The modeled and observed response of Lake Spokane hypolimnetic dissolved oxygen concentrations to phosphorus inputs

How Well Does the Mechanistic Water Quality Model CE‐QUAL‐W2 Represent Biogeochemical Responses to Climatic and Hydrologic Forcing?

“…However, our climate and flow sensitivity analyses showed that the model was rather insensitive to cold and warm conditions and a very wide range in flows. The model underestimated the observed DO values in Lake Spokane by ~1.2 to 2.3 mg/L (average ~26%) for the wide range of flows tested, which is similar to the modeled results previously reported for a wide range of TP IN concentrations in the Lake Spokane (~30% underestimation; Brett et al, ). When comparing the relationship between DO MIN and each variable (TP, Chl a , and temperature) with flow (Figures b, , b, and d), the modeled Chl a and temperature responses had the poorest correspondence with the observed data.…”

“…When comparing the relationship between DO MIN and each variable (TP, Chl a , and temperature) with flow (Figures b, , b, and d), the modeled Chl a and temperature responses had the poorest correspondence with the observed data. A possible reason for this underestimation is that the model's DO outcomes are poorly related to the TP‐Chl a ‐DO kinetic process representation in the model (Brett et al, ). For example, the overestimated TP R and Chl a concentrations could account for more DO depletion by organic matter decay and algal respiration in the flow sensitivity scenarios.…”

“…Higher winter peak flows will increase sediment and particulate nutrient transport, and lower summer base flows will concentrate dissolved nutrients due to reduced dilution of point source discharges. For example, in the Spokane River effluent discharges from wastewater treatment plants are about 5% of the total river flow, but the total phosphorus concentrations in these effluent discharges are 30 to 40 times higher than the background river concentrations (Brett et al, ). Therefore, if summer flows decrease, the riverine phosphorus concentrations will increase markedly due to reduced effluent dilution.…”

“…To address this issue, we conducted a series of climate and hydrologic sensitivity analyses using a version of the water quality model CE‐QUAL‐W2 (Cole & Wells, ) which has been parameterized for the Spokane River and Lake Spokane (a reservoir on the Spokane River also known as Long Lake) system during a low flow scenario. Our analysis builds on a long‐term hydrologic record for the Spokane River and a long‐term observational record for water quality in Lake Spokane (Brett et al, ; Welch et al, ). Previous studies have assessed whether the Lake Spokane Water Quality Model (Berger et al, ) is able to mechanistically represent the sensitivity of the reservoir's hypolimnetic DO concentrations to summer input total phosphorus (TP IN ) concentrations (Brett et al, ).…”

“…Our analysis builds on a long‐term hydrologic record for the Spokane River and a long‐term observational record for water quality in Lake Spokane (Brett et al, ; Welch et al, ). Previous studies have assessed whether the Lake Spokane Water Quality Model (Berger et al, ) is able to mechanistically represent the sensitivity of the reservoir's hypolimnetic DO concentrations to summer input total phosphorus (TP IN ) concentrations (Brett et al, ). Wells and Berger () and Brett and Arhonditsis () also debated ways whereby this model could be improved as a water quality management tool.…”