suspended sediment

Long‐term patterns and short‐term dynamics of stream solutes and suspended sediment in a rapidly weathering tropical watershed

Shanley, J. B., W. H. McDowell, and R. F. Stallard (2011), Long‐term patterns and short‐term dynamics of stream
solutes and suspended sediment in a rapidly weathering tropical watershed, Water Resour. Res., 47, W07515,
doi:10.1029/2010WR009788

Abstract: 
The 326 ha Río Icacos watershed in the tropical wet forest of the Luquillo Mountains, northeastern Puerto Rico, is underlain by granodiorite bedrock with weathering rates among the highest in the world. We pooled stream chemistry and total suspended sediment (TSS) data sets from three discrete periods: 1983–1987, 1991–1997, and 2000–2008. During this period three major hurricanes crossed the site: Hugo in 1989, Hortense in 1996, and Georges in 1998. Stream chemistry reflects sea salt inputs (Na, Cl, and SO4), and high weathering rates of the granodiorite (Ca, Mg, Si, and alkalinity). During rainfall, stream composition shifts toward that of precipitation, diluting 90% or more in the largest storms, but maintains a biogeochemical watershed signal marked by elevated K and dissolved organic carbon (DOC) concentration. DOC exhibits an unusual “boomerang” pattern, initially increasing with flow but then decreasing at the highest flows as it becomes depleted and/or vigorous overland flow minimizes contact with watershed surfaces. TSS increased markedly with discharge (power function slope 1.54), reflecting the erosive power of large storms in a landslide‐prone landscape. The relations of TSS and most solute concentrations with stream discharge were stable through time, suggesting minimal long‐term effects from repeated hurricane disturbance. Nitrate concentration, however, increased about threefold in response to hurricanes then returned to baseline over several years following a pseudo first‐order decay pattern. The combined data sets provide insight about important hydrologic pathways, a long‐term perspective to assess response to hurricanes, and a framework to evaluate future climate change in tropical ecosystems.

Water, Sediment, and Nutrient Discharge Characteristics of Rivers in Puerto Rico, andtheir Potential Influence on Coral Reefs

Warne, A.G., Webb, R.M.T., and Larsen, M.C., 2005, Water, Sediment, and Nutrient Discharge Characteristics of Rivers in Puerto Rico, and their Potential Influence on Coral Reefs: U.S. Geological Survey Scientific Investigations Report
2005-5206, 58 p.

Abstract: 
Data from 29 streamflow-gaging stations, including 9 stations with daily suspended-sediment concentration, and data from 24 water-quality stations were compiled and analyzed to investigate the potential effects of river sediment and nutrient discharges on the coral reefs of Puerto Rico. The largely mountainous watersheds of the 8,711-square-kilometer island of Puerto Rico are small, channel gradients are steep, stream valleys tend to be well-incised and narrow, and major storms tend to be intense but brief; hence flooding is rapid with peak discharges several orders of magnitude above base discharge, and flood waters recede quickly. Storm runoff transports a substantial part of fluvial suspended sediment from uplands to the coast, as indicated by sediment data from a set of nine streamflow-gaging stations representative of runoff from watersheds considered typical of conditions in Puerto Rico. For example, the highest recorded daily sediment discharge is 1 to 3.6 times the annual suspended-sediment discharge, and runoff from major storms induces sediment transport 1 to 32 times the median annual sediment load. Precipitation associated with Hurricane Georges in September 1998 is estimated to have averaged 300 millimeters across the island, which is equivalent to a volume of about 2.6 billion cubic meters. Analysis of runoff and sediment yield from Hurricane Georges indicates that more than 1.0 billion cubic meters of water and at least 2.4 million metric tonnes of sediment (and as much as 5 to 10 million metric tonnes), were discharged to the coast and shelf as a result of this major storm. Because of their relatively small size, dams and reservoirs of Puerto Rico have relatively little effect on total discharge of water and sediment to the coastal marine waters during major storms. The presence of reservoirs, however, may be detrimental to coral reefs for two reasons: (1) coarse sediments deposited in the reservoir can be replaced by finer sediments scoured, if available, from the river channels and flood plains below the dam; and (2) the loads of phosphorus and ammonia reaching the coastal waters may increase as organic matter decomposes in the anoxic bottom waters of the reservoir. Rainfall, water discharge, sediment discharge, and sediment yield vary across the island. Mean annual runoff for the island is estimated to be 910 millimeters, about 57 percent of mean annual precipitation (1,600 millimeters). Mean annual suspended-sediment discharge from Puerto Rico into surrounding coastal waters is estimated to range from 2.7 to 9.0 million metric tonnes. Hydrologic and sediment data associated with Hurricane Georges indicate that sediment yield is generally proportional to the depth of storm runoff. Discharge and sediment-concentration data indicate that during this storm, river water and sediment that discharged into the marine environment generally formed hypopycnal plumes (buoyant suspension layers). Generally, hyperpycnal (density) plumes can develop in areas with high discharges and sediment concentrations. Both hypopycnal and hyperpycnal plumes distribute suspended sediment over broad areas of the Puerto Rico shelf and shelf slope. Comparison of long-term suspended-sediment discharge and watershed characteristics for Puerto Rico with those of other river systems around the world indicates that Puerto Rico rivers are similar to temperate and tropical upland river systems.

CONTINUOUS AUTOMATED SENSING OF STREAMFLOW DENSITY AS A SURROGATE FOR SUSPENDED-SEDIMENT CONCENTRATION SAMPLING

Larsen, M. C., Alamo, C. F., Gray, J. R., and Fletcher, W. (2001).
„Continuous automated sensing of streamflow density as a
surrogate for suspended-sediment concentration sampling.‰
Proceedings of the 7th Federal Interagency Sedimentation
Conference, March 25–29, 2001, Reno, Nevada, Vol. I, pp. III-
102–III-109.

Abstract: 
A newly refined technique for continuously and automatically sensing the density of a water-sediment mixture is being tested at a U.S. Geological Survey streamflow-gaging station in Puerto Rico. Originally developed to measure crude oil density, the double bubbler instrument measures fluid density by means of pressure transducers at two elevations in a vertical water column. By subtracting the density of water from the value measured for the density of the water-sediment mixture, the concentration of suspended sediment can be estimated. Preliminary tests of the double bubbler instrument show promise but are not yet conclusive.

The Effect of Land Use on Soil Erosion in the Guadiana Watershed in Puerto Rico

LÓPEZ, TANIA DEL MAR; AIDE, T. MITCHELL; SCATENA F. N. 1998. The Effect of Land Use on Soil Erosion in the Guadiana Watershed in Puerto Rico. Caribbean Journal of Science, Vol. 34, No. 3-4, 298-307, 1998.

Abstract: 
The Revised Universal Soil Loss Equation (RUSLE) was used in conjunction with a Geographic Information System to determine the influence of land use and other environmental factors on soil erosion in the Guadiana watershed in Puerto Rico. Mean annual erosion, suspended sediment discharge, and the rainfall-erosion factor of the RUSLE increased with annual rainfall. Median soil erosion rates varied among the seven land uses: bare soil (534 Mg ha-1 yr-1), open canopy forest (26 Mg ha-1 yr-1), agriculture (22 Mgha-1 yr -1), pasture (17 Mg ha -1 yr -1), less dense urban (15 Mg ha-1 yr -1), closed canopy forest (7 Mg ha -1 yr -1), and dense urban (1 Mg ha-1 yr -1). The differences between open canopy forest, agriculture, pasture, and less dense urban were not significantly different but median values for open canopy forests were slightly greater because they occurred on steep slopes. The five-year average sediment delivery ratio for the basin was 0.17, which is comparable to delivery ratios estimated for watersheds of similar size. Simulations of different land use configurations indicate that reforestation of 5% of the watershed with the highest erosion rates would decrease basin wide erosion by 20%. If the entire watershed was reforested, soil erosion would be reduced by 37%.

Export of Carbon, Nitrogen, and Major Ions from Three Tropical Montane Watersheds

Export of Carbon, Nitrogen, and Major Ions from Three Tropical Montane Watersheds
William H. McDowell and Clyde E. Asbury
Limnology and Oceanography
Vol. 39, No. 1 (Jan., 1994), pp. 111-125

Abstract: 
Annual exports of suspended sediment, dissolved and particulate C and N, dissolved N and P, and major cations and anions were measured in three montane tropical rain forest watersheds in Puerto Rico during 1983-1986. Organic C was primarily exported in the form of DOC, and DOC export (33-94 kg ha-1 yr-1) was similar to values in larger tropical watersheds with similar runoff. Particulate and dissolved organic N accounted for 60-70% of the 4-9 kg ha-1 yr-1 of total N exported. Export of base cations and rates of weathering varied with bedrock geology in the three watersheds. Concentrations of suspended sediment, particulate C and N, and DOC increased as a function of discharge in all three streams. NH4+, NO3-, SO42-, and K+ concentrations showed little or no response to variations in discharge; Na-, Ca2+, Mg2+, Cl-, HCO3-, and SiO2 all decreased at high flows. After removing the effects of discharge, residual NO, concentrations in each stream were inversely related to estimated rates of leaf fall. On a watershed basis, export and accumulation of N in biomass were greater than precipitation inputs, suggesting that unmeasured inputs (8-16 kg ha-1 yr-1) were large.

ESTIMATING THE HUMAN IMPACT ON SUSPENDED SEDIMENT LOADS IN MOUNTAIN STREAMS OF EASTERN PUERTO RICO

Abstract: 
Worldwide, suspended sediment flux entering the oceans each year is increasing due to human activities. Land development in Puerto Rico is causing the island’s mean annual suspended sediment flux to increase. These high suspended sediment yields are unfavorable for human recreation, water treatment, as well as habitat protection. This study attempts to determine if land development in eastern Puerto Rico is significantly impacting suspended sediment (SS) levels. In eastern Puerto Rico lies the Luquillo Experimental Forest (LEF), which has been protected from land development since the 1940s. With 75 percent of the Island’s virgin forests, seven streams (Quebrada 1, Quebrada 2, Quebrada 3, Rio Mameyes Puente Roto, Quebrada Sonadora, Quebrada Guaba, and Rio Icacos) were chosen to represent “natural” suspended sediment conditions. Examining the relationships between drainage area, discharge, suspended sediment concentrations, slope and suspended sediment loads, these seven streams were compared to one stream outside of the national forest, Rio Mameyes Rt. 3, whose watershed is partially used for agriculture and urban purposes. Results indicate that humans are not significantly impacting suspended sediment levels in the Rio Mameyes Rt. 3. Because land development in the Rio Mameyes Rt. 3 watershed is relatively small, less than 20 percent, it is feasible that the agriculture and urban development are not drastically affecting SS levels or the sampling period did not capture the full range of variability. A principal component analysis of drainage area, suspended sediment load, discharge, slope and temperature of the eight streams shows that only one factor, which contains all components, can be extracted.
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