Multiscale Drivers of Water Level Fluctuations and Nutrient Availability in Small Intermittent Reservoirs in the US Midwest

Authors

Uchechukwu Vitus Chinatu Ogbenna, University of Nebraska-LincolnFollow

First Advisor

Jessica Corman

Committee Members

Aaron Mittlestet, Daniel Uden, David Admiraal, Shannon Bartelt‑Hunt

Date of this Version

8-2025

Document Type

Thesis

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Natural Resource Sciences

Under the supervision of Professor Jessica Corman

Lincoln, Nebraska, August 2025

Comments

Copyright 2025, Uchechukwu Vitus Chinatu Ogbenna. Used by permission

Abstract

Reservoir dynamics are often captured by fluctuations in water surface elevation, which reflect a complex interplay of hydrologic inputs, climatic conditions, land use influences, and management operations. While water level fluctuations (WLFs) in large regulated systems have been widely studied, the drivers and implications of WLFs in small reservoirs, particularly those fed by intermittent streams, remain poorly understood. Here, we examined the temporal dynamics of WLFs and their relationship with nutrient availability in nine small reservoirs in the U.S. Midwest using daily records from 1985 to 2022. We used water level classification from modified Zscore and transition matrices to assess the stability and dynamics of water levels across daily, monthly, and annual timescales. Mixed-effects and generalized linear models were used to identify the physical and environmental drivers of water level changes, as well as their relationships to summer nutrient concentrations. Results indicate strong stability in low (< -1.5 zscore) and normal (–1.5 < Z < 1.5) water level conditions across timescales, with elevated dynamics at the monthly scale. In addition to lag effect, precipitation was the dominant driver at daily and annual scales, while seasonal and land use interactions were more influential at monthly resolution. Agricultural land use appeared to moderate the immediate effects of rainfall, likely due to increased infiltration and catchment retention. The results from our nutrient models revealed that nitrate and phosphate concentrations were significantly associated with water surface elevation, windspeed, and temperature, pointing to both external loading and internal processes such as sediment-water interaction and oxygen availability through mixing events. Our findings demonstrates the need to integrate scale-dependent hydrologic variability into reservoir management and nutrient mitigation strategies, particularly in small reservoirs fed by intermittent streams and are subject to climatic and landscape control.

Advisor: Jessica Corman