Skip to main content
ARS Home » Pacific West Area » Tucson, Arizona » SWRC » Research » Research Project #422899

Research Project: Ecohydrological Processes, Scale, Climate Variability, and Watershed Management

Location: Southwest Watershed Research Center

2016 Annual Report


1a. Objectives (from AD-416):
1. Improve watershed management by developing the capacity to more accurately predict soil and plant water dynamics utilizing a combination of remote sensing, modeling and in-situ measurements. 2. Quantify how seasonal, annual, and decadal-scale variations in climate (including climate forecasts) and plant community composition impact the cycling of energy, water and carbon in semiarid rangelands. 3. Develop improved watershed model components and decision support systems that more fully utilize and assimilate economic and remotely sensed data for parameterization, calibration and model state adjustment. 4. As part of the LTAR network, and in concert with similar long-term, land-based research infrastructure in Southwestern U.S., use the WGEW LTAR to improve the observational capabilities and data accessibility of the LTAR network, to support research to sustain or enhance agricultural production and environmental quality in agroecosystems characteristic of the Southwestern U.S., as per the LTAR site responsibilities and other information outlined in the 2011 USDA Long- LTAR Network Request for Information (RFI) to which the location successfully responded, and 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.


1b. Approach (from AD-416):
Methods of investigation include field and laboratory experimentation, as well as the development and use of state-of-the-science watershed models and the use of remote sensing for watershed characterization. Multiple methods and techniques will be employed to improve the prediction of plant-water soil dynamics under objective one. They include data assimilation techniques to incorporate both in-situ and remotely sensed measurements into simulation models. In addition, algorithms for enhanced retrieval of watershed characteristics and state variables will be developed. Results from this research are critical for extension of results to large-areas using remote sensing and critical for improved inputs and parameter estimates for the models addressed under objective three. Research undertaken to address objective two more closely focuses on ecohydrology and determining changes in the cycling of energy, water and carbon as well as changes in the composition of plant communities across a wide range of time scales. This includes global change impacts on ecohydrologic processes (including water, nutrient and energy cycles) that underpin ecosystem structure and function. Thus, objective two also focuses on the relationship between global change, ecohydrology and watershed response, which will allow the evaluation of the combined impacts of climate change, intensive land use and species invasions on ecohydrological processes that are critical to maintaining ecosystems. It includes three Multi-Location Projects (MLPs) led or co-led by scientists in this research unit, which will examine observations across decadal and continental scales using observations from USDA’s national network of experimental watersheds, ranges and forests. To address objective three we will develop tools and methods to enhance watershed and rangeland management through wider accessibility of databases from our long-term experimental watersheds, and by development and testing of watershed and decision support models which can assimilate remotely sensed data and incorporate economic and ecosystem service information.


3. Progress Report:
This project is carried out under National Program (NP) 211 - Water Availability & Watershed Management. The first objective of this project is associated with National Action Plan Components 1 and 4; the second objective with Component 4, the third objective with Components 3 and 4; and the fourth objective with Components 2, 3, and 4. This report summarizes progress through FY16 corresponding to the 60th month milestones listed in the five-year project plan. In addition to this project, scientists in the Management Unit (MU) provided significant contributions to the NP 211 Action Plan for the new 211 project cycle. The MU currently has two projects under NP 211. This project and Project No: 2022-12660-005-00D, "Soil Erosion, Sediment Yield, and Decision Support Systems for Improved Land Management on Semiarid Rangeland on Semiarid Rangeland Watersheds". In consultation with our Area Director and the NP 211 National Program Leader we agreed that these two projects will be combined in the next 211 cycle. A combined project plan has been developed and reviewed by the Area office and is currently pending review through the Office of Scientific Quality Review. From a personnel perspective, one long open category 1 scientist vacancy was filled this fiscal year. The new scientist reported to our laboratory in August. A lead contributor to Objective 1 research retired early in the fiscal year and the MU is in the process a hiring a replacement. Progress on Objective 1 was made using remotely sensed data coupled with in-situ observations and models to more accurately predict soil and plant water dynamics. Numerous publications are in the pipeline from as a result of the 2015 intensive field experiment Soil Moisture Active Passive (SMAP) Validation Experiment 2015, SMAPVEX15, that was undertaken in collaboration with National Aeronautics and Space Administration (NASA), the Hydrology and Remote Sensing Laboratory, and numerous university collaborators. Earlier research conducted under this objective on ecosystem resilience under altered hydro climatic conditions was extended to international sites in Australia and China. Significant progress was made on Objective 2. An analysis of land-atmosphere carbon dioxide (CO2) and water exchange data from a network of eddy covariance sites across southern Arizona was completed and published to determine the role of interannual fluctuations in precipitation and evapotranspiration on net CO2 exchange and to examine how the productivity of ecosystems in this region have responded to the long-term, turn-of-the-21st-century drought. A broader scale analysis across the Southwest region was also conducted to examine how ecosystem productivity in this region, and across a broad range in annual precipitation, responds to fluctuations in water availability. Additionally, we continue to maintain and operate numerous eddy covariance flux towers in this region. These data have been vital to our analyses as well as to the analyses of many other researchers worldwide and for the ARS Long Term Agro-ecosystems Research (LTAR) network discussed further under Objective 4 who are interested in inventorying, measuring and modeling water and carbon cycling. An investigation on precipitation intensification at sub-daily time scales at the MU's Walnut Gulch Experimental Watershed (WGEW) LTAR site was completed. While no significant trends in precipitation were detected, many observed events on WGEW from rain gauges not used in the trend analysis are much greater than the estimated 100-year event indicating that the dense WGEW gauge network captures a substantially greater number of low-frequency, extreme rainfall events not typically observed in sparse networks. Simulated runoff from these storms was up to four times as large as the largest observed runoff event of record indicating greater storm water protection from high-intensity storm bursts covering a small area may be warranted for high values structures (hospitals, power stations, etc.) that have a small footprint. Continued progress was made under Objective 3 to further improve the Automated Geospatial Watershed Assessment (AGWA) tool and the KINematic runoff and EROSion model (KINEROS2) rainfall-runoff-erosion model. Improvements included: 1) Incorporation of the dynamic version of the Rangeland Hydrology and Erosion Model (RHEM) hillslope runoff and erosion engine; 2) The ability to derive a representative 2-D hillslope ground surface profile from 3-D digital topographic data; 3) Incorporate methods to represent the impacts of military training activities on rangelands; 4) Coupling of AGWA and the facilitator decision support tool to enable AGWA and other decision concerned to be compared in a structured manner; 5) Ingest radar-rainfall estimates from new National Weather Service (NWS) dual-pole radars as well as near-real time, telemetered rain gauge intensity measurements to improve flash flood forecasting including testing at several NWS Weather Forecast Offices; 6) Testing post-fire model parameterization and rainfall representation for identification of at risk watershed areas for Burned Area Emergency Response (BAER) teams to target wildfire mitigation efforts; and 7) Successfully developed an internet version of several of the key functions of AGWA. Objective 4 was added in Fiscal Year 2014 as a result of increased funding for the Walnut Gulch Experimental Watershed Long Term Agroecosystem Research site. Progress this year was primarily to: 1) Complete pre-treatment characterization of LTAR watersheds to be treated with herbicide to reduce mesquite and brush encroachment as part of our “aspirational” agriculture common experiment with Univ. of Arizona collaborators; 2) Arrange and complete aerial herbicide application of two LTAR watersheds; 3) Provide overall LTAR leadership through the Field Leadership Team; 4) Co-chair the meteorological observations committee to provide guidelines to other Long Term Agricultural Research (LTAR) locations on instrumentation and co-chair the data committee to develop procedures for transmitting meteorological observations and phenocam images from LTAR sites to the National Agricultural Library; 5) Spearhead efforts to identify and test management and collaboration software platforms for the LTAR network; and 6) Develop methods for analysis for trends of precipitation intensification under stationary and non-stationary conditions and complete the analysis for the WGEW LTAR and the Coshocton ARS experimental watershed.


4. Accomplishments
1. Consistent ecosystem productivity response to changes in water availability across the Southwest. Semiarid regions, such as in the Southwest, are important in understanding the Earth as a biogeochemical system because more than one-third of Earth’s land area is semiarid, and biological activity fluctuates with the highly variable water availability. Unfortunately, the interaction of climate and the carbon cycle in semiarid agroecosystems is poorly understood, because long-term water and carbon cycle measurements are scarce for these regions. To address this challenge, ARS scientists in Tucson, Arizona, have been making measurements of water, carbon dioxide (CO2), and energy fluxes in diverse semiarid ecosystems since 2004. Using data from four ARS-studied ecosystems and an additional 16 sites across the Southwest, ARS scientists performed an analysis to understand how water availability affects productivity. Specifically, the study found that 1) net and gross productivity in grasslands to woodlands and forests, showed very similar temporal responses to interannual changes in water availability; and 2) these temporal responses at a site were the same as spatial relationships across the climatic gradient of sites, increasing our confidence that temporal responses from several years of data can be extrapolated to predict productivity under anticipated climate change. We found that, on average, the expected climatic drying in the Southwest should lead to a reduction in net ecosystem production of 64 grams of cargon per square meter per year for a hypothetical 100 mm reduction in water availability.

2. Grasslands absorb more carbon dioxide than shrublands in the current climate of the Southwest. Semiarid regions are very important for regulating the amount of atmospheric carbon dioxide taken up by the global land surface. However, we lack a detailed understanding of how climate shifts, such as the ongoing decadal-scale drought in the Southwest, impact the uptake of carbon in semiarid ecosystems and how this response may vary with differing vegetation composition. Therefore, ARS scientists in Tucson, Arizona, used measurements of land-atmosphere water and carbon dioxide exchange from the last ten years and investigated the response of the ecosystem carbon uptake to changes in water availability in four Southwest U.S. ecosystems varying in relative shrub, tree and grass abundance. A precipitation-induced “pivot point” in the annual carbon balance was identified where the carbon uptake turned from carbon uptake in wet years to carbon loss in dry years. At sites with larger amounts of grass cover, pivot points were closer to the drought-period mean precipitation, suggesting that these grassier ecosystems have more quickly adjusted to the decadal-scale drought than the ecosystems with shrubs. As the climate in Southwest U.S. is expected to become drier in this century, these results suggest that the carbon uptake of ecosystems with more grass and less woody vegetation will adjust quicker to these changes and result in a faster return to carbon sequestration and the mitigation of climate change.

3. Unusually extreme rainfall intensities measured in the Walnut Gulch ARS Long Term Agro-Ecosystem Research Network (LTAR) watershed. Hydrologists are concerned with high intensity rainfall and peak runoff rates for soil conservation and stormwater infrastructure designs. ARS scientists and staff in Tucson, Arizona, operate the 149 square kilometer Walnut Gulch Experimental Watershed (WGEW) LTAR in southeast Arizona with 88 rain gauges and a database of 60 years of sub-daily rainfall intensities and runoff rates. This data was used to provide updated, rainfall intensity-duration-frequency relations that agreed relatively well with National Oceanic and Atmospheric Administration (NOAA) estimates from relatively few rain gauges in Arizona. However, across the range of durations, many observed events on WGEW from gauges not used in the frequency analysis are much greater than the estimated 100-year event indicating that the dense WGEW gauge network captures a substantially greater number of low-frequency, extreme rainfall events not typically observed in sparse networks. To assess the hydrologic consequences of these extreme events they were used as input to a well-tested watershed model for a small gauged watershed which did not experience events of similar magnitude. Simulated runoff volumes and peak discharge rates were up to four times as large as the largest observed runoff event of record indicating greater storm water protection may be warranted from high-intensity storm bursts covering a small area for high values structures (hospitals, power stations, etc.) that have a small footprint.

4. Recent tree die-off has little effect on streamflow. Over the last two decades, beetle infestation has killed billions of trees and affected millions of acres of forest in the Rocky Mountains of North America, many of which are critical sources of human water supply. ARS scientists in Tucson, Arizona, in collaboration with others, quantified annual streamflow responses in the decade following mortality and compared them to 25 to 40 years of pre-mortality records. In contrast to streamflow increases predicted by historical paired catchment studies and recent modeling, observed streamflow changes were weak, variable, and more frequently showed declines than increases. Although initially surprising, these results are consistent with the growing body of literature documenting increased snow sublimation and evaporation from the sub-canopy following die-off in forests of the Rocky Mountains. These forests are at the headwaters of rivers vital to the reliable water supply of the western U.S., and so these results indicate that water supplies across this region will likely not change due to this remarkable die-off of trees.

5. Simple rainfall representation can be used in post-fire watershed risk assessment modeling. Representation of precipitation is one of the most difficult aspects of modeling post-fire runoff and erosion as the impact of post-fire storms depends on the overlap between locations of high intensity rainfall and areas of high severity burns. One of the most useful applications of models in post-fire situations is risk-assessment to quantify peak flow and identify areas at high risk to flooding and erosion. ARS scientists in Tucson, Arizona, and scientists at the University of Arizona, used the KINematic runoff and EROSion (KINEROS2)Automated Geospatial Watershed Assessment (AGWA) model to compare several spatial and temporal rainfall representations of post-fire rainfall-runoff events at Zion National Park and Bandelier National Monument to determine the effect of differing representations on modeled peak flow and determining at-risk locations within a watershed. Results showed that rainfall representation greatly affected modeled peak flow, but using a simple uniform rainfall did not significantly alter the model’s predictions for high-risk locations. This has important implications for post-fire assessments prior to a flood-inducing rainfall event, or for post-storm assessments in areas with low-gage density or lack of radar rainfall data.

6. Monetary valuation of non-market riparian ecosystem services. Conservation of freshwater ecosystems in the semiarid Southwest is a critical issue as these systems support habitat for wildlife and provide consumptive use for humankind. Economists have utilized stated preference techniques to value non-marketed goods and services such as freshwater ecosystems for much of the last four decades. Working collectively, ARS scientists from Tucson, Arizona, and a team of physical and social scientists from the University of New Mexico, South Dakota State, U.S. Geological Survey, Wesleyan University, and Adams State University, developed a set of ecological endpoints for two river regions in the southwestern U.S. and used these ecological endpoints in a contingent valuation survey to obtain willingness to pay values for restoration and preservation alternatives. The results demonstrate statistically significant preservation estimates for the Upper San Pedro ($52.42/person) and restoration estimates for the Middle Rio Grande ($61.88/person) ecosystems.

7. Modeling mitigation of wildfire impact on hydrology with fuel treatments. Wildfire severity impacts post-fire hydrological response and fuel treatments can be a useful tool for land managers to moderate this response but current models focus on only one aspect of the fire-watershed linkage at a time (fuel treatments, fire behavior, fire severity, or watershed responses). This study, conducted by ARS scientists in Tucson, Arizona, and scientists at the University of Arizona, developed a spatial modeling approach that couples three models used sequentially to allow managers to model the effects of fuel treatments on post-fire hydrological impacts. Case studies involving a planned prescribed fire at Zion National Park and a planned mechanical thinning at Bryce Canyon National Park were used to demonstrate the approach. The first model was used to estimate the effects of fuel treatments, and the second was used to model wildfires in treated and untreated landscapes. Post-wildfire hydrological response was then modeled using KINematic runoff and EROSion (KINEROS2) within the Automated Geospatial Watershed Assessment Tool (AGWA). This coupled model approach can help land managers estimate the impact of planned fuel treatments on wildfire severity and post-wildfire runoff/erosion, and compare various fuel treatment scenarios to optimize resources and maximize mitigation results.

8. Plants absorbed more carbon in the warm spring of 2012, offsetting reductions during the summer drought. Climate change is predicted to result in warmer temperatures throughout the year and an increased prevalence of drought. In 2012, the contiguous U.S. experienced exceptionally warm temperatures, along with the most severe drought since the 1930s Dust Bowl. An ARS scientist in Tucson, Arizona, collaborated with others to use an extensive network of ecosystem measurements combined with satellite remote sensing observations and atmospheric measurements to examine the impact of a warmer spring followed by a summer drought on ecosystem photosynthesis and water use. They found that earlier vegetation activity increased spring carbon uptake, which partially compensated for the reduced uptake from summer drought, ultimately resulting in an 11% decrease in net annual carbon uptake. The warm and early spring also depleted soil water earlier, which reduced evaporative cooling and increased surface heating during summer. These results suggest that the potentially harmful effects of future climate change-induced summer droughts on ecosystem carbon storage may be partially mitigated by warming-induced increases in spring carbon uptake, but warmer springs may further increase summer heating, potentially leading to increased prevalence of summer heat waves.


5. Significant Activities that Support Special Target Populations:
ARS researchers in Tucson, Arizona, have employed a U.S. Veteran part-time, a senior at the University of Arizona in Rangeland Science, as a field technician.


Review Publications
Shen, W., Jenerette, G., Hui, D., Scott, R.L. 2016. Precipitation legacy effects on dryland ecosystem carbon fluxes: direction, magnitude and biogeochemical carryovers Biogeosciences. 13:425-439. https://doi.org/10.5194/bg-13-425-2016.
Broadbent, C., Brookshire, D., Goodrich, D.C., Dixon, M., Brand, A., Thacher, J., Stewart, S. 2015. Valuing preservation and restoration alternatives for ecosystem services in the southwestern U.S. Ecohydrology. 8:851-862. https://doi.org/10.1002/eco.1628.
Wagle, P., Xiao, X., Scott, R.L., Kolb, T., Cook, D., Brunsell, N., Baldocchi, D., Basara, J., Matamala, R., Zhou, Y., Bajgain, R. 2015. Biophysical controls on carbon and water vapor fluxes across a grassland climatic gradient in the United States. Agricultural and Forest Meteorology. 214-215:293-305. https://doi.org/10.1016/j.agrformet.2015.08.265.
Biederman, J.A., Scott, R.L., Somor, A., Harpold, A., Gutmann, E., Gochis, D., Breshears, D., Troch, P., Brooks, P., Meddens, A. 2015. Recent tree die-off has little effect on streamflow in contrast to expected increases from historical studies. Water Resources Research. 51:9775-9789. https://doi.org/10.1002/2015WR017401.
Scott, R.L., Biederman, J.A., Hamerlynck, E.P., Barron-Gafford, G. 2015. The carbon balance pivot point of southwestern U.S. semiarid ecosystems: Insights from the 21st century drought. Journal of Geophysical Research-Biogeosciences. 120:2612-2624. https://doi.org/10.1002/2015JG003181.
Holifield Collins, C.D., Kautz, M., Ponce-Campos, G., Hottenstein, J., Metz, L. 2015. Development of an integrated multi-platform approach for assessing brush management conservation efforts in semiarid rangelands. Journal of Applied Remote Sensing (JARS). 9:1-15. https://doi.org/10.1117/1.JRS.9.096057.
Nourani, V., Fard, A., Naizi, F., Gupta, H., Goodrich, D.C., Valizadeh, K. 2015. Implication of remotely sensed data to incorporate land cover effect into a linear reservoir-based rainfall-runoff model. Journal of Hydrology. 529:94-105. https://doi.org/10.1016/j.jhydrol.2015.07.020.
Sidman, G., Guertin, D., Goodrich, D.C., Unkrich, C.L., Burns, I. 2015. Risk-assessment of post-wildfire hydrological response in semi-arid basins: The effects of varying rainfall representations in the KINEROS2/AGWA model. International Journal of Wildland Fire. 25(3):268-278. https://doi.org/10.1071/WF14071.
Cendrero-Mateo, M., Carmo Silva, A.E., Nearing, G., Porcar-Castell, A., Hamerlynck, E.P., Papuga, S., Moran, M.S. 2015. Dynamic response of plant chlorophyll fluorescence to light, water and nutrient availability. Functional Plant Biology. 42:746-757. doi: 10.1071/FP15002.
Fu, C., Wang, G., Goulden, M., Scott, R.L., Bible, K., Carbon, Z. 2016. Combined measurement and modeling of the hydrological impact of hydraulic redistribution using CLM4.5 at eight AmeriFlux sites. Hydrology and Earth System Sciences. 20:2001-2018. https://doi.org/10.5194/hess-20-2001-2016.
Biederman, J.A., Scott, R.L., Goulden, M., Vargas, R., Litvak, M., Kolb, T., Yepez, E., Oechel, W., Blanken, P., Bell, T., Garatuza-Payan, J., Maurer, G., Dore, S., Burns, S. 2016. Terrestrial carbon balance in a drier world: the effects of water availability in southwestern North America. Global Change Biology. 22(5):1867-1879. https://doi.org/10.1111/gcb.13222.
Sidman, G., Guertin, D., Goodrich, D.C., Thoma, D., Falk, D., Burns, I. 2016. A coupled modeling approach to assess the impact of fuel treatments on post-wildfire runoff and erosion. International Journal of Wildfire. 25:351-362. https://doi.org/10.1071/WF14058.
Barnes, M., Moran, M.S., Scott, R.L., Kolb, T., Ponce Campos, G.E., Moore, D., Ross, M., Mitra, B., Dore, S. 2016. Vegetation productivity responds to sub-annual climate conditions across semiarid biomes. Ecosphere. 7(5):e01339. https://doi.org/10.1002/ecs2.1339.
Wolf, S., Keenan, T., Fisher, J., Baldocchi, D., Desai, A., Richardson, A., Scott, R.L., Law, B., Litvak, M., Brunsell, N., Peters, W., Van Der Laan-Luijkx, I. 2016. Warm spring reduced carbon cycle impact of the 2012 US summer drought. Proceedings of the National Academy of Sciences. 113:5880-5885. https://doi.org/10.1073/pnas.1519620113.
Nguyen, U., Glenn, E., Nagler, P., Scott, R.L. 2014. Long-term decrease in satellite vegetation indices in response to environmental variables in an iconic desert riparian ecosystem: the Upper San Pedro, Arizona, USA. Ecohydrology. 8:610-625. https://doi.org/10.1002/eco.1529.
Crow, W.T., Lei, F., Anderson, M.C., Hain, C., Scott, R.L., Billesbach, D., Arkebauer, T. 2015. Robust estimates of soil moisture and latent heat flux coupling strength obtained from triple collocation. Geophysical Research Letters. 42:8415-8423. doi: 10.1002/2015GL065929.
Hufkens, K., Keenan, T., Flanagan, L., Scott, R.L., Bernacchi, C.J., Joo, E., Brunsell, N., Verfaillie, J., Richardson, A. 2016. Productivity of North American grasslands is increased under future climate scenarios despite rising aridity. Nature Climate Change. 6:710-716. doi:10.1038/nclimate2942.
Keefer, T.O., Renard, K., Goodrich, D.C., Heilman, P., Unkrich, C.L. 2015. Quantifying extreme precipitation events and their hydrologic response in Southeastern Arizona. American Society of Civil Engineers. 21(1):1-10. doi: 10.1061/(ASCE)HE.1943-5584.0001270, 04015054.