H44C-01 INVITED
Hydroecology/ Ecohydrology: Development and Recent Advances
Ecohydrology and hydroecology are making a mark on the environmental agenda, as evidenced by the proliferation of these terms in the academic literature, a new subject-specific journal/ text book, and wide occurrence of dedicated sessions at major conferences. In a practical context, consideration of hydroecological/ ecohydrological interactions is required to make management decisions about the water needs of riverine and wetland ecosystems versus people. In this presentation, we provide a perspective on the development of this ‘emerging discipline' by reviewing the scientific literature, categorising bibliographic search data and using examples of current research to focus attention on a range of issues that require further evaluation and thought. Examples are drawn from studies of: alpine river systems, river flow variability and ecological response, and hydrological disturbance of exposed riverine sediment beetle communities. We suggest that a potential impediment to the development of ecohydrology/ hydroecology is the lack of a clear subject definition to acts as a focal point to unite the research community. Most importantly, we assert that it not simply the integration of hydrology and ecology that will determine the future prospects for ecohydrology/ hydroecology but the way in which this integrative science is conducted. We advocate a truly interdisciplinary (as opposed to multi-disciplinary) approach in which ecologists and hydrologists benefit from true synergy by embracing advances at the cutting- edge of both sciences. Such an approach should provide more perceptive answers to hydroecological/ ecohydrological problems and management questions.
H44C-02
The Control of Salmonid Populations by Hydrological Connectivity: an Analysis at the Local, Reach and Watershed scales
Amid concern over the decline of salmonid and other fish populations, the traditional basis of much stream restoration has been identification of degraded river sections and localised attempts to restore them. Research has demonstrated that fine sediment, solutes and organic matter also influence instream aquatic ecosystems and these may be influenced by upstream watershed land use. However, results from modelling the role played by land use impacts are contradictory. This is not surprising for three reasons. First, measurements of river ecology taken at any one point will be influenced by processes operating at scales ranging from the local (e.g. presence of suitable spawning habitat) through the reach-scale to the tributary and watershed scales. Second, upstream land use is only important if it can transmit or deliver its signal (e.g. a fine sediment source) to the river network, as moderated by the extent (frequency, duration) to which it connects with the river network. Third, once a signal is transmitted to a river, its importance can only be judged with respect to other signals, as a result of dilution and/or accumulation effects. These latter two issues are often overlooked by studies of land use impacts upon aquatic ecology. In this paper we bring together two critical research developments: system-scale semi-quantitative electrofishing of salmonids; and risk based modeling of hydrological connectivity; for a 2300 km2 catchment. We support these developments with quantification of both local- and reach-scale influences on salmonid habitat. We use multivariate inference to show that an index of delivery based upon hydrological connection is a first order control upon the presence/absence and the abundance of juvenile salmonid fry: the topographic control of watershed hydrological response exerts a fundamental filtering effect upon the spatial structure of salmonid fry when evaluated at the catchment-scale. We also show that the nature of this relationship is scale dependent, varying between and within sub-watersheds. We conclude that if topographic control mediates the watershed to stream linkage, land use impacts can only be understood with respect to their position in the landscape. The spatial organisation of landscape elements becomes crucial to understanding the ecological impacts of particular management activities, with hydrological flow paths providing the functional linkage. This is of practical importance as locations of high connectivity should be a primary objective in targeting watershed restoration measures to where they will deliver most instream benefits.
H44C-03
Regional Trends in Streamflow in Central Texas: Reconciling the Effects of Degradation and Woody Plant Encroachment
The extent and density of woodlands and shrublands have expanded greatly on drylands across the globe—a phenomena described as woody plant encroachment. Rangelands in Texas for example have been largely converted from grasslands and savannas to woodlands in the last 150 years. In Central Texas—a landscape with a predominantly karst geology—Ashe juniper and oak have come to dominate much of the landscape. These changes were set in motion by the severe overgrazing that occurred at the turn of the last century. The vegetation has transitioned from healthy savanna to degraded grasslands and now finally to woodlands. The hydrological consequences of this conversion have been evaluated from the plot to the small catchment scale using a variety of techniques. The general consensus from this body of work is that recharge and streamflow are around 50 mm lower when woody cover predominates. One would expect then that long terms trends in streamflow would reflect a declining trend as woody plant coverage has progressively increased in the last century. Surprisingly, however, we find no declining trends in regional streamflow during the period of record (around 80-100 years) and in fact there are indications that the baseflow component of streamflow has increased slightly during this time. We propose that these counterintuitive results (increasing streamflow with conversion from degraded grasslands to woodland) are the result of a general improvement in rangeland condition that has occurred in the last 50 years or so.
H44C-04 INVITED
Transient coupling of water, carbon and nutrient cycling patterns over hillslope gradients: Co- evolution of catenae in different landscapes
Interest and activity in the mechanistic linkage of hydrologic and ecological processes has seen a dramatic increase over the past two decades. The majority of research has addressed water stress as a major determinant of ecological patch characteristics, including canopy density, root profiles and life form, with an emphasis on water limited environments. Optimization methods, following Eagleson's&ppioneering work, have been developed to pose simple principles such as productivity maximization or water stress minimization to estimate ecological patch structure within the context of climate and soil conditions. However, ecosystems are characterized by recurring or continual disturbance which reset or have significant restructuring effects on ecological form, and can produce long periods of transient adjustment of the full ecosystem that are "sub-optimal" relative to the simple criteria posed. In addition, the connectivity of patches along hydrologic flowpaths creates a space/time sequence of ecosystem states and complex hillslope to catchment level behavior showing elements of self organization which are transient and conditional on both current ecohydrological processes and the history of disturbance. Soil and canopy memory to a range of disturbance time scales and interannual climate variability may be demonstrated by both empirical measurements and modeled catchment ecohydrologic behavior. We explore examples of this behavior in humid landscapes with a gradient of natural to anthropomorphic disturbance in two LTER sites: Coweeta which has a long term data set characterizing control and manipulated catchments, and the Baltimore Ecosystem Study with instrumented catchments in a range of land use and management.
H44C-05
Ecohydrology and Carbon Cycling in a Headwater Catchment: Identifying the Hydrologic, Biologic and Abiotic Processes Expressed in Terrestrial-Aquatic Connectivity
Dissolved carbon dioxide in headwater streams has been suggested as a tool for assessing whole catchment soil respiration. However, the dynamics of dissolved CO2 in streams are complex, and are driven by many processes, some of which are hydrologically mediated, others are biologically mediated, and still others are mediated by additional abiotic factors. We employed high frequency water quality and hydrometric monitoring in a forested headwater catchment in coastal British Columbia with the goal of elucidating controls over stream water CO2 dynamics at a range of temporal scales. Direct measurements of dissolved CO2 were made in situ using an infrared gas analyzer, while a multiparameter water quality sensor monitored dissolved oxygen, pH, temperature and conductivity. Using these data we went about teasing out the hydrological, biological, and abiotic processes expressed by the concentration dynamics of the dissolved gasses. Dissolved oxygen concentrations in streamwater were inversely related to dissolved CO2 concentrations at diurnal and seasonal scales. However, the behavior of dissolved gasses during rainfall-runoff events varied seasonally, with springtime rainfall events characterized by opposite trends for CO2 and O2, with CO2 concentrations inversely related to storm discharge, while O2 concentrations increased on the rising limbs and decreased on the falling limbs. However, the dissolved gasses were correlated during summer events, with both CO2 and O2 positively related to storm discharge. During the five-month study, the range of observed values was much greater for dissolved CO2 (from 2.7 to 12.0 mg/L for CO2) than for dissolved O2 (7.4 to 10.6 mg/L). The stream was always undersaturated with respect to O2 (flow-weighted mean of 82 percent of oxygen saturation), but supersaturated with CO2. The excess CO2 (EpCO2), which is the amount of dissolved CO2 in streamwater compared to atmospheric equilibrium, ranged from 2.7 to 15.4, with a flow-weighted mean EpCO2 of 6.1. Inputs of CO2 derived from terrestrial soil respiration are clearly evident in the stream chemistry, as seen in the amplitude of diurnal periodicity of CO2, which was greater than that of O2. This research contributes to identifying the terrestrial and aquatic components of stream respiration, which is of fundamental importance to quantifying what is a significant, though poorly constrained, portion of the net ecosystem carbon balance.
H44C-06
Spatially explicit simulation of hydrologically controlled carbon and nitrogen cycles and associated feedback mechanisms in a boreal ecosystem in Eastern Canada.
Current estimates of terrestrial carbon overlook the effects of topographically-driven lateral flow of soil water. We hypothesize that this component, which occur at a landscape or watershed scale have significant influences on the spatial distribution of carbon, due to its large contribution to the local water balance. To this end, we further developed a spatially explicit ecohydrological model, BEPS-TerrainLab V2.0. We simulated the coupled hydrological and carbon cycle processes in a black spruce-moss ecosystem in central Quebec, Canada. The carbon stocks were initialized using a long term carbon cycling model, InTEC, under a climate change and disturbance scenario, the accuracy of which was determined with inventory plot measurements. Further, we simulated and validated several ecosystem indicators such as ET, GPP, NEP, water table, snow depth and soil temperature, using the measurements for two years, 2004 and 2005. After gaining confidence in the model's ability to simulate ecohydrological processes, we tested the influence of lateral water flow on the carbon cycle. We made three hydrological modeling scenarios 1) Explicit, were realistic lateral water routing was considered 2) Implicit where calculations were based on a bucket modeling approach 3) NoFlow, where the lateral water flow was turned off in the model. The results showed that pronounced anomalies exist among the scenarios for the simulated GPP, ET and NEP. In general, Implicit calculation overestimated GPP and underestimated NEP, as opposed to Explicit simulation. NoFlow underestimated GPP and overestimated NEP. The key processes controlling GPP were manifested through stomatal conductance which reduces under conditions of rapid soil saturation ( NoFlow ) or increases in the Implicit case, and, nitrogen availability which affects Vcmax, the maximum carboxylation rate. However, for NEP, the anomalies were attributed to differences in soil carbon pool decomposition, which determine the heterotrophic respiration and the resultant nitrogen mineralization which affects GPP and several other feedback mechanisms. These results suggest that lateral water flow does play a significant role in the terrestrial carbon distribution. Therefore, regional or global scale terrestrial carbon estimates could have significant errors if proper hydrological constrains are not considered for modeling ecological processes due to large topographic variations on the Earth's surface. For more info please visit: http://ajit.govind.googlepages.com/agu2007 http://ajit.govind.googlepages.com/agu2007
H44C-07
Linking Soil Moisture, Micro-climate, and Transpiration in a Headwater Catchment
Evapotranspiration is a major determinant of streamflow in forested basins. However, the role topography plays in forest water relations is poorly understood. To date, many hydrological models use only a single value for transpiration across a catchment. Quantifying the variation in forest water use with regards to slope position is central to understanding controls on water quantity and quality in hydro-ecological models and is critical to predicting the hydrologic impacts of various forestry operations. We measured transpiration, soil moisture, and foliar pre-dawn water potential in 4 plots across a ridge to ridge transect throughout the summers of 2005 and 2006 in a headwater catchment in western Oregon. Additionally, we measured deuterium and 18O of xylem water and soil water to track changes in the depth of transpiration source water throughout the summers. From May through October 2006, daily average transpiration in upslope plots was approximately 40% greater than that of valley bottom plots (1.0 mm day-1 vs. 0.6 mm day-1, respectively). Minimum pre-dawn water potential values ranged from -0.8 to -1.3 MPA in late August with north-facing plots having the lowest values. Stable isotope data indicates that transpiration rates remained higher longer in the growing season in plots where trees were able to access water deeper in the soil profile. Preliminary data suggest that topographic gradients influencing soil depth, soil moisture retention, and micro-climate result in large variation in forest water use over very small distances.