A STRIP MODEL APPROACH TO PARAMETERIZE A COUPLED GREEN‐AMPT KINEMATIC WAVE MODEL1

ABSTRACT: Infiltration processes at the plot scale are often described and modeled using a single effective hydraulic conductivity (Kg) value. This can lead to errors in runoff and erosion prediction. An integrated field measurement and modeling study was conducted to evaluate: (1) the relationship among rainfall intensity, spatially variable soil and vegetation characteristics, and infiltration processes; and (2) how this relationship could be modeled using Green and Ampt and a spatially distributed hydrologic model. Experiments were conducted using a newly developed variable intensity rainfall simulator on 2 m by 6 m plots in a rangeland watershed in southeastern Arizona. Rainfall application rates varied between 50 and 200 mm/hr. Results of the rainfall simulator experiments showed that the observed hydrologic response changed with changes in rainfall intensity and that the response varied with antecedent moisture condition. A distributed process based hydrologic simulation model was used to model the plots at different levels of hydrologic complexity. The measurement and simulation model results show that the rainfall runoff relationship cannot be accurately described or modeled using a single Kg value at the plot scale. Multi‐plane model configurations with infiltration parameters based on soil and plot characteristics resulted in a significant improvement over single‐plane configurations.     

APPLICABILITY OF THE GREEN AND AMPT INFILTRATION EQUATION TO RANGELANDS1

ABSTRACT: The objective of this study was to investigate the use of Green and Ampt infiltration equation parameters (determined by least squares fitting of field infiltration data or predicted from soil texture properties) to characterize infiltration on spatially varying rangeland sites. It was found that a least squares regression approach reduces the physically based parameters in the Green and Ampt to empirical coefficients since negative coefficients are obtained, particularly on plots with low infiltration rates. Green and Ampt parameters predicted from soil texture data describe infiltration rates less than 3 cm/hr. The applicability of these Green and Ampt parameters appears limited to sites with lower infiltration rates. Results indicate that soil texture predictive triangles, developed to describe infiltration on agricultural soils, need revision to adequately describe infiltration patterns on rangelands.     

EFFECTS OF IMPROVED PRECIPITATION ESTIMATES ON AUTOMATED RUNOFF MAPPING: EASTERN UNITED STATES1

ABSTRACT: We evaluated maps of runoff created by means of two automated procedures. We implemented each procedure using precipitation estimates of both 5-km and 10-km resolution from PRISM (Parameter-elevation Regressions on Independent Slopes Model). Our goal was to determine if using the 5-km PRISM estimates would improve map accuracy. Visual inspection showed good general agreement among our runoff maps, as well as between our maps and one produced using a manual method. A quantitative uncertainty analysis comparing runoff interpolated from our maps with gage data that had been withheld showed slightly smaller actual and percentage interpolation errors for the 5-km PRISM-based maps. Our analyses suggest a modest region-wide improvement in runoff map accuracy with the use of PRISM-based precipitation estimates of 5-km (compared to 10-km) resolution.     

RUNOFF AND EROSION RESPONSE OF SIMULATED WASTE BURIAL COVERS IN A SEMI-ARID ENVIRONMENT1

ABSTRACT: Control of runoff (reducing infiltration) and erosion at shallow land burials is necessary in order to assure environmentally safe disposal of low-level radioactive-waste and other waste products. This study evaluated the runoff and erosion response of two perennial grass species on simulated waste burial covers at Idaho National Engineering and Environmental Laboratory (INEEL). Rainfall simulations were applied to three plots covered by crested wheatgrass [Agropyron desertorum(Fischer ex Link) Shultes], three plots covered by streambank wheatgrass [Elymus lanceolatus(Scribner and Smith) Gould spp. lanceolaus], and one bare plot. Average total runoff for rainfall simulations in 1987, 1989, and 1990 was 42 percent greater on streambank wheatgrass plots than on crested wheatgrass plots. Average total soil loss for rainfall simulations in 1987 and 1990 was 105 percent greater on streambank wheatgrass plots than on crested wheatgrass plots. Total runoff and soil loss from natural rainfall and snowmelt events during 1987 were 25 and 105 percent greater, respectively, on streambank wheatgrass plots than on crested wheatgrass plots. Thus, crested wheatgrass appears to be better suited in revegetation of waste burial covers at INEEL than streambank wheatgrass due to its much lower erosion rate and only slightly higher infiltration rate (lower runoff rate).     

A REVISED VERSION OF PnET‐II TO SIMULATE THE HYDROLOGIC CYCLE IN SOUTHEASTERN FORESTED AREAS1

ABSTRACT: The PnET‐II model uses hydroclimatic data on maximum and minimum temperatures, precipitation, and solar radiation, together with vegetation and soil parameters, to produce estimates of net primary productivity, evapotranspiration (ET), and runoff on a monthly time step for forested areas. In this study, the PnET‐II model was employed to simulate the hydrologic cycle for 17 Southeastern eight‐digit hydrologic unit code (HUC) watersheds dominated by evergreen or deciduous tree species. Based on these control experiments, model biases were quantified and tentative revision schemes were introduced. Revisions included: (1) replacing the original single soil layer with three soil layers in the water balance routine; (2) introducing calibrating factors to rectify the phenomenon of overestimation of ET in spring and early summer months; (3) parameterizing proper values of growing degree days for trees located in different climate zones; and (4) adjusting the parameter of fast‐flow (overland flow) fraction based on antecedent moisture condition and precipitation intensity. The revised PnET‐II model, called PnET‐II3SL in this work, substantially improved runoff simulations for the 17 selected experimental sites, and therefore may offer a more powerful tool to address issues in water resources management.     

INPUTS OF COPPER‐BASED CROP PROTECTANTS TO COASTAL CREEKS FROM PASTICULTURE RUNOFF1

ABSTRACT: Inputs of copper‐based crop protectants from tomato fields grown under plastic mulch agriculture (plasticulture) to an estuarine creek were investigated. Copper was measured in runoff from diverse land‐uses including conventional agriculture, plasticulture, residences, and natural areas. Water column and sediment copper concentrations were measured in plasticulture and control (nonagriculture) watersheds. Copper concentrations in plasticulture‐impacted creeks exceeded background levels episodically. High concentrations occurred during or immediately after runoff‐producing rains. Concentrations of 263 μg/L total copper and 126 μg/L dissolved copper were measured in a tidal creek affected by plasticulture; concentrations exceeded the shellfish LC₅₀ values and the water quality criteria of 2.9 μg/L dissolved copper. Control watersheds indicated background water column levels of ≤ 4 μg/L dissolved copper with similar copper levels during periods with and without rain. The copper concentrations in tomato plasticulture field runoff itself contained up to 238 μg/L dissolved copper. Copper concentrations in runoff from other land‐uses were less than 5 μg/L dissolved copper. Creek sediment samples adjacent to a plasticulture field contained significantly higher copper concentrations than sediments taken from nonplasticulture watersheds.     

TEMPORAL RESPONSES OF SURFACE‐WATER AND GROUND‐WATER TO PRECIPITATION IN ILLINOIS1

ABSTRACT: Illinois data from 168 months (1986–1999) were investigated to determine the responses of surface‐water and ground‐water resources to precipitation. Such responses were generally within the month of occurrence or one to two months later, with recovery being reached another one to three months into the future, depending on season of the year. Although the drought of 1988 immediately impacted surface‐water and ground‐water resources, the time of recovery was substantially longer compared to those of individual dry months, generally continuing for several months. The extremely wet summer of 1993 resulted in elevated responses in water resources almost immediately, but in this instance continued through the following fall and winter, into the spring of 1994.     

GENETIC PROGRAMMING AND ITS APPLICATION IN REAL‐TIME RUNOFF FORECASTING1

ABSTRACT: Genetic programming (GP), a relatively new evolutionary technique, is demonstrated in this study to evolve codes for the solution of problems. First, a simple example in the area of symbolic regression is considered. GP is then applied to real‐time runoff forecasting for the Orgeval catchment in France. In this study, GP functions as an error updating scheme to complement a rainfall‐runoff model, MIKE11/NAM. Hourly runoff forecasts of different updating intervals are performed for forecast horizons of up to nine hours. The results show that the proposed updating scheme is able to predict the runoff quite accurately for all updating intervals considered and particularly for updating intervals not exceeding the time of concentration of the catchment. The results are also compared with those of an earlier study, by the World Meteorological Organization, in which autoregression and Kalman filter were used as the updating methods. Comparisons show that GP is a better updating tool for real‐time flow forecasting. Another important finding from this study is that nondimensionalizing the variables enhances the symbolic regression process significantly.     

HYDROLOGIC IMPLICATIONS OF GREATER GROUND‐WATER RECHARGE TO LAS VEGAS VALLEY,NEVADA1

ABSTRACT: Published estimates of natural recharge in Las Vegas Valley range between 21,000 and 35,000 acre‐feet per year. This study examined the underlying assumptions of previous investigations and evaluated the altitude‐precipitation relationships. Period‐of‐record averages from high altitude precipitation gages established in the 1940s through the 1990s, were used to determine strong local altitude‐precipitation relationships that indicate new total precipitation and natural recharge amounts and a new spatial distribution of that recharge. This investigation calculated about 51,000 acre‐feet per year of natural recharge in the Las Vegas Hydrographic Basin, with an additional 6,000 acre‐feet per year from areas tributary to Las Vegas Valley, for a total of 57,000 acre‐feet per year. The total amount of natural recharge is greater than estimates from earlier investigations and is consistent with a companion study of natural discharge, which estimated 53,000 acre‐feet per year of outflow. The hydrologic implications of greater recharge in Las Vegas Valley infer a more accurate ground‐water budget and a better understanding of ground‐water recharge that will be represented in a ground‐water model. Thus model based ground‐water management scenarios will more realistically access impacts to the ground‐water system.     

ACCURACY AND CONSISTENCY OF WATER‐CURRENT MIETERS1

ABSTRACT: The measurement of discharge in natural streams requires hydrographers to use accurate meters that have consistent performance among meters of the same model. This paper presents the results of an investigation into the accuracy and consistency of four models of current meters‐Price Type‐AA, Price Pygmy, Marsh McBirney 2000, and Swoffer 2100. Test results for six meters of each model are presented. Variation of meter performance within a model is used as an indicator of consistency, and percent velocity error that is computed from a measured reference velocity is used as an indicator of meter accuracy. Velocities measured by each meter are also compared to the manufacturer's published or advertised accuracy limits. The investigation found the Price models to be more accurate and consistent than the other models. The Price models met their respective accuracy limits over the range of test velocities better than the other models. The Marsh McBirney model usually measured within its accuracy specification. The Swoffer meters did not meet the stringent Swoffer accuracy limits for all the velocities tested. The Swoffer model had accuracies similar to the Price Type‐AA model when individual meter rating equations were computed and used. Every model tested had meters that did not meet manufacturer accuracy limits. Because current meters are not consistently accurate within a model, hydrographers should periodically check meters against a velocity standard.     

TREE‐RING BASED RECONSTRUCTIONS OF INTERANNUAL TO DECADAL SCALE PRECIPITATION VARIABILITY FOR NORTHEASTERN UTAH SINCE 1226 A.D.1

ABSTRACT: Samples from 107 piñon pines (Pinns edulis) at four sites were used to develop a proxy record of annual (June to June) precipitation spanning the 1226 to 2001 AD interval for the Uinta Basin Watershed of northeastern Utah. The reconstruction reveals significant precipitation variability at interannual to decadal scales. Single‐year dry events before the instrumental period tended to be more severe than those after 1900. In general, decadal scale dry events were longer and more severe prior to 1900. In particular, dry events in the late 13th, 16th, and 18th Centuries surpass the magnitude and duration of droughts seen in the Uinta Basin after 1900. The last four decades of the 20th Century also represent one of the wettest periods in the reconstruction. The proxy record indicates that the instrumental record (approximately 1900 to the Present) underestimates the potential frequency and severity of severe, sustained droughts in this area, while over representing the prominence of wet episodes. In the longer record, the empirical probability of any decadal scale drought exceeding the duration of the 1954 through 1964 drought is 94 percent, while the probability for any wet event exceeding the duration of the 1965 through 1999 wet spell is only 1 percent. Hence, estimates of future water availability in the Uinta Basin and forecasts for exports to the Colorado River, based on the 1961 to 1990 and 1971 to 2000 “normal” periods, may be overly optimistic.     

Generalized Nondimensional Depth‐Discharge Rating Curves Tested on Florida Streamflow1

Abstract: A discharge rating is a relationship between stage and discharge at a specific point in a river stream or lake outlet structure. Rating curves are useful for interpolating and perhaps extrapolating flow measurements and for use directly in storage routing models. However, rating data and stations are limited. A generalized nondimensional mathematical expression that describes the rating relation of depth and discharge has been developed and tested against observations from 46 stations in West‐Central Florida. Three approaches were tested in sequence to select the best fit. The proposed model is a log‐linear equation with zero intercept and a slope that fits more than 50% of the stations were analyzed. The model is normalized by the depth and discharge values at 10% exceedance from data published by the U.S. Geological Survey. For ungauged applications, Q₁₀ and d₁₀ were derived from a relationship shown to be reasonably well correlated to the watershed drainage area. The average relative error for this parameter set shows that for the flow range up to the Q₁₀ discharge, better than 30% agreement with the USGS rating data can be expected for about 50% of the stations. Further analysis is required to determine why so many stations exhibit such similar behavior and to identify the criteria or parameters governing the differences.     

IRRIGATION METHODS AND TRANSPORT OF IMAZAMETHABENZ‐METHYL TO GROUND WATER: GREENFIELDS BENCH,MONTANA1

ABSTRACT: A series of gravel terraces support a shallow aquifer that is the sole source of drinking water for three public water supplies and more than 400 private wells on the Greenfields Bench in west‐central Montana. Farming practices on the Greenfields Bench include irrigation of malting barley and the yearly application of herbicides for the control of weeds. The most commonly used herbicide (imazamethabenz‐methyl, U.S. trade name Assert®) has been found in the ground water on the Greenfields Bench. An experiment was conducted in 2000 and 2001 to characterize the transport of Assert and its acid metabolite to ground water under three irrigation methods: flood, wheel line sprinkler, and center pivot sprinkler. Results show that Assert concentrations in ground water are controlled by hydraulic loading rates of each irrigation method, Assert persistence in soil, hydraulic characteristics of the aquifer, and adsorption/desorption of Assert onto clay particles and organic matter.