Populus Fremontii Tree Ring Analysis and Semi-Arid River Water Source Variability over Time, San Pedro River, Arizona

Abstract

Summer floods are an important source of sustained streamflow in arid and semi-arid rivers of the American Southwest and Northwest Mexico. The degree to which natural function versus human alterations influence the system is subject to debate. Environmental information in the tree ring cellulose of Populus can be used to investigate the variation in water sources over time in these areas. Past research has shown that streamflow sources in the San Pedro Basin of Arizona vary isotopically between a source water of basin ground water and a summer flood water source. This study uses isotopic analyses of Populus fremontii and atmospheric data in the San Pedro Basin to estimate the water source of the trees and the river water source condition. After analyzing weather data within the basin, an inversion of the Barbour oxygen isotope model using tree ring cellulose isotopes was used to obtain the water source isotopic composition. The variation in water source composition inferred from the model was then compared to the river composition over time. It was initially found that each site’s water source isotopic composition was significantly different from the source water. However, several water source isotopic compositions were found to be more negative than the known basin groundwater signature in each of the study sites. Following sensitivity analyses on various parameters within the model, it was seen that relative humidity has a strong influence on the determination of source water. Therefore, relative humidity must be an accurate measurement and is not considered to be so in this study. Furthermore, in order to understand the degree to which natural function versus human alterations influence the system, older Populus fremontii tree ring isotopes are needed, posing a question regarding the reliability of the species.