Ground Water / Surface Water Responses to Global Climate Simulations , Santa Clara - Calleguas Basin , Ventura , California 1

Abstract

Climate variations can play an important, if not always crucial, role in successful conjunctive management of ground water and surface water resources. This will require accurate accounting of the links between variations in climate, recharge, and withdrawal from the resource systems, accurate projection or predictions of the climate variations, and accurate simulation of the responses of the resource systems. To assess linkages and predictability of climate influences on conjunctive management, global climate model (GCM) simulated precipitation rates were used to estimate inflows and outflows from a regional ground water model (RGWM) of the coastal aquifers of the Santa ClaraCalleguas Basin at Ventura, California, for 1950 to 1993. Interannual to interdecadal time scales of the El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) climate variations are imparted to simulated precipitation variations in the Southern California area and are realistically imparted to the simulated ground water level variations through the climate-driven recharge (and discharge) variations. For example, the simulated average ground water level response at a key observation well in the basin to ENSO variations of tropical Pacific sea surface temperatures is 1.2 m/oC, compared to 0.9 m/oC in observations. This close agreement shows that the GCM-RGWM combination can translate global scale climate variations into realistic local ground water responses. Probability distributions of simulated ground water level excursions above a local water level threshold for potential seawater intrusion compare well to the corresponding distributions from observations and historical RGWM simulations, demonstrating the combination's potential usefulness for water management and planning. Thus the GCM-RGWM combination could be used for planning purposes and – when the GCM forecast skills are adequate – for near term predictions. (KEY TERMS: ground water hydrology; surface water hydrology; climate change; climate cycles; modeling; water management.) Hanson, Randall T. and Michael D. Dettinger, 2005. Ground Water/Surface Water Responses to Global Climate Simulations, Santa Clara-Calleguas Basin, Ventura, California. Journal of the American Water Resources Association (JAWRA) 41(3):517-536. INTRODUCTION Climate variations have important roles to play in the successful management of many ground water resources (Alley, 2001). They drive significant variations of recharge, discharge, and withdrawal of the resources in concert with, and independently from, the climatic influences on surface water resources. Ground water hydrologists and water managers for too long have neglected these climate induced variations or have treated them as purely random. Too often they have neglected the variations in near-term policy and operational decision making on seasonal to interannual time scales and have ignored them in long-term policy and capital investment decisions on interdecadal time scales (Gleick and Adams, 2000). Thus, short-term and long-term perspectives are needed on the relation of climate variability as it relates to the use, development, and sustainability of ground water resources (Alley et al., 1999). Recent studies (Hanson et al., 2002, 2004) have identified quasi-periodic variations in hydrologic time series that appear to reflect a wide range of quasiperiodic climatic forcings (Dettinger et al., 1998). For example, observed ground water levels and streamflow rates in the Santa Clara-Calleguas Basin of coastal Southern California reflect climatic forcings on time scales that range from days to decades (Hanson and Dettinger, 1996; Hanson et al., 2002). These quasi-periodic hydrologic variations represent teleconnections that appear to be driven by climatic forcings that are recurrent and persistent climatic patterns 1Paper No. 03162 of the Journal of the American Water Resources Association (JAWRA) (Copyright © 2005). Discussions are open until December 1, 2005. 2Respectively, Research Hydrologist, U.S. Geological Survey, Water Resources Division, California District, 5735 Kearny Villa Road, Suite O, San Diego, California 92123; and Research Hydrologist, Scripps Institution of Oceanography and U.S. Geological Survey, Climate Research Group, University of California, San Diego, California 92093 (E-Mail/Hanson: rthanson@usgs.gov). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 517 JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JUNE AMERICAN WATER RESOURCES ASSOCIATION 2005 GROUND WATER/SURFACE WATER RESPONSES TO GLOBAL CLIMATE SIMULATIONS, SANTA CLARA-CALLEGUAS BASIN, VENTURA, CALIFORNIA1 Randall T. Hanson and Michael D. Dettinger,2 over large parts of the Earth’s surface. For example, in the coastal Southern California basins, the largest variations in ground water levels and streamflow appear to be related to the PDO (Latif and Barnett, 1994; Mantua et al., 1997; Gershunov et al., 1999; Chao et al., 2000; Mantua and Steven, 2002) and to the ENSO (Diaz and Pulwarty, 1994; Jiang et al., 1995; Kumar et al., 2000; Dettinger et al., 2001). These variations combined can have profound effects on hydrologic systems. This is due in part to the magnitude and phase relation of the interannual and interdecadal climatic forcings that, when combined, can cause average or extreme climatic forcings that may not be obvious from any single climatic component (e.g., Minobe, 1999). The time scales and amplitudes of the hydrologic responses to climate variations depend on the time scales and mechanisms of the climate forcings, on how closely the ground water and surface water systems are coupled to each other and to climate variations, and whether the overall hydrologic responses in a given setting depend more on slower aquifer responses or more rapid streamflow responses. In some settings, especially where resources have been allocated to the brink of overdraft and where surface and ground water are tightly coupled, foreknowledge of likely climatic variations a season or more in advance (i.e., in the short term) may be a useful addition to ground water planning and management. Modern seasonal climate forecast systems are achieving levels of forecast skill that may be useful for ground water management, especially in regions where seasonal climate variations are significantly conditioned by interannual ocean/atmosphere processes such as ENSO (Gershunov et al., 1999; Stern and Easterling, 1999; Koster et al., 2000). In this paper, the results from a GCM linked to an RGWM of the Santa Clara-Calleguas Basin (Figure 1) are presented. Ten-member ensembles of simulated atmospheric circulation and attendant conditions from 1950 to 1993 from the climate model have been used to explore the range of climatic response of the RGWM and to evaluate the potential predictability of ground water systems on seasonal time scales. Simulated ground water level and streamflow responses to the ensemble climate simulations were compared with the observed hydrologic variations during the same period and with hydrologic simulations driven by observed climate variations. Climate Forecasts for Ground Water Models Most models that simulate ground water and surface water flow at the regional scale of complete watersheds are developed to assess the impacts of ground water developments and operational aspects of various resource management schemes. For this reason, most of the models eventually are used to simulate projections of realistic future conditions. The challenge of forecasting combined ground water and surface water conditions is knowing the distribution of supply and demand beforehand. Future demand is based largely on anthropogenic effects and typically is uncertain. In turn, the future variations of supplies of ground water natural recharge and surface water infiltration are driven largely by variations in precipitation. Some variations of agricultural demands for irrigation water supplied from ground water also are based on these same climatic fluctuations of precipitation. Statistical syntheses of these fluctuations are possible (Hanson and Dettinger, 1996; Hanson et al., 2002), but an alternative is to use dynamic climate forecasts from global and regional climate models to provide the driving forces for forecasting the inflows to regional river and aquifer systems (Hanson and Dettinger, 1999). Global climate forecasts and simulations include seasonal to interdecadal climate variations of the sort that are present in hydrologic time series (Hanson and Dettinger, 1996; Hanson et al., 2004) and thus can provide realistic climate series for assessing the outcomes of ground water management. On seasonal and slightly longer time scales, climate forecasts may even support forecasting with ground water flow models. The simulated linkages presented here demonstrate potential mechanisms for water purveyors and resource managers to systematically assess the impact of human decision making on the sustainability of the water resources in light of climate forecasts. This has already been demonstrated by linking this RGWM to evaluate specific future management scenarios (Hanson and Dettinger, 1996; Hanson, 1998; Hanson et al., 2002) and by linking the RGWM with simplified response rules through the use of optimization techniques (Reichard, 1995). Although the present focus is on making linkages between near term climate simulations and forecasts with RGWMs, these same linkages provide avenues for assessing impacts on ground water by global climate change (York et al., 2002; Younger et al., 2002; Yusoff et al., 2002). The purpose of this study was to establish the linkage, to assess the transmission of climate variability between models, and to evaluate the feasibility of such GCMRGWM linkages for seasonal to interannual forecasting of ground water/surface water systems. This study addresses two basic questions about the linkage between GCMs and RGWMs. First, on what time scales and where within the ground water/surface water system can the GCM and RGWM JAWRA 518 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION HANSON AND DETTINGER

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@inproceedings{Hanson2005GroundW, title={Ground Water / Surface Water Responses to Global Climate Simulations , Santa Clara - Calleguas Basin , Ventura , California 1}, author={R. T. Hanson and Michael D. Dettinger}, year={2005} }