THE SIGINIFICANCE OF MODEL STRUCTURE IN ONE-DIMENSIONAL STREAM SOLUTE TRANSPORT MODELS WITH MULTIPLE TRANSIENT STORAGE ZONES

Open Access
Author:
Kerr, Patrick Corbitt
Graduate Program:
Civil Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 07, 2011
Committee Members:
  • Mike Gooseff, Thesis Advisor
  • Michael N Gooseff, Thesis Advisor
Keywords:
  • transient storage
  • modeling
  • hyporheic
  • stream
Abstract:
Transient storage models are used to simulate solute transport in streams. The processes characterized and the parameters interpreted from these models are important to our understanding of biogeochemical cycling. Single storage zone (1-SZ) models lump portions of the stream with different biogeochemical and hydrodynamic characteristics into one zone. Recent progress has been made to develop transient storage zone models with two-storage zones (2-SZ) to discriminate between surface and subsurface (hyporheic) exchange. Surface transient storage (STS) is the non-advective portion of channels that surround and sometimes divide the main channel, while hyporheic transient storage (HTS) is the wetted subsurface area adjacent to the stream. These storage zones are subject to different physical and biogeochemical conditions. Thus, to advance biogeochemical models, we want to separate the effects of these storage zones. In models with multiple transient storage zones, each storage zone can exchange with the stream, another storage zone, or both. In a competing model structure both zones are connected to the stream at the same location and the stream interacts with the STS and HTS separately; whereas in a nested model structure the STS interacts directly with the stream and the HTS interacts only with the STS. Residence time metrics used to compare single transient storage models were adapted for use with the competing and nested 2-SZ models and are model-structure based. We investigated the transient storage characteristics of a 460-m reach of Laurel Run, a first-order stream in central Pennsylvania, using a 1-SZ, nested 2-SZ, and competing 2-SZ model configurations. This study modeled the solute breakthrough curves from three salt tracer experiments ranging from high to low flows and compared the results. Results show that both the competing and nested 2-SZ models iv may accurately simulate the BTCs obtained in the STS and main channel. The calibrated model parameters differ for the three model structures, and therefore so does the interpretation of associated transient storage metrics. The new 2-SZ metrics were applied to this data and the results suggest that the interpretation of transient storage processes and its relationship with biogeochemical cycling processes is dependent on model structure.