Annual Sediment Yield Research Articles

Sediment Yield Estimation and Identifying the Soil Erosion Prone Areas in Koysha Dam Watershed of Omo-Gibe Basin

Sedimentation is huge problems that have threatened many reservoirs in Ethiopia. Koysha dam watershed is conditioned densely populated with intensive traditional agricultural practice and low soil and water conservation practice in left side. The left side of watershed is gentle slope terrain topography. This study has been conducted to estimate mean annual sediment yield of watershed, to identify and prioritize the most sensitive sub-watersheds with the help of Arc SWAT 2012 for planning reservoir sedimentation mitigating strategies at the watershed level. Based on a digital elevation model the catchment was divided in to 23 sub-basins using the dam axis as the main outlet. The current LULC map was downloaded from satellite. The pre-processing and both unsupervised and supervised classification was conducted in ERDAS Imagine 2015. By overlaying land use, soil and slope maps, sub-watersheds were further divided in to 241 HRUs. Arc SWAT Model was calibrated and validated using SUFI-2 SWAT-CUP optimization algorithms for stream flow rate and sediment yield data observed at dam axis, which transposed from other gauging stations. The model performance was evaluated by using both stream flow and sediment yield data. The study has revealed that Koysha dam residual watershed has mean annual sediment yield of 7.22 t/ha/year. Out of the 23 sub-watersheds, seven sub-basins produce above average sediment yields ranging from 8.79 -56.70 t/ha/yr, while the others yield below the average value. Out of the 23 sub-basins two sub-basins were prioritized for immediate implementation of watershed management interventions. The maximum sediment outflow of these two sub-basin are 26.33 and 56.7 t/ha/year and are characterized dominantly by cultivated land with average land slope of 31.45% and 28.74% respectively and with the dominant soil type of Humic Nitisols. The soil erosion was sensitive to cultivation land use and terrain steepness. Keywords: Koysha dam watershed, Sediment yield, SWAT, Prioritization DOI: 10.7176/CER/14-2-01 Publication date: April 30 th 2022

Effects of a cascade reservoir system on runoff and sediment yields in a River Basin of southwestern China

In the upper reach of the Yangtze River Basin in China, changes of the runoff and sediment yield in the middle reach of the Jinsha River Basin have been given close attentions because of their declination trends over recent years. Then, the impact factors for the variations of runoff and sediment yield were urgent need to be investigated through a distributed hydrological modelling under multi spatiotemporal scales. The results showed that climate change had a significant impact on runoff and sediment yield of the basin before the operation of cascade reservoirs. The contribution rate of the reservoirs operation to the reduction of the annual runoff and sediment yield in the watershed was 48.90% and 28.75% respectively. The Ludila reservoir played an important role in reducing annual runoff and sediment yield of the basin. The location of reservoirs with the sequence of Ludila, Longkaikou, Jin'anqiao, and Ahai from upstream to downstream had the greatest effect on the annual sediment yield of the basin. The function of multiple reservoirs on the monthly runoff and sediment yield of the basin was not the superposition of a single one. Under the simultaneous change of temperature and precipitation, the precipitation had a greater impact on runoff and sediment yield of the basin than temperature. Spatially, the annual runoff and sediment yield in the middle and upper reaches of the basin were sensitive to changes of temperature and precipitation. This research provides scientific support for management of water and sediment resources in watersheds with cascade reservoirs.

Disproportionate and chronic sediment delivery from a fluvially controlled, deep‐seated landslide in Aotearoa New Zealand

AbstractPast research has highlighted the importance of sediment delivery from multiple‐occurrence regional landslide events triggered by storms or earthquakes. Herein, we examine delivery from a more persistent source of sediment, that of a large, slow‐moving landslide in the soft‐rock hill country of Aotearoa New Zealand. We map and monitor the 80‐ha Rangitikei Landslide from 2015 to 2019 using time‐lapse photography, ground surveys, photogrammetry, and piezometers. We show that the landslide can be divided into several zones with distinctive movement patterns, but all zones respond to river erosion. The fastest zone moves more than 10 m per year in a flow‐like fashion, while other zones move 0.01 m per year via slow sliding. Movement occurs all year round, but is two to three times faster in winter and spring. While rainfall and associated groundwater change are commonly attributed to landslide movement patterns, our data show that river flow correlates closely with the weekly to seasonal variability in movement of the landslide toe. This suggests that fluvial erosion can play an important role in the movement dynamics of highly coupled landslides. We estimate an annual sediment yield to the Rangitikei River of at least 40 000 tonnes, in this first quantification of sediment delivery from an active soft‐rock landslide in Aotearoa. This volume implies 7% of the total catchment suspended sediment yield is derived from 0.03% of the contributing catchment area, demonstrating the disproportionate effect of this (and likely other) deep‐seated landslide(s) as a source of sediment in the Rangitikei catchment. Sediment delivery is more continuous than the episodic supply of multiple‐occurrence regional landslide triggering events, and by delivering mostly fine‐grained sediment, it has a potentially large impact on water quality.

Impacts of mixed forests on controlling soil erosion in China

Soil erosion poses a major threat to the sustainability and stability of global terrestrial ecosystems. Mixed forests have more advantages in restoring degraded land and enhancing soil fertility than monocultures, but their influence on soil erosion remains uncertain. Therefore, we presented a meta-analysis consisting of 261 paired observations of mixed forests and the corresponding monocultures from 65 studies to evaluate the effect of tree diversity on soil erosion in China. The results showed that compared with monocultures, mixed forests significantly reduced the annual runoff and sediment yield by 17.4% and 23.6%, respectively. Tree-shrub mixtures showed more efficiency in reducing runoff (24.2%) and sediment yield (32.1%) than tree-tree mixtures. However, the positive effect of mixed forests on soil erosion control was significant > 20 years after stand establishment. Mixed forests showed the highest capacity to control soil erosion at slope angles of 16°-25° (runoff: 27.7%; sediment yield: 37.7%). In terms of soil textures, mixed forests performed better with respect to soil erosion control on sandy soils (runoff: 21.4%; sediment yield: 30.1%), whereas significant differences were not observed between different stand types on clayey soils. In addition, in the dry areas (humidity index (HI) < 30), mixed forests conferred the greatest benefits in reducing runoff and sediment yield (25.8% and 33.0%, respectively). Our results indicate that increasing tree species richness is a suitable approach to controlling soil erosion in China. This research provides a scientific basis for the management and sustainable development of forest ecosystems in the future.

Asynchronized erosion effects due to climate and human activities on the central Chinese Loess Plateau during the Anthropocene and its implications for future soil and water management

AbstractVariations and regional differences in land surface processes during the Anthropocene have directly impacted local economic development and social stability. Nevertheless, little is known about the historical erosion effects and human–environment interactions at annual and decadal scales on the central Chinese Loess Plateau (CLP) over the past 200 years, when human activities and annual sediment discharge both reached historical peaks. Located in the northcentral hilly–gully and southcentral plateau–gully regions, respectively, the Jingbian and Heshui landslide‐dammed reservoirs were ideal settings to reconstruct the erosion flux and precipitation over A.D. 1850–2000 due to vertical stacking of interannual couplets. During this period, the mean annual specific sediment yield reached 1.17 × 104 t/(km2 a) in the Jingbian catchment (precipitation threshold of 30 mm), while it reached 1.13 × 104 t/(km2 a) in the Heshui catchment (precipitation threshold of 40 mm) according to sediment yield with frustum and extrapolation of historical precipitation. Interannual El Niño–Southern Oscillation (ENSO) events were deemed a primary climatic factor inducing soil erosion over the central CLP. This should remind decision‐makers that it is necessary to prevent large‐scale floods and severe erosion before future ENSO events. Changes in population, policies, and human activities also affected land surface processes on the central CLP during 1850–2000. In addition, the results revealed that abandoned farmlands in the southcentral regions with 600 mm and more precipitation had a strong ecological resilience, while this was weak in the northcentral drier region. These findings imply that future ecological restoration of the central CLP should consider the ecological resilience of farmlands that have been recently abandoned due to urbanization. These novel results provide new insights into regional soil and water management under China's development agenda of ‘Ecological Conservation and High‐quality Development of the Yellow River Basin’.

IDENTIFYING EROSION HOT SPOT AREAS AND EVALUATION OF BEST MANAGEMENT PRACTICES IN THE TOBA WATERSHED, ETHIOPIA

Ethiopian highlands have been increasingly exposed to the risk of soil erosion and evaluations of how various management practices can reduce the risk of soil erosion are still limited. Here, the calibrated and validated Soil and Water Assessment Tool (SWAT) is used to estimate soil loss rates, identify erosion hotspot areas and evaluate effective best management practices (BMP) to curb the risk of soil erosion. The annual sediment yield (SY) in Toba watershed varies from 0.09 t ha-1 yr-1 to 44.8 t ha-1 yr-1 with an average SY of 22.7 t ha-1 yr-1. Cultivated lands on steep slopes are the sources of extensive soil loss rate, whereas areas with good vegetation cover have low SY. The increased population pressure, increased cultivation of steep slope and uncontrolled grazing are the causes of high SY in the watershed. 17 sub-basins with SY higher than the tolerable erosion of Ethiopia (2-18 t ha-1 yr-1) are considered for the application of BMP scenarios. Implementing all BMPs could reduce the extent of SY but with varying degrees and combination of the BMPs are more pronounced and desirable. Reforestation with vegetative strips was the most effective management (87.8% reduction) followed by soil/stone bund with vegetative strips (83.7% reduction). These findings are important to ensure sustainable land management and promote sustainable agricultural production in a rapidly changing agricultural watershed. In general, the result highlights the need for regional developments and cooperation to urge for strong BMPs strategies for the rapid land and water resources degradation.

Assessment of Spatial Soil Erosion Using RUSLE Model Integration with GIS and RS Tools: The Case of Gilgel Gibe-I Catchment, South West Ethiopia

Water-induced soil erosion is one of the serious environmental, agricultural, and socioeconomic problems in Ethiopian highlands. Accurate information on the rates of soil erosion helps environment protection and socio-economic development efforts of the nation. The objective of this research was to estimate annual soil loss, sediment yield, and map erosion risk areas of Gilgel Gibe-I (GG-I) catchment via integrating Revised Universal Soil Loss Equation (RULSE) model with Geographical Information System (GIS) and Remote Sensing (RS) technologies. The model inputs variables; rainfall erosivity (R), soil erodibility (K), topographic (LS), land cover (C) and land management (P) were derived from meteorological stations, Ethio-soil map, and satellite image of the catchment. The annual soil loss (t^-1ha^-1yr) was estimated using pixel-by-pixel ArcGIS map overlays to ensure the accuracy of RULSE output. The model output revealed on average 12.52 (t^-1ha^-1yr) soils was lost from GG-I catchment through sheet and rill erosion. The rates of soil loss were varying in the catchment, 59.8% of the catchment exposed to low rate (<5 t^-1ha^-1yr), 12.2% to moderate rate (5-12 t^-1ha^-1yr), 11.7% to high rate (12-30 t^-1ha^-1yr), and 6.6% to severe (>30 t^-1ha^-1yr). The annual sediment yield capacity of the catchment was 2.54 t^-1ha and delivery ration estimated 0.203% transported to outlet of the catchment-GGI hydropower dam. To combat the problems of GG-I hydropower dam siltation, land degradation, and low agricultural productivity an integrated natural resource management intervention is required throughout the catchment particularly in high and severe erosion risk areas.

Determination of Soil Erosion and Sediment Yield in the Bonsa River Basin Using GIS and Revised Universal Soil Loss Equation (RUSLE)

The Bonsa river is an important tributary of the Ankobra river in the Western Region of Ghana. The catchment of the Bonsa river has been undergoing rapid land cover changes due to human activities such as farming, illegal mining, population growth, among others which are likely to promote soil erosion and sediment yield in the river basin. To estimate the amount of soil eroded over a period and subsequent sediment yielded along the Bonsa river basin, the Revised Universal Soil Loss Equation (RUSLE) was integrated with Geographic Information System (GIS) to model the spatial distribution patterns in soil erosion and sediment yield within the catchment. Data used included annual rainfall records, soil map, Digital Elevation Model (DEM) and land-use map of the study area. Parameters of the model were determined and converted into raster layers using the raster calculator tool in ArcMap to produce a soil erosion map. The concept of Sediment Delivery Ratio (SDR) was applied to determine the annual sediment yield by combining a raster SDR layer with soil erosion map. The predicted soil loss and sediment yield values were found to be low. This may be due to high soil protective cover provided by vegetation as well as low topographic relief in the river basin. Though, the elements and processes responsible for soil erosion and sediment yield prevailing in the basin was found to be low, adverse situations could be developed with time if the prevailing conditions are not checked, as soil erosion is a natural gradual slow process. The gains made could be sustained by putting measures in place to control human activities, particularly, illegal mining (galamsey) in the basin, indiscriminate cutting down of trees and farmining activities along the Bansa river basin. This study will support monitoring, planning of water resources and help to improve sustainable water quality.

Application of SWAT, Random Forest and artificial neural network models for sediment yield estimation and prediction of gully erosion susceptible zones: study on Mayurakshi River Basin of Eastern India

The study aims to estimate annual sediment yields of Mayurakshi River Basin using SWAT tool of ArcGIS. Climatic aspects, soil, land use and slope maps were integrated within SWAT tool to predict sediment yield capacity using MUSLE method. Alongside, the study attempts to identify spatial distribution of major gully erosion prone sites using ANN and RF models. Twelve Gully Erosion Conditioning Factors namely, elevation, curvature, aspect, runoff, TWI, slope, geology, stream frequency, rainfall erosivity, NDVI, LS-factor and LULC were selected. A gully erosion inventory map was prepared using 128 gully sites and divided into training (70%) and testing (30%) classes. Results obtained from the study shows that sediment yield capacity is very high in north-western and south-western parts of the basin due to high probability of gully erosion. Both ANN (ROC 0.96, Kappa 0.92) and RF (ROC 0.97, Kappa 0.94) models have high prediction accuracy and strong interrater reliability with MUSLE (0.60 and 0.75).

Sediment Production in French Alpine Rivers

AbstractGlobal compilations of river sediment loads show that mountainous areas produce a high proportion of the sediment transported to the oceans. However, because of the effort involved in measuring sediment fluxes in mountain river systems, the loads of these rivers are generally unknown. Here, we present estimates of contemporary sediment loads of 16 gravel bed rivers draining the Ecrins‐Pelvoux Massif in southeast France. Sediment production in this part of the Alps is relatively high and many river segments are either wandering or braided. We model sediment fluxes and annual sediment yields by coupling hydraulic‐based relations for sediment transport with hydrologic‐based relations for flow frequency. Bed load fluxes are modeled for a range of discharges using a function that relates transport rates to excess shear stress. Fluxes are then weighted by the frequency of individual discharges and summed to get the annual bed load for each site. The suspended load is estimated empirically as a fraction of the bed load. Results suggest that bed load fluxes at channel‐forming flows scale almost linearly with downstream increases in discharge. In addition, it appears that annual sediment loads (bed load + suspended load) scale linearly with drainage area. A complementary relation for specific sediment yield suggests that the load per unit drainage area is constant across the range of basins studied. The modeled sediment yields are comparable to previous field‐based estimates of modern sediment yields, and generally lower than estimates developed from analyses of cosmogenic radionuclides.

Multi-site calibration of SWAT for the spatial distribution of sediment yield, Middle Awash Dam watershed, Ethiopia

Multi-site calibration of sediment yield has a significant effect on evaluating the spatial distribution of sediment yield of watershed and reservoirs sedimentation. Multi-site calibration was conducted at three gauging stations of the Proposed Middle Awash Dam watershed. The sequential Uncertainty fitting (SUFI-2) calibration uncertainty program (SWAT-CUP) has been used to calibrate and validate flow and sediment parameters using monthly flow and sediment rate observed data. P-factor and R-factor measured model calibration and uncertainty where the P-factor recorded 0.56-0.86/0.54-0.77 and R-factor 0.52-0.93/0.68-0.84 values respectively during the calibration and validation period for the three gauging stations. Model result showed the performance model was excellent during the calibration period with the coefficient of determination R2=0.78-0.82, Nash-Sutcliffe Efficiency ENS=0.77-0.79, Observation Standard Deviation Ratio RSR=0.44-0.48 and percent bias PBIAS=-13.3 to +14.3. Following the calibration process, the model estimated mean annual spatial distribution of sediment yield 7.23 ton/ha/yr at the outlet. Sediment yields spatial distribution showed that among the 19 sub-watersheds ranked based on their sediment yield contributions, eight sub-watersheds have a slope greater than 5%, which is relatively steeper and contributed average annual sediment yields of 16 ton/ha/yr. The temporal variability hydrograph showed 70.8 % of yearly sediment yield in the study area during the rainy season. The study results informed to development of watershed management strategies to minimize the sediment problems in the entire watershed. Keywords : Hydrological modelling, Middle Awash Dam, sediment yield, spatial distribution, SWAT model

Assessing The Impact Of Sediments On Best Management Practices (Bmps) For Highway Runoff Control At Wolaita Sodo Town

The goals of this work are to estimate the amount of erosion and to evaluate the impacts of sediments on Best Management Practices (BMPs) for highway runoff and also used to estimate the annual erosion and sediment yield from the construction sites considering various erosion control management methods that might be used in the sites. Once the sediment yield is found, a model was designed to estimate the sediment capture efficiency of different Best Management Practices (BMPs) and to evaluate the service year of each BMP before it is filled with sediment. The two major objectives of this project are: to predict sediment yield from highway construction sites under different erosion management conditions and secondly to estimate the service year of sediment control BMPs treating runoff from highway construction sites. The BMPs selected for evaluation were: detention ponds, infiltration trenches, grass lined swales, grass lined swales with rock check dams, and bio retention areas. Each respective BMP requires different measures of efficiency to accurately assess its effectiveness and service year.

Impacts of Management Scenarios on Sediment Yield Simulation in Upper and Middle Awash River Basin, Ethiopia

Sediment transport is a complex hydrological phenomenon that influences water resources, ecosystems, soil and water quality. The evaluation of sediment yield and mitigation practices are the mechanism to enhance the water resource potential. The study was aimed at simulating the sediment yield and the impact of three best management scenarios in the Upper and Middle Awash River Basin (UMARB). The Soil and Water Assessment Tool (ArcSWAT 2012) was used to simulate and identify high sediment yield prone areas in the catchment by the implementation of three management practices such as filter strips, terraces, and grassed waterways. The model results indicated a good agreement between observed and simulated streamflow and sediment yield in the baseline scenario. The mean annual sediment yield was 21.43 ton/ha/yr. The implementation of 5 and 10 m filter strips reduced the mean annual sediment yield by 31.68% and 46.6%, respectively. Terraces and grassed waterways were showed a reduction of sediment yield by 19.56% and 16.64%, respectively. Filter strips were found to provide the highest reduction of sediment yield in each sub-basin and the outlet of the catchment. Besides, the combination of the three management scenarios has reduced the values of sediment yield by 50.88% from the baseline, and the mean annual sediment yield becomes 10.53 ton/ha/yr. In these results, the efficiency of soil conservation practices would increase by applying more than two sediment management scenarios in the study area. The study delivered important information for the reduction of sediment yield and implementation of sustainable watershed management in the UMARB.

Watershed-based soil erosion and sediment yield modeling in the Rib watershed of the Upper Blue Nile Basin, Ethiopia

Soil erosion is a major environmental problem that degrades topsoil and agricultural land productivity in mountainous areas such as Ethiopia. Soil erosion estimation is critical for the planning of erosion control plans in an agriculture-dependent economy. The current study used the Revised Universal Soil Loss Equation (RUSLE) model to evaluate the spatial distribution of annual soil loss and sediment yield in the Rib watershed. The main factors affecting soil erosion and sediment yield include vegetation cover, topography, soil, and climate. Soil erosion and sediment yield maps should be produced with these elements in account to describe locations with high soil erosion and sediment yield risks and to ensure improved erosion mitigation strategies in watersheds. The five input parameters were rainfall erosivity (R), soil erodibility (K), slope length-steepness (LS), cover-management factor (C), and support practice (P). The RUSLE model, when combined with remote sensing technology, has the ability to provide accurate and low-cost erosion and sediment yield risk maps in the Rib watershed. The estimated average annual soil loss was 25.53 t ha−1 yr−1. The soil loss rate was higher in the steeper and topographically dissected part of the watershed. Sediment delivery ratio ranges between 0 and 0.82. The outcome undoubtedly aids in the identification of priority locations for the adoption of sustainable land management and soil erosion mitigation conservation strategies.

Response of sediment concentration and load to removal of juniper woodland and subsequent establishment of grasslands – A paired experimental watershed study

Encroachment of woody plants into grasslands is a global phenomenon of environmental concern. Mechanical removal is often necessary to re-establish herbaceous dominance for heavily encroached watersheds, but its impact on water quality and quantity of runoff into streams and reservoirs has not been vigorously studied. The sediment concentration and load following mechanical removal of juniper (Juniperus virginiana, L.) woodland and subsequent re-establishment of tallgrass prairie or switchgrass (Panicum virgatum L.) biomass production were quantified at the experimental watershed scale in the south-central Great Plains, USA. Impact analysis was used to evaluate the effects of watershed treatment and phase of land use conversion. The annual sediment yield from juniper woodland watersheds averaged<0.10 t ha−1 yr−1 before treatment and increased to 0.28 t ha−1 yr−1 after juniper were cut and left on site. In the second year, removing the dried trees increased the annual sediment yield to 1.14 t ha−1 yr−1 in the prairie restoration watershed and 13.29 t ha−1 yr−1 in the switchgrass watershed that was sprayed with herbicide in preparation for no-till planting. The annual sediment loads averaged 0.44 t ha−1 yr−1 from the restored prairie and 0.29 t ha−1 yr−1 from the established switchgrass, comparable to 0.73 t ha−1 yr−1 from the intact juniper woodland during the third and fourth years after initiation of treatments.While restored grassland watersheds had elevated peak flows and longer flow duration leading to greater runoff, lower sediment yields were due to reduced mean and peak sediment concentration compared to the juniper watershed.Therefore, restoring juniper woodland to native prairie or switchgrass biomass production systems may increase water yield without increasing sediment yield, especially in years with extreme storm events.

Evaluation of Ecosystem-Based Adaptation Measures for Sediment Yield in a Tropical Watershed in Thailand

Ecosystem-based adaptation (EbA) can potentially mitigate watershed degradation problems. In this study, various EbA measures were evaluated using a bio-physical model called the Soil and Water Assessment Tool (SWAT), in a small, forested watershed named Hui Ta Poe, in the northeastern region of Thailand. The developed watershed model was first used to investigate the effect of various degraded watersheds due to land-use changes on the sediment yield in the study area. The most degraded watershed produced an annual average sediment yield of 13.5 tons/ha. This degraded watershed was then used to evaluate the effectiveness of various EbA measures such as reforestation, contouring, filter strips, and grassed waterways in reducing the sediment yield. Under all individual and combined EbA scenarios analyzed, there was a significant reduction in sediment yield; however, the maximum reduction of 88% was achieved with a combined scenario of reforestation, grassed waterways, and filter strips. Reforestation alone was found to be the second-best option, which could reduce the sediment yield by 84%. Contouring alone was the least effective, with a reduction in sediment yield of only 23%. This study demonstrates the usefulness of implementing EbA measures for sediment management strategies to address watershed degradation, which is a severe problem across the globe.

Runoff and sediment yield in relation to precipitation, temperature and glaciers on the Tibetan Plateau

The riverine sediment is an essential carrier for nutrients and pollutants delivery and is considered as an important indicator of land degradation and environmental changes. With growing interest in environmental changes over the Tibetan Plateau, this study investigated mean annual runoff and sediment yield from eight headwater catchments in relation to annual precipitation, air temperature, and glacier area ratio, etc. Results show that runoff (Q) is positively correlated with both precipitation (P) and temperature (T), i.e., Q = 0.357P+20.3T-6.4, indicating combined water supply from rainfall and meltwater, increase of which may exceed the evapotranspiration water loss caused by temperature raise. Sediment yield (S) shows an inverted parabolic relationship with precipitation and at the same time positive correlation with glacier area ratio (Ag), i.e., S = 0.000609 P2-0.470P+48.5 Ag+202.53, indicating that sediment yield is a minimum at about 500–600 mm of precipitation, increasing sharply on both sides of this minimum in one case owing to decreased vegetation protection and in the other to enhanced erosive power and that erosion rate in the glacierized area is generally higher than non-glacierized area. The variation in sediment yield with precipitation can be explained by the operation of two factors, i.e., rainfall erosive action that increases continuously with increase in precipitation, and vegetation protective action that is unity for zero precipitation and decreases with increases in precipitation. The above results may be useful in visualizing not only variations in rates of erosion among climatic zones on the Tibetan Plateau but also the probable changes of erosion during a climatic change.

The fuzzy effect of soil conservation practices on runoff and sediment yield from agricultural lands at the catchment scale

Intensive soil degradation of agriculture lands during the past decades led local authorities in the Harod Catchment, northern Israel, to implement soil and water conservation practices. Herein, for the first time in Israel, we quantified the impact of these practices on water discharge, runoff/rainfall, and sediment yields at the catchment scale. We monitored two neighboring tributaries of the Harod River: Shkedim and Shunem. Both are dominated by intensive agriculture, are similar with respect to soils and rain patterns, but differ in terms of tillage and soil conservation practices - implemented in abundance in the Shkedim catchment. Inner-catchment processes were studied at the plot scale using a structure for motion photogrammetry, utilizing an unmanned aerial vehicle. At the catchment scale, we used airborne LIDAR to provide a better understanding of the extent to which different geomorphometric characteristics might influence flood discharge, suspended sediment concentration, and yield. We monitored the impact of temporal changes in vegetation cover with remote sensing. Intense storms occurring in early winter when the soil was bare and freshly tilled generated much runoff and sediment. During three field seasons, measured mean annual sediment yields were 820 and 2,000 t km−2 y-1 for Shkedim and Shunem, respectively. The benefits of soil conservation practices applied in the Shkedim catchment are manifested by lower runoff yields and peak discharges. Conservation practices together with milder topography were identified to limit flow continuity and to promote sinks, while in the Shunem, tillage parallel to contour lines induced higher hydro-geomorphic connectivity. Still, Shkedim soil loss is high due to cultivation of riparian zones, lack of maintenance of engineered structures, and a larger area of bare soil during winter due to rotational cropping. These blur the efficiency of soil conservation practices at the catchment scale.

Climate Change Impact on the Hydrologic Regimes and Sediment Yield of Pulangi River Basin (PRB) for Watershed Sustainability

The impacts of climate change are increasingly threatening the sustainability of ecosystems around the world. The Pulangi River Basin (PRB) in the Philippines is experiencing sedimentation beyond the tolerable amount (11.2 tons/ha/yr) due to land conversion and the effects of climate change. Changes in precipitation and temperature due to climate change are likely to further affect the annual runoff and sediment yield of PRB. In this study, the Soil and Water Assessment Tool (SWAT) was employed to simulate various scenarios of twelve downscaled climate projections from three Global Circulation Models (GCM) of CMIP5 under two Representative Concentration Pathways (RCP 4.5 and 6.0) for 2040–2069 and 2070–2099 timeframes, and the results were compared to a baseline period (1975–2005). This study revealed that the maximum mean annual precipitation is expected to increase by 39.10%, and the minimum and maximum temperatures are expected to increase by 3.04 °C and 3.83 °C, respectively. These observed changes correspond to an increase in runoff (44.58–76.80%) and sediment yield (1.33–26.28%) within the sub-basins. These findings suggest a general increase in the threat of severe flooding and excessive soil loss, leading to severe erosion and reservoir sedimentation throughout the PRB.

Effect of watershed disturbance and river-tributary confluences on watershed sedimentation dynamics in the Western Allegheny Plateau

Harmful algal blooms (HABs) have increased in the Ohio River Basin over the past decade. Fine sediment dynamics are now recognized to play an important role in the proliferation and toxicity of HABs. As a result, the fate of sediment at confluences of tributaries and regulated river systems may be important zones for sediment retention and burial. The objective of this study was to improve understanding of watershed sediment loading dynamics and backwater inundation impacts on sedimentation within confluence watersheds. The study site selected for this work was the Fourpole Creek watershed in the Western Allegheny Plateau ecoregion of Appalachia, which is a mixed-use watershed with predominantly forested and urban landcover with a backwater floodplain wetland at the confluence with the Ohio River. Approximately 16 months of high-frequency (15-minute) turbidity data was used as a surrogate for total suspended solids at upstream and downstream nodes of the backwater confluence feature. Continuous sediment yield estimates suggested an annual sediment yield of 82 t/km2 and was found to be higher than sediment yields reported elsewhere in Appalachian watersheds without mining disturbances. A modified sediment rating curve using both seasonal and annual regression approaches were found to underpredict sediment yield by 29% and 80%, respectively. A hysteresis-based source unmixing method and hysteresis index were used to quantify sediment source and flow pathways during events and suggested high variability in sediment source contributions both seasonally and within-seasons, suggesting three likely sediment sources in the watershed including in-channel/gully erosion, resuspension of recently deposited sediment in the fluvial network, and erosion of subsurface macropores from forested uplands. Upstream-downstream monitoring of the confluence floodplain suggested 34% of the annual sediment inputs were retained during the continuously monitored 2019 calendar year. Our results suggest that confluence floodplains play an important role in intercepting and storing sediment inputs. Efforts to limit downstream sediment loads should ensure the function of confluence floodplains is maintained or enhanced when considering watershed management strategies in these landscapes.