New Mexico Geological Society Annual Spring Meeting — Abstracts

A long-term water balance study in an experimental watershed of the Sacramento Mountains monitors the impact of thinning vegetation on groundwater recharge. The study objective is to evaluate if the tree thinning will increase groundwater recharge in the mountains to provide larger regional flows to aquifers in surrounding basins. In the semi-arid Southwest, evapotranspiration (ET) makes up 75 to 95 % or more of the total water budget. The standard methods for its quantification have large uncertainties, especially for heterogeneous land surfaces. The variability of daily vegetation transpiration and solar radiation with time of year and the effects of complex terrain on the amount of solar radiation reaching sloped land surfaces of different aspects create a seasonal and spatial variability of ET that is not well quantified in mountainous regions.

Through applying the remote sensing model METRIC (Mapping Evapotranspiration with high Resolution and Internalized Calibration) to satellite imagery from the LANDSAT satellite, we calculate high-resolution maps of ET for the Sacramento Mountains watershed area to quantify spatially-distributed estimates of ET before and after thinning. METRIC calculates ET through applying an energy balance that is calculated spatially across an image to estimate ET for each pixel that represent an area 30 meters by 30 meters. In the Sacramento Mountains watershed there are 4 sets of paired plots consisting of a control plot adjacent to the thinned plot. Differences in ET are calculated between thinned and control plots in the watershed before and after thinning. To determine the net impact of thinning on ET for an image and account for differences not due to thinning, pre-thinning ET differences between each pair of control and thinned plots are subtracted from the post-thinning differences between that same pair of control and thinned plots.

Estimates of ET from METRIC indicate a significant net decrease in ET in the first year after thinning for all of the thinned plots but show significant variability between areas with different terrain characteristics. Future analysis with more images over the course of the season in later years will help to more precisely evaluate the time-scale and rate of change of the effect of thinning on ET. Future insights based on such estimates could also help in identifying areas that have the largest response to land management practices such as tree thinning.