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Dive into the research topics where Pamela L. Nagler is active.

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Featured researches published by Pamela L. Nagler.


Remote Sensing of Environment | 2000

Plant litter and soil reflectance.

Pamela L. Nagler; Craig S. T. Daughtry; Samuel N. Goward

The presence of plant litter on the soil surface influences the flow of nutrients, carbon, water, and energy in terrestrial ecosystems. Quantifying plant litter cover is important for interpreting vegetated landscapes and for evaluating the effectiveness of conservation tillage practices. Current methods of measuring litter cover are subjective, requiring considerable visual judgment. Reliable and objective methods are needed. The spectral reflectance (0.4-2.5 μm) of wet and dry soils (six types) and plant litters (2 crops, 14 forest, and 2 grasses) of different ages were measured. Discrimination of plant litters from the soils was ambiguous in the visible and near-infrared (0.4-1.1μm) wavelength region. An absorption feature associated with cellulose and lignin was observed at 2.1 μm in the spectra of dry plant litter, which was not present in the spectra of soils. A new spectral variable, cellulose absorption index (CAI), was defined using the relative depth of the reflectance spectra at 2.1 μm. CAI of dry litter was significantly greater than CAI of soils. CAI generally decreased with age of the litter. Water absorption dominated the spectral properties of both soils and plant litter and significantly reduced the CAI of the plant litters. Nevertheless, the CAI of wet litter was significantly greater then CAI of wet soil. This study provides a new methodology to discriminate plant litter from soils by differences in spectral reflectance produced by their physical and chemical attributes. This remote sensing method should improve quantification of plant litter cover and thus improve estimates of phytomass production, surface energy balance, and the effectiveness of soil conservation practices. Plant litter reflectance is a verifiable component in vegetative landscapes and should be labeled and modeled separately from soils in landscape studies.


Agricultural and Forest Meteorology | 2003

Comparison of transpiration rates among saltcedar, cottonwood and willow trees by sap flow and canopy temperature methods

Pamela L. Nagler; Edward P. Glenn; T. Lewis Thompson

Transpiration (Et), measured by stem sap flow gauges, and canopy and air temperature differential ( Tc − Ta )o fPopulus fremontii (cottonwood), Salix gooddingii (willow) and Tamarix ramosissima (saltcedar) were compared to determine if remotely sensed canopy temperatures could be used to estimate Et or water stress in these trees in desert riparian zones of the United States and Mexico. Controlled experiments were conducted in which containerized plants were placed closely together and allowed to grow into a single, dense canopy over a summer in a desert climate. At the end of the growth period, two canopies of each species were measured for Et and Tc − Ta over 11 days, first under unstressed conditions then under water or salt stress. Et and Tc − Ta were significantly ( P< 0.05) correlated for all species. Correlation coefficients improved when a radiation term was included in the equation predicting Et from Tc − Ta. During the non-stress part of the experiment, canopies of all three species had similar rates of Et, but saltcedar maintained higher Et rates and lower rsv than the native trees on the stress treatments. For each species, models were developed, using both meteorological data and a canopy, energy-balance equation, to predict daily Et and stomatal resistance (rsv); these models had standard errors of 15–22% when compared with measured Et over the unstressed portion of the experiment.


Critical Reviews in Plant Sciences | 2011

Distribution and Abundance of Saltcedar and Russian Olive in the Western United States

Pamela L. Nagler; Edward P. Glenn; Catherine S. Jarnevich; Patrick B. Shafroth

Over the past century, two introduced Eurasian trees, saltcedar (Tamarix spp.) and Russian olive (Elaeagnus angustifolia) have become wide spread on western United States of American (U.S.) rivers. This paper reviews the literature on the following five key areas related to their distribution and abundance in the western United States: (1) the history of introduction, planting, and spread of saltcedar and Russian olive; (2) their current distribution; (3) their current abundance; (4) factors controlling their current distribution and abundance; and (5) models that have been developed to predict their future distribution and abundance. Saltcedar and Russian olive are now the third and fourth most frequently occurring woody riparian plants and the second and fifth most abundant species (out of 42 native and non-native species) along rivers in the western United States. Currently there is not a precise estimate of the areas that these species occupy in the entire West. Climatic variables are important determinants of their distribution and abundance. For example, saltcedar is limited by its sensitivity to hard freezes, whereas Russian olive appears to have a chilling requirement for bud break and seed germination, and can presumably survive colder winter temperatures. Either species can be dominant, co-dominant or sub-dominant relative to native species on a given river system. A number of environmental factors such as water availability, soil salinity, degree of streamflow regulation, and fire frequency can influence the abundance of these species relative to native species. Numerous studies suggest that both species have spread on western rivers primarily through a replacement process, whereby stress-tolerant species have moved into expanded niches that are no longer suitable for mesic native pioneer species. Better maps of current distribution and rigorous monitoring of distributional changes though time can help to resolve differences in predictions of potential future spread. An adequate understanding does not yet exist of what fraction of western riparian zones is resistant to dominance by either of these species, what fraction is at risk and could benefit from intervention, and what fraction has been altered to the point that saltcedar or Russian olive are most likely to thrive.


Remote Sensing | 2009

An Empirical Algorithm for Estimating Agricultural and Riparian Evapotranspiration Using MODIS Enhanced Vegetation Index and Ground Measurements of ET. I. Description of Method

Pamela L. Nagler; Kiyomi Morino; R. Scott Murray; John Osterberg; Edward P. Glenn

We used the Enhanced Vegetation Index (EVI) from MODIS to scale evapotranspiration (ETactual) over agricultural and riparian areas along the Lower Colorado River in the southwestern US. Ground measurements of ETactual by alfalfa, saltcedar, cottonwood and arrowweed were expressed as fraction of potential (reference crop) ETo (EToF) then regressed against EVI scaled between bare soil (0) and full vegetation cover (1.0) (EVI*). EVI* values were calculated based on maximum and minimum EVI values from a large set of riparian values in a previous study. A satisfactory relationship was found between crop and riparian plant EToF and EVI*, with an error or uncertainty of about 20% in the mean estimate (mean ETactual = 6.2 mm d−1, RMSE = 1.2 mm d−1). The equation for ETactual was: ETactual = 1.22 × ETo-BC × EVI*, where ETo-BC is the Blaney Criddle formula for ETo. This single algorithm applies to all the vegetation types in the study, and offers an alternative to ETactual estimates that use crop coefficients set by expert opinion, by using an algorithm based on the actual state of the canopy as determined by time-series satellite images.


Remote Sensing | 2014

High Spatial Resolution WorldView-2 Imagery for Mapping NDVI and Its Relationship to Temporal Urban Landscape Evapotranspiration Factors

Hamideh Nouri; Simon Beecham; Sharolyn Anderson; Pamela L. Nagler

Evapotranspiration estimation has benefitted from recent advances in remote sensing and GIS techniques particularly in agricultural applications rather than urban environments. This paper explores the relationship between urban vegetation evapotranspiration (ET) and vegetation indices derived from newly-developed high spatial resolution WorldView-2 imagery. The study site was Veale Gardens in Adelaide, Australia. Image processing was applied on five images captured from February 2012 to February 2013 using ERDAS Imagine. From 64 possible two band combinations of WorldView-2, the most reliable one (with the maximum median differences) was selected. Normalized Difference Vegetation Index (NDVI) values were derived for each category of landscape cover, namely trees, shrubs, turf grasses, impervious pavements, and water bodies. Urban landscape evapotranspiration rates for Veale Gardens were estimated through field monitoring using observational-based landscape coefficients. The relationships between remotely sensed NDVIs for the entire Veale Gardens and for individual NDVIs of different vegetation covers were compared with field measured urban landscape evapotranspiration rates. The water stress conditions experienced in January


Aquaculture | 2003

Effects of fertilization treatment and stocking density on the growth and production of the economic seaweed Gracilaria parvispora (Rhodophyta) in cage culture at Molokai, Hawaii

Pamela L. Nagler; Edward P. Glenn; Stephen G. Nelson; Sherman Napolean

Abstract The edible red seaweed, Gracilaria parvispora Abbott, was pulse-fertilized in tanks containing fish-culture water or chemical fertilizer, then cultured in floating cages in a low-nutrient, ocean lagoon in Molokai, HI. Small, daily additions of ammonium sulfate and ammonium diphosphate were the only additions needed to stimulate growth. Fish-culture water was as effective as chemical fertilizer in supporting growth. Thalli fertilized for 7 days in tanks contained 2.5–5% nitrogen in tissues by the end of the treatment period; upon transfer to low-nutrient water, nitrogen content decreased to 1% as the nitrogen was mobilized to support growth. Thalli grew rapidly over the first 14 days after transfer from fertilizer tanks to the ocean, achieving relative growth rates of 8–10% day −1 and producing 39–57 g dry wt. m −2 day −1 . However, by 21 days after transfer, growth ceased due to depletion of stored nutrients. The optimal stocking density was 2 kg m −3 based on growth rates. Nearly all net growth occurred in the cages rather than in the fertilizer tanks, which serve only to introduce nitrogen into the thalli. The yields obtained here are four times higher than achieved previously with this species and are comparable to high-yielding, intensive tank cultures.


Remote Sensing | 2009

An Empirical Algorithm for Estimating Agricultural and Riparian Evapotranspiration Using MODIS Enhanced Vegetation Index and Ground Measurements of ET. II. Application to the Lower Colorado River, U.S.

R. Scott Murray; Pamela L. Nagler; Kiyomi Morino; Edward P. Glenn

Abstract: Large quantities of water are consumed by irrigated crops and riparian vegetation in western U.S. irrigation districts. Remote sensing methods for estimating evaporative water losses by soil and vegetation (evapotranspiration, ET) over wide river stretches are needed to allocate water for agricultural and environmental needs. We used the Enhanced Vegetation Index (EVI) from MODIS sensors on the Terra satellite to scale ET over agricultural and riparian areas along the Lower Colorado River in the southwestern U.S., using a linear regression equation between ET of riparian plants and alfalfa measured on the ground, and meteorological and remote sensing data, with an error or uncertainty of about 20%. The algorithm was applied to irrigation districts and riparian areas from Lake Mead to the U.S./Mexico border. The results for agricultural crops were similar to results produced by crop coefficients developed for the irrigation districts along the river. However, riparian ET was only half as great as crop coefficient estimates set by expert opinion, equal to about 40% of reference crop evapotranspiration. Based on reported acreages in 2007, agricultural crops (146,473 ha) consumed 2.2 × 10


Environmental Management | 2008

Reconciling Environmental and Flood Control Goals on an Arid-Zone River: Case Study of the Limitrophe Region of the Lower Colorado River in the United States and Mexico

Edward P. Glenn; Kate Hucklebridge; Osvel Hinojosa-Huerta; Pamela L. Nagler; Jennifer Pitt

Arid zone rivers have highly variable flow rates, and flood control projects are needed to protect adjacent property from flood damage. On the other hand, riparian corridors provide important wildlife habitat, especially for birds, and riparian vegetation is adapted to the natural variability in flows on these rivers. While environmental and flood control goals might appear to be at odds, we show that both goals can be accommodated in the Limitrophe Region (the shared border between the United States and Mexico) on the Lower Colorado River. In 1999, the International Boundary and Water Commission proposed a routine maintenance project to clear vegetation and create a pilot channel within the Limitrophe Region to improve flow capacity and delineate the border. In 2000, however, Minute 306 to the international water treaty was adopted, which calls for consideration of environmental effects of IBWC actions. We conducted vegetation and bird surveys within the Limitrophe and found that this river segment is unusually rich in native cottonwood and willow trees, marsh habitat, and resident and migratory birds compared to flow-regulated segments of river. A flood-frequency analysis showed that the existing levee system can easily contain a 100 year flood even if vegetation is not removed, and the existing braided channel system has greater carrying capacity than the proposed pilot channel.


Sensors | 2018

Application and Comparison of the MODIS-Derived Enhanced Vegetation Index to VIIRS, Landsat 5 TM and Landsat 8 OLI Platforms: A Case Study in the Arid Colorado River Delta, Mexico

Christopher J. Jarchow; Kamel Didan; A. Barreto-Munoz; Pamela L. Nagler; Edward P. Glenn

The Enhanced Vegetation Index (EVI) is a key Earth science parameter used to assess vegetation, originally developed and calibrated for the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra and Aqua satellites. With the impending decommissioning of the MODIS sensors by the year 2020/2022, alternative platforms will need to be used to estimate EVI. We compared Landsat 5 (2000–2011), 8 (2013–2016) and the Visible Infrared Imaging Radiometer Suite (VIIRS; 2013–2016) to MODIS EVI (2000–2016) over a 420,083-ha area of the arid lower Colorado River Delta in Mexico. Over large areas with mixed land cover or agricultural fields, we found high correspondence between Landsat and MODIS EVI (R2 = 0.93 for the entire area studied and 0.97 for agricultural fields), but the relationship was weak over bare soil (R2 = 0.27) and riparian vegetation (R2 = 0.48). The correlation between MODIS and Landsat EVI was higher over large, homogeneous areas and was generally lower in narrow riparian areas. VIIRS and MODIS EVI were highly similar (R2 = 0.99 for the entire area studied) and did not show the same decrease in performance in smaller, narrower regions as Landsat. Landsat and VIIRS provide EVI estimates of similar quality and characteristics to MODIS, but scale, seasonality and land cover type(s) should be considered before implementing Landsat EVI in a particular area.


international geoscience and remote sensing symposium | 1996

Shortwave infrared spectral reflectance of plant litter and soils

Pamela L. Nagler; C.S.T. Daughtry; Samuel N. Goward

Acquiring and understanding the spectral reflectance of two non-photosynthetically active materials, plant litter and soils, is important for interpreting vegetative landscapes. The goal of this research is to develop robust remote sensing techniques to discriminate plant litter from soils. Spectral reflectance of wet and dry soil and litter (crop, forest, and grass) in the 0.4-2.5 /spl mu/m wavelength range was measured. An absorption feature at 2.1 /spl mu/m in the spectrum of dry litter, associated with cellulose, was not present in soils. Water absorption dominated the spectral properties of both soils and litter, but discrimination of wet litter from wet soil was possible. Plant litter reflectance is a verifiable component in vegetative landscapes and should be labeled and modeled separately from soils in landscape studies.

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John Osterberg

United States Bureau of Reclamation

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