High resolution characterization of headwaters for CO₂ emission modeling

Abstract

Most information available for river mapping is based on satellite images and flyover photographs, with only limited field measurements. These methods have proven sufficient for characterizing larger (wider) rivers; however, smaller sections of river, such as headwater systems, have been historically understudied during the mapping of the watersheds. New satellite technology allows for higher resolution imaging, however, vegetative cover can obstruct the view of the satellites for smaller rivers systems which can often be found in forested areas. Without sufficient data for small river networks, it remains difficult to correctly parameterize or validate watershed hydrological and biogeochemical models. In particular, carbon dioxide emission models for rivers, which are crucial for assessing the contribution and response of ecosystems to changing climate, are heavily dependent on river surface area and flow conditions. This work investigates in-situ characteristics of a number of small river networks with the goal of recommending a generalized mathematical approach for modeling river surface area based on other more easily assessed watershed parameters. In this study, small-scale watersheds were characterized using high resolution field measurements of river width, depth, flow velocity, and GPS routing. A customized river surveying apparatus was designed and built to efficiently measure the physical parameters simultaneously. Wide field-of-view photographs were taken to quantify vegetative cover, as well as localized flow conditions and river width characterization. Twenty-three headwater reaches in Massachusetts, New Hampshire, and Virginia were characterized for evaluation of the statistical distribution of width for small rivers. Parameters such as river depth, elevation, distance upstream, and drainage area were analyzed to determine relevant trends relating to the width of the river. In general, elevation and the distance upstream from the major branch displayed inverse correlations to width, while the drainage area had a positive correlation to the width. Additionally, high resolution stream width data was studied to determine the largest measurement interval resulting in reliable overall stream width characterization. It was determined that a measurement interval of 2 m was the largest acceptable interval for first order rivers to maintain accuracy within 5 % of the mean width calculated using high resolution data (1 m intervals); the largest acceptable interval for second order rivers was 3 m.