Variability of DOC and nitrate responses to storms in a small Mediterranean forested catchment

Bernal, S., A. Butturini, and F. Sabater (2002), Variability of DOC and
nitrate responses to storms in a small Mediterranean forested catchment,
Hydrol. Earth Syst. Sci., 6, 1031– 1041.

Severe drought periods followed by intense rainfall often leads to major floods in Mediterranean catchments. The resulting hydrology is complex and the response of solutes in the streams is often unpredictable. This study aimed to identify the most relevant factors controlling the hydrological responses to storms of an intermittent Mediterranean stream and to link those factors with dissolved organic carbon (DOC) and nitrate during storm events. Measurements of climate, hydrology, DOC and nitrate concentrations during 26 storm events over three hydrological years were analysed. The contribution of the storm events to the total DOC and nitrate annual export was also calculated. Nitrate was mainly mobilised during high flow, while most of the DOC export occurred during baseflow. Solute concentrations peaked after drought periods and the solute export was maximal during the largest rainfalls (i.e.>100 L m-2). One single large storm contributed some 22% of the total annual export of DOC, and about 80% of that of nitrate. Discharge was a good predictor of neither DOC nor nitrate responses, so variables other than discharge were considered. Factor Analysis was used to identify the main factors controlling the biogeochemical responses. Antecedent moisture conditions and the magnitude of the storm event were the most relevant factors and accounted for 63% of the total variance. Solute responses during high flow were highly variable. However, solute concentration changes showed a significant and moderate relationship with the factors controlling the hydrological responses (i.e. Δ DOC v. the antecedent moisture conditions and Δ NO3-N v. the magnitude of the storm event).

Hydrologic flowpaths influence inorganic and organic nutrient leaching in a forest soil

Asano, Y., Compton, J. E. & Church, R. M. Hydrologic flowpaths influence
inorganic and organic nutrient leaching in a forest soil. Biogeochem. 81,
191-204 (2006).

Hydrologic pathways through soil affect element leaching by determining the relative importance of biogeochemical processes such as sorption and decomposition. We used stable hydrogen isotopes of water (dD) to examine the influence of flowpaths on soil solution chemistry in a mature spruce–hemlock forest in coastal Oregon, USA. Soil solutions (50 cm depth, n = 13) were collected monthly for 1 year and analyzed for dD, major ions and dissolved organic carbon (DOC) and nitrogen (DON). We propose that the variability of dD can be used as an index of flowpath length and contact time. Throughfall variability in dD was much greater than soil solution variability, illustrating that soil solution integrates the variation in inputs. Lysimeters with greater variation in dD presumably have a greater proportion of flow through rapid flowpaths such as macropores. The variation in soil solution dD for individual lysimeters explained up to 53% of the variation in soil solution chemistry, and suggests that flowpaths influence leaching of some constituents. Soil solutions from lysimeters with greater dD variation had higher DOC and DON (r2 = 0.51 and 0.37, respectively), perhaps because transport via macropores reduces interaction of DOM with the soil matrix. In contrast, nitrate concentrations were highest in lysimeters with a small variation in dD, where long contact time and low DOC concentrations may yield higher net nitrification. Our results demonstrate the utility of stable isotopes to link flowpaths and soil solution chemistry, and illustrate how the spatial complexity of soils can influence ecosystem- level nutrient losses.

Landslides Triggered by Hurricane Hugo in Eastern Puerto Rico, September 1989

Larsen, M. C., and Torres-Sánchez, A. J., 1992, Landslides triggered by Hurricane Hugo in eastern Puerto Rico, September 1989: Caribbean Journal of Science, vol. 28, no. 3-4, p. 113-125.

On the morning of September 18, 1989, a category-four hurricane struck eastern Puerto Rico with a sustained wind speed in excess of 46 m/s. The 24-h rainfall accumulation from the hurricane ranged from 100 to 339 mm. Average rainfall intensities ranging from 34 to 39 mm/h were calculated for 4 and 6 h periods, respectively, at a rain gage equipped with satellite telemetry, and at an observer station. The hurricane rainfall triggered more than 400 landslides in the steeply sloping, highly dissected mountains of eastern Puerto Rico. Of these landslides, 285 were mapped from aerial photography which covered 6474 ha. Many of the mapped landslides were on northeast- and northwest-facing slopes at the eastern terminus of the mountains, nearest the hurricane path. The surface area of individual landslides ranged from 18 m2 to 4500 m2, with a median size of 148 m2. The 285 landslides disturbed 0.11% of the land surface in the area covered by aerial photographs. An approximate denudation rate of 164 mm/1000 y was calculated from the volume of material eroded by landsliding and the 10-y rainfall recurrence interval.

A rainfall intensity duration threshold for landlisdes in a humid-tropical environment, puerto rico

A Rainfall Intensity-Duration Threshold for Landslides in a Humid-Tropical Environment, Puerto Rico
Matthew C. Larsen and Andrew Simon
Geografiska Annaler. Series A, Physical Geography
Vol. 75, No. 1/2 (1993), pp. 13-23

Landslides are triggered by factors such as heavy rainfall, seismic activity, and construction on hill-slopes. The leading cause of landslides in Puerto Rico is intense and/or prolonged rainfall. A rainfall threshold for rainfall-triggered landsliding is delimited by 256 storms that occurred between 1959 and 1991 in the central mountains of Puerto Rico, where mean annual rainfall is close to or in excess of 2,000 mm. Forty one of the 256 storms produced intense and/or prolonged rainfall that resulted in tens to hundreds of landslides. A threshold fitted to the lower boundary of the field defined by landslide-triggering storms is expressed as I = 91.46 D-0.82 where I is rainfall intensity in millimeters per hour, and D is duration in hours. Landslide-producing storms occurred at an average rate of 1.2 per year. In general the landslides triggered by short-duration, high-intensity rainfall events were mainly shallow soil slips and debris flows, while the long-duration, low-intensity rainfall produced larger, deeper debris avalanches and slumps. For storms that had durations of up to 10 h, landsliding did not occur until rainfall intensity was as much as three times as high as the rainfall intensity reported as sufficient to trigger landsliding in temperate regions. As storm durations approach 100 h, the rainfall conditions necessary to initiate landsliding in Puerto Rico converge with those defined for temperate regions. A comparison of the Puerto Rico threshold with rainfall data from other humid-tropical regions suggests that the threshold developed for Puerto Rico may be applicable to other similar environments throughout the world.



Accurate assessment of water budgets is critical for effective management of water resources, especially on small, densely-populated islands with extremely limited storage capacity such as Puerto Rico. A water budget defines a balance between inputs, outputs, and storage. The water budgets described herein provide a generalized summary of the inputs, extractions, and outputs from four watersheds in and near the Luquillo mountains using rainfall, runoff, and public-supply extraction data as well as estimates of groundwater losses and inputs such as cloud drip and infiltration from septic tanks. Mean annual rainfall accumulation during a 7-year study (1991 to 1997) ranged from 1,722 mm in the Canóvanas watershed, to 4,235 mm in the Icacos and Mameyes watersheds; the Cayaguás watershed had 2,172 mm. Combined runoff, groundwater flow and withdrawals ranged from 47 to 73 percent of inputs (combined rainfall, cloud drip and septic tank infiltration). Evapotranspiration, calculated as the water budget residual, amounted to 27, 40, 44, and 53 percent of total moisture inputs in the Icacos, Cayaguás, Mameyes, and Canóvanas watersheds, respectively.

Evaluation of temporal and spatial factors that control the susceptibility to rainfall-triggered landslides

Larsen, M.C., 2001, Evaluation of temporal and spatial factors that control the susceptibility to rainfall-triggered landslides, in Gruntfest, E., and Handmer, J., eds., Coping with Flash floods: Kluwer Academic Publishers, p. 277-288.

Research plan for the investigation of water, energy, and biogeochemical budgets in the Luquillo mountains, Puerto Rico

Larsen, M.C., Collar, P.D., and Stallard, R.F., 1993, Research plan for the investigation of water, energy, and biogeochemical budgets in the Luquillo mountains, Puerto Rico: U.S. Geological Survey Open-file Report 92-150, 19 p.

The Luquillo mountains of eastern Puerto Rico are the site of U. S. Geological Survey (USGS) research into biogeochemical and geomorphic processes that control the movement and transformation of water, energy, bedrock weathering products, and nutrients in the earth-surface environment. This study was begun in 1990 and is scheduled to last three years, with the possibility of being extended for further data collection. The study area for this research effort includes the 113 square kilometers Luquillo Experimental Forest (LEF) that is administered by the U. S. Forest Service. The LEF has been the site of ongoing research since 1988 as part of the National Science Foundation's Long Term Ecological Research program. In addition, comparative studies are being conducted in the Río Grande de Loíza basin (Loíza basin), an urban and agriculturally developed 600 square kilometers watershed located immediately to the west of the LEF. The principal elements of the study described in the report are as follows: Determination of biogeochemical budgets: water, energy, carbon, nutrient, ion, sediment, and gas budgets will be calculated in two LEF watersheds instrumented with meteorologic, soil, hydrologic, and ground-water monitoring equipment. A biweekly time series of samples is being collected. In addition, intensive sampling is undertaken during selected storms. Study of weathering, erosion, and mass-wasting processes in undeveloped watersheds of contrasting lithology: chemical-weathering, erosion, and mass-wasting processes in watersheds underlain by the two dominant rock types, volcaniclastic and quartz diorite, are being compared. The effects of mass wasting on biogeochemical cycling in each rock type will be evaluated through a compilation of physical, chemical, and mineralogic properties for a chronosequence of landslides. Water and sediment budgets will be used to develop a conceptual model of hillslope hydrology and landform evolution. Comparison of weathering and gas flux in developed and forested watersheds: paired basins were selected and gaged in the relatively undisturbed LEF and in the agriculturally developed Loíza basin. Budgets of all aqueous constituents will be compared and contrasted in the developed and forested basins of similar lithology. Gas-flux differences (carbon dioxide, nitrogen dioxide, methane) between developed and undeveloped areas will be evaluated using chamber techniques and the results related to land-use differences. Measurement of reservoir and agricultural pond gas fluxes: methane production is being measured in selected reservoirs and agricultural ponds in and near the Loíza basin and LEF. A regional methane budget will be calculated.


Larsen, M.C., Wieczorek, G.F., Eaton, L.S., Morgan, B.A., Torres-Sierra, H., 2001,
Natural Hazards on Alluvial Fans: the Venezuela debris-flow and flash-flood disaster: U.S. Geological Survey Fact Sheet, FS 103-01, 4 p.

In December 1999, rainstorms induced thousands of landslides along the Cordillera de la Costa, Vargas, northern Venezuela. Rainfall on December 2-3 totaled 200 millimeters (8 inches) and was followed by a major storm (911 millimeters, or 36 inches) on December 14 through 16. Debris flows and flash floods on alluvial fans inundated coastal communities, caused severe property destruction, and resulted in a death toll estimated at 19,000 people. Because most of the coastal zone in Vargas consists of steep mountain fronts that rise abruptly from the Caribbean Sea, the alluvial fans are the only areas where slopes are not too steep to build. Rebuilding and reoccupation of these areas requires careful determination of potential hazard zones to avoid future loss of life and property.


Larsen M.C., Wieczorek G.F., Eaton L.S., Torres-Sierra H. (2001) – The rainfall-triggered landslide and flash-flood disaster in northern Venezuela, December 1999. Proceedings of the Seventh Federal Interagency Sedimentation Conference, Reno, NV, IV, 9-16.

A combination of climatologic, geologic, and demographic factors makes the Caribbean coast of Venezuela in the state of Vargas highly susceptible to episodic debris flows and flash floods. An extremely steep, tectonically active mountain front forms the boundary with a tropical sea. Easterly tradewinds can force moist air masses upslope and precipitate large rainfall volumes, creating conditions for high-magnitude debris flows and flash floods. The population of several hundred thousand people that reside at the base of the mountains is inevitably vulnerable to hydrologic disasters that seem to recur once or twice per century. The flash flood-debris flow process combination is highly destructive in populated areas. Without careful planning of human settlements, the impacts of these types of disasters are likely to increase in the future.

Venezuela debris-flow and flash-flood disaster of 1999 studied

Larsen, M.C., Wieczorek, G.F., Eaton, L.S., Morgan, B.A., Torres-Sierra, H., 2001, Venezuela debris-flow and flash-flood disaster of 1999 studied: EOS, Transactions: American Geophysical Union, v. 82, no. 47, p. 572-573.

Alluvial fans in urban and rural areas are sites of episodic, rainfall-induced natural hazards [Garner, 1959; Campbell, 1975; Wieczorek et al., 2001;]. Debris flows, hyper-concentrated flows, and flash floods that occur episodically in these alluvial fan environments place many communities at high risk during intense and prolonged rainfall. Although scientists have become better able to define areas of high natural hazard, population expansion and development pressures in such areas have put more people at risk than ever before. Recognition of the magnitude and distribution of debris -flow and flash-flood hazards is therefore a critically important area of natural hazard research. In December 1999, rainstorms induced thousands of landslides in such an area along the Cordillera de la Costa, Vargas, located north of Caracas, Venezuela; an area of dense human settlement located at the base of steep mountains. Flash floods and debris flows caused severe property destruction on alluvial fans at the mouths of the coastal mountain drainage network. Rainfall accumulation on December 2 and 3 totaled 293 mm and was followed by a major storm that dropped 911 mm of rain from December 14 through 16. More than 8000 individual residences and 700 apartment buildings were destroyed or damaged and roads, telephone, electricity, water, and sewage systems were severely disrupted [Salcedo, 2000]. Total economic losses are estimated at US$1.79 billion [Salcedo, 2000]. The debris flows and floods inundated coastal communities and resulted in a catastrophic death toll of as many as 19,000 people [USAID, 2000]. The landslides and flash floods also changed hill slopes, stream channels, and alluvial fan morphology. The alluvial fans along this Caribbean coastline are dynamic zones of high geomorphic activity. Because most of the coastal zone in Vargas consists of steep mountain fronts that rise abruptly from the Caribbean Sea—rising to elevations in excess of 2,600 m--the alluvial fans provide practically the only flat areas upon which to build. Rebuilding and reoccupation of these areas requires careful determination of hazard zones to avoid future loss of life and property.
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