SEDIMENT YIELD - SURFACE TOPOGRAPHY RELATIONSHIPS
FOR SELECTED MISSISSIPPI SOILS
M. J. M. Römkens [1], K. Helming [2] and S. N. Prasad [3]
Microtopography is an important surface characteristic of upland areas that affect erosion processes of detachment, transport, and runoff. Yet, little quantitative information is available about the relationship between surface microtopography and sediment yield. A laboratory rainfall simulation study was conducted to determine for four different erosion susceptible soils the changes in surface microtopography and sediment yield during a series of 6 to 8 rainstorms of 0.75 h duration and 60 mm h-1 intensity each. The soils chosen were the Ap-materials of a Grenada sil (Glossic Fragiudalfs), Atwood sil (Typic Paleudalfs), and a Forestdale sicl (Typic Ochraqualfs), as well as the C-material, a Glauconitic sediment, of a Ruston sil (Typic Paleudalfs). Soil beds were prepared in a flume with a seedbed-like surface condition. Before all and after each rainstorm, the surface microtopography was determined using a laser microreliefmeter. Microtopography, expressed in terms of the mean local topographic gradient, and runoff data indicate a very similar pattern among the four soils. The data show an initially rapid increase in the sediment concentration, which reached quickly a maximum and then gradually decreased to a near constant value at the end of the storm series. Sediment yield followed closely the sediment concentration trend due to a near constant runoff rate. Surface microtopography changed rapidly during the first rainstorm but then decreased more gradual to an approximate constant value for most of the later rainstorms in the sequence. Three distinct phases in the sediment yield-microtopography relationship are recognized: (1) a preponding phase, (2) a post ponding-increased sediment yield phase, and (3) a post ponding-decreased sediment yield phase. These phases reflect changes in the relative importance of soil erosion processes of roughness dissipation, rill development, and soil surface matrix stabilization.
Kay Words: Surface microrelief, sediment yield, sediment concentration, surface roughness, microtopography, runoff.
POLYMER CHARGE AND MOLECULAR WEIGHT EFFECTS
ON TREATED IRRIGATION FURROW PROCESSES
R. D. Lentz[4], R. E. Sojka and C.W. Ross[5]
Application of 5-10 mg L-1 water soluble anionic polyacrylamide (PAM) to furrow irrigation water during flow advance substantially reduces sediment loss and increases net infiltration. We hypothesized that PAM_s solvated molecular conformation influences its irrigation-management efficacy. The study was conducted in Kimberly, Idaho, on Portneuf silt loam (Durinodic Xeric Haplocalcids); under furrow-irrigated beans (Phaseolus vulgaris ) at a 1.5% slope. Polyacrylamides with contrasting molecular weight (anionic: 4-7, 12-15 and 14-17 MDa, i.e. Mg mol-1), charge type (neutral, anionic, cationic), and charge density (8, 19, 35 mol %) were tested in two studies. Inflow rate was 23 L min-1 during furrow advance, and 15 L min-1 for the remaining set. Anionic and neutral PAMs were twice as effective as cationic PAMs for controlling sediment loss in new furrows. The order of effectiveness for overall soil-loss control was: anionic > neutral > cationic PAM, and efficacy increased with increasing charge density and/or molecular weight. Net furrow infiltration increased by 14 to 19% when PAM treatment molecular weight was reduced from 17 to 4 MDa. General trends suggested that medium and high charge density anionic and neutral PAM produced the greatest increase in infiltration compared with controls. Compared with untreated furrows, neutral PAM gave the greatest season-long net infiltration gains (5%); while charged PAMs tended to increase net infiltration early in the season on new furrows but decreased infiltration on repeat-irrigated furrows later in the season.
Key Words: Polymer charge, Molecular weight effect, Soil loss control, Charge Density, Infiltration capacity
rocesses of Ephemeral Gully Erosion
Javier Casalí[6], Sean J. Bennett[7], and Kerry M. Robinson[8]
The formation of ephemeral gullies can significantly increase soil loss from agricultural lands and severely impact farm productivity. Erosion prediction technology and conservation management techniques would be greatly improved if the contribution from ephemeral gullies could be more accurately quantified. Field research in Mississippi, U.S.A. and Spain has revealed three categories of ephemeral gullies. Classic ephemeral gullies formed by concentrated flow erosion from runoff occurring within the same field. Drainage ephemeral gullies formed by concentrated flow erosion from runoff originating from areas upstream of where the gully occurs. Discontinuity ephemeral gullies formed in areas where management practices have created a sudden change in slope, such as field boundaries adjacent to roads. Despite the large differences in climate, watershed size, hydrology, and geography, the ephemeral gullies observed in Spain were morphologically similar to those in Mississippi. Using an experimental flume, ephemeral gully erosion proceeded primarily through bed incision, gully widening, and bank steepening, and total sediment load depended upon whether the flow was detachment- or transport-limited.
Key Words: Ephemeral gully; Soil loss; Erosion prediction technology; Conservation management; Bed incisions; Total sediment load
WATERSHED CONSIDERATIONS FOR INTEGRATED
STREAM MODELING
Charles W. Slaughter[9], Peter Goodwin[10] and Rick Marbury[11]
ABSTRACT
Water, sediment and many water quality constituents for rivers are typically derived from upland contributing watersheds as well as from lower-elevation streamside zones and banks. This is particularly evident for the topographically complex landscapes of the interior Pacific Northwest and Great Basin regions, where meltwater from high-elevation snowpacks is the primary water source for rivers traversing extensive semiarid lowlands. While river basin management has commonly focused on downstream high-order reaches, natural resource managers are increasingly concerned with small, low-order stream systems and riparian environments in the headwaters of river basins. The need for understanding headwaters hydrology is demonstrated for a rangeland watershed system in which hydrologic regime of headwaters and mid-elevation sectors is intimately linked to streamflow and channel processes in low-elevation, higher-order stream reaches. Comprehensive, successful river and watershed management and simulation model application requires adequately understanding hydrologic and ecosystem characteristics of the source watershed.
Key Words: Water quality; Watershed, River basin management; Rangeland watershed system; channel process; Ecosystem characteristics
WATERSHED SEDIMENT YIELD AND RANGELAND HEALTH
Leonard J. Lane, Mary R. Kidwell, and Mark A. Weltz[12]
Much of the western United States is considered to be rangeland. Rangeland areas produce a diverse mix of benefits and products, and their overall health, in an ecosystem context, is of national importance. Because sediment yield from a watershed is an integrated expression of all soil erosion and sedimentation processes occurring within it, it is logical that we seek to quantify and interpret sediment yield in the context of soil/site stability and watershed function as measures of rangeland health. Depth integrated suspended sediment samples were combined with runoff measured using flumes to calculate sediment discharge and yield from two experimental watersheds in the southwestern USA. Sediment yield estimates for individual runoff events were summed to produce estimates of annual sediment yield from these two rangeland watersheds. Estimated annual sediment yield data were then combined with the concepts of sediment delivery ratio and soil loss tolerance to assess soil/site stability at the watershed scale. Analyses suggest that using sediment yield estimates from distributed watershed processes with time-space averaged soil loss tolerance values is inconsistent. Thus, new distributed soil/site stability criteria are needed to replace the soil loss tolerance concept in assessing the health of rangeland watersheds. Sediment transport/yield models are used at interior points in a watershed to simulate distributed sedimentation processes. However, application of these models requires calibration and validation data and is thus dependent upon the availability of sediment concentration and yield databases. Therefore, additional efforts are required to build sediment yield databases through rescue of historical data along with continued measurement and monitoring at existing and new sampling sites.
Key Words: Sediment yield, Rangalang health, Ecosystem contex, Sedimentation processes, Sediment delivery ratio
LONG TERM SEDIMENT YIELD AND MITIGATION IN
A SMALL SOUTHERN PIEDMONT WATERSHED
D. M. Endale[13], H. H. Schomberg1, and J. L. Steiner1
ABSTRACT
Southern Piedmont lands suffer moderate to severe erosion when farmed under single?crop, conventional till systems consisting of moldboard plowing, disking or harrowing. This is primarily due to high soil erodibility, high energy spring?summer storms, low residue cover, and poor management factors. A winter season with no crop often leaves soil unprotected from rainfall impacts. Conservation cropping systems that minimize tillage and leave a growing crop and crop residues on the surface both in summer and winter protect soil from erosive effects and sustain productivity. In this paper we present and discuss 26-years of soil loss, runoff and residue production data from a 2.71 ha catchment typical of small Southern Piedmont watersheds. The catchment was first managed in conventional tillage system of summer soybean (Glycine max (L.) Merrill) and winter fallow from 1972 to 1974. It was then converted to conservation cropping systems of summer soybean, sorghum (Sorghum bicolor (L.) Moench), or cotton (Gossypium hirsutum (L.)) and winter barley (Hordeum vulgare (L.)), wheat (Triticum aestivum (L.)), or clover (Trifolium incarnatum (L.)) which have continued to the present. Conservation cropping systems had immediate and residual effects in controlling erosion and runoff in both summer and winter. Destructive soil erosion, from high energy storms was significantly reduced. Residue production increased from about 2 Mg ha-1 yr-1 under conventional tillage to 9.88 Mg ha-1 yr-1 under conservation cropping systems over 20 years.
Key Words: Soil erodibility, Conservation cropping system, Tillage system, Erosive effect
Processes of Headcut Growth and Migration in Rills and Gullies
Kerry M. Robinson[14], Sean J. Bennett[15], Javier Casalí[16], and Gregory J. Hanson1
The formation and upstream migration of headcuts significantly increases soil losses and sediment yield from agricultural lands, threatens the structural integrity of earthen dams, and can undermine roads and bridges. Recent research using unique experimental facilities and methodologies has provided new insights on these erosion processes. Under controlled experimental conditions, steady-state soil erosion due to migrating headcuts has been simulated for rills and along crop furrows. During migration, headcut shape, size, rate of movement, and sediment yield remained constant. Downstream of the headcut, a soil bed was constructed where slope was dependent upon the sediment yield from the headcut and the flow transport capacity. Soil erosion processes were also examined in a large outdoor facility simulating gully headcuts. Using a compacted cohesive soil, headcut migration rate was observed to decrease as the average density and average unconfined compressive strength increased. While the flow rate and overfall height were not observed to have a major impact on advance rates, a sand layer at the base of an overfall did have a dramatic influence on advance rates. The erosion processes and flow structure within the large gully headcuts were strikingly similar to those in rills and crop furrows. The commonality of form and process in these soil erosion phenomena suggests that erosion prediction technology and mitigation measures may be developed and widely applied.
Key Words: Headcut, Soil loss, Sediment yield, Soil Erosion process, Rill, Gully
woody vegetation and debris for in-channel sediment control
F. Douglas Shields, Jr. and C. M. Cooper[17]
Large woody debris and woody vegetation can exert a major influence on channel hydraulics and morphology, particularly in smaller (< 30 m wide) streams. Although debris and vegetation have long been used for channel erosion and sedimentation control, scientifically-based guidelines for designers are scarce. Recent