|Science Results (Summer 2009)|
Potts, D.L. Buffalo State University
Because water is so important to life on this planet, knowing the amount of moisture in the soil is important for predicting the response of plants in natural ecosystems to events such as global warming, increases in atmospheric carbon dioxide, and changing land-use patterns. Dry lands worldwide are currently experiencing an increase in the density and cover of woody plants. Whatever the cause, woody plants influence the spatial and temporal availability of soil moisture with potentially important effects on local and regional hydrologic cycling. We monitored soil moisture at a semiarid savanna near Tucson, AZ to determine the effect that the trees had on the amount of soil moisture in the soil. We found that the tree canopy reduced the amount of precipitation input into the soil, but also that the shade of the canopy had the tendency to reduce evaporative losses so that the soil dried less quickly than soil out in the open. These findings are consistent with, and may help to explain, the results of other investigations that have examined the role of woody plants in enhancing nutrient cycling and altering carbon cycling in dryland ecosystems.
Jenerette, G.D. University of Arizona
The dynamics of vegetation greenness, including its timing, peak, and total growth exhibit substantial complexity in both space and time. This variation directly affects individual plants, vegetation communities and ecosystem functioning and is implicated as a critical biological response to global changes. The authors’ objective in this paper was to evaluate hypothesized functional relationships between precipitation and plant/ecosystem greenness in the northern Sonoran Desert using remotely-observed greenness patterns. Both summer and winter peak greenness responded positively to precipitation; however, the timing of greenness, its interactions with prior season precipitation, and distribution in space and time varied substantially. These complex analyses of greenness-precipitation relationships help elucidate the complexity in regional vegetation dynamics for a region which has two distinct rainfall seasons.
Schafer, K.V.R. Rutgers University
Development and rigorous testing of process-based carbon sequestration models is a critical feature for accurate carbon accounting. In forest systems, disturbances that remove large amounts of standing biomass are extremely important in the ability of a forest to sequester carbon. However, most large-scale process-based models do not directly account for the important micrometeorological effects such disturbances have on forest/atmosphere exchange. This study parameterized and tested the sensitivity of a forest carbon assimilation model, the 4C-A model, which utilizes whole-plant water use to better constrain estimations of canopy conductance, the feature most likely to be impacted by disturbances. Comparing model results to biomass and eddy-covariance estimates of forest carbon sequestration, the parameterized 4C-A model was found to accurately assess changes in carbon source/sink activity following wide-scale defoliation by gypsy moths. Thus, the 4C-A model has the power and sensitivity to accurately assess carbon dynamics across large spatial and temporal scales, making it a powerful tool in future carbon accounting efforts.
Nichols, M.H. Southwest Watershed Research Center
A robotic camera system is being tested for collecting information on rangelands in the southwestern US. The robotic mount allows a conventional digital camera to be used to collect hundreds of high resolution photographs which can be stitched together to create a very high resolution panoramic image. Three rangeland resources were chosen as examples to demonstrate the use of the system. Very high resolution panoramas showing riparian areas, wildlife, and invasive species were created and can be viewed on the Internet at http://gigapan.orgby searching for “rangelands”. Research is ongoing to link this technology with conventional rangeland monitoring methods to improve data collection and interpretation.
Hamerlynck, E.P. Southwest Watershed Research Center
The dominance and spread of Lehmann lovegrass into desert grasslands is in part due to its higher annual productivity and standing biomass compared to native grasses. However, field studies have shown that carbon uptake in plots dominated by Lehmann lovegrass are less efficient than native grasses at utilizing rainfall pulses. This study seeks to reconcile these conflicting findings by tracking soil water content, leaf level gas exchange, and ecosystem-level evapotranspiration (ET), net ecosystem carbon exchange (NEE) and its components, ecosystem respiration (Reco) and ecosystem photosynthesis (GEP) in Lehmann lovegrass and a native bunchgrass, bush muhly, over the course of a monsoon growing season. We found that soils under lovegrass always had higher water contents, yet ET were similar, possibly reflecting differences in canopy structure that affect rain interception and transmission to the soil. Moister soils likely allowed lovegrass to rapidly develop leaves, have lower stomatal limitations to photosynthesis, allowing greater carbon uptake, especially with drier soils later in the season. Lower and more dynamic Reco in lovegrass plots, which might reflect lower allocation to root growth, may allow more aboveground growth, which could facilitate its spread and dominance in semiarid grasslands.
Hamerlynck, E.P. Southwest Watershed Research Center
Interception and evaporation from canopies of large trees in savannas can potentially limit the amount of water infiltrating into under-canopy soils, while at the same time, lower temperatures and reduced solar radiation under canopies could limit soil evaporation losses, facilitating wetter subcanopy soils. To better understand the net effect of tree cover on soil moisture and plant performance dynamics, we measured soil water content at two depths (5 and 25 cm), and leaf-level and whole-plant water use of the perennial grass, bush muhly, growing under and between mesquite canopies at the Santa Rita Experimental Range. Overall, we found that drier soil conditions prevailed under mesquite trees, and these limited whole-plant photosynthesis, primarily by limiting the total amount of photosynthetically active area, not lower leaf-level gas exchange rates. Thus, bush muhly may dominate understory locations due to greater drought tolerance, not from direct benefits from mesquite cover.
Wei, H. University of Arizona
Runoff and erosion rates predicted from models for rangelands are important quantitative indicators for rangeland health and for assessing the effectiveness of conservation practices. Government agencies, rangeland managers, conservationists and rangeland scientists are in need of a technology that will allow them to estimate these values. In this study we developed a new technology for predicting infiltration, runoff, and soil erosion specifically for rangelands of the western United States. The new model is scientifically rigorous, in that it is based on state-of-the-art understanding of infiltration, runoff, and soil erosion processes on rangelands. It is based on an extensive set of measured data that has been collected over the past 20 years. Also, it accessible to the average user via the internet, and requires only information that is commonly collected by or available to rangeland scientists and managers. This new technology will enable improved estimation of hydrology and erosion by water on rangelands across the western United States, which will lead to an improved ability to manage this extensive and sometimes fragile natural resource.
Bayley, T. University of Arizona
Global climate change is occurring now. Historical weather records over the last century show that precipitation is increasing both in terms of the number of days we have rain and the intensities of rain. Statistical analyses of the records have indicated that there is a less than one in thousand chance that the changes in these patterns of precipitation could have occurred under a stable climate. We also have good scientific reason to believe that the changes will continue into the next century as well. Parts of the country are expected to become wetter, and parts may become drier. As rainfall changes, so does soil erosion. In this study we developed a computer simulation model to look at how changes in precipitation and temperatures might affect erosion rates in the United States. This web-based tool is called the Water Erosion Prediction Project Climate Assessment Tool (WEPPCAT). The impact of this research will be a better understanding of how climate change will affect coil erosion, as well as how we can change land use management practices such as reduced-impact tillage systems to improve conservation strategies in a future of non-stationary climate.
Zhang, Y. University of Arizona
Global climate change is occurring now. Historical weather records over the last century show that precipitation is increasing both in terms of the number of days we have rain and the intensities of rain. Statistical analyses of the records have indicated that there is a less than one in thousand chance that the changes in these patterns of precipitation could have occurred under a stable climate. We also have good scientific reason to believe that the changes will continue into the next century as well. In this study we looked at projected changes in mean annual precipitation and the power of rainfall to cause erosion (erosivity) for time periods 2030-2059 and 2070-2099 in Northeast China using future precipitation predicted from six GCM models under three green-house gas emissions scenarios (high, medium, and low). Changes were compared to 1960-1999 conditions. Changes in rainfall erosivity were 91%, 71%, and 59% under the three scenarios. Future rainfall erosivity changes may have severe detrimental impacts on soil and water resources in Northeastern China. The impact of this research will be a better understanding of how climate change will affect soil erosion, as well as point out the need for improved land management practices to improve conservation strategies in a future of non-stationary climate.
Vásquez-Méndez, R. University of Queretaro, Queretaro, México
Runoff and erosion rates from rangelands are important quantitative indicators for rangeland health and for understanding how to better apply conservation practices. Government agencies, rangeland managers, conservationists and rangeland scientists are in need of better information and data on the amount of runoff and erosion that occurs under different vegetation types in arid and semi-arid environments. In this study we measured runoff and erosion on plots with different types of vegetation in central Mexico. We found that soil surface physical conditions were different between low vegetation cover conditions and greater vegetation cover conditions, indicating a positive effect of vegetation on the regulation of surface hydrological processes. This new information will enable improved knowledge of hydrology and erosion by water on rangelands across Mexico as well as the southwestern United States, which will lead to an improved ability to manage this extensive and sometimes fragile natural resource.
Scott, R. L. Southwest Watershed Research Center
Measurements of the energy and mass exchange at the land-atmosphere interface are critical for determining local, regional and global budgets, model testing, and understanding ecosystem processes. The eddy covariance technique has become the “gold-standard” to quantify these exchanges. So, there is a continual need to assess the accuracy of these measurements as opportunities arise. This study evaluates the accuracy of evaporation measurements by eddy covariance by comparing them to an evaporation estimate derived from a seasonal or annual water balance estimate over nearly twelve years and at three sites in Arizona, USA. Though there were differing results at each site, results indicate that eddy covariance estimates were highly accurate with a mean disagreement of less than 3% in comparison with the watershed estimates. This study gives confidence to the accuracy of the data collected at these sites and to the equipment and procedures used with this measurement technique.
Nearing, G.S. University of Arizona
Mapping soil moisture over large areas is important for a variety of applications including fire, flood, and drought prediction, weather forecasting, and crop yield prediction, and a high resolution product is desirable for watershed-scale applications. Continuous and distributed soil moisture estimates can be obtained using a land surface hydrology model calibrated to soil moisture states observed by orbital radar remote sensing devices. Radar remote sensing estimates of soil moisture are corrupted by speckle which must be averaged out over large land areas, thus adversely affecting the spatial resolution of the information product. Here we develop and demonstrate a method for probabilistically accounting for speckle while simultaneously calibrating the land surface model in a way which derives a more robust information signal from the remote sensing data, thereby improving the spatial resolution of the model/image soil moisture estimation product.
Yuill, B. University of New Orleans
Sediment on the channel bed is a primary source of transported material during flash flows in dryland ephemeral channels. Often, channel bed sediment is arranged in patches that are discernable by particle size and texture. A field experiment was conducted to determine whether and how these patches evolve during successive flows. Twelve coarse (gravel-cobble) sediment patches distributed throughout the channel network within a 4.53 ha watershed in southeastern Arizona were monitored for 2 years. Changes in patch area and grain size were measured, and painted patch grains were monitored to confirm that patch grains were mobilized during flow. Individual coarse bed material patches exhibited variable persistence during flows. While no patch fully dispersed during the study period, two new patches formed. Most coarse patches remained relatively stable in area and grain-size distribution. In general, patch grains that were lost through transport were sufficiently replaced with sediment from upstream. These results provide important information for understanding the sources of sediment transported during flows.
Buono, J. University of Arizona
Overland flow velocity is an important variable in controlling the rates and amounts of erosion. However, on rangelands, it is difficult to measure because the flow is shallow and can vary considerably across the width of the flow surface. A common method to estimate the mean flow velocity has been to measure the surface velocity by injecting a dye into the flow, measuring the time the dye front takes to travel a known distance, and using a constant multiplier to convert the dye or surface velocity to mean flow velocity. Based on one comprehensive field study done in 1970, we hypothesized that the relationship between the mean flow and dye velocity was not a constant or linear as has been reported in the literature but non-linear. We conducted rainfall simulator experiments on flow surfaces representing those commonly found on rangelands in southeastern Arizona. Using digital imagery to capture the progression of the dye front with time, we computed both the fastest surface flow velocity and the average flow velocity of the dye front. The mean flow velocity was computed using salt injected into the flow and measuring the change in electrical conductivity of the runoff water with time. We found that the best predictor of the mean flow velocity was the average dye front velocity and that the relationship was non-linear. In addition, the relationship appeared to be independent of the dominant vegetation type (grass or shrub) or flow surface characteristics (slope, amount of ground cover). The experimental design and results will be aid in characterizing flow characteristics and hydraulic parameters important in erosion prediction on rangelands.
Serrat-Capdevila, A. University of Arizona
The quantification of climate change impacts on hydrology has focused on how changes in precipitation and temperature can affect runoff, with less emphasis on how evapotranspiration (ET)-- the dominant water loss from many watersheds—will change. This study focuses on estimating climate induced changes in the ET. We analyzed ET and meteorological data from three riparian sites located in a semiarid watershed in southern Arizona and found that a relatively simple model could be developed that would allow us to estimate future ET rates given climate model projections. Climate predictions for this region indicate that atmospheric evaporative demand will be greater, but actual ET rates at the studied field sites will remain largely unchanged due to plant mechanisms that will constrain the loss of water from their leaves. However, the length of the growing season is projected to increase due to warmer temperatures and this will result in a greater annual riparian water use. These findings of increased riparian water use will likely lead to greater groundwater deficits and decreased streamflow and have important implications for water management in semiarid regions.
McAuliffe, J. R. Desert Botanical Gardens
Individual studies have shown that recent protracted drought has resulted in widespread adult plant mortality across the arid Southwestern U.S., but to date, no widespread study has been undertaken to establish the patterns of regional variation in drought effects. We found that plant mortality was far greater in deciduous shrubs that rely primarily on cool season (Oct. – March) than those that rely mainly on warm season (April – Sept) precipitation, and that only the most severely drought-impacted areas of southern eastern California and southwestern Arizona showed appreciable mortality of the evergreen drought-tolerant creosotebush (Larrea tridentata). In addition, overall mortality and canopy die back best matched long-term metrics of drought intensity, which suggested plant mortality played out over areas with 5 years of dramatically reduced precipitation. Evidence of past mortality, likely from the 1950’s drought, showed that mortality in these desert communities are distinctly pulsed in nature. This is important because it suggests population and community structure in these water limited systems are not due to demographic processes that are in equilibrium, but are highly episodic in nature.