Relative scales of time and effectiveness of watershed processes in a tropical montane rain forest of puerto rico

Scatena FN. 1995. Relative scales of time and effectiveness of watershed
processes in a tropical montane rain forest of Puerto Rico. Pages 103–111
in Costa JE, Miller AJ, Potter KW, Wilcock PR, eds. Natural and Anthropogenic
Influences in Fluvial Geomorphology.Washington (DC):
American Geophysical Union. Geophysical Monograph 89.

Streams of the Montane Humid Tropics

Tropical montane streams produce a disproportionately large amount of the sediment and carbon that reaches coastal regions and have often been considered to be distinct fluvial systems. They typically drain orogenic terrains that have not been recently glaciated, but have undergone climatic changes throughout the Pleistocene and currently receive 2000–3000 mm or more of precipitation each year. Steep gradient reaches with numerous boulders, rapids, and waterfalls that alternate with lower gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium characterize these streams. Although their morphology and hydrology have distinctive characteristics, they do not appear to have diagnostic landforms that can be solely attributed to their low-latitude locations. While they are relatively understudied, an emerging view is that their distinctiveness results from a combination of high rates of chemical and physical weathering and a high frequency of significant geomorphic events rather than the absolute magnitudes of individual floods or other geomorphic processes. Their bedrock reaches and abundance of large and relatively immobile boulders combined with their ability to transport finer-grained sediment also suggest that the restorative processes in these systems may be less responsive than in other fluvial systems.


Multivariate analysis of water quality and physical
characteristics of selected watersheds in Puerto Rico.
Journal of the American Water Resources Association 39:

Multivariate analyses were used to develop equations that could predict certain water quality (WQ) conditions for unmonitored watersheds in Puerto Rico based on their physical characteristics. Long term WQ data were used to represent the WQ of 15 watersheds in Puerto Rico. A factor analysis (FA) was performed to reduce the number of chemical constituents. Cluster analysis (CA) was used to group watersheds with similar WQ characteristics. Finally, a discriminant analysis (DA) was performed to relate the WQ clusters to different physical parameters and generate predicting equations. The FA identified six factors (77 percent of variation explained): nutrients, dissolved ions, sodium and chloride, silicacious geology, redox conditions, and discharge. From the FA, specific conductance, sodium, phosphorous, silica, and dissolved oxygen were selected to represent the WQ characteristics in the CA. The CA determined five groups of watersheds (forested, urban polluted, mixed urban/rural, forested plutonic, and limestone) with similar WQ properties. From the five WQ clusters, two categories can be observed: forested and urban watersheds. The DA found that changes in forest cover, percent of limestone, mean annual rainfall, and watershed shape factor were the most important physical features affecting the WQ of watersheds in Puerto Rico.



To evaluate the current status of instream flow practices in streams that drain into the Caribbean Basin, a voluntary survey of practising water resource managers was conducted. Responses were received from 70% of the potential continental countries, 100% of the islands in the Greater Antilles, and 56% of all the Caribbean island nations. Respondents identified ‘effluent discharges’, ‘downstream water quality’ and ‘existing extraction permits’ to be the most common sources of instream flow conflicts. In 75% of the countries, some type of ‘formal procedures’ exist for reviewing permit applications for freshwater extraction. In 82% of the countries, effluent discharge permits state the amount of effluent that can be discharged into a water body while only 69% require that surface water extraction permits explicitly state the quantity of water that can be extracted. In setting instream flow requirements, record low flow is used over 83% of the time. Freshwater fish were identified as the most important aquatic organism but no country ‘always’ considers the ecology or habitat requirements of aquatic species in their instream flow determinations and nearly 70% of the respondents indicated that multivariate, ecological-based methods are ‘never’ used in their country. Survey responses also indicate there is a notable lack of public involvement during the issuing of water permits. Moreover, over 80% of the countries do not provide public announcements or hearings during the permit process. In summary, this survey indicates that while there is a widespread recognition of the need for instream flows, there is a general lack of regionally based information and public involvement regarding stream flow determination.

Lithological and fluvial controls on the geomorphology of tropical montane stream channels in Puerto Rico

Pike, Andrew S.; Scatena, F.N.; Wohl, Ellen E. 2010. Lithological and fluvial controls on the geomorphology of tropical montane stream channels in Puerto Rico. Earth Surface Processes and Landforms. DOI: 10.1002/esp.1978.

An extensive survey and topographic analysis of fi ve watersheds draining the Luquillo Mountains in north-eastern Puerto Rico was conducted to decouple the relative infl uences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels. The Luquillo Mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic infl uence on the stream channels. However, the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment. GIS-based topographic analysis was used to examine channel profi les, and survey data were used to analyze downstream changes in channel geometry, grain sizes, stream power, and shear stresses. Results indicate that the longitudinal profi les are generally well graded but have concavities that refl ect the infl uence of multiple rock types and colluvial-alluvial transitions. Non-fl uvial processes, such as landslides, deliver coarse boulder-sized sediment to the channels and may locally determine channel gradient and geometry. Median grain size is strongly related to drainage area and slope, and coarsens in the headwaters before fi ning in the downstream reaches; a pattern associated with a mid-basin transition between colluvial and fluvial processes. Downstream hydraulic geometry relationships between discharge, width and velocity (although not depth) are well developed for all watersheds. Stream power displays a mid-basin maximum in all basins, although the ratio of stream power to coarse grain size (indicative of hydraulic forcing) increases downstream. Excess dimensionless shear stress at bankfull fl ow wavers around the threshold for sediment mobility of the median grain size, and does not vary systematically with bankfull discharge; a common characteristic in self-forming ‘threshold’ alluvial channels. The results suggest that although there is apparent bedrock and lithologic control on local reach-scale channel morphology, strong fluvial forces acting over time have been suffi cient to override boundary resistance and give rise to systematic basin-scale patterns.

Riparian indicators of flow frequency in a tropical montane stream network

Pike, A. S., and Scatena, F. N., 2010, Riparian indicators of flow frequency in a tropical montane stream
network: Journal of Hydrology, v. 382, p. 72–87, doi:10.1016/j.jhydrol.2009.12.019.

Many field indicators have been used to approximate the magnitude and frequency of flows in a variety of streams and rivers, yet due to a scarcity of long-term flow records in tropical mountain streams, little to no work has been done to establish such relationships between field features and the flow regime in these environments. Furthermore, the transition between the active channel of a river and the adjacent flood zone (i.e. bankfull) is an important geomorphologic and ecological boundary, but is rarely identifiable in steep mountain channels that lack alluvial flood plains. This study (a) quantifies relationships between field indicators and flow frequency in alluvial and steepland channels in a tropical mountain stream network and (b) identifies a reference active channel boundary in these channels, based on statistically defined combinations of riparian features, that corresponds to the same flow frequency of the bankfull stage and the effective discharge in adjacent alluvial channels. The relative elevation of transitions in riparian vegetation, soil, and substrate characteristics were first surveyed at nine stream gages in and around the Luquillo Experimental Forest in Northeastern Puerto Rico. The corresponding discharge, flow frequency, and recurrence intervals associated with these features was then determined from longterm 15-min discharge records and a partial duration series analysis. Survey data indicate that mosses and short grasses dominate at a stage often inundated by sub-effective flows. Herbaceous vegetation is associated with intermediate discharges that correspond to the threshold for sediment mobilization. Near-channel woody shrubs and trees establish at elevations along the channel margin inundated by a less frequent discharge that is coincident with the effective discharge of bed load sediment transport. Our data demonstrate that in alluvial channels in the study, both the bankfull stage (as marked by a flood plain) and the channel-forming (effective) discharge are associated with the presence of fine-grained substrate and soil, and tall, mature woody vegetation. In montane reaches that lack a flood plain, a boundary that is characterized by the incipient presence of soil, woody shrubs, and trees corresponds to the same flow frequency as the bankfull discharge of nearby alluvial channels. The reference discharge based on these riparian features in steepland sites has an average exceedance probability between 0.09% and 0.30%, and a recurrence interval between 40 and 90 days. We conclude that flows with similar frequencies influence the establishment of riparian vegetation, soil development, and substrate characteristics along channel margins in similar ways. Thus, these riparian features can be used as an indicator of hydrogeomorphic site conditions to identify active channel boundaries that occur at a constant flow frequency throughout the study stream network.

Water Quality Trading in the Lower Delaware River Basin: A Resource for Practitioners

Scatena, F. N., Curley, D., Laskowski, S., Abbot, K., Barkin, H., Shieh, W., and Johson, J. (2005). "Water Quality Trading in the Lower Delaware River Basin: A Resource for Practitioners." Institution of Environmental Studies, University of Pennsylvania, Philadelphia.

This document addresses technical and administrative issues related to the establishment of water quality trading (WQT) systems in the Lower Delaware River (LDR) Basin and is based on a series of workshops, public seminars, and analysis conducted in 2005. The report is designed for practitioners interested in developing WQT systems and provides a series of specific recommendations and steps to promote the development of WQT in the area. Pollutant trading systems allow dischargers to obtain environmental protection in an economically-efficient manner by substituting, or trading, inexpensive controls at one location for expensive controls elsewhere. Although pollutant trading has been discussed for decades, there are few WQT systems operating in the United States and none have a long track record. Those that are in operation are limited in scope and were initiated by state, federal, or private funding. Nevertheless, WQT is expected to increase in the future and has already created incentives for dischargers to examine their operations and reduce their discharges beyond what the conventional methods would provide.

Instream-Flow Analysis for the Luquillo Experimental Forest, Puerto Rico: Methods and Analysis

Scatena, F.N.; Johnson, S.L. 2001. Instream-Flow Analysis for the Luquillo Experimental Forest, Puerto Rico: Methods and Analysis. Gen. Tech. Rep. IITF-GTR-11. Rio Piedras, PR: U.S. Department of Agriculture, Forest Service, International Institute of Tropical Forestry. 30 p.

This study develops two habitat-based approaches for evaluating instream-flow requirements within the Luquillo Experimental Forest in northeastern Puerto Rico. The analysis is restricted to instream-flow requirements in upland streams dominated by the common communities of freshwater decapods. In headwater streams, pool volume was the most consistent factor in predicting the abundance of common freshwater shrimp. In second- and third-order tributaries, both water depth and velocity can be used to define their habitats. The most common species of shrimp are reclusive during the day; at night they prefer areas of low velocity (<0.09 m/s) and areas shallower than 0.4 m. In headwater streams, total usable shrimp habitat declines rapidly when water depth in the deepest pools is less than 0.5 m. In second-and third-order tributaries, the amount of habitat declines rapidly when discharge is within one standard deviation of the average annual 7-day minimum flow. These dis-charges are typically exceeded between 95 and 99 percent of the time. Analysis of habitat loss associated with different instream-flow constraints showed that habitat loss increases greatly when water extraction is equal to or greater than Q98. Among-reach differences in the amount of usable habitat resulting from differences in channel morphology can be as high as 35 percent. Therefore, site-specific studies should be conducted when using habitat-preference relations in a particular area.

Natural disturbances and the hydrology of humid tropical forests

Scatena FN, Planos-Gutiérrez EO, Schellekens J. 2004. Natural disturbances
and the hydrology of humid tropical forests. In Forests,
Water, and People in the Humid Tropics, Bonell M, Bruijnzeel LA
(eds). Cambridge University Press: Cambridge; 5–28.

Humid tropical forests are highly dynamic ecosystems that are affected by a wide array of environmental processes and disturbances (Figure 19.1). Quantifying the magnitude, frequency, and impacts of natural disturbances is essential for designing hydraulic structures, developing water management strategies, and distinguishing between natural variation and man-made influences. A disturbance can be defined as any discrete event that transfers mass and energy from one part of a system to another in a manner that disrupts ecosystem, community, or population structure and changes resource availability or the physical environment (see White and Pickett 1985 for a detailed discussion). Natural disturbances can be driven by both external factors – hurricanes, meteor impacts, etc. – and the biological properties of the system such as senescence, pathogens, etc. The natural disturbances specified by the United Nations in the International Decade of Natural Disaster Reduction were earthquakes, windstorms, tsunamis, floods, landslides, volcanic eruptions, wildfires, grasshopper and locust infestations, drought, and desertification (Board on Natural Disasters, 1999). Additional natural disturbances known to affect the hydrology of humid tropical forests are tree falls, pathogens, exotic invasions and meteor impacts. Quantifying the effects of disturbances on landform morphology and ecosystem development have been major themes in geomorphology and ecology (Wolman and Miller, 1960, Connell, 1978). This approach has led to the paradigm that landscapes are structured by the processes acting upon them (O’Neill et al., 1986, Urban et al., 1987, Scatena, 1995). It is now generally recognised that the ability of a disturbance to affect the morphology of a landscape or the structure of an ecosystem depends on: (1) the type of disturbance (e.g., flood, fire, landslide, biologic, anthropogenic etc.); (2) the force exerted (e.g. wind velocity and duration, rainfall magnitude and intensity, earthquake magnitude etc.); (3) the ecosystem component that is impacted directly (e.g. soil, biomass, leaf area etc.); (4) the area affected and the spatial distribution of impacts; (5) the return period or frequency of the event; (6) the condition of the system at the time of the disturbance (e.g. structure, regeneration phase, time since last disturbance); and (7) the magnitude of the constructive or restorative processes that occur between disturbances.

Hydrometeorology of tropical montane cloud forests: emerging patterns

Bruijnzeel LA, Mulligan M, Scatena FN. 2010. Hydrometeorology of
tropical montane cloud forests: emerging patterns. Hydrological
Processes. DOI: 10.1002/hyp.7974.

altitudinal limits between which TMCF generally occur (800–3500 m.a.s.l. depending on mountain size and distance to coast) their current areal extent is estimated at ¾215 000 km2 or 6Ð6% of all montane tropical forests. Alternatively, on the basis of remotely sensed frequencies of cloud occurrence, fog-affected forest may occupy as much as 2Ð21 Mkm2. Four hydrologically distinct montane forest types may be distinguished, viz. lower montane rain forest below the cloud belt (LMRF), tall lower montane cloud forest (LMCF), upper montane cloud forest (UMCF) of intermediate stature and a group that combines stunted sub-alpine cloud forest (SACF) and ‘elfin’ cloud forest (ECF). Average throughfall to precipitation ratios increase from 0Ð72 š 0Ð07 in LMRF (n D 15) to 0Ð81 š 0Ð11 in LMCF (n D 23), to 1Ð0 š 0Ð27 (n D 18) and 1Ð04 š 0Ð25 (n D 8) in UMCF and SACF–ECF, respectively. Average stemflow fractions increase from LMRF to UMCF and ECF, whereas leaf area index (LAI) and annual evapotranspiration (ET) decrease along the same sequence. Although the data sets for UMCF (n D 3) and ECF (n D 2) are very limited, the ET from UMCF (783 š 112 mm) and ECF (547 š 25 mm) is distinctly lower than that from LMCF (1188 š 239 mm, n D 9) and LMRF (1280 š 72 mm; n D 7). Field-measured annual ‘cloud-water’ interception (CWI) totals determined with the wet-canopy water budget method (WCWB) vary widely between locations and range between 22 and 1990 mm (n D 15). Field measured values also tend to be much larger than modelled amounts of fog interception, particularly at exposed sites. This is thought to reflect a combination of potential model limitations, a mismatch between the scale at which the model was applied (1 ð 1 km) and the scale of the measurements (small plots), as well as the inclusion of near-horizontal wind-driven precipitation in the WCWB-based estimate of CWI. Regional maps of modelled amounts of fog interception across the tropics are presented, showing major spatial variability. Modelled contributions by CWI make up less than 5% of total precipitation in wet areas to more than 75% in low-rainfall areas. Catchment water yields typically increase from LMRF to UMCF and SACF–ECF reflecting concurrent increases in incident precipitation and decreases in evaporative losses. The conversion of LMCF (or LMRF) to pasture likely results in substantial increases in water yield. Changes in water yield after UMCF conversion are probably modest due to trade-offs between concurrent changes in ET and CWI. General circulation model (GCM)-projected rates of climatic drying under SRES greenhouse gas scenarios to the year 2050 are considered to have a profound effect on TMCF hydrological functioning and ecology, although different GCMs produce different and sometimes opposing results. Whilst there have been substantial increases in our understanding of the hydrological processes operating in TMCF, additional research is needed to improve the quantification of occult precipitation inputs (CWI and wind-driven precipitation), and to better understand the hydrological impacts of climate- and land-use change. Copyright  2010 John Wiley & Sons, Ltd.
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