UBC Theses and Dissertations
Development of guidelines for design of sampling programs to predict groundwater discharge Cahn, Lorie Selma
The objective of this study is to develop guidelines for the design of sampling programs to predict groundwater discharge. A method for choosing a preferred sampling strategy from a set of alternatives is presented. A framework is outlined, in the form of an objective function, that incorporates both the cost of collecting data and the worth of data. A monetary value is assigned to the worth of hydraulic conductivity data by examining the economic losses associated with the uncertainty in predictions of groundwater discharge. The method is applied to the problem of designing a sampling program that measures hydraulic conductivity for predicting discharge from a rapid infiltration pond. Hydraulic conductivity data are generated for hypothetical hill slopes using a stochastic finite element model. A set of sampling strategies are selected. For each sampling strategy, the value and location of measurements and the uncertainty in the spatial variation of hydraulic conductivity are incorporated using conditional simulations. Estimates of pond discharge are calculated from the stream function solution and compared to the actual value of pond discharge for the hypothetical site. The root mean square error is used to quantify the uncertainty in discharge predictions. A set of alternative sampling strategies are compared using the objective function. Prediction uncertainty, measured by the root mean square error, is sensitive to both the structure of the heterogeneities and the location of measurements. Sampling schemes that lead to good estimates of the ensemble mean and standard deviation will not necessarily lead to good predictions of discharge. The goal of sampling schemes should be to collect data in key locations of the flow domain and to identify the spatial variation in hydraulic conductivity in a cost effective manner. For predicting discharge from a rapid infiltration pond, locating one or two initial boreholes below the pond is the preferred strategy for a majority of the cases tested. When the measurements are spaced evenly throughout the flow domain, important shallow layers may be missed that have a large influence on pond discharge. Increasing the number of boreholes does not necessarily lead to more certain predictions of pond discharge or to lower values of the objective function. Considerable uncertainty in discharge predictions can exist even with a relatively large number of measurements. While an optimal strategy exists, there is potential for significant variation in prediction uncertainty at individual sites.
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