Post-drought Groundwater Storage Recovery in 1 California ’ s Central Valley 2 3

21 Groundwater depletion is a major threat to agricultural and municipal water supply in California’s 22 Central Valley. Recent droughts during 2007–2009 and 2012–2016 exacerbated chronic groundwater 23 depletion. However, it is unclear how much groundwater storage recovered from drought-related 24 overdrafts during post-drought years, and how climatic conditions and water management affected 25 recovery times. We estimated groundwater storage change in the Central Valley for April 2002 through 26 September 2019 using four methods: GRACE satellite data, a water balance approach, a hydrologic 27 simulation model, and monitoring wells. We also evaluated the sensitivity of drought recovery to different 28 climate scenarios (recent climate ± droughts) and future climate change scenarios (20 GCMs and 2 29 RCPs). Central Valley groundwater loss ranged from 19 km (2007 – 2009) to 26 km (2012 – 2016) 30 (median of four methods). Aquifer storage recovery was 34% and 13% of the overdraft during the 2010– 31 2011 and 2017–2019 post-drought years. Numerical experiments show that recovery times are sensitive to 32 Confidential manuscript prepared for Water Resources Research 2 climate forcing, with longer recovery times for a future climate scenario that replicates historical 33 climatology relative to historical forcing with no-drought. Recovery times for groundwater pumping 34 restrictions at 30 to 50 percentiles of historic groundwater depletion were reduced by ~2× relative to no 35 pumping restrictions under no-drought future climatology. This study highlights the importance of 36 considering water management within the context of climate change scenarios to determine future 37 drought recoveries. 38 39 Plain language summary: 40 California’s Central Valley has experienced chronic groundwater depletion over the past few 41 decades, the rate of which has been amplified by droughts in 2007-2009 and 2012-2016. There is 42 limited knowledge as to how much of the drought-caused groundwater depletion has recovered 43 during post-drought years and how climate and water management affect overdraft recovery 44 times. We address these issues by estimating groundwater storage changes using four methods 45 and conducting numerical experiments with varying climatic conditions and water management 46 options. We find that less than one-third of the groundwater overdraft from the most recent 47 droughts was recovered during post-drought years. Projected overdraft recovery times vary 48 greatly depending on the climate scenarios and water management strategies, and future droughts 49 are likely to cause overdrafts from which recovery is unlikely given the current level of 50 groundwater extractions. However, management measures such as capping groundwater 51 pumping could reduce recovery times by a factor of two or more depending on the groundwater 52 extraction cap and post-drought climate. 53 54 55 56 57 58 59 60 61 Confidential manuscript prepared for Water Resources Research