Search

Agriculture in the United States must respond to escalating demands for productivity and efficiency, as well as pressures to improve its stewardship of natural resources. Growing global population and changing diets, combined with a greater societal awareness of agriculture's role in delivering ecosystem services beyond food, feed, fiber, and energy production, require a comprehensive perspective on where and how US agriculture can be sustainably intensified, that is, made more productive without exacerbating local and off-site environmental concerns. The USDA's Long-Term Agroecosystem Research (LTAR) network is composed of 18 locations distributed across the contiguous United States working together to integrate national and local agricultural priorities and advance the sustainable intensification of US agriculture. We explore here the concept of sustainable intensification as a framework for defining strategies to enhance production, environmental, and rural prosperity outcomes from agricultural systems. We also elucidate the diversity of factors that have shaped the past and present conditions of cropland, rangeland, and pastureland agroecosystems represented by the LTAR network and identify priorities for research in the areas of production, resource conservation and environmental quality, and rural prosperity. Ultimately, integrated long-term research on sustainable intensification at the national scale is critical to developing practices and programs that can anticipate and address challenges before they become crises.

Many concepts have been proposed to explain hydrologic connectivity of hillslopes with streams. Hydrologic connectivity is most often defined by qualitative assessment of spatial patterns in perched water tables or soil moisture on hillslopes without a direct linkage to water flow from hillslopes to streams. This form of hydrologic connectivity may not explain the hydrologic response of catchments that have network(s) of preferential flow paths, e.g. soil pipes, which can provide intrinsic connectivity between hillslopes and streams. Duplex soils are known for developing perched water tables on hillslopes and fostering lateral flows, but the connectivity of localized perched water tables on hillslopes with soil pipes has not been fully established. The objectives of this study were to characterize pipeflow dynamics during storm events, the relationships between perched water tables on hillslopes and pipeflows, and their threshold behavior. Two well-characterized catchments in loess soil with a fragipan were selected for study because they contain multiple, laterally extensive (over 100 m) soil pipe networks. Hillslopes were instrumented with shallow wells adjacent to the soil pipes, and the wells and pipe collapse features were equipped with pressure transducers. Perched water tables developed on hillslopes during a wetting up period (October – December) and became well connected spatially across hillslope positions throughout the high flow period (January – March). The water table was not spatially connected on hillslopes during the drying out (April-June) and low flow (July-September) periods. Even when perched water tables were not well-connected, water flowing through soil pipes provided hydrologic connectivity between upper hillslopes and catchment outlets. Correlations between soil pipeflow and perched water tables depended on the size and location of soil pipes. The threshold relationship between available soil-moisture index plus storm precipitation (ASI + P) and pipeflow was dependent on the season and strongest during dry periods and not high flow seasons. This study demonstrated that soil pipes serve as a catchment backbone of preferential flow paths that provide intrinsic connectivity between upper hillslopes and streams.

Quantifying the effectiveness of erosion control practices in watersheds remains a difficult problem. Determination of recent sediment accumulation rates for lake sediments in agricultural watersheds using radioisotopes, such as lead-210 (210Pb) and cesium-137 (137Cs), is potentially a valuable means of assessing the effectiveness of soil conservation practices at the watershed scale. The predominance of sediment arriving in runoff from the watershed, variable sedimentation rates, and mechanical mixing of soil in nearby fields all present challenges in the conversion of radioisotope data to sedimentation rates. Four sediment cores from Beasley Lake, Mississippi, were used to demonstrate the application of the Constant Initial Concentration (CIC) model for calculating sediment age from the distribution of 210Pb bottom sediments. The activity of 137Cs was used to supplement the 210Pb data by providing a benchmark date within the core to calibrate the CIC model. Three of the four cores showed reductions in sediment accumulation rate within the 30 years prior to core collection in 2008 and 2011 by at least 50% relative to rates from before the adoption of soil conservation measures in the watershed. The most recently resolved rates were approximately 0.5 cm y−1 (0.2 in yr−1). The study demonstrates the application of the CIC model for developing sediment accumulation chronologies in agricultural catchments.

In many regions precipitation does not reliably meet crop water demands – a situation that climate change will likely exacerbate. Supplemental irrigation can help enhance and stabilize crop yields, but the need of water for irrigation has often led to groundwater over-exploitation. On-farm ponds can provide a more sustainable water source. Their use has often been promoted by local authorities, but, by converting a portion of cultivated area to water storage and reducing water availability downstream, on-farm ponds also imply constraints and trade-offs. For an effective exploitation of their potential benefits, they must be carefully designed and managed based on the local edaphic and climate conditions – a non trivial, task because of the cascading effects of rainfall unpredictability. Here we identify the most suitable on-farm pond size, according to two criteria: maximization of average yield (i.e., production maximization) and achievement of a minimum acceptable yield (i.e., risk minimization, accounting for the farmer’s risk aversion). To this aim, we develop a minimalist model, requiring few, physically based parameters, coupling crop biomass, soil moisture, and water stored in the pond. While general, the model is here applied to a case-study in the Lower Mississippi River Basin (USA). Simulations show that yield maximization and risk minimization are goals hard to reconcile, regardless of climatic conditions, soil type and irrigation strategy, with smaller ponds allowing the maximum average yield at the cost of reducing its stability from year to year. Stress avoidance irrigation ensures higher yields than deficit irrigation, even if it implies a faster use of the stored water. Future, more extreme climates will result in lower maximum average yields and narrower ranges of pond sizes ensuring desirable minimum yields.

Soil pipes are important subsurface flow pathways in many soil erosion phenomena. However, limited research has been performed on quantifying and characterizing their flow and transport characteristics. The objectives of this research were to determine the applicability of a streamflow model with transient storage in deriving flow and transport characteristics of soil pipes. Tracer data from pulse inputs were collected in four different soil pipes after a fluorescein dye was injected in the upstream end of each soil pipe network in three branches (west, middle, and east) of a main catchment and a back catchment in Goodwin Creek Experimental Watershed in Mississippi. Multiple sampling stations were positioned along each soil pipe network. The transient storage zone model OTIS-P was executed inversely to estimate transport parameters by soil pipe reach such as the soil pipe cross-sectional area (A), soil storage zone cross-sectional area (As), and exchange rate between the soil pipe and the soil storage zone (αs). Model convergence was achieved, and simulated breakthrough curves of the reaches were in good agreement with actual tracer data for eight of the nine reaches of the three branches of the Main Catchment and five of the seven reaches of the Back Catchment soil pipe. Simulation parameters for the soil pipe networks were similar to the range of values reported for flow and transport characteristics commonly observed in streams. Inversely, estimated soil pipe flow velocities were higher with increased tortuosity, which led to a smaller cross-sectional areas predicted for the soil pipe flowpaths, while other parameters were not sensitive to tortuosity. In general, application of One-Dimensional Transport with Inflow and Storage-P to this unique soil pipe condition suggested larger transient storage (As and αs) compared with most stream systems. This was hypothesized to be because of relatively higher ratio of the wetted perimeter to flow area in the soil pipe, the hydraulic roughness of the soil pipe, potential retention in collapsed portions of the pipe, and interaction with smaller preferential flow systems. Copyright © 2015 John Wiley & Sons, Ltd.

Concentrated surface runoff, such as associated with ephemeral gully channels, increases erosion and transfers fine sediment and associated agrichemicals from upland areas to stream channels. Ephemeral gully erosion on cropland may contribute up to 40 % or more of the sediment delivered to the edge of agricultural fields, significantly threatening the health of downstream ecological services. Typically, conservation practices developed for sheet and rill erosion are also expected to treat ephemeral gully erosion, but technology and tools are needed to account for the separate benefits and effects of practices on various sediment sources. Without improved research studies, subjective observations will continue to be used to satisfy quality criteria in lieu of scientifically defensible, quantitative methods to estimate the impact of gully erosion. Ephemeral gully channels evolve by one, or combination of, complex physical process in the form of incision, headcut migration upslope, and channel sidewalls expansion (widening). This study focused on the latter, ephemeral gully channel widening relationships. The impact of various width functions on erosion can be very significant and is dependent on discharge, slope, soil properties, and management conditions. A description is provided on six ephemeral gully widening relationships, followed by recommended improvements, comparative application, and identification of future research needs. Improvements in the development of ephemeral gully width algorithms are critical to understanding the impact of conservation practices on controlling ephemeral gully erosion. Tools are needed to predict and quantify ephemeral gully erosion, including the capability to evaluate the effect of conservation practices to control erosion. An improved critical shear stress equation was developed and described that provides an approach to incorporating impacts of management practices on the resulting gully erodibility. Conservation management planning by organizations needs a systematic approach to determining the extent of ephemeral gully erosion problems on a field, watershed, or national basis, or to predict the recurring or new locations of ephemeral gullies prior to their development.

The local role that land-atmosphere interactions play in the rainfall process has been often explored by investigating the initiation of moist convection as the top of the atmospheric boundary layer (ABL) crosses the lifting condensation level (LCL). However, this LCL crossing alone is not a sufficient indicator of the probability and intensity of subsequent convective precipitation, which is instead better characterized by the added consideration of the so-called convective available potential energy (CAPE). In this study, both the LCL crossing and CAPE are jointly considered as the primary indicators of the occurrence and intensity of moist convection in order to analyze the land-atmosphere interactions through a simple soil-plant system and a zero-dimensional mixed-layer model. The approach is explored using the free atmospheric conditions observed at the Central Facility in the Southern Great Plains, where the ABL analysis shows both dry and wet soil can be conducive to early moist convection depending on atmospheric conditions but CAPE always tends to be larger under wetter soil conditions. The combination of the two indicators, LCL crossing and CAPE, further allows us to classify free atmosphere and soil moisture regimes into positive and negative feedback regimes for moist convection.

Experiments were conducted in a laboratory flume channel to evaluate the effects of increasing amounts of sand on its transport over and through an immobile coarse gravel bed. Detailed measurements of sand transport rate, bed texture, and bed topography were collected for four different discharges at approximately the same flow depth of 0.2 m for 11 different elevations of sand in the gravel bed. Sand transport was measured using both physical samples and a density cell. For a given flow rate, increases in the elevation of sand relative to gravel resulted in decreases of bed shear stress from 32–44% and increases in sand transport by three orders of magnitude. For the highest two discharges, the sand merged into a small number of long and low bed forms that translated through and over the gravel bed. A collapse of the transport data was accomplished by relating the sand transport rate to the bed shear stress scaled by the cumulative probability distribution function of the gravel surface evaluated...

Bed load is part of the total sediment load of a stream, comprising the coarsest fraction of mobile material that is frequently in contact with the stream bed. Sediment conveyed downstream may fill reservoirs and channels, impeding navigation, increasing the likelihood of flooding, and degrading water quality and aquatic habitats. Local erosion and deposition of bed material may also cause instability of channel banks. Any long-term program of channel stabilization or improvement must account for bed-load transport in order to avoid channel aggradation or degradation. The determination of bed load rates has been concentrated in four general methods (Hubbell 1964): (1) direct measurement by collecting physical samples, (2) using bed-load transport relations, (3) measuring the erosion or deposition of bed material sediment in a confined area, or (4) measuring bed-load transport indirectly using surrogate technologies. None of these techniques is suitable for a wide range of uses. No single bed-load transport relation has been shown to have general applicability (e.g., Gomez and Church 1989; Vanoni 2006, p. 221–222; Scheer et al. 2002). Many streams do not have a convenient area to carry out erosion or deposition measurements. Accurate determination of bed load using direct measurement methods is difficult because the rate and size of sediment in transport varies dramatically in time at a point and spatially at a given time over a channel cross section of a channel even when the flow is steady (Ehrenberger 1932; Hubbell et al. 1985; Whiting et al. 1988; Kuhnle and Southard 1988; Kuhnle et al. 1989; Gray et al. 1991; Bunte and Abt 2005). Matters are further complicated by unsteady flows. Designing a device that will sample with an equal efficiency over widely varying transport rates is itself a significant challenge. An additional challenge is to collect enough samples at a point and in a cross section to adequately define the mean rate of bed-load transport for a given flow strength. Recent improvements in technology have allowed the development of devices that do not collect physical samples of the sediment but collect indirect or surrogate information that is directly related to the transport of bed-load sediment. These surrogate methods have been divided into active and passive types by Gray et al. (2010), depending on whether the sensors emit signals and record properties of the reflected sound or passively record naturally generated sound emissions. Examples of active sensors consist of acoustic Doppler current profilers that track motion of the bed sediment (Rennie et al. 2002) and active tracking of tagged sediment particles (Emmett et al. 1996; Nichols 2004). Examples of passive sensors include impact pipes (Papanicolaou et al. 2009; Mizuyama et al. 2010), impact plates (Krein et al. 2008; Moen et al. 2010; Rickenmann et al. 2014), geophones (Tsakiris et al. 2014), and devices capable of sensing sediment-generated noise (SGN) associated with the transport of gravel and coarser sediment (Thorne 1986a; Barton et al. 2010). The following paragraphs will focus on passive acoustic techniques for measuring SGN using hydrophones deployed in the transporting flow. Bed load made up of gravel (diameter >2 mm) and larger particles has been shown in previous studies to generate sound through particle collisions and bed collisions, the amplitude of which is related to the particle transport rate (e.g., Thorne 1985, 1986a; Barton 2006). Passive acoustic instrumentation has several characteristics that make it attractive for use in measuring bed load. It is well suited for remote deployment because the hardware needed to sense and record sound is relatively inexpensive. Passive acoustic methods, which record sound that is generated at some distance from a hydrophone, is nonintrusive so the phenomenon being measured is minimally affected by the measurement process. Passive acoustic instrumentation integrates sound from a finite area, decreasing bias caused by spatial heterogeneity of bed-load transport. Continuous monitoring of bed-load transport in a minimally invasive manner is readily accomplished during infrequent high-magnitude events when coarse gravel and cobble sediments are in motion. These events are infrequent but have been shown to be responsible for transporting the majority of bed-load sediment in some streams (Kuhnle 1992). Challenges of using passive acoustics to measure bed load include the generally unknown size of the measurement volume and the acoustics of fluvial environments. This paper reviews the current state of knowledge supporting the use of passive acoustic technology for bed load monitoring using hydrophones deployed in the transporting flow to directly record interparticle collisions. It is shown that while prior SGN work has addressed the generation and characteristics of SGN signals and qualitative/empirical approaches to quantifying bed load using SGN in field settings, a lack of work to date on the propagation of SGN signals in the riverine environment limits SGN as a broadly applicable methodology. In the following section, the concept of using SGN to measure bed load is introduced, along with the development of the method to date. The acoustic characteristics of the riverine setting are then presented, including propagation, transmission losses, and boundary effects. Since sediment transport may not be the most dominant source of underwater sound in a river, the

While rainfall intermittency is a dynamical phenomenon, little progress has been made in the literature on the link between rainfall intermittency and atmospheric dynamics. We present the basic dynamical models of intermittency that are phenomenologically most similar to rainfall: Pomeau–Manneville Type-III and On–Off. We then illustrate each type with both a 1-D iterative map and a corresponding stochastic process stressing the appearance of these dynamics in high-dimensional (stochastic) systems as opposed to low-dimensional chaotic systems. We show that the pdf of rainfall intensities, the pdf of “laminar phases” (periods of zero rainfall intensity), and the spectrum of the rainfall series all have power-law behavior that is broadly consistent with intermittency in the classic types. Using a seasonal analysis, we find that summer convective rainfall at daily and sub-daily scales seems consistent with features of Type-III intermittency. The correspondence with Type-III intermittency and a preliminary entropic analysis further suggest that rainfall may be an example of sporadic randomness, blending deterministic and stochastic components.

Improving the prediction of sand transport downstream of dams requires characterization of the interaction between turbulent flow and near-surface interstitial sands. The advanced age and impending decommissioning of many dams have brought increased attention to the fate of sediments stored in reservoirs. Sands can be reintroduced to coarse substrates that have available pore space resulting from periods of sediment-starved flow. The roughness and porosity of the coarse substrate are both affected by sand elevation relative to the coarse substrate; therefore, the turbulence characteristics and sediment transport over and through these beds are significantly altered after sediment is reintroduced. Past work by the writers on flow over sand-filled gravel beds revealed that the fine-sediment level controls the volume of material available for transport and the area of sediment exposed to the flow. The present work expands on the gravel-bed experiments by conducting similar measurements of turbulent f...