Watershed Model Parameterization for Assessing Impacts due to Climate Change

Abstract:

The Chesapeake Bay (CB) Total Maximum Daily Load (TMDL) program drives water quality policy and management in parts of six states — Delaware, Maryland, New York, Pennsylvania, Virginia, and West Virginia — along with the District of Columbia to achieve water quality standards in the Bay through reductions in nutrient and sediment pollution. The HSPF Watershed Model (WSM) is used as an accounting tool in the development of the TMDL to track progress and guide implementations of best management practices.

Published research has shown that precipitation has increased in the US during 20^th century by about ten percent, and half of the increase is due to changes in frequency and intensity in the upper tenth percentile of the distribution. Projections from global climate models suggest that these trends are anticipated to continue over the next century. Our analysis of climate data over the last three decades show similar trends in observed precipitation in the CB Watershed. The impact of climate change on the CB TMDL will be examined in a 2017 assessment of progress in the State and Federal partnership of the Chesapeake Bay Program. This is consistent with the CB Executive Order of May 12, 2009 mandates assessment of the impacts of climate change on the CB TMDL.

The WSM has a simulation period of more than 3 decades from 1985 to 2011. Over the simulation period precipitation intensity, temperatures, and CO₂ levels are increasing. A study conducted by Najjar et al. (2010) that included regional climate projections suggests that pollutant loads in the CB region will increase over the next century. Butcher et al. (2014) demonstrated that a watershed model parameter needs to be adjusted to compensate for the effect of elevated CO₂ concentrations on plant transpiration in climate projection applications. This raises the question of whether parameters within a watershed model calibrated using historical climate data are sufficient for assessing hydrologic and water quality impacts of climate change. Here we present an analysis of HSPF model parameters and calibrations to examine parameter sensitivity to a changing climate and their associations with trends in climate. This study explores ways to improve confidence in model predictions to support management actions.