ESTIMATION OF EVAPOTRANSPIRATION OF COTTONWOOD TREES IN THE CIBOLA NATIONAL WILDLIFE REFUGE, CIBOLA, ARIZONA

by JETTON, AMITY J.

This study used sap flow measurements and satellite imagery to estimate water use by cottonwood (Populus fremontii S. Wats. ssp) trees in an irrigated restoration plot at Cibola National Wildlife Refuge on the Lower Colorado River. Several thousand hectares of irrigated plots of this type are planned to improve riparian habitat on the river, hence it is important to know how much water the trees require. In this study, the ET rates for 20 Freemont cottonwood trees, from an 8 ha plot, were monitored over a 30-day period. ET rates were estimated by measuring sap flow through branches of the trees. Biometric scaling was used to project ET at branch to ET at tree and plot level through the ratio of basal trunk area with the cross-sectional area of the branches. The mean biometric ratio exhibited a 1:1 relationship. Sap flow ET results showed that the cottonwood tree consumed 6-11 mm day-1 of water. My main contribution in this project was working with vegetation indices from MODIS and Landsat 5 TM (TM) time-series imagery and air temperature data. I developed projected ET rates over annual cycles, based on an empirical method calibrated against moisture flux tower data in previous studies. ET estimates from satellite data were similar to concurrent measurements of ET by sap flow methods. Annual estimates of ET from satellite data were approximately 1,200 mm yr-1, with an error or uncertainty of 20-30% inherent in both the ground and remote sensing methods.