Most Recent News & Announcements

Publications: Article accepted in JGR-Atmospheres

Mon, 25 Jun 2012 13:23:39 -0400

Stephen’s article, “Error estimates in the assessment of the isotopic composition of surface fluxes: A direct comparison of techniques using laser-based water vapor isotope analyzers” has just been accepted for publication in JGR-Atmospheres. This article is derived from the analysis of a suite of observations and intercomparisons between eddy covariance, flux gradient, and Keeling-plot type methods for assessing the isotopic composition of evapotranspiration at landscape scales. After working through the sources of error inherent in each of the methods (while neglecting instrument error), Stephen is able to show that methods which depend on the covariance of water vapor concentration and isotopic composition directly (i.e. Keeling plot/mixing model approaches) perform better than eddy covariance approaches, which introduce additional uncertainty due to their inclusion of inherently low correlations between vertical wind and water vapor concentration. While we don’t think that this will be the last word on how to estimate landscape-scale isotopic composition of evapotranspiration using the current suite of closed-path optical spectroscopy, it’s nice to finally have some initial assessment of the quantitative uncertainty inherent in the varying strategies being used to tackle this issue.

Publications: Two articles recently accepted in Hydrology and Earth System Sciences

Mon, 25 Jun 2012 13:10:35 -0400

Our group has had two articles recently accepted into Hydrology and Earth System Science (HESS). Both articles are parts of different special issues. The first article, led by Lixin Wang (former group member and soon-to-be Asst. Professor at IUPUI) is focused on identifying critical issues in dryland ecohydrology and reviewing recent work related to these topics.

The second article, led by Lizzie King and Frances O’Donnell, examines frameworks for integrating hydrological sciences and critical social/ecological questions within the context of undergraduate engineering education. The course is based on our experiences teaching a field ecohydrology course at the Mpala Research center in central Kenya, where we focus on applying basic hydrological methods and approaches to complicated applied topics related to emerging dryland water resource issues.

Publications: Article accepted in Vadose Zone Journal

Thu, 12 Apr 2012 15:10:08 -0400

Keir’s review of soil water isotopic methods has been accepted for publication in Vadose Zone Journal. The article, “Stable isotopes of water vapor in the vadose zone: A review of measurement and modeling techniques” examines use of soil water vapor isotopes as a tracer of hydrologic processes occurring in the vadose zone. The measurement of soil water vapor isotopic composition is challenging due to difficulties inherent in sampling vadose zone airspace in situ. Historically, the soil water vapor isotope signature (δ18O, δ2H) has therefore been modeled as opposed to directly measured, and typically is treated of as being in equilibrium with liquid soil water isotope values. We present a review of the measurement and modeling of soil water vapor isotopes, with implications for studies of the soil-plant-atmosphere continuum at various scales. A contribution of this work is to introduce the use and effects of soil water potential on kinetic fractionation during soil evaporation within the Craig-Gordon modeling framework. Additionally, we assert that the recent development of laser-based isotope analytical systems may allow for the regular in situ measurement of the vadose zone isotopic composition of water in the vapor phase. Some sampling difficulties remain, but indications are that with low flow rates, authentic water vapor samples can be obtained without inducing evaporation in the liquid soil water.

Publications: Article accepted in PLoS ONE

Wed, 28 Mar 2012 09:24:40 -0400

An article led by Abinash Bhattachan and co-authored by Kebonye Dintwe, Mokganedi Tathlego, Frances O’Donnell, Kelly, Greg Okin, and Paolo D’Odorico has been accepted for publication in PLoS One. This article, “Evaluating ecohydrological theories of woody root distribution in the Kalahari” is the result of three summers of field work examining the below-ground structure of woody plants across a climate gradient in the Kalahari desert of Botswana.

Publications: Article accepted in JGR-Biogeosciences

Wed, 28 Mar 2012 09:15:57 -0400

An article written by Frances O’Donnell and Kelly has been accepted for publication in JGR-Biogeosciences. This article, “A model-based evaluation of woody plant encroachment effects on coupled carbon and water cycles” presents a model of soil carbon dynamics based on the interactions between plant spatial pattern and soil moisture dynamics in drylands.

Frances developed a model of coupled steady-state soil moisture and carbon dynamics that accounts for the effects of structural heterogeneity at the scale of vegetation patches by representing the local effects of root systems and canopies. She applied the model to paired grasslands and woody encroachment-produced shrublands spanning a climate gradient in the American Southwest and results were consistent with measured values. She found that woody encroachment increases heterogeneity in soil carbon decomposition rates, and has the net effect of increasing soil carbon residence times by 3.6-4.9 years. Her model represents an important step towards a mechanistic and quantitative understanding of the effect of vegetation structure on soil moisture and carbon dynamics and highlights the need for a better description of belowground vegetation structure in ecosystem models.

Publications: Article accepted in Water Resources Research

Mon, 23 Jan 2012 10:01:03 -0500

An article written by Trenton Franz Wang (along with co-authors Lizzie King, Kelly, Jan Norbotten, Mike Celia, and Ignacio Rodriguez-Iturbe) has just been accepted for publication in Water Resources Research. The article, “Ecohydrological determinants of landscape structure and function in a degraded Kenyan savanna” presents a model of hillslope vegetation dynamics that includes vegetation impacts on both patch-based water balance dynamics as well landscape flow patterns. Trenton asked a cery straight-forward, but complicated question:

“Given a set of simple rules about how trees alter local water balance, what patterns of hillslope vegetation distribution are most beneficial to the overall patterns of water use and availability at our site in central Kenya?”

Because the effect of vegetation on micro-topography aren’t well known, Trenton explored how changes in the strength of vegetation-topography interactions led to alternative “optimal” hillslope patterns. The most interesting result from Trenton’s model is that metrics of landscape organization lead to alternative outcomes in terms of optimality - suggesting that how choose to characterize the system strongly influences our predictions regarding how it organizes. This is deeply troubling, but the sort of result that only a simplified model could reveal.

The patch-scale water balance model is based on empirical analysis of runoff and soil moisture dynamics collected way back in 2008, when Trenton was just starting his PhD. He has since graduated and moved on to a postdoc position at the University of Arizona, and we’re all happy to see this work from his PhD get published.

Note: The article is now on-line and can be found on our website here.

Research: Caylor group research presentations at the 2011 AGU Meeting

Mon, 14 Nov 2011 12:19:27 -0500

The leaves are off the trees in Princeton and Thanksgiving is around the corner... which means it is time to start prepping slides and posters for the 2011 Fall Meeting of AGU. This year, 7 members of our group are involved in 8 different presentations, all during the first three days of the meeting. In addition to these presentations, Kelly’s Ph.D. advisor, Prof. Hank Shugart, is being honored as one of only 2 new fellows (out of 60 total 2011 fellows) inducted into the Global Environmental Change Focus Group on Thursday evening.

Caylor Group Presentations
(current/former group members bolded in author lists)

MONDAY, DECEMBER 05, 2011

B11A. Belowground Carbon Allocation and Retention: Frontiers in Biogeosciences and Climate Change I Posters 8:00 AM - 12:20 PM; Halls A-C

1. B11A-0452. Soil organic carbon and nitrogen content in savannas: Potential impacts of climate change Kebonye Dintwe; Greg Okin; Paolo D'Odorico; Kelly K. Caylor; Frances C. O'Donnell; Abinash Bhattachan

2. B11A-0467. Influence of rainfall climatology and coarse root structure on water use and carbon uptake by savanna trees, Frances C. O'Donnell; Kelly K. Caylor

B11H. Stable Isotope Fluxes in the Carbon and Water Cycles of Terrestrial Ecosystems I 8:00 AM - 10:00 AM; Room 2008

9:45 AM - 10:00 AM
3. B11H-08. The effect of warming on grassland evapotranspiration partition using laser-based isotope monitoring techniques, Lixin Wang; Shuli Niu; Xuhui Zhou; Jianyang Xia; Yiqi Luo; Stephen P. Good; Kelly K. Caylor; Matthew F. McCabe

GC11C. Climate Change, Food, and Water: Biophysical and Economic Impacts I 8:00 AM - 10:00 AM; Room 3005

9:20 AM - 9:40 AM
4. GC11C-05. Drought, Climate Change and Potential Agricultural Productivity (Invited), Justin Sheffield; Julio E. Herrera-Estrada; Kelly K. Caylor; Eric F. Wood

H11E. Simplifying Complexity: Characterizing Dynamics of Ecohydrological Systems I Posters 8:00 AM - 12:20 PM; Halls A-C

8:00 AM - 8:00 AM
5. H11E-1106. Groundwater-vegetation interactions in the small-scale dynamics of the Okavango Delta, Gabriele Manoli; Diana Chavarro-Rincon; Marco Marani; Mario Putti; Piotr Wolski; Michael Murray-Hudson; Kelly K. Caylor

TUESDAY, DECEMBER 06, 2011

B23E. Process-Based Studies of Biosphere-Atmosphere Carbon and Water Fluxes in Terrestrial Ecosystem II 1:40 PM - 3:40 PM; Room 2002

3:10 PM - 3:25 PM
6. B23E-07. Savanna Grassland Transpiration Fluxes After A Water Pulse Using Stable Isotope And Eddy Covariance Techniques, Kelly K. Caylor; Stephen P. Good; Keir Soderberg; Elizabeth King

B24B. Laser-Based Isotope Techniques in Biogeosciences I 4:00 PM - 6:00 PM; Room 2008

4:15 PM - 4:30 PM
7. B24B-02. Uncertainty in estimates of the isotopic composition of water vapor fluxes: direct comparison of techniques using laser-based analyzers, Stephen P. Good; Keir Soderberg; Lixin Wang; Kelly K. Caylor

WEDNESDAY, DECEMBER 07, 2011

EP31G. Predictive Understanding of Coupled Interactions Among Water, Life, and Landforms II (Video On-Demand) 8:00 AM - 10:00 AM; Rooms 2022-2024

8:00 AM - 8:15 AM
8. EP31G-01. Metabolic principles of river basin organization (Invited), Ignacio Rodriguez-Iturbe; Kelly K. Caylor; Andrea Rinaldo

Publications: Article accepted in Agricultural and Forest Meteorology

Tue, 1 Nov 2011 11:58:48 -0400

An article written by Lixin Wang (along with co-authors Stephen Good, Kelly, and Lucas Cernusak) has been accepted for publication in the journal Agricultural and Forest Meteorology. The article, “Direct quantification of leaf transpiration isotopic composition” has been kicking around the review process for a little while and we’re happy to see it is finally heading to press. The article reports on some new methods that Steve and Lixin developed to characterize transpiration and the isotopic composition of leaf water flux using an flow-through chamber design coupled to a integrated cavity output spectrometer from Los Gatos, Inc. This work was conducted both at our field site in Kenya as well as in the lab at Princeton. We’re very grateful that Lucas got involved in the manuscript, and because of him we’re even able to show that our method can achieve rapid measurements of leaf isotopic water flux with minimal impact on plant leaf water enrichment. We’re really excited about this method - not only does it give us a new tool for understanding aspect of plant water use and transpiration water sources at our field site, but it has also opened a door for us to use the same general approach for characterizing other in-situ water vapor fluxes and compositions within the soil-vegetation-atmosphere continuum. We’re already working on expanding the methods... so more to come soon!

Note: The paper has now been published, and can be found on our website here.

Research: 2nd Mpala-based COSMOS probe now calibrated

Sat, 15 Oct 2011 12:11:01 -0400

Our 2nd COSMOS probe is now operational at our flux tower site located at the Mpala Research Center in central Kenya. The probe is located within the Kenya Long-term Exclosure Experiment (KLEE), which is a flat savanna composed of Acacia drepanolobium and grass understory. Soils are black clay vertisols ("black cotton"), which go through significant shrink swelling and cracking. Here’s a link to the real-time probe data, which has already been calibrated:

http://cosmos.hwr.arizona.edu/Probes/StationDat/055/index.php

This probe now gives us a second soil moisture observation within a strongly contrasting soil (our first probe was deployed at the Mpala Flux Tower, which is underlain by sandy soils). It will be extremely interesting to see how these probes respond to rainfall heterogeneity over the coming years. The COSMOS project is run by the University of Arizona, and former Caylor lab member Trenton Franz is a postdoctoral research associate working on probe deployment and calibration.

News: Professor Dave Breshears visits the Mpala Research Center

Sat, 10 Sep 2011 09:26:37 -0400

In August and September, the Caylor Lab in Kenya hosted Dave Breshears from the University of Arizona during his sabbatical trip around the globe. Dave was comparing hemispherical photos to micrometeorological variables near the ground surface along a woody gradient of Acacia gerrardii in central Mpala. If the hemi-photos turn out to be a good predictor of near-surface winds and temperature, they will help identify spatial patterns of potential evapotranspiration. This method will then be useful to understand the sources of ET from the fairly open mixed savanna within the footprint of our flux tower.

Publications: Article to be published in Ecohydrology

Fri, 9 Sep 2011 15:25:45 -0400

We’ve just gotten news that an article written by Lizzie King (along with co-authors Trenton Franz, and Kelly) has been accepted for publication in the journal Ecohydrology. The article, “Ecohydrological interactions in a degraded two-phase mosaic dryland: implications for regime shifts, resilience, and restoration” is known more familiarly in the Caylor group as “Plants Behaving Badly”. It is the culmination of 3-4 years of thinking, measuring, analyzing, and occasionally cursing the hydrological and ecological factors leading to expansion of a native succulent species of Sanseveria in the communal group ranches adjacent to our research site in central Kenya. Along the way to completion of this work we learned a lot about this plant, the soils it lives in, the difficulty of walking through thick glades of it carrying heavy electromagnetic induction sensors, and its ability to thrive in the absence of water or any affection/interest from any other forms of life. We also figured out that the story of how this plant expands into degraded landscapes allowed us to make substantial headway towards a real-life, fully-quantified application of ecosystem management based on conceptual models arising from resilience theory coupled to scale-dependent feedbacks. As usual, Lizzie did a bang-up job telling this story, and we’re excited to see it finally coming to the pages of a journal in the near future. As for the Sanseveria... well, let’s just say we’re looking forward to applying our management suggestions for its suppression as soon as possible!

Research: COSMOS soil moisture probe now operational at the Mpala tower site

Thu, 8 Sep 2011 09:29:51 -0400

Our first of two COSMOS probes (and the first probe deployed in Africa) is now operational at our flux tower site located at the Mpala Research Center in central Kenya. Here’s a link to the real-time probe data (currently uncalibrated):

http://cosmos.hwr.arizona.edu/Probes/StationDat/050/index.php

We’re excited about this new observation, as it will provide us with landscape-scale observations of soil moisture within (and beyond) our flux tower footprint. The COSMOS project is run by the University of Arizona, and former Caylor lab member Trenton Franz is a postdoctoral research associate working on probe deployment and calibration. Thanks Trenton for your help getting this probe to our site! 

Research: Rain gauge and soil moisture probe deployment in Zambia

Fri, 2 Sep 2011 10:23:33 -0400

Last month Stephanie, Diana, and Molly were in Zambia deploying a suite of rain gauges and soil moisture probes as part of our NSF-funded collaboration on climate, resilience, and food security with Tom Evans at Indiana University and the Zambia Agricultural Research Institute. With the great help of ZARI researchers and students, they were able to get 20 sets of instruments deployed in communities across the southern and eastern provinces. These gauges and instruments will be coupled with detailed surveys and focus group activities. The combination of social and biophysical data will allow us to understand how both social and physical dynamics interact to affect the resilience of subsistence dryland farmers and their families across a range of geographical settings within each province. We’ll be back out in Zambia in late December, 2011 to conduct measurements on crop water use efficiency.

News: Princeton Grand Challenge Interns, Summer 2011

Wed, 31 Aug 2011 09:22:54 -0400

From June through August 2011 the Caylor Lab in Kenya hosted three undergraduate interns from the Princeton Environmental Institute’s Grand Challenges Internship Program – Kathleen Ryan, Hannah Safford, and Alice Suh. The interns advanced our research in three areas: rainfall patterns, soil vapor isotopes, and vegetation monitoring.

Kathleen compiled all of the rainfall data for Mpala Research Centre (the record is now continuous for the past 40 years), and calibrated each of the active rainfall gauges. She also initiated a long-term project of using atmospheric back-trajectories from NOAA’s HYSPLIT model (http://www.arl.noaa.gov/HYSPLIT.php) to help interpret the isotope signatures of rain samples that we have been collecting since 2008. The rainfall database will be available to all Mpala researchers, and the back-trajectory analysis will be presented at the Isoscapes conference at Purdue University, September 2011.

Hannah put together an experiment to monitor the soil vapor isotope signal during a soil drying event for comparison with the corresponding liquid isotope values. This work helps to refine our understanding of isotope fractionation during evaporation from the soil, and the experimental setup serves as a prototype for a continuous soil vapor monitoring system to be installed at our flux tower.

Alice developed a non-destructive biomass monitoring technique and performed a comparison of instrumentation for leaf-level photosynthesis and plant water potential measurements. The water potential comparison showed good agreement between the somewhat cumbersome “pressure bomb” technique and the much more portable WP4T dew point potentiometer. The latter will help in our monitoring of water stress and the effect of water potential on the transpiration water vapor isotope signature.

Overall, we had a very productive summer, and it was a pleasure to host these three young scholars.

Funding: Interdisciplinary Science award from NASA

Fri, 8 Jul 2011 09:47:11 -0400

Our group has received - along with the University of Virginia (Paolo D’Odorico and Todd Scanlon) and lead institution UCLA (Greg Okin and Yongkang Xue) - new support to advance the understanding of feedbacks between energy/water fluxes and vegetation/fire dynamics in savannas at multiple scales. The coupled nature of fires and water/energy fluxes at the Earth’s surface is perhaps nowhere more evident than in savanna ecosystems. Frequently stressed and sensitive to change, savanna ecosystems are responsive to climate variability over relatively short time scales, and both water and fires are the main driving forces in shaping their vegetation distribution, composition, and dynamics. Fires also have major impacts on the flux of water and energy from savanna ecosystems, both indirectly by controlling vegetation structure and plant community composition, and directly by altering soil moisture, surface temperature, and other surface physical characteristics. The coupling between fires and vegetation dynamics is complex. It involves patch-scale interactions with hydrological processes, landscape-scale feedbacks with tree-grass plant communities, and land-atmosphere feedbacks through the impact of fire scars on the dynamics of the near surface atmosphere. We will perform targeted field work in southern Africa to address hypotheses about the linkages between vegetation and water-fire coupling across scales, use remote sensing data to characterize spatial and temporal relationships between fire and vegetation, develop models that will include embedded feedback effects, and evaluate the effect of these feedbacks on the sensitivity of savanna ecosystems to changing climate forcing. The project will provide over $1,000,000 in support to UCLA, UVA, and Princeton over the next three years.