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The sustainability of irrigated agriculture in many arid and semiarid areas of …

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Model Environment
- Sustainability of irrigated agriculture in the San Joaquin Valley, California

The adapted modeling approach is based on the coupling of a soil chemistry module (20) with a regional-scale hydrology model (21) to yield an integrated approach for simulating three-dimensional variably saturated subsurface flow and reactive salt transport (13). The horizontal boundaries of the model domain coincided with the hydrologic boundaries of an earlier regional groundwater flow model (6), defined by the trough of the San Joaquin Valley on the east, the Coast Range foothills in the west, and no-flow boundaries in the north and south of the regional flow domain (Fig. 1 A). The model domain was discretized into a regular finite difference grid of 2,960 square cells of 805-m (0.5 mi) side length and 64-ha area, corresponding to a typical field size. In the vertical direction, the model domain extended from the land surface to the top of the Corcoran clay, using 17 layers of increasing thickness from the surface downwards. The total number of active model grid cells was 36,040. Hydrologic flows and salt transport were simulated for a 57-year period, from 1940 to 1997, using annual average boundary conditions and grid cell-specific soil parameters (Figs. 1 B and C and 5). The salinity module included reactions such as cation exchange and precipitation and dissolution of gypsum and calcite (22, 23). By using historical crop acreage and water delivery records for each water district, crops and irrigation amounts were randomly distributed, leading to the annual assignment of a single crop to each grid cell. Other required boundary conditions were needed to quantify vertical (across Corcoran clay and into deep groundwater) and lateral (toward San Joaquin River) water flow and salt fluxes and exchange between the simulated domain and its surroundings (13), so that an annual salt balance could be estimated. Spatially distributed water flow and salinity reaction and transport parameters were obtained from soil survey data and 242 well logs (more information is available in Supporting Text). Hydrological parameter values were either optimized (15, 24) or obtained from existing information (see Tables 1 and 2, which are published as supporting information on the PNAS web site).

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