The role of interior watershed processes in improving parameter estimation and performance of watershed models

Authors: YEN, HAW - Texas Agrilife Research; BAILEY, RYAN - Colorado State University; ARABI, MAZDAK - Colorado State University; AHMADI, MEHDI - Colorado State University; White, Michael; Arnold, Jeffrey

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2013
Publication Date: 8/24/2014
Publication URL: http://handle.nal.usda.gov/10113/60445
Citation: Yen, H., Bailey, R.T., Arabi, M., Ahmadi, M., White, M.J., Arnold, J.G. 2014. The role of interior watershed processes in improving parameter estimation and performance of watershed models. Journal of Environmental Quality. 43:1601-1613.

Interpretive Summary: Watershed models are typically evaluated only through comparison with measured data, with little attention paid to the “reasonableness” of internal model processes which govern its predictions. Circumstances may arise in which the model appears mimic measured data well, but is internally incorrect. In this study, rules of thumb are used to constrain internal model processes while also matching available in-stream measured data. Results show that a model can meet these constrains while matching in-stream data. Without these additional constraints, predicted nutrient abatement strategies could be misleading, even with a model that seems to perform well using traditional criteria.

Technical Abstract: Watershed models typically are evaluated solely through comparison of in-stream water and nutrient fluxes with measured data using established performance criteria, whereas processes and responses within the interior of the watershed that govern these global fluxes often are neglected. Due to the large number of parameters at the disposal of these models, circumstances may arise in which excellent global results are achieved using inaccurate magnitudes of these “intra-watershed” responses. When used for scenario analysis, a given model hence may inaccurately predict the global, in-stream effect of implementing land-use practices at the interior of the watershed. In this study, data regarding internal watershed behavior are used to constrain parameter estimation to maintain realistic intra-watershed responses while also matching available in-stream monitoring data. The methodology is demonstrated for the Eagle Creek Watershed in central Indiana, USA. Streamflow and nitrate (NO3) loading are used as global in-stream comparisons, with two processes, the annual mass of denitrification and the ratio of NO3 losses from subsurface and surface flow, used to constrain parameter estimation. Results show that imposing these constraints not only yields realistic internal watershed behavior, but also provides good in-stream comparisons. Results further demonstrate that in the absence of incorporating intra-watershed constraints, evaluation of nutrient abatement strategies could be misleading, even though typical performance criteria are satisfied. Incorporating intra-watershed responses yields a watershed model that more accurately represents the observed behavior of the system, and hence a tool that can be used with confidence in scenario evaluation.