John J. Ramirez-Avila

Applicability of models to predict phosphorus losses in drained fields: a review.

Most phosphorus (P) modeling studies of water quality have focused on surface runoff loses. However, a growing number of experimental studies have shown that P losses can occur in drainage water from artificially drained fields. In this review, we assess the applicability of nine models to predict this type of P loss. A model of P movement in artificially drained systems will likely need to account for the partitioning of water and P into runoff, macropore flow, and matrix flow. Within the soil profile, sorption and desorption of dissolved P and filtering of particulate P will be important. Eight models are reviewed (ADAPT, APEX, DRAINMOD, HSPF, HYDRUS, ICECREAMDB, PLEASE, and SWAT) along with P Indexes. Few of the models are designed to address P loss in drainage waters. Although the SWAT model has been used extensively for modeling P loss in runoff and includes tile drain flow, P losses are not simulated in tile drain flow. ADAPT, HSPF, and most P Indexes do not simulate flow to tiles or drains. DRAINMOD simulates drains but does not simulate P. The ICECREAMDB model from Sweden is an exception in that it is designed specifically for P losses in drainage water. This model seems to be a promising, parsimonious approach in simulating critical processes, but it needs to be tested. Field experiments using a nested, paired research design are needed to improve P models for artificially drained fields. Regardless of the model used, it is imperative that uncertainty in model predictions be assessed.

Southern Phosphorus Indices, Water Quality Data, and Modeling (APEX, APLE, and TBET) Results: A Comparison

Phosphorus (P) Indices in the southern United States frequently produce different recommendations for similar conditions. We compared risk ratings from 12 southern states (Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, and Texas) using data collected from benchmark sites in the South (Arkansas, Georgia, Mississippi, North Carolina, Oklahoma, and Texas). Phosphorus Index ratings were developed using both measured erosion losses from each benchmark site and Revised Universal Soil Loss Equation 2 predictions; mostly, there was no difference in P Index outcome. The derived loss ratings were then compared with measured P loads at the benchmark sites by using equivalent USDA-NRCS P Index ratings and three water quality models (Annual P Loss Estimator [APLE], Agricultural Policy Environmental eXtender [APEX], and Texas Best Management Practice Evaluation Tool [TBET]). Phosphorus indices were finally compared against each other using USDA-NRCS loss ratings model estimate correspondence with USDA-NRCS loss ratings. Correspondence was 61% for APEX, 48% for APLE, and 52% for TBET, with overall P index correspondence at 55%. Additive P Indices (Alabama and Texas) had the lowest USDA-NRCS loss rating correspondence (31%), while the multiplicative (Arkansas, Florida, Louisiana, Mississippi, South Carolina, and Tennessee) and component (Georgia, Kentucky, and North Carolina) indices had similar USDA-NRCS loss rating correspondence-60 and 64%, respectively. Analysis using Kendalls modified Tau suggested that correlations between measured and calculated P-loss ratings were similar or better for most P Indices than the models.

Comparing an Annual and a Daily Time-Step Model for Predicting Field-Scale Phosphorus Loss

A wide range of mathematical models are available for predicting phosphorus (P) losses from agricultural fields, ranging from simple, empirically based annual time-step models to more complex, process-based daily time-step models. In this study, we compare field-scale P-loss predictions between the Annual P Loss Estimator (APLE), an empirically based annual time-step model, and the Texas Best Management Practice Evaluation Tool (TBET), a process-based daily time-step model based on the Soil and Water Assessment Tool. We first compared predictions of field-scale P loss from both models using field and land management data collected from 11 research sites throughout the southern United States. We then compared predictions of P loss from both models with measured P-loss data from these sites. We observed a strong and statistically significant ( < 0.001) correlation in both dissolved (ρ = 0.92) and particulate (ρ = 0.87) P loss between the two models; however, APLE predicted, on average, 44% greater dissolved P loss, whereas TBET predicted, on average, 105% greater particulate P loss for the conditions simulated in our study. When we compared model predictions with measured P-loss data, neither model consistently outperformed the other, indicating that more complex models do not necessarily produce better predictions of field-scale P loss. Our results also highlight limitations with both models and the need for continued efforts to improve their accuracy.

Evaluation of the APEX Model to Simulate Runoff Quality from Agricultural Fields in the Southern Region of the United States

The Agricultural Policy Environmental eXtender (APEX) model has been widely applied to assess phosphorus (P) loss in runoff water and has been proposed as a model to support practical decisions regarding agricultural P management, as well as a model to evaluate tools such as the P Index. The aim of this study is to evaluate the performance of APEX to simulate P losses from agricultural systems to determine its potential use for refinement or replacement of the P Index in the southern region of the United States. Uncalibrated and calibrated APEX model predictions were compared against measured water quality data from row crop fields in North Carolina and Mississippi and pasture fields in Arkansas and Georgia. Calibrated models satisfactorily predicted event-based surface runoff volumes at all sites (Nash-Sutcliffe efficiency [NSE] > 0.47, |percent bias [PBIAS]| < 34) except Arkansas (NSE < 0.11, |PBIAS| < 50) but did not satisfactory simulate sediment, dissolved P, or total P losses in runoff water. The APEX model tended to underestimate dissolved and total P losses from fields where manure was surface applied. The model also overestimated sediments and total P loads during irrigation events. We conclude that the capability of APEX to predict sediment and P losses is limited, and consequently so is the potential for using APEX to make P management recommendations to improve P Indices in the southern United States.

Phosphorus in runoff from two highly weathered soils of the tropics

Ramírez-Ávila, J. R., Sotomayor-Ramírez, D., Martínez-Rodríguez, G. A. and Pérez-Alegría, L. R. 2011. Phosphorus in runoff from two highly weathered soils of the tropics. Can. J. Soil Sci. 91: 267-277. Agricultural fields with high soil phosphorus (P) content are important contributors to surface water degradation. Two consecutive simulated rainfall events were conducted on two Ultisols previously amended with inorganic P fertilizer or broiler litter. Soil test P (Bray 1 and Olsen) levels evaluated ranged from 1 to 350 mg kg-1. Surface runoff concentrations of total P (TP) and dissolved P (DP) generated by a 30-min runoff event were quantified. Runoff DP concentrations ranged from 0.08 to 3.98 mg L-1 in fertilizer P-amended soils and from 0.08 to 4.93 mg L-1 in broiler litter-amended soils. A single exponential model adequately described the relationships between soil test P and DP concentrations in runoff. For each soil, the soil test P-DP concentration relationships were positively influenced by soil organic matter and negatively influenced by antecedent soil moisture (P<0.05). For both soils, the soil test P-DP concentration relationships were positively influenced by groundcover percentage and negatively influenced by slope. Environmental soil test P critical levels corresponding to a runoff threshold of 1 mg L-1 DP, ranged between 176 and 296 mg kg-1 (Olsen) and 143 to 276 mg kg-1 (Bray 1) in soils amended with fertilizer-P. In broiler litter-amended soils, threshold values were 88 and 111 mg kg-1 using Olsen and Bray 1, respectively. Differences in surface runoff-P concentrations due to amendment sources, antecedent soil moisture content, soil organic matter, groundcover and slope suggest that these factors need to be considered in P management decisions at the farm level.

Nutrient and Sediment Production, Watershed Characterization, and Land Use in the Town Creek Watershed, Mississippi

Sediment and nutrient impairment of streams is a predominant condition for watersheds in the Tombigbee River basin, which drains through the Mobile River Basin into the Gulf of Mexico. Currently, a study is underway in Town Creek watershed (MS) as a pilot watershed physically representative in geology, physiographic, and climate of Ecoregion 65 watersheds into the Tombigbee River Basin. The study monitors surface water discharge and concentration of Dissolved Phosphorus (DP), Total Phosphorus (TP), Suspended Sediments (SSC), and associated physical parameters (pH, turbidity, Dissolved Oxygen, Electric Conductivity). Monitoring has been done in 7 grab sampling stations and two automated sampling stations along the approximated 45 miles principal channel, and 17 grab sampling stations on 10 tributaries into the 1769 km2 watershed. Analyses performed in the study area (which is composed by more than 50% for agriculture lands (cropland and pasture), 39 % for forest, and 10% for urban area), has shown that more than 40% of the water quality samples obtained since summer 2008 are above 0.1 mg TP/L (established nutrient criteria for streams). The highest TP concentrations (0.35 mg/l) in the principal channel were observed after the input of tributaries discharges from the urban area (TP˜0.35). Highest SSC in the principal channel during the studied period were observed in areas influenced by structures in construction, tributaries from urban areas, and streams in agricultural areas with deficiencies or absence of a riparian zone and evidence of streambank erosion processes. Tributaries located downstream of the urban area did not contribute significant concentrations of TP (<0.05mg/l), allowing a dilution effect observed in a 10% increase of the ratio DP/TP at the watershed outlet.

Streambank erosion assessment in southeastern plains ecoregion channels using in situ monitoring and submerged jet testing.

Channel width adjustment due to stream bank erosion is a common mode of channel form adjustment as streams respond to changes in runoff and sediment supply from the surrounding landscape. This is particularly observed in the highly disturbed watersheds of Northeastern Mississippi. Research is being conducted in the Town Creek watershed, MS to better understand how streambank erosion is affected by the position and characteristics of the eroding streambank in the watershed. Measurements of streambank profile adjustment using erosion pins and topographic surveys, flow velocity, and streambank soil erodibility using a jet test device was conducted at three different locations within the watershed. Soil chemical and physical properties were determined to evaluate effects on streambank soil erodibility. Erosion pins were monitored over a one year period. Topographic surveying has been performed on headwater unstable channels with actively eroding streambanks for six months. Streambank erosion and deposition processes were observed simultaneously at erosion pin locations. Erosion depths ranged from 1 mm to 560 mm, whereas sediment deposition depths varied between 1 and 360 mm. Preliminary survey results indicate erosion varying between 3 and 600 mm due to mass wasting and basal clean-out during and after storm runoff events. Soil streambank critical shear stress (τ c ) and detachment rate coefficient (k d ) determined by jet testing indicate a wide range of erodibility throughout the test locations.

A Sediment Budget for Town Creek Watershed: Preliminary Results from Streambank Erosion Processes and Rates Assessment.

Increased streambank erosion results not only in accelerated sediment yield, but also destabilizes streams with associated changes in stream type. This document reports preliminary results from a study which main objective is to evaluate streambank erosion rates and generate empirical correlations to estimate streambank erosion involving physical, geometric, and morphologic variables in the Town Creek watershed in Mississippi. A combination of methods is used for the study including field reconnaissance and detailed data collection, surveying, and channel modeling. Higher rates of increase in sediment load and yield from the northern headwaters were observed from unstable active streambank profiles near agricultural lands and limited or with no presence of riparian vegetation. Streambank failure events at these channels tend to be periodic, most frequently occurred during stormflow events season. Channel morphology changes from incised V-shaped channels to wide U-shaped channels with an increase in riparian vegetation density along the middle 20-km of the principal channel. Streambank erosion pins were installed on two representative places along the middle 20-km of the principal channel. Streambank erosion rate was assessed by measuring the length of the exposed pins after stormflow events on each plot. Assessment showed sediment deposition in most of the pin erosion plots. Jet testing results described streambank soils with high and very high potential to be eroded. Combined preliminary results, watershed characterization and field observation initially point to season and channel morphology as the more important factors affecting streambank erosion and deposition rates on streambanks and streambeds.

Grass filter strips evaluation for reducing sediment and nutrient exportation from grasslands under manure applications in Puerto Rico

Sediment and nutrients losses from agricultural lands have been identified as a significant environmental problem. Vegetative filter strips are commonly used as a best management practice (BMP) to control agricultural pollution. This study was conducted to test the hypothesis that grass filter strips were effective in reducing sediment and nutrient exportations in runoff from grazed pastures amended with irrigated dairy manure. The experiment was carried out under conventional management practices in a dairy farm in Puerto Rico. Natural runoff events were diverted into runoff collection devices placed at 0, 10, and 20 m within a grass filter barrier. Collected samples were analyzed for Total Suspended Solids (TSS), Total Kjeldahl Nitrogen (TKN), Dissolved Phosphorus (DP), and Total Phosphorus (TP). TSS concentrations in runoff entering the filter strips were minimal, indicating that TSS losses are not significant from pasture fields exhibiting high vegetative coverage. The higher TP and TKN concentrations were

Caribbean Phosphorus Index Validation and Management Practices Evaluation on Fields under Manure Applications

Phosphorus (P) losses from agricultural soils are a major cause of fresh water quality impairment. The objective of this study was to validate, at field level, the Caribbean Phosphorus Index (CAPI). The CAPI is a tool developed for the identification of agricultural soils with high risk of P losses to surface waters. P runoff concentrations measured under natural rainfall condition on various fields of dairy and poultry farms in Puerto Rico were compared with risk levels determined using CAPI. Average Olsen soil test P (STP) values were 41 mg P kg-1 and 152 mg P kg-1 for the dairy and the poultry farms, respectively. Average total phosphorus (TP) and dissolved P (DP) concentration losses in runoff were 2.29 mg TP L-1, and 1.79 mg DP L-1 for the dairy farm, and 5.87 mg TP L-1 and 4.82 mg DP L-1 for the poultry farm. The CAPI ranked both dairy farm fields as fields having a Medium potential for P movement and poultry farm fields with Medium and High potential. The current CAPI version underestimated the impact of nutrients on the surrounding waters because runoff concentrations greater than 1 mg L-1 from fields less than 30 m from a surface water body should be ranked in the Very High category. A modification of the CAPI excluding the soil erosion factor from the CAPI matrix yielded results that were more attuned to the observed runoff concentration losses. This exclusion is justified from evidence indicating that soil erosion is minimal in fields exhibiting abundant (>80%) grass cover.