Urbanization
Urban influences on the nitrogen cycle in Puerto Rico
Fri, 07/29/2011 - 12:56 — ckassOrtiz-Zayas, J. R., E. Cuevas, O. L. Mayol-Bracero, L.
Donoso, I. Trebs, D. Figueroa-Nieves, and W. H. Mcdowell.
2006. Urban influences on the nitrogen cycle in Puerto Rico.
Biogeochemistry 79:109–133.
Abstract:
Anthropogenic actions are altering fluxes of nitrogen (N) in the biosphere at unprecedented
rates. Efforts to study these impacts have concentrated in the Northern hemisphere, where
experimental data are available. In tropical developing countries, however, experimental studies are
lacking. This paper summarizes available data and assesses the impacts of human activities on N
fluxes in Puerto Rico, a densely populated Caribbean island that has experienced drastic landscape
transformations over the last century associated with rapid socioeconomic changes. N yield calculations
conducted in several watersheds of different anthropogenic influences revealed that disturbed
watersheds export more N per unit area than undisturbed forested watersheds. Export of N
from urban watersheds ranged from 4.8 kg ha)1 year)1 in the Rı´o Bayamo´ n watershed to
32.9 kg ha)1 year)1 in the highly urbanized Rı´o Piedras watershed and 33.3 kg ha)1 year)1 in the
rural-agricultural Rı´o Grande de An˜ asco watershed. Along with land use, mean annual runoff
explained most of the variance in fluvial N yield. Wastewater generated in the San Juan Metropolitan
Area receives primary treatment before it is discharged into the Atlantic Ocean. These
discharges are N-rich and export large amounts of N to the ocean at a rate of about
140 kg ha)1 year)1. Data on wet deposition of inorganic N (NHþ4
þ NO
3 ) suggest that rates of
atmospheric N deposition are increasing in the pristine forests of Puerto Rico. Stationary and
mobile sources of NOx (NO+NO2) and N2O generated in the large urban centers may be
responsible for this trend. Comprehensive measurements are required in Puerto Rico to quantitatively
characterize the local N cycle. More research is required to assess rates of atmospheric N
deposition, N fixation in natural and human-dominated landscapes, N-balance associated with
food and feed trade, and denitrification.
FLASHINESS INDICES FOR URBAN AND RURAL STREAMS IN PUERTO RICO
Sun, 07/24/2011 - 10:35 — ckassAbstract:
Urbanization and increases in impervious area are known to increase stream runoff and flashiness and several
indices have been developed to quantify flashiness in temperate streams. The effectiveness of these indices in the humid
tropics and how urbanization influences the flashiness of tropical streams that are known for their inherent flashiness is
poorly understood and documented. This study compares two existing flashiness indices and a new approach that quantifies
flashiness by the average time between large events on 13 urban to forested streams in Northeastern Puerto Rico. The average
time between events of specific magnitudes, the Richards-Baker Flashiness index, and a Frequency of Stage Change
approach were calculated and compared using average daily, hourly, and 15 minute discharge data. All analysis was based on
USGS discharge records for the same 10 year period, 2000 through 2009. The results indicate that when flashiness is based
on average daily stream flow, there was little to no discernible difference in the flashiness of the urban and forested streams.
This results because average daily discharge records miss or underestimates the magnitude, duration, and the frequency of
most events. When comparing the time between events of a given magnitude using hourly or 15 minute discharge series, the
urban drainages have a shorter time period between peaks of a given magnitude than rural drainages. The Frequency of Stage
Change approach also indicates that high density urban drainages are flashier than drainages with forested or mixed land use.
For the average days and the stage frequency change method, flashiness indices based on hourly time series are similar to
those based on the 15 minute series. The analysis indicates that tropical urban streams are flashier than their rural
counterparts; however the difference is less than has been reported in temperate studies and is only statistically apparent
when using high resolution discharge records.
