Water Resources Research Act Program

Details for Project ID 2008NV139B

Uncertainty and Sensitivity of Ground-Water Discharge Estimates for the Shrublands in the Great Basin Area

Institute: Nevada
Year Established: 2008 Start Date: 2008-03-01 End Date: 2011-02-28
Total Federal Funds: $17,834 Total Non-Federal Funds: $35,800

Principal Investigators: Jianting Zhu

Project Summary: Limited water supply in Nevada and the American Southwest in general will not be enough to support its rapidly growing population growth. With future needs of water resources in mind, local water authorities are looking northeast to the Great Basin. One plan among many others proposes to pump water from the Great Basin aquifers. Before such action can be taken, it is necessary to investigate the way in which the water budget for the area is influenced by pumping and the potential for long-term pumping to affect water availability to phreatophytic vegetation. This proposed project focuses on the water budget component of ground-water discharge by evapotranspiration (ET). The volume of water lost to the atmosphere through ET can be computed as the product of the ET rate and the area of combined vegetation, open water, and moist soil that contribute to ET. The three main variables that determine the ground-water discharge by ET are ET rates of individual ET units, the areas associated with the ET units and precipitation rates for the areas. Given the large area that contributes to the ground-water discharge by ET (a total of more than 4 billion m2 in 12 valleys), and the dearth of previous studies of the valleys, ground-water discharge through ET was estimated using a rather sparse dataset. As a result, the ET rate and area uncertainty have significant influence on the ground-water discharge estimates. Because discharge estimates are expected to be an important component of the overall water budget, it is beneficial to quantify the uncertainty associated with the ET estimates in order to help maximize future data collection efforts. The major effort of this project will be to identify the most influential variables that contribute most to estimate uncertainty of the ground-water discharge by ET. The analysis will be based on more physically based conditions of input variable correlations and the restraint of deterministic total area value for every valley in the area. The findings of this study will enable one to utilize limited resources more efficiently on the most influential variables in order to better understand and reduce the estimation uncertainty of ground-water discharge by ET, so that the limited resources can be used in reducing uncertainty to greatest extent for future efforts. In contrary to the previous study of ground-water discharge uncertainty, we will consider the constraint of total area in each valley and the possible correlations among the input variables, which in reality is the case. The other important novel aspect in this project is that we will use a suite of probability density functions aimed at quantifying the significance of distribution effects on ground-water discharge estimation uncertainty. This will in turn help decide the most influential features required to be characterized in future data collection efforts. In addition, we will also perform uncertainty analysis by systematically varying the variances of the input variables in each category to examine the sensitivity of the total ground-water discharge estimate to the three input variable categories (i.e., ET rates, areas, and precipitation rates) in general.