Contours of Corcoran Clay Depth in feet from Page (1986) for the Central Valley Hydrologic Model (CVHM)

This digital dataset defines the depth of the Corcoran Clay Member of the Tulare Formation. The complex hydrologic system of the Central Valley is simulated using the USGS numerical modeling code MODFLOW-FMP (Schmid and others, 2006b). This simulation is referred to here as the Central Valley Hydrologic Model (CVHM) (Faunt, 2009). Utilizing MODFLOW-FMP, the CVHM simulates groundwater and surface-water flow, irrigated agriculture, land subsidence, and other key processes in the Central Valley on a monthly basis from 1961-2003. Numerous lenses of fine-grained sediments are distributed throughout the southern Central Valley (San Joaquin Valley) and generally constitute more than 50 percent of the total thickness of the valley fill. Generally, these lenses are discontinuous and not vertically extensive or laterally continuous. However, the Corcoran Clay is a low-permeability, areally extensive, lacustrine deposit (Johnson and others, 1968) as much as 200-ft thick (Davis and others, 1959; Page, 1986). This continuous clay divides the groundwater-flow system of the western San Joaquin Valley into an upper semi-confined zone and a lower confined zone (Williamson and others, 1989; Belitz and Heimes, 1990; Burow and others, 2004). The extent and distribution of the Corcoran Clay was defined for use in the CVHM because the unit is one of the few deposits in the Valley that can be regionally mapped. The CVHM is the most recent regional-scale model of the Central Valley developed by the U.S. Geological Survey (USGS). The CVHM was developed as part of the USGS Groundwater Resources Program (see "Foreword", Chapter A, page iii, for details).

本数字化数据集明确了图莱里组（Tulare Formation）科科兰黏土层段的埋深。本研究采用美国地质调查局（U.S. Geological Survey, USGS）数值模拟程序MODFLOW-FMP（Schmid等，2006b），对中央谷复杂水文系统开展模拟。本模拟即为本文所称中央谷水文模型（Central Valley Hydrologic Model, CVHM）（Faunt，2009）。该模型依托MODFLOW-FMP程序，以月为时间步长，模拟了1961年至2003年间中央谷的地下水与地表水径流、灌溉农业、地面沉降及其他关键水文过程。

中央谷南部（圣华金谷，San Joaquin Valley）广泛分布着大量细粒沉积透镜体，其总厚度通常占河谷充填物总厚度的50%以上。此类透镜体通常不连续，垂向延伸范围有限且横向展布不连贯。但科科兰黏土（Corcoran Clay）属于低渗透性、区域展布广泛的湖相沉积（Johnson等，1968），最大厚度可达200英尺（Davis等，1959；Page，1986）。这套连续黏土层将圣华金谷西部的地下水流系统划分为上部半承压含水层带与下部承压含水层带（Williamson等，1989；Belitz与Heimes，1990；Burow等，2004）。

由于科科兰黏土层是该谷地少数可进行区域填图的沉积单元之一，因此明确其展布范围与分布特征被纳入中央谷水文模型的建模需求。中央谷水文模型是美国地质调查局（USGS）迄今发布的最新中央谷区域尺度水文模型。该模型系美国地质调查局地下水资源计划（详见《前言》，A章节，第iii页）的研发成果之一。