Decrease In Sediment Yield Research Articles

EFFECTS OF WATERSHED REPRESENTATION ON RUNOFF AND SEDIMENT YIELD MODELING1

ABSTRACT: This paper evaluates the effects of watershed geometric representation (i.e., plane and channel representation) on runoff and sediment yield simulations in a semiarid rangeland watershed. A process based, spatially distributed runoff erosion model (KINEROS2) was used to explore four spatial representations of a 4.4 ha experimental watershed. The most complex representation included all 96 channel elements identifiable in the field. The least complex representation contained only five channel elements. It was concluded that oversimplified watershed representations greatly influence runoff and sediment yield simulations by inducing excessive infiltration on hillslopes and distorting runoff patterns and sediment fluxes. Runoff and sediment yield decrease systematically with decreasing complexity in watershed representation. However, less complex representations had less impact on runoff and sediment‐yield simulations for small rainfall events. This study concludes that the selection of the appropriate level of watershed representation can have important theoretical and practical implications on runoff and sediment yield modeling in semiarid environments.

Regional patterns in recent trends in sediment yields of Eurasian and Siberian rivers

A multipurpose hydrological and statistical analysis of long-term changes in suspended sediment yield has been made for large Russian rivers discharging to the Arctic Ocean. Factors affecting sediment yield formation, e.g., snow pack duration, mean air temperature and mean water discharge, have been analyzed as well. The duration of observations at 13 selected meteorological stations varies from 56 to 113 years and covers the period from 1883 to 1995. The duration of water discharge measurements at 22 hydrological stations varies from 20 to 120 years; measurements were made from 1881 to 2000. The duration of sediment yield observations is shorter (from 1936 to 2000) and varies from 14 to 62 years. The air temperature rise is evident for 10 stations while temperature fall is observed at 3 stations (Dickson Island, Narjan Mar, Kiusiur). These three stations are established in the coastal area of the Arctic Ocean. The most impressive coincidence of trends towards an increase of mean air temperature and mean annual water discharge is observed in the Pechora, Angara, Lena, Aldan, Yana and Indugirka river basins, and in the lower reaches of the Ob and Yenisei river basins. A decrease of water discharge is observed in the Severnaya Dvina river, probably refracting by the effect of cut forest on water availability in the river. The decrease of water discharges in the upper parts of the Ob and Yenisei river basins refracts reservoirs. Changes in suspended sediment yield depend more on man's activity than on climate change. Construction of reservoirs in the upper reaches of the Ob and Yenisei rivers explains the decreased sediment yield. An increase of sediment yield in the Kolyma river basin has been observed due to the gold mining there.

Channel network morphology and sediment dynamics under alternating periglacial and temperate regimes: a numerical simulation study

The occurrence of permafrost in a highly permeable catchment has a profound effect on runoff generation. The presence of permafrost effectively makes the subsoil impermeable. Therefore, overland flow can be the dominant runoff-generating process during periglacial conditions. The absence of permafrost will promote subsurface drainage and, therefore, saturation excess overland flow can become the dominant runoff-generating process during temperate conditions. In this paper, we present a numerical modelling study in which the effect of alternating climate-related phases of permafrost and nonpermafrost on catchment hydrology and geomorphology is investigated. Special attention is given to the characteristics of the channel network being formed, and the sediment yield from these catchments. We find that channel networks expand under permafrost conditions and contract under nonpermafrost conditions. A change from permafrost to nonpermafrost conditions is characterised by a decrease in sediment yield, while a change towards permafrost conditions is marked by a peak in sediment yield. This peak is explained by the build-up of a reservoir of erodible sediment during the nonpermafrost phase. The driving force behind this reservoir build-up may be local base-level change due to tectonic uplift or eustacy. We present a number of experiments, which show the details of this process. The results are in line with existing reconstructions of climate and fluvial dynamics during the Pleistocene in Europe and offer a new explanation to these observations.

A study on responses of soil erosion and sediment yield to closing cultivation on sloping land in a small catchment using 137Cs technique in the Rolling Loess Plateau, China

By comparison of volumes and 137Cs contents of the deposited sediments before and after 1993, changes of specific sediment yields and relative sediment contributions from the gully area and from the inter gully area after closing cultivation on the later area on a small catchment of Zhaojia Gully, in the Rolling Loess Plateau, are analyzed in this paper. Closing cultivation in a large scale has not resulted in decrease but increase of specific sediment yield of the catchment, in sharp decrease of the sediment yield and the relative sediment contribution of the inter-gully area, and in increase of the sediment yield and the contribution of the gully area, for a short term. The mean specific sediment yield of 29650 t km−2 a−1 of 1994–1996 in the catchment was 2.2 times the average value of 13413 t km−2 a−1 for a long term. The specific sediment yield of the inter-gully area decreased from the 14335 t km−2 a−1 in 1994 to 7034 t km−2 a−1 in 1995 and 3517 t km−2 a−1 in 1996, while the yield of the gully area varied between 44944 and 62136 t km−2 a−1 during the period of 1994–1996 which was much greater than the value of 21118 t km−2 a−1 before 1993. The relative sediment contribution from the inter-gully area decreased from 23% in 1994 to 15% in 1995 and 6% in 1996, while the contribution from the gully area increased from 77% in 1994 to 85% in 1995 and 94% in 1996. It is suggested that compacting of ploughed soils resulted in increasing of the erosion resistance but in decreasing of the precipitation infiltration, therefore, the soil erosion reduced but the runoff amount increased on the inter-gully area. Increase of delivering runoff from the inter-gully area to the gully area should result in activeness of gully erosion and mass movements, consequently, in increase of the total sediment yield from the catchment.

Soil Erosion and Forestry Management After Wildfire in a Mediterranean Woodland, Mt. Carmel, Israel

Forestry management is crucial in mitigating erosion processes after extensive fires in Mediterranean woodlands. Fire alters forest ecosystems, causing an increase in rates of geomorphic processes. The September 1989 fire in Mt. Carmel covered an area of 4 km2 in the main recreation area of a typical Mediterranean forest area. Six experimental plots, covering an area of 300-500 m(2) each, were established in order to determine the effect of forestry management practice on soil erosion after the fire; runoff and sediments were collected during and after each rainstorm. Rainfall was measured by two recorders and several rain gauges at the study site. During the first year following the fire, sediment yield was 100-500 times higher than on vegetated slopes. Sediment movement was increased mainly by the lack of vegetation. Results show that there is a clear trend of decrease in sediment yield between the first year and the next five. After this period the burnt areas recovered to similar rates of erosion as in the unburnt areas. The clearings of burnt logs by machine and cable sliding increased the sediment yield in the experimental plots. After vegetation development in the second season following the fire, no significant differences were found among the types of management practice plots.

Rill sediment transport on a Prince Edward Island (Canada) fine sandy loam

In order to determine the temporal variation of sediment yield under conditions of freezing, rill erosion tests were conducted on beds of a fine sandy loam soil subjected to surface flow under various conditions of freezing, mulching, surface compaction, and slope. During a 20-min test period, runoff samples were collected at six sampling times to measure sediment yield. Freezing, mulching, and slope had significant ( P< =0.05) effects on sediment yield. Generally, sediment yield decreased with time, although treatment units subjected to freeze-thaw (frozen at the start of the tests) or compaction showed an initial rise. The sediment yield of the latter treatments were best described by non-linear ( R 2=0.807 to 0.969) regression models. In examining sediment size grades <600 μm, only the <38 μm fraction showed significant differences in individual sample mass among sampling times. For this size grade there was a significant decrease in sediment yield for the 7% slope ( R 2=0.901) and for bare surface ( R 2=0.614) at later sampling.

Sediment Yield under Different Land Uses in the Spanish Pyrenees

The effects of different land uses on soil conservation and sediment yield were studied by means of experimental plots and other direct and indirect evidence in the Central Spanish Pyrenees. The purpose was to compare the traditional land uses with the present ones, in order to explain some of the landscape characteristics and, especially, the state of soil conservation. The work attempts to identify the land uses that contribute more to the sustainability of a mountain area affected by significant and generalized land-use changes in recent decades. The results obtained suggest that the traditional cereal agriculture greatly contributes to soil erosion and responds rapidly to precipitation. This probably accounts for the state of damage of many soils used for cereal cultivation over many centuries. Meadows yield much water and few sediments, and dense shrub cover yields very little water and very few sediments. The burning of the dense shrub cover produces a sudden increase in runoff and erosion, although a few months later sediment yield reverts back to rates similar to those prevailing before the fire. Now, the reduction of the area of cultivated land, the abandonment of shifting agriculture, the replacement of cereals by meadows, and the expansion of bushes in the former cultivated fields will cause a decrease in sediment yield.

Rates of fluvial erosion in basins with a Mediterranean type climate

Rates of erosion in mediterranean type climate areas are variable and they depend on climatic, physiographic and human factors. Available data from four mediterranean climatic areas are analysed, in order to find a relationship between sediment yield trends and precipitation values within each area and to compare trends along different areas. It may be inferred that there is a decrease in sediment yield with increasing precipitation; two distinctive curves are noticed: one for Israel and Spain, with a sediment peak at 300 mm of annual precipitation and a decreasing rate of 50 t/km −2/yr −1 for 100 mm of increasing annual rainfall, and the other for the Pacific coast areas — California and Chile — with a sediment peak at 400 mm and a faster decreasing rate of 100 t/km −2/yr −1 every 100 mm increase of rainfall. Non climatic factors, such as relief and human impact, may explain the higher rate of sediment yield for the Pacific coastal areas. The general trend of the “Langbein-Schumm curve”, determining an established correlation between mean annual precipitation and sediment yield, is valid for the analysed areas.

Runoff and Erosion Following Mechanical and Chemical Control of Creosotebush (Larrea tridentata)

Runoff and sediment yield were monitored from 1983 through 1986 on a range site dominated by creosotebush. The site was rootplowed and seeded, treated with tebuthiuron at 36 kg ai ha-1, or left untreated. Runoff from rootplowed and herbicide-treated plots was no different from untreated plots for 1 yr, but sediment yield from treated plots was lower than that from the untreated plots. Rootplowing and seeding increased sediment yield in the second year (1984), whereas treating with herbicide decreased sediment yield. Runoff and sediment yield during 1985 and 1986 were greatest from the untreated areas and least from the rootplowed and herbicide-treated areas. Total cover was nearly the same for each treatment. The untreated plots had 13% shrub cover; the others had none. Between shrubs in untreated plots, bare ground occupied large interconnected areas that contributed to the higher runoff and erosion rates.

Rainfall, floods and river channel changes in the upper Hunter

Since 1946, there has been a significant increase in annual rainfall in the Hunter Valley, especially for stations in the western sector. Contemporaneous increases in rainfall intensities of frequent storms and flood peaks have also been recorded. From analyses of the data, together with a review of recent environmental changes in the Hunter Valley, it is concluded that human disturbance of the catchment has played no significant part in the increase in flood magnitude. Instead, the change in floods may be completely attributed to the variation in rainfall regime. The altered hydrologic regime combined with a decrease in sediment yield from extra‐channel sources appears to be the primary cause of recent river channel changes.

Impact of climatic change on an arid watershed: Nahal Yael, southern Israel

The Nahal Yael basin is underlain chiefly by schist, amphibolite, and granite. Thin (generally <1 m thick), grussy colluvium which covered the lower portions of granitic hillslopes in the late Pleistocene has now been stripped completely, causing marked contrasts in outcrop morphologies, even where there is no contrast of fracture density or petrologic characteristics. Formerly mantled slopes are now smooth and crumbly, and lack desert varnish. Previously unmantled slopes are rough and craggy, and varnished but little weathered. Such stripping suggests a change from a semiarid to a drier and/or warmer climate. Slopes underlain by amphibolite responded similarly to the climatic change, but the amphibolite was more deeply weathered, and the colluvium was only partially stripped. The least stripping of colluvium occurred on schist hillslopes, partly because schist outcrops require more rain to generate runoff, and partly because angular blocks of schist require larger flows for transport, compared to other slope lithologies. The stream subsystem responded to the climatically induced changes in the discharge of water and sediment from the hillslopes. Increase in sediment yield caused valley alluviation in the early Holocene, and a decrease in sediment yield later in the Holocene caused entrenchment of the valley fill. More granite and amphibolite gravel-size particles are transported now than when the hillslopes were extensively mantled. Dense networks of trails are not common on Holocene geomorphic surfaces, but are present on remnants of Pleistocene surfaces.

Sedimentation history of the Mundi Mundi alluvial fans, Western New South Wales

The Mundi Mundi alluvial fans lie along the Mundi Mundi fault scarp in arid far western New South Wales. The fans consist of a number of well-defined stratigraphic units which can be traced both across the fans and down the fans. The fans are presently dissected and sedimentation is proceeding at intersection points on the larger fans, and most of the small fans are incised so that a net sediment loss is occurring. The oldest stratigraphic unit recognised is the Umberumberka unit which was deposited by bedload transportation in partly braided distributary channels and by slurry-flow deposition over temporarily abandoned channel deposits. The accumulation of the unit was a response to a relatively wetter climate at a time of depressed temperatures prior to 16,000 years B.P., and the end of accumulation was caused by increasing aridity. The Umberumberka unit is capped by a calcareous palaeosol, the Belmont Palaesol, which formed on the stable surfaces of the fans immediately after the peak of desiccation. The Korkora unit overlies the palaeosol and was deposited by shallow channels and slurries. The Mundi Mundi unit overlies the Korkora unit and was deposited in the same way as the Umberumberka unit. The Mundi Mundi unit was deposited between about 6000 and 3000 years B.P. as a response to an increase in rainfall in the Australian interior. The end of accumulation of the Mundi Mundi unit was followed by dissection of the fans, and this change in fan regimen was a result of climatic deterioration. A minor period of alluviation in the trenches cut into the fans (the Tackaringa unit) began about 1000 years B.P. as a result of a slight increase in moisture. The modern streams are dissecting the Thackaringa unit, but floods with a low recurrence interval continue to contribute sediment to the unit. Climatic fluctuations have been responsible for the timing of periods of aggradation and degradation in the alluvial fans, but the major stratigraphic units record a progressive decrease in the mud (< 63 μm) content of the sediments. This decrease may reflect a long-term decrease in sediment yield from the fan catchments. The effect of this decrease in sediment yield on the behaviour of the fans is not yet clear.