Kenneth W. Potter

Hydrological impacts of changing land management practices in a moderate‐sized agricultural catchment

Since the mid-1930s a variety of soil conservation practices have been applied to agricultural lands throughout the United States. While intended to reduce soil erosion, if effective, these practices should alter the hydrology of streams which drain the treated lands. This hypothesis was explored for the East Branch of the Pecatonica River, a gaged 221 square mile agricultural catchment in southwestern Wisconsin. On the basis of the analysis of peak and daily flow data there has been a decrease in flood peaks and in winter/spring flood volumes and an increase in hydrologic rise times and in the contribution of winter/spring snowmelt events to base flow. These changes do not appear to be due to climatic variations, reservoir construction, or major land use changes. Instead, they appear to have resulted from the adoption of various soil conservation practices, particularly those involving the treatment of gullies and the adoption of conservation tillage.

Two-way cluster analysis of geochemical data to constrain spring source waters

Abstract The purpose of the study was to use geochemical characteristics and apparent ages of sampled groundwaters to determine which of the two regionally extensive bedrock aquifers, the lower bedrock aquifer or the upper bedrock aquifer, is a more likely source of water discharging to springs in the Nine Springs watershed. The use of summary statistics and our knowledge of the regional hydrostratigraphy resulted in the identification of three groups of monitoring points that are representative of groundwaters with distinct geochemical characteristics. Two-way cluster analysis of the geochemical data supports these groupings and further identifies subtle geochemical characteristics of the groups. One spring, which is representative of smaller springs and seeps found in the watershed, belongs to a group that is characterized by variable nitrate and chloride concentrations. Water discharging from this spring has a groundwater residence time of approximately 8 years based on the tritium/helium 3 dating method. The water discharging to this small spring is thought to have traveled primarily through the unlithified aquifer, as opposed to either of the major bedrock aquifers. Most of the springs in the watershed belong to a group that is characterized by elevated, but consistent, nitrate, sodium, and chloride concentrations. In addition, cluster analysis revealed that potassium and alkalinity concentrations are somewhat low. Apparent groundwater ages for this group range from 10 to 15 years. The water discharging from the majority of the springs in the watershed is thought to have traveled primarily through the unlithified aquifer and the upper bedrock aquifer before discharging into the former glacial lakebed wetland complex. Due to the relatively short groundwater residence times, spring water quality and flow in the Nine Springs watershed are likely to be vulnerable to the rapid urban expansion occurring within the watershed.

A watershed modeling analysis of fluvial geomorphologic influences on flood peak attenuation

Flood peak attenuation caused by storage of flood water on overbank surfaces effectively reduces the magnitude of peak discharges in some, but not all watersheds. Several geomorphic factors that affect the storage and conveyance of flood water were investigated to assess their quantitative influence on downstream peak discharges. The MIKE11 rainfall-runoff and hydrodynamic models were calibrated for the Grant River watershed, southwestern Wisconsin. Alternative geomorphic conditions were modeled and compared to the original case. Results indicate that channel-floodplain-terrace morphology, valley width, stream slope, and hydraulic roughness each influence peak discharges, especially for moderate magnitude (5- to 50-year recurrence interval) floods. Peak discharges varied by as much as 49% between simulations depending on geomorphic conditions. Watersheds that effectively attenuate produce peak discharges that are strongly correlated with total runoff. Watersheds that attenuate little produce peak discharges that exhibit greater variance due to storm intensity and duration.

Flood frequency analysis of simulated flows

When conventional statistical techniques are used with model-simulated flows to estimate flood frequencies for different basin scenarios or for regulated flow conditions, the resulting estimates can appear inconsistent with the simulated flood data. In this paper we describe a new approach for flood frequency analysis of model-simulated flows. Rather than fitting a probability distribution directly to simulated peak discharges, separate distributions are estimated for runoff volumes and for peak discharges conditioned on runoff volumes. The latter estimation is based on rainfall-runoff modeling using historical rainfall data for the basin of interest as well as data from extreme storms that have occurred in the region. The modeling of extreme events forces consistency in the resulting flood frequency estimates. An example illustrates the proposed method.

Estimating the upper tail of flood‐peak frequency distributions using hydrometeorological information

In its 1988 report, the National Research Council Committee on Estimating the Probabilities of Extreme Floods identified principles for improving the estimation of floods with annual exceedance probabilities of the order of 10−3 or smaller. This paper presents a methodology for estimating the probability of extreme floods which makes use of these principles. Its core is the “peak-to-volume” method [Bradley and Potter, 1992], which is based on separately estimating the probability distributions of flood volumes and flood peaks conditioned on volumes. In its original form it relies on extrapolating a conventionally estimated probability distribution, the distribution of flood volumes. To strengthen this step, we propose use of the gradient of extreme values (GRADEX) method [Guillot and Duband, 1967], in which the upper tail of the flood volume distribution is deduced from that Of precipitation. We enhance the GRADEX method by exploiting the work of Smith [1989], who developed a regional model for estimating the upper tail of a frequency distribution based on extreme order statistics. An example illustrates the proposed method.

Mixed Flood Distributions in Wisconsin

Traditional flood frequency analysis is predicted on the assumption that the annual flood series can be considered to be a sample from a single population. In Wisconsin this is not a valid assumption. Wisconsin floods are of two types, which are hydrologically and statistically distinct.

Impacts of Agriculture on Aquatic Ecosystems in the Humid United States

Agricultural development of the United States profoundly altered hydrologic processes, resulting in severe degradation of aquatic ecosystems. The conversion of natural landscapes to agricultural lands increased the peak and volume of surface runoff, causing massive soil erosion, stream channel enlargement, and introduction of enormous quantities of fine-grained sediment to lakes, streams, floodplains, and wetlands. During the years following the creation of the U.S. Soil Conservation Service in 1935, the gradual adoption of conservation practices resulted in significant reductions in surface runoff, soil erosion and sediment transport, as well as increases in baseflow. This amelioration of agricultural impacts is not well documented in most areas. More recently, increasing yield pressures, loss of soil fertility, and poor nutrient management have resulted in excess agricultural fertilizer use. Today, by adopting appropriate agricultural practices, it is possible to greatly reduce, and in some cases eliminate, adverse impacts of agriculture. However, the historical use of damaging agricultural practices has left a lasting legacy in many regions. Some of these legacies, such as channel straightening, can and have been reversed by restoration. Others, such as fine-grained sediment in streams, channels disconnected from their floodplains, and eutrophication due to excess nutrients, are much more difficult to reverse and represent a fundamental change in riparian habitat. Restoration of this habitat is infeasible in many situations.

Estimating the Probability Distribution of Annual Maximum Levels on the Great Lakes

Abstract The traditional approach to estimating the probability distribution of annual maximum water levels on the Great Lakes has been to treat the observed water level series as random samples, ignoring the fact that annual maximum lake levels are correlated in time. This paper provides an approach for estimating lake level probabilities which explicitly accounts for this correlation. The approach is based on decomposing annual maximum water levels into two components, which separately account for hydrologic variations and storm effects. By conditioning the distribution of the hydrologic component on an initial level and by convoluting the resulting distribution with the distribution of storm effects, the conditional probability distribution of annual maximum water levels is obtained for any number of years in the future. The approach is illustrated using data from Lake Erie.

Modelling the Error in Flood Discharge Measurements

Abstract The measurement of peak discharge is typically composed of three distinct processes. Low magnitude floods are determined with an established rating curve. Intermediate magnitude floods are inferred by extrapolating the established rating curve. High magnitude floods are usually determined through a field survey. The measurement error characteristics for each process are different, a phenomenon termed discontinuous measurement error (dme). Monte Carlo experiments with an error model that approximates the peak measurement process reveal a bias in the measured coefficients of variation, skewness, and kurtosis. This bias is significant and has important implications with regard to-flood frequency analysis.

CUAHSI supports workshops on the future of hydrologic science

With support from the National Science Foundation, the Consortium for the Advancement of Hydrologic Science, Inc. (CUAHSI) was formed in 2001 to develop a broad-based research and education agenda for hydrologic science in U.S. universities, and to facilitate the acquisition of the resources needed to implement this agenda. At the time of this article, 60 universities had become members. Early in 2002, CUAHSI conducted a series of six regional workshops, in Chicago, Atlanta, Phoenix, San Francisco, Philadelphia, and Boston, to introduce the hydrologic research community to CUAHSI, solicit input to the CUAHSI committees developing infrastructure and science plans, and foster a sense of community among hydrologic scholars. CUAHSI also encouraged member universities to conduct local workshops patterned after the regional ones; to date, such workshops have been held at the University of Wisconsin, the University of Colorado, and the University of California at Riverside. Summaries of the activities at each of these workshops are posted on the CUAHSI Web site (www.cuahsi.org).