RESEARCHRESEARCHMy research has sought to better understand how land-use practices alter water resources. Distinctly interdisciplinary, my work with human altered systems spans the fields of Hydrology and Tree Physiology to advance understanding of how ecosystems function and how land management affects ecosystem water quality and water use.
Riparian ecohydrology in Arundo donax infested reaches on the Rio Grande
Leon River Restoration Project - Watersheds
Freeman Ranch Project - Carbon and Water Tradeoffs
Transpirational Water Loss from Salt Cedar Communities
Relationships between brush management, fuel reduction, and river ecology in Texas and New Mexico
Co-PI: Li Kui and Fan Li
Local hydrologic budget in arid and semiarid area can be drastically altered by canopy. Bulk rainfall can be partitioned into canopy interception, litter interception, stemflow and throughfall. Arundo donax grows very fast and has dense canopy. So its canopy can effectively reduce the amount of rainfall reaching the soil surface. The rain that is intercepted can either be evaporated back to the atmosphere or it can flow down the outside of the stem as stemflow. The redistribution of precipitation via stemflow can result in a concentration of water near the stem. Whether this portion of water is used by roots directly or going downward to recharge groundwater is important. However, I’m still considering the probability of measuring stemflow on Arundo, because most measurements were conducted in rainforests or on brushes.
Collaborator: John Goolsby
Although non-native to North America, giant reed (Arundo donax) is common to wetland and riparian systems in the lower half of the United States. Introduced by Spanish immigrants in the early 1800’s, Arundo has formed dense thickets lining waterways in the Lower Rio Grande basin. Despite its proximity to water resources, evidence of Arundo water consumption is lacking. Lead by research scientist John Goolsby, the USDA ARS in Weslaco, TX, has ongoing greenhouse experiments with three prospective biological control agents that use Arundo as their host: a wasp (Tetramesa romana), a fly (Cryptonevra spp.), and a root scale (Rhizaspidiotus donacis). Insect damage can lead to reduced growth and transpiration. This may lessen the impact Arundo has on the landscape, both in terms of reduced competition with native plants and reduced water consumption. The objectives of this project are to use a two-phase approach to measuring Arundo water use that combines gas exchange measurements in potted plants in a greenhouse environment with in situ measurements at two field sites. Phase one involves conducting a controlled experiment that contrasts growth rates and transpiration of plants infected with the three prospective biological control agents. The second phase involves the development and utilization of new scaling methodology to estimate transpiration in a natural stand.
Specific research objectives:
This project was funded by the Rio Grande Basin Initiative and the Texas Water Resources Institute.
Co-PI: M. Keith Owens
In the winter garden area, including La Salle County, aquifer levels in South Texas have dropped by approximately 100 feet since 1920. Rangelands cover a large proportion of the county (~91%, according to SCS 1994). Brush management in rangelands may enhance water resources by reducing evapotranspiration, but results of scientific studies so far have been inconclusive (Dugas and Mayeux 1991,Weltz and Blackburn 1995). Early studies in shallow, karstic soils indicate little improvement because of the unique hydrogeology (Wilcox et al. In Press). Sporadic, high intensity rain events are thought to be the only source of water for aquifer recharge. During such events, water is quickly transmitted through fractured rock below the rooting zone.
In contrast, because common rangeland soils of La Salle County are known to have very slow percolation rates, after large rain events, it is possible that water remains in the rooting zone for long enough to be taken up by plants. The presence of saline layers in these soils further suggest that flushing of the soil profile during large rain events occurs rarely. These areas are dominated by mesquite, whose roots likely access soil layers much deeper than forage plants. Therefore this study aims to test whether the removal of mesquite and associated brush by root plow results in less evapotranspiration and greater forage production. Because post treatment vegetation re-growth occurs quickly, we will follow trends for two years in a chronosequence of rangeland sites.
Questions of Interest
Does more water infiltrate below the rooting zone after brush removal?
What is the recovery rate of the herbaceous vegetation and how much is forage biomass increased?
How long do the potential benefits of brush removal last?
Co-PI's: Wayne Hamilton, Fred Smeins, Richard Conner, Neal Wilkins
The Leon River Restoration Project (LRRP) is an ongoing project on private lands in the Leon River watershed in Hamilton and Coryell Counties to improve the quality and quantity of water through removal of Ashe juniper (Juniperus ashei), while improving wildlife habitat and forage production for livestock. Wildlife species of particular concern include the federally listed Black-capped Vireo (BCVI) and Golden-cheeked Warbler (GCWA).
Phase I of the watershed component consisted of locating springs and small watersheds, installing monitoring equipment, and collecting pretreatment data. Phase II of the watershed study will include juniper removal on one spring of each pair. One of the three “micro” watersheds will have juniper removed, all brush will be removed on a second, and the third will serve as a control with no treatment. One of the two small watersheds on Coryell Creek will have all juniper removed and the other will be the control. Low level discharges on Bullard Creek (partially treated) and a small tributary within the Bullard Creek watershed (control) will be monitored on a continuous basis. These data will be used in basin scale modeling efforts.
Pretreatment runoff records from paired basins will be compared across multiple scales of time and place to provide inferences of hydrological process controls. The relative degree of coupling between rainfall and runoff and lag times between them provide unique signatures for each basin that will be used to make predictive models that allow for comparisons between treated and control basins. Such comparisons make it possible to quantify changes in runoff that occur after juniper removal (evapotranspiration and moisture storage/release from soils).
Recharge is affected by both vegetation water uptake (i.e. evapotranspiration) and soil permeability. In this study we will account for differences due to soil permeability and manipulate vegetation cover in order to determine the direct effect of vegetation on recharge. An improved understanding of deep drainage is needed to better inform soil water balance modeling, which in turn can be used to measure likely impacts of changing land use on groundwater recharge and quality. Using chloride mass-balance methods to measure deep drainage are a useful tool as they are fairly cost effective for examining large number of sites with standard soil sampling equipment and can make use of existing data and archived samples. Chloride is considered an environmental tracer (Scanlon 1994), being applied on the surface by precipitation.
The objectives for this study are to obtain preliminary estimates of soil chloride levels and electrical conductivity in order to provide necessary information to plan a future extensive study. We hope to find differences in deep drainage between vegetation treatments. Something that may emerge from the data is the relationship and/or affects between chloride accumulation and the position and thickness of the caliche layer, thereby helping to quantify any affect on water movement that this layer might have.
This project was funded by the Wintergarden Groundwater Conservation District and the Texas Water Resources Institute.
The focus of my research is to understand the mechanisms underlying runoff and stream flow processes in karstic rangelands for improved water quality and quantity. Since 2003, stream flow and precipitation data have been collected for a total of 11 subwatersheds within the Leon River basin. Soil characteristics and water infiltration information, along with vegetation data, collected in the field will supplement existing data and will be used to determine which factor (s) are most influential in predicting runoff. Given this information, along with topography and livestock stocking rates, areas that are considered to be especially sensitive to bacterial loading can be identified and managed appropriately.
The focus of this project is to quantify differences in soil water uptake following rain events between restored prairie and juniper-dominated sites in the Leon River basin. Four juniper-dominated and four grass-dominated plots were established, for a total of 8 20x20 meter plots. Soil moisture access tubes were installed to monitor soil moisture at 10-cm increments to a depth of 40 cm. A complete weather station at the Little Bullard site continuously monitors air temperature, humidity, wind speed and direction, net radiation, and precipitation. Soil and climate data will parameterize a plant water uptake model that will be used to compare initial wetness and rates of uptake between grass and juniper areas following rain events. Modeler Karin Rebel has agreed to collaborate in this effort.
This project was funded by the Texas Department of Agriculture.
Co-PI: M. Keith Owens
Salt cedar (Tamarix spp.) is a widespread, invasive shrub on riparian areas in west Texas and New Mexico and has become the dominant vegetation along much of the Pecos River. These phreatophytic trees are able to access water from the river bed or shallow ground water, which enables them to grow throughout much of the spring and summer seasons. Current studies report water use by these trees as high as 7.7 acre-feet per year, which would make them some of the most prodigious water users recorded. These extremely high water use estimates include subsurface lateral flow and drainage, as well as evaporation, so actual water use by the trees may be lower than originally thought. To determine if salt cedar trees can use high amounts of water and to quantify that amount, we need to measure actual transpirational water loss from mature trees during an entire growing season. The primary objective of this project is to quantify actual rates and amounts of water used by salt cedar trees to establish direct linkages between groundwater drawdown and salt cedar water use at a variety of temporal scales.
This project was funded by the Rio Grande Basin Initiative and the Texas Water Resources Institute.
Co-PI: M. Keith Owens
In the last century, riparian areas throughout the Western US have been impacted by the introduction of the invasive species, saltcedar (Tamarix sp.). Simultaneous with such shifts in species composition, western US rivers, such as the Rio Grande and Pecos, have been impacted by periodic drought and increased water withdrawal, subsequently suffering declining flows.
As part of a larger study that examines the effects of exotic plant removal, our objectives were to a) compare water use in individual stems of saltcedar and cottonwood, b) contrast flow rates of water in saltcedar stands with and without a mature cottonwood overstory, and c) contrast flow rates of water in cottonwood stands with and without an exotic saltcedar understory.
This project was funded by the USFS Rocky Mountain Research Station, Albuquerque, NM with support from the Bosque del Apache National Wildlife Refuge.