Seasonal estimates of actual evapo-transpiration from Tamarix ramosissima stands using three-dimensional eddy covariance

Abstract

This study addresses the pattern of evapotranspiration (ET) throughout the growing season for Tamarix ramosissima between regularly flooded and unflooded sites. Spatial and temporal ET 2 patterns along the Middle Rio Grande demonstrated considerable variability. ET at the unflooded site was 61% of ET at the flooded site, totaling 74 and 122 cm yr -1 at the unflooded and flooded sites, respectively. The seasonal coefficient of variability was 37% and 38% at the flooded and unflooded sites, respectively. Spatial variability was 39%. Determining ET patterns with respect to the spatial, ecological and temporal setting improves riparian zone ET depletion predictions. Keywords: Rio Grande, flooding, seasonal variability, cottonwood, saltcedar, exotic species, restoration 3 IntroductionRiparian areas are prominent features within arid and semi-arid landscapes. They provide important economic, recreational and natural resources in disproportion to their limited spatial extent. Riparian water resources in the southwestern U.S. are in great demand for use in agriculture, urban development and ecosystem services (Jackson et al., 2001). Such conflicts of interests are especially acute during La Niña years, when water resources are commonly scarce (Molles & Dahm, 1990). Along the Middle Rio Grande, equitable assignment of water to competing groups and ecosystems has been hampered by the elusiveness of an accurate water budget. One of the main components lacking good quantification in the Middle Rio Grande water budget is evapotranspiration (ET) from the riparian forest, or bosque as it is known in New Mexico.Presently, ET estimates used in water budgets of the Middle Rio Grande bosque are based upon the modified Blaney-Criddle crop coefficient model (cf Hansen & Gorbach, 1997):where u is the consumptive water use, i is the month of the year, k is the empirical crop coefficient, T is the mean monthly temperature, and p is the mean monthly proportion of daylight hours (Jensen et al., 1990). Semi-empirical methods like the Blaney-Criddle model perform well over uniform vegetation like crops, but spatial, inter-annual and even seasonal variability is often poorly represented in monthly temperature and crop coefficient estimates, assuming of course that temperature controls ET in such a simple manner. Further limitations on such models are related to the applicability of crop coefficient data sources. For example, applied a model to the Middle Rio Grande bosque for Populus deltoides subsp. wislizenii (S. Wats.) Eckenw. in which the crop coefficient was determined from a three-year-old, well-watered Populus fremontii S. Watson plantation. It is currently unknown whether crop coefficients developed from juveniles provide an accurate estimate of ET above mature stands that are exposed to seasonal cycles of flooding and drought. Such variability likely limits the accuracy and utility of crop coefficient-based ET estimates.Determining sources of spatial and temporal variability in bosque ET will contribute to a more a...

Groundwater-dependent ecosystems: recent insights from satellite and field-based studies

“…Likewise, the principle drivers of water use by vegetation in GDEs were aquifer attributes (S y , regional groundwater flow), meteorology (solar radiation, vapour pressure deficit), environmental stress, and vegetation attributes (LAI, species composition) (Cleverly et al, 1997;Perkins and Sophocleous, 1999;Dahm et al, 2002;Cleverly et al, 2006;Butler et al, 2007;Lautz, 2008;Abudu et al, 2010). In general, these controls are observed in the wider literature on the controls of vegetation water use Whitley et al, 2009).…”

“…Stress tolerance, opportunistic use of groundwater and use of multiple water sources (e.g. soil moisture) have contributed to the invasive success of Tamarix (Busch et al, 1992;Cleverly et al, 1997;Di Tomaso, 1998;Nippert et al, 2010). Consequently, Tamarix inhabit sites with variable depth-togroundwater (Lite and Stromberg, 2005), which results in an amount of ET that is equivalently variable in time and space .…”

“…Of further risk to riparian GDEs and agriculture, groundwater extraction and land use change threaten riparian ecosystems (Scott et al, 1999;Nippert et al, 2010;Pert et al, 2010). Thus, many studies have been undertaken over several decades to investigate the water use of GDEs in south-western North America (van Hylckama, 1970;Gay and Fritschen, 1979;Sala et al, 1996;Devitt et al, 1998;Goodrich et al, 2000a;Cleverly et al, 2002;Scott et al, 2004;Nagler et al, 2005b). Sunlight is plentiful in the south-western USA; thus, riparian GDEs are strong carbon sinks (Kochendorfer et al, 2011).…”

Multiscale remote-sensing retrieval in the evapotranspiration of Haloxylon ammodendron in the Gurbantunggut desert, China

“…At the canopy scale, the optimum index model was established by the iteration method instead of the typical measurement (Wullschleger et al 2001;Cleverly et al 2002;Xu et al 2008) and estimation (Penuelas et al 1993;Bodner et al 2007). The hyperspectral response characteristics of the sap flow rate were first explored from the semi-empirical perspective.…”

“…At stand scale, the thermal pulse method has been effectively used to measure the stem sap flow of sub-deciduous (Wullschleger et al 2001;Zhang and Gong 2004;Zhang et al 2005). Cleverly et al (2002) estimated the seasonal actual ET from Tamarix ramosissima stands using three-dimensional eddy covariance. Moore et al (2008) estimated nocturnal transpiration in riparian Tamarix thickets authenticated by sap flux, eddy covariance and leaf gas exchange measurements.…”