1 Proportion of baseflow in annual streamflow, 2 Baseflow transit time distributions, 3 Baseflow recession characteristics, 4 Chemicals (e.g. silica or radon-222) that reflect interaction with rocks, 5 Catchment hydrological balances, 6 Groundwater modelling. Of these, the most clearly diagnostic of deep groundwater flows to streams is long baseflow mean transit times (2). Baseflow MTTs can be estimated using tritium concentrations (by radioactive decay) or oxygen-18 (by smoothing of natural variations). NZ has advantages for tritium dating because the tritium input function is well characterised, and measurements can be made with very high precision, but the input history of tritium can cause difficulties. Oxygen-18 may only give minimum values. Pukemanga Catchment (area 3 ha, on deeply weathered siltstone and sandstone) has a baseflow contribution of 85%. The baseflow MTT was 6-14 yr (tritium) and >4 years (18O), leading to a conceptual model of the catchment dominated by deep groundwater flow (Stewart et al, 2007). Many Taupo catchments (on air-fall volcanic deposits) have high baseflow contributions (90%) and their runoff is also dominated by deep groundwater flows. Their baseflow MTTs range from 40-84 yr (tritium) (Morgenstern, 2007). One of them, Tutaeuaua Catchment (area 13.5 km2, on Oruanui Ignimbrite) has baseflow MTT of 40 yr. Groundwater flow dynamics enabling the baseflow records of both streams to be matched are consistent with the sizes of the respective groundwater bodies, and the porosities and hydraulic conductivities of the subsurface materials (Bidwell et al, 2008, Hadfield, 2007). These remarks are made to suggest that process conceptualization in catchments should often include deep groundwater flowpaths, that integration across scales can be assisted by consideration of both near-surface and deep ground water flow systems, and that contaminant transport modelling should generally include deep groundwater flowpaths because of delayed effects. References: Stewart et al. 2007 (HP 21, 3340-56), Morgenstern 2007 (Environment Waikato Tech. Rept 2007/26), Bidwell et al. 2008 (HESS 12, 975-87), Hadfield 2007 (EWTR 2007/39).
H21L-03
Are Headwaters Just the Sum of Hillslopes? Evidence from Geochemical and Isotopic Observation
The spatial patterns of stream solute concentrations in a catchment reflect its hydrology and biogeochemistry. The spatial relationship between drainage areas and solute concentrations may be asymptotic or convergent (large variation in small streams that becomes dampened as they merge into larger streams). Because spatial patterns in stream solute concentrations are likely affected by landscape features, and such effects may differ depending on solute type, it is important for studies to apply first-order control over the spatial pattern of each solute. Thus, we investigated the concentration spatial patterns of dissolved silica and major ions (Na+, K+, Mg2+, Ca2+, NO3-, SO42-, and Cl-) in steep, humid headwaters of a 4.27 km2 catchment on Tanakami Mountain, central Japan. Because the catchment had a nearly homogenous landscape of incised valley and granitic bedrock covered in forest, we could eliminate any potential effects of variable landscape features. We found convergent relationships between drainage area and solute concentrations for all solutes except K+. The concentrations of all solutes were similar among sampling locations with an area of more than 0.5 ~ 1.0 km2. When considering only the longitudinal profile, solute concentrations showed an asymptotic pattern, indicating that longitudinal profiles with asymptotic patterns form convergent solute spatial patterns. We also examined quantitatively the observed convergent relationships between drainage area and solute concentrations and showed that the observed spatial patterns can be described by central limit theorem. Taken together, these results suggest that variation in hillslope discharges and the mixing of water from a variety of hillslopes controls downstream chemistry. That is, it looks like that the streams are just the sum of hillslope discharges.
H21L-04
Effects of groundwater springs on spatial sources of baseflow in the Catskill Mountains, New York State, USA
A multi-evidence study examines the influence of hillslope hydrologic processes on meso-scale watershed functioning in Town Brook watershed (36.8 square km) in New York State. Data from groundwater levels, spring discharge, surface saturation maps, and geophysical surveys reveal process heterogeneity at the hillslope-scale (<0.5 square km). At the small watershed scale (2.5 square km), end-member mixing analysis (EMMA) indicates that three hydrologic sources are capable of explaining the conservative tracer response: saturated areas, near-stream groundwater, and upslope springs. The small watershed results show that during dry summer conditions, hillslope groundwater springs maintain baseflow and control upslope saturation patterns. Synoptic sampling of O-18 and geochemical tracers confirm that upslope groundwater springs influence baseflow water chemistry. The sampling reveals baseflow transit times that are longer in the most upstream sampling locations (0.01 to 0.1 square km) than at larger spatial scales (<36.8 square km). Explaining a longer transit time in upslope areas is not possible with surface topography-based models (e.g. TOPMODEL) that assume flowpath length increases with spatial scale. Instead, the influence of groundwater springs on baseflow requires an alternative 'hydrogeomorphologic' conceptualization that includes the spatial distribution of hydrologic processes. The conceptualization recognizes two 'geomorphologic scales' that control baseflow in Town Brook: 1. the proximity to upslope areas with minimal till, 2. the location and extent of glacially derived valley-bottom material. Using spatial information, derived from soils, geology, topography, and aerial images, the conceptual model can explain the range of conservative tracer concentrations found in Town Brook. The results of this study emphasize the need to identify dominant hydrologic processes when determining source areas and flowpaths across spatial scales.
H21L-05
Spatio-temporal Variability in Midwinter Snowmelt Generated by Ground Heat Flux: Implications for Catchment Hydrology
Ground heat flux is commonly ignored in the modelling of snowpack energy exchanges and snowmelt runoff due to its perceived insignificance relative to other energy sources. Snowmelt at the base of a snowpack was continuously measured during the winters of 2006/07 and 2007/08 with 4 m2 lysimeters at six sites within a 3.5 km2 continental, mountainous catchment in southeast British Columbia. Soil wetness and soil, air, and snow temperatures were also continuously measured at each site. During the 2006/07 winter season, accumulated snowmelt during a three month midwinter period with sub- zero air temperatures ranged from 11 to 107 mm, comprising 3 to 36 % as much as the annual peak snow water accumulation. Daily snowmelt regularly exceeded 1 mm at several sites while daily maximum air temperatures were well below 0°C suggesting that ground heat flux generated the midwinter snowmelt (i.e. ground melt). Temporal variability of melt was strongly associated with air temperature, even at sub-zero temperatures. Spatial variability of melt was strongly associated with soil wetness, and wetness levels at wetter sites were maintained or increased through ground melt inputs. During the 2007/08 midwinter period, accumulated ground melt did not exceed 10 mm due to extensive soil freezing prior to snowpack development. The results suggest that a positive feedback response loop exists between soil wetness, ground heat flux, and ground melt due to the association between soil thermal conductivity and soil wetness. Pre-winter soil wetness and soil temperature, and winter meteorology influence the amount of midwinter ground melt because they control the relative amounts of ground heat flux that are used for melt, soil warming, or snowpack heat conduction. Pre-winter soil hydro-thermal dynamics and midwinter ground melt might be important controls on the spatial pattern of winter/spring catchment wetness and subsequent runoff response if antecedent soil conditions and ground melt inputs are spatially organized. A conceptual model is presented that describes the processes controlling midwinter ground melt.
H21L-06
Investigating Meadow Hydrology and Hyporheic Exchange
Using a combination of heat and geochemical tracers, the groundwater-surface water interactions in the stream/meadow complex running the length of Long Meadow (1.5 km long) in Sequoia National Park were investigated. Specifically we were quantifying subsurface discharge, an important source of meadow moisture following the cessation of snowmelt. A distributed temperature system (DTS), with two-meter spatial resolution and two-minute temporal resolution, was deployed in the stream, enabling the capture of diurnal swings in surface water-temperatures over the course of five days. Point temperature sensors (Hobo Tidbits) were deployed to also assess the vertical temperature profile and stratification within a number of the stream pools. Water samples were collected to determine the activity of radon-222 (3.8 day half-life) as a proxy for groundwater discharge. Temperature and pressure head data were collected from existing monitoring wells and piezometers. The high spatial and temporal resolution of the DTS enabled us to locate specific thermal anomalies in the longitudinal stream temperature profile, indicative of groundwater discharge. Some of these anomalies occurred in pools, up to 130 cm deep. In spite of the small size of the pools, temperature profiles exhibited diurnal polymictic behavior with mixing at night and stratification developing during the day. Cooler temperatures at the pond bottoms were attributed to groundwater upwelling. This was confirmed in a large pool through observation of high radon activity near the pool bottom. Highest radon activity in the pool was found in bottom sediment porewater, low activity was found near the pool surface, and intermediate activity was observed in deeper water. Due to the fractured rock system underlying the meadow, groundwater upwelling is spatially heterogeneous. As the stream dries, it becomes intermittent, with wet portions corresponding to upwelling regions. Well/piezometer clusters strategically placed in upwelling zones in the meadow also indicate groundwater discharge at the time of this experiment. Using an energy and mass balance, preliminary estimates have been made of groundwater upwelling.
H21L-07
Towards an Energy-based Runoff Generation Theory for Tundra Landscapes
Runoff hydrology has a large historical context concerned with the mechanisms and pathways of how water is transferred to the stream network. Despite this, there has been relatively little application of runoff generation theory to cold regions, particularly the expansive treeless environments where tundra vegetation, permafrost, and organic soils predominate. Here, the hydrological cycle is heavily influenced by 1) snow storage and release, 2) permafrost and frozen ground that restricts drainage, and 3) the water holding capacity of organic soils. While previous research has adapted temperate runoff generation concepts such as variable source area, transmissivity feedback, and fill-and-spill, there has been no runoff generation concept developed explicitly for tundra environments. Here, we propose an energy-based framework for delineating runoff contributing areas for tundra environments. Aerodynamic energy and roughness height control the end of winter snow water equivalent, which varies orders of magnitude across the landscape. Radiant energy in turn controls snowmelt and ground thaw rates. The combined spatial pattern of aerodynamic and radiant energy control flow pathways and the runoff contributing areas of the catchment, which are persistent on a year-to-year basis. Examples will be presented from several tundra catchments highlighting the importance of including aerodynamic and radiant energy in predicting runoff contributing areas. While ground surface topography obviously plays an important role in the assessment of contributing areas, the close coupling of energy to the hydrological cycles in arctic and alpine tundra environments dictates a new paradigm.