Bulletins, 1965-1995

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Commercial Navigation on the Upper Mississippi River: An Economic Review of its Development and Public Policy Issues Affecting Minnesota
(Water Resources Research Center, University of Minnesota, 1974-10) Christianson, Rodney W.
Development of the Upper Mississippi River and its major tributaries have been of great importance for the economic and social well-being of Minnesota. Water development projects such as the nine-foot navigation channel have provided a cheap transportation route in which important commodities such as coal and petroleum can be shipped into Minnesota, white the abundant harvest of grain in Minnesota and surrounding states can be moved out to domestic and foreign markets at low freight costs. In addition, development of the Upper Mississippi River has increased recreational opportunities, and has provided a more productive and usable habitat for fish and wildlife. However, the Upper Mississippi has been developed primarily as a transportation artery, and important considerations such as ecological processes and environmental values of America's largest and most unique river system often have been neglected and even destroyed in a few areas. Opposition to further development is strong. Inland water transport is a significant carrier of domestic cargo, accounting for about 14% of the total traffic. During the past decade the inland waterways increased their cargo carried by 46% (62% when the Great Lakes are excluded). By increasing the absolute amount of freight carried greater than the average (42%), the inland waterway's relative share of total freight traffic has also grown over the past decade. Also development of the Upper Mississippi River into a major inland waterway has been even more significant for Minnesota and the Midwest than for much of the rest of the nation. Past and present development of the Upper Mississippi River and its tributaries, both by the public and private sectors, has been extensive,. Public funds (mostly federal) have not only provided the nine-foot navigation channel, but also numerous harbors and fleeting areas along the Upper Mississippi and its tributaries. Private investment in terminals, towing vessels, and barges has been and continues to be substantial. In spite of these huge costs, the real cost of inland waterway shipment is still lower on most waterways per ton-mile than the least cost alternative. A transportation model, based on competitive assumptions and employing a derived demand analysis, is presented. The model predicts that there will be an increase in demand for transportation services in general, and barge services in particular. However, derivation of the elasticity of demand for barge services revealed that the demand for barge services will become more elastic in the future. Planning and feasibility studies of commercial navigation projects on the Upper Mississippi River (and on the waterways) have emphasized economic developmental values and expansion of waterway capacity while giving little attention or even ignoring environmental values and ecological processes. Planning has been undertaken at three government levels--national, regional, and state. The national study recommended better evaluation procedures, more equitable cost-sharing policies (user charges), and coordination of all modes of transportation to achieve a more efficient national transportation system. However, regional and state plans are often formulated to meet conditions and needs that existed in the past. These studies have tended to emphasize continued subsidization and expansion of water development projects to aid commercial navigation. The current issues in commercial navigation which affect Minnesota basically involve a resolution of the conflict between developmental and environmental values. Such is the issue of dredging to maintain the 9-foot navigation channel on the Upper Mississippi River. Without maintenance dredging barge traffic would come to a halt on the Upper Mississippi causing irreparable economic harm. But environmentalists contend that the Corps' present method of dredging and depositing of dredge spoil causes irreparable environmental damage by blocking the flow of backwater sloughs. The dredging issue reduces to one of what cost is the public willing to bear to preserve environmental values being destroyed by present methods of dredging and placement of dredge spoil. However, instead of the general public bearing the cost, users of the 9-foot navigation channel could be required to pay the full costs of environmentally sound dredge spoil disposal. With a system of user charges in effect, environmental values would be better accounted for and barge transportation would be assigned to its most efficient position in the national transportation system. There needs to be a better balance between developmental and environmental values in present and future commercial navigation projects. Administrative-legal procedures and institutions, which include environmental impact statements, inter-agency cooperation and agreement, and lawsuits, will help insure that environmental interests (both government agencies and citizen groups) effectively communicate their values into the decision-making process. However, the market, or pricing system, is also an effective communications device. Establishing a pricing system through the imposition of user charges which requires that users of inland waterways pay the full costs (economic and environmental) of providing navigation facilities would insure that developmental and environmental values are better balanced in future commercial navigation projects.
An Approach to Groundwater Monitoring Well Network Design Relying on Numerical Techniques and Public-Domain Information
(Water Resources Research Center, University of Minnesota, 1994-05) Mooers, Howard D.; Pfannkuch, Hans-Olaf; Quinn, John J.
Uncertainties in the hydrogeology of a study area and in the transport properties of potential contaminants challenge the designers of groundwater monitoring well networks- Numerical flow modeling is a useful tool for guiding the placement of wells, but it requires justified boundary conditions and sufficient knowledge of aquifer parameters. In a graciated terrain, additional problems arise because of complex spatial arrangements of aquifers and aquitards. The transport of conservation contaminant tracers is normally calculated as a simple function of average values of hydraulic conductivity, hydraulic gradient, and effective porosity. But are the results of this straightforward method always valid? In order to address these topics, this investigation focused on a landfill on the Anoka sandplain of east-central Minnesota. The purpose of this study was to determine the proper placement of an initial group of monitoring wells at the landfill using only offsite public-domain data. The results may then be applied to the siting of other wells. Finite-difference flow modeling was supported by an abundance of inexpensive public-domain information and by the construction of a detailed, sub-regional glacial geologic map. A two-dimensional kriging analysis refined the model by determining the cell-by-cell best estimates of the basal elevation of the surficial aquifer. Particle tracking results indicated the expected pathway of landfill leachate. Based on the results, one well upgradient of the landfill and several downgradient wells were selected from the database of actual monitoring wells, and the head data from these shallow wells were used to calibrate the model. The calibrated hydraulic conductivity of the sandplain aquifer agrees closely with values obtained through grain-size analyses and pump tests. Numerical analyses of boundary conditions support the validity of the flow model. Other case studies of unconfined outwash aquifers suggest that predicted plumes of conservative tracers are often greater than the actual extents. Compared to the chloride data for monitoring wells at the Anoka site, particle tracking results have an accurate orientation but a length at least two times too long. Uncertainties, such as the effect of longitudinal dispersion and the transient nature of the leachate's initial concentration and source area, suggest an even greater difference' This conceptual understanding of plume migration provides guidance for the placement of additional downgradient wells. The described application of models and inexpensive offsite data to monitoring well network design is a methodology that may be effective for the monitoring of solutes from existing or proposed potential contamination sources.
Spatial and Temporal Analysis of Groundwater Recharge with Application to Sampling Design
(Water Resources Research Center, University of Minnesota, 1995-07) Andricevic, Roko; Foufoula-Georgiou, Efi; Jankovic, Igor
The most influential process for the groundwater shallow aquifer monitoring is groundwater recharge. This study focuses on defining and quantifying the groundwater recharge as a process primarily resulting from precipitation. The spatial and temporal distribution of the recharge was analyzed through a numerical experiment designed to realistically describe a shallow aquifer case. The spatial and temporal structure of the recharge was examined and quantified as a function of precipitation events and soil heterogeneity. The quantified spatial and temporal distribution of the groundwater recharge then was used to demonstrate its influence on sampling network design. It was found that the temporal variability of the recharge has a major impact on the design of groundwater flow monitoring networks.
Lake and Groundwater Paleohydrology: Use of Groundwater Flow Theory to Explain Past Lake Levels in West-Central Minnesota
(Water Resources Research Center, University of Minnesota, 1989-06) Almendinger, James E.
Investigation of a simple analytic model of an interfluvial water table demonstrates that a shift in groundwater recharge N changes the water table elevation the most near the middle of the interfluve. Consequently, lakes lying farthest from rivers are most vulnerable to lake-level change. The partial derivative of groundwater head with respect to N, the "positional sensitivity," is quantified for the simple model as a function of position across the interfluve. Despite its simplicity, the positional sensitivity of the model has some predictive value for water-table and lake-level changes in a sandplain in west-central Minnesota. Lake levels are also a function of surficial hydrology. "Lake pumping" is symbolized by y and defined as the net removal of water from a lake by hydrologic processes acting at the lake surface, namely evaporation minus direct precipitation and minus any input from overland runoff that reaches the lake. Investigation of a simple analytic groundwater model of a circular Jake next to an infinitely long river shows that the sensitivity of the lake level to a change in Y is proportional to the radius of the lake and its distance from the river. The analysis also indicates that lakes lying in highly permeable substrates are not very sensitive to changes in 1. The response of a lake level to a shift in climate depends on characteristics of surficial and groundwater hydrology that are unique to that lake. Determination of the past levels of several lakes, rather than just one, should help provide a more nearly unique reconstruction of past hydrology and climate. Analysis of the sediments of several closed-basin lakes lying in the Parkers Prairie sandplain in west-central Minnesota indicates that lake levels were lowest about 8.5 to 8 ka. I manipulate the N and y of a steady-stare analytic-element groundwater model such that the modeled water table coincides with the paleo-lake levels for a given past time. Model results indicate that lake levels at 8.5 to 8 ka can be explained primarily by reducing N to 4O% of the modern value, coupled with a y of about 20 to 30 cm yrl. By 6 ka N had increased to 50 to 80% of the modern value allowing most lakes to rise in level, but y may also have increased forcing at least one lake to remain nearly dry.
On-site Biodegradation of Organic Pollutants in Contaminated Soils and Groundwater
(Water Resources Research Center, University of Minnesota, 1987-06) Maier, Walter J.
Widespread use and improper disposal of chlorinated organic materials have resulted in contamination of soils and groundwater throughout the nation. This research focused on development of remedial technology to rehabilitate contaminated soil and groundwater. Biological treatment was chosen because it is environmentally sound and less costly than methods involving physical-chemical processes. Pentachlorophenol (PCP) was used as a model compound. A mixed microbial culture capable of utilizing PCP as a source of carbon and energy was developed from municipal wastewater treatment plant sludge. The culture was grown in a continuous flow chemostat reactor and used as the inoculum in batch and continuous flow experiments. PCP removal from contaminated groundwater and synthetic media was studied in batch tests. Complete removal and stoichiometric release of chloride ions was observed. The effects of different size sand particles on PCP biodegradation were tested to determine the rates of removal. Short and long sand columns also were tested to determine the rates of PCP removal under saturated and unsaturated flow conditions. The presence of sand particles enhanced biodegradation; small size sand particles (<25 pn) had the greatest effect. The presence of sand particles shortened the lag phase observed when the culture was inoculated into medium containing high concentrations (-100 mg/L) of PCP. Sand columns operated in unsaturated flow conditions removed PCP more effectively than saturated flow columns. The greater effectiveness of unsaturated flow conditions appears to be due to greater availability of molecular oxygen as well as a difference in flow pattern. Enhanced adsorption of microbial cells on sand and flow of water along grain surfaces resulted in better contact between active microorganisms and PCP under unsaturated flow conditions.
Effects of Agricultural Runoff Upon Natural Wetland Ecosystems
(Water Resources Research Center, University of Minnesota, 1983-09) Vanamburg, Gerald L.
Fiscal Year 1984 Program Report
(Water Resources Research Center, University of Minnesota, 1985-09) Water Resources Research Center
The Program Report of the University of Minnesota Water Resources Research Center presents a summary of activities for fiscal year 1984, covering the period of October 1, 1984 through September 30, 1985. The Report describes the research activities of the Center, its involvement in training water scientists and its efforts information in transfer and the findings of six sponsored projects. These findings include: 1. Unsaturated flow patterns are shown to have higher rates of removal of pentachlorophenol than saturated flows which limit oxygen. 2. Atrazine and nitrates have been detected in water samples examined from two Karst springs in southeasternMinnesota. 3. Coliform bacteria are shown to enter groundwater in we1ls in Olmsted County, Minnesota, following rainfall events that exceed 1/4 inch. 4. Drought tolerance in crop plants showed: that lower water potentials of young expanding leaves are the result of solute accumulation, which contributes to the high turgor pressure necessary for growth; that the leaf elongation rate and stomatal conductivity relationship is species characteristic; that accumulation in stem cells of drought adapted plants was greater than non-adapted plants, and that a drought adaptation includes reducing the vascular bundles and vessels. 5. Laboratory experiments on the effects of acid precipitation on Minnesota lakes and in situ porewater sampling have identified sulfate reduction and calcium- cation exchange is major mechanisms of internal alkalinity generation. 6. Efforts to improve liaison between the Water Resources Research Center and State and Federal agencies has resulted in the involvement of eight university researchers from four departments in a cooperative research project with Minnesota Department of Natural Resources and state Planning Agency, the United States Geological survey, and the Natural Resources Research Institute, University of Minnesota, Duluth.
Water Relations and Drought Adaptations in Pisum sativum
(Water Resources Research Center, University of Minnesota, 1985-10) Lee-Stadelmann, O.Y.; Stadelmann, E.J.
The work reported here concerns the analysis of some aspects of the water relations of the adapted plants, The pressure chamber technique during and after water stress was used and light microscopical observations were made to study the cellular and anatomical adaptations. Pisum sativum plants were used ocr this work as a model system for drought adapted plants. We reported earlier that these plants are able to attain drought adaptation. Important features identified in this study as indicators for drought adaptation in this study are the ability of the plants to: 1) maintain high water potential (-6 to -8 bars) and turgor pressure at water deficit conditions; 2) rapid close stomata under water shortage conditions; 3) accumulate solute during a long recovery period: 4) accumulate K+ in the intercellular space, 5) retain more living stem parenchyma cells under water deficit conditions, and to develop smaller vascular bundles and xylem vessels. Despite the development of these features during adaptation, small changes in internal water balance (both water and osmotic potential) were found to be related to larger changes in membrane properties especially those involved in K+ uptake from the medium. This is in line with our earlier observation that membrane adaptation increased lipophily which could be an important factor for drought resistance.
Groundwater Recharge Rates in Minnesota as Related to Precipitation
(Water Resources Research Center, University of Minnesota, 1983-03) Kanivetsky, Roman; Palen, Barbara
Quantitative estimates of recharge rates and storage coefficients in sufficient detail are essential to any meaningful state water-resources management, policies, plans and legislation. A methodology was developed to derive quantitative estimates of groundwater recharge rates and storage coefficients for unconfined aquifers in the various hydrologic regions of Minnesota. Recharge estimates are based on information from groundwater hydrographs. Statistical analysis of groundwater level fluctuations and their correlation with precipitation, evaporation, stream discharge were evaluated. Harmonic analysis and autoregression analysis of water levels were done. Storage coefficients were estimated from pumping test data, soil moisture method and empirical equations. Groundwater recharge rates were determined by combining data on groundwater level fluctuations and storage coefficient evaluation for unconfined aquifers in Minnesota. This work is fundamental to the ultimate goal of an interactive water resources model of the state.
Maximum Application Rates for Land Treatment of Septage
(Water Resources Research Center, University of Minnesota, 1983-05) Anderson, James L.; Clanton, C.J.; Hansel, M.J.; Machmeier, R.E.
During 1980, septage was applied in rates of 1120 and 1500 kg of nitrogen per hectare to three different soil textures in an attempt to determine maximum loading rates. These rates resulted in increased concentrations of nitrates in the soil water for a Hubbard loamy Sanci, Waukegan silt loam and Lester clay loam, indicating that the application rates exceeded the maximum rate that the soils could treat. The first year's results indicate that soil type, application rates and soil depth resulted in no significant difference in total Kjeldahl nitrogen, ammonia, fecal streptococcus and fecal coliforms in the soil water samples. Nitrate concentrations, however, were significantly different between the soils, application rates and soil depths. For the Hubbard loamy sand, rainfall had a larger effect on nitrate concentrations and movement within the soil profile than for the Waukegan silt loam or Lester clay 1oam. 0n the Waukegan silt loam and Lester clay loam there was relatively little change in the nitrate concentration in the soil profile during the period when septage was applied twice a week. After the design loading had been applied to the soil and no further applications made, a sharp increase in nitrate concentrations was observed in the soil profile. This probably resulted from changing the anaerobic surface layer to an aerobic condition resulting in nitrification and subsequent movement of nitrates through the profile following a rainfall event. With no additional septage application, the second year's data indicate a significant difference in nitrate-N between soils, application rates, and depths. Generally, the nitrate concentrations in the Hubbard loamy sand and Waukegan silt loam were less than the first year, but the concentrations in the Lester clay loam were higher than the first year. This indicates that nitrification and nitrate movement in the Lester clay loam are slower than the other two soils. Application resulted in a significant increase in the concentration of soil water calcium, magnesium, sodium and potassium during the first year of the study. However, there was no increase in the phosphorus content of the soil water.
Low Energy Filtration Using Buoyant Media
(Water Resources Research Center, University of Minnesota, 1983-08) Brezonik, P.L.; Chiesa, S.; Semmens, M.J.
The use of a spherical buoyant media in filtration has many potential advantages. The media stratify during backwashing such that the largest spheres are at the top of the bed and the smallest are at the bottom. During upflow filtration solids may be removed by both straining on the underside of the filter and by depth filtration. The accumulated solids may be readiiy dislodged by a brief downflow backwash and gravity assists i n this cleaning process. Basic studies on the removal of coagulated clay suspensions indicated that the clay was removed throughout the depth (D.3m) of the filter for media sizes between 0.6 and 1.0 mm. Straining was not observed to be significant but it would become more important for smaller media sizes or more flocculant solids. Headloss development was monitored and followed the expected behavior for depth filtration. Low headlosses could be maintained by using shallow bed of media and backwashing frequently. These preliminary studies indicate that if the buoyant media are to be used in an effective low energy filter, the system design wi11 be very important. A straining mechanism may be preferred since the solids are retained on the underside of the filter and gravity wi1'l assist in cleaning the bed during backwash. However, to avoid rapid headloss development depth filtration must be used also. Additional studies are needed to characterize the backwashing behavior of the media and to identify the best operating strategies for long-term use.
Policy and Organizational Considerations for Improving Transfer of University Research Information and Effecting Availability of Expertise to State Agencies
(Water Resources Research Center, University of Minnesota, 1984-07) Blake, G.R.
The troubled state of financing of the Water Resources Research Center (WRRC) precipitated a review of the WRRC, its role, its relation to other water resources units or programs in the University, its administrative structure and its financing. An ad hoc committee appointed by the University Vice President reported back with its reommendations on the role of WRRC, in research, in coordination, and in communication and outreach. The ad hoc report was supportive of the need for strengthening and financially supporting an expanded role for WRRC. The University administration sensing a need for developing a coordinate mission for Natural Resources on its various colleges and campuses took action to set up a structure for that purpose. This consisted of appointing an Executive Council, a technical advisory committee and four resources sub-committees of which the Water Resources Subcommittee was one. The Water Resources Subcommittee if activated will probably set the goals and define the future role and authority of not only WRRC but also of other water Centers in the University.
Sulfur Metabolism and Organic Acids: Implications for the Natural and Anthropogenic Acidification of Peatlands and Colored Lakes
(Water Resources Research Center, University of Minnesota, 1989-06) Eisenreich, Steven J.; Urban, Noel R.
On-Site Biodegradation of Organic Pollutants in Contaminated Soils and Groundwater
(Water Resources Research Center, University of Minnesota, 1987-06) Maier, W.J.
Widespread use and improper disposal of chlorinated organic materials have resulted in contamination of soils and groundwater throughout the nation. This research focused on development of remedial technology to rehabilitate contaminated soil and groundwater. Biological treatment was chosen because it is environmentally sound and less costly than methods involving physical-chemical processes. Pentachlorophenol (PCP) was used as a model compound. A mixed microbial culture capable of utilizing PCP as a source of carbon and energy was developed from municipal wastewater treatment plant sludge. The culture was grown in a continuous flow chemostat reactor and used as the inoculum in batch and continuous flow experiments. PCP removal from contaminated groundwater and synthetic media was studied in batch tests. Complete removal and stoichiometric release of chloride ions was observed. The effects of different size sand particles on PCP biodegradation were tested to determine the rates of removal. Short and long sand columns also were tested to determine the rates of PCP removal under saturated and unsaturated flow conditions. The presence of sand particles enhanced biodegradation; small size sand particles (<25 pn) had the greatest effect. The presence of sand particles shortened the lag phase observed when the culture was inoculated into nedium containing high concentrations (-100 mg/L) of PCP. Sand columns operated in unsaturated flow conditions removed PCP more effectively than saturated flow columns. The greater effectiveness of unsaturated flow conditions appears to be due to greater availability of molecular oxygen as well as a difference in flow pattern. Enhanced adsorption of microbial cells on sand and flow of water along grain surfaces resulted in better contact between active microorganisms and PCP under unsaturated flow conditions.
Midseason Soil Water Recharge for Corn in the Northwestern Corn Belt
(Water Resources Research Center, University of Minnesota, 1983-03) Blake, George R.; Johnson, Bradley S.; Nelson, Wallace W.
Incomplete soil water recharge between growing seasons and insufficient growing-season precipitation limit plant growth in southwestern Minnesota. Field experiments were conducted from 1979-81 on a Nicollet clay loam soil (Aquic Hapludoll) to determine the effects of midseason soil water recharge on corn (Zea mays L.) production. Six treatments consisting of timing and amount variables of supplemental water addition were studied. "Daily precipitation and air- temperatures were measured. Soil water contents and potentials were measured with a neutron probe and tensiometers, respectively. Progressive developmnent of soil water deficiencies occurred during the three-year study. Following a wet year in 1979, a shallow-receding water table was present during the 1980 growing season. Extensive soil water depletion to 90 cm occurred in both 1980 and 81, the water table could not be detected within 200 cm of the soil surface in 1981. Despite these diverse conditions, positive and significant grain yield responses to the addition of supplemental water were observed in each year of the study. Grain yield resulting from the midseason application of 7.6 cm of water exceeded grain yield with natural precipitation by 1808, 2130, and IB47 kg/ha tn 1979, 80, and 81, respectively. Grain production was enhanced as effectively by single-midseason applications of 7.6 cm of water as by "optimum irrigation" (application of 3.8 cm at 50 percent depletion of plant-available water to a soil depth of 90 cm). In the presence of a receding water table (1980), 90 percent of the grain yield variability and 92 percent of the variability in total dry matter-production (TMDP) was accounted for by the amount of supplemental water added and early-season water table depth. Response to the addition of supplemental water diminished with decreasing early-season water table depth.
An Experimental Investigation of Evapotranspiration Estimation Methods
(Water Resources Research Center, University of Minnesota, 1982-11) Baker, Donald G.; Ljungkull, Jon Eric
Using meteorological data collected during the 1978, 1979, and 1980 growing seasons at St. Paul, Minnesota, estimates of daily potential evapotranspiration were made using thirteen different calculation methods. These estimates were then compared to measurements of actual evapotranspiration from a cropped surface obtained from the University of Minnesota's weighing lysimeter. The crop was soybeans in 1978 and 1979, and alfalfa in 1980. Simpie linear regression techniques were used to compare the estimated and measured values of evapotranspiration. Scatter plots showing the relationships between the predicted and observed values and histograms of the differences between the two are presented. Summary statistics for each regression and set of difference values are reported. It was found that pan evaporation was most successful at predicting daily evapotranspiration. The methods which combine energy balance and aerodynamic functions were the next best, followed by the methods using radiation and temperature as inputs. The methods based on temperature alone performed most poorly.
Geomorphologic Study of River Basins and Hydrologic Response
(Water Resources Research Center, University of Minnesota, 1992-06) Foufoula-Georgiou, Efi; Helmlinger, Keith R.
It has long been recognized that catchment geomorphology relationships can be used as predictors of catchment flood characteristics. These geomorphologic relationships can be determined for river networks which have been automatically extracted from digital elevation data. If scaling properties exist in a catchment or river network then laws which hold at one scale (for example, basins with horizontal length scale of 1 kilometer) can be extrapolated with appropriate scaling to other scales (such as basins with horizontal length scales of tens of kilometers). This research has examined several basins for the purpose of (1) differentiating between the hillslope and channel scales from digital elevation data, and (2) identifying the presence of scaling in river networks and estimating the scaling laws. The ultimate goal of such research is to relate the findings about scaling in river networks to measures of hydrologic response of the river basin. There is evidence that river networks are fractals which means that small basins, such as sub basins of a larger basin, have statistically similar structure with larger basins. Two methods were used to estimate the fractal dimensions of the terrain surface and the river networks: (1) the variation method, and (2) the box-counting method. Artificial river networks were generated from Iterated Function Systems (IFS) for verification of the box-counting results. Neither of the two methods for determining the fractal dimension of a surface were capable of predicting the breakpoint between the hillslope and channel scales, at least from the resolution at which the digital elevation data were available for this study. The fractal dimension of the branching structure of a river network can be expressed as log RB /log RL (where RB and RL are Horton'8 bifurcation and length ratios) which is equivalent to the fractal dimension of the river network when individual streams have a fractal dimension of unity. In this research we investigated the reliability of estimating the fractal dimension of river networks based on Horton's ratios RB and RL as opposed to estimation from indirect methods such as the box-counting method. It was found that Horton's ratios can be difficult to estimate, and that they remain constant when the threshold area defining the network sources is varied and when the resolution of the digital elevation data is varied. For the two river networks studied (the fractal dimensions of individual streams in both networks were unity) the boxcounting method was shown to be in agreement with the fractal dimension of the branching structure estimated from Horton's ratios. Future research should address the following two problems: (1) Determination of the hillslope scale from morphometric properties of the river network based on the assumption that threshold area does not remain constant over the basin but changes with local slope, and (2) Exploration of Diffusion Limited Aggregation (DLA) models for studying the evolution and structure of river networks and their hydrologic response.
Evaluation of Computed Tomography to Determine the Distribution of Macropores in Soil
(Water Resources Research Center, University of Minnesota, 1991-08) Nieber, John L.; Warner, Glenn S.
Preferential flow paths in some soils result in the rapid movement of water and associated chemicals to groundwater. Macropores are one type of preferential path. Examples of macropores include earthworm tunnels, decayed root channels, shrink/swell cracks, tillage cracks, etc. Water movement through a soil can potentially be modeled if the number, size and extent of the macropores are known. At present no easy and accurate method of characterizing macropores has been developed. X-ray computed tomography (Cf), developed for medical purposes, uses scan techniques to secure multiple views of an object and provides ameans of obtaining nondestructive internal cross sections of objects. The spatial resolution of modern scanners is on the order of I mm, providing a potential means of detecting small air-filled pores within the soil. Undisturbed soil cores, 200 mm in diameter, were taken in cultivated and uncultivated areas. A medical Cf scanner was used to scan these cores at depth intervals varying from 10 mm to 50 mm. Data for each scan were analyzed using microcomputers to display images and to determine the size and number of macropores for each scan image. Cores were physically sectioned at scan locations to visually compare the size, location and continuity of macropores in the sections with those shown in the scan images. Laboratory cores packed with soil were scanned as standards to determine the response and contrast resolution of the Cf scanner for dense material containing sharp density discontinuities. Cores were packed with different types and densities of soil. Artificial "macropores" were formed in some packed cores using glass tubes and holes formed with wire probes to assess the resolution of the scanner. Macropores of various sizes and types were found in all of the field cores scanned with the majority of macropores being associated with earthworm tunnels. The number and size of macropores were approximately the same for all cores below a depth of approximately 50 mm. The number of macropores found above this depth was affected by a large number of roots in the grass-surfaced core and by tillage in the bare-surfaced core. Many macropores were continuous to depths of 600 mm or more. Some passed completely through the core. Dye tests and physical sampling revealed that some of the macropores functioned as preferential flow paths and had very high flow rates. Analysis of the packed cores revealed that the detection of small holes in a dense medium by Cf depends not only on the size of hole but also on the density of the medium. Various attenuation values were examined to determine the optimal threshold value to use for the detection and measurement of macropores in the field cores. The average attenuation value for air in the holes in the cores was not zero as was initially assumed, but varied with the size of hole. The results showed that a CT scanner can accurately determine pore locations and sizes for pores I mm or larger. Accurate determinations of macropore characteristics require that proper values of parameters for the medium and conditions being analyzed be assessed. Use of microcomputers greatly enhanced the ability to display images, analyze features of interest, detect and characterize individual macropores. The main conclusion of the study was that the Cf scanner when coupled with use of computers is a potentially valuable tool for characterizing macropores.
Effects of Climate Variability and "Greenhouse Effect"--Scenarios on Minnesota's Water Resources
(Water Resources Research Center, University of Minnesota, 1992-06) Baker, Donald G.; Newman, Lynn E.; Skaggs, Richard H.
We examined the sensitivity of Minnesota's water resources to predicted climate changes due to global wanning. Statistical models, for the current climate, of the relationship between seasonal moisture surplus (Thornthwaite water budget method) and seasonal river discharge were built for nine watersheds in Minnesota. Global Climate Model predicted temperature changes and precipitation ratios were used to adjust the observed climatic record. The moisture surplus values were recalculated for this doubled C02 world and became the predictor variables in the statistical models. The results indicate a general and substantial decrease in river discharge. However, there is great spatial variation in the magnitude of the decrease. Mean annual discharge decreases ranged from 0.3% for the St. Louis River to 18% for the Blue Earth River. The individual annual discharges were about evenly split between increases and decreases for the St. Louis River while there were only four years (out of 47) with increased discharge for the Blue Earth River. All nine basins showed increased discharge during the first season (three months) of the year largely because increased temperatures shifted the snow melt period to an earlier month. The second season (April through June) had most extensive and largest decreases. In the third season (July through September) decreases were large (13% to 31 %) for the southern and central river basins but were modest (2% to 7%) for the northern basins. During the last three months of the year, five basins had increased discharge and four basins, especially the Mississippi at both Anoka and St. Paul, had decreases. Thus, the southern and central portions of Minnesota would experience substantial decreases in discharge but the northern third of the state would see only modest decreases.
Water Quality Modeling: Terrain Analysis and the Agricultural Non-Point Source Pollution (AGNPS) Model
(Water Resources Research Center, University of Minnesota, 1991-05) Water Resources Research Center

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