Actual evapotranspiration (ETa) is an essential variable in linking energy cycles, carbon, and water, yet challenging to measure. Inputs uncertainty and deficiencies in the key elements of hydrologic models are fundamental challenges for optimizing model performance. Furthermore, the performance of land surface model-based ETa, reanalysis, and remote sensing products varies with spatiotemporal scales. Here, we evaluate sources of bias in the regional Wisconsin Irrigation and Scheduling Program (WISP) model and develop a correction using eddy covariance (EC) observations. ETa, observations were made for five years (2018–2022) using EC systems in agricultural fields in Wisconsin. WISP model ETa bias was linked to underestimation of net longwave radiation (LWnet) that was traced to incorrect specification of effective clear sky atmospheric emissivity (εa,clr). Applying a correction to the εa,clr led to reduced WISP model percent bias (pbias) and error for both LWnet and ETa. The calibrated model more accurately represented observed ETa. The results indicate that explicit treatment of the LWnet balance decreases the uncertainty of model parameters and improves the WISP model performance at independent sites. Applying this improved model parameterization reduced the bias of LWnet radiation from 62.8% to -6.2%, which improved the Nash-Sutcliffe Efficiency (NSE) from -0.08 to 0.52 for ETa on training sites. Additionally, overall pbias was significantly reduced (p = 0.035) for validation sites after WISP correction. Hence, WISP performance improved for different crop types when optimal regional parameters were used, confirming the physical parameters' reliability. Our results highlight that model development should focus on energy balance parameterizations to improve ET simulation and the accuracy of hydrologic and climatic simulations for understanding critical processes underlying hydrologic and climatic variability and change over land.

The use of atmospheric fallout radionuclides as sediment tracers can provide valuable information on sediment transport processes within watersheds. Research was conducted in cropland and mixed land use subwatersheds of the Pleasant Valley watershed (50 km(2)) in south-central Wisconsin to identify sources of suspended sediment using cesium-137 (Cs-137) and excess lead-210 (Pb-210(xs)). In the cropland-dominated subwatershed, contributions to suspended sediment from in-stream and agricultural sources ranged from 36% to 81% and from 19% to 64%, respectively. In the mixed land use subwatershed, contributions to suspended sediment from in-stream and agricultural sources ranged from 68% to 100% and from 0% to 32%, respectively. The relative contribution to suspended sediment from agricultural sources was greater in the cropland subwatershed compared to the mixed land use subwatershed. The phosphorus (P) concentration in suspended sediment and fine sediment deposited on the streambed varied from 601 to 1545 mg kg(-1), and cropland areas were the dominant sources of P-enriched sediment in streams. The legacy P and sediment in this watershed can result in a lag time between implementation of best management practices and attainment of desired water quality improvement. Depending on the fingerprinting properties considered (metals vs. fallout radionuclides) and suspended sediment source considerations, the source apportionment results may vary.

This study tested a focused strategy for reducing phosphorus (P) and sediment loads in agricultural streams. The strategy involved selecting small watersheds identified as likely to respond relatively quickly, and then focusing conservation practices on high-contributing fields within those watersheds. Two 5,000 ha (12,360 ac) watersheds in the Driftless Area of south central Wisconsin, previously ranked in the top 6% of similarly sized Wisconsin watersheds for expected responsiveness to conservation efforts to reduce high P and sediment loads, were chosen for the study. The stream outlets from both watersheds were monitored from October of 2006 through September of 2016 for streamflow and concentrations of sediment, total P, and, beginning in October of 2009, total dissolved P. Fields and pastures having the highest potential P delivery to the streams in each watershed were identified using the Wisconsin P Index (Good et al. 2012). After three years of baseline monitoring (2006 to 2009), farmers implemented both field- and farm-based conservation practices in one watershed (treatment) as a means to reduce sediment and P inputs to the stream from the highest contributing areas, whereas there were no out-of-the-ordinary conservation efforts in the second watershed (control). Implementation occurred primarily in 2011 and 2012. In the four years following implementation of conservation practices (2013 through 2016), there was a statistically significant reduction in storm-event suspended sediment loads in the treatment watershed compared to the control watershed when the ground was not frozen (p = 0.047). While there was an apparent reduction in year-round suspended sediment event loads, it was not statistically significant at the 95% confidence level (p = 0.15). Total P loads were significantly reduced for runoff events (p < 0.01) with a median reduction of 50%. Total P and total dissolved P concentrations for low-flow conditions were also significantly reduced (p < 0.01) compared to the control watershed. This study demonstrated that a strategy that first identifies watersheds likely to respond to conservation efforts and then focuses implementation on relatively high-contributing fields within those watersheds can be successful in reducing stream P concentrations and loads.

Grassed waterways (GWWs) transport sediment and nutrients from upland source areas to receiving waters. Watershed planners have acritical need to understand GWW sediment delivery to optimally target source area management practices. Better physically based tools are needed to estimate sediment delivery by GWWs. This study developed several distributed sediment delivery ratio (SDR) regressions for GWWs using the process-based Water Erosion Prediction Project (WEPP) model to provide simple equations to estimate sediment delivery for planning applications. Water Erosion Prediction Project was calibrated and validated for runoff and sediment yield for large 30.2-ha and smaller 5.7-ha nested watersheds with terraces and a common GWW outlet. A crop rotation of corn, oat and alfalfa and fall tillage using chisel plow were used in the nested watersheds. A hypothetical management case without terraces using corn, oat and alfalfa rotation with chisel plow as fall tillage was also evaluated for the 5.7-ha watershed and the GWW. The length, slope, Manning's roughness coefficient and infiltration rate for the GWW were varied and SDRs calculated for 30 representative (in terms of daily rainfall) days over a 20-year period of simulated climate. Regressions were developed for the existing (terraced) and hypothetical (non-terraced) management scenarios for early (April-July), late (August-October) and full (April-October) growing seasons. Equations developed for the non-terrace watershed had higher R-2 values compared to the terraced watershed suggesting that channel and rainfall parameters were better able to explain the variation in SDR for the non-terraced watershed. Manning's roughness coefficient was the most significant parameter for predicting SDR for both the terraced and non-terraced watersheds. The equations developed here can be used to estimate SDRs for watersheds that are drained via GWWs having similar physical characteristics: slope (1-5%), Mannings's roughness coefficient (0.1-0.3), length (0.15-1km) and infiltration rate (0.025-25 mm h(-1)). The SDRs can be used to estimate sediment yield, which is an essential element for making land management decisions but is rarely measured. Copyright (C) 2017 John Wiley & Sons, Ltd.

Identification of areas contributing disproportionately high amount of pollutants (i.e., critical source areas (CSAs)) to streams is important to efficiently and effectively target best management practices (BMPs). Process-based models are commonly used to identify CSAs and evaluate the impact of alternative management practices on pollutant load reductions. The objective of this study was to use the Soil and Watershed Assessment Tool (SWAT) to identify CSAs at the subwatershed level and evaluate the impact of alternative BMPs on sediment and total phosphorus (TP) load reductions in the Pleasant Valley watershed (50km2) in South Central Wisconsin (USA). The Nash-Sutcliffe efficiency, percent bias, and coefficient of determination ranged from 0.58 to 0.71, −12.87 to 38.33, and 0.67 to 0.79, respectively, indicating that SWAT was able to predict stream flow, sediment and TP loadings at a monthly time-step with sufficient accuracy. Based on the SWAT simulation results, annual average (2006–2012) subwatershed yield for sediment and TP ranged from 0.06 to 3.14tonsha−1yr−1 and 0.04 to 1.9kgha−1yr−1, respectively. The croplands were the major source of sediment and TP in this watershed (≥84%). Reduction in sediment and TP loading ranged from 66% to 99% at the subwatershed level after conversion of croplands to Conservation Reserve Program (CRP) grasslands in subwatersheds identified as CSAs. On the other hand, reduction in sediment and TP loading with implementation of no-till practices ranged from only 14% to 25%. At the watershed outlet, sediment and TP loading reduction was ≤15% after conversion of croplands to CRP grasslands and implementation of no-till practices because only about 8% of the watershed area was targeted for BMPs and/or resuspension of sediment deposited on the stream bed masked the downstream improvements in water quality.

Manure losses from crop fields via surface and subsurface pathways following land application can lead to eutrophication and habitat destruction in surface waters. High solids manure has less susceptibility to runoff and leaching, which is particularly important in high risk fields with increased loss potential. Unfortunately, the determination of manure solids content is typically conducted by laboratory analysis where results are provided after field application has occurred. An inexpensive solids tester (less than $40 to construct) was developed out of readily available materials for manure applicators to determine the solids content of liquid dairy manure in-field and in real time prior to application. As manure samples are passed through the screen, solids are captured on the screen while more dilute manure passes through. In order to evaluate the solids tester, 33 manure samples were collected from 6 separate dairies with a variety of bedding types (3 farms used sand and 3 farms used recycled digested solids for bedding) and manure handling and processing systems. Results indicated that the manure volume that is retained on the screen can be related to the solids content of dairy manure from 3% to 9%. An equation was produced to relate manure solids to the volume retained on the screen, TS=1.7953e(0.0046Vr), with an r-squared value of 0.82. Manure samples with less than 3% solids were shown to pass through the screen completely, therefore results were inaccurate for low TS content. The results did not vary significantly for dairies which used sand bedding and those that used recycled manure solids. This quick in-field measurement of total solids content allows for application to appropriate fields, thus reducing the risk of manure losses to nearby waterways.

Phosphorus (P) loss from agriculture can compromise the quality of receiving water bodies. For cattle farms, P can be lost from cropland, pastures, and outdoor animal lots. We developed a new model that predicts annual runoff, total solids loss, and total and dissolved P loss from cattle lots. The model requires input for annual precipitation, lot surface type, soil test P for earthen lots, cattle number and type, frequency of cleaning, and percent vegetative cover. The model estimates annual runoff using a precipitation dataset and curve number, annual solids loss based on annual runoff, annual particulate P loss based on solid loss and manure and soil P content, and annual dissolved P loss for each runoff event. Testing showed that the model reliably estimated runoff, solids loss, and P loss from a wide variety of lots and was more accurate than other, currently used models. The new model provides a valuable tool for developing whole-farm estimates of P loss and more effectively targeting P loss mitigation practices.

Overlap loss in harvesting machinery has been observed as a necessary inefficiency for many years. Each time a non-row-crop machine makes a pass in the field it is favorable for the operator to overlap slightly into the previous pass, for instance where the crop has already been cut, as opposed to undercutting and leaving a strip of crop standing. In this study, overlap loss was explored through a controlled experiment as well as through on-farm survey. The experimental portion employed a factorial design to study the influence of two drivers and three cutting speeds (6.4, 9.7, and 12.9 km/h) of a self-propelled windrower. Driver experience and mowing speed were not found to have a significant effect on overlap. However, the interaction of inexperienced operators and higher mowing speeds increased overlap. The on-farm survey included three mower configurations: pull-type, self-propelled windrower, and mounted, as well as; cutting widths ranging from 4.01 to 9.60 m and a variety of field conditions on 15 farms. Three of the mounted mowers and one self-propelled windrower utilized automatic guidance. Surveyed overlap ranged from 0.4 to 16.13% of machine cutting width. Average loss was 5% of cutting width. Automatic guidance has been purported to improve efficiency by eliminating time spent covering already mowed ground, reducing operator fatigue, and ensuring a uniform cutting pattern and swath density. The use of a GNSS-based guidance system to steer the mower was shown to reduce overlap loss from 5.03% to 2.34%.

The Wisconsin Phosphorus Index (WPI) is one of several P indices in the United States that use equations to describe actual P loss processes. Although for nutrient management planning the WPI is reported as a dimensionless whole number, it is calculated as average annual dissolved P (DP) and particulate P (PP) mass delivered per unit area. The WPI calculations use soil P concentration, applied manure and fertilizer P, and estimates of average annual erosion and average annual runoff. We compared WPI estimated P losses to annual P loads measured in surface runoff from 86 field-years on crop fields and pastures. As the erosion and runoff generated by the weather in the monitoring years varied substantially from the average annual estimates used in the WPI, the WPI and measured loads were not well correlated. However, when measured runoff and erosion were used in the WPI field loss calculations, the WPI accurately estimated annual total P loads with a Nash-Sutcliffe Model Efficiency (NSE) of 0.87. The DP loss estimates were not as close to measured values (NSE = 0.40) as the PP loss estimates (NSE = 0.89). Some errors in estimating DP losses may be unavoidable due to uncertainties in estimating on-farm manure P application rates. The WPI is sensitive to field management that affects its erosion and runoff estimates. Provided that the WPI methods for estimating average annual erosion and runoff are accurately reflecting the effects of management, the WPI is an accurate field-level assessment tool for managing runoff P losses.

Phosphorus (P) is an essential nutrient for plant and livestock growth. However, P loss in agricultural runoff can increase the frequency of toxic algal blooms and fish kills in receiving waters. Agricultural P loss occurs in both dissolved and particulate (sediment bound) forms. Suspended sediments play an important role in the transport of particulate P from fields to surface waters. Implementing appropriate management practices to control soil erosion and subsequent sediment delivery requires quantification of the relative contribution of sediment sources (e.g. stream bed, stream bank and upland areas under various land uses). Sediment fingerprinting using atmospheric fallout radionuclides can be used to apportion sediment sources, and thus provide valuable guidance for management decisions. Due to their long half-lives, the fallout radionuclides 137Cs and unsupported 210Pb are ideally suited for evaluating sediment transport processes that occur over long time scales. This fingerprinting method is independent of soil and rock type and can be used to differentiate between surficial and channel sources of suspended sediments. The objective of this study was to identify sources of in-stream suspended sediment in an agricultural watershed using the atmospheric fallout radionuclides 137Cs and 210Pb. The study was conducted in the non-glaciated region of southwestern Wisconsin in the Sugar Pecatonica River Basin, which is part of the Upper Mississippi River Basin. The watershed is approximately 5000 ha in size and contains primarily agriculture, forest, and grass land cover. The average watershed slope is about 11% with silt loam soils. Fieldwork included collection of both source materials (upland, streambed, and stream bank) and in-stream suspended sediments. In-stream suspended sediment samples were collected monthly for four months using passive time integrated in-stream tube samplers (Phillips et al., 2000). The samplers consist of a 10.2 cm diameter PVC tube with 0.4 cm diameter inlet and outlet, and collect a sample that is statistically representative of the grain size distribution in small streams. All source material samples were collected from the top 2.5 cm. Upland soil samples were collected from fields that represented various combinations of land use, soil type, and slope within the watershed. Upland samples were collected in a 20 m x 20 m grid with 5 m spacing and composited for analysis. Representative samples were also collected from the top 2.5cm of stream beds and eroding stream banks. All samples collected were stored at 40 C and analyzed for organic matter content (percent volatile solids) and 137Cs and unsupported 210Pb. Radionuclide analysis was done through low background gamma counters. Over a four month period (mid-April through mid-August, 2010), results indicate that approximately two-thirds of in-stream suspended sediment originated from eroding stream banks and the remainder from upland areas. Within the upland categories (cultivated, pasture, woodland, grassland), cultivated lands followed by woodlands were significant contributors to in-stream sediments.

Phosphorus (P) export from agricultural lands above known threshold levels can result in adverse impacts to receiving water quality. Phosphorus loss occurs in dissolved and sediment‐bound, or particulate phosphorous (PP), forms, with the latter often dominating losses from row‐cropped systems. To target practices, land managers need good computer models and model developers need good monitoring data. Sediment monitoring data (e.g. radiometric finger printing and sediment P sorption capacity) can help identify sediment source areas and improve models, but require more sediment mass than is typically obtained by automatic sampling. This study compares a simple suspended sediment sampler developed at the University of Exeter (UE) with automatic sampling in intermittent channels draining corn and alfalfa fields. The corn field had a greater runoff coefficient (27%) than alfalfa (11%). No differences were found in enrichment ratios (sediment constituent/soil constituent) in PP (PPER) or percent loss on ignition (LOIER) between paired UE samplers on corn. The median LOIER for the UE samplers (1·9%) did not differ significantly (p > 0·13) from the automatic sampler (2·0%). The PPER from the UE samplers was on average 20% lower than the automatic samplers. A correlation (r2 = 0·75) was found between sediment PP and % LOI from automatic samplers and UE samplers for particles < 50 µm, while for > 50 µm PP concentration did not change with changes in % LOI. Sediment ammonium‐oxalate extractable metals were similarly related to LOI, with the strongest correlation for iron (r2 = 0·71) and magnesium (r2 = 0·70). Copyright © 2011 John Wiley & Sons, Ltd.

Limited comparisons of sediment and phosphorus (P) loss dynamics from agricultural fields under snowmelt and rainfall runoff conditions exist despite significant differences in underlying particle detachment and transport processes during these two periods. A systems approach was used on three hydrologically isolated hillslope tracts from which rainfall runoff and snowmelt data were collected over an 18-month period under different residue and manure management systems: corn-grain, corn-silage, and corn-silage with fall manure application. Particulate-bound P dominated overall losses for all the monitored events. While higher mass loads of sediments and P were exported in rainfall runoff, the concentrations of dissolved P forms and organic matter were higher in snowmelt. During the rainfall runoff period, both sediment and P losses were inversely related to the percent residue cover, with the highest coverage corn-grain site producing the lowest levels. In contrast, manure P input prior to melt events, rather than percent residue, dictated P loss patterns during the snowmelt period. Consequently, median P and volatile solids concentrations in snowmelt were higher for the manured site than from the non-manured sites. Importantly, the dissolved P load from the manured field was higher during the snowmelt period compared to the rainfall runoff period. Differences in organic matter sources (manure vs. crop residue) produced sediments with contrasting solids-P signatures, with those from manured corn-silage site enriched both in volatile solids and P Interestingly, the residue cover-manure interplay and the mode of runoff generation had no significant effect on the sediment and P mass distribution in various particle size classes. Our results are expected to improve understanding of P loss pathways and facilitate development of better predictive tools for P transport by enhancing insight on sediment and P mass distribution in different size classes under contrasting row-crop production systems and from different modes of runoff generation.

The Conservation Reserve Program (CRP) is a federal program that encourages the planting of cool- or warm-season grass cover on highly erodible croplands and along stream corridors. We sought to determine whether fish community structure in coldwater streams was associated with CRP and other agricultural land use changes in southwestern Wisconsin. We compared coldwater fish index of biotic integrity (IBI) scores and species richness in streams located in areas of relatively high (21.3% of land area; high-CRP area) versus relatively low (12.1% of land area; low-CRP area) CRP participation. All of the streams were sampled in the 1970s before implementation of the CRP and again at the same locations after implementation, from 2000 to 2005. Pre-CRP fish communities were characterized by a relatively high diversity of eurythermal species and low coldwater IBI scores. We found significant increases in coldwater IBI scores over time in streams within the high-CRP area relative to streams within the low-CRP area. Fish populations in streams within the high-CRP area shifted from eurythermal and tolerant species before CRP implementation to stenothermal, cool- and coldwater species after implementation. Ecological responses within the high-CRP streams also included a reduction in species richness. Without intensive monitoring of watershed nutrients, the fish community changes cannot be mechanistically linked to specific land use practices. However, we demonstrate that IBI scores and species richness were correlated with phosphorus loading estimates and that predicted phosphorus reductions were greater within the high-CRP grassland area. The estimated phosphorus loading declines reflected reduced cropland areas and reduced density of dairy farms. The estimates did not capture all environmental factors, such as trends in production of hogs and cattle or streamflow regime changes associated with conservation practices. We conclude that the combination of extensive grassland management, livestock reductions, and other long-term agricultural land use changes benefited the coldwater fish communities within the high-CRP area.

The impact of field surface conditions and erosion processes on runoff volume, soil loss and sediment particle size during the rainfall runoff period was investigated. Results are reported for multiple events and from within individual events (intra-event) for sites with different corn (Zea mays L.) management systems (i.e., grain (CG), silage (CS), and silage-manure (CSM)). The multi-event bulk runoff volume and soil loss for CG were less than that for CS and CSM due to higher residue levels increasing surface roughness, ponding and infiltration. The aggregate stability for CG treatment was greater than that for CS and CSM and aggregate size peaks were identified at 5.4, 32,160 and 570 pm. Size peaks at 32 and 570 pm had the highest combined frequency (64%). Intra-event continuous monitoring covered silage plots with crop-rows oriented up-and-down the slope (CS) and along the contour (CScont). The crop-row orientation significantly influenced both runoff and sediment loss (concentration, load, size-distribution) patterns. The runoff volume and sediment concentration for CS was twice that of CScont. While no treatment difference attributable to residue coverage was evident for particle-size dynamics, crop-row orientation had a significant effect with finer-sized particles exported from the contoured site. Surface sealing, more pronounced at the silage sites, occurred after the 1st major storm in a season for all monitoring periods, types, and treatments, and it significantly influenced runoff generation, sediment load, and size distribution characteristics. Under crusted conditions, a storm with slightly higher rainfall depth but significant lower erosive potential, generated 53% more runoff and twice as much sediment compared to an early-season event. During crust development finer particles dominated sediment composition, later shifting to larger particles due to rill erosion once a stable crust was established. These results are expected to improve our understanding and, hence, predictive capability for transport of particulate-bound contaminants from row-crop systems, especially under conditions promoting surface crust formation. (c) 2008 Elsevier B.V. All rights reserved.

Although conservation planning for agricultural lands has evolved to its current relatively stable form over many decades, conservation planning for construction sites is still in its infancy. This project drew on the resources of various agencies and researchers to develop a conservation planning tool specifically geared towards meeting construction site planning needs in Wisconsin, basing it on the Revised Universal Soil Loss Equation, version 2 (RUSLE2). The project began by deconstructing the planning process itself to determine possible approaches and critical elements, and used those pieces to build a rational new approach to construction site conservation planning. Although no changes were required to the erosion or sediment delivery calculations in RUSLE2, this new tool required substantial changes to the database, the interface, and how the results were presented and packaged. The lessons learned in this effort should be instructive to both a general discussion of the construction site planning process and to attempts to develop other tools that meet this need.

A principal focus of water quality management efforts in the U.S. is related to nutrient, specifically nitrogen (N) and phosphorus (P), export in runoff from agricultural lands. The focus of this discussion is to investigate the influence of residue levels and manure addition on particulate P delivery by runoff. Rainfall runoff samples were collected from three hydrologically isolated hillslope tracts in conservation tillage with the following treatments: corn-grain (CG); corn-silage (CS); and corn-silage with fall manure addition (SM). Rainfall-runoff, frost free (FF) events were sampled from May 2004 through September 2005. Samples were analyzed for solids mass, P in the dissolved and particulate forms, sediment P-mass distribution in five different particle-size classes along with particle and aggregate size distributions, and aggregate stability. This discussion is limited to soil and total phosphorus (TP) loss and the distribution of TP mass over five particle size classes in the sediment.

The effects of bottom water withdrawals were evaluated within and below two southwestem Wisconsin impoundments. Like many man-made lakes in the unglaciated area of Wisconsin, Twin Valley Lake and White Mound Lake were constructed during the late 1960s for flood control and recreation. Both impoundments, which are located in large agricultural watersheds, were originally designed to release cold bottom water with the intended goal of managing trout below the darns. However, recent water quality monitoring results revealed that the streams became degraded due to frequent dissolved oxygen criterion violations and excessive filamentous bacteria growths. Organic loading from the bottom discharges is the likely reason that trout stream habitat was not successfully created below the dams as originally intended. Despite the accelerated phosphorus removal from the long-term withdrawals, blue-green algal blooms continued to be a problem in both impoundments. While maximizing total phosphorus export can improve take water quality conditions, discharge rates from these impoundments were found to be excessively high, resulting in disturbance of their thermo-structure and entrainment of nutrients into the surface waters. In 2005, we blocked the bottom gate at Twin Valley Lake and monitored water quality and thermal responses. As a result, lake and stream water quality improved significantly while the downstream fish community structure did not change. The impoundment thermo-structure was restored with well-defined hypolimnion and epilimnion. We conclude that managing impoundments is often a balancing act between seemingly disparate goals of achieving optimum conditions above or below a dam, with undesirable consequences often occurring if the focus is disproportionately on a single goal.

This work describes a simple, passive sampling system for measuring runoff, sediment, and chemical losses from typical agricultural fields. The sampler consists of a 5 to 7 m wide runoff collector connected to a series of multislot divisors. These divisors split the flow into aliquots, providing a continuous sampling during the runoff event. Divisors were located in a wooden box below ground level. With an adequate pump, this system can operate in fields with a slope gradient as low as 2%, and can stay in the field during winter to record first snowmelt-generated runoff. A radio transmitter reports by telemetry the occurrence and magnitude of any runoff event, and indicates when the system should be sampled and emptied. This article includes a description of the equipment, advantages, and disadvantages based on 2 yr of operation, and examples of data collected.

There is a growing interest in corn silage utilization due to changes in animal farmdynamics and favorable economics compared to alfalfa. The extent of residue cover influencesrunoff production and soil losses, and hence, these changes will affect the off-site migration ofphosphorus (P). Since high-cut silage will increase residue cover, this method could conceivablyminimize water quality degradation that would otherwise result from harvesting corn for silage. Weexamined cropping system effects on sediment and P losses from no-till fields planted in corn.Treatments included conventional corn grain (CG) and silage (CS-L) and non-conventional, high-cut (24-26) silage (CS-H). Each treatment received one of three manure treatments: no manure,application in fall or spring. Simulated rainfall (76 mm/hr; 1 h) experiments were performed in springand fall 2002, runoff from 2.0 m x 1.5 m plots collected and a sub-sample analyzed for totalsediments, dissolved reactive phosphorus (DRP), and total phosphorus (TP). Compared to CS-L, CS-H was effective in reducing sediment and P losses; the reductions weregreatest in fall runoff and were enhanced by manure addition. Runoff depth, sediment load, DRPload, and TP load were reduced by 50% when no manure was added and by 85% with manureapplied in either season. Although sediment, DRP, and TP concentrations were higher undermanure application, the secondary effect of enhanced infiltration resulted in lower values on a loadbasis. Reductions were smaller following spring rainfall. DRP concentration was sensitive tomanure application timing while TP concentration was not. Compared to the no manure treatment,DRP concentrations in runoff from all crop treatments were five times greater following springmanure application, while TP concentrations were highest under no manure. Preliminary data onparticle size distribution in transported sediments reveal an increased concentration of fines (< 2 m)on manure-applied plots. Recently applied spring manure protected the soil the most, but leads tothe preferential selection of finer particles.

We evaluated the reductions in P loading needed to control blue-green algal blooms in Lake Mendota, Wisconsin. After developing a 21-year loading data set, we used a P mass balance model expressed as a difference equation with an annual time step indexed from mid-April. We defined and estimated a loss parameter lambda as the proportion of the lake's April P concentration lost through sedimentation and outflow during the following year. Using the distribution of annual lambda 's and input loadings, we predicted the steady-state distribution of April P concentrations that would result from scenarios of altered inputs due to changes in management practices. These results were then linked to the probability of summer blue-green algal blooms. For no load reduction, the probability of a bloom (>2 mg algae ·L-1) on any summer day is about 60%. This probability decreases to 20% with a load reduction of 50%. Our approach illustrates how managers can consider reducing the frequency of extreme events like algal blooms, which may correspond more to the public's perception of lake water quality than average conditions.