The Southwest Watershed Research Center (SWRC) of the United States Department of Agriculture-Agricultural Research Service has been conducting arid and semiarid (dryland) watershed research since 1953. This included establishment and continuous operation of the Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona. The 149 km2 ephemeral watershed is one of the most densely instrumented dryland research catchments in the world with a drainage area greater than 10 km2. This instrumentation captures many aspects of the hydrological cycle including how precipitation is partitioned into soil moisture, runoff and evapotranspiration and its subsequent effects on sediment transport, vegetation productivity, carbon sequestration, and groundwater recharge. The long-term, high-resolution record of observations on the WGEW enables understanding of the mean and variability of the hydrological processes, not well characterized with shorter term records, that fail to capture the large variability common to dryland regions. This presentation will highlight trends in temperature, precipitation, and runoff over the WGEW observation period. Additional research findings made by the SWRC and collaborators on erosion; plant productivity and carbon sequestration; the facilitation of soil water redistribution by plant roots; and groundwater recharge will also be presented.

Soil erosion is a worldwide agricultural and environmental problem that threatens food security and ecosystem viability. In arable environments, the primary cause of soil loss is short and intense storms that are characterized with high spatiotemporal variability. The complex nature of these erosive events imposes a great challenge for erosion modeling and risk analysis. Accurate high-resolution measurement of rain intensity is often lacking or sparsely available. As a result, many studies rely on coarser-resolution rainfall data that often fail to address the impact of intra-storm properties. In this study, based on a novel statistical method, we quantify the discrete and cumulative multiannual impact of rainstorm regime on runoff and soil erosion to better understand the most important rainstorm properties on erosion rate and amount, and, to provide storm-scale risk analyses. Central to our analyses is the coupling of a processes-based crop-land erosion model, Dynamic Water Erosion Prediction Project (DWEPP), with a stochastic rainfall generator that produces localized rainfall statistics at 5-min resolution (CoSMoS). To our knowledge, this is the first study that calibrated DWEPP runoff and sediment at the plot-scale on cropland. The model had an acceptable fit with measured event runoff and sediment data collected in northern Israel (NSE = 0.79 - 0.82). We then generated 300-year stochastic simulations of event-based runoff and sediment yield and used them to estimate erosion risk and calibrate a state-of-the-art frequency analysis method that explicitly accounts for rainstorms occurrence and properties. Results indicate that in the study area, high erosion rate events are characterized by intense rain bursts of short duration (shorter than the usually adopted erosivity index of 30-min), and not necessary by events of large volume accumulation or long duration. On these bases, we proposed an optimal rainfall erosivity index that combines intra-storm properties for the study area. As changes in rainstorm properties are expected under a changing climate, we expect our methodology to be a valuable tool for investigating the global concerns about future changes in soil erosion rate.

The Walnut Gulch Experimental Watershed (WGEW) is the primary outdoor hydrologic laboratory for the USDA-ARS’ Southwest Watershed Research Center (SWRC). This site represents the Southwest semiarid environment within the Long-term Agroecosystem Research (LTAR) network. The SWRC maintains a collection of long-term hydro-climatic measurements from WGEW, featuring an extensive archive of rainfall and runoff observations from an ephemeral network of streams within the 149 km2 watershed. The WGEW was established in 1953, and has continually developed and improved quality assurance and quality control procedures to aid in the accuracy and curation of the constantly growing datasets obtained from more than 100 rain gauges and 18 flumes, weirs, and gauged ponds. These efforts have led to the development of a state-of-the-art database and data visualization tools to aid in the curation of research-grade hydrometeorologic datasets. This required development of automated quality assurance and quality control tools to check and maintain the data for 21st century research needs. We developed five tools to improve the quality of rainfall and runoff database based on conventional hydrologic principles and the relationships between them: 1) precipitation is spatially correlated; 2) there is a temporal relation between rainfall and runoff; 3) runoff is only a limit portion of rainfall; and, 4) closer took of extreme events. Hence, we developed the following methods that included the analysis of interpolated rainfall maps at a daily time step, the association between rainfall and runoff events, lag time, runoff coefficients, and multiple regression methods to identify problematic events in the data archive. To visually inspect and verify the errors, we developed a graphical tool that displays relevant event hyetographs and hydrographs within a specific time window. After flagging anomalous events, we evaluated the types of errors using the original records and metadata information. The implementation of these approaches resulted in developing a suite of semi-automated QAQC tools that correctly detected 813 rainfall and 24 runoff events with erroneous timestamps that had passed all previous quality checks.