Location: Southwest Watershed Research2011 Annual Report
1a. Objectives (from AD-416)
The existing project objectives (as given below) reflect the redirection towards quantification of climate change effects: 1. Develop methods and techniques for quantifying natural and anthropogenic induced ephemeral-channel runoff and subsequent recharge in cooperation with U.S. Geological Survey Tucson Science Center under current and projected climate scenarios. 2. Develop methods and techniques to quantify and predict water budgets of riparian ecosystems under current and projected climate scenarios through direct measurements of evaporation and plant transpiration and predict water savings by removal of invasive mesquite vegetation. 3. Develop methods and techniques to explicitly quantify the spatial and temporal distribution of vegetation, land use, and infiltration reduction using remotely sensed methods to improve prediction of basin scale semi-arid water budget components.
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. Satellite derived rainfall will be evaluated using raingages for large area rainfall estimation, the enhancement of recharge due to urbanization will be examined in adjacent, well instrumented, natural and residentially developed catchments. High-resolution remotely sensing and rainfall simulator experiments will be used to evaluate the capability to remote estimate infiltration rates on compacted and constructed surfaces common to development at the urban-rural interface. Remote spectral surface responses will be combined with energy balance models and radiative transfer theory to estimate surface water, carbon and energy fluxes based on observations from a network of five eddy-covariance and two Bowen ratio towers. A number of modeling components for the Automated Geospatial Watershed Assessment (AGWA) will be developed or enhanced to enable a more realistic representation of watershed processes and best management practices. AGWA will be migrated to both the internet and ARCGIS platforms to enhance usability and access. In addition we will quantify the physical mechanisms and component fluxes that are responsible for the observed ecosystem-scale water and CO2 fluxes. Scientists will carry out this research at sites located across both a riparian and an upland woody plant encroachment gradient. Continue existing research under objective #3, and add activities to develop methods and techniques to quantify and predict water budgets under current and projected climate scenarios through direct measurements of evaporation and plant transpiration, and predict water savings by removal of invasive mesquite vegetation.
3. Progress Report
A number of important advances were made on all three of our objectives, which fall under NP211. Under Objective 1 the management unit (MU) continued its membership and regular participation with Upper San Pedro Partnership (USPP-http://www.usppartnership.com/) to provide it with research and technical assistance. In addition to on-going water cycle observations for the USPP the MU has worked on obtaining improved estimates of impervious area using MU research and high resolution remotely-sensed imagery. This information will be coupled with prior runoff modeling studies to refine the estimate of groundwater recharge resulting from urbanization. Under Objective 2 research results contradicted the commonly held hypothesis that there is a positive feedback between canopy cover and soil moisture that then explains the stability of woody vegetation distribution patterns - a key finding to understand desertification. Projected hotter, dryer weather should lead to increased riparian plant water use (ET) when access to groundwater is not limited. However, actual daily ET at studied field sites would remain largely unchanged due to constraining plant mechanisms, but the longer growing season will increase riparian plant water use. In the case of woody plant encroachment into arid and semiarid lands, the current paradigm that more ecosystem carbon sequestration would occur under such conditions was not supported as we found an increase in the release of CO2 due to enhanced soil microbial respiration. Under Objective 3 a new approach was developed to map soil moisture patterns over large areas using satellites coupled with models. Activities that support all 3 Objectives include MU scientists being awarded an EPA-RARE grant from EPA-Region 8 (Denver) to use AGWA for their watershed and management and cumulative impacts assessments, which will result in approximately $100,000 being transferred to ARS. The large fires in Arizona and New Mexico prompted a request to the MU from an interagency fire science Burn Area Response team to conduct post-fire watershed assessments identifying places of increased flooding risk and targeting on-ground remediation efforts. An OECD travel fellowship was completed to Spain and an MU scientist continues to supervise a Univ. of Arizona Ph.D. student from Spain who matriculated at the UA as a result of a prior OECD travel fellowship to Spain. The unit hosted two colleagues from Kazakhstan who worked collaboratively on a Russian language web site and database which will utilize MU research to explore the effects of climate and soils on vegetation cover over roughly 60,000 square miles of rangeland in Kazakhstan.
1. Quantifying the potential for vegetation change in arid and semiarid lands. The distribution of post-storm soil moisture in arid lands is critical to understanding the feedback between moisture distribution and vegetation growth. Scientists at the Southwest Watershed Research Center found that root-zone soil moisture was significantly higher between shrubs than under shrubs, due largely to greater root density there. Results contradict the commonly held hypothesis that there is a positive feedback between canopy cover and soil moisture that explains the stability of woody vegetation distribution patterns. These findings are key for modeling and management practices related to desertification because other mechanisms likely dominate the observed worldwide expansion of woody vegetation into arid and semiarid grasslands.
2. Riparian water use and climate change. Studies on the impacts of climate change on hydrology have focused on how changes in precipitation and temperature affect runoff, with less emphasis on how plant water use (evapotranspiration or ET), which is the dominant water loss from many arid and semiarid watersheds, will change. An ARS scientist at the Southwest Watershed Research Center along with university colleagues analyzed ET and weather data from three riparian sites located in a semiarid watershed in southern Arizona and developed a simple model to estimate future plant water use (ET) rates given climate model projections. Climate predictions for this region indicate that hotter and dryer weather conditions could increase plant water use, but actual ET rates at the studied field sites will remain largely unchanged due to plant mechanisms that 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 plant water use. These findings of increased riparian water use may lead to greater groundwater deficits and decreased streamflow, further stressing water management institutions in semiarid regions.
3. A new approach for mapping soil moisture at the watershed scale. Satellite imagery can be used for landscape mapping. As part of a team effort, an ARS scientist at Tucson, AZ, merged satellite imagery with a Land Surface Model to increase the reliability of modeled soil moisture predictions at fine spatial resolutions. The approach utilized a method that enables calibrating the land surface models directly to satellite-based measurements in a way which simultaneously accounted for model parameter and measurement induced uncertainty. At resolutions finer than 100m x100m, the proposed algorithm predicted surface level soil mositure to within 4% volumetric water content 95% of the time. This result can aid the U.S. Army in planning routes and transporting personnel and supplies to save fuel and avoid unsafe, wet soils.
4. Drought-induced vegetation change affects carbon exchange in semiarid grasslands. Severe drought can lead to a change in plant community structure, which, in turn, may yield differences in how water and carbon dioxide are cycled in ecosystems. Scientists at the Southwest Watershed Research Center studied how the exchange of carbon dioxide between the atmosphere and a grassland in southern Arizona responded to a severe drought. When the drought ended the native grass species were replaced by an invasive African grass. The grassland was a source of carbon dioxide to the atmosphere during the drought and then became a sink when the drought ended and the exotic grass moved in. When another dry growing season occurred after the invasion, the grassland still took in more carbon than was released to the atmosphere. This study shows that invasive species may lead to more carbon sequestration in certain ecosystems and environments.
5. Woody encroachment affects soil carbon dioxide releases. Because carbon dioxide (CO2) released from bacterial decomposition in soils is a dominant component of ecosystem carbon dynamics, it is important to quantify how temperature, moisture, and plant activity influence this CO2 release or uptake. Scientists at the Southwest Watershed Research Center used automated measurement systems to quantify soil respiration under shrubs, grasses, and bare soil in a semiarid grassland invaded by woody plants and related patterns of CO2 uptake or release to environmental conditions. The condition immediately adjacent to the measurements (microhabitat) dramatically influenced the results. CO2 released under the mesquite trees was much larger than under grasses or bare soil. CO2 release (respiration) was not influenced by temperature in the way predicted by a commonly-used theory. As woody plants continue to expand into former grasslands, the increased carbon uptake by the more productive ecosystems commonly found in these ecosystems may be partially negated or even entirely offset by the increased carbon losses from the soil.
6. Plant/soil interactions in southwestern U.S. savannah. Recent research has suggested that tree cover in savanna systems improves under-canopy soil moisture, sustaining higher biomass in these locations. However, data suggests under-canopy plant communities are dominated by plants with well-developed drought-tolerant traits. By coupling measurements of soil water with whole-plant carbon and water fluxes, ARS scientists at the Southwest Watershed Research Center were able to show that understory soil water conditions, though less variable than between canopy spaces, were more limiting to plant carbon uptake. This finding suggests higher soil water under savanna tree canopies is very transient, and high plant densities in these microhabitats follows a reduction in temperature and light stress due to shading for plants capable of tolerating prolonged dry conditions. These findings are important in understanding controls to spatial patterns of productivity common to semiarid savanna systems.
Serrat-Capdevila, A., Scott, R.L., Shuttleworth, W.J., Valdez, J.B. 2011. Estimating evapotranspiration under warmer climates: Insights from a semiarid riparian system. Journal of Hydrology. 399: 1-11.