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Dive into the research topics where Christopher J. Watts is active.

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Featured researches published by Christopher J. Watts.


Environmental Monitoring and Assessment | 2000

A Landscape Approach for Detecting and Evaluating Change in a Semi-Arid Environment

William G. Kepner; Christopher J. Watts; Curtis M. Edmonds; John K. Maingi; Stuart E. Marsh; Gonzalo Luna

Vegetation change in the American West has been a subject of concern throughout the twentieth century. Although many of the changes have been recorded qualitatively through the use of comparative photography and historical reports, little quantitative information has been available on the regional or watershed scale. It is currently possible to measure change over large areas and determine trends in ecological and hydrological condition using advanced space-based technologies. Specifically, this process is being tested in a community-based watershed in southeast Arizona and northeast Sonora, Mexico using a system of landscape pattern measurements derived from satellite remote sensing, spatial statistics, process modeling, and geographic information systems technology. These technologies provide the basis for developing landscape composition and pattern indicators as sensitive measures of large-scale environmental change and thus may provide an effective and economical method for evaluating watershed condition related to disturbance from human and natural stresses. The project utilizes the database from the North American Landscape Characterization (NALC) project which incorporates triplicate Landsat Multi-Spectral Scanner (MSS) imagery from the early 1970s, mid 1980s, and the 1990s. Landscape composition and pattern metrics have been generated from digital land cover maps derived from the NALC images and compared across a nearly 20-year period. Results about changes in land cover for the study period indicate that extensive, highly connected grassland and desertscrub areas are the most vulnerable ecosystems to fragmentation and actual loss due to encroachment of xerophytic mesquite woodland. In the study period, grasslands and desertscrub not only decreased in extent but also became more fragmented. That is, the number of grassland and desertscrub patches increased and their average patch sizes decreased. In stark contrast, the mesquite woodland patches increased in size, number, and connectivity. These changes have important impact for the hydrology of the region, since the energy and water balance characteristics for these cover types are significantly different. The process demonstrates a simple procedure to document changes and determine ecosystem vulnerabilities through the use of change detection and indicator development, especially in regard to traditional degradation processes that have occurred throughout the western rangelands involving changes of vegetative cover and acceleration of water and wind erosion.


Geophysical Research Letters | 2008

Observed relation between evapotranspiration and soil moisture in the North American monsoon region

Enrique R. Vivoni; Hernan A. Moreno; Giuseppe Mascaro; Julio C. Rodríguez; Christopher J. Watts; Jaime Garatuza-Payan; Russell L. Scott

] Soil moisture control on evapotranspiration is poorlyunderstood in ecosystems experiencing seasonal greening.In this study, we utilize a set of multi-year observations atfour eddy covariance sites along a latitudinal gradient invegetation greening to infer the ET-q relation during theNorth American monsoon. Results reveal significantseasonal, interannual and ecosystem variations in theobserved ET-q relation directly linked to vegetationgreening. In particular, monsoon-dominated ecosystemsadjust their ET-q relation, through changes in unstressedET and plant stress threshold, to cope with differences inwater availability. Comparisons of the observed relations tothe North American Regional Reanalysis dataset reveallarge biases that increase where vegetation greening is moresignificant. The analysis presented here can be used to guideimprovements in land surface model parameterization inwater-limited ecosystems.


Journal of Hydrometeorology | 2003

Derivation of an Effective Height for Scintillometers: La Poza Experiment in Northwest Mexico

O.K. Hartogensis; Christopher J. Watts; Julio C. Rodríguez; H. A. R. De Bruin

The large-aperture scintillometer (LAS) is by now a generally accepted device for routinely obtaining the area-averaged sensible heat flux, H, on a scale of up to 10 km. It is an optical instrument that consists of a transmitter and receiver. In practice, the LAS beam height often varies along the path due to a variety of reasons. This study will explain what effective height to use in such situations, when analyzing scintillometer data to derive H. Several aspects are covered: a slanted path over flat terrain, structured terrain, and varying path height due to the curvature of the earth’s surface. To test the derived effective height formulation the authors present LAS data taken in September and October 1996 at a rangeland site in Sonora, Mexico. In experiment 1, the LAS was set up over a slant path, ranging roughly between 10 and 45 m above the surface over a 3200-m path. In experiment 2, a horizontal LAS path was used at approximately 30 m over a pathlength of 1100 m. The resulting sensible heat fluxes were compared with eddy-covariance data and show satisfactory results for both the full and one of the approximate formulations of the effective height.


Agricultural and Forest Meteorology | 2003

The understory and overstory partitioning of energy and water fluxes in an open canopy, semiarid woodland

Russell L. Scott; Christopher J. Watts; Jaime Garatuza Payan; Eric Edwards; David C. Goodrich; David G. Williams; W. James Shuttleworth

Eddy flux studies have traditionally focused on total ecosystem exchanges of energy and water by making measurements in the well-mixed surface layer, but this approach does not provide information about the partitioning of the total ecosystem fluxes between overstory and understory sources and sinks. In more open canopy environments, information about partitioning of fluxes is often required in order to understand the relative importance and functioning of key ecosystem components and their response to climate forcing. In this paper, we present results from a series of experiments carried out in a riparian mesquite (Prosopis velutina) woodland. Three eddy covariance systems were deployed before, during, and after the onset of the summer rainy season to measure energy and water fluxes. One eddy covariance system was installed on a tower to measure whole ecosystem fluxes. The other two were installed at a height of 2 m, one in a relatively closed understory patch and the other in a more open understory patch. Our results indicate that the understory and overstory moisture sources were mostly decoupled. The trees apparently had access to deep moisture sources, and thus, their water use was relatively insensitive to local precipitation. In contrast, the contribution of the understory to the total ecosystem fluxes was highly variable due to the presence or absence of near-surface soil moisture.


Monthly Weather Review | 2004

Analysis of 2002 and 2003 Warm-Season Precipitation from the North American Monsoon Experiment Event Rain Gauge Network

David J. Gochis; Alejandro Jimenez; Christopher J. Watts; Jaime Garatuza-Payan; W. James Shuttleworth

Analyses of rainfall characteristics and their linkage to physiographic features are made from the North American monsoon experiment (NAME) Event Rain Gauge Network (NERN) in northwest Mexico. The findings are based on the network configuration for the 2002 and 2003 warm seasons. Despite the relatively short record used, a clearer structure of core-region monsoon rainfall is beginning to emerge. In agreement with earlier, coarser-scale studies, the seasonal precipitation maximum overlies the western slope of the Sierra Madre Occidental but does not strictly parallel a particular elevation band. It is shown that the distance to the Gulf of California and, potentially, the configuration of the terrain profile may also play an important role in determining where the axis of maximum precipitation lies. The diurnal cycles of precipitation frequency and intensity are shown to have distinct relationships to terrain elevation that are qualitatively similar to those observed over the Front Range of the Rocky Mountains in the central-western United States. The relationship between precipitation and gulf surge events occurring during the summer of 2003 is also explored.


Agricultural and Forest Meteorology | 2000

Estimation of heat and momentum fluxes over complex terrain using a large aperture scintillometer.

A. Chehbouni; Christopher J. Watts; Jean-Pierre Lagouarde; Yann Kerr; J.-C. Rodriguez; Jean-Marc Bonnefond; F. Santiago; Gérard Dedieu; David C. Goodrich; Carl L. Unkrich

A comprehensive experimental plan has been designed to further investigate the potential and the limitations associated with the use of a large aperture scintillometer (LAS) to infer path average sensible and momentum fluxes over complex surfaces as part of the Semi-Arid Land-Surface-Atmosphere (SALSA) Program. The complexity of the terrain is associated with the type and the cover of the vegetation canopy as well as with changes in topography. Scintillometer based estimates of sensible heat flux and friction velocity are compared to those measured by eddy correlation systems over a grassland patch, a mesquite patch, and over a transect spanning both patches. The results show that considering the complexity of the surface, the overall performance of the scintillometer is relatively good.


Journal of Climate | 2007

Variation of hydrometeorological conditions along a topographic transect in Northwestern Mexico during the North American monsoon

Enrique R. Vivoni; Hugo A. Gutiérrez-Jurado; Carlos A. Aragon; Luis A. Méndez-Barroso; Alex Rinehart; Robert L. Wyckoff; Julio C. Rodríguez; Christopher J. Watts; John D. Bolten; V. Lakshmi; Thomas J. Jackson

Abstract Relatively little is currently known about the spatiotemporal variability of land surface conditions during the North American monsoon, in particular for regions of complex topography. As a result, the role played by land–atmosphere interactions in generating convective rainfall over steep terrain and sustaining monsoon conditions is still poorly understood. In this study, the variation of hydrometeorological conditions along a large-scale topographic transect in northwestern Mexico is described. The transect field experiment consisted of daily sampling at 30 sites selected to represent variations in elevation and ecosystem distribution. Simultaneous soil and atmospheric variables were measured during a 2-week period in early August 2004. Transect observations were supplemented by a network of continuous sampling sites used to analyze the regional hydrometeorological conditions prior to and during the field experiment. Results reveal the strong control exerted by topography on the spatial and tem...


Journal of Climate | 2007

Changes in Vegetation Condition and Surface Fluxes during NAME 2004

Christopher J. Watts; Russell L. Scott; Jaime Garatuza-Payan; Julio C. Rodríguez; John H. Prueger; William P. Kustas; Michael William Douglas

Abstract The vegetation in the core region of the North American monsoon (NAM) system changes dramatically after the onset of the summer rains so that large changes may be expected in the surface fluxes of radiation, heat, and moisture. Most of this region lies in the rugged terrain of western Mexico and very few measurements of these fluxes have been made in the past. Surface energy balance measurements were made at seven sites in Sonora, Mexico, and Arizona during the intensive observation period (IOP) of the North American Monsoon Experiment (NAME) in summer 2004 to better understand how land surface vegetation change alters energy flux partitioning. Satellite data were used to obtain time series for vegetation indices and land surface temperature for these sites. The results were analyzed to contrast conditions before the onset of the monsoon with those afterward. As expected, precipitation during the 2004 monsoon was highly variable from site to site, but it fell in greater quantities at the more sou...


Agricultural Water Management | 1999

Use of satellite data to estimate radiation and evaporation for northwest Mexico

J.B. Stewart; Christopher J. Watts; Julio C. Rodríguez; H. A. R. De Bruin; A.R. van den Berg; Jaime Garatuza-Payan

Abstract Incoming solar radiation was estimated from visible band data obtained by the GOES satellite over northwest Mexico. Comparisons against ground-based measurements of incoming solar radiation showed good agreement, particularly in months with low cloud cover. The data from an automatic weather station installed within the Yaqui Valley Irrigation Scheme was used to estimate potential evaporation from a formula based on incoming solar radiation and climatological values of temperature. The success of this formula was assessed by comparison against potential evaporation estimated using the Penman and Penman–Monteith formulae and measurements of net radiation.


Agricultural and Forest Meteorology | 2001

Estimation of surface sensible heat flux using dual angle observations of radiative surface temperature

Abdelghani Chehbouni; Y. Nouvellon; J.P. Lhomme; Christopher J. Watts; Gilles Boulet; Yann Kerr; M.S. Moran; David C. Goodrich

Abstract In this study, dual angle observations of radiative surface temperature have been used in conjunction with a two-layer model to derive sensible heat flux over a sparsely vegetated surface. Data collected during the semi-arid-land-surface-atmosphere program (SALSA) over a semi-arid grassland in Mexico were used to assess the performance of the approach. The results showed that this approach led to reasonable estimates of the observed fluxes. The mean average percentage difference (MAPD) between observed and simulated fluxes was about 23%, which is not statistically different from the expected 20% scatter, when different flux measuring devices are compared over the same site. However, the sensitivity analysis indicated that the approach was rather sensitive to uncertainties in both measured radiative temperatures and aerodynamic characteristics of the vegetation. Finally, the issue of using dual angle observations of surface temperature for characterizing the difference between aerodynamic and nadir viewing radiative temperature has been examined. The results showed that this difference is linearly correlated with the difference between nadir and oblique radiative temperatures. Based on this finding, we expressed sensible heat flux in terms of the (nadir) radiative-air temperature gradient and a corrective term involving the nadir–oblique temperature differences. This formulation has been successfully tested. The resulting MAPD was about 33%.

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David C. Goodrich

Agricultural Research Service

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David J. Gochis

National Center for Atmospheric Research

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Russell L. Scott

Agricultural Research Service

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Yann Kerr

University of Toulouse

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