|Nagler, P. - UNIVERSITY OF ARIZONA|
|Westenburg, C. - USGS|
|Cleverly, J. - UNIVERSITY OF NEW MEXICO|
|Glenn, E. - UNIVERSITY OF ARIZONA|
|Huete, A. - UNIVERSITY OF ARIZONA|
Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 8, 2005
Publication Date: August 15, 2005
Citation: Nagler, P.L., Scott, R.L., Westenburg, C., Cleverly, J.R., Glenn, E.P., Huete, A.R. 2005. Evapotranspiration on western u.s. rivers estimated using the enhanced vegetation index from modis and data from eddy covariance and bowen ratio flux towers. Remote Sensing of Environment 97(3): 337-351. Interpretive Summary: Evapotranspiration (ET) by riparian vegetation is an important component of the water budget of arid and semi-arid watersheds. While accurate estimates of riparian zone ET are needed to properly and soundly apportion river water for human and environmental needs, these estimates are not widely available and are costly to produce using current on-the-ground technologies. This study develops and applies a method that uses information readily available from satellites to estimate ET over portions of three western U.S. rivers. The method is shown to accurately reproduce estimates made from on the ground instruments and to derive river reach estimates that fit within the range of recent estimates made by other approaches. The results of this study suggest that this relatively simple approach might provide accurate and cost effective riparian ET estimates in basins with similar climate and vegetation.
Technical Abstract: We combined remote sensing and in-situ measurements to estimate evapotranspiration (ET) from riparian vegetation over large reaches of western U.S. rivers and ET by individual plant types. ET measured from nine flux towers (eddy covariance and Bowen ratio) established in plant communities dominated by five major plant types on the Middle Rio Grande, Upper San Pedro River, and Lower Colorado River was strongly correlated with Enhanced Vegetation Index (EVI) values from the Moderate Resolution Imaging Spectrometer (MODIS) sensor on the NASA Terra satellite. The inclusion of maximum daily air temperatures (Ta) measured at the tower sites further improved this relationship. Sixteen-day composite values of EVI and Ta were combined to predict ET across species and tower sites (r2 = 0.74); the regression equation was used to scale ET for 2000–2004 over large river reaches with Ta from meteorological stations. Measured and estimated ET values for these river segments were moderate when compared to historical, and often indirect, estimates and ranged from 851–874 mm yr' 1. ET of individual plant communities ranged more widely. Cottonwood (Populus spp.) and willow (Salix spp.) stands generally had the highest annual ET rates (1100–1300 mm yr' 1), while mesquite (Prosopis velutina) (400–1100 mm yr' 1) and saltcedar (Tamarix ramosissima) (300–1300 mm yr' 1) were intermediate, and giant sacaton (Sporobolus wrightii) (500–800 mm yr' 1) and arrowweed (Pluchea sericea) (300–700 mm yr' 1) were the lowest. ET rates estimated from the flux towers and by remote sensing in this study were much lower than values estimated for riparian water budgets using crop coefficient methods for the Middle Rio Grande and Lower Colorado River.