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[1] This study reports sediment yields from seven small (0.18-5.42 ha) watersheds in Southern Arizona measured from 1995 to 2005. Sediment concentrations and total event sediment yields were related to storm-runoff characteristics, and statistical relationships were developed to estimate sediment yields for events with missing data. Precipitation ranged from 263 to 298 mm yr À1 , runoff ranged from 8.2 to 26.4 mm yr À1, and sediment yields ranged from 0.07 to 5.7 t ha À1 yr À1 , with an areal average of 2.2 t ha À1 yr À1 . For six of the seven watersheds, between 6 and 10 events produced 50% of the total sediment yields over the 11-year period. On the seventh watershed, two storms produced 66% of the sediment because of differences in the geomorphology and vegetation characteristics of that area. Differences between sediment yields from all watersheds were attributable to instrumentation, watershed morphology, degree of channel incision, and vegetation.

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[1] Watershed research is critical for quantifying the unique characteristics of hydrologic processes worldwide and especially in semiarid regions. In 1953, the United States Department of Agriculture established the Walnut Gulch Experimental Watershed (WGEW) near Tombstone, Arizona, to conduct hydrologic and erosion research. This manuscript (1) provides a historical context summarizing the evolution of the Southwest Watershed Research Center research program, (2) describes significant contributions to instrumentation development and contributions to science, and (3) describes the current WGEW data collection program in the context of contemporary research questions. The development of specialized flumes for streamflow measurement and the establishment of the core monitoring networks are described. WGEW data have been used to quantify semiarid rainfall, runoff, infiltration, and transmission losses; to develop and validate simulation models; and to support broader, regional, basin-scale research. Currently, rainfall, runoff, sediment, meteorology, and flux data collection continue at the WGEW, but the monitoring network has been expanded, and data use has evolved to support several multiple government agencies, universities, and international research programs.

An extensive precipitation database at the ∼149 km2 Walnut Gulch Experimental Watershed (WGEW) has been developed over the past 53 years with the first records starting in August 1953 and continuing to the present. The WGEW is a tributary of the San Pedro River, is located in southeastern Arizona, and surrounds the town of Tombstone. Average annual precipitation for the period of 1956–2005, as measured with six gauges, is roughly 312 mm, with approximately 60% falling during the summer monsoon. From a historical high of 95 rain gauges, a current network of 88 gauges is operational. This constitutes one of the densest rain gauge networks in the world (∼0.6 gauges/km2) for watersheds greater than 10 km2. Through 1999, the network consisted of analog recording weighing rain gauges. In 2000, a newly designed digital gauge with telemetry was placed adjacent (∼1 m) to the analog gauges. Both the analog and digital networks of gauges were in operation from 2000 to 2005 to enable a comparative analysis of the two systems. The analog data were digitized from paper charts and were stored in breakpoint format. The digital data consist of rainfall depths at 1‐min intervals during periods of rainfall. All these data can be obtained in a variety of formats and were accumulated over various time intervals (daily, monthly, and annual) via a web interface at http://www.tucson.ars.ag.gov/dap/.

Unmanned aerial vehicles (UAVs) provide a new research tool to obtain high spatial and temporal resolution imagery at a reduced cost. Rapid advances in miniature sensor technology are leading to greater potentials for ecological research. We demonstrate one of the first applications of UAV lidar and hyperspectral imagery and a fusion method for individual plant species identification and 3D characterization at submeter scales in south-eastern Arizona, USA. The UAV lidar scanner characterized the individual vegetation canopy structure and bare ground elevation, whereas the hyperspectral sensor provided species-specific spectral signatures for the dominant and target species at our study area in leaf-on condition. We hypothesized that the fusion of the two different data sources would perform better than either data type alone in the arid and semiarid ecosystems with sparse vegetation. The fusion approach provides 84-89% overall accuracy (kappa values of 0.80-0.86) in target species classification at the canopy scale, leveraging a wide range of target spectral responses in the hyperspectral data and a high point density (50 points/m 2 ) in the lidar data. In comparison, the hyperspectral image classification alone produced 72-76% overall accuracies (kappa values of 0.70 and 0.71). The UAV lidar-derived digital elevation model (DEM) is also strongly correlated with manned airborne lidar-derived DEM (R 2 = 0.98 and 0.96), but was obtained at a lower cost. The lidar and hyperspectral data as well as the fusion method demonstrated here can be widely applied across a gradient of vegetation and topography to monitor and detect ecological changes at a local scale.

[1] This work investigates spatial patterns of hillslope erosion and sediment yields in a semiarid ecosystem considering influences of vegetation, slope, rocks, and landscape morphology. The 137 Cs inventories were measured on one shrub and one grassed watershed in southeastern Arizona. Calculated mean erosion rates in eroding areas were 5.6 and 3.2 t ha À1 yr À1 , and net erosion rates for the entire watershed, including depositional areas, were 4.3 and nearly 0 t ha À1 yr À1 for the shrub and grass watersheds, respectively, over the past four decades. Differences in hillslope erosion rates between the two watersheds were apparently due to vegetation: while on the shrub site, runoff pathways were unobstructed, on the grass site, runoff was obstructed by vegetation patches and litter. Hillslope erosion rates within the watersheds were not correlated to slope gradient or curvature but were correlated to rocks in the upper soil profile. These results are interpretable in terms of slope-velocity equilibrium wherein overland flow velocities became independent of slope gradient because of differential rock cover, which evolved as a result of preferential erosion of fine material on the steeper slopes prior to 137 Cs deposition. Watershed morphology and channel incision controlled sediment yield. Most of the eroded soil was deposited in swales of the grassed watershed. Most of the soil eroded in the shrub watershed was exported from the watershed outlet by way of a well-incised channel system. The study shows that measurement of sediment yield from a watershed can be a poor indicator of erosion taking place within the watershed.Citation: Nearing, M. A., A. Kimoto, M. H. Nichols, and J. C. Ritchie (2005), Spatial patterns of soil erosion and deposition in two small, semiarid watersheds,