Yakov A. Pachepsky

Effect of soil organic carbon on soil water retention

Abstract Reports about the relationship between soil water retention and organic carbon content are contradictory. We hypothesized that this relationship is affected by both proportions of textural components and amount of organic carbon. To test the hypothesis, we used the U.S. National Soil Characterization database and the database from pilot studies on soil quality as affected by long-term management. Regression trees and group method of data handling (GMDH) revealed a complex joint effect of texture and taxonomic order on water retention at −33 kPa. Adding information on taxonomic order and on taxonomic order and organic carbon content to the textural class brought 10% and 20% improvement in water retention estimation, respectively, as compared with estimation from the textural class alone. Using total clay, sand and silt along with organic carbon content and taxonomic order resulted in 25% improvement in accuracy over using textural classes. Similar but lower trends in accuracy were found for water retention at −1500 kPa and the slope of the water retention curve. At low organic carbon contents, the sensitivity of the water retention to changes in organic matter content was highest in sandy soils. Increase in organic matter content led to increase of water retention in sandy soils, and to a decrease in fine-textured soils. At high organic carbon values, all soils showed an increase in water retention. The largest increase was in sandy and silty soils. Results are expressed as equations that can be used to evaluate effect of the carbon sequestration and management practices on soil hydraulic properties.

MODELING BACTERIA FATE AND TRANSPORT IN WATERSHEDS TO SUPPORT TMDLS

Fecal contamination of surface waters is a critical water-quality issue, leading to human illnesses and deaths. Total Maximum Daily Loads (TMDLs), which set pollutant limits, are being developed to address fecal bacteria impairments. Watershed models are widely used to support TMDLs, although their use for simulating in-stream fecal bacteria concentrations is somewhat rudimentary. This article provides an overview of fecal microorganism fate and transport within watersheds, describes current watershed models used to simulate microbial transport, and presents case studies demonstrating model use. Bacterial modeling capabilities and limitations for setting TMDL limits are described for two widely used watershed models (HSPF and SWAT) and for the load-duration method. Both HSPF and SWAT permit the user to discretize a watershed spatially and bacteria loads temporally. However, the options and flexibilities are limited. The models are also limited in their ability to describe bacterial life cycles and in their ability to adequately simulate bacteria concentrations during extreme climatic conditions. The load-duration method for developing TMDLs provides a good representation of overall water quality and needed water quality improvement, but intra-watershed contributions must be determined through supplemental sampling or through subsequent modeling that relates land use and hydrologic response to bacterial concentrations. Identified research needs include improved bacteria source characterization procedures, data to support such procedures, and modeling advances including better representation of bacteria life cycles, inclusion of more appropriate fate and transport processes, improved simulation of catastrophic conditions, and creation of a decision support tool to aid users in selecting an appropriate model or method for TMDL development.

Escherichia Coli and Fecal Coliforms in Freshwater and Estuarine Sediments

It has been known for some time that substantial populations of fecal coliforms and E. coli are harbored in freshwater bottom sediments, bank soils, and beach sands. However, the relative importance of sediments as bacterial habitats and as a source of water-borne fecal coliforms and E. coli has not been recognized until recently, when a large number of publications have shown that in many cases the resuspension of sediment, rather then runoff from surrounding lands, can create elevated E. coli concentrations in water. This review is an attempt to develop the first comprehensive single source of existing information about fecal coliforms and E. coli in sediments and adjacent soils and to outline the knowledge gaps and research needs. The authors summarize available information on variability and environmental correlations of E. coli and FC concentrations in sediments, genetic diversity of E. coli in sediments, survival of E. coli and FC in sediments, release with resuspended sediment and settling of E. coli and FC, modeling of sediment effects on fate and transport of E. coli in surface waters, and implications for monitoring and management of microbiological water quality. The demonstrated role of pathogenic E. coli strains in food and water quality challenges reinforces the need in better understanding ecological and hydrological factors that affect functioning of sediments as E. coli reservoirs.

Hydropedology: Synergistic integration of pedology and hydrology

This paper presents a vision that advocates hydropedology as an advantageous integration of pedology and hydrology for studying the intimate relationships between soil, landscape, and hydrology. Landscape water flux is suggested as a unifying precept for hydropedology, through which pedologic and hydrologic expertise can be better integrated. Landscape water flux here encompasses the source, storage, flux, pathway, residence time, availability, and spatiotemporal distribution of water in the root and deep vadose zones within the landscape. After illustrating multiple knowledge gaps that can be addressed by the synergistic integration of pedology and hydrology, we suggest five scientific hypotheses that are critical to advancing hydropedology and enhancing the prediction of landscape water flux. We then present interlinked strategies for achieving the stated vision. It is our hope that by working together, hydrologists and pedologists, along with scientists in related disciplines, can better guide data acquisition, knowledge integration, and model-based prediction so as to advance the hydrologic sciences in the next decade and beyond.

Irrigation Waters as a Source of Pathogenic Microorganisms in Produce. A Review

There is increasing evidence that consumption of raw fresh produce is a major factor contributing to human gastrointestinal illness. A wide variety of pathogens contribute to food-borne illnesses, including bacteria (e.g., Salmonella, pathogenic Escherichia coli), protozoa (e.g., Cryptosporidium, Giardia), and viruses (e.g., noroviruses). Large-scale production of produce typically requires some form of irrigation during the growing season. There is a rapidly growing body of research documenting and elucidating the pathways of produce contamination by water-borne pathogens. However, many gaps still exist in our knowledge and understanding. The purpose of this review is to provide a comprehensive approach to the issue, including the most recent research. Topics covered include: temporal and spatial variability, and regional differences, in pathogen and indicator organism concentrations in water; direct and circumstantial evidence for contaminated water as a source of food-borne pathogens; fate and transport of pathogens and indicator organisms in irrigation systems, and the role of environmental microbial reservoirs; and current standards for irrigation water quality, and risk assessment. A concerted effort by researchers and practitioners is needed to maintain food safety of fresh produce in an increasingly intensive food production system and limited and declining irrigation water resources.

Temporal Stability of Soil Water Contents: A Review of Data and Analyses

Temporal stability (TS) of soil water content (SWC) has been observed throughout a wide range of spatial and temporal scales. Yet, the evidence with respect to the controlling factors on TS SWC remains contradictory or nonexistent. The objective of this work was to develop the first comprehensive review of methodologies to evaluate TS SWC and to present and analyze an inventory of published data. Statistical analysis of mean relative difference (MRD) data and associated standard deviations (SDRD) from 157 graphs in 37 publications showed a trend for the standard deviation of MRD (SDMRD) to increase with scale, as expected. The MRD followed generally the Gaussian distribution with R 2 ranging from 0.841 to 0.998. No relationship between SDMRD and R 2 was observed. The smallest R 2 values were mostly found for negatively skewed and platykurtic MRD distributions. A new statistical model for temporally stable SWC fields was proposed. The analysis of the published data on seven measurement-, terrain-, and climate-related potentially controlling factors of TS SWC suggested intertwined effects of controlling factors rather than single dominant factors. This calls for a focused research effort on the interactions and effects of measurement design, topography, soil, vegetation and climate on TS SWC. Research avenues are proposed which will lead to a better understanding of the TS phenomenon and ultimately to the identification of the underlying mechanisms.

A comparative modeling study of soil water dynamics in a desert ecosystem

We compared three different soil water models to evaluate the extent to which variation in plant growth form and cover and soil texture along a topographic gradient interact to affect relative rates of evaporation and transpiration under semiarid conditions. The models all incorporated one-dimensional distribution of water in the soil and had separate functions for loss of water through transpiration and soil evaporation but differed in the degree of mechanism and emphasis. PALS-SW (Patch Arid Lands Simulator-Soil Water) is a mechanistic model that includes soil water fluxes and emphasizes the physiological control of water loss by different plant life forms along the gradient. 2DSOIL is a mechanistic model that emphasizes the physical aspects of soil water fluxes. SWB (Soil Water Budget) is a simple water budget model that has no soil water redistribution and includes simplified schemes for soil evaporation and transpiration by different life forms. The model predictions were compared to observed soil water distributions at five positions along the gradient. All models predicted soil water distributions reasonably well and, for the most part, predicted similar trends along the transect in the fractions of water lost as soil evaporation versus transpiration. Transpiration was lowest (about 40% of total evapotranspiration (ET)) for the creosote bush community, which had the lowest plant cover (30% peak cover). The fraction of ET as transpiration increased with increasing plant cover, with 2DSOIL predicting the highest transpiration (60% of total ET) for the mixed vegetation community (60% peak cover) on relatively fine textured soil and PALSr SW predicting highest transpiration (69% of total ET) for the mixed vegetation community (70% peak cover) on relatively coarse textured soil. The community type had an effect on the amount of water lost as transpiration primarily via depth and distribution of roots. In this respect, PALS-SW predicted greatest differences among stations as related to differences in plant community types. However, since PALS-SW did not provide as good of fit with the soil moisture data as did 2DSOIL, the differences in the morphology and physiology of the life-forms may be secondary to the overall control of water loss by the primary factors accounted for in 2DSOIL: vertical distribution of soil moisture, degree of canopy cover, and evaporative energy budget of the canopy. Soil texture interacted with the amount and type of plant cover to affect evaporation and transpiration, but the effect was relatively minor.

Survival of manure-borne E. coli in streambed sediment: effects of temperature and sediment properties.

Escherichia coli bacteria are commonly used as indicator organisms to designate of impaired surface waters and to guide the design of management practices to prevent fecal contamination of water. Stream sediments are known to serve as a reservoir and potential source of fecal bacteria (E. coli) for stream water. In agricultural watersheds, substantial numbers of E. coli may reach surface waters, and subsequently be deposited into sediments, along with fecal material in runoff from land-applied manures, grazing lands, or wildlife excreta. The objectives of this work were (a) to test the hypothesis that E. coli survival in streambed sediment in the presence of manure material will be affected by sediment texture and organic carbon content and (b) to evaluate applicability of the exponential die-off equation to the E. coli survival data in the presence of manure material. Experiments were conducted at three temperatures (4 degrees C, 14 degrees C, and 24 degrees C) in flow-through chambers using sediment from three locations at the Beaverdam Creek Tributary in Beltsville, Maryland mixed with dairy manure slurry in the proportion of 1000:1. Indigenous E. coli populations in sediments ranged from ca. 10(1) to 10(3)MPNg(-1) while approx 10(3) manure-borne E. coli MPNg(-1) were added. E. coli survived in sediments much longer than in the overlaying water. The exponential inactivation model gave an excellent approximation of data after 6-16 days from the beginning of the experiment. Slower inactivation was observed with the increase in organic carbon content in sediments with identical granulometric composition. The increase in the content of fine particles and organic carbon in sediments led not only to the slower inactivation but also to lower sensitivity of the inactivation to temperature. Streambed sediment properties have to be documented to better evaluate the role of sediments as reservoirs of E. coli that can affect microbiological stream water quality during high flow events.

Effect of Bovine Manure on Fecal Coliform Attachment to Soil and Soil Particles of Different Sizes

ABSTRACT Manure-borne bacteria can be transported in runoff as free cells, cells attached to soil particles, and cells attached to manure particles. The objectives of this work were to compare the attachment of fecal coliforms (FC) to different soils and soil fractions and to assess the effect of bovine manure on FC attachment to soil and soil fractions. Three sand fractions of different sizes, the silt fraction, and the clay fraction of loam and sandy clay loam soils were separated and used along with soil samples in batch attachment experiments with water-FC suspensions and water-manure-FC suspensions. In the absence of manure colloids, bacterial attachment to soil, silt, and clay particles was much higher than the attachment to sand particles having no organic coating. The attachment to the coated sand particles was similar to the attachment to silt and clay. Manure colloids in suspensions decreased bacterial attachment to soils, clay and silt fractions, and coated sand fractions, but did not decrease the attachment to sand fractions without the coating. The low attachment of bacteria to silt and clay particles in the presence of manure colloids may cause predominantly free-cell transport of manure-borne FC in runoff.

Generalized Richards' equation to simulate water transport in unsaturated soils

Simulations of water transport in soil are ubiquitous, and the Richards’ equation introduced in 1931 is the main tool for that purpose. For experiments on water transport in soil horizontal columns, Richards’ equation predicts that volumetric water contents should depend solely on the ratio (distance)/(time) q where q ¼ 0:5: Substantial experimental evidence shows that value of q is significantly less than 0.5 in some cases. Donald Nielsen and colleagues in 1962 related values of q , 0.5 to ‘jerky movements’ of the wetting front, i.e. occurrences of rare large movements. The physical model of such transport is the transport of particles being randomly trapped and having a power law distribution of waiting periods. The corresponding mathematical model is a generalized Richards’ equation in which the derivative of water content on time is a fractional one with the order equal or less than one. We solved this equation numerically and fitted the solution to data on horizontal water transport. The classical Richards’ equation predicted a decrease of the soil water diffusivity for the same water content as infiltration progressed whereas the generalized Richards’ equation described all observations well with a single diffusivity function. Validity of the generalized Richards’ equation indicates presence of memory effects in soil water transport phenomena and may help to explain scale-dependence and variability in soil hydraulic conductivity encountered by researchers who applied classical Richards’ equation. Published by Elsevier Science B.V.