Progress 07/01/03 to 06/30/09

Outputs
OUTPUTS: There were several independent studies during the project, conducted over different ecosystems. But they shared a central theme, the evapotranspiration (ET) and CO2 fluxes between the surface and atmosphere. The eddy covariance technique was used to measure the fluxes, and additional measurements of net radiation and soil heat flux completed the entire energy balance. Flux measurements were made over a period of about four years in a rangeland site that included three plant communities: crested wheatgrass, sagebrush, and Utah juniper. The remote location and weather resulted in occasional periods of missing data, mainly in winter. But results were adequate to characterize the seasonal changes in water use and carbon flux for these plant communities which are typical of the Great Basin. Another experiment was part of a large field study of soil moisture and ET variations over an agricultural watershed. Fluxes of heat, water vapor and CO2 were measured over several crops, and at a number of locations during the summer season. These results were integrated with remote sensing information obtained by other investigators. The combination allowed estimates of the spatial distribution of ET over the watershed, as well as changes over time due to soil moisture variations caused by intermittent rain events. A study of CO2 fluxes and soil respiration was conducted over a cheatgrass site in southern Idaho. In addition to the eddy covariance estimates, CO2 fluxes from the soil were measured continuously with chambers, and CO2 concentrations were measured in the soil continuously. Each of these approaches led to separate estimates of the CO2 emissions at the soil surface. The study allowed us to quantify the seasonal variations of CO2 exchanges from the cheatgrass ecosystem, the role of the soil emissions, and the net carbon balance for the year. Finally, work was begun on a collaborative project with others at USU, looking at irrigation and water use of turfgrass. Since this plant represents a large fraction of urban water use in the Great Basin, it is critical to determine how much water is actually used by this landscape under various environmental conditions and irrigation schedules. This requires obtaining reliable estimates of water use and exploring the biophysics that governs ET of turfgrass. The project conducted eddy covariance measurements of the ET and energy balance of a large turfgrass plot at a USU experimental farm. The data were used to: determine daily values of actual ET; compare with reference ET values currently used; quantify the role of advection of heat from dry surroundings on ET; and examine which biophysical factors govern ET under various weather conditions. PARTICIPANTS: Collaborators: Dr. Ron Ryel, Wildland Resources, USU; May Myklebust, Wildland Resources, USU; Dr. Kelly Kopp, Plants, Soils & Climate, USU; Dr. Roger Kjelgren, Plants, Soils & Climate, USU; Dr. Scott Jones, Plants, Soils & Climate, USU; Bruce Bugbee, Plants, Soils & Climate, USU. Graduate Student: Lynda Fenton, Plants, Soils & Climate, USU. TARGET AUDIENCES: Researchers studying the role of terrestrial ecosystems in the carbon balance; researchers and water managers intereted in the water use of turfgrass in urban areas PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
The seasonal changes in ET and CO2 exchanges were reported for the three rangeland sites. Since there were nearly no earlier reliable measurements over such a span for these ecosystems, all the findings were new. The ET was greatest in the late spring, when soil water was largest and climate grows warm and dry. Crested wheatgrass senesced in summer and showed the largest decline in ET, which stayed very small except for soil evaporation after intermittent rain events. However, sagebrush was able to maintain a larger base rate during the dry summers, though much smaller than the spring maximum. Finally, juniper maintained the largest ET during the how and dry summer period. Responses of the plant communities to CO2 showed a similar pattern. The largest net uptake at the surface was observed in the spring. The infrequent summer showers, produced an ET response lasting 2 - 3 days. The CO2 fluxes in sagebrush and juniper also responded to the showers. In the study over cheatgrass, the three approaches to estimate soil respiration were evaluated and compared for a range of environmental conditions and in different seasons. For a large range of conditions the approaches produced estimates that were in good agreement when there was little or no vegetation present. The estimate for annual soil efflux was 406 +-73 grams of carbon per square meter in 2005. Results indicated that eddy covariance measurements underestimate nocturnal fluxes of CO2 when LAI is near one. The chamber method did not work in snow, and the gradient method was not reliable during summer rain. In order to estimate soil efflux of such a site a combination of all three methods is recommended. In the experiment over the agricultural watershed, collaboration with other investigators led to a model based on remote sensing data to predict the spatial distribution of ET over the study region. These estimates agreed well with the measured values at the eddy covariance sites. A lidar system in the study measured the growth of the boundary layer, and used this to estimate the surface heat flux over a region. The results were compared to the array of values measured at the eddy covariances sites, and there was good agreement. The turfgrass experiment yelded daily ET values for the growing seasons of 2007 and 2008. The values varied with changes of the season and enviromental conditions, ranging from about 1 to 5 mm per day. The ratio of actual to reference ET was determined, and varied with environmental conditions. When winds were present, advection of heat from dry surroundings during the afternoon enhanced saturation deficit and ET values. A modified form of the Penman-Monteith equation was used to determine the relative importance of available energy vs. saturation deficit and stomatal conductance to ET. Initial results show that ET is mainly energy limited during the morning, and the importance of saturation deficit grows in the afternoon, especially during advection. An M.S. thesis to be completed in january 2010, will report all the results for the turfgrass ET study. More advanced studies will continue in a new AES project.

Publications

• No publications reported this period
• Hipps, L. 2007. Review of: Desert Meteorology, T. Warner. Cambridge University Press. Agricultural and Forest Meteorology 142:85-86.

Progress 01/01/08 to 12/31/08

Outputs
OUTPUTS: The research focused on two land surfaces. Seasonal CO2 exchanges for cheatgrass (Bromus tectorum) were measured last year. In addition, eddy covariance measurements of the evapotranspiration of turfgrass were continued during the growing season of 2008. The cheatgrass data were collected in southern Idaho, and the turfgrass experiment is conducted at the Greenville Experimental Farm near Logan, UT. Data for the cheatgrass study included three independent estimates of CO2 fluxes: a method using soil CO2 concentrations at several depths gradient, chamber measurements, and eddy covariance. Special interest was on nighttime periods where previous studies have shown that eddy covariance measurements sometimes underestimate fluxes when turbulence is very low. Calculations of fluxes were performed for each method over the seasons where the surface varied from snow to bare soil, to a plant canopy. Eddy covariance values received corrections for coordinate rotation, high frequency losses and density effects of heat and water vapor. The resulting flux estimates were compared between the three approaches. In the turfgrass study, eddy covariance measurements of sensible heat and water vapor fluxes were made in the summer of 2008 for the period of April through October. Measurements were also made of net radiation and soil heat flux. The large time series data sets from the eddy covariance are checked for continuity and errors. Then the various corrections described above are performed to yield the flux values. The software for this was written in our lab. The data are being analyzed to determine the net radiation, soil heat flux, sensible heat flux and evapotranspiration or ET values at hourly and daily intervals. These values are used to quantify the ability to close the energy balance equation. The values of the energy balance closure can be used to assess the reliability of the flux estimates, and to indicate whether it might be appropriate to add to the flux values in order to force the conservation of energy. Reference ET values using the well-known FAO 56 equations are calculated for each hour. By comparing the reference ET values with actual ET the so-called crop coefficients can be determined and compared with the approximations used in current operational models for turfgrass. The connections between environmental properties such as soil moisture, radiation, wind and saturation deficit, with the hourly and daily changes of ET are examined, with the aid of the Penman-Monteith equation. This allows a determination of which factors govern the ET of turfgrass under various conditions. Finally, several dry down experiments were performed in the latter part of the summer. During these periods, irrigation was systematically reduced to a level where water stress should appear. Additional measurements during these intensive studies included: spatial changes in soil moisture, foliage temperature, stomatal conductance and spectral reflectance of radiation in many visible and near infrared bands. Other investigators and research projects were involved in these studies. PARTICIPANTS: Collaborators: Dr. Ron Ryel, Wildland Resources, USU; May Myklebust, Wildland Resources, USU; Dr. Kelly Kopp, Plants, Soils & Climate, USU; Dr. Roger Kjelgren, Plants, Soils & Climate, USU; Dr. Scott Jones, Plants, Soils & Climate, USU; Bruce Bugbee, Plants, Soils & Climate, USU. Graduate Student: Lynda Fenton, Plants, Soils & Climate, USU. TARGET AUDIENCES: Researchers studying the role of terrestrial ecosystems in the carbon balance; researchers and water managers interetsed in the water use of turfgrass in urban areas. PROJECT MODIFICATIONS: More focus has been placed on water use and energy balance of turfgrass, since urban landscapes are an important component of the water balance of land surfaces in this region.

Impacts
Estimates of soil respiration and CO2 exchange in the cheatgrass plant community for each of the three methods were evaluated and compared for a range of conditions. During periods when no canopy was present, the methods produced values that were in general agreement. This suggests the approaches are sound under these conditions. However, during nighttime conditions when there was a significant plant canopy the eddy covariance estimates were well below the soil respiration values of the other two approaches, even considering the limits of uncertainty. It is not yet clear why the eddy covariance has problems with nocturnal conditions only when vegetation is present. The gradient method did not produce valid results during rain events, as would be expected. It is concluded that the best approach would be to use all three methods to estimate CO2 fluxes in this type of ecosystem. The results of this study are described in a manuscript currently in press in Agricultural and Forest Meteorology. The analyses of ET values for turfgrass in 2008 are nearly completed. Preliminary results show a range of daily values similar to those of 2007, varying from 1 to 5 mm per day depending upon conditions. The Reference ET values have been calculated for each hour, and show values and variations with environmental conditions that are similar to what would be expected.

Publications

• Wang, S.Y., Gillies, R.R., Jin, J. and L.E. Hipps 2008. The recent rainfall cycle in the intermountain region as a quadrature amplitude modulation from the Pacific decadal oscillation. Geophysical Research Letters :In Press.
• Prueger, J.H., Eichinger, W.E., Hipps, L.E. and J.L. Hatfield 2008. Air flow distortion and turbulence statistics near an animal facility. Atmospheric Environment 42:3301-3314.
• Leffler, A.J. , Peek, M.S., Hipps, L .E., Ivans, S., Ryel, R.J. and M.M. Caldwell 2007. Potential contribution of respiration by Anabrus simplex (Mormon crickets) to net CO2 exchange in three Great Basin ecosystems. Western North American Naturalist 67:109-119.
• Cooper, D.I, Eichinger, W.E., Archuleta, J., Hipps, L.E., Neale, C.M.U. and J.H. Prueger 2007. An advanced method for deriving latent energy flux from a scanning Raman lidar. Agronomy Journal 99:272-284.

Progress 01/01/07 to 12/31/07

Outputs
OUTPUTS: The data from several experiments were collected and used to study the exchanges of water vapor and CO2 for different ecosystems in this region. Although the experiments are at different locations, they share a common goal of quantifying the surface fluxes of water vapor and CO2, as well as determining the key properties and processes which govern these exchanges in each ecosystem. The study sites included cheatgrass, sagebrush, Utah Juniper and turfgrass. In each location eddy covariance measurements were made over the entire year, though were some gaps in the data. Also, the experiment at the cheatgrass site ended in the fall. The cheatgrass study was conducted in southern Idaho, and made estimates of soil respiration using three approaches. These included soil fluxes of CO2 with a gradient method that uses measured soil CO2 concentrations at several depths, traditional chamber measurements made periodically over soil and vegetation, and eddy covariance. These measurements were conducted over an entire year. The eddy covariance values received all the necessary corrections to yield final flux values, so that all the methods could be compared and integrated as needed. The raw eddy covariance values were collected and stored for the sagebrush and Utah Juniper site for the entire year. But they have not yet received any further processing. Eddy covariance measurements of sensible heat and water vapor fluxes were made over a turfgrass site for the period of April through October. In addition, the net radiation and soil heat flux values were also measured. The data are being analyzed to determine the net radiation, soil heat flux, sensible heat flux and evapotranspiration or ET values at hourly and daily intervals. These values can be combined to check the ability to close the energy balance equation. The values of the energy balance closure can be used to assess the reliability of the flux estimates, and to indicate whether it might be appropriate to add to these values in order to force the equation to balance. In addition, reference ET values are being calculated for each hour, to allow the actual ET values to be compared with the traditional ET models currently used for turfgrass. All of the eddy covariance measurements in the above studies produce large data sets that must be checked for errors, and then used to calculate fluxes after applying several critical corrections. The software for these analyses has been written and tested. Finally, an eddy covariance system was sited over a meadow at the USU T.W. Daniels Experimental Forest. This is part of a larger and integrated study of hydrological and carbon exchange processes in the soil and vegetation in several ecosystems of a typical mountain watershed in the Great Basin region. The data from this eddy covariance system will be checked to see if the site is suitable for such measurements. PARTICIPANTS: Collaborators: Dr. Ron Ryel, Wildland Resources, USU; May Myklebust, Wildland Resources, USU; Dr. Kelly Kopp, Plants, Soils & Climate, USU; Dr. Roger Kjelgren, Plants, Soils & Climate, USU; Dr. Scott Jones, Plants, Soils & Climate, USU. Graduate Students: Lynda Fenton, Plants, Soils & Climate, USU. TARGET AUDIENCES: Researchers studying the role of terrestrial ecosystems in the carbon balance; researchers and water managers interetsed in the water use of turfgrass in urban areas.

Impacts
The three approaches to estimate soil respiration in the cheatgrass plant community were evaluated and compared for a range of environmental conditions and in different seasons. Each provided an independent estimate of soil respiration. For a large range of conditions the approaches produced estimates that were in good agreement when there was little or no vegetation present. Overall, the automated measurements with the soil chamber averaged 93% and manual soil chamber averaged 105% of the values resulting from the soil gradient method. The estimate for the annual value of soil efflux was equal to 406 plus or minus 73 grams of carbon per square meter in 2005. However, nighttime eddy covariance measurements were only 67% of the soil gradient measurements when canopy leaf area index, LAI, was near maximum. This suggests that eddy covariance measurements underestimate nocturnal fluxes of CO2 when LAI is near one in this system. The reasons for the apparently low eddy covariance estimates at night when vegetaton cover increased are not yet understood. The chamber method was apparently incorrect in snow, and the gradient method did not reflect surface fluxes during summer rain. In order to estimate soil efflux of such a site a combination of all three methods is recommended. The preliminary findings of the turfgrass study have yelded daily ET values for the growing season of 2007. The values vary greatly as one would expect with changes of the season and enviromental conditions. Over the April to October period they ranged from just over 1 mm per day to 5 mm per day. Under hot and dry conditions they are quite large. The energy balance closure values have only been computed for a section of the data. However, the preliminary results indicate that during the daytime hours values are typically above 0.80, and sometimes near 0.90. These are similar to others reported in the literature. There were some lower values and variability from hour to hour. The results will be more conclusive when the remainder of the data are analyzed.

Publications

• Hipps, L. 2007. Review of: Desert Meteorology, T. Warner. Cambridge University Press. Agricultural and Forest Meteorology 142:85-86.

Progress 01/01/06 to 12/31/06

Outputs
One of the major activities involved looking at the ability of eddy covariance and related measurements to close the surface energy balance for sites in an agricultural watershed. The estimates of the water vapor and sensible heat fluxes should balance the available energy. The ratio of turbulence fluxes to available energy is ideally near 1.0. Historically, the measured fluxes of heat and water vapor tend to be smaller than available energy, suggesting that the eddy covariance approach underestimates the exchanges. The energy balance was examined closely for several sites during an intensive experiment. The results displayed a fair amount of variability, but suggested the turbulence fluxes were usually underestimated to at least some extent. Inexplicably some short periods presented energy balance closure values under 0.7. The goals of the study were; to determine if lower closure values were associated with any particular atmospheric conditions, and to assess the effects of increasing the averaging period on the flux estimates. Examination of the depth of the atmospheric boundary layer, indicated no connections with the ability to close the energy balance. Power spectra and cospectra were calculated, and generally were unremarkable. However, the calculated cospectra for temperature and humidity did have an odd appearance during a period of low closure with warm and dry air advection. This is being investigated further. Possible problems with low frequency events were considered by increasing averaging periods up to 3 hours. But this resulted in no significant changes. More investigation continues, but at present it is concluded that fluxes are underestimated, and this likely includes CO2. Decisions about whether or not to force the closure of the energy balance need to be made based upon further discussion. The project also studied the effects of temporal changes in precipitation on water and CO2 fluxes for three plant communities in the Great Basin, which included crested wheatgrass, sagebrush and juniper. The first results were published this year, and showed different responses of the three communities to both seasonal or low frequency variations in precipitation, and intermittent rain events. In spring soil water was available, and the water vapor and CO2 fluxes responded only slightly to rain events. As precipitation waned in the summer, the fluxes were reduced in general. The response to periodic summer rain events was greatest in juniper, followed by sagebrush, and least at the crested wheatgrass, which usually becomes senescent at some point in the summer. However, during one summer with more frequent rain events, crested wheatgrass was able to remain active and responded more to individual events. The juniper was most responsive to summer rains, and typically doubled the assimilation of CO2 for a few days after each event. The results suggest that the effects of precipitation events on the net ecosystem exchanges of water and CO2 depend upon the history of the precipitation. So the temporal distribution of precipitation is an important factor affecting the water and carbon balances of these plant communities.

Impacts
We need to understand how agricultural lands and rangelands respond to climate, in order to manage them more effectively. In addition, it must be determined how sensitive they are to climatic fluctuations, in order to predict how the water balance and productivity of these lands may respond to future climate changes.

Publications

• Ivans, S., Hipps, L.E., Leffler, A.J. and C.A. Ivans. 2006. Response of water vapor and CO2 fluxes in semi-arid lands to seasonal and intermittent precipitation pulses. Journal of Hydrometeorology, 7: 995-1010.
• Peek, M.S., Leffler,A.J., Hipps, L.E., Ivans, S.,Ryel, R.J.and M.M. Caldwell. 2006. Significant root turnover and relocation in the soil profile in response to seasonal soil moisture in a natural stand of Utah juniper (Juniperus osteosperma). Tree Physiology, 26: 1469-1476.

Progress 01/01/05 to 12/31/05

Outputs
Several of the studies of this project including energy balance, as well as the water vapor and CO2 fluxes over an agricultural watershed, have been published. One manuscript involved the development of a model using remote sensing data to predict spatial changes in evapotranspiration. The results of the model were validated by the good agreement with flux measurements made at the surface stations. Another paper examined the connections between the surface heat fluxes and the growth of the atmospheric boundary layer using an elastic lidar sensor that measures atmospheric particulates. A published boundary layer model was used to relate the growth of the boundary layer over the region to the surface heat flux. The measurements of boundary layer growth were input into the model to predict the surface heat flux. The results agreed well with surface eddy covariance measurements, and the differences were not much larger that the uncertainty of the surface estimates. Finally, the spatial distribution of evapotranspiration and CO2 fluxes over the watershed were quantified by examining the flux measurements from an array of surface eddy covariance systems. The three Ameriflux sites near Rush Valley in Utah, continue monitoring net ecosystem exchanges water vapor and CO2 for the crested wheatgrass, sagebrush, and juniper communities. The responses of both water vapor and CO2 fluxes during the growing season to seasonal and intermittent precipitation events were examined in each vegetation type. A major observation was that the temporal distribution of precipitation is just as important as the seasonal total in governing the responses of the plant communities to rain events. When early summer precipitation was low, the grass species senesced early, and was then unable to respond to rain events later. The sagebrush and juniper were always able to show a response of both water vapor and CO2 exchanges to rain events anytime in the growing season, as they stay physiologically active. When rain events continued periodically during the summer, all the communities including the crested wheatgrass were able to stay physiologically active and respond to precipitation. Generally, the responses in water vapor and CO2 to individual summer rain events lasted two to five days. Sometimes a summer rain event initially caused a large increase in soil respiration that dominated the net exchange of CO2. Later, the increase in assimilation by the plants became more evident, and caused the net CO2 flux to become more negative, towards the surface. The actual changes in water and carbon exchanges in response to pulses of precipitation depended upon the state of the plants and soil at the time of the event. So the history of precipitation at any site at least partially governs the response of the plant community to any particular event.

Impacts
The studies here will help to document how various vegetation types in the Great Basin act as sources or sinks for carbon dioxide. This information will be needed in the long-term goal of determining the carbon balance for North America, and its role in climate change. But the results of this project will also help develop the ability to predict how these vegetation communities are likely to respond to climate change.

Publications

• Eichinger, W.E., Holder, H.E., Cooper, D.I., Hipps, L.E., Knight, R. Kustas W.P., Nichols, J. and J.H. Prueger. 2005. Lidar measurement of boundary layer evolution to determine sensible heat fluxes. Journal of Hydrometeorology, 6(6): 840-853.
• Chavez, J.L., Neale, C.M.U., Hipps, L.E., Prueger, J.H. and W.P. Kustas. 2005. Comparing aircraft-based remotely sensed energy balance fluxes with eddy covariance tower data using heat flux source area functions. Journal of Hydrometeorology, 6(6): 923-940.
• Prueger, J.H., Hatfield, J.L., Kustas, W.P., Hipps, L.E., MacPherson, J.I., Neale, C.M.U., Eichinger, W.E., Cooper, D.I. and T.B. Parkin. 2005. Tower and aircraft eddy covariance measurements of water vapor, energy, and carbon dioxide fluxes during SMACEX. Journal of Hydrometeorology, 6(6): 954-960.

Progress 01/01/04 to 12/31/04

Outputs
Analyses of data continued for the large experiment conducted in 2002 over the agricultural watershed in Iowa. This year the energy balance results and surface fluxes for sensible heat and water vapor were integrated with remote sensing data. Development of a model to predict surface evaporation, from aircraft remote sensing data. An innovative aspect of the model was the use of flux footprint analyses to determine which remote sensing pixels to use, and how much to weight tem in the comparison with the surface fluxes. The results were compared to measured surface fluxes, and good agreement was obtained. In addition, the measured surface fluxes were integrated into a study using Lidar to determine the relationship between surface heat flux and the development of the atmospheric boundary layer. The three Ameriflux sites continued to measure fluxes of water vapor and CO2 over three different plant communities near Rush Valley in Utah. The responses of the water vapor and carbon exchanges of the plant communities to both seasonal changes in precipitation, and individual events were examined. In spring, when soils were wet, the water vapor fluxes and CO2 fluxes were largest. The fluxes did not respond much to individual events in this case. However, during summer and early fall, rain events had the greatest effect on fluxes in juniper, followed by sagebrush, and finally crested wheatgrass. Juniper was most able to utilize intermittent rain events to increase assimilation of carbon, Sagebrush was able to do this as well, though to a lesser extent. Since crested wheatgrass often becomes senescent in summer, it was not able to respond as well to periodic summer rain events, especially in dry years. The results suggest that these communities will respond differently to yearly variations in precipitation. Finally, a study partially supported by this project was completed over riparian vegetation. The transpiration rate of Tamarisk along a reach of the Rio Grande in New Mexico was studied using eddy covariance measurements for two growing seasons. The total transpiration for the 1999 season was 1060 mm, while 1016 mm was measured in 2001. Daily values were variable, but could reach 9 mm per day. A model for daily water use was developed. This model integrated a model for stomatal conductance, as well as advection of warm dry air from the surroundings and the regional atmospheric boundary layer. The model needed some parameterization using the first year of data, and was validated using the second and independent year of results. The agreement between the model prediction and measured values of daily water use was very good, and within the uncertainty of the eddy covariance measurements.

Impacts
Knowledge and quantitative measurements of water and carbon flows in entire agricultural regions is lacking. This project will help combine various measurements and develop models to allow the spatial variation of water and carbon exchanges to be determined in an agricultural watershed. In addition, knowledge of how different plant communities in the Great Basin respond to variations in precipitation, will aid in understanding how any future changes in prcipitation might alter the composition of vegetation, and effects on the water and carbon balances in this region. Finally, the the model developed for daily water use of Tamarisk is being tested by the Bureau of Reclamation, for use in their decision support system for allocating water resources.

Publications

• No publications reported this period

Progress 01/01/03 to 12/31/03

Outputs
Data have been collected and analyzed for 14 different sites consisting of corn and soybean crops in a watershed in Iowa, and for crested wheatgrass, sagebrush and juniper plant communities in Utah. The hourly values of the fluxes of water vapor and CO2 were determined for each site. In 2003, the manufacturer of the water vapor and CO2 sensor used in all these measurements disclosed they had made an error in the timing of the instrument. So the measurements were not being made at the time that was indicated. This resulted in improper synchronization with the other measurements, and caused the computed fluxes to be in error. The original data had to be reanalyzed to determine what the correct offset in time should have been for the sensors. This has been completed for all the data. The new and corrected fluxes have been determined for the studies of the crested wheatgrass, sagebrush and juniper sites. The Iowa data are nearly complete. Results for the sites in Utah indicate significant differences in the response of each plant community to seasonal changes of precipitation. Crested wheat grass exhibited a net assimilation of CO2 only during a short period in the spring when soil water was still available near the surface due to winter precipitation. The vegetation then senesced, and this community always acted as a source of CO2 for the remainder of the season. The sagebrush showed the largest assimilation of carbon during the spring, but was able to continue for at least part of each day for the entire summer. This is related to its ability to access water from deeper in the soil. Juniper showed the greatest ability to maintain transpiration and assimilation of carbon throughout the entire season. Intermittent rain events in the summer caused an increase in efflux of CO2 in the crested wheatgrass, lasting for several days. No green vegetation was present to allow any assimilation in the summer. However, to some extent the sagebrush, and especially the juniper were able to increase the assimilation for several days following rain events in the summer. It is clear that the seasonal distribution of precipitation will affect the plant communities in different ways. The seasonal totals for water vapor and carbon fluxes are being calculated for each plant community. The Iowa data will soon be finished, and will be examined to determine the spatial distribution of water and CO2 fluxes over the watershed. The connections between these fluxes and the spatial variations of precipitation, as well as atmospheric properties will be examined.

Impacts
The research addresses the water and carbon balance of various vegetation communities, and how they are affected by the environment. In order to understand the global carbon balance, the amount of CO2 exchange with the atmosphere must be documented for land surfaces. This work will provide some of that information.

Publications

• Ivans, S.and L.E. Hipps. 2003. CO2 flux response to precipitation events in Juniperus osteosperma and Artemisia tridentata ecosystems using eddy covariance measurements. Absracts of European Geophysical Society Meetings, April, 2003, Nice, France.