Enrique R. Vivoni

Flow structure in depth‐limited, vegetated flow

Aquatic vegetation controls the mean and turbulent flow structure in channels and coastal regions and thus impacts the fate and transport of sediment and contaminants. Experiments in an open-channel flume with model vegetation were used to better understand how vegetation impacts flow. In particular, this study describes the transition between submerged and emergent regimes based on three aspects of canopy flow: mean momentum, turbulence, and exchange dynamics. The observations suggest that flow within an aquatic canopy may be divided into two regions. In the upper canopy, called the “vertical exchange zone”, vertical turbulent exchange with the overlying water is dynamically significant to the momentum balance and turbulence; and turbulence produced by mean shear at the top of the canopy is important. The lower canopy is called the “longitudinal exchange zone” because it communicates with surrounding water predominantly through longitudinal advection. In this region turbulence is generated locally by the canopy elements, and the momentum budget is a simple balance of vegetative drag and pressure gradient. In emergent canopies, only a longitudinal exchange zone is present. When the canopy becomes submerged, a vertical exchange zone appears at the top of the canopy and deepens into the canopy as the depth of submergence increases.

Precipitation Recycling Variability and Ecoclimatological Stability—A Study Using NARR Data. Part II: North American Monsoon Region

This work studies precipitation recycling as part of the dynamic North American monsoon system (NAMS) to understand how moisture and energy fluxes modulate recycling variability at the daily-tointraseasonal time scale. A set of land–atmosphere variables derived from North American Regional Reanalysis (NARR) data are used to represent the hydroclimatology of the monsoon. The recycling ratio is estimated using the Dynamic Recycling Model, which provides recycling estimates at the daily time scales. Multichannel singular spectrum analysis (M-SSA) is used to extract trends in the data while at the same time selecting only the variability common to all of the variables. The 1985–2006 climatological analysis of NAMS precipitation recycling reveals a positive feedback mechanism between monsoon precipitation and subsequent increase in precipitation of recycled origin. Recycling ratios during the monsoon are consistently above 15% and can be as high as 25%. While monsoon precipitation and evapotranspiration are predominantly located in the seasonally dry tropical forests in the southwestern part of the domain, recycling is enhanced northeast of this region, indicating a relocation of soil moisture farther inland to drier regions in the northeast. The three years with the longest monsoons in the 22-yr period present an asynchronous pattern between precipitation and recycling ratio. The longest monsoons have a characteristic double peak in precipitation, with enhanced recycling ratios during the intermediate dry period. This indicates that, even when large-scale moisture advection decreases, evapotranspiration provides moisture to the overlying atmosphere, contributing to precipitation. Through the negative feedback present during long monsoons and by relocation of soil moisture, precipitation recycling brings favorable conditions for vegetation sustenance in the NAMS region.

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

] 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.

GIS-based NEXRAD Stage III precipitation database: automated approaches for data processing and visualization

This study develops a geographical information system (GIS) approach for automated processing of the Next Generation Weather Radar (NEXRAD) Stage III precipitation data. The automated processing system, implemented by using commercial GIS and a number of Perl scripts and C/C++ programs, allows for rapid data display, requires less storage capacity, and provides the analytical and data visualization tools inherent in GIS as compared to traditional methods. In this paper, we illustrate the development of automatic techniques to preprocess raw NEXRAD Stage III data, transform the data to a GIS format, select regions of interest, and retrieve statistical rainfall analysis over user-defined spatial and temporal scales. Computational expense is reduced significantly using the GIS-based automated techniques. For example, 1-year Stage III data processing (~9000 files) for the West Gulf River Forecast Center takes about 3 days of computation time instead of months of manual work. To illustrate the radar precipitation database and its visualization capabilities, we present three application examples: (1) GIS-based data visualization and integration, and ArcIMS-based web visualization and publication system, (2) a spatial-temporal analysis of monsoon rainfall patterns over the Rio Grande River Basin, and (3) the potential of GIS-based radar data for distributed watershed models. We conclude by discussing the potential applications of automated techniques for radar rainfall processing and its integration with GIS-based hydrologic information systems.

Connectivity in dryland landscapes: shifting concepts of spatial interactions

Dryland ecosystems are often characterized by patchy vegetation and exposed soil. This structure enhances transport of soil resources and seeds through the landscape (primarily by wind and water, but also by animals), thus emphasizing the importance of connectivity – given its relation to the flow of these materials – as a component of dryland ecosystem function. We argue that, as with the fertile-islands conceptual model before it, the concept of connectivity explains many phenomena observed in drylands. Further, it serves as an organizing principle to understand dryland structure and function at scales from individual plants to entire landscapes. The concept of connectivity also helps to organize thinking about interactions among processes occurring at different scales, such as when processes at one scale are overridden by processes at another. In these cases, we suggest that state change occurs when fine-scale processes fail to adjust to new external conditions through resource use or redistribution at the finer scale. The connectivity framework has practical implications for land management, especially with respect to decision making concerning the scale and location of agricultural production or habitat restoration in the worlds drylands.

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

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...

Extending the Predictability of Hydrometeorological Flood Events Using Radar Rainfall Nowcasting

Abstract The predictability of hydrometeorological flood events is investigated through the combined use of radar nowcasting and distributed hydrologic modeling. Nowcasting of radar-derived rainfall fields can extend the lead time for issuing flood and flash flood forecasts based on a physically based hydrologic model that explicitly accounts for spatial variations in topography, surface characteristics, and meteorological forcing. Through comparisons to discharge observations at multiple gauges (at the basin outlet and interior points), flood predictability is assessed as a function of forecast lead time, catchment scale, and rainfall spatial variability in a simulated real-time operation. The forecast experiments are carried out at temporal and spatial scales relevant for operational hydrologic forecasting. Two modes for temporal coupling of the radar nowcasting and distributed hydrologic models (interpolation and extended-lead forecasting) are proposed and evaluated for flood events within a set of nes...

Real-time streaming of environmental field data

Field measurements in the environmental sciences still depend upon the pencil and paper notebook for data collection. Although robust, this method is labor-intensive and susceptible to recording and georeferencing errors during transcription. Recent advances in mobile computing and wireless communications allow the geoscientist to process and transmit data while still in the field, thereby minimizing human errors and time delays. We describe an integrated system developed for environmental and geolocation data acquisition that is intended to streamline the collection process. The system consists of software applications and hardware components that enable wireless, mobile and Internet computing during field campaigns. In particular, two-way transfer and display of collected data is achieved between the field site and a remote location, a concept referred to as field data streaming. A prototype system has been tested in field trials in Cambridge, Massachusetts, USA and Newcastle, New South Wales, Australia. Field studies demonstrate the noticeable gains in efficiency and precision achieved with the use of the field streaming technology. Potential applications include biogeochemical and hydrologic studies, water quality monitoring, emergency response to water-borne disasters and intensive field sampling campaigns.

Submesoscale Spatiotemporal Variability of North American Monsoon Rainfall over Complex Terrain

Abstract The authors analyze information from rain gauges, geostationary infrared satellites, and low earth orbiting radar in order to describe and characterize the submesoscale (<75 km) spatial pattern and temporal dynamics of rainfall in a 50 km × 75 km study area located in Sonora, Mexico, in the periphery of the North American monsoon system core region. The temporal domain spans from 1 July to 31 August 2004, corresponding to one monsoon season. Results reveal that rainfall in the study region is characterized by high spatial and temporal variability, strong diurnal cycles in both frequency and intensity with maxima in the evening hours, and multiscaling behavior in both temporal and spatial fields. The scaling parameters of the spatial rainfall fields exhibit dependence on the rainfall rate at the synoptic scale. The rainfall intensity exhibits a slightly stronger diurnal cycle compared to the rainfall frequency, and the maximum lag time between the two diurnal peaks is within 2.4 h, with earlier pe...

Predictability of Evapotranspiration Patterns Using Remotely Sensed Vegetation Dynamics during the North American Monsoon

AbstractThe links between vegetation, evapotranspiration (ET), and soil moisture (SM) are prominent in western Mexico—a region characterized by an abrupt increase in rainfall and ecosystem greenup during the North American monsoon (NAM). Most regional-scale land surface models use climatological vegetation and are therefore unable to capture fully the spatiotemporal changes in these linkages. Interannually varying and climatological leaf area index (LAI) were prescribed, both inferred from the space-borne Moderate Resolution Imaging Spectroradiometer (MODIS), as the source of vegetation parameter inputs to the Variable Infiltration Capacity (VIC) model applied over the NAM region for 2001–08. Results at two eddy covariance tower sites for three summer periods were compared and evaluated. Results show that both vegetation greening onset and dormancy dates vary substantially from year to year with a range of more than half a month. The model using climatological LAI tends to predict lower (higher) ET than t...