Kenneth Belitz

Verification, Validation, and Confirmation of Numerical Models in the Earth Sciences

Verification and validation of numerical models of natural systems is impossible. This is because natural systems are never closed and because model results are always nonunique. Models can be confirmed by the demonstration of agreement between observation and prediction, but confirmation is inherently partial. Complete confirmation is logically precluded by the fallacy of affirming the consequent and by incomplete access to natural phenomena. Models can only be evaluated in relative terms, and their predictive value is always open to question. The primary value of models is heuristic.

Alternative to Agricultural Drains in California's San Joaquin Valley: Results of a Regional‐Scale Hydrogeologic Approach

The occurrence of selenium in agricultural drainage water derived from the central part of the western San Joaquin Valley has focused concern on alternatives to agricultural drains for managing shallow, poor-quality groundwater. A transient, three-dimensional simulation model was developed to evaluate the response of the water table to alternatives that affect recharge to or discharge from the groundwater flow system. The modeled area is 551 mi2 (1 mi2 = 2.59 km2) and includes both the semiconfined and confined zones above and below the Corcoran Clay Member of the Tulare Formation of Pleistocene age. The simulation model was calibrated using hydrologic data from 1972 to 1988, and was extended to the year 2040 to forecast for various management alternatives, including maintenance of present practices, land retirement, reduced recharge, increased groundwater pumping, and combinations of these alternatives. Maintenance of present practices results in a worsening of the situation: the total area subject to bare-soil evaporation increases from 224 mi2 in 1990 to 344 mi2 in 2040, and drain flow increases from 25,000 ac ft/yr (1 ac ft = 1234 m3) to 28,000 ac ft/yr. Although land retirement results in elimination of bare-soil evaporation and drain flow in the areas retired, it has little to no effect in adjacent areas. In contrast, regional-scale changes in recharge and pumping are effective for regional management. The area subject to bare-soil evaporation can be reduced to 78 mi2, and drain flow to 8000 ac ft/yr if (1) recharge is reduced by 15% (26,000 ac ft/yr) in areas that currently use surface and groundwater (362 mi2); (2) recharge is reduced by 40% (28,000 ac ft/yr) in areas that currently use only surface water (137 mi2); and (3) pumping rates are uniformly incremented by 0.5 ft/yr (160,000 ac ft/yr) in both areas. If these water budget changes were to be implemented in the study area, and in adjacent areas with similiar Hydrogeologic characteristics, then approximately 400,000 ac ft/yr of surface water would be made available. Thus a shift in the hydrologic budget in the central part of the western San Joaquin Valley improves the prospects for sustaining agriculture in the area, and could provide substantial water resources for other uses.

Slug tests in elastic, unconfined aquifers: Incorporating the free surface

In this paper we evaluate slug tests in shallow, fully screened wells that completely penetrate homogeneous, anisotropic, elastic, unconfined aquifers. If the free surface is incorporated into the initial boundary value problem, the dimensionless head (HD) is a function of five parameters: dimensionless time (tD), system geometry and anisotropy (η), elastic storativity (S), specific yield (Sy), and the size of the initial perturbation relative to the initial saturated thickness (λ). If the water table is assumed to be a constant head boundary, the dependence on Sy and λ is lost. Numerical solution of the problem indicates that the dimensionless response of the well (HD versus tD) is largely a function of λ, Sy, and η the response is less sensitive to S. Application of type curves derived for wells that are cased above a fixed-head water table and screened beneath will result in an overestimate of transmissivity. The overestimate will be minimized if λ and Sy are small.

Distribution of wells in the central part of the western San Joaquin Valley, California

Information from 5,860 wells in the central part of the western San Joaquin Valley, California, was collected from several sources and compiled into a common data base. Only 2,547 wells had sufficient information for classification into four categories based on the hydrogeology: wells perforated in the semiconfined zone at depths less than or equal to 50 feet, wells perforated in the semiconfined zone at depths greater than 50 feet, wells perforated in the semiconfined and confined zones, and wells perforated only in the confined zone. Additionally, wells perforated in the semiconfined zone at depths greater than 50 feet were classified by the type of deposits in which they were perforated (Coast Range alluvium or Sierran sand). A computerized data base system was developed to manage well information and to facilitate characterizing the nature and distribution of the wells. Wells perforated in the semiconfined zone at depths less than or equal to 50 feet are evenly distributed over part of the study area underlain by shallow ground water. These wells generally are used as observation wells. Most wells perforated in the semiconfined zone at depths greater than 50 feet are perforated in the Sierran sand. This concentration of wells perforated in the Sierran sand indicates a tendency for using the Sierran sand, where it exists, as a source of water. There are 533 wells perforated in both the semiconfined and confined zones and 410 wells perforated only in the confined zone. Most of these wells are upslope of the valley trough in areas where the Sierran sand is not present. Wells perforated only in the confined zone are concentrated near the creeks.