1. Timber harvest effect on soil moisture in the southern Sierra Nevada: Is there a measurable impact? Matthew W Meadows 1, Roger Bales 1, Martha Conklin 1, Michael Goulden 2, Peter Hartsough 3, Jan Hopmans 3, Carolyn Hunsaker 4, Ryan Lucas 1, Armen Malazian 3 1 Sierra Nevada Research Institute, University of California, Merced; 2 Earth System Science, University of California, Irvine; 3 Department of Land, Air and Water Resources, University of California, Davis; 4 Pacific Southwest Research Station, USDA Forest Service, Fresno, CA H11B-1165 Research is supported by the National Science Foundation through the Southern Sierra Critical Zone Observatory, a Major Research Instrumentation grant, and in cooperation with the COSMOS program and USDA Forest Service Pacific Southwest Research Station. Acknowledgments http://criticalzone.org/sierra http://cosmos.hwr.arizona.edu http://www.fs.fed.us/psw/topics/water/kingsriver Introduction We monitored precipitation, evapotranspiration, streamflow and soil-moisture in a Sierra Nevada mixed-conifer forest during water years 2010-2013. We measured volumetric water content using a COsmic-ray Soil Moisture Observing Systems (COSMOS) to estimate shallow soil-moisture storage and an eddy- covariance flux tower to measure evapotranspiration, covering an area of about 0.2 km 2. Discharge was measured in the upper Providence Creek (P301) basin, with an area of 0.54 km 2. Timber harvest occurred during the summer of 2012, involving uneven-age thinning limited to trees less than 76.2 cm diameter at breast height (DBH). Timber harvest intensity varied by tree size class: approximately 39% of the trees 0 to 25.5 cm DBH, 21% of the trees 25.5 to 50.8 cm DBH, and 4% of trees 50.8-76.2 cm DBH. Merchantable timber removed from the site was about 81-100 m 3 /ha. Findings Water year 2011 was one of the wettest years on record - approximately 196 cm of precipitation - while WY2012 and 2013 were two of the driest with 60 and 67 cm of precipitation. Each year, total summer precipitation was only 4-6 cm. Discharge during one storm in WY2011 was about the same as total annual Q in WY2012 and 2013. Annual ET was similar for all four years, averaging about 80 cm each year, despite large variability in annual precipitation amounts. Annual evapotranspiration in WY2013, following harvest, was about 10% lower than the prior 3-year average. However, 2012 and 2013 were both dry years. About one-third of the annual total ET for WY2010-2013 occurred prior to snow disappearance. Both WY2012 and 2013 had deficit annual storage. However, the four year average shows surplus water availability. Soil water content from COSMOS. Each year soil desiccation immediately followed snow-cover depletion, dropping from field capacity by about 20% volumetric water content over a 3-month period. In WY2012 and 2013 the dates of snow disappearance were 2-3 months earlier than in WY2011; occurring approximately on June 10, 2011; April 20, 2012; and March 12, 2013. Conclusions Water storage in soil and deeper regolith is derived from snowmelt and rainfall, providing moisture for evapotranspiration in the mixed-conifer forests of the Sierra Nevada, in addition to streamflow. Data to date are not enough to show significant effect on ET and soil moisture due to harvest treatment. The timing of soil desiccation largely depends on the amount of annual precipitation and the timing of snow disappearance, while ET varied little from year to year. Spring discharge WY2011 Mixed-conifer forest Eddy-covariance flux tower Increasing local respiration Measurements took place at the Southern Sierra Critical Zone Observatory (CZO) in California, USA. Eddy-flux tower and COSMOS are collocated in the upper P301 basin. The rate of soil-water loss was the same for all years. Both WY2011 and 2013 experienced small spring storms which increased soil moisture during this time period. However, this did not affect the drying trends. Water balance annual totals and four-year average 2010201120122013Average P (cm)1311966067114 Q (cm)12334213 ET (cm)7875736773 ∆S (cm)4089-17-227 Four-year water balance O N D J F M A M J J A S eddy-flux tower and COSMOS stream gauge