Dry Antecedent Moisture Conditions Research Articles

High-resolution modelling of the grass swale response to runoff inflows with Mike SHE

The feasibility of simulating the hydrological response of a grass swale to runoff inflows was examined using the hydrological model Mike SHE and the available input data from 12 irrigation events mimicking runoff from block rainfalls. The test swale channel had a trapezoidal cross-section, bottom slope of 1.5%, length of 30 m, and was built in loamy fine sand. The irrigation events consisted in releasing two equal constant inflows to the swale: a concentrated longitudinal flow at the upstream end and a distributed lateral inflow along the swale side slope adjacent to the contributing drainage area. The total inflows approximated runoff from two events with return periods of 2 months and 3 years, respectively, for durations of 30 min. Irrigation experiments were done for two states of the initial soil moisture, dry or wet antecedent moisture conditions (AMC). Mike SHE has been extensively used on catchments of various sizes, but rarely for small stormwater management facilities and their detailed topography investigated in this study. The latter application required high spatial and temporal resolutions, with computational cells of 0.2 × 0.2 m and time steps as short as 0.6 s to avoid computational instabilities. For dominant hydrological processes, the following computational options in Mike SHE were chosen: Soil infiltration – the van Genuchten equation, unsaturated zone flow – the one-dimensional Richards equation, and overland flow – the diffusive wave approximation of the St. Venant equations. For study purposes, the model was calibrated for single events representing one of four combinations of low and high inflows, and dry and wet AMC, and then applied to the remaining 11 events. This was complemented by calibration for two events, representing high inflow on wet AMC and low inflow in dry AMC. The goodness of fit was statistically assessed for observed and simulated peak flows, hydrograph volumes, Nash-Sutcliffe model efficiencies (NSE), and soil water content (SWC) in swale soil layers. The best fit (NSE > 0.8) was obtained for high inflows and wet AMC (i.e., when the primary swale function is flow conveyance); the least fit was noted for low inflows and dry AMC, when the primary swale function is flow attenuation. Furthermore, this observation indicates the overall importance of correct modelling of the soil infiltration. The effects of spatial variation of SWC on the swale discharge hydrograph could not be confirmed from simulation results, but high topographical accuracy was beneficial for reproducing well the locations of the observed water ponding. No significant increases in simulated SWC at 0.3 m or greater depths were noted, which agreed with field observations. Overall, the results indicated that Mike SHE was effective in process-oriented small-scale modelling of grass swale flow hydrographs.

A lab in the field: high-frequency analysis of water quality and stable isotopes in stream water and precipitation

Abstract. High-frequency measurements of solutes and isotopes (18O and 2H) in rainfall and streamflow can shed important light on catchment flow pathways and travel times, but the workload and sample storage artifacts involved in collecting, transporting, and analyzing thousands of bottled samples severely constrain catchment studies in which conventional sampling methods are employed. However, recent developments towards more compact and robust analyzers have now made it possible to measure chemistry and water isotopes in the field at sub-hourly frequencies over extended periods. Here, we present laboratory and field tests of a membrane-vaporization continuous water sampler coupled to a cavity ring-down spectrometer for real-time measurements of δ18O and δ2H combined with a dual-channel ion chromatograph (IC) for the synchronous analysis of major cations and anions. The precision of the isotope analyzer was typically better than 0.03 ‰ for δ18O and 0.17 ‰ for δ2H in 10 min average readings taken at intervals of 30 min. Carryover effects were less than 1.2 % between isotopically contrasting water samples for 30 min sampling intervals, and instrument drift could be corrected through periodic analysis of secondary reference standards. The precision of the ion chromatograph was typically ∼ 0.1–1 ppm or better, with relative standard deviations of ∼ 1 % or better for most major ions in stream water, which is sufficient to detect subtle biogeochemical signals in catchment runoff. We installed the coupled isotope analyzer/IC system in an uninsulated hut next to a stream of a small catchment and analyzed stream water and precipitation samples every 30 min over 28 days. These high-frequency measurements facilitated a detailed comparison of event-water fractions via endmember mixing analysis with both chemical and isotope tracers. For two events with relatively dry antecedent moisture conditions, the event-water fractions were < 21 % based on isotope tracers but were significantly overestimated (40 to 82 %) by the chemical tracers. These observations, coupled with the storm-to-storm patterns in precipitation isotope inputs and the associated stream water isotope response, led to a conceptual hypothesis for runoff generation in the catchment. Under this hypothesis, the pre-event water that is mobilized by precipitation events may, depending on antecedent moisture conditions, be significantly shallower, younger, and less mineralized than the deeper, older water that feeds baseflow and thus defines the pre-event endmember used in hydrograph separation. This proof-of-concept study illustrates the potential advantages of capturing isotopic and hydrochemical behavior at a high frequency over extended periods that span multiple hydrologic events.

Effects of irrigation intensity on preferential solute transport in a stony soil

ABSTRACTIf irrigation intensity exceeds soil infiltration capacity, water may flow preferentially down cracks and large pores. In this situation, solute transport will involve only a fraction () of the soil’s water and leaching rate may be affected. To assess whether irrigation intensity affects preferential solute flow, an experiment was performed at Lincoln using 12 steel-encased lysimeters with a Lismore Stony Silt Loam soil under two irrigation intensities, 5 and 20 mm h–1. Burns’ equation was used to describe the measurements of non-reactive tracer concentration as a function of drainage. Under dry antecedent moisture conditions, bromide transport was not significantly different under the different irrigation rates, even though strong preferential leaching occurred, with of 0.23. For chloride, was 0.85 and 0.58, for 5 and 20 mm h–1 respectively, sufficient evidence to confirm the effect of irrigation intensity (P < 0.05). By assuming to be 1.00 for the median rainfall at Lincoln, an exponential function was fitted to the data, suggesting a lower limit of 0.35 for under moist conditions. Implications for nutrient leaching are discussed.

Modelling of Spatially Distributed Surface Runoff and Infiltration in the Olifants River Catchment/Water Management Area Using GIS

This study aimed to model the amount of rainfall leaving the Olifants River Catchment area as surface runoff and entering into the subsurface as infiltration (as part of these waters may contribute to the water ingress in the abandoned/closed mines) using the RINSPE model implemented in ArcView GIS 3.3. The runoff and infiltration depths and the total volumes were calculated by the model for 7 scenarios using National Land-Cover 2000 Dataset. Scenarios 1 & 2: used one inch uniform rainfall to predict the expected runoff and infiltration at any location in the catchment for average and dry antecedent moisture conditions (AMCII & AMCI); scenario 3 used annual rainfall and assumed 40 rainfall events in a year for AMCII; scenarios 4 to 6 used annual rainfall and assumed 35, 40 and 46 rainfall events in a year for AMCI, and scenario 7 assumed 40 rainfall events in a year for AMCI and excluded the catchments areas of Letaba and Shingwedzi Rivers. Scenarios 1 and 2 show that runoff and infiltration are respectively 11.36 & 59.83% and 6.16% & 65.03% of the rainfall. Scenario 3 predicted a total infiltration of 6,449.793 million cubic meters or MCM (14.46% of total rainfall); the total runoff predicted is 16,589.1 MCM (37.2% of total rainfall). Scenarios 4, 5, 6 and 7 showed infiltrations of respectively 22.24%, 24.57%, 19.3% and 22.71% of total rainfall whereas the surface runoff predicted are respectively 13,120.85, 14,776.52, 11,310.57 and 10,748.07. MCMs (29.43%, 33.14%, 25.37% and 31.49% of total rainfall). Spatially distributed runoff and infiltration maps will help to understand the amount of rainfall leaving mined areas as polluted runoff and the amount of water infiltrating into the subsurface horizons, which may later contribute to water ingress or appear as part of the acid mine drainage formed in the catchment.

Subsurface flow processes in sloping cropland of purple soil

Subsurface flow is a prominent runoff process in sloping lands of purple soil in the upper Yangtze River basin. However, it remains difficult to identify and quantify. In this study, in situ runoff experimental plots were used to measure soil moisture dynamics using an array of time domain reflectometry (TDR) together with overland flow and subsurface flow using isolated collecting troughs. Frequency of preferential flow during rainfall events and the controls of subsurface flow processes were investigated through combined analysis of soil properties, topography, rainfall intensity, initial wetness, and tillage. Results showed that subsurface flow was ubiquitous in purple soil profiles due to well-developed macropores, especially in surface soils while frequency of preferential flow occurrence was very low (only 2 cases in plot C) during all 22 rainfall events. Dry antecedent moisture conditions promoted the occurrence of preferential flow. However, consecutive real-time monitoring of soil moisture at different depths and various slope positions implied the possible occurrence of multiple subsurface lateral flows during intensive storms. Rainfall intensity, tillage operation, and soil properties were recognized as main controls of subsurface flow in the study area, which allows the optimization of management practices for alleviating adverse environmental effects of subsurface flow in the region.

Hydrology of Small Field Plots Used to Study Phosphorus Runoff under Simulated Rainfall

Use of small plots and rainfall simulators to extrapolate trends in runoff water quality requires careful consideration of hydrologic process represented under such conditions. A modified version of the National Phosphorus Runoff Project (NPRP) protocol was used to assess the hydrology of paired 1 x 2 m plots established on two soils with contrasting hydrologic properties (somewhat poorly drained vs. well drained). Rain simulations (60 mm h(-1)) were conducted to generate 30 min of runoff. For the somewhat poorly drained soil, simulations were conducted in October and May to contrast dry conditions typically targeted by NPRP protocols with wet conditions generally associated with natural runoff. For the well-drained soil, only dry conditions (October) were evaluated. Under dry antecedent moisture conditions, an average of 64 mm of rainfall was applied to the somewhat poorly drained soil to generate 30 min of runoff, as opposed to 96 mm to the well-drained soil. At an extreme, differences in rainfall were equivalent to a 50-yr rainfall-return period. An absence of detectable spatial trends in surface soil moisture suggests uniformity of runoff processes within the plots. No differences in applied rainfall were evident between wet and dry antecedent conditions for the somewhat poorly drained soil. However, significant differences in runoff generation processes were observed in dissolved P concentrations between wet and dry conditions. As natural runoff from the somewhat poorly drained soil is largely under wet antecedent conditions, this study highlights the need for care in interpreting findings from generalized protocols that favor infiltration-excess runoff mechanisms.

Hydrologic Impacts of Mechanical Seeding Treatments on Sagebrush Rangelands

In and around the Great Basin, United States, restoration of shrub steppe vegetation is needed where rangelands are transitioning to annual grasslands. Mechanical seedbed preparation can aid native species recovery by reducing annual grass competition. This study was designed to investigate the nature and persistence of hydrologic and erosion impacts caused by different mechanical rangeland seeding treatments and to identify interactions between such impacts and related soil and vegetation properties. A cheatgrass (Bromus tectorum L.)–dominated site was burned and seeded with native grasses and shrubs in the fall of the year. An Amazon-drill and a disk-chain seeder were used to provide varying levels of surface soil disturbance. An undisturbed broadcast seeding was used as a control. Simulated rainfall was applied to 6 large (32.5-m2) plots per treatment over 3 growing seasons at a rate of 63.5 mm · h-1. Rainfall was applied for 60 minutes under dry antecedent moisture conditions and for 30 minutes, 24 hours later under wet antecedent moisture conditions. The disk-chain created the largest reduction in infiltration and increase in sediment yield, which lasted for 3 growing seasons posttreatment. The Amazon-drill had a lesser impact, which was insignificant after the second growing season posttreatment. Surface soil properties showed little correlation with treatment-induced hydrologic and erosion impacts. Hydrologic recovery was strongly correlated with litter dynamics. The seeding treatments were unsuccessful at establishing seeded plant species, and the site once again became dominated by cheatgrass. A continuous upward trend in biomass production and surface litter cover was observed for all treatments between the beginning and end of the study because of cheatgrass invasion. Although the initial goal of using mechanical seeding treatments to enhance recovery of native grass species failed, cheatgrass production provided sufficient biomass to rapidly replenish surface litter cover necessary for rapid hydrologic stability of the site. Adentro y alrededor de la Gran Cuenca, Estados Unidos, se necesita la restauración de la vegetación de estepa arbustiva, donde los pastizales son una transición a zacatales anuales. La preparación mecánica de la cama de siembra puede ayudar a la recuperación de las especies nativas al reducir la competencia de zacates anuales. Este estudio se diseño para investigar la naturaleza y persistencia de los impactos hidrológicos y de erosión causados por diferentes tratamientos mecánicos de siembra de pastizales y para identificar interacciones entre tales impactos y las propiedades relacionadas del suelo y vegetación. Un sitio dominado por “Cheatgrass” (Bromus tectorum L.) fue quemado y sembrado con zacates nativos y arbustos en el otoño. Se usaron una sembradora amazona y una de cadena y discos para crear diferentes niveles de disturbio en la superficie del suelo. Una siembra al voleo en un sitio sin disturbio se usó como control. Se aplicó lluvia simulada, a una tasa de 63.5 mm · h-1, en seis parcelas grandes por tratamiento a lo largo de tres estaciones de crecimiento. La lluvia se aplicó por 60 minutos bajo condiciones secas del suelo y por 30 minutos bajo condiciones húmedas del suelo. La sembradora de cadena y disco produjo la mayor reducción en la infiltración y aumentó la producción de sedimentos, efecto que duró las tres estaciones de crecimiento posteriores a la aplicación de los tratamientos. La sembradora amazona tuvo un impacto menor, el cual fue insignificante después de la segunda estación de crecimiento posterior a la aplicación del tratamiento. Las propiedades de la superficie del suelo mostraron una baja correlación con los impactos hidrológicos y de erosión inducidos por los tratamientos. La recuperación hidrológica estuvo altamente correlacionada con la dinámica del mantillo. Los tratamientos de siembra no fueron exitosos para el establecimiento de plantas de las especies usadas, y el sitio fue dominado nuevamente por el “Cheatgrass.” Se observó una tendencia ascendente continua en la producción de biomasas y la cobertura de mantillo de la superficie en todos los tratamientos entre el inicio y fin del estudio, a consecuencia de la invasión del “Cheatgrass.” Mientras que la meta inicial de usar tratamientos de siembra mecánica para mejorar la recuperación de las especies nativas fracasó, la producción del “Cheatgrass” suministró suficiente biomasa para reponer rápidamente la cobertura de mantillo de la superficie del suelo necesaria para una rápida estabilidad hidrológica del sitio.

Soil water repellency in a Japanese cypress plantation restricts increases in soil water storage during rainfall events

AbstractForest soils in Japan are often water repellent. Substantial water repellency frequently occurs and impedes water infiltration into the soil matrix, but continuous overland flow is not necessarily observed because forest soils usually have macropores through which the water can enter the subsoil. Although this flow pattern may influence the manner of water storage in forest soils at the solum scale, field evidence has not yet indicated this process. We monitored soil water storage during natural rainfall events in a 60‐cm deep solum using time domain reflectometry (TDR) moisture sensors, and observed stained flow patterns in the soil following simulated rainfall containing a colour dye, on a slope planted with Japanese cypress (Chamaecyparis obtusa). The surface soil at the research plot exhibited strong water repellency at water contents lower than the threshold critical water content of 0·29 m3 m−3. Under dry antecedent moisture conditions, increases in soil water storage were small compared to the cumulative rainfall, despite the low wetness of the soil matrix. In contrast, under moderate moisture conditions, increases in the water content corresponded to the cumulative rainfall. Under dry conditions, rainwater may have entered the subsoil at a few limited locations connected with continuous vertical macropores, such as decayed root channels or interstructural voids. Therefore, the water seemed to bypass a large part of the soil matrix away from the macropores. Such preferential water flow was confirmed by the stained flow patterns after the rainfall simulation. The flow patterns visualized by the dye were discontinuous and scattered under dry conditions and diffuse under moderate moisture conditions. Repellency induced preferential flow led to restricted increases in solum scale water storage during rainfall events, reflecting a physical nonequilibrium in soil water storage. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Within-storm soil surface dynamics and erosive effects of rainstorms

This study deals with the characteristics which make a rainstorm an event that can produce intense erosion and even trigger the formation of a badlands site. In order to keep the presentation closely linked to a real situation, a rainstorm which took place on an experimental farm equipped for soil erosion studies was selected. The effects of the erosive rainstorm, which fell on dry antecedent moisture conditions, are given in terms of total rill erosion and rill cross-section along the slope. Unfortunately, the data collected did not answer the basic question, i.e., what combination of factors makes a rainstorm critical? A set of rainfall simulation experiments was therefore carried out, in the field and in the laboratory, in order to evaluate the soil surface variations caused by the rainstorm. All the experiments were performed on dry antecedent soil moisture conditions. It was confirmed that the characteristics of the infiltration curve are modified considerably during such rain events. The saturated conductivity of the first thin top-layer is also modified and it can easily decrease by a factor of 10 due to drop impact forces. The runoff coefficient is also influenced by the raindrop impacting energy and it increases sharply with cumulate energy until a maximum value is reached. The surface micro-relief dynamics was also studied. It was very clearly shown that impacting drop kinetic energy is the rainfall characteristic which is linked to random roughness decay. Cumulative rainfall was not able to align all the data in a single trend. The effect of surface micro-relief decay on the rainstorm erosive power was examined using two equations, thus linking Manning's hydraulic roughness to random roughness. Using a simulated runoff over the field plots that were particularly eroded by the rainstorm, it was possible to observe that the runoff drag forces reached values of between 3 and 100 times the ones which would have been calculated if the random roughness had been constant during the same event. Many of the soil surface characteristics that are modified interact with one another and with erosion. Examining each of them in isolation cannot explain the drastic increase in erosivity of a rainstorm, as the latter is the result of the combined effects of all the surface modifications.

EVALUATION OF CURVE NUMBER PROCEDURES TO PREDICT RUNOFF IN GLEAMS1

ABSTRACT: Proper selection of curve number values will improve the capability of the SCS‐Curve Number procedure in predicting runoff. Both CREAMS and GLEAMS models use the Smith and Williams (1980) approach of converting CNII (curve number value for average antecedent moisture conditions) into CNI (curve number value for dry antecedent moisture conditions) in calculating the soil retention parameter (S). CREAMS and GLEAMS have been found to under predict runoff because of the internal conversion of CNII to CNI. This study shows modifications of the GLEAMS model using CMI without converting it to CM and it also shows the seasonal curve number approaches with and without converting CNII to CNI. Results indicate that using CNII without internal conversion to CNI provides better runoff and erosion predictions than the original version of GLEAMS and versions with seasonal curve numbers when tested with four years of field data in the Coastal Plain physiographic region of Maryland.

An assessment of a conceptual rainfall‐runoff model's ability to represent the dynamics of small hypothetical catchments: 2. Hydrologic responses for normal and extreme rainfall

Results from five numerical experiments to test a conceptual rainfall‐runoff model (the soil moisture accounting component, SMA, used by the U.S. National Weather Service), comprising combinations of three sets of input data (hourly precipitation and evapotranspiration data) and four hypothetical two‐layered small catchments (areas ≤0.2 km2, saturated hydraulic conductivity from 0.02 to 0.2 m/h, soil depth from 0.8 to 1.6 m, hillslope length from 100 to 250 m, and slope from 0.04 to 0.10), representing a wide range of flow mechanisms, are presented. The calibrated SMA model could simulate flow volumes corresponding to broad‐scale flow mechanisms but generally performed poorly during dry to wet catchment transitions. SMA was found to be unreliable in forecasting extreme floods, especially under dry antecedent moisture conditions, and for catchments where calibration data were comprised primarily of base flow. For each case, the sum of the SMA conceptual storages differed from the maximum physical water storage capacity of the hypothetical small catchment. The calibrated SMA model parameters were climate dependent indicating that a calibrated SMA model should be used with caution when predicting hydrologic response to a changed climate.

Vertical and Lateral Components of Soil Nutrient Flux in a Hillslope

AbstractThe vertical and lateral components of chemical flux during storm events were investigated in a Typic Hapludult to assess their importance in understanding the effects of atmospheric deposition on hillslope sites. Vertical and lateral water flux were calculated from soil water potential data and hydraulic conductivity curves. Throughfall, stemflow, forest floor leachate, and soil solution from the BA, Bt, and BC horizons were sampled and analyzed for SO4, NO3‐N, Cl, HCO3, H, K, Ca, Mg, and Na. Total lateral flow as a ratio of total vertical flow averaged 0.23 and 0.30 in the A and BA horizons, respectively, indicating lateral fluxes were an important path of nutrient movement in the surface horizons. The highest lateral flow occurred in the A horizon during dry antecedent moisture conditions and in the BA horizon during wet antecedent moisture conditions. Fluxes of all ions except HCO3, NO3‐N, and H peaked in the forest floor leachate and the BA soil solution, then decreased with depth. Decreases of SO4 flux between the BA and BC horizons could not be explained by the lag of solute movement or by lateral solute losses, demonstrating the system was an effective SO4 sink.