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 Science Results (Winter 2006)headline bar **Field application performance of multiple soil moisture sensors**  Paige, G.B.University of Wyoming Keefer, T.O.SouthwestWatershedResearchCenter  Various types of soil moisture sensors have been developed for automated and unattended use for scientific and land management applications.These sensors are being installed and used for a wide range of soil moisture applications such as drought forage prediction or validation of large scale remote sensing instruments.The convergence of three different research projects at the USDA-ARS Walnut Gulch Experimental Watershed in southeastern Arizona facilitated the evaluation and comparison of three commercially available soil moisture probes under field application conditions.Previous studies have compared multiple sensors under laboratory or controlled field conditions, or compared individual sensors to physically measured soil moisture, however, few have directly compared the performance of different probes under field applications as they are being employed for long term monitoring of soil moisture.The sensors all use an electronic technique to indirectly measurethe soil moisture content.Sensor responses are evaluated over a range of conditions using water balance and infiltration simulation models.Each of the sensors responded to the majority of precipitation events; however, they varied greatly in response time and magnitude from each other.The results of a soil moisture computer simulation model were compared to the measured sensor values, but no distinct or consistent trend was detected.The results of this analysis underscore the need to recognize the limitations of soil moisture sensors and the factors that can affect their accuracy in predicting soil moisture in situ.  **Long-term meteorological and soil-dynamics database WGEW, Arizona, USA**  Keefer, T.O.SouthwestWatershedResearchCenter Moran, M.S.SouthwestWatershedResearchCenter Paige, G.B.University of Wyoming  The USDA-ARSSouthwestWatershedResearchCenter has accumulated weather data and soil-dynamics data over a seventeen year (1990-2006) period at the Walnut Gulch Experimental Watershed in southeastern Arizona.Data have been acquired at multiple locations including three automated weather stations, five soil-profile trench sites and nineteen locations dispersed across the watershed co-located with recording raingages.Weather elements measured at the weather stations include air temperature, relative humidity, wind speed, wind direction, solar radiation, net radiation, photosynthetically active radiation and barometric pressure. Soil-dynamic properties measured at the weather stations, trenches and raingages include soil moisture, soil temperature, soil-heat flux and soil-surface temperature.Weather elements consist of two sub-daily sampling periods, one hour from 1990 to 1996 and 20 minute from 1996 to current.Soil-dynamics records are of multiple sampling frequencies ranging from bi-weekly to 20 minute, at surface, near-surface (5 cm) or in soil profiles to 2 m.This dataset offers researchers access to valuable information for management, modeling and analysis of the hydrometeorology of rangelands of the southwestern US.These data can be accessed from the United States Department of Agriculture, Agricultural Research Service, SouthwestWatershedResearchCenter database ftp site (ftp://www-ftp.tucson.ars.ag.gov/databases).  **Multiyear riparian evapotranspiration and groundwater use for a semiarid watershed**  Scott, R.L.SouthwestWatershedResearchCenter Cable, W.L.SouthwestWatershedResearchCenter Huxman, T.E.University of Arizona Nagler, P.L.USGS Goodrich, D.C.SouthwestWatershedResearchCenter  Riparian corridors provide valuable habitat in semiarid regions such as the southwestern United States. Rural and urban development often impact the vitality of riparian areas by changing land use and by diverting water and lowering the water table. Accurate information about riparian water use is needed to improve water budgets within these basins so that stakeholders will have accurate information to make water resource management decisions. This scientific paper reports on efforts to estimate annual riparian water use for the Sierra Vista Subwatershed in southeastern Arizona.Riparian water use was determined by developing a model that coupled remote sensing data from satellites with local evaporation measurements.The model was then applied over the entire riparian region within the watershed determine evaporation and groundwater use for the years 2001 through 2005. This paper provides the first yearly estimates of groundwater use by riparian vegetation in the SanPedroValley, which in turn will improve the groundwater budgets used by scientists and management agencies to better understand the regions water resources. It also provides a provocative approach that improves scientific understanding of how better to estimate riparian water use in other basins.  **Ecological sites on Walnut Gulch**  Heilman, P.SouthwestWatershedResearchCenter Stone, J.J.SouthwestWatershedResearchCenter Robinett, D.NRCS  The West is changing. Land management objectives of publicly owned rangelands will change to reflect the interests of a different, and larger, population. Hydrologists in action agencies will increasingly be asked to quantify the expected effects of rangeland management on hydrologic variables as part of interdisciplinary teams, based on rangeland management concepts. A key rangeland management concept is a land unit with the ability to produce a given vegetation community, known as an ecological site. This paper lists and maps the ecological sites on Walnut Gulch and describes the two most common sites. Ecological sites are a fairly mature concept, but state and transition models for sites are relatively new. A state and transition model for the Loamy Upland ecological site is provided. Researchers interested in ecohydrology or improving rangeland management at the ranch or small watershed scale should explore how to incorporate ecological sites into their work.  **Partitioning Evapotranspiration in Semiarid Grassland and Shrubland Ecosystems Using Diurnal Surface Temperature Variation**  Moran, M.S.SouthwestWatershedResearchCenter Soctt, R.L.SouthwestWatershedResearchCenter Keefer, T.O.SouthwestWatershedResearchCenter Paige, G.B.University of Wyoming Emmerich, W.E.SouthwestWatershedResearchCenter Cosh, M.H.USDA-ARS Hydrology and Remote Sensing Lab O'Neill, P.E.NASA  Encroachment of woody plants in grasslands has become a common phenomenon across the Western U.S. over the past 150 years.Shrublands have a different water demand from grasslands, manifesting in different water availability for plant transpiration (T).In turn, these shifts in evaporation (E) versus T related to vegetation change can impact management strategies in dryland ecosystems associated with land use and climate change.To study this phenomenon, a network of towers is in place to measure evapotranspiration (ET) in grass- and shrub-dominated ecosystems throughout the Western U.S.A method is described and tested here to partition the daily measurements of ET into E and T based on simple measurements of surface temperature.The method was tested at Walnut Gulch Experimental Watershed in southeast Arizona over three years, 2003-2005.Results showed that reasonable estimates of daily T were obtained for a multi-year period with ease of operation and minimal cost.With known season-long daily T, E and ET, it is possible to determine the soil water availability associated with grass- and shrub-dominated sites and better understand the hydrologic impact of regional woody plant encroachment.  **Value of long-term data collection for understanding and predicting ecosystem dynamics**  Moran, M.S.SouthwestWatershedResearchCenter Peters, D.C.USDA-ARS Jornada Experimental Range McClaren, M.P.University of Arizona Nichols, M.H.SouthwestWatershedResearchCenter  Federally established watersheds, rangelands and forests have produced long-term records of soil, water, and vegetation conditions that span decades and centuries across the U.S.Why is this important?Understanding the landscape response to disturbance allows resource managers to develop realistic goals for long-term site stability and restoration.Understanding historic variability helps us manage current, limited resources.The goal of this review is to express the value of such long-term data for understanding and predicting ecosystem dynamics and the importance of continued long-term data collection.The basic conclusion is that the current understanding of ecology would simply not be possible without these multi-decadal datasets.As a result, long-term data have influenced management strategies for the most important activities and events affecting our natural resources, including livestock grazing, erosion control, logging, urbanization, disease, flood, drought, fire, desertification, and non-native plant invasion.There is a movement across the U.S. to develop a network of networks to study trends in both time and space.New long-term data collection efforts have been designed to coordinate with existing networks, and existing long-term data collection networks have adapted their measurements to address new science issues.This flexibility and foresight has made, and continues to make, long-term data collection sustainable, relevant and inherently valuable.  **Precipitation thresholds for CO2 uptake in grass and shrub plant communities on Walnut Gulch Experimental watershed**  Emmerich, W.E.SouthwestWatershedResearchCenter Verdugo, C.LSouthwestWatershedResearchCenter  Precipitation is the driving force for plant growth and carbon uptake from the atmosphere in arid environments.The timing, frequency, and amount of precipitation influence how grass and shrub communities grow.Precipitation thresholds for a carbon dioxide uptake response for a shrub and grass plant community were determined.Precipitation timing and frequency were found to be more important than amount for thresholds and for total carbon uptake.This study will allow scientists to better understand how shrub and grass plant communities interact and estimate what plant communities will prosper and which will not with climate change.   **Long-term carbon dioxide and water flux database, Walnut Gulch Experimental Watershed, Arizona USA**  Emmerich, W.E.SouthwestWatershedResearchCenter Verdugo, C.LSouthwestWatershedResearchCenter  The American Geophysical Union has a new thrust to make high-quality data available for research by publishing metadata for a data set that will be available on the web in an AGU-approved web site.This is one of a set of thirteen manuscripts describing data from fifty years of research and data collection at the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona, USA.This work describes carbon dioxide and water flux data along with associated meteorological data collected on the WGEW from 1997 through 2006.This report provides background information on the collection and archiving of this data set, and contact information for obtaining copies of the data files.Data and metadata are available via the U.S. Department of Agriculture, Agricultural Research Service, Southwest Watershed Research Center (SWRC) at http://www.tucson.ars.ag.gov/dap/.  **Geomorphic and physiographic characteristics and processes of the Walnut Gulch Experimental Watershed, Arizona, United States**  Osterkamp, W.R.USGS Nichols, M.H.SouthwestWatershedResearchCenter  Knowledge of landscape features and forms, and their relation to underlying geology and overlying soils is important for understanding how hydrologic and erosion processes shape a watershed. This manuscript describes large and small scale landforms on The Walnut Gulch Experimental Watershed, southeastern Arizona, USA. The watershed is located in the Basin and Range Physiographic Province. Major geomorphic features of the watershed are results of tectonics, mostly long-term faulting, and erosion processes that have displaced and modified rocks ranging in age from Precambrian to Recent. Channel network development and geologic controls on erosion in the watershed are described. Finally, recent erosion and geomorphic research is described.This information is useful for understanding the potential for land management to alter the current landscape.  **The Southwest Watershed Research Center Data Access Project**  Nichols, M.H.Southwest Watershed Research Center Anson, E.Zardick Computing  The American Geophysical Union has a new thrust to make high-quality data available for research by publishing metadata for a data set that will be available on the web in an AGU-approved web site.This manuscript presents an overview of the USDA-ARS Southwest Watershed Research Center's web-based data access project. The goal of the project is to promote analyses and interpretations of historic and current data by improving data access. Access to hydrologic data, including rainfall and runoff data, collected on experimental watersheds operated by the Agricultural Research Service in southern Arizona is provided through a public website (http://tucson.ars.ag.gov/dap). The website was released to the public in October 2003, and since that time the online data access website has received more that 4500 visitors.  **Geomorphic adjustments of Walnut Gulch, Arizona, USA 1935 - 2005**  Nichols, M.H.Southwest Watershed Research Center Renard, K.G.Retired ARS  Channels convey both water and sediment. As runoff travels through a channel, the channel width and depth adjust by scouring and filling in response to the forces imparted by the water and the sediment. Sediment deposits can become colonized by vegetation that may stabilize the channel bottom. Changes in channel width and depth, and the evolution of vegetation islands were evaluated along the main channel on the Walnut Gulch Experimental Watershed in southeastern Arizona. From 1935 to 1974 the area occupied by vegetation in the channel decreased 7 %. From 1974 to 2005, the area occupied by vegetation increased 79%. In addition, adjustments to channel width and depth have resulted in a decrease in shear forces on the channel when runoff is conveyed. These changes were compared to runoff measured measurements. From 1974 through 2000, there was a decrease in the number and magnitude of measured runoff events in comparison with runoff measured from the late 1950s through 1973. Understanding how channels in semiarid watersheds change in response to runoff is important for making land management decisions in the face of both drought and periods of above average rainfall and runoff.  **Sediment database, Walnut Gulch Experimental Watershed, Arizona, United States**  Nichols, M.H.Southwest Watershed Research Center Stone, J.J.Southwest Watershed Research Center Nearing, M.A.Southwest Watershed Research Center  The American Geophysical Union has a new thrust to make high-quality data available for research by publishing metadata for a data set that will be available on the web in an AGU-approved web site.This is one of a set of thirteen manuscripts describing data from fifty years of research and data collection at the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona, USA. This manuscript describes sediment data collected on the WGEW. Sampling instruments and techniques are described, including the traversing slot sediment sampler, pump samplers on both v-notch weirs and H-flumes, and sedimentation surveys of stock tanks. Sediment data are used by scientists and others to understand erosion and sedimentation processes within semiarid rangeland watersheds. Data are available via the U.S. Department of Agriculture, Agricultural Research Service, Southwest Watershed Research Center at http://www.tucson.ars.ag.gov/dap/ .  **Long-term remote sensing database, Walnut Gulch Experimental Watershed, Arizona, USA**  Moran, M.S.Southwest Watershed Research Center Holifield Collins, C.Southwest Watershed Research Center Goodrich, D.C.Southwest Watershed Research Center Qi, J.Michigan State University Shannon, D.T.Programmer, Tucson, AZ  The American Geophysical Union (AGU) has a new thrust to make high-quality data available for research by publishing metadata for a data set that will be available on the web in an AGU-approved web site.This is one of a set of thirteen manuscripts describing data from fifty years of research and data collection at the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona, USA.This work describes a collection of images from satellite- and aircraft-based sensors dating back to 1990 with ancillary ground-based measurements archived with each image.This report provides background information on the collection and archiving of this data set, and contact information for obtaining copies of the image and data files.Many images are available in the University of Arizona, Arizona Regional Image Archive (ARIA) at http://aria.arizona.edu.Metadata are available via the U.S. Department of Agriculture, Agricultural Research Service, Southwest Watershed Research Center (SWRC) at http://www.tucson.ars.ag.gov/dap/ .  **Using Remotely-Sensed Estimates of Soil Moisture to Infer Soil Texture and Hydraulic Properties across a Semi-arid Watershed**  Santanello, Jr., J.A.UMCP & NASA-GSFC Peters-Lidard, C.D.NASA-GSFC Garcia, M.E.NASA-GSFC Mocko, D.M.NASA-GSFC Tischler, M.A.US ARMY Moran, M.S.Southwest Watershed Research Center Thoma, D.P.University of Arizona  Soil moisture is a critical component of atmospheric, land-surface, and hydrologic models that impacts weather forecasts on daily to seasonal timescales.Because detailed maps of soil properties are difficult to obtain, approximations of general soil types and properties are routinely implemented in forecast models and lead to incorrect predictions of soil moisture andn meteorological conditions.In this study, the problem of estimating soil moisture and soil properties is approached from a unique perspective.The first testbed for this experiment is the Walnut Gulch Experimental Watershed in southeastern Arizona, where 6 daily estimates of near-surface soil moisture across the watershed were derived from passive microwave data using established techniques.Then, a land-surface model was run to determine which soil types and properties are required in the model to simulate the soil moisture conditions that match those from satellite.By adjusting the sand, clay, and silt contents (i.e. the properties that control the flow of moisture) of the soil in a physically consistent manner, errors in model simulated versus observed soil moisture were minimized.Using the resultant soil types to simulate another time period clearly demonstrates the improvement in soil moisture simulations over those using coarse or default soil property maps.Results also show that this methodology can be successful with as few as 2 satellite overpasses that capture the typical range of soil moisture variability for a given region.Overall, this study demonstrates the potential to gain physically meaningful and much-needed soils information at high-resolution using few but appropriately timed satellite retrievals of soil moisture in models.Ultimately, such information will improve weather forecasts on daily to climatic timescales.  **Vegetation data, Walnut Gulch Experimental Watershed, Arizona, USA**  Skirvin, S.University of Arizona Kidwell, M.R.Tucson, AZ Biedenbender, S.H.USFS Henley, F.P.US ARMY King, D.M.Southwest Watershed Research Center Holifield Collins, C.Southwest Watershed Research Center Moran, M.S.Southwest Watershed Research Center Weltz, M.A.Southwest Watershed Research Center  The American Geophysical Union has a new thrust to make high-quality data available for research by publishing metadata for a data set that will be available on the web in an AGU-approved web site.This is one of a set of thirteen manuscripts describing data from fifty years of research and data collection at the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona, USA.In this work, we described the detailed WGEW vegetation data collected at over 50 permanent transects in 1967, 1994-2000, and 2005, including repeat photography of transects. A vegetation map was developed in 2002 and is illustrated here.This is the first published description of the detailed WGEW vegetation data and we are pleased to be able to make this resource available to researchers and other interested parties.Data are available at http://www.tucson.ars.ag.gov/dap/ .  **The history of ARS Watershed Research and Modeling in Arizona and New Mexico**  Renard, K.G.Retired ARS Nichols, M.H.Southwest Watershed Research Center Woolhiser, D.A.Retired ARS Osborn, H.B.Retired ARS  Watershed research is important for understating rainfall patterns and their relation to runoff that carries sediment to downstream points. In the southwestern US, these processes are difficult to measure. Early attempts to collect data were impeded by limitations of measurement instruments that were developed for more humid areas. Field measurements are critical for understanding watershed processes. Characterizing these processes requires long term data records. This manuscript describes the evolution of watershed research in non-forested regions of the southwestern US, with a focus on semiarid rangelands. Instrumented watersheds in Arizona and New Mexico are described. The development of specialized supercritical runoff measuring flume, and the limitations of other runoff measuring methods are described. Watershed data have been used to develop natural resource models that are important tools for land management decision making. This manuscript provides an historic perspective on ARS watershed research in the southwestern US.  **Sediment transfer and storage in dryland headwater streams**  Powell, D.M.Univ. of Leicester Brazier, R.Univ. of Sheffield Parsons, A.Univ. of Leicester Wainwright, J.Univ. of Sheffield Nichols, M.H.Southwest Watershed Research Center  Sediment is conveyed through normally dry channels in the southwestern US in response to summer thunderstorms. The sediment may come from surrounding hillslopes, or from the channel bed. During high velocity flash flows, sediment on the channel bed is scoured and transported. As the flow recedes, sediment is deposited. A study was conducted on the Walnut Gulch Experimental Watershed to measure scour and fill patterns in three small channels. Scour chains were buried and after each flow, they were measured to determine the depth of scour or fill. Individual chain measurements were used to compute the total volume of scour and fill. The total amount of scour and fill was found to be related to flood magnitude.Scour and fill at any given point were found to be compensating and overall, the channels were not aggrading or degrading during the study period. These results are useful for understanding how sediment is transferred from watershed uplands to downstream receiving waters, and are of particular interest to those concerned about water quality.  **Appropriate scale of soil moisture retrieval from high-resolution radar imagery for bare and minimally vegetated soils**  Thoma, D.National Park Service Moran, M.S.Southwest Watershed Research Center Bryant, R.Statistical Research Inc. Rahman, M.Saskatchewan Environment Planning and Risk Analysis Division Holifield Collins, C.Southwest Watershed Research Center Keefer, T.O.Southwest Watershed Research Center Noriega, R.University of Arizona Osman, I.University of Arizona Skirvin, S.University of Arizona Tischler, M.US ARMY Bosch, D.Southeast Watershed Research Center Starks, P.Great Plains Agroclimate and Natural Resources Research Unit Peters-Lidard, C.NASA-GSFC  The distribution of near-surface soil moisture is an important factor in a wide range of decision making related to flood prediction, prescribed burns, animal stocking rates, rangeland health and off road trafficability.It is possible to map surface soil moisture using satellite sensors that measure the radar backscatter from a non-vegetated soil surface.Studies have reported mixed results from this satellite-based approach because it appears that the accuracy depends on the scale of the map.This study offers a protocol for determining the minimum spatial resolution for soil moisture retrieval from radar imagery with known confidence.It is an image-based approach that doesn't require any a-priori information about the surface.This work explained the poor performance of soil moisture retrieval approaches using SAR imagery at site scales and the good performance at watershed scales.This opens the door to use of soil moisture maps for many resource management applications that require both fine spatial resolution and high input accuracy.  **Prognosis for Upper San Pedro River (USA): Can the Riparian Ecosystem be Sustained?**  Stromberg, J.Arizona State University Dixon, M.D.Arizona State University Scott, R.L.Southwest Watershed Research Center Maddock, T.University of Arizona Barid, K.University of Arizona Tellman, B.University of Arizona  Rivers are products of their watersheds. Thus, a riparian preserve can be affected by off-site activities that alter the hydrologic cycle. This issue is of increasing national and global concern and is exemplified by the case of the Upper San Pedro River.Much of the Upper San Pedro watershed is under state and private ownership and is steadily urbanizing. There are concerns that the riparian ecosystem is being affected by human actions, notably groundwater pumping, occurring beyond the riparian conservation area borders.In this chapter we review scientific issues that underpin the San Pedro riparian conservation challenge. We also summarize results of modeling exercises that project effects of watershed development on riparian groundwater levels and stream flow, and effects of hydrologic change on riparian vegetation; such studies can help to determine workable solutions to water management challenges. We conclude by discussing the role of long-term monitoring in determining whether management actions are achieving desired outcomes.  **Long-term precipitation database, Walnut Gulch Experimental Watershed, Arizona, United States**  Goodrich, D.C.Southwest Watershed Research Center Keefer, T.O.Southwest Watershed Research Center Unkrich, C.L.Southwest Watershed Research Center Nichols. M.H.Southwest Watershed Research Center Osborn, H.B.Retired ARS Stone, J.J.Southwest Watershed Research Center Smith, J.R.Southwest Watershed Research Center  The American Geophysical Union has a new thrust to make high-quality data available for research by publishing metadata for a data set that will be available on the web in an AGU-approved web site. This is one of a set of thirteen manuscripts describing data from fifty years of research and data collection at the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona, USA. This work describes the long-term precipitation observations made at the WGEW, the network of raingages used to make these observations, and the associated data processing and quality control procedures employed. Several examples of the spatial and temporal characteristics of long-term precipitation observations are presented as well as a partial list of uses of these observations in a variety of research endeavors.All the data are available via the U.S. Department of Agriculture, Agricultural Research Service, Southwest Watershed Research Center (SWRC) at http://www.tucson.ars.ag.gov/dap/.  **Transpiration by seepwillow (Baccharis salicifolia) shrubs under a semiarid riparian forest canopy**  McGuire, R.R.Southwest Watershed Research Center Scott, R.L.Southwest Watershed Research Center  An important part of the water budget in arid and semi-arid watersheds is evapotranspiration (ET) by riparian vegetation. While previous studies have examined riparian ET at the canopy level, understory riparian vegetation has been overlooked due to the belief that this quantity was negligible. We used sensor technology to measure whole plant transpiration by the understory woody shrub seepwillow under two environments: shrubs located between cottonwood canopy cover ("open"); and shrubs directly beneath a dense cottonwood canopy ("closed") at a perennial reach of the San Pedro River in southeastern Arizona. Although atmospheric conditions were different at both locations, transpiration levels were very similar and seldom restricted by climate, with total seepwillow transpiration measured at 448 mm during this study. Transpiration by seepwillow shrubs during the growing season (DOY 91-309) estimated using a simple empirical model, yielded 819 mm which was well in excess of precipitation totals of 188 mm and similar to that for dominant riparian cottonwood trees (966 mm). These results suggest that transpiration by riparian understory vegetation can be a significant contributor to the riparian water budget, especially in regions like the western U.S. where evaporative demand is often high.  **GIS Database, Walnut Gulch Experimental Watershed, Arizona, USA**  Heilman, P.Southwest Watershed Research Center Nichols, M.H.Southwest Watershed Research Center Goodrich, D.C.Southwest Watershed Research Center Miller, S.N.University of Wyoming Guertin, D.P.University of Arizona  This paper describes the spatial information available about the Walnut Gulch Experimental Watershed. The data are available as Geographic Information System (GIS) files. Available layers are listed, and each layer is described. All data are available as either images or separate GIS layers (vector or raster format) via the U.S. Department of Agriculture, Agricultural Research Service, Southwest Watershed Research Center (SWRC) at http://www.tucson.ars.ag.gov/dap/.  **Hydrogeomorphic Response to an Extreme Precipitation Event in Southern Arizona**  Magril, C.S.USGS Webb, R.H.USGS Schaffner, M.NWS Lyon, S.W.University of Arizona Griffiths, P.G.USGS Shoemaker, C.NWS Unkrich, C.L.Southwest Watershed Research Center Yatheendradas, S.University of Arizona Troch, P.A.University of Arizona Pytlak, E.NWS Goodrich, D.C.Southwest Watershed Research Center Desilets, S.L.E.University of Arizona Youberg, A.AZ State Geological Survey Pearthree, P.A.AZ State Geological Survey  Heavy rainfall on July 27-31, 2006, led to record flooding and an unprecedented outbreak of debris flows in the Santa Catalina Mountains north of Tucson, Arizona. At least 240 hillslope failures spawned debris flows in an area where no more than 10 had been documented historically. More than 18 debris flows destroyed infrastructure in the heavily used Sabino Canyon Recreation Area. In three adjacent canyons, debris flows reached the heads of alluvial fans on the edge of the Tucson metropolitan area. While land-use planners and water-resource managers in southeastern Arizona evaluate the potential of this previously-discounted hazard to development along the mountain front, an interdisciplinary group of scientists has collaborated to better understand this extreme event. The study describes the results of these extreme events and it was also found that a watershed model developed by the ARS was successful in simulating record runoff event with high-quality ground-based rainfall data.However, more research is needed to understand the hazard from debris-flow activity in southeastern Arizona, especially with respect to the frequency of debris-flows and excessive sedimentation, which may compromise flood-control structures designed without accommodation for rapid channel filling.  **Spatial interpolation of precipitation in a dense gauge network for monsoon storm events in the southwestern U.S.**  Garcia, M.University of Maryland Peters-Lidard, C.D.NASA Goddard Goodrich, D.C.Southwest Watershed Research Center  Arid and semi-arid regions account for approximately one-third of the land mass of earth.These regions are experiencing continued pressure from population growth in many parts of the world.Water is a critical resource in these regions and is often in short supply.Detailed study of water resources and the hydrology of semi-arid regions is important if we are to continue to populate and use these regions. In many cases rainfall in these area varies greatly from one place to another. Inaccuracy in the rainfall can directly impact our ability to predict runoff, soil moisture, evaporation and plant water use. This paper examines the results ofmethods to interpolate monsoon season rainfall over the USDA-ARS Walnut Gulch Experimental Watershed in the SE Arizona which has a high density of rain gauges. Several methods were tested and it was found that the most commonly used method for operational analyses and for engineering assessments, is inferior to the slightly more sophisticated methods tested.Using these methods we can better estimate spatial rainfall patterns and improve the prediction of runoff for both flooding and water yield.  **Sensitivity of mesquite shrubland carbon exchange to precipitation in contrasting****physiographic settings**  Potts, D.L.UC Irvine Scott, R.L.Southwest Watershed Research Center Cable, J.M.University of Wyoming Huxman, T.E.University of Arizona Williams, D.G.University of Wyoming  The ranges of woody plants have expanded in many semiarid areas around the globe. With this change in ecosystem structure, associated changes in how ecosystems cycle carbon and water are expected but largely unknown.This study examined the sensitivity of the carbon cycling in ecosystems invaded by the woody shrub, mesquite, to precipitation in southern Arizona.The sensitivity was examined for a range of shrub sizes and in a riparian and upland setting. For mesquite plants in the upland setting, water status and photosynthetic gas exchange were closely linked to the onset of the North American monsoon. In contrast, the smallest size classes of riparian mesquite were physiologically responsive to monsoonal rainfall while the larger size classes of riparian mesquite were physiologically insensitive to monsoonal rains suggesting they were accessing alluvial groundwater. Soil respiration was greatest beneath mesquite canopies and was coupled to shallow soil moisture associated with the monsoon. These results show that woody plant encroachment in riparian areas will have a bigger impact on ecosystem carbon cycling due to the deeper roots of shrubs which access the groundwater more readily.   **Simulated Changes in Runoff and Sediment in Developing Areas Near Benson, Arizona**  Levick, L.R.University of Arizona Semmens, D.US-EPA Goodrich, D.C.Southwest Watershed Research Center Kepner, W.G.US-EPA Brush, J.US-EPA Leidy, R.A.US-EPA Goldmann, E.US-EPA  The unprecedented speed and scale of development throughout the American Southwest presents special challenges to the review and permitting process as required under the Clean Water Act (CWA) and the National Environmental Policy Act (NEPA).Many areas undergoing rapid development are in arid and semiarid regions whose watersheds and associated streams exhibit ephemeral or intermittent flow.The standard process for CWA permitting for new development rarely considers the special attributes and circumstances encountered in these environments.In addition, rapid urbanization can present a challenge in assessing cumulative impacts when permitting is conducted piecemeal over multiple parcels in the same region.The study area, located near Benson, Arizona, in the San Pedro Valley, will convert approximately 8,200 acres of previously undisturbed land that drains directly into the San Pedro River to a planned development.The San Pedro River, nationally known as being one of the last free-flowing rivers in the Southwest, is a critical migration corridor for hundreds of bird species and serves as important habitat for many other regionally-declining species of plants, fish, and wildlife.Upstream from the proposed development is the San Pedro Riparian National Conservation Area (SPRNCA), created in 1988 to protect nearly 40 miles of river and riparian area, and its biological, educational, recreational and cultural resources.Although not federally protected as an RNCA, the San Pedro River downstream (north) of the study area also contains many of the same highly valued attributes and is critical to maintaining the ecological integrity of upstream areas.Changes to the hydrology of that environment, such as increases or decreases in flow or sediment volumes, can have serious impacts on the aquatic ecosystem and the health of those areas.  This study examines the effects of development on the hydrology of this portion of the San Pedro River watershed.The proposed development resulted in substantial relative changes in runoff and sediment yield that are attributable to modifications in land cover, as predicted by the AGWA/KINEROS2 rainfall-runoff-erosion model.Consistent with established principles, the relative changes are largest for the smallest design storms and decrease with increasing design-storm size.In all cases, net runoff and sediment yield increased at the watershed outlets due to urbanization and more impermeable surfaces.Expected adverse environmental consequences from such increases may include degraded water quality from sediment and pollutant transport, erosion and alteration of the stream channel, habitat destruction, decreased biological diversity, and increased flooding.  **Intraseasonal variation in water and carbon dioxide flux components in asemiarid riparian woodland**  Yepez, E.A.University of New Mexico Scott, R.L.Southwest Watershed Research Center Cable, W.L.University of Wyoming Williams, D.G.University of Wyoming  Natural and anthropogenic changes in the climate system anticipated during this century will have large impacts on ecosystem structure and function. Within this context, there is a compelling need to understand how precipitation influences sources and sinks of carbon dioxide in seasonally dry ecosystems because these ecosystems would likely be more sensitive to changes in precipitation than to other global changes. In this paper, we investigated how the distribution of precipitation over a growing season influenced the dynamics of carbon and water cycling in semiarid riparian mesquite woodland. We made measurements of carbon dioxide exchange between the woodland and the atmosphere as well as evaporation of water and its partitioning into its component fluxes of soil evaporation, understory transpiration, and overstory transpiration.We found that in this ecosystem where the trees access groundwater, gross ecosystem production (GEP) and ecosystem respiration (Reco) were often uncoupled because of their different sensitivities to growing season rainfall.Also, the majority of the precipitation that fell on this ecosystem is returned to the atmosphere via plant uptake and transpiration.These results suggest that future changes precipitation will likely have the greatest impact on the carbon cycling of riparian woodlands with increases in precipitation likely leading to reduced carbon storage and decreases leading to storage gains.  **Using airborne lidar to predict leaf area index in cottonwood trees and refine riparian water use estimates**  Farid. A.University of Arizona Goodrich, D.C.Southwest Watershed Research Center Bryant, R.University of Arizona Sorooshian, S.University of California, Irvine  Arid and semi-arid regions account for approximately one-third of the land mass of earth. These regions are experiencing continued pressure from population growth in many parts of the world. Water is a critical resource in these regions and is often in short supply. To maintain the economic, social, and ecological viability of these areas it is essential that decision makers and resource managers have a solid scientific basis on which to make watershed based decisions including management of riparian vegetation. Riparian trees use water in proportion to their size, and are especially large users of water in flood plains along rivers in semi-arid environments. It's difficult to measure tree size and their density of leaves using traditional ground-based techniques over large areas to estimate reach level channel water water use. For this reason new techniques that are more accurate and efficient need to be developed. This study demonstrated that a lidar (light detecting and ranging) system mounted in an airplane can accurately measure features of forest canopy and leaf areas that are related to water use.Such information is not readily available from other remote sensing methods. The leaf area derived from lidar, and weather information was used in this study to estimate riparian cottonwood water use.The lidar derived estimates compared very well with those derived from ground-based methods. This information can be used in many forestry, ecological and hydrologic applications that will improve management of hydrologic resources and ecological models. **A remote sensing approach for estimating distributed daytime net carbon dioxide flux in semiarid grasslands**  Holifield Collins, C.Southwest Watershed Research Center Emmerich, W.E.Southwest Watershed Research Center Moran, M.S.Southwest Watershed Research Center Bryant, R.University of Arizona Verdugo, C.L.Southwest Watershed Research Center  Semiarid grasslands cover a significant portion of the Earth's land surface and may play an important role in absorbing atmospheric carbon dioxide (CO2).Atmospheric CO2 is increasing and the potential impact on climate change has caused an increased effort to more accurately determine how much CO2 is being released and absorbed by land surfaces.Local scale measurements of CO2 flux (release and absorption) are being made using micrometeorological instrumentation at various locations around the world, but measurements at a larger scale are needed.Information that can be obtained from satellite imagery has the potential to provide measurements at this larger scale.In this study, Landsat satellite imagery was combined with local measurements of CO2 flux to create maps of net CO2 flux estimates for the daytime period (6 a.m - 6 p.m.) over an area of semiarid grassland spanning 2,224 acres in southeastern Arizona.These regional maps have great potential as a tool for better understanding the role of semiarid grasslands in the carbon cycle.  **Assessing vegetation change temporally and spatially in Southeastern Arizona**  King, D.Southwest Watershed Research Center Skirvin, S.University of Arizona Holifield Collins, C.Southwest Watershed Research Center Moran, M.S.Southwest Watershed Research Center Biedenbender, S.USFS Kidwell, M.Tucson, AZ Weltz, M.A.USDA-ARS-NPS Diaz-Guiterrez, A.IFAPA, Cordoba Spain  Vegetation change has been occurring over the past 100 years in grasslands across the Sonoran and Chihuahan deserts in the Southwestern United States.There is a great deal of controversy about why these changes have been occurring (e.g. grazing, drought, fire, climate).As a result, end users such as cattle growers and producers, physical scientists, land management agencies and extension agents do not have the information needed to make informed decisions to protect the land and its resources.In this study, vegetation data, photography, and satellite imagery were examined to assess vegetation change and some of the factors behind its occurrence within a 150 km2 watershed in southeastern Arizona.This study showed that shrub encroachment did not appear to be occurring in the watershed over the past 100 years.We also found that while shrubs have suffered as a result of drought conditions over the last decade, grass cover has been maintained by fewer but larger, plants.Information of this nature may be beneficial to cattle growers and land management agencies as they develop management plans that help foster rangeland sustainability.  **A retrospective analysis of model uncertainty for forecasting hydrologic change**  Semmens, D.US-EPA Hernandez, M.University of Arizona Goodrich, D.C.Southwest Watershed Research Center Kepner, W.G.US-EPA  A relatively new planning process called "Alternative Futures" is being more widely applied for local and regional planning and decision making.In this process a future landscape is developed e.g. 20 years in the future) for the area of interest typically under three scenarios (e.g. restricted development, development as currently conducted, and unrestricted development).These future landscapes can then be used as input into watershed computer models to evaluate potential environmental and watershed impacts.Care must be exercised in this analysis as errors can, and typically do exist in watershed computer models.In this study, historical landscape change over a 24-year period was used to measure the watershed model error with known (i.e. observed alternative landscapes). Results demonstrate that if future land-use/cover and climate conditions are known precisely, the model does a satisfactory job of predicting observed conditions almost 25 years into the future.Unfortunately, future land-use/cover and climate conditions can never be known with certitude.The goal of regional planning efforts is to explore desired outcomes, and it is assumed that policy can be used to shape future change and guide it towards a particular outcome.As a result, climate conditions are the primary unknown in projecting future hydrologic response.Results of the present study indicate that by holding climate constant, it is possible to evaluate qualitatively the broad spatial patterns of hydrologic response to landscape change within a basin. Given the sensitivity of hydrologic response to climatic conditions, future research will focus more attention on the use of climate scenarios to characterize hydrologic response for a range of climatic conditions.  **At what scales do climate variability and land cover change impact on flooding and low flows?**  Bloschl, G.Vienna University of Technology Ardoin, S.Hydrologue IRD Bonell, M.Unesco Division Water Sciences Dorninger, M.University of Vienna Goodrich, D.C.Southwest Watershed Research Center Gutknecht, D.Vienna University of Technology Matamoros, D. FIMCM-ESPOL Merz, B.GeoForschungsZentrum Shand, P.British Geological Survey Szolgay, J.Slovak University of Technology  Land cover change and climatic variability can have profound effects on extreme hydrological events such as floods and low-flow during periods of drought.The United Nations UNESCO Division of Water Sciences has recognized the importance of these effects and has formed an expert working group to summarize the state of the science and develop key scientific challenges to identify the relative role of climatic variability and land cover change on floods and low flows as a function of watershed size. This paper summarizes a five-year research strategy and a series of workshops to test the approaches in selected basins.   **Climate variability and land cover change impact on flooding and low flows - at what scales?**  Bloschl, G.Vienna University of Technology Ardoin, S.Hydrologue IRD Bonell, M.Unesco Division Water Sciences Dorninger, M.University of Vienna Goodrich, D.C.Southwest Watershed Research Center Gutknecht, D.Vienna University of Technology Matamoros, D. FIMCM-ESPOL Merz, B.GeoForschungsZentrum Shand, P.British Geological Survey Szolgay, J.Slovak University of Technology  Land cover change and climatic variability can have profound effects on extreme hydrological events such as floods and low-flow during periods of drought.The United Nations UNESCO Division of Water Sciences has recognized the importance of these effects and has formed an expert working group to summarize the state of the science and develop key scientific challenges to identify the relative role of climatic variability and land cover change on floods and low flows as a function of watershed size. This paper summarizes a five-year research strategy and a series of workshops to test the approaches in selected basins.  **Hydrologic model uncertainty associated with simulating future land-cover/use scenarios.A Retrospective analysis**  Semmens, D.US-EPA Hernandez, M.University of Arizona Goodrich, D.C.Southwest Watershed Research Center Kepner, W.G.US-EPA  A relatively new planning process called "Alternative Futures" is being more widely applied for local and regional planning and decision making.In this process a future landscape is developed e.g. 20 years in the future) for the area of interest typically under three scenarios (e.g. restricted development, development as currently conducted, and unrestricted development).These future landscapes can then be used as input into watershed computer models to evaluate potential environmental and watershed impacts.Care must be exercised in this analysis as errors can, and typically do exist in watershed computer models.In this study, historical landscape change over a 24-year period was used to measure the watershed model error with known (i.e. observed alternative landscapes). Results demonstrate that if future land-use/cover and climate conditions are known precisely, the model does a satisfactory job of predicting observed conditions almost 25 years into the future.Unfortunately, future land-use/cover and climate conditions can never be known with certitude.The goal of regional planning efforts is to explore desired outcomes, and it is assumed that policy can be used to shape future change and guide it towards a particular outcome.As a result, climate conditions are the primary unknown in projecting future hydrologic response.Results of the present study indicate that by holding climate constant, it is possible to evaluate qualitatively the broad spatial patterns of hydrologic response to landscape change within a basin. Given the sensitivity of hydrologic response to climatic conditions, future research will focus more attention on the use of climate scenarios to characterize hydrologic response for a range of climatic conditions.  **Soils relative to geology and landforms in Walnut Gulch Experimental Watershed, Arizona, USA**  Osterkamp, W.USGS Breckenfeld, D.Retired NRCS Stone, J.J.Southwest Watershed Research Center  The American Geophysical Union has a new thrust to make high-quality data available for research by publishing metadata for a data set that will be available on the web in an AGU-approved web site.This is one of a set of thirteen manuscripts describing data from fifty years of research and data collection at the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona, USA.This work describes the soils data available for the WGEW.Relationships between the soils and geology and landforms are described.Three scales of maps and associated data bases including STATSGO (3 map units), SSURGO (18 map units), and a more detailed version of SSURGO (25 map units) specifically conducted at WGEW are available via a web interface at http://www.tucson.ars.ag.gov/dap/.  **Long-term runoff database, Walnut Gulch Experimental Watershed, Arizona, United States**  Stone, J.J.Southwest Watershed Research Center Nichols, M.H.Southwest Watershed Research Center Goodrich, D.C.Southwest Watershed Research Center Buono, J.University of Arizona  The American Geophysical Union has a new thrust to make high-quality data available for research by publishing metadata for a data set that will be available on the web in an AGU-approved web site.This is one of a set of thirteen manuscripts describing data from fifty years of research and data collection at the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona, USA.This work describes the long-term runoff data set of the WGEW.The runoff data are from 30 watersheds ranging in size from .18 to 14933 ha.This report provides background information on the type of measurement structures, period of record, data quality, and examples of use of the data.Runoff records including hydrographs and summary data are available in several formats via a web interface at http://www.tucson.ars.ag.gov/dap/.  **A multi-year, multi-gauge event based comparison of two types of automated-recording weighing-bucket raingages**  Keefer, T.O.               Southwest Watershed Research Center Unkrich, C.L.              Southwest Watershed Research Center Smith, J.R.                 Southwest Watershed Research Center Goodrich, D.C.           Southwest Watershed Research Center Simanton, J.R.           Retired ARS  An improved electronic type of recording raingage was installed at Walnut Gulch Experimental Watershed replacing the mechanical type that has been in operation there for the past 50 years.  From 2000 through 2004 the electronic and mechanical types of raingages were operated side-by-side so that an exhaustive analysis of the comparable measurements could be undertaken.  The results showed 1) that there is virtually no difference between individual electronic raingages, 2) the data processing procedures of the mechanical raingage may introduce errors affecting the measured peak rainfall intensity but only at short time intervals of less than 5 minutes, 3) the electronic and mechanical raingages are equivalent for most measures of rainfall such as depth and intensity measured over time intervals of 5 minutes or more, and 4) computer simulations of runoff will only be affected in limited cases on very small watershed areas, less than 1 acre.  **Long-term soil erodibility database, Walnut Gulch Experimental Watershed, Arizona, USA**  Rhoton, F.E.                       Water Quality and Ecology Research Emmerich, W.E.                  Southwest Waterhsed Research Center Goodrich, D.C.                    Southwest Watershed Research Center Miller, S.N.                          University of Wyoming McChesney, D.S.                Water Quality and Ecology Research  Soil erosion is generally a greater problem in semiarid environments where the depth of soil development is often minimal, vegetative cover is sparse, and soil components responsible for soil aggregate stability occur in low concentrations.  The distribution of these soil properties in watersheds varies with slope position, gradient, and aspect.  Since soil erodibility varies accordingly, an approach is needed for quantifying aggregate stability that will accurately assess soil susceptibility to runoff and erosion losses under different slope conditions at watershed scales.  We used total clay and water dispersible clay contents to calculate an aggregation index which is an indicator of soil erodibility.  Using this approach, we have demonstrated that soil erodibility can be mapped at watershed scales for purposes of identifying areas that are most erosion prone, and that may require a change in management practices.


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