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ARS Home Pacific West Area Tucson, Arizona SWRC Research





- Scott, Russell - Russ
- Goodrich, David - Dave
- Heilman, Philip - Phil
- Nichols, Mary
- Williams, Christopher - Jason
- Biederman, Joel
- 2022
- 2021
- 2020
- 2019
- 2018
- 2017
- Publications
- Water Availability and Watershed Management (211)
- Soil and Air (212)
- Grass, Forage, and Rangeland Agroecosystems (215)

Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management **Location:
                            Southwest Watershed Research Center** 2021 Annual Report **Objectives**
                                

1:As part of the LTAR network, and in concert with similar long-term, land-based research infrastructure in the region, use the Walnut Gulch LTAR site to improve the observational capabilities and data accessibility of the LTAR network and support research to sustain or enhance agricultural production and environmental quality in agroecosystems characteristic of the semiarid Southwest region. Research and data collection are planned and implemented based on the LTAR site application and in accordance with the responsibilities outlined in the LTAR Shared Research Strategy, a living document that serves as a roadmap for LTAR implementation. Participation in the LTAR network includes research and data management in support of the ARS GRACEnet and/or Livestock GRACEnet projects. 
1.1:Improve & continue long-term measurements & analysis of water budgets on WGEW & Santa Rita Experimental Range (SRER) watersheds. 
1.2:Expand variables measured on WGEW & SRER watersheds based on recommendations of the LTAR Meteorology, Hydrology, CO2, Non-CO2 Gas, Soil, Biology, & Wind Erosion Committees. 
1.3:Develop a long-term monitoring program. 
1.4:Implement an experiment on the SRER watersheds to quantify the effects of brush management on a set of ecosystem services. 
1.5:Compute trends in sub-daily & daily precipitation intensity across LTAR sites. 
1.6:Evaluate National Weather Service dual pole radar precipitation data & its ability to improve flash flood forecasting. 

2:Quantify how seasonal, annual, and decadal-scale variations in climate, plant community composition, and management impact processes controlling the cycling of water, energy, and carbon in semiarid rangelands 
2.1:Determine how changes in vegetation structure & climate affect ecosystem-atmosphere water vapor & CO2 exchange using long-term flux tower observations.
2.2:Use isotopes in pond deposition sediments to understand & quantify erosion & sediment yields in semiarid landscapes as a function of ecological sites. 
2.3:Quantify the impact of erosion control structures on runoff & sediment transfers in semiarid landscapes. 
2.4:Estimate annual production & minimum total foliar cover using Landsat & MODIS satellite.
2.5:Develop methods to assess climate impacts on rangeland vegetation composition & production across the West. 

3:Develop a new conceptual framework and corresponding experimental methods to understand and model the dynamics of semiarid upland and channel erosion processes.
3.1:Conduct experiments to quantify the effects of surface condition. 
3.2:Conduct experiments to develop a remote sensing method to estimate hydraulic roughness. 

4:Improve hillslope (RHEM) and AGWA/KINEROS2 watershed models and develop methods to incorporate new remotely sensed, meteorologic, & land surface information.
4.1:Complete development & post-disturbance testing of the RHEM for application in Western rangelands.
4.2:Develop a mechanism to extend the findings from the Walnut Gulch LTAR site across Arizona & New Mexico & support collaborative vegetation management of public lands to improve watershed function. 
4.3:Incorporate a variety of KINEROS2 (K2) / AGWA model enhancements. **Approach**
                                

Objective 1: 
1. Use co-located rain gauges to quantify uncertainties in long-term precipitation datasets. 
2. Use radar stage measurements to test remote methods to measure runoff stage
3. Deploy mobile x-band Doppler radar and compare with Dual Pole radar rainfall rain-gauge observations, and runoff observations on the WGEW. 
4. Meet LTAR objectives by: a) using observational datasets to quantify the individual components of the watershed water balance in Walnut Gulch Experimental Watershed WGEW), b) using satellite and ground measurements of vegetation to document changes in watershed vegetation, c) determining trends and magnitude of precipitation intensities and precipitation extremes across the continental US, and d) implementing the LTAR common experiment to assess the effects of brush management on a set of ecosystem services.

Objective 2:
1. Use long-term flux tower observations to determine how changes in vegetation structure and climate affect ecosystem-atmosphere water vapor and carbon dioxide exchange.
2. Use 210Pb pond stratigraphy to determine erosion rates and their historical dynamics on small watersheds over the past 50-100 years.
3. Quantify runoff and sediment yields on watersheds to quantify the impact of erosion control structures on runoff and sediment transfers.
4. Use satellite, climate, site productivity and management data to estimate annual production and minimum total foliar cover.
5. Use LiDAR, point cloud, and new satellite datasets to construct canopy height models to assess climate impacts on rangeland vegetation composition and production.

Objective 3:
1. Use rainfall simulator experiments to quantify the effects of surface condition on infiltration, runoff, concentrated flow dynamics, sediment transport processes, and surface evolution.
2. Use radar backscatter roughness and hydraulic roughness at a laboratory, rainfall simulator, and small watershed scales using airborne and satellite active radar imagery to develop a remote sensing methods to estimate hydraulic roughness. 

Objective 4:
1. Complete development and post-disturbance testing of the Rangeland Hydrology and Erosion Model (RHEM) for application in Western rangelands.
2. Create a web interface to identify problem areas in watersheds, compare across watersheds, and assess trends in time prior to KINEROS2 modeling.
3. Incorporate RHEM, improved process model representations, and higher-resolution, model inputs, sub-surface and variable width routing, and interstorm processes into KINEROS2. **Progress Report**
                                

This project is the sole focus of the management unit (MU), and this annual report describes progress for the 4th year of the 5-year project cycle. Major challenges faced during the year included major restrictions due to COVID-19 protocols and the construction of the Unit’s new building. Pandemic protocols mainly complicated field work that included travel to the field and working side-by-side collecting environmental data. The construction of the new building has finally resumed, and while office space is not currently a limiting factor for research, there will be no room for additional domestic or international collaborators until the new building is completed. On a positive note, the Unit’s two scientist vacancies at the start of this project cycle have been filled for several years now, and their contributions have complemented many of the plan objectives, primarily through rainfall simulation and rainfall manipulation experiments.

Under Objective 1, a major effort was completed and published by all of the early ARS Experimental Watersheds, now part of the Long Term Agroecosystem Research (LTAR) sites. This effort cataloged the major scientific findings over six-plus decades and identified the major societal benefits arising from sustained research that supported the long-term data collection at the watersheds. Under Sub-objective 1.1, substantial improvements were made in observational capabilities at the Walnut Gulch Experimental Watershed (WGEW) site by employing radar to measure runoff at flumes 4 and 6. Also, in collaboration with other LTAR locations, analysis of runoff samples for dissolved organic matter, collected from grass and shrub-dominated WGEW sub-watersheds, has been completed and are being incorporated into a journal manuscript. Fluorescence spectra were used to infer whether organic matter was derived from plant litter or soil microbial decomposition. Coupling these concentration data with the high-quality water balance measurements in these sub-watersheds allowed the determination of dissolved mass fluxes. For Sub-objectives 1.2 and 1.3, in collaboration with the Soil Health Institute, samples were taken to quantify the health of WGEW and Santa Rita Experimental Range (SRER) soils using a wide variety of soil metrics. Field samples were taken and results are being analyzed. For vegetation monitoring using small aerial drones, a manuscript was published describing a workflow for automated Structure-from-Motion processing and analysis of drone imagery. Lastly, analysis for six LTAR Experimental Watersheds, testing for trends for sub-daily rainfall intensification was completed and is being written up for a journal manuscript.

Under Objective 2, we continued to make substantial progress on quantifying how seasonal, annual, and decadal-scale variations in climate, plant community composition, and management impact processes controlling the cycling of water, energy, and carbon in semiarid rangelands. Accordingly, a paper on quantifying the effects of brush management on land-atmosphere water and carbon exchanges was drafted and submitted to a journal for peer review. Research continued on evaluating the impact of extreme drought on riparian vegetation water use (evapotranspiration or ET), as well as the drought effects across the entire southwestern U.S., on ecosystem carbon and water cycling. Moreover, in collaboration with the University of Arizona, experimental manipulations of rainfall were initiated at a recently completed experimental facility in a semiarid rangeland ecosystem. Sixty plots were planted with native perennial bunchgrass seedlings inside six covered high tunnels to exclude natural rainfall. Each tunnel was fitted with rainfall harvesting equipment, with the harvested rainfall applied to plots according to one of four rainfall scenarios. Automated remote cameras tracked vegetation greenness. Soil moisture observations and modeling were carried out to quantify the severity and duration of drought stress in the root zone. A portable tent was fabricated and used to measure exchanges of water and carbon dioxide with the atmosphere on each plot. Automated soil chambers measured soil respiration. A root zone camera was used to track belowground growth.  First-season results were presented to several stakeholder groups, one national, and one international conference. Several manuscripts have been prepared and will be submitted for journal review. In support of the rainfall manipulation experiment, historical weather station analysis was completed to look at recent changes in climate. A manuscript was submitted and published. Finally, under this objective, substantial progress has been made in identifying and mapping legacy conservation structures in multiple watersheds in southern Arizona. The condition and function of the structures affecting erosion, gullying and surface runoff alterations were quantified based on high-resolution light detection and ranging (LiDAR) data and aerial imagery. Database development and process interpretations are being supported through the Conservation Effects Assessment Project .

Under Objective 3, a new conceptual framework and corresponding experimental methods were developed to understand and model the dynamics of semiarid upland and channel erosion processes. For Sub-objective 3.1, the effects of surface condition (distribution of vegetation, slope steepness, and roughness) on infiltration, runoff, concentrated flow dynamics, sediment transport processes, and surface evolution were quantified. 

Under Objective 4, the widely-used hillslope Rangeland Hydrology and Erosion Model (RHEM) and watershed Automated Geospatial Watershed Assessment (AGWA) models were improved by developing methods to incorporate remotely-sensed, meteorological, and land surface information into them. In support of Sub-objective 4.1, a procedure for return-frequency-based risk assessment has been developed and implemented on the RHEM interactive website. Also, a rainfall event data generator was developed that can automatically convert available regional/global climate products into inputs that can be used by the model. Also, a tool was created to generate customized reports containing remotely sensed information and climate data for public land allotments in southern Arizona to highlight areas with anomalously low cover and grass growth to focus attention by public rangeland managers. Under Sub-objective 4.3, a between-storm component treating evaporation, transpiration, and soil water redistribution has been developed, tested and incorporated into the KINEROS2 (K2) model. **Accomplishments**
                                

1. Increasing dry intervals between rainfall events across the western United States. It is well-established that annual precipitation totals have declined over much of the western United States since the 1970s. However, it is unknown whether the decline is mainly the result of smaller storms or less frequent precipitation. ARS researchers in Tucson, Arizona, collaborated with university partners to analyze daily weather station records from 337 stations across the western U.S. since the 1970s. They found that over the last five decades, the longest dry period between subsequent rainstorms increased by an average of as much as 60%, from 20 to 32 days. The most severe changes were found in the Desert Southwest, while portions of the Pacific Northwest saw decreased drought intervals. These results demonstrate that reduced annual rainfall results mainly from fewer storms with longer intervening droughts. Impacts of longer droughts include changes in plant species, reduced plant productivity and ecosystem carbon uptake, reduced forage availability for livestock, increased wildfire activity, and less certain water availability for human consumption. Concurrent press releases by ARS and university partners generated coverage in ca. 200 media outlets including multiple radio and television outlets and the Associated Press. 2. Scientific and societal benefits from the USDA-Agricultural Research Service’s Experimental Watershed Network. Sustaining water resources, water quality, and watershed health are essential for life and food and fiber production. In the wake of the Dust Bowl’s devastating impacts to watersheds and agriculture, the U.S. Department of Agriculture established a series of experimental watersheds to better understand how erosion, runoff, and water quality vary in response to different agricultural practices. More than a half-century worth of high-resolution observations from this network have led to unprecedented scientific insights that underpin dozens of watershed models and methods that guide billions of dollars’ worth of conservation measures, infrastructure investments, and have led to a host of substantial societal benefits. The LTAR (Long-Term Agro-ecosystems Research) network subsumes many of the ARS Experimental Watersheds and expands their mission to sustainable intensification of agricultural production while improving ecosystem services and increasing rural prosperity. 3. Forests remove carbon dioxide from the atmosphere and act as an important buffer against climate change. It is important to characterize how mountain forests sequester carbon dioxide, particularly in the southwestern United States where mountain forests play an important role in regional sequestration. ARS researchers in Tucson, Arizona, used carbon sequestration measurements from six mountain forest sites in the southwestern United States to investigate how forest sequestration might respond to regionally forecasted warming and drying. Results suggest that the abundant winter snow at higher elevation forests can sustain sequestration for longer into the growing season, but sequestration at lower elevation forests declines with reduced snow and rain. However, changes in future precipitation remain highly uncertain. As a result, we conclude that semi-arid mountain forest carbon uptake is likely to remain stable for the foreseeable future barring dramatic changes in precipitation and temperature. 4. Pinyon and juniper removal enhance vegetation and infiltration on sagebrush rangelands. Pinyon and juniper range expansion has altered the vegetation structure and hydrologic function of more than 170,000 km2 of sagebrush steppe rangelands throughout the western United States. Owners and managers of these landscapes are challenged with selecting effective pinyon and juniper removal treatments to re-establish sagebrush vegetation structure and associated hydrologic function. ARS researchers from Tucson, Arizona, and Boise, Idaho, assessed the long-term impacts of pinyon and juniper tree removal by fire, cutting, and shredding on vegetation and hydrology at two sagebrush sites. The treatments over a 13-year period enhanced the vegetation, ground-surface conditions, and soil hydrologic properties that promote infiltration and limit runoff generation. However, various ecological tradeoffs were evident across the different treatment alternatives. The results provide an improved understanding of the long-term ecological impacts of pinyon and juniper removal practices and provide science-based knowledge for guiding the management of critically important imperiled sagebrush ecosystems in the western United States. 5. New instrument efficiently and accurately monitors surface runoff. Current methods to quantify rangeland runoff rates such as artificial rainfall and overland flow simulations are effective but are challenging to execute, expensive, and resource demanding. Land managers commonly use surface runoff rates as an indicator of rangeland health and to guide management decisions. These resource professionals need easy, inexpensive, and reliable approaches to monitor runoff rates. ARS researchers from Tucson, Arizona, and scientists from New Mexico State University, Las Cruces, New Mexico, assessed a relatively new, under-utilized, and inexpensive method for monitoring rangeland runoff, Upwelling Bernoulli Tubes (UBe Tubes). The study found that properly calibrated UBe Tubes provide relatively accurate runoff measurements for rangeland applications and can deliver near real-time data during runoff events. Given the ease of implementation, UBe Tubes provide a new approach for monitoring spatially variable runoff rates along hillslopes and a potential source for fundamental information in assessing rangeland conditions and guiding management. 6. Brush management re-establishes grassland conditions and improves hydrologic function. A substantial component of rangeland in the southwestern United States has been converted from grass- to shrub-dominated vegetation. This vegetation degradation commonly results in increased runoff and long-term soil loss and poses negative implications on delivery of ecosystem services. Land managers commonly apply herbicide-based brush management treatments to reduce shrub cover and re-establish grass cover and associated ecological structure and function, but such effectiveness of these treatments can vary substantially with site characteristics and pre-treatment conditions. ARS researchers in Tucson, Arizona, and Boise, Idaho, and University of Arizona, scientists in Tucson, Arizona, conducted vegetation and rainfall simulation experiments within adjacent herbicide treated and untreated areas to evaluate effectiveness of herbicide application to re-establish grassland conditions and associated hydrologic function. The herbicide treatment substantially reduced shrub cover, enhanced grass cover, improved overall hydrologic function, and reduced hillslope sediment delivery. The results contribute improved understanding of brush management treatment effects for guiding management and advance understanding of vegetation effects on hydrologic and erosion processes for the respective rangeland systems. 7. Scaling up collection and processing of drone imagery for rangeland monitoring. The high cost of labor compared to the large area of allotments and ranches means that managers often lack monitoring information. Satellite remote sensing can cover large areas, but that information is typically very coarse. Drones can provide the intermediate scale of monitoring needed to link field and remote sensing information, but until now the quantity of photographic data made it difficult to process and interpret information from large areas with drones. ARS researchers in Tucson, Arizona, along with scientists from the University of Arizona, developed a workflow capable of processing drone imagery from hundreds of acres using a high-performance computer system. The resulting digital vegetation cover and height products can be used directly in further analysis. This new workflow takes less than one-quarter of the processing time and enables much more representative sampling to support remotely sensed monitoring of rangelands. **Review Publications**
                            *Nichols, M.H., Brandau, W., Shaw, F. 2021. Unintended consequences of rangeland conservation structures. International Soil and Water Conservation Research. 9(1):158-165. https://doi.org/10.1016/j.iswcr.2020.11.006.*
                                *Fullhart, A.T., Nearing, M.A., Weltz, M.A. 2021. Temporarily downscaling precipitation intensity factors for KÖPPEN Climate Regions in the U.S.. Journal of Soil and Water Conservation. 76(1):39-51. https://doi.org/10.2489/jswc.2021.00156.*
                                *Robles, M., Hammond, J., Kampf, S., Biederman, J.A., Demaria, E.M. 2020. Winter inputs buffer streamflow sensitivity to snowpack losses in the Salt River Watershed in Lower Colorado River Basin. Water. 13(3). https://doi.org/10.3390/w13010003.*
                                *Goodrich, D.C., Heilman, P., Anderson, M.C., Baffaut, C., Bonta, J.V., Bosch, D.D., Bryant, R.B., Cosh, M.H., Endale, D.M., Veith, T.L., Havens, S.C., Hedrick, A., Kleinman, P.J., Langendoen, E.J., Mccarty, G.W., Moorman, T.B., Marks, D.G., Pierson Jr, F.B., Rigby Jr, J.R., Schomberg, H.H., Starks, P.J., Steiner, J., Strickland, T.C., Tsegaye, T.D. 2020. The USDA-ARS experimental watershed network – Evolution, lessons learned, societal benefits, and moving forward. Water Resources Research. 57(2). Article e2019WR026473. https://doi.org/10.1029/2019WR026473.*
                                *Nichols, M.H., Degginger, T. 2021. The landscape impact of unmaintained rangeland water control structures in southern Arizona, USA. Catena. 201, Article 105201. https://doi.org/10.1016/j.catena.2021.105201.*
                                *Macbean, N., Scott, R.L., Biederman, J.A., Ottle, C., Vuichar, N., Kolb, T., Dore, S., Litvak, M., Ducharne, A., Moore, D. 2020. Testing water fluxes and storage from two hydrology configurations within the ORCHIDEE land surface model across US semi-arid sites. Hydrology and Earth System Sciences. 24:5203-5230. https://doi.org/10.5194/hess-24-5203-2020.*
                                *Nelson, J., Perez-Priego, O., Zhou, S., Poyatos, R., Zhang, Y., Blanken, P., Gimeno, T., Wohlfahrt, G., Desai, A., Gioli, B., Limousin, J., Bonal, D., Paul-Limoges, E., Scott, R.L., Varlagin, A., Fuchs, K., Montagnani, L., Wolf, S., Delpierre, N., Berveiller, D., Gharun, M., Marchesini, L., Gianelle, D., Sigut, L., Mammarella, I., Siebicke, L., Black, T., Knohl, A., Hortnagl, L., Magliulo, V., Carvalhais, N., Migliavacca, M., Reichstein, M., Jung, M. 2020. Ecosystem transpiration and evaporation: Insights from three water flux partitioning methods across FLUXNET sites. Global Change Biology. 26:6916-6930. https://doi.org/10.1111/gcb.15314.*
                                *Dwivedi, R., Eastoe, C., Knowles, J.F., Meixner, T., Mcintosh, J., Ferre, P., Castro, C., Wright, W., Niu, G., Minor, R., Barron-Gafford, G., Abramson, N., Mitra, B., Stanley, M., Chorover, J. 2021. An improved practical approach for estimating catchment-scale response functions through wavelet analysis. Hydrological Processes. 35(3), Article e14082. https://doi.org/10.1002/hyp.14082.*
                                *Fullhart, A.T., Nearing, M.A., Armendariz, G.A., Weltz, M.A. 2021. Climate benchmarks and input parameters representing locations in 68 countries for a stochastic weather generator, CLIGEN. Earth System Science Data. 123(2):435-446. https://doi.org/10.5194/essd-13-435-2021.*
                                *Bond-Lamberty, B., Christianson, D., Malhotra, A., Pennington, S., Sihi, D., Aghakouchak, A., Anjileli, H., Arain, M., Armesto, J., Ashraf, S., Ataka, M., Baldocchi, D., Black, T., Buchmann, Carbone, M., Chang, S., Crill, P., Curtis, P., Davidson, E., Desai, A., Drake, J., El-Madany, T., Gavazzi, M., Görres, C., Gough, C., Goulden, M., Gregg, J., Gutiérrez Del Arroyo, O., He, J., Hirano, T., Hopple, A., Hughes, H., Järveoja, J., Jassal, R., Jian, J., Kan, H., Kaye, J., Kominami, Y., Liang, N., Lipson, D., Macdonald, C., Maseyk, K., Mathes, K., Mauritz, M., Mauritz, M., Mcnulty, S., Miao, G., Migliavacca, M., Miller, S., Nietz, J., Nilsson, M., Noormets, A., Norouz, H., O’Connell, C., Osborne, B., Oyonarte, C., Pang, Z., Peich, M., Pendall, E., Perez-Quezada, J., Phillips, C.L., Phillips, R., Raich, J., Renchon, A., Ruehr, N., Sanchez-Canete, E., Saunders, M., Savage, K., Schrumpf, M., Scott, R.L., Seibt, U., Silver, W., Sun, T., Sun, W., Szutu, D., Takagi, K., Takagi, M., Teramoto, M., Tjoelker, M., Trumbore, S., Ueyama, M., Ueyama, R., Varner, R., Verfaillie, J., Vogel, C., Wang, J., Winston, G., Wood, T., Wu, J., Wutzler, T., Zeng, J., Zha, T., Zhang, Q., Zou, J. 2020. COSORE: A community database for continuous soil respiration and other soil-atmosphere greenhouse gas flux data. Global Change Biology. 26:7268-7283. https://doi.org/10.1111/gcb.15353.*
                                *Knowles, J.F., Scott, R.L., Biederman, J.A., Blanken, P., Burns, S., Dore, .S., Kolb, T., Litvik, M., Barron-Gafford, G. 2020. Montane forest productivity across a semi-arid climatic gradient. Global Change Biology. 26(12):6945-6958. https://doi.org/10.1111/gcb.15335.*
                                *Zhang, F., Biederman, J.A., Dannenberg, M., Yan, D., Reed, S., Smith, W. 2021. Five decades of observed daily precipitation reveal longer and more variable drought. Geophysical Research Letters. 48(7), Article 092293. https://doi.org/10.1029/2020GL092293.*
                                *Doane, T., Pelletier, J., Nichols, M.H. 2021. Hack distributions of rill networks and nonlinear slope length–soil loss relationships. Earth Surface Dynamics. 9:317-331. https://doi.org/10.5194/esurf-9-317-2021.*
                                *Belmonte, A., Sankey, T., Biederman, J.A., Bradford, J., Goetz, S., Kolb, T. 2021. UAV-based estimate of snow cover dynamics: Optimizing semi-arid forest structure for snow persistence. Remote Sensing. 13(5):1036. https://doi.org/10.3390/rs13051036.*
                                *Scott, R.L., Knowles, J.F., Nelson, J., Gentine, P., Li, X., Bryant, R.B., Biederman, J.A. 2021. Water availability impacts on evapotranspiration partitioning. Agricultural and Forest Meteorology. 297, Article 108251. https://doi.org/10.1016/j.agrformet.2020.108251.*
                                *Yang, J., Magney, T., Yan, D., Knowles, J.F., Smith, W., Scott, R.L., Barron-Gafford, G. 2020. The Photochemical Reflectance Index (PRI) captures the ecohydrologic sensitivity of a semi-arid mixed conifer forest. Journal of Geophysical Research-Biogeosciences. 125(11), Article 005624. https://doi.org/10.1029/2019JG005624.*
                                *Chu, H., Luo, X., Ouyang, Z., Chan, W., Dengel, S., Biraud, S., Torn, M., Metzger, S., Kumar, J., Arain, M., Arkebauer, T., Baldocchi, D., Bernacchi, D., Black, T., Blanken, P., Bohrer, G., Bracho, R., Brown, S., Brunsell, N., Chen, J., Chen, X., Clark, K., Desai, A., Duman, T., Durden, T., Fares, S., Forbrich, I., Gamon, J., Griffis, T., Helbig, M., Hollinger, D., Humphreys, E., Ikawa, H., Iwata, H., Ju, Y., Knowles, J.F., Knox, S., Kobayashi, H., Kolb, T., Law, B., Lee, X., Litvak, M., Liu, H., Munger, J., Noormets, A., Novick, K., Oberbauer, S., Oechel, W., Oikawa, P., Papuga, S., Pendall, E., Prajapati, P., Prueger, J.H., Quinton, W., Richardson, A., Russell, E., Scott, R.L., Starr, G., Staebler, R., Stoy, P., Stuart-Haëntjens, E., Sonnentag, O., Sullivan, R., Suyker, A., Ueyama, M., Vargas, Wood, J., Zona, D. 2021. Representativeness of Eddy-Covariance flux footprints for areas surrounding AmeriFlux sites. Agricultural and Forest Meteorology. 301-302, Article 108350. https://doi.org/10.1016/j.agrformet.2021.108350.*
                                *Li, L., Kang, X., Biederman, J.A., Wang, W., Qian, R., Zheng, Z., Zhang, B., Ran, Q., Xu, C., Liu, W., Che, R., Xu, Z., Cui, X., Hao, Y., Wang, Y. 2021. Nonlinear carbon cycling responses to precipitation variability in a semiarid grassland. Science of the Total Environment. 781, Article 147062. https://doi.org/10.1016/j.scitotenv.2021.147062.*
                                *Bean, A.R., Coffin, A.W., Arthur, D.K., Baffaut, C., Holifield Collins, C.D., Goslee, S.C., Ponce Campos, G.E., Sclater, V., Strickland, T.C., Yasarer, L.M. 2021. Regional frameworks for the USDA Long-Term Agroecosystem research (LTAR) Network: Preliminary concepts and potential indicators. Frontiers in Sustainable Food Systems. 4:612785. https://doi.org/10.3389/fsufs.2020.612785.*
                                *Johnson, J., Williams, C.J., Guertin, D., Archer, S., Heilman, P., Pierson Jr, F.B., Wei, H. 2021. Restoration of a shrub-encroached semiarid grassland: implications for structural, hydrologic, and sediment connectivity. Ecohydrology. 14(4). Article e2281. https://doi.org/10.1002/eco.2281.*
                                *Li, X., Xiao, J., Kimball, J., Reichle, R., Scott, R.L., Litvak, M., Bohrer, G., Frankenberg, C. 2020. Synergistic use of SMAP and OCO-2 data in assessing the responses of ecosystem productivity to the 2018 U.S. drought. Remote Sensing of Environment. 251, Article 112062. https://doi.org/10.1016/j.rse.2020.112062.*
                                *Lee, E., Kumar, P., Knowles, J.F., Minor, R., Tran, N., Barron-Gafford, G., Scott, R.L. 2021. Convergent hydraulic redistribution and groundwater access supported facilitative dependency between trees and grasses in a semi-arid environment. Water Resources Research. 57(6), Article 028103. https://doi.org/10.1029/2020WR028103.*
                                *Roby, M., Scott, R.L., Moore, D. 2020. High vapor pressure deficit decreases the productivity and water-use-efficiency of rain-induced pulses in semiarid ecosystems. Journal of Geophysical Research-Biogeosciences. 125(10),Article 005665. https://doi.org/10.1029/2020JG005665.*
                                *Barron-Gafford, G.A., Knowles, J.F., Perez Sanchez-Canete, E., Minor, R.L., Lee, E., Sutter, L., Tran, N., Murphy, P., Hamerlynck, E.P., Kumar, P., Scott, R.L. 2020. Hydraulic redistribution buffers climate variability and regulates grass-tree interactions in a semiarid riparian savanna. Ecohydrology. 14(3). Article e2271. https://doi.org/10.1002/eco.2271.*
                                *Vivoni, E., Perez-Ruiz, E., Keller, Z.T., Escoto, E., Templeton, R., Templeton, N., Anderson, C., Schreiner-Mcgraw, A., Mendez-Barroso, L., Robles-Morua, A., Scott, R.L., Archer S.R., Peters, D.C. 2021. Long-term research catchments to investigate differential woody plant encroachment in the Sonoran and Chihuahuan deserts: Santa Rita and Jornada experimental ranges. Hydrological Processes. 35(2), Article e14031. https://doi.org/10.1002/hyp.14031.*
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