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At least 1,153 records · Page 64Linked to original sources

USGS1973

Thermal conductivity of carbonate rocks

The thermal conductivities of several well-defined carbonate rocks were determined near 40°C. Values range from 1.2 W m −1 C −1 for a highly porous chalk to 5.1 W m −1 C −1 for a dolomite. The thermal conductivity of magnesite (5.0) is at the high end of the range, and that for Iceland Spar Calcite (3.2) is near the middle. The values for limestones decrease linearly with increasing porosity. Dolomites of comparable porosity have greater thermal conductivities than limestones. Water-sorbed samples have expected greater thermal conductivities than air-saturated (dry) samples of the same rock. An anomalously large increase in the thermal conductivity of a water-sorbed clayey dolomite over that of the same sample when dry is attributed to the clay fraction, which swells during water inhibition, causing more solid-to-solid contacts within the dolomite framework. Measurements were made with a Colora Thermoconductometer. Chemical and mineralogical analyses were made and tabulated. Porosity of the rocks was determined by mercury porosimetry and also from density measurements. The Iceland Spar Calcite and magnesite were included for reference.

Engineering Geology↗

USGS1971

C13 and O18 compositions in some fresh-water carbonates associated with ultramafic rocks and serpentinites: Western United States

All carbonates associated with the ultramafic rocks and serpentinites of the western United States are shown by their stable isotope ratios to be of near-surface, low-temperature origin. These include vein materials that have been previously classified as hydrothermal. New laboratory and natural data were obtained on the equilibrium isotope relations between hydromagnesite and water. The origins of travertines in the ultramafic area are easily distinguished on the basis of their stable isotope ratios. The extremely heavy isotope ratios of nesquehonites suggest an intricate evaporating-film mechanism of formation.

Geochimica et Cosmochimica Acta↗

USGS1976

Carbon, hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece

At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C. The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ 18 O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ 18 O H2O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite. The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δD H2O values for the metamorphic fluid decrease from −5‰ in the glaucophane zone to an average of about −70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend. Theδ 18 O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism. Detailed data on 20 marble units show systematic variations of δ 18 O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ 18 O gradients at the margins and large δ 18 O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ 18 O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates. Almost all the δ 13 C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ 13 C values of most marble units show a general correlation with δ 18 O values. The CO 2 H 2 O "> CO 2 H 2 O mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ 13 C values of the CO 2 range from −8.0 to 3.6‰ and indicate that at some localities CO 2 in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.

Geochimica et Cosmochimica Acta↗

USGS1994

Variations in the chemical and stable isotope composition of carbon and sulfur species during organic-rich sediment alteration: An experimental and theoretical study of hydrothermal activity at Guaymas Basin, Gulf of California

Organic-rich diatomaceous ooze was reacted with seawater and a Na-Ca-K-Cl fluid of seawater chlorinity at 325–400°C, 400–500 bars, and fluid/sediment mass ratios of 1.56–2.35 to constrain factors regulating the abundance and stable isotope composition of C and S species during hydrothermal alteration of sediment from Guaymas Basin, Gulf of California. Alteration of inorganic and organic sedimentary components resulted in extensive exchange reactions, the release of abundant H 2 S, CO 2 , CH 4 , and C organic , to solution, and recrystallization of the sediment to an assemblage containing albitic plagioclase, quartz, pyrrhotite, and calcite. The δ 34 S cdt "> δ 34 S cdt values of dissolved H 2 S varied from −10.9 to +4.3‰ during seawater-sediment interaction at 325 and 400°C and from −16.5 to −9.0‰ during Na-Ca-K-Cl fluid-sediment interaction at 325 and 375°C. In the absence of seawater SO 4 , H 2 S is derived from both the transformation of pyrite to pyrrhotite and S released during the degradation of organic matter. In the presence of seawater SO 4 , reduction of SO 4 contributes directly to H 2 S production. Sedimentary organic matter acts as the reducing agent during pyrite and SO 4 reduction. Requisite acidity for the reduction of SO 4 is provided by Mg fixation during early-stage sediment alteration and by albite and calcite formation in Mg-free solutions. Organically derived CH 4 was characterized by δ 13 C pdb "> δ13Cpdb values ranging between −20.8 and −23.1‰, whereas δ 13 C pdb "> δ 13 C pdb values for dissolved C organic ranged between −14.8 and −17.7%. Mass balance calculations indicate that δ 13 C values for organically derived CO 2 were ≥ − 14.8%. Residual solid sedimentary organic C showed small (≤ 0.7‰) depletions in 13 C relative to the starting sediment. The experimental results are consistent with the isotopic and chemical composition of natural hydrothermal fluids and minerals at Guaymas Basin and permit us to better constrain sources and sinks for C and S species in subseafloor hydrothermal systems at sediment-covered spreading centers. Our data show that the sulfur isotope composition of hydrothermal Sulfide minerals in Guaymas Basin can be explained by derivation of S from diagenetic sulfide and seawater sulfate. Basaltic S may also contribute to hydrothermal sulfide precipitates but is not required to explain their isotopic composition. Estimates of seawater/ sediment mass ratios based on sulfur isotopic composition of sulfide minerals and the abundance of dissolved NH 3 in vent fluids range from 3–29 during hydrothermal circulation. Sources of C in Guaymas Basin hydrothermal fluids include thermal degradation of organic matter, bacteriogenic methane production, and dissolution of diagenetic carbonate.

Baja California Sur↗

USGS1992

Morphology and genesis of carbonate soils on the Kyle Canyon fan, Nevada, U.S.A.

The physical and chemical properties of soils formed in an arid climate on calcareous alluvium of the Kyle Canyon alluvial fan, southern Nevada, were studied in order to infer the rates and relative importance of various soil-forming processes. These studies included field and microscopic observations and analyses of thin sections, major oxides, extractable iron, and clay minerals. The results are interpreted to reflect five major pedogenic processes: (1) The calcic horizons and calcretes of Kyle Canyon soils form by precipitation of CaCO 3 , derived from eolian dust and alluvium, as clast coats, matrix cement, and massive layers. (2) The A and uppermost B horizons are essentially dust-derived, for they contain large amounts of detrital material not present in the alluvial parent material, and their major-oxide content is similar to that of modern dust. (3) Clay particles are translocated from A into B horizons. (4) Iron-bearing minerals in the near-surface B horizons are slowly oxidized. (5) Carbonate and aluminosilicate grains are both displaced and replaced by pedogenic CaCO 3 ; the silica released by replacement of aluminosilicates may be locally precipitated as amorphous or opaline silica and (or) incorporated into newly formed palygorskite and sepiolite. Rates of soil development at Kyle Canyon are approximate due to uncertainties in age estimates. Some soil field properties change at rates that are similar to rates for soils formed in rhyolitic parent material near Mercury, Nevada. The rate of accumulation of CaCO 3 (3–5 g m −2 yr −1 ) at Kyle Canyon is an order of magnitude faster than that near Mercury, but is comparable to rates calculated for soils in southern New Mexico and Utah.

Nevada↗

USGS1970

Comparison of chemical hydrogeology of the carbonate peninsulas of Florida and Yucatan

Aquifers of the peninsulas of Florida and northern Yucatan are Tertiary marine carbonate formations showing many lithologic and faunal similarities. In addition, the tropical to subtropical climates of the two areas are similar, each having annual rainfall of about 1000 to 1500 mm. Despite similarities in these fundamental controls, contrasts in the hydrologic and geochemical systems are numerous and striking. For example, Florida has many rivers; Yucatan has none. Maximum thickness of fresh ground water in Florida is about 700 meters; in the Yucatan it is less than 70 meters. In Florida the gradient of the potentiometric surface averages about 1 meter per kilometer; in the Yucatan it is exceedingly low, averaging about 0.02 meter per kilometer. In Florida the chemical character of water changes systematically downgradient, owing to solution of minerals of the aquifer and corresponding increases in total dissolved solids, sulfate, calcium, and Mg-Ca ratio; in the Yucatan no downgradient change exists, and dominant processes controlling the chemical character of the water are solution of minerals and simple mixing of the fresh water and the body of salt water that underlies the peninsula at shallow depth. Hydrologic and chemical differences are caused in part by the lower altitude of the Yucatan plain. More important, however, these differences are due to the lack of an upper confining bed in Yucatan that is hydrologically equivalent to the Hawthorn Formation of Florida. The Hawthorn cover prevents recharge and confines the artesian water except where it is punctured by sinkholes, but sands and other unconsolidated sediments fill sinkholes and cavities and impede circulation. In the Yucatan the permeability of the entire section is so enormous that rainfall immediately infiltrates to the water table and then moves laterally to discharge areas along the coasts.

Florida Peninsula, Yucatan Peninsula↗

USGS1971

Effects of karst features on circulation of water in carbonate rocks in coastal areas

The normal balance between fresh water in coastal aquifers and sea water applies also to carbonate-rock aquifers that have been karstified, but there are local modifications in the balance that need to be considered. Uneven distribution of permeability, expressed by a network of solution channels bounded by relatively impermeabler rock, causes an uneven distribution of head of the environmental water along the seacoast. Where sinkholes and (or) vertical solution shafts below sea level penetrate the aquifer, the fresh ground water may discharge through these karst features if the fresh-water head is greater than that of the salt water. However, under some conditions the salt-water head may exceed that of the fresh water, and the direction of movement is reversed as sea water flows into the aquifer. This sea-water flow into the aquifer occurs (1) where sinkholes, acting as “cased wells,” penetrate less permeable rock before reaching a lateral solution channel and (2) where (or when) the fresh-water head is less than that required to balance the salt water. On Andros Island, Bahamas, the range in tide (as much as 5 feet) from low tide to high tide is sufficient to cause such a reversal locally. During low tide the salt-water head becomes sufficiently low that the ground-water head exceeds that of the sea water, and the ground water flows through the sinkholes to the ocean floor; during high tide sea water flows in the sinkholes. In the Adriatic Sea along the coast of Yugoslavia, apparently the fresh-water head is sufficient to produce perennial springs in some localities, but in other areas, as in the Bay of Kastela near Split, the fresh-water head becomes low enough during some seasons that the flow is reversed and salt water enters the aquifer through the karst features. The development of sinkholes and other karst features near the present coast and extending below sea level occurred generally during a low stand of the Pleistocene sea when the top of the saturated zone stood lower than the bottom of the deepest sinkholes or natural wells. Integrated evaluations of (1) the distribution of permeability in coastal karst regions and (2) the principles relating to the dynamic balance between fresh aquifer water and sea water are leading to better knowledge of methods that may salvage much karst water which is lost to the sea.

Split↗

USGS1993

Use of principal-component, correlation, and stepwise multiple-regression analyses to investigate selected physical and hydraulic properties of carbonate-rock aquifers

Correlation analysis in conjunction with principal-component and multiple-regression analyses were applied to laboratory chemical and petrographic data to assess the usefulness of these techniques in evaluating selected physical and hydraulic properties of carbonate-rock aquifers in central Pennsylvania. Correlation and principal-component analyses were used to establish relations and associations among variables, to determine dimensions of property variation of samples, and to filter the variables containing similar information. Principal-component and correlation analyses showed that porosity is related to other measured variables and that permeability is most related to porosity and grain size. Four principal components are found to be significant in explaining the variance of data. Stepwise multiple-regression analysis was used to see how well the measured variables could predict porosity and (or) permeability for this suite of rocks. The variation in permeability and porosity is not totally predicted by the other variables, but the regression is significant at the 5% significance level.

Journal of Hydrology↗

USGS1987

Kinetics and mechanisms of iron sulfide reductions in hydrogen and in carbon monoxide

The reduction of iron sulfides by hydrogen and by carbon monoxide has been studied using plug flow and thermogravimetric methods. The reactions were studied in the 523–723°K temperature range and were found to be first-order processes. Plug flow studies were used to correlate reaction rates between pyrite and the gases as a function of the surface area of the pyrite. The rate of H 2 S formation increases with the surface area of the pyrite sample. The results of thermogravimetric experiments indicate that the reactions consist of several steps. Rate constants for the pyrite reduction by H 2 and by CO were obtained. The activation energies increased with degree of reduction. Values of E a were 113.2 (step I) and 122.5 kJ/mole (step II) for pyrite reduction with CO and 99.4 (step I), 122.4 (step II), 125.2 (step III), and 142.6 kJ/mole (step IV) for pyrite reduction with hydrogen.

Journal of Solid State Chemistry↗

USGS1996

Overview of a simple model describing variation of dissolved organic carbon in an upland catchment

Hydrological mechanisms controlling the variation of dissolved organic carbon (DOC) were investigated in the Deer Creek catchment located near Montezuma, CO. Patterns of DOC in streamflow suggested that increased flows through the upper soil horizon during snowmelt are responsible for flushing this DOC-enriched interstitial water to the streams. We examined possible hydrological mechanisms to explain the observed variability of DOC in Deer Creek by first simulating the hydrological response of the catchment using TOPMODEL and then routing the predicted flows through a simple model that accounted for temporal changes in DOC. Conceptually the DOC model can be taken to represent a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out when infiltrating meltwaters cause the water table to rise into this “reservoir”. Concentrations of DOC measured in the upper soil and in streamflow were compared to model simulations. The simulated DOC response provides a reasonable reproduction of the observed dynamics of DOC in the stream at Deer Creek.

Ecological Modelling↗

USGS1977

Techniques for the conversion to carbon dioxide of oxygen from dissolved sulfate in thermal waters

The fractionation of oxygen isotopes between dissolved sulfate ions and water provides a useful geothermometer for geothermal waters. The oxygen isotope composition of dissolved sulfate may also be used to indicate the source of the sulfate and processes of formation. The methods described here for separation, purification and reduction of sulfate to prepare carbon dioxide for mass spectrometric analysis are modifications of methods by Rafter (1967), Mizutani (1971), Sakai and Krouse (1971), and Mizutani and Rafter (1969). ?? 1976.

Geothermics↗

USGS1994

Microfossil biostratigraphy of prograding Neogene platform-margin carbonates, Bahamas: Age constraints and alternatives

Benthic and planktic foraminifera and calcareous nannofossils were recovered in shallow-water carbonate rock cores from two continuous boreholes drilled 7.5 km apart on the west platform margin of the Great Bahama Bank. The microfossils define six biostratigraphic units in each hole. One unit in each hole represents a correlative condensed section. Seven foraminiferal biozones are recognized in 11 of the units between the holes: middle Miocene Globorotalia fohsi robusta Zone N12, late Miocene G. acostaensis Zone N16 and G. humerosa Zone N17, early Pliocene G. margaritae evoluta Subzone N19, late Pliocene G. exilis Subzone N21 and, tentatively, G. tosaensis tosaensis Zone N21, and early Pleistocene G. crassaformis viola Subzone N22. The twelfth unit is inferred to be of G. crassaformis viola Subzone N22 age. The oldest unit is onshore, the youngest is offshore. As presently interpreted, the nannofossil and foraminiferal zonations are partially correlative. Although the microfossils unequivocally constrain the series ages of the sediments, the incompleteness of the fossil record allows for alternative biozonal age models within the series. The Miocene and Pliocene biozones are common to both holes, but the greatest similarities between the holes are the significant mixing of middle and late Miocene, and late Miocene-early Pliocene faunas, the greatly condensed intervals at the Miocene/Pliocene boundary, and the early Pliocene influx of deep-water benthic and pelagic foraminifera. Of particular importance is the tentative recognition of late Pliocene G. tosaensis tosaensis Zone N21 in one borehole. Subsequent data not available to this phase of the study indicate that much of the zone is likely missing. Its absence will lend support to speculations of a regional unconformity in the Bahamas. The microfossils indicate that (1) several transgressions occurred from the middle Miocene to at least the earliest Pleistocene (greater than 11.5–greater than 0.46 Ma), during which banktop-derived sediments accumulating at the margin prograded the platform seaward; (2) a condensed interval on the bank top may represent a late Miocene lowstand, a period of sediment bypassing, or a lack of accommodation space; (3) the slope received thin layers of pelagic sediments in a condensed interval during the late Miocene and early Pliocene, while the bank top accommodated early Pliocene (4.2–3.4 Ma) deep-water indicators prior to a likely period of exposure (2.35 - 1.89 Ma); (4) two cycles of banktop sediment production and starvation occurred during the Pliocene; (5) the Pliocene transgression was punctuated by stillstands or low-amplitude reversals during which parts of biozones did not accumulate; and (6) the sediments containing the most complete microfossil-datum record are the thin pelagic strata that mark interruptions in the regular shedding of transgressive deposits from the platform. Sedimentation-rate patterns varied but were generally higher offshore than onshore.

Marine Micropaleontology↗

USGS1992

Redistribution of soil nitrogen, carbon and organic matter by mechanical disturbance during whole-tree harvesting in northern hardwoods

To investigate whether mechanical mixing during harvesting could account for losses observed from forest floor, we measured surface disturbance on a 22 ha watershed that was whole-tree harvested. Surface soil on each 10 cm interval along 81, randomly placed transects was classified immediately after harvesting as mineral or organic, and as undisturbed, depressed, rutted, mounded, scarified, or scalped (forest floor scraped away). We quantitatively sampled these surface categories to collect soil in which preharvest forest floor might reside after harvest. Mechanically mixed mineral and organic soil horizons were readily identified. Buried forest floor under mixed mineral soil occurred in 57% of mounds with mineral surface soil. Harvesting disturbed 65% of the watershed surface and removed forest floor from 25% of the area. Mechanically mixed soil under ruts with organic or mineral surface soil, and mounds with mineral surface soil contained organic carbon and nitrogen pools significantly greater than undisturbed forest floor. Mechanical mixing into underlying mineral soil could account for the loss of forest floor observed between the preharvest condition and the second growing season after whole-tree harvesting.

New Hampshire↗

USGS1988

Acid neutralizing processes in an alpine watershed front range, Colorado, U.S.A.-1: Buffering capacity of dissolved organic carbon in soil solutions

Soil interstitial waters in the Green Lakes Valley, Front Range, Colorado were studied to evaluate the capacity of the soil system to buffer acid deposition. In order to determine the contribution of humic substances to the buffering capacity of a given soil, dissolved organic carbon (DOC) and pH of the soil solutions were measured. The concentration of the organic anion, Ai − , derived from DOC at sample pH and the concentration of organic anion, Ax − at the equivalence point were calculated using car☐yl contents from isolated and purified humic material from soil solutions. Subtracting Ax − from Ai − yields the contribution of humic substances to the buffering capacity (A equiv. − ). Using this method, one can evaluate the relative contribution of inorganic and organic constituents to the acid neutralizing capacity (ANC) of the soil solutions. The relative contribution of organic acids to the overall ANC was found to be extremely important in the alpine wetland (52%) and the forest-tundra ecotone (40%), and somewhat less important in the alpine tundra sites (20%). A failure to recognize the importance of organic acids in soil solutions to the ANC will result in erroneous estimates of the buffering capacity in the alpine environment of the Front Range, Colorado.

Colorado↗

USGS1995

Decadal and annual changes in biogenic opal and carbonate fluxes to the deep Sargasso Sea

Analyses of samples from a 14-year series of sediment-trap deployments in the deep Sargasso Sea reveal a significant trend in the ratio of the sinking fluxes of biogenic calcium carbonate and silica. Although there are pronounced seasonal cycles for both flux components, the overall opal/CaCO 3 ratio changed by 50% from 1978 to 1991 (largely due to a decrease of opal flux), while total flux had no significant trend. These results suggest that plankton communities respond rapidly to subtle climate change, such as is evident in regional variations of wind speed, precipitation, wintertime ventilation and midwater temperatures. If the trends we observe in the makeup of sinking particulate matter occur on a large scale, they may in turn modify climate by modulating ocean-atmosphere CO 2 exchange and albedo over the ocean.

Deep Sea Research Part A, Oceanographic Research P↗

USGS1984

Response of northern San Francisco Bay to riverine inputs of dissolved inorganic carbon, silicon, nitrogen and phosphorus

Estuarine processes can be effective in modifying (filtering) distributions of dissolved inorganic forms of carbon (DIC), silicon (DIS), nitrogen (DIN), and phosphorus (DIP) in northern San Francisco Bay. During winter, high inflow from the Sacramento-San Joaquin river system supplied these nutrients to the estuary at rates that exceeded potential rates of estuarine supply and removal processes. During spring and summer, when inflow rates were lower, the estuary was an effective “filter” of the river inflow “signal” because rates of estuarine processes were high relative to river and other supply rates. At lower inflow rates, the river apparently influenced estuarine hydrodynamic features that controlled rates of phytoplankton nutrient removal. Largest biological removal effects were localized in San Pablo Bay during spring and Suisun Bay during summer, and they were generally more pronounced in shallow water areas of the bays. In San Pablo Bay, effects of biological removal appeared soon after river inflow decreased from high winter rates, but persisted for only a short time. During the following summer months, DIN and DIP distributions in San Pablo Bay indicated that estuarine sources contributed to higher concentrations of these nutrients.

California↗

USGS2011

Are soils of Iowa USA currently a carbon sink or source? Simulated changes in SOC stock from 1972 to 2007

Upscaling the spatial and temporal changes in carbon (C) stocks and fluxes from sites to regions is a critical and challenging step toward improving our understanding of the dynamics of C sources and sinks over large areas. This study simulated soil organic C (SOC) dynamics within 0–100 cm depth of soils across the state of Iowa in the USA from 1972 to 2007 using the General Ensemble biogeochemical Modeling System (GEMS). The model outputs with variation coefficient were analyzed and assembled from simulation unit to the state scale based upon major land use types at annual step. Results from this study indicate that soils (within a depth of 0–100 cm) in Iowa had been a SOC source at a rate of 190 ± 380 kg C ha −1 yr −1 . This was likely caused by the installation of a massive drainage system which led to the release of SOC from deep soil layers previously protected under poor drainage conditions. The annual crop rotation was another major force driving SOC variation and resulted in spatial variability of annual budgets in all croplands. Annual rate of change of SOC stocks in all land types depended significantly on the baseline SOC levels; soils with higher SOC levels tended to be C sources, and those with lower levels tended to be C sinks. Management practices (e.g., conservation tillage and residue management practices) slowed down the C emissions from Iowa soils, but could not reverse the general trend of net SOC loss in view of the entire state due mainly to a high level of baseline SOC stocks.

Agriculture, Ecosystems and Environment↗

USGS2013

The role of irrigation runoff and winter rainfall on dissolved organic carbon loads in an agricultural watershed

We investigated the role of land use/land cover and agriculture practices on stream dissolved organic carbon (DOC) dynamics in the Willow Slough watershed (WSW) from 2006 to 2008. The 415 km 2 watershed in the northern Central Valley, California is covered by 31% of native vegetation and the remaining 69% of agricultural fields (primarily alfalfa, tomatoes, and rice). Stream discharge and weekly DOC concentrations were measured at eight nested subwatersheds to estimate the DOC loads and yields (loads/area) using the USGS developed stream load estimation model, LOADEST. Stream DOC concentrations peaked at 18.9 mg L −1 during summer irrigation in the subwatershed with the highest percentage of agricultural land use, demonstrating the strong influence of agricultural activities on summer DOC dynamics. These high concentrations contributed to DOC yields increasing up to 1.29 g m −2 during the 6 month period of intensive agricultural activity. The high DOC yields from the most agricultural subwatershed during the summer irrigation period was similar throughout the study, suggesting that summer DOC loads from irrigation runoff would not change significantly in the absence of major changes in crops or irrigation practices. In contrast, annual DOC yields varied from 0.89 to 1.68 g m −2 yr −1 for the most agricultural watershed due to differences in winter precipitation. This suggests that variability in the annual DOC yields will be largely determined by the winter precipitation, which can vary significantly from year to year. Changes in precipitation patterns and intensities as well as agricultural practices have potential to considerably alter the DOC dynamics.

California↗