Integrated Watershed Sciences Symposium
Abstracts

David Eaton, LBJ School of Public Affairs, University of Texas at Austin
THE FUTURE OF MEXICO-TEXAS RIO GRANDE/RIO BRAVO RELATIONS

This paper discusses the origin of the controversy over Mexico's "water debt" to Texas associated with the 1944 treaty between the U.S. and Mexico on the use of border rivers. The current status of water relations between the nations is assessed. Means for resolving the disputes and water debt are discussed. Prospects for future water relations are considered, both with respect to water allocation and water quality management.


R. L. Kiesling, Environmental Science Institute, University of Texas and USGS
THE ROLE OF CHANGING LAND USE AND AGRICULTURAL PRACTICES ON THE TROPHIC STATUS AND STRUCTURE OF AQUATIC COMMUNITIES: WATERSHED EUTROPHICATION AND ITS CONSEQUENCES

Eutrophication in the North Bosque River watershed (NBR) has resulted from high loadings of phosphorus and nitrogen associated with specific types of agricultural land use and point-source discharges associated with urban land use, in contrast to background loadings from rangelands. Specific land use practices associated with dairies have generated disproportionate nonpoint-source loadings of nutrients; these loads have come to dominate total instream loading in the upper North Bosque River watershed (UNBR). The effect of high nutrient loads on ecosystem function within the NBR was documented by investigating corresponding changes in biological communities. In situ stream bioassays documented a major response in stream primary production and trophic status along the instream nutrient enrichment gradient. In situ stream primary production was a significant function of ambient limiting-nutrient concentrations, and primary production conformed to a theoretical production model based on resource-based population growth. Annual average ambient chlorophyll concentrations for phosphorus-limited sites varied as a function of annual average instream orthophosphate concentrations as did periphyton and macro-benthic algal biomass. Upstream sites nearest to urban and dairy land uses were at maximum productivity compared to a rangeland reference site, indicating eutrophic conditions.

The NBR drains into a terminal water supply reservoir, Lake Waco. Previous studies of Lake Waco, a eutrophic central Texas reservoir, have suggested that the reservoir trophic status has increased with increased nutrient loading from agricultural NPS. Elemental concentrations and isotopic ratios of reservoir sediment cores are consistent with this hypothesis. Cores collected from Lake Waco, which receives nutrient loads from its north and south basins, indicate increasing total and soluble-reactive phosphorus concentrations in recent sediments. This pattern is most pronounced in the main reservoir body. Carbonate-carbon isotope ratios in the main reservoir body and the northern arm become lighter by about one per mil PDB in the topmost sediments compared to the bottom sediments, whereas the depletion is less pronounced in the southern arm. A likely explanation for this is that increased production of organic carbon and its decomposition provide light carbon to precipitating calcite. Oxygen isotopic compositions of calcite also indicate depletion of one per mil PDB in the topmost sediments in the main reservoir body, which is also consistent with increased eutrophication. Detailed biological and geochemical analyses are underway to further test the hypothesis


Jordan Furnans and Ben R. Hodges, Department of Civil Engineering, University of Texas at Austin
DISPERSION AND MIXING PROCESSES IN TEXAS COASTAL EMBAYMENTS

Describing and quantifying mixing and dispersive properties of the Texas coastal embayments is potentially improved through the use of Lagrangian surface drifters and numerical modeling. Lagrangian drifters passivley follow the water currents, and studies of the drifter trajectories provide insight into the properties of the surface currents. In this presentation, a new type of Lagrangian drifter is discussed, and results from its use in Lake Kinneret, Israel and Marmion Marine Park, Australia are presented. Numerical modeling of the Lagrangian drifters is also introduced, and its applications in contaminant transport and field/model validation are considered.


Paul F. Hudson, Department of Geography, University of Texas at Austin
SEDIMENT TRANSPORT IN TEXAS RIVERS: IS THERE A PROBLEM?

Fluvial sediments are important to the formation and functioning of a diverse array of river and riparian environments, as well as coastal estuaries. Texas has over 3,700 rivers, which drain a variety of climatic and geologic settings. All Texas' large coastal flowing rivers are regulated by dams, which were primarily constructed between the 1930s and 1980s. Dams and reservoirs trap and store riverine sediments. However, it remains difficult to establish whether sediment loads have declined downstream of dams, or even whether dams are impacting riverine and estuarine environments.

This paper offers several reasons why it is difficult to quantify the impact of dams on Texas rivers. For a number of rivers the sediment gauging stations are located too far downstream to detect changes in sediment loads, which could be due to the channel reworking the floodplain and causing sediment loads to recover, or the supply of sediments from downstream tributaries. Many of the sediment records do not span an adequate length of time before and after dam construction. Additionally, most river systems have headwater dams as well as main-stem dams in the lower reaches of the basin. These dams were imposed on the system over different times, which makes it difficult to establish a predisturbance or "natural" period. Several large rivers in Texas, such as the Guadalupe, still flood their floodplains and continue to deposit large amounts of sediment onto the floodplain. The paper concludes that it is not possible to make a general statement about the impact of dams on the sediment load of Texas' coastal draining rivers.


Venkatesh Merwade and David Maidment, Department of Civil Engineering and Center for Research in Water Resources, University of Texas at Austin
GEOSPATIAL REPRESENTATION OF RIVER CHANNELS

The representation of river channels using Geographic Information Systems (GIS) is gaining momentum due to the availability of high quality data. The data are collected using depth sounding measurements from a boat combined with GPS location of the measuring device to generate a set of scattered (x,y,z) points over the river bed. GIS procedures to represent the data using raster and vector formats and their use in hydraulic modeling are discussed. The detailed channel data are expensive and are collected on small reaches. To describe the river channels on a regional scale, a GIS framework is developed that uses the field data collected on small reaches and hydraulic geometry relationships. The application of this framework to a study reach of 4.5 miles long along the Brazos River in Texas will be presented.


John E. McCray, Dept. of Geological Sciences, UT-Austin
Paula Jo Lemonds, HDR Inc., Denver CO
MODELING HYDROLOGY AND WASTEWATER POLLUTANT TRANSPORT IN AN ALPINE WATERSHED

Lake Dillon in the Blue River Watershed, Colorado, is a primary drinking-water reservoir for Denver. Eutrophication of the lake is a concern, primarily from phosphorus (P) loading. Local officials have attributed the P loading to onsite wastewater systems (OWS). The watershed model, SWAT, is used to construct a hydrologic and transport model of the Blue River watershed to better understand the potential influence of various point and nonpoint sources of P in the watershed. The watershed model was calibrated to measured flow rates and the model's performance with respect to pollutant transport was evaluated by comparing results to measured P concentrations. The hydrogeologic model results are most sensitive to the physical parameters associated with snowmelt, as well as orographic effects on precipitation and evapotranspiration. However, uncertainties in chemical-transport parameters preclude a rigorous assignment of relative contributions of various P sources. Rather, the effort has elucidated what P transport parameters that are most crucial to accurate simulations for this watershed. The model was most sensitive to the P sorption coefficient, the P availability index, and the P enrichment ratio (a measure of P in runoff sediments compared to immobile sediments). Modeling results indicate that OWS are likely not the most significant sources of P to Lake Dillon.


Paul Montagna, UT-Marine Science Institute, University of Texas at Austin
IMPORTANCE OF FRESHWATER INFLOW TO TEXAS COASTAL RESOURCES

The ecology of Texas estuaries is strongly influenced by latitudinal ecotones that exist along the northwestern Gulf of Mexico coastline. Climatic variability and differences in freshwater inflow among the ecosystems structure communities and maintains secondary production. There are seasonal, interannual and latitudinal patterns of inflow, and these patterns are regulating community structure, population dynamics and secondary production. Freshwater mixing with seawater is responsible for the enormous productivity of estuaries. This mixing enhances productivity because of introduced nutrients, lowered salinity, and creation of wetland habitats. Many estuarine organisms require low salinities at specific times during the year as a cue for reproduction or to support the next crop of young, so inflow timing is critical as well. Freshwater is a limiting resource in many parts of Texas. Freshwater allocation generally is reserved for agricultural, industrial, and municipal purposes. Recently, the ecological need for freshwater inflows to downstream estuaries has also been recognized and implemented. This allocation has caused conflict in south Texas where water is scarce and droughts common. The general public view is that water run to the sea is water run to waste. UT researchers have worked closely with resource managers at local, state, and federal agencies to help protect key marine living resources.


John Sharp, Geological Sciences, University of Texas at Austin
STREAM-GROUNDWATER INTERACTIONS - GROUNDWATER FLOW SYSTEMS

Evaluation of stream-aquifer systems and the associated groundwater flow systems is important in studies of water availability, environmental impact, and aquifer remediation. Alluvial aquifers are a relatively untapped water resource because in much of the USA, surface waters are totally allocated. This is most acute in the western states, but restrictions on surface water are spreading because of requirements to maintain minimum instream flow for navigation, recreation, fish and wildlife, agriculture, and power generation. Alluvial valleys contain thick, productive sand and gravel deposits that occur in clearly defined bands and are in hydrologic contact with stream. Streams can be either gaining (effluent) or losing (influent) with respect to groundwater, but transient conditions of bank storage can be significant. Groundwater chemistry can vary because of many factors, but the hyporheic zone is defined as the groundwater that is chemically similar to the stream and which may contain similar fauna.

Most major rivers are underfit. Their valleys are filled with alluvium, and the rivers are not fully penetrating. The alluvium is subdivided into a sand-gravel substratum deposited by vertical accretion, and topstratum deposited by lateral accretion. The highly variable topstratum is divided into a channel belt, a meander belt, and a flood basin. A variety of groundwater response zones are defined - zones of: 1) rapidly fluctuating groundwater levels (that correspond to the hyporheic zone), 2) long-term stability or slow groundwater response, 3) predominantly down-valley flow (analogous to underflow), 4) persistent groundwater highs, and 5) pumping-induced groundwater lows. Published head data are not always available. When data are absent (or ignored), it is commonly assumed that groundwater and dissolved solutes flow directly to the river. Analysis of alluvial systems and digital simulations indicate that, in some cases, this is only valid immediately adjacent to an effluent river. Groundwater flow elsewhere may be dominated by downstream flow or underflow, which has legal implications in Texas. In alluvial systems, two end-member groundwater flux components can be defined: 1) baseflow that flows perpendicular to the river and 2) underflow that flows parallel to the river and the stream flow. Alluvial stream-aquifer systems can be classified as underflow-dominated, baseflow-dominated, or mixed. This can be inferred from geomorphologic data, including channel slope, river sinuosity, degree of penetration, width-to-depth ratio, and the fluvial depositional system.

Man has altered the alluvial systems by shortening the rivers, stabilizing banks, eliminating islands and natural meandering, dredging, construction (of wing dikes, locks, dams, and artificial cutoffs), changing stream flow regimen, trapping stream sediments in reservoirs, diversion of stream flow, urbanization, and intensive cultivation. The long-term effects of these alterations are unknown.


Zong-Liang Yang and Guo-Yue Niu, UT - Department of Geological Sciences
Modeling Hydrological Processes in the Coupled Hydrological and Meteorological Models

Our past and ongoing work has been focused on the development of soil-vegetation-atmosphere transfer (SVAT) schemes for use in hydrological modeling, weather forecasting, and climate simulation studies. The SVAT models use input of meteorological variables (precipitation, downward solar radiation, downward longwave radiation, surface pressure, air temperature, wind, and humidity) and produce output of soil moisture, soil temperature, canopy temperature, canopy interception, evapotranspiration, runoff, photosynthesis, respiration, CO2 fluxes, surface albedo, and other energy fluxes. The models can be used at a point, a catchment, a watershed, a continent, or global land, provided that the meteorological input (or forcing) data and the land surface characteristics (e.g., terrain, soil porosity and leaf area index) are available. The models can serve as a lower boundary condition in 3-dimentional atmospheric models, an upper boundary condition in hydrological models, and an important component in ecological models. The remote sensing data can be assimilated into the SVAT models as well. In this talk, the research in each of these areas will be highlighted, with a focus on watershed and regional scale hydrological simulations.